    Network Working Group                                         J. Lazzaro
    Request for Comments: 4695                                  J. Wawrzynek
    Category: Standards Track                                    UC Berkeley
                                                               November 2006

                          RTP Payload Format for MIDI

    Status of This Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The IETF Trust (2006).

    Abstract

       This memo describes a Real-time Transport Protocol (RTP) payload
       format for the MIDI (Musical Instrument Digital Interface) command
       language.  The format encodes all commands that may legally appear on
       a MIDI 1.0 DIN cable.  The format is suitable for interactive
       applications (such as network musical performance) and content-
       delivery applications (such as file streaming).  The format may be
       used over unicast and multicast UDP and TCP, and it defines tools for
       graceful recovery from packet loss.  Stream behavior, including the
       MIDI rendering method, may be customized during session setup.  The
       format also serves as a mode for the mpeg4-generic format, to support
       the MPEG 4 Audio Object Types for General MIDI, Downloadable Sounds
       Level 2, and Structured Audio.

    Table of Contents

       1. Introduction ....................................................4
          1.1. Terminology ................................................5
          1.2. Bitfield Conventions .......................................6
       2. Packet Format ...................................................6
          2.1. RTP Header .................................................7
          2.2. MIDI Payload ..............................................11
       3. MIDI Command Section ...........................................12
          3.1.  Timestamps ...............................................14
          3.2.  Command Coding ...........................................16

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       4. The Recovery Journal System ....................................22
       5. Recovery Journal Format ........................................24
       6. Session Description Protocol ...................................28
          6.1. Session Descriptions for Native Streams ...................29
          6.2. Session Descriptions for mpeg4-generic Streams ............30
          6.3. Parameters ................................................33
       7. Extensibility ..................................................34
       8. Congestion Control .............................................35
       9. Security Considerations ........................................35
       10. Acknowledgements ..............................................36
       11. IANA Considerations ...........................................37
          11.1. rtp-midi Media Type Registration .........................37
               11.1.1. Repository Request for "audio/rtp-midi" ...........40
          11.2. mpeg4-generic Media Type Registration ....................41
               11.2.1. Repository Request for Mode rtp-midi for
                       mpeg4-generic .....................................44
          11.3. asc Media Type Registration ..............................46
       A. The Recovery Journal Channel Chapters ..........................48
          A.1. Recovery Journal Definitions ..............................48
          A.2. Chapter P: MIDI Program Change ............................52
          A.3. Chapter C: MIDI Control Change ............................53
               A.3.1. Log Inclusion Rules ................................54
               A.3.2. Controller Log Format ..............................55
               A.3.3. Log List Coding Rules ..............................57
               A.3.4. The Parameter System ...............................60
          A.4. Chapter M: MIDI Parameter System ..........................62
               A.4.1. Log Inclusion Rules ................................64
               A.4.2. Log Coding Rules ...................................65
                     A.4.2.1. The Value Tool .............................67
                     A.4.2.2. The Count Tool .............................70
          A.5. Chapter W: MIDI Pitch Wheel ...............................71
          A.6. Chapter N: MIDI NoteOff and NoteOn ........................71
               A.6.1. Header Structure ...................................73
               A.6.2. Note Structures ....................................74
          A.7. Chapter E: MIDI Note Command Extras .......................75
               A.7.1. Note Log Format ....................................76
               A.7.2. Log Inclusion Rules ................................76
          A.8. Chapter T: MIDI Channel Aftertouch ........................77
          A.9. Chapter A: MIDI Poly Aftertouch ...........................78
       B. The Recovery Journal System Chapters ...........................79
          B.1. System Chapter D: Simple System Commands ..................79
               B.1.1. Undefined System Commands ..........................80
          B.2. System Chapter V: Active Sense Command ....................83
          B.3. System Chapter Q: Sequencer State Commands ................83
               B.3.1. Non-compliant Sequencers ...........................85
          B.4. System Chapter F: MIDI Time Code Tape Position ............86
               B.4.1. Partial Frames .....................................88

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          B.5. System Chapter X: System Exclusive ........................89
               B.5.1. Chapter Format .....................................90
               B.5.2. Log Inclusion Semantics ............................92
               B.5.3. TCOUNT and COUNT Fields ............................95
       C. Session Configuration Tools ....................................95
          C.1. Configuration Tools: Stream Subsetting ....................97
          C.2. Configuration Tools: The Journalling System ..............101
               C.2.1. The j_sec Parameter ...............................102
               C.2.2. The j_update Parameter ............................103
                     C.2.2.1. The anchor Sending Policy .................104
                     C.2.2.2. The closed-loop Sending Policy ............104
                     C.2.2.3. The open-loop Sending Policy ..............108
               C.2.3. Recovery Journal Chapter Inclusion Parameters .....110
          C.3. Configuration Tools: Timestamp Semantics .................115
               C.3.1. The comex Algorithm ...............................115
               C.3.2. The async Algorithm ...............................116
               C.3.3. The buffer Algorithm ..............................117
          C.4. Configuration Tools: Packet Timing Tools .................118
               C.4.1. Packet Duration Tools .............................119
               C.4.2. The guardtime Parameter ...........................120
          C.5. Configuration Tools: Stream Description ..................121
          C.6. Configuration Tools: MIDI Rendering ......................128
               C.6.1. The multimode Parameter ...........................129
               C.6.2. Renderer Specification ............................129
               C.6.3. Renderer Initialization ...........................131
               C.6.4. MIDI Channel Mapping ..............................133
                     C.6.4.1. The smf_info Parameter ....................134
                     C.6.4.2. The smf_inline, smf_url, and smf_cid
                              Parameters ................................136
                     C.6.4.3. The chanmask Parameter ....................136
               C.6.5. The audio/asc Media Type ..........................137
          C.7. Interoperability .........................................139
               C.7.1. MIDI Content Streaming Applications ...............139
               C.7.2. MIDI Network Musical Performance Applications .....142
       D. Parameter Syntax Definitions ..................................150
       E. A MIDI Overview for Networking Specialists ....................156
          E.1. Commands Types ...........................................159
          E.2. Running Status ...........................................159
          E.3. Command Timing ...........................................160
          E.4. AudioSpecificConfig Templates for MMA Renderers ..........160
       References .......................................................165
       Normative References .............................................165
       Informative References ...........................................166

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    1.  Introduction

       The Internet Engineering Task Force (IETF) has developed a set of
       focused tools for multimedia networking ([RFC3550] [RFC4566]
       [RFC3261] [RFC2326]).  These tools can be combined in different ways
       to support a variety of real-time applications over Internet Protocol
       (IP) networks.

       For example, a telephony application might use the Session Initiation
       Protocol (SIP, [RFC3261]) to set up a phone call.  Call setup would
       include negotiations to agree on a common audio codec [RFC3264].
       Negotiations would use the Session Description Protocol (SDP,
       [RFC4566]) to describe candidate codecs.

       After a call is set up, audio data would flow between the parties
       using the Real Time Protocol (RTP, [RFC3550]) under any applicable
       profile (for example, the Audio/Visual Profile (AVP, [RFC3551])).
       The tools used in this telephony example (SIP, SDP, RTP) might be
       combined in a different way to support a content streaming
       application, perhaps in conjunction with other tools, such as the
       Real Time Streaming Protocol (RTSP, [RFC2326]).

       The MIDI (Musical Instrument Digital Interface) command language
       [MIDI] is widely used in musical applications that are analogous to
       the examples described above.  On stage and in the recording studio,
       MIDI is used for the interactive remote control of musical
       instruments, an application similar in spirit to telephony.  On web
       pages, Standard MIDI Files (SMFs, [MIDI]) rendered using the General
       MIDI standard [MIDI] provide a low-bandwidth substitute for audio
       streaming.

       This memo is motivated by a simple premise: if MIDI performances
       could be sent as RTP streams that are managed by IETF session tools,
       a hybridization of the MIDI and IETF application domains may occur.

       For example, interoperable MIDI networking may foster network music
       performance applications, in which a group of musicians, located at
       different physical locations, interact over a network to perform as
       they would if they were located in the same room [NMP].  As a second
       example, the streaming community may begin to use MIDI for low-
       bitrate audio coding, perhaps in conjunction with normative sound
       synthesis methods [MPEGSA].

       To enable MIDI applications to use RTP, this memo defines an RTP
       payload format and its media type.  Sections 2-5 and Appendices A-B
       define the RTP payload format.  Section 6 and Appendices C-D define
       the media types identifying the payload format, the parameters needed
       for configuration, and how the parameters are utilized in SDP.

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       Appendix C also includes interoperability guidelines for the example
       applications described above: network musical performance using SIP
       (Appendix C.7.2) and content-streaming using RTSP (Appendix C.7.1).

       Another potential application area for RTP MIDI is MIDI networking
       for professional audio equipment and electronic musical instruments.
       We do not offer interoperability guidelines for this application in
       this memo.  However, RTP MIDI has been designed with stage and studio
       applications in mind, and we expect that efforts to define a stage
       and studio framework will rely on RTP MIDI for MIDI transport
       services.

       Some applications may require MIDI media delivery at a certain
       service quality level (latency, jitter, packet loss, etc).  RTP
       itself does not provide service guarantees.  However, applications
       may use lower-layer network protocols to configure the quality of the
       transport services that RTP uses.  These protocols may act to reserve
       network resources for RTP flows [RFC2205] or may simply direct RTP
       traffic onto a dedicated "media network" in a local installation.
       Note that RTP and the MIDI payload format do provide tools that
       applications may use to achieve the best possible real-time
       performance at a given service level.

       This memo normatively defines the syntax and semantics of the MIDI
       payload format.  However, this memo does not define algorithms for
       sending and receiving packets.  An ancillary document [RFC4696]
       provides informative guidance on algorithms.  Supplemental
       information may be found in related conference publications [NMP]
       [GRAME].

       Throughout this memo, the phrase "native stream" refers to a stream
       that uses the rtp-midi media type.  The phrase "mpeg4-generic stream"
       refers to a stream that uses the mpeg4-generic media type (in mode
       rtp-midi) to operate in an MPEG 4 environment [RFC3640].  Section 6
       describes this distinction in detail.

    1.1.  Terminology

       In this document, the key words "MUST", "MUST NOT", "REQUIRED",
       "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
       and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
       [RFC2119].

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    1.2.  Bitfield Conventions

       In this document, the packet bitfields that share a common name often
       have identical semantics.  As most of these bitfields appear in
       Appendices A-B, we define the common bitfield names in Appendix A.1.

       However, a few of these common names also appear in the main text of
       this document.  For convenience, we list these definitions below:

         o R flag bit.  R flag bits are reserved for future use.  Senders
           MUST set R bits to 0.  Receivers MUST ignore R bit values.

         o LENGTH field.  All fields named LENGTH (as distinct from LEN)
           code the number of octets in the structure that contains it,
           including the header it resides in and all hierarchical levels
           below it.  If a structure contains a LENGTH field, a receiver
           MUST use the LENGTH field value to advance past the structure
           during parsing, rather than use knowledge about the internal
           format of the structure.

    2.  Packet Format

       In this section, we introduce the format of RTP MIDI packets.  The
       description includes some background information on RTP, for the
       benefit of MIDI implementors new to IETF tools.  Implementors should
       consult [RFC3550] for an authoritative description of RTP.

       This memo assumes that the reader is familiar with MIDI syntax and
       semantics.  Appendix E provides a MIDI overview, at a level of detail
       sufficient to understand most of this memo.  Implementors should
       consult [MIDI] for an authoritative description of MIDI.

       The MIDI payload format maps a MIDI command stream (16 voice channels
       + systems) onto an RTP stream.  An RTP media stream is a sequence of
       logical packets that share a common format.  Each packet consists of
       two parts: the RTP header and the MIDI payload.  Figure 1 shows this
       format (vertical space delineates the header and payload).

       We describe RTP packets as "logical" packets to highlight the fact
       that RTP itself is not a network-layer protocol.  Instead, RTP
       packets are mapped onto network protocols (such as unicast UDP,
       multicast UDP, or TCP) by an application [ALF].  The interleaved mode
       of the Real Time Streaming Protocol (RTSP, [RFC2326]) is an example
       of an RTP mapping to TCP transport, as is [RFC4571].

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    2.1.  RTP Header

       [RFC3550] provides a complete description of the RTP header fields.
       In this section, we clarify the role of a few RTP header fields for
       MIDI applications.  All fields are coded in network byte order (big-
       endian).

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | V |P|X|  CC   |M|     PT      |        Sequence number        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           Timestamp                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                             SSRC                              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                     MIDI command section ...                  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                       Journal section ...                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 1 -- Packet format

       The behavior of the 1-bit M field depends on the media type of the
       stream.  For native streams, the M bit MUST be set to 1 if the MIDI
       command section has a non-zero LEN field, and MUST be set to 0
       otherwise.  For mpeg4-generic streams, the M bit MUST be set to 1 for
       all packets (to conform to [RFC3640]).

       In an RTP MIDI stream, the 16-bit sequence number field is
       initialized to a randomly chosen value and is incremented by one
       (modulo 2^16) for each packet sent in the stream.  A related
       quantity, the 32-bit extended packet sequence number, may be computed
       by tracking rollovers of the 16-bit sequence number.  Note that
       different receivers of the same stream may compute different extended
       packet sequence numbers, depending on when the receiver joined the
       session.

       The 32-bit timestamp field sets the base timestamp value for the
       packet.  The payload codes MIDI command timing relative to this
       value.  The timestamp units are set by the clock rate parameter.  For
       example, if the clock rate has a value of 44100 Hz, two packets whose
       base timestamp values differ by 2 seconds have RTP timestamp fields
       that differ by 88200.

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       Note that the clock rate parameter is not encoded within each RTP
       MIDI packet.  A receiver of an RTP MIDI stream becomes aware of the
       clock rate as part of the session setup process.  For example, if a
       session management tool uses the Session Description Protocol (SDP,
       [RFC4566]) to describe a media session, the clock rate parameter is
       set using the rtpmap attribute.  We show examples of session setup in
       Section 6.

       For RTP MIDI streams destined to be rendered into audio, the clock
       rate SHOULD be an audio sample rate of 32 KHz or higher.  This
       recommendation is due to the sensitivity of human musical perception
       to small timing errors in musical note sequences, and due to the
       timbral changes that occur when two near-simultaneous MIDI NoteOns
       are rendered with a different timing than that desired by the content
       author due to clock rate quantization.  RTP MIDI streams that are not
       destined for audio rendering (such as MIDI streams that control stage
       lighting) MAY use a lower clock rate but SHOULD use a clock rate high
       enough to avoid timing artifacts in the application.

       For RTP MIDI streams destined to be rendered into audio, the clock
       rate SHOULD be chosen from rates in common use in professional audio
       applications or in consumer audio distribution.  At the time of this
       writing, these rates include 32 KHz, 44.1 KHz, 48 KHz, 64 KHz, 88.2
       KHz, 96 KHz, 176.4 KHz, and 192 KHz.  If the RTP MIDI session is a
       part of a synchronized media session that includes another (non-MIDI)
       RTP audio stream with a clock rate of 32 KHz or higher, the RTP MIDI
       stream SHOULD use a clock rate that matches the clock rate of the
       other audio stream.  However, if the RTP MIDI stream is destined to
       be rendered into audio, the RTP MIDI stream SHOULD NOT use a clock
       rate lower than 32 KHz, even if this second stream has a clock rate
       less than 32 KHz.

       Timestamps of consecutive packets do not necessarily increment at a
       fixed rate, because RTP MIDI packets are not necessarily sent at a
       fixed rate.  The degree of packet transmission regularity reflects
       the underlying application dynamics.  Interactive applications may
       vary the packet sending rate to track the gestural rate of a human
       performer, whereas content-streaming applications may send packets at
       a fixed rate.

       Therefore, the timestamps for two sequential RTP packets may be
       identical, or the second packet may have a timestamp arbitrarily
       larger than the first packet (modulo 2^32).  Section 3 places
       additional restrictions on the RTP timestamps for two sequential RTP
       packets, as does the guardtime parameter (Appendix C.4.2).

       We use the term "media time" to denote the temporal duration of the
       media coded by an RTP packet.  The media time coded by a packet is

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       computed by subtracting the last command timestamp in the MIDI
       command section from the RTP timestamp (modulo 2^32).  If the MIDI
       list of the MIDI command section of a packet is empty, the media time
       coded by the packet is 0 ms.  Appendix C.4.1 discusses media time
       issues in detail.

       We now define RTP session semantics, in the context of sessions
       specified using the session description protocol [RFC4566].  A
       session description media line ("m=") specifies an RTP session.  An
       RTP session has an independent space of 2^32 synchronization sources.
       Synchronization source identifiers are coded in the SSRC header field
       of RTP session packets.  The payload types that may appear in the PT
       header field of RTP session packets are listed at the end of the
       media line.

       Several RTP MIDI streams may appear in an RTP session.  Each stream
       is distinguished by a unique SSRC value and has a unique sequence
       number and RTP timestamp space.  Multiple streams in the RTP session
       may be sent by a single party.  Multiple parties may send streams in
       the RTP session.  An RTP MIDI stream encodes data for a single MIDI
       command name space (16 voice channels + Systems).

       Streams in an RTP session may use different payload types, or they
       may use the same payload type.  However, each party may send, at
       most, one RTP MIDI stream for each payload type mapped to an RTP MIDI
       payload format in an RTP session.  Recall that dynamic binding of
       payload type numbers in [RFC4566] lets a party map many payload type
       numbers to the RTP MIDI payload format; thus a party may send many
       RTP MIDI streams in a single RTP session.  Pairs of streams (unicast
       or multicast) that communicate between two parties in an RTP session
       and that share a payload type have the same association as a MIDI
       cable pair that cross-connects two devices in a MIDI 1.0 DIN network.

       The RTP session architecture described above is efficient in its use
       of network ports, as one RTP session (using a port pair per party)
       supports the transport of many MIDI name spaces (16 MIDI channels +
       systems).  We define tools for grouping and labelling MIDI name
       spaces across streams and sessions in Appendix C.5 of this memo.

       The RTP header timestamps for each stream in an RTP session have
       separately and randomly chosen initialization values.  Receivers use
       the timing fields encoded in the RTP control protocol (RTCP,
       [RFC3550]) sender reports to synchronize the streams sent by a party.
       The SSRC values for each stream in an RTP session are also separately
       and randomly chosen, as described in [RFC3550].  Receivers use the
       CNAME field encoded in RTCP sender reports to verify that streams
       were sent by the same party, and to detect SSRC collisions, as
       described in [RFC3550].

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       In some applications, a receiver renders MIDI commands into audio (or
       into control actions, such as the rewind of a tape deck or the
       dimming of stage lights).  In other applications, a receiver presents
       a MIDI stream to software programs via an Application Programmer
       Interface (API).  Appendix C.6 defines session configuration tools to
       specify what receivers should do with a MIDI command stream.

       If a multimedia session uses different RTP MIDI streams to send
       different classes of media, the streams MUST be sent over different
       RTP sessions.  For example, if a multimedia session uses one MIDI
       stream for audio and a second MIDI stream to control a lighting
       system, the audio and lighting streams MUST be sent over different
       RTP sessions, each with its own media line.

       Session description tools defined in Appendix C.5 let a sending party
       split a single MIDI name space (16 voice channels + systems) over
       several RTP MIDI streams.  Split transport of a MIDI command stream
       is a delicate task, because correct command stream reconstruction by
       a receiver depends on exact timing synchronization across the
       streams.

       To support split name spaces, we define the following requirements:

         o  A party MUST NOT send several RTP MIDI streams that share a MIDI
            name space in the same RTP session.  Instead, each stream MUST
            be sent from a different RTP session.

         o  If several RTP MIDI streams sent by a party share a MIDI name
            space, all streams MUST use the same SSRC value and MUST use the
            same randomly chosen RTP timestamp initialization value.

       These rules let a receiver identify streams that share a MIDI name
       space (by matching SSRC values) and also let a receiver accurately
       reconstruct the source MIDI command stream (by using RTP timestamps
       to interleave commands from the two streams).  Care MUST be taken by
       senders to ensure that SSRC changes due to collisions are reflected
       in both streams.  Receivers MUST regularly examine the RTCP CNAME
       fields associated with the linked streams, to ensure that the assumed
       link is legitimate and not the result of an SSRC collision by another
       sender.

       Except for the special cases described above, a party may send many
       RTP MIDI streams in the same session.  However, it is sometimes
       advantageous for two RTP MIDI streams to be sent over different RTP
       sessions.  For example, two streams may need different values for RTP
       session-level attributes (such as the sendonly and recvonly
       attributes).  As a second example, two RTP sessions may be needed to
       send two unicast streams in a multimedia session that originate on

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       different computers (with different IP numbers).  Two RTP sessions
       are needed in this case because transport addresses are specified on
       the RTP-session or multimedia-session level, not on a payload type
       level.

       On a final note, in some uses of MIDI, parties send bidirectional
       traffic to conduct transactions (such as file exchange).  These
       commands were designed to work over MIDI 1.0 DIN cable networks may
       be configured in a multicast topology, which use pure "party-line"
       signalling.  Thus, if a multimedia session ensures a multicast
       connection between all parties, bidirectional MIDI commands will work
       without additional support from the RTP MIDI payload format.

    2.2. MIDI Payload

       The payload (Figure 1) MUST begin with the MIDI command section.  The
       MIDI command section codes a (possibly empty) list of timestamped
       MIDI commands, and provides the essential service of the payload
       format.

       The payload MAY also contain a journal section.  The journal section
       provides resiliency by coding the recent history of the stream.  A
       flag in the MIDI command section codes the presence of a journal
       section in the payload.

       Section 3 defines the MIDI command section.  Sections 4-5 and
       Appendices A-B define the recovery journal, the default format for
       the journal section.  Here, we describe how these payload sections
       operate in a stream in an RTP session.

       The journalling method for a stream is set at the start of a session
       and MUST NOT be changed thereafter.  A stream may be set to use the
       recovery journal, to use an alternative journal format (none are
       defined in this memo), or not to use a journal.

       The default journalling method of a stream is inferred from its
       transport type.  Streams that use unreliable transport (such as UDP)
       default to using the recovery journal.  Streams that use reliable
       transport (such as TCP) default to not using a journal.  Appendix
       C.2.1 defines session configuration tools for overriding these
       defaults.  For all types of transport, a sender MUST transmit an RTP
       packet stream with consecutive sequence numbers (modulo 2^16).

       If a stream uses the recovery journal, every payload in the stream
       MUST include a journal section.  If a stream does not use
       journalling, a journal section MUST NOT appear in a stream payload.
       If a stream uses an alternative journal format, the specification for
       the journal format defines an inclusion policy.

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       If a stream is sent over UDP transport, the Maximum Transmission Unit
       (MTU) of the underlying network limits the practical size of the
       payload section (for example, an Ethernet MTU is 1500 octets), for
       applications where predictable and minimal packet transmission
       latency is critical.  A sender SHOULD NOT create RTP MIDI UDP packets
       whose size exceeds the MTU of the underlying network.  Instead, the
       sender SHOULD take steps to keep the maximum packet size under the
       MTU limit.

       These steps may take many forms.  The default closed-loop recovery
       journal sending policy (defined in Appendix C.2.2.2) uses RTP control
       protocol (RTCP, [RFC3550]) feedback to manage the RTP MIDI packet
       size.  In addition, Section 3.2 and Appendix B.5.2 provide specific
       tools for managing the size of packets that code MIDI System
       Exclusive (0xF0) commands.  Appendix C.5 defines session
       configuration tools that may be used to split a dense MIDI name space
       into several UDP streams (each sent in a different RTP session, per
       Section 2.1) so that the payload fits comfortably into an MTU.
       Another option is to use TCP.  Section 4.3 of [RFC4696] provides
       non-normative advice for packet size management.

    3.  MIDI Command Section

       Figure 2 shows the format of the MIDI command section.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |B|J|Z|P|LEN... |  MIDI list ...                                |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 2 -- MIDI command section

       The MIDI command section begins with a variable-length header.

       The header field LEN codes the number of octets in the MIDI list that
       follow the header.  If the header flag B is 0, the header is one
       octet long, and LEN is a 4-bit field, supporting a maximum MIDI list
       length of 15 octets.

       If B is 1, the header is two octets long, and LEN is a 12-bit field,
       supporting a maximum MIDI list length of 4095 octets.  LEN is coded
       in network byte order (big-endian): the 4 bits of LEN that appear in
       the first header octet code the most significant 4 bits of the 12-bit
       LEN value.

       A LEN value of 0 is legal, and it codes an empty MIDI list.

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       If the J header bit is set to 1, a journal section MUST appear after
       the MIDI command section in the payload.  If the J header bit is set
       to 0, the payload MUST NOT contain a journal section.

       We define the semantics of the P header bit in Section 3.2.

       If the LEN header field is nonzero, the MIDI list has the structure
       shown in Figure 3.

          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  Delta Time 0     (1-4 octets long, or 0 octets if Z = 1)     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  MIDI Command 0   (1 or more octets long)                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  Delta Time 1     (1-4 octets long)                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  MIDI Command 1   (1 or more octets long)                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                              ...                              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  Delta Time N     (1-4 octets long)                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  MIDI Command N   (0 or more octets long)                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 3 -- MIDI list structure

       If the header flag Z is 1, the MIDI list begins with a complete MIDI
       command (coded in the MIDI Command 0 field, in Figure 3) preceded by
       a delta time (coded in the Delta Time 0 field).  If Z is 0, the Delta
       Time 0 field is not present in the MIDI list, and the command coded
       in the MIDI Command 0 field has an implicit delta time of 0.

       The MIDI list structure may also optionally encode a list of N
       additional complete MIDI commands, each coded in a MIDI Command K
       field.  Each additional command MUST be preceded by a Delta Time K
       field, which codes the command's delta time.  We discuss exceptions
       to the "command fields code complete MIDI commands" rule in Section
       3.2.

       The final MIDI command field (i.e., the MIDI Command N field, shown
       in Figure 3) in the MIDI list MAY be empty.  Moreover, a MIDI list
       MAY consist a single delta time (encoded in the Delta Time 0 field)
       without an associated command (which would have been encoded in the
       MIDI Command 0 field).  These rules enable MIDI coding features that
       are explained in Section 3.1.  We delay the explanations because an
       understanding of RTP MIDI timestamps is necessary to describe the
       features.

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    3.1.  Timestamps

       In this section, we describe how RTP MIDI encodes a timestamp for
       each MIDI list command.  Command timestamps have the same units as
       RTP packet header timestamps (described in Section 2.1 and
       [RFC3550]).  Recall that RTP timestamps have units of seconds, whose
       scaling is set during session configuration (see Section 6.1 and
       [RFC4566]).

       As shown in Figure 3, the MIDI list encodes time using a compact
       delta-time format.  The RTP MIDI delta time syntax is a modified form
       of the MIDI File delta time syntax [MIDI].  RTP MIDI delta times use
       1-4 octet fields to encode 32-bit unsigned integers.  Figure 4 shows
       the encoded and decoded forms of delta times.  Note that delta time
       values may be legally encoded in multiple formats; for example, there
       are four legal ways to encode the zero delta time (0x00, 0x8000,
       0x808000, 0x80808000).

       RTP MIDI uses delta times to encode a timestamp for each MIDI
       command.  The timestamp for MIDI Command K is the summation (modulo
       2^32) of the RTP timestamp and decoded delta times 0 through K.  This
       cumulative coding technique, borrowed from MIDI File delta time
       coding, is efficient because it reduces the number of multi-octet
       delta times.

       All command timestamps in a packet MUST be less than or equal to the
       RTP timestamp of the next packet in the stream (modulo 2^32).

       This restriction ensures that a particular RTP MIDI packet in a
       stream is uniquely responsible for encoding time starting at the
       moment after the RTP timestamp encoded in the RTP packet header, and
       ending at the moment before the final command timestamp encoded in
       the MIDI list.  The "moment before" and "moment after" qualifiers
       acknowledge the "less than or equal" semantics (as opposed to
       "strictly less than") in the sentence above this paragraph.

       Note that it is possible to "pad" the end of an RTP MIDI packet with
       time that is guaranteed to be void of MIDI commands, by setting the
       "Delta Time N" field of the MIDI list to the end of the void time,
       and by omitting its corresponding "MIDI Command N" field (a syntactic
       construction the preamble of Section 3 expressly made legal).

       In addition, it is possible to code an RTP MIDI packet to express
       that a period of time in the stream is void of MIDI commands.  The
       RTP timestamp in the header would code the start of the void time.
       The MIDI list of this packet would consist of a "Delta Time 0" field

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       that coded the end of the void time.  No other fields would be
       present in the MIDI list (a syntactic construction the preamble of
       Section 3 also expressly made legal).

       By default, a command timestamp indicates the execution time for the
       command.  The difference between two timestamps indicates the time
       delay between the execution of the commands.  This difference may be
       zero, coding simultaneous execution.  In this memo, we refer to this
       interpretation of timestamps as "comex" (COMmand EXecution)
       semantics.  We formally define comex semantics in Appendix C.3.

       The comex interpretation of timestamps works well for transcoding a
       Standard MIDI File (SMF) into an RTP MIDI stream, as SMFs code a
       timestamp for each MIDI command stored in the file.  To transcode an
       SMF that uses metric time markers, use the SMF tempo map (encoded in
       the SMF as meta-events) to convert metric SMF timestamp units into
       seconds-based RTP timestamp units.

       The comex interpretation also works well for MIDI hardware
       controllers that are coding raw sensor data directly onto an RTP MIDI
       stream.  Note that this controller design is preferable to a design
       that converts raw sensor data into a MIDI 1.0 cable command stream
       and then transcodes the stream onto an RTP MIDI stream.

       The comex interpretation of timestamps is usually not the best
       timestamp interpretation for transcoding a MIDI source that uses
       implicit command timing (such as MIDI 1.0 DIN cables) into an RTP
       MIDI stream.  Appendix C.3 defines alternatives to comex semantics
       and describes session configuration tools for selecting the timestamp
       interpretation semantics for a stream.

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            One-Octet Delta Time:

               Encoded form: 0ddddddd
               Decoded form: 00000000 00000000 00000000 0ddddddd

            Two-Octet Delta Time:

               Encoded form: 1ccccccc 0ddddddd
               Decoded form: 00000000 00000000 00cccccc cddddddd

            Three-Octet Delta Time:

               Encoded form: 1bbbbbbb 1ccccccc 0ddddddd
               Decoded form: 00000000 000bbbbb bbcccccc cddddddd

            Four-Octet Delta Time:

               Encoded form: 1aaaaaaa 1bbbbbbb 1ccccccc 0ddddddd
               Decoded form: 0000aaaa aaabbbbb bbcccccc cddddddd

                      Figure 4 -- Decoding delta time formats

    3.2.  Command Coding

       Each non-empty MIDI Command field in the MIDI list codes one of the
       MIDI command types that may legally appear on a MIDI 1.0 DIN cable.
       Standard MIDI File meta-events do not fit this definition and MUST
       NOT appear in the MIDI list.  As a rule, each MIDI Command field
       codes a complete command, in the binary command format defined in
       [MIDI].  In the remainder of this section, we describe exceptions to
       this rule.

       The first MIDI channel command in the MIDI list MUST include a status
       octet.  Running status coding, as defined in [MIDI], MAY be used for
       all subsequent MIDI channel commands in the list.  As in [MIDI],
       System Common and System Exclusive messages (0xF0 ... 0xF7) cancel
       the running status state, but System Real-time messages (0xF8 ...
       0xFF) do not affect the running status state.  All System commands in
       the MIDI list MUST include a status octet.

       As we note above, the first channel command in the MIDI list MUST
       include a status octet.  However, the corresponding command in the
       original MIDI source data stream might not have a status octet (in
       this case, the source would be coding the command using running
       status).  If the status octet of the first channel command in the
       MIDI list does not appear in the source data stream, the P (phantom)
       header bit MUST be set to 1.  In all other cases, the P bit MUST be
       set to 0.

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       Note that the P bit describes the MIDI source data stream, not the
       MIDI list encoding; regardless of the state of the P bit, the MIDI
       list MUST include the status octet.

       As receivers MUST be able to decode running status, sender
       implementors should feel free to use running status to improve
       bandwidth efficiency.  However, senders SHOULD NOT introduce timing
       jitter into an existing MIDI command stream through an inappropriate
       use or removal of running status coding.  This warning primarily
       applies to senders whose RTP MIDI streams may be transcoded onto a
       MIDI 1.0 DIN cable [MIDI] by the receiver: both the timestamps and
       the command coding (running status or not) must comply with the
       physical restrictions of implicit time coding over a slow serial
       line.

       On a MIDI 1.0 DIN cable [MIDI], a System Real-time command may be
       embedded inside of another "host" MIDI command.  This syntactic
       construction is not supported in the payload format: a MIDI Command
       field in the MIDI list codes exactly one MIDI command (partially or
       completely).

       To encode an embedded System Real-time command, senders MUST extract
       the command from its host and code it in the MIDI list as a separate
       command.  The host command and System Real-time command SHOULD appear
       in the same MIDI list.  The delta time of the System Real-time
       command SHOULD result in a command timestamp that encodes the System
       Real-time command placement in its original embedded position.

       Two methods are provided for encoding MIDI System Exclusive (SysEx)
       commands in the MIDI list.  A SysEx command may be encoded in a MIDI
       Command field verbatim: a 0xF0 octet, followed by an arbitrary number
       of data octets, followed by a 0xF7 octet.

       Alternatively, a SysEx command may be encoded as multiple segments.
       The command is divided into two or more SysEx command segments; each
       segment is encoded in its own MIDI Command field in the MIDI list.

       The payload format supports segmentation in order to encode SysEx
       commands that encode information in the temporal pattern of data
       octets.  By encoding these commands as a series of segments, each
       data octet may be associated with a distinct delta time.
       Segmentation also supports the coding of large SysEx commands across
       several packets.

       To segment a SysEx command, first partition its data octet list into
       two or more sublists.  The last sublist MAY be empty (i.e., contain
       no octets); all other sublists MUST contain at least one data octet.
       To complete the segmentation, add the status octets defined in Figure

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       5 to the head and tail of the first, last, and any "middle" sublists.
       Figure 6 shows example segmentations of a SysEx command.

       A sender MAY cancel a segmented SysEx command transmission that is in
       progress, by sending the "cancel" sublist shown in Figure 5.  A
       "cancel" sublist MAY follow a "first" or "middle" sublist in the
       transmission, but MUST NOT follow a "last" sublist.  The cancel MUST
       be empty (thus, 0xF7 0xF4 is the only legal cancel sublist).

       The cancellation feature is needed because Appendix C.1 defines
       configuration tools that let session parties exclude certain SysEx
       commands in the stream.  Senders that transcode a MIDI source onto an
       RTP MIDI stream under these constraints have the responsibility of
       excluding undesired commands from the RTP MIDI stream.

       The cancellation feature lets a sender start the transmission of a
       command before the MIDI source has sent the entire command.  If a
       sender determines that the command whose transmission is in progress
       should not appear on the RTP stream, it cancels the command.  Without
       a method for cancelling a SysEx command transmission, senders would
       be forced to use a high-latency store-and-forward approach to
       transcoding SysEx commands onto RTP MIDI packets, in order to
       validate each SysEx command before transmission.

       The recommended receiver reaction to a cancellation depends on the
       capabilities of the receiver.  For example, a sound synthesizer that
       is directly parsing RTP MIDI packets and rendering them to audio will
       be aware of the fact that SysEx commands may be cancelled in RTP
       MIDI.  These receivers SHOULD detect a SysEx cancellation in the MIDI
       list and act as if they had never received the SysEx command.

       As a second example, a synthesizer may be receiving MIDI data from an
       RTP MIDI stream via a MIDI DIN cable (or a software API emulation of
       a MIDI DIN cable).  In this case, an RTP-MIDI-aware system receives
       the RTP MIDI stream and transcodes it onto the MIDI DIN cable (or its
       emulation).  Upon the receipt of the cancel sublist, the RTP-MIDI-
       aware transcoder might have already sent the first part of the SysEx
       command on the MIDI DIN cable to the receiver.

       Unfortunately, the MIDI DIN cable protocol cannot directly code
       "cancel SysEx in progress" semantics.  However, MIDI DIN cable
       receivers begin SysEx processing after the complete command arrives.
       The receiver checks to see if it recognizes the command (coded in the
       first few octets) and then checks to see if the command is the
       correct length.  Thus, in practice, a transcoder can cancel a SysEx
       command by sending an 0xF7 to (prematurely) end the SysEx command --
       the receiver will detect the incorrect command length and discard the
       command.

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       Appendix C.1 defines configuration tools that may be used to prohibit
       SysEx command cancellation.

       The relative ordering of SysEx command segments in a MIDI list must
       match the relative ordering of the sublists in the original SysEx
       command.  By default, commands other than System Real-time MIDI
       commands MUST NOT appear between SysEx command segments (Appendix C.1
       defines configuration tools to change this default, to let other
       commands types appear between segments).  If the command segments of
       a SysEx command are placed in the MIDI lists of two or more RTP
       packets, the segment ordering rules apply to the concatenation of all
       affected MIDI lists.

              -----------------------------------------------------------
             | Sublist Position |  Head Status Octet | Tail Status Octet |
             |-----------------------------------------------------------|
             |    first         |       0xF0         |       0xF0        |
             |-----------------------------------------------------------|
             |    middle        |       0xF7         |       0xF0        |
             |-----------------------------------------------------------|
             |    last          |       0xF7         |       0xF7        |
             |-----------------------------------------------------------|
             |    cancel        |       0xF7         |       0xF4        |
              -----------------------------------------------------------

                   Figure 5 -- Command segmentation status octets

       [MIDI] permits 0xF7 octets that are not part of a (0xF0, 0xF7) pair
       to appear on a MIDI 1.0 DIN cable.  Unpaired 0xF7 octets have no
       semantic meaning in MIDI, apart from cancelling running status.

       Unpaired 0xF7 octets MUST NOT appear in the MIDI list of the MIDI
       Command section.  We impose this restriction to avoid interference
       with the command segmentation coding defined in Figure 5.

       SysEx commands carried on a MIDI 1.0 DIN cable may use the "dropped
       0xF7" construction [MIDI].  In this coding method, the 0xF7 octet is
       dropped from the end of the SysEx command, and the status octet of
       the next MIDI command acts both to terminate the SysEx command and
       start the next command.  To encode this construction in the payload
       format, follow these steps:

         o  Determine the appropriate delta times for the SysEx command and
            the command that follows the SysEx command.

         o  Insert the "dropped" 0xF7 octet at the end of the SysEx command,
            to form the standard SysEx syntax.

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         o  Code both commands into the MIDI list using the rules above.

         o  Replace the 0xF7 octet that terminates the verbatim SysEx
            encoding or the last segment of the segmented SysEx encoding
            with a 0xF5 octet.  This substitution informs the receiver of
            the original dropped 0xF7 coding.

       [MIDI] reserves the undefined System Common commands 0xF4 and 0xF5
       and the undefined System Real-time commands 0xF9 and 0xFD for future
       use.  By default, undefined commands MUST NOT appear in a MIDI
       Command field in the MIDI list, with the exception of the 0xF5 octets
       used to code the "dropped 0xF7" construction and the 0xF4 octets used
       by SysEx "cancel" sublists.

       During session configuration, a stream may be customized to transport
       undefined commands (Appendix C.1).  For this case, we now define how
       senders encode undefined commands in the MIDI list.

       An undefined System Real-time command MUST be coded using the System
       Real-time rules.

       If the undefined System Common commands are put to use in a future
       version of [MIDI], the command will begin with an 0xF4 or 0xF5 status
       octet, followed by an arbitrary number of data octets (i.e., zero or
       more data bytes).  To encode these commands, senders MUST terminate
       the command with an 0xF7 octet and place the modified command into
       the MIDI Command field.

       Unfortunately, non-compliant uses of the undefined System Common
       commands may appear in MIDI implementations.  To model these
       commands, we assume that the command begins with an 0xF4 or 0xF5
       status octet, followed by zero or more data octets, followed by zero
       or more trailing 0xF7 status octets.  To encode the command, senders
       MUST first remove all trailing 0xF7 status octets from the command.
       Then, senders MUST terminate the command with an 0xF7 octet and place
       the modified command into the MIDI Command field.

       Note that we include the trailing octets in our model as a cautionary
       measure: if such commands appeared in a non-compliant use of an
       undefined System Common command, an RTP MIDI encoding of the command
       that did not remove trailing octets could be mistaken for an encoding
       of "middle" or "last" sublist of a segmented SysEx commands (Figure
       5) under certain packet loss conditions.

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              Original SysEx command:

                  0xF0 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0xF7

              A two-segment segmentation:

                  0xF0 0x01 0x02 0x03 0x04 0xF0

                  0xF7 0x05 0x06 0x07 0x08 0xF7

              A different two-segment segmentation:

                  0xF0 0x01 0xF0

                  0xF7 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0xF7

              A three-segment segmentation:

                  0xF0 0x01 0x02 0xF0

                  0xF7 0x03 0x04 0xF0

                  0xF7 0x05 0x06 0x07 0x08 0xF7

             The segmentation with the largest number of segments:

                  0xF0 0x01 0xF0

                  0xF7 0x02 0xF0

                  0xF7 0x03 0xF0

                  0xF7 0x04 0xF0

                  0xF7 0x05 0xF0

                  0xF7 0x06 0xF0

                  0xF7 0x07 0xF0

                  0xF7 0x08 0xF0

                  0xF7 0xF7

                         Figure 6 -- Example segmentations

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    4.  The Recovery Journal System

       The recovery journal is the default resiliency tool for unreliable
       transport.  In this section, we normatively define the roles that
       senders and receivers play in the recovery journal system.

       MIDI is a fragile code.  A single lost command in a MIDI command
       stream may produce an artifact in the rendered performance.  We
       normatively classify rendering artifacts into two categories:

         o Transient artifacts.  Transient artifacts produce immediate but
           short-term glitches in the performance.  For example, a lost
           NoteOn (0x9) command produces a transient artifact: one note
           fails to play, but the artifact does not extend beyond the end of
           that note.

         o Indefinite artifacts.  Indefinite artifacts produce long-lasting
           errors in the rendered performance.  For example, a lost NoteOff
           (0x8) command may produce an indefinite artifact: the note that
           should have been ended by the lost NoteOff command may sustain
           indefinitely.  As a second example, the loss of a Control Change
           (0xB) command for controller number 7 (Channel Volume) may
           produce an indefinite artifact: after the loss, all notes on the
           channel may play too softly or too loudly.

       The purpose of the recovery journal system is to satisfy the recovery
       journal mandate: the MIDI performance rendered from an RTP MIDI
       stream sent over unreliable transport MUST NOT contain indefinite
       artifacts.

       The recovery journal system does not use packet retransmission to
       satisfy this mandate.  Instead, each packet includes a special
       section, called the recovery journal.

       The recovery journal codes the history of the stream, back to an
       earlier packet called the checkpoint packet.  The range of coverage
       for the journal is called the checkpoint history.  The recovery
       journal codes the information necessary to recover from the loss of
       an arbitrary number of packets in the checkpoint history.  Appendix
       A.1 normatively defines the checkpoint packet and the checkpoint
       history.

       When a receiver detects a packet loss, it compares its own knowledge
       about the history of the stream with the history information coded in
       the recovery journal of the packet that ends the loss event.  By
       noting the differences in these two versions of the past, a receiver
       is able to transform all indefinite artifacts in the rendered

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       performance into transient artifacts, by executing MIDI commands to
       repair the stream.

       We now state the normative role for senders in the recovery journal
       system.

       Senders prepare a recovery journal for every packet in the stream.
       In doing so, senders choose the checkpoint packet identity for the
       journal.  Senders make this choice by applying a sending policy.
       Appendix C.2.2 normatively defines three sending policies: "closed-
       loop", "open-loop", and "anchor".

       By default, senders MUST use the closed-loop sending policy.  If the
       session description overrides this default policy, by using the
       parameter j_update defined in Appendix C.2.2, senders MUST use the
       specified policy.

       After choosing the checkpoint packet identity for a packet, the
       sender creates the recovery journal.  By default, this journal MUST
       conform to the normative semantics in Section 5 and Appendices A-B in
       this memo.  In Appendix C.2.3, we define parameters that modify the
       normative semantics for recovery journals.  If the session
       description uses these parameters, the journal created by the sender
       MUST conform to the modified semantics.

       Next, we state the normative role for receivers in the recovery
       journal system.

       A receiver MUST detect each RTP sequence number break in a stream.
       If the sequence number break is due to a packet loss event (as
       defined in [RFC3550]), the receiver MUST repair all indefinite
       artifacts in the rendered MIDI performance caused by the loss.  If
       the sequence number break is due to an out-of-order packet (as
       defined in [RFC3550]), the receiver MUST NOT take actions that
       introduce indefinite artifacts (ignoring the out-of-order packet is a
       safe option).

       Receivers take special precautions when entering or exiting a
       session.  A receiver MUST process the first received packet in a
       stream as if it were a packet that ends a loss event.  Upon exiting a
       session, a receiver MUST ensure that the rendered MIDI performance
       does not end with indefinite artifacts.

       Receivers are under no obligation to perform indefinite artifact
       repairs at the moment a packet arrives.  A receiver that uses a
       playout buffer may choose to wait until the moment of rendering
       before processing the recovery journal, as the "lost" packet may be a
       late packet that arrives in time to use.

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       Next, we state the normative role for the creator of the session
       description in the recovery journal system.  Depending on the
       application, the sender, the receivers, and other parties may take
       part in creating or approving the session description.

       A session description that specifies the default closed-loop sending
       policy and the default recovery journal semantics satisfies the
       recovery journal mandate.  However, these default behaviors may not
       be appropriate for all sessions.  If the creators of a session
       description use the parameters defined in Appendix C.2 to override
       these defaults, the creators MUST ensure that the parameters define a
       system that satisfies the recovery journal mandate.

       Finally, we note that this memo does not specify sender or receiver
       recovery journal algorithms.  Implementations are free to use any
       algorithm that conforms to the requirements in this section.  The
       non-normative [RFC4696] discusses sender and receiver algorithm
       design.

    5.  Recovery Journal Format

       This section introduces the structure of the recovery journal and
       defines the bitfields of recovery journal headers.  Appendices A-B
       complete the bitfield definition of the recovery journal.

       The recovery journal has a three-level structure:

         o Top-level header.

         o Channel and system journal headers.  These headers encode
           recovery information for a single voice channel (channel journal)
           or for all systems commands (system journal).

         o Chapters.  Chapters describe recovery information for a single
           MIDI command type.

       Figure 7 shows the top-level structure of the recovery journal.  The
       recovery journals consists of a 3-octet header, followed by an
       optional system journal (labeled S-journal in Figure 7) and an
       optional list of channel journals.  Figure 8 shows the recovery
       journal header format.

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           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |            Recovery journal header            | S-journal ... |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                      Channel journals ...                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 7 -- Top-level recovery journal format

                  0                   1                   2
                  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 |S|Y|A|H|TOTCHAN|   Checkpoint Packet Seqnum    |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 8 -- Recovery journal header

       If the Y header bit is set to 1, the system journal appears in the
       recovery journal, directly following the recovery journal header.

       If the A header bit is set to 1, the recovery journal ends with a
       list of (TOTCHAN + 1) channel journals (the 4-bit TOTCHAN header
       field is interpreted as an unsigned integer).

       A MIDI channel MAY be represented by (at most) one channel journal in
       a recovery journal.  Channel journals MUST appear in the recovery
       journal in ascending channel-number order.

       If A and Y are both zero, the recovery journal only contains its 3-
       octet header and is considered to be an "empty" journal.

       The S (single-packet loss) bit appears in most recovery journal
       structures, including the recovery journal header.  The S bit helps
       receivers efficiently parse the recovery journal in the common case
       of the loss of a single packet.  Appendix A.1 defines S bit
       semantics.

       The H bit indicates if MIDI channels in the stream have been
       configured to use the enhanced Chapter C encoding (Appendix A.3.3).

       By default, the payload format does not use enhanced Chapter C
       encoding.  In this default case, the H bit MUST be set to 0 for all
       packets in the stream.

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       If the stream has been configured so that controller numbers for one
       or more MIDI channels use enhanced Chapter C encoding, the H bit MUST
       be set to 1 in all packets in the stream.  In Appendix C.2.3, we show
       how to configure a stream to use enhanced Chapter C encoding.

       The 16-bit Checkpoint Packet Seqnum header field codes the sequence
       number of the checkpoint packet for this journal, in network byte
       order (big-endian).  The choice of the checkpoint packet sets the
       depth of the checkpoint history for the journal (defined in Appendix
       A.1).

       Receivers may use the Checkpoint Packet Seqnum field of the packet
       that ends a loss event to verify that the journal checkpoint history
       covers the entire loss event.  The checkpoint history covers the loss
       event if the Checkpoint Packet Seqnum field is less than or equal to
       one plus the highest RTP sequence number previously received on the
       stream (modulo 2^16).

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S| CHAN  |H|      LENGTH       |P|C|M|W|N|E|T|A|  Chapters ... |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 9 -- Channel journal format

       Figure 9 shows the structure of a channel journal: a 3-octet header,
       followed by a list of leaf elements called channel chapters.  A
       channel journal encodes information about MIDI commands on the MIDI
       channel coded by the 4-bit CHAN header field.  Note that CHAN uses
       the same bit encoding as the channel nibble in MIDI Channel Messages
       (the cccc field in Figure E.1 of Appendix E).

       The 10-bit LENGTH field codes the length of the channel journal.  The
       semantics for LENGTH fields are uniform throughout the recovery
       journal, and are defined in Appendix A.1.

       The third octet of the channel journal header is the Table of
       Contents (TOC) of the channel journal.  The TOC is a set of bits that
       encode the presence of a chapter in the journal.  Each chapter
       contains information about a certain class of MIDI channel command:

          o  Chapter P: MIDI Program Change (0xC)
          o  Chapter C: MIDI Control Change (0xB)
          o  Chapter M: MIDI Parameter System (part of 0xB)
          o  Chapter W: MIDI Pitch Wheel (0xE)
          o  Chapter N: MIDI NoteOff (0x8), NoteOn (0x9)
          o  Chapter E: MIDI Note Command Extras (0x8, 0x9)

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          o  Chapter T: MIDI Channel Aftertouch (0xD)
          o  Chapter A: MIDI Poly Aftertouch (0xA)

       Chapters appear in a list following the header, in order of their
       appearance in the TOC.  Appendices A.2-9 describe the bitfield format
       for each chapter, and define the conditions under which a chapter
       type MUST appear in the recovery journal.  If any chapter types are
       required for a channel, an associated channel journal MUST appear in
       the recovery journal.

       The H bit indicates if controller numbers on a MIDI channel have been
       configured to use the enhanced Chapter C encoding (Appendix A.3.3).

       By default, controller numbers on a MIDI channel do not use enhanced
       Chapter C encoding.  In this default case, the H bit MUST be set to 0
       for all channel journal headers for the channel in the recovery
       journal, for all packets in the stream.

       However, if at least one controller number for a MIDI channel has
       been configured to use the enhanced Chapter C encoding, the H bit for
       its channel journal MUST be set to 1, for all packets in the stream.

       In Appendix C.2.3, we show how to configure a controller number to
       use enhanced Chapter C encoding.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|D|V|Q|F|X|      LENGTH       |  System chapters ...          |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 10 -- System journal format

       Figure 10 shows the structure of the system journal: a 2-octet
       header, followed by a list of system chapters.  Each chapter codes
       information about a specific class of MIDI Systems command:

          o  Chapter D: Song Select (0xF3), Tune Request (0xF6), Reset
                        (0xFF), undefined System commands (0xF4, 0xF5, 0xF9,
                        0xFD)
          o  Chapter V: Active Sense (0xFE)
          o  Chapter Q: Sequencer State (0xF2, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC)
          o  Chapter F: MTC Tape Position (0xF1, 0xF0 0x7F 0xcc 0x01 0x01)
          o  Chapter X: System Exclusive (all other 0xF0)

       The 10-bit LENGTH field codes the size of the system journal and
       conforms to semantics described in Appendix A.1.

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       The D, V, Q, F, and X header bits form a Table of Contents (TOC) for
       the system journal.  A TOC bit that is set to 1 codes the presence of
       a chapter in the journal.  Chapters appear in a list following the
       header, in the order of their appearance in the TOC.

       Appendix B describes the bitfield format for the system chapters and
       defines the conditions under which a chapter type MUST appear in the
       recovery journal.  If any system chapter type is required to appear
       in the recovery journal, the system journal MUST appear in the
       recovery journal.

    6.  Session Description Protocol

       RTP does not perform session management.  Instead, RTP works together
       with session management tools, such as the Session Initiation
       Protocol (SIP, [RFC3261]) and the Real Time Streaming Protocol (RTSP,
       [RFC2326]).

       RTP payload formats define media type parameters for use in session
       management (for example, this memo defines "rtp-midi" as the media
       type for native RTP MIDI streams).

       In most cases, session management tools use the media type parameters
       via another standard, the Session Description Protocol (SDP,
       [RFC4566]).

       SDP is a textual format for specifying session descriptions.  Session
       descriptions specify the network transport and media encoding for RTP
       sessions.  Session management tools coordinate the exchange of
       session descriptions between participants ("parties").

       Some session management tools use SDP to negotiate details of media
       transport (network addresses, ports, etc.).  We refer to this use of
       SDP as "negotiated usage".  One example of negotiated usage is the
       Offer/Answer protocol ([RFC3264] and Appendix C.7.2 in this memo) as
       used by SIP.

       Other session management tools use SDP to declare the media encoding
       for the session but use other techniques to negotiate network
       transport.  We refer to this use of SDP as "declarative usage".  One
       example of declarative usage is RTSP ([RFC2326] and Appendix C.7.1 in
       this memo).

       Below, we show session description examples for native (Section 6.1)
       and mpeg4-generic (Section 6.2) streams.  In Section 6.3, we
       introduce session configuration tools that may be used to customize
       streams.

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    6.1.  Session Descriptions for Native Streams

       The session description below defines a unicast UDP RTP session (via
       a media ("m=") line) whose sole payload type (96) is mapped to a
       minimal native RTP MIDI stream.

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 rtp-midi/44100

       The rtpmap attribute line uses the "rtp-midi" media type to specify
       an RTP MIDI native stream.  The clock rate specified on the rtpmap
       line (in the example above, 44100 Hz) sets the scaling for the RTP
       timestamp header field (see Section 2.1, and also [RFC3550]).

       Note that this document does not specify a default clock rate value
       for RTP MIDI.  When RTP MIDI is used with SDP, parties MUST use the
       rtpmap line to communicate the clock rate.  Guidance for selecting
       the RTP MIDI clock rate value appears in Section 2.1.

       We consider the RTP MIDI stream shown above to be "minimal" because
       the session description does not customize the stream with
       parameters.  Without such customization, a native RTP MIDI stream has
       these characteristics:

         1. If the stream uses unreliable transport (unicast UDP, multicast
            UDP, etc.), the recovery journal system is in use, and the RTP
            payload contains both the MIDI command section and the journal
            section.  If the stream uses reliable transport (such as TCP),
            the stream does not use journalling, and the payload contains
            only the MIDI command section (Section 2.2).

         2. If the stream uses the recovery journal system, the recovery
            journal system uses the default sending policy and the default
            journal semantics (Section 4).

         3. In the MIDI command section of the payload, command timestamps
            use the default "comex" semantics (Section 3).

         4. The recommended temporal duration ("media time") of an RTP
            packet ranges from 0 to 200 ms, and the RTP timestamp difference
            between sequential packets in the stream may be arbitrarily
            large (Section 2.1).

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         5. If more than one minimal rtp-midi stream appears in a session,
            the MIDI name spaces for these streams are independent: channel
            1 in the first stream does not reference the same MIDI channel
            as channel 1 in the second stream (see Appendix C.5 for a
            discussion of the independence of minimal rtp-midi streams).

         6. The rendering method for the stream is not specified.  What the
            receiver "does" with a minimal native MIDI stream is "out of
            scope" of this memo.  For example, in content creation
            environments, a user may manually configure client software to
            render the stream with a specific software package.

       As in standard in RTP, RTP sessions managed by SIP are sendrecv by
       default (parties send and receive MIDI), and RTP sessions managed by
       RTSP are recvonly by default (server sends and client receives).

       In sendrecv RTP MIDI sessions for the session description shown
       above, the 16 voice channel + systems MIDI name space is unique for
       each sender.  Thus, in a two-party session, the voice channel 0 sent
       by one party is distinct from the voice channel 0 sent by the other
       party.

       This behavior corresponds to what occurs when two MIDI 1.0 DIN
       devices are cross-connected with two MIDI cables (one cable routing
       MIDI Out from the first device into MIDI In of the second device, a
       second cable routing MIDI In from the first device into MIDI Out of
       the second device).  We define this "association" formally in Section
       2.1.

       MIDI 1.0 DIN networks may be configured in a "party-line" multicast
       topology.  For these networks, the MIDI protocol itself provides
       tools for addressing specific devices in transactions on a multicast
       network, and for device discovery.  Thus, apart from providing a 1-
       to-many forward path and a many-to-1 reverse path, IETF protocols do
       not need to provide any special support for MIDI multicast
       networking.

    6.2.  Session Descriptions for mpeg4-generic Streams

       An mpeg4-generic [RFC3640] RTP MIDI stream uses an MPEG 4 Audio
       Object Type to render MIDI into audio.  Three Audio Object Types
       accept MIDI input:

         o General MIDI (Audio Object Type ID 15), based on the General MIDI
           rendering standard [MIDI].

         o Wavetable Synthesis (Audio Object Type ID 14), based on the
           Downloadable Sounds Level 2 (DLS 2) rendering standard [DLS2].

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         o Main Synthetic (Audio Object Type ID 13), based on Structured
           Audio and the programming language SAOL [MPEGSA].

       The primary service of an mpeg4-generic stream is to code Access
       Units (AUs).  We define the mpeg4-generic RTP MIDI AU as the MIDI
       payload shown in Figure 1 of Section 2.1 of this memo: a MIDI command
       section optionally followed by a journal section.

       Exactly one RTP MIDI AU MUST be mapped to one mpeg4-generic RTP MIDI
       packet.  The mpeg4-generic options for placing several AUs in an RTP
       packet MUST NOT be used with RTP MIDI.  The mpeg4-generic options for
       fragmenting and interleaving AUs MUST NOT be used with RTP MIDI.  The
       mpeg4-generic RTP packet payload (Figure 1 in [RFC3640]) MUST contain
       empty AU Header and Auxiliary sections.  These rules yield mpeg4-
       generic packets that are structurally identical to native RTP MIDI
       packets, an essential property for the correct operation of the
       payload format.

       The session description that follows defines a unicast UDP RTP
       session (via a media ("m=") line) whose sole payload type (96) is
       mapped to a minimal mpeg4-generic RTP MIDI stream.  This example uses
       the General MIDI Audio Object Type under Synthesis Profile @ Level 2.

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
       config=7A0A0000001A4D546864000000060000000100604D54726B0000
       000600FF2F000

       (The a=fmtp line has been wrapped to fit the page to accommodate memo
       formatting restrictions; it comprises a single line in SDP.)

       The fmtp attribute line codes the four parameters (streamtype, mode,
       profile-level-id, and config) that are required in all mpeg4-generic
       session descriptions [RFC3640].  For RTP MIDI streams, the streamtype
       parameter MUST be set to 5, the "mode" parameter MUST be set to
       "rtp-midi", and the "profile-level-id" parameter MUST be set to the
       MPEG-4 Profile Level for the stream.  For the Synthesis Profile,
       legal profile-level-id values are 11, 12, and 13, coding low (11),
       medium (12), or high (13) decoder computational complexity, as
       defined by MPEG conformance tests.

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       In a minimal RTP MIDI session description, the config value MUST be a
       hexadecimal encoding [RFC3640] of the AudioSpecificConfig data block
       [MPEGAUDIO] for the stream.  AudioSpecificConfig encodes the Audio
       Object Type for the stream and also encodes initialization data (SAOL
       programs, DLS 2 wave tables, etc.).  Standard MIDI Files encoded in
       AudioSpecificConfig in a minimal session description MUST be ignored
       by the receiver.

       Receivers determine the rendering algorithm for the session by
       interpreting the first 5 bits of AudioSpecificConfig as an unsigned
       integer that codes the Audio Object Type.  In our example above, the
       leading config string nibbles "7A" yield the Audio Object Type 15
       (General MIDI).  In Appendix E.4, we derive the config string value
       in the session description shown above; the starting point of the
       derivation is the MPEG bitstreams defined in [MPEGSA] and
       [MPEGAUDIO].

       We consider the stream to be "minimal" because the session
       description does not customize the stream through the use of
       parameters, other than the 4 required mpeg4-generic parameters
       described above.  In Section 6.1, we describe the behavior of a
       minimal native stream, as a numbered list of characteristics.  Items
       1-4 on that list also describe the minimal mpeg4-generic stream, but
       items 5 and 6 require restatements, as listed below:

         5. If more than one minimal mpeg4-generic stream appears in a
            session, each stream uses an independent instance of the Audio
            Object Type coded in the config parameter value.

         6. A minimal mpeg4-generic stream encodes the AudioSpecificConfig
            as an inline hexadecimal constant.  If a session description is
            sent over UDP, it may be impossible to transport large
            AudioSpecificConfig blocks within the Maximum Transmission Size
            (MTU) of the underlying network (for Ethernet, the MTU is 1500
            octets).  In some cases, the AudioSpecificConfig block may
            exceed the maximum size of the UDP packet itself.

       The comments in Section 6.1 on SIP and RTSP stream directional
       defaults, sendrecv MIDI channel usage, and MIDI 1.0 DIN multicast
       networks also apply to mpeg4-generic RTP MIDI sessions.

       In sendrecv sessions, each party's session description MUST use
       identical values for the mpeg4-generic parameters (including the
       required streamtype, mode, profile-level-id, and config parameters).
       As a consequence, each party uses an identically configured MPEG 4
       Audio Object Type to render MIDI commands into audio.  The preamble
       to Appendix C discusses a way to create "virtual sendrecv" sessions
       that do not have this restriction.

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    6.3.  Parameters

       This section introduces parameters for session configuration for RTP
       MIDI streams.  In session descriptions, parameters modify the
       semantics of a payload type.  Parameters are specified on an fmtp
       attribute line.  See the session description example in Section 6.2
       for an example of a fmtp attribute line.

       The parameters add features to the minimal streams described in
       Sections 6.1-2, and support several types of services:

         o  Stream subsetting.  By default, all MIDI commands that are legal
            to appear on a MIDI 1.0 DIN cable may appear in an RTP MIDI
            stream.  The cm_unused parameter overrides this default by
            prohibiting certain commands from appearing in the stream.  The
            cm_used parameter is used in conjunction with cm_unused, to
            simplify the specification of complex exclusion rules.  We
            describe cm_unused and cm_used in Appendix C.1.

         o  Journal customization.  The j_sec and j_update parameters
            configure the use of the journal section.  The ch_default,
            ch_never, and ch_anchor parameters configure the semantics of
            the recovery journal chapters.  These parameters are described
            in Appendix C.2 and override the default stream behaviors 1 and
            2, listed in Section 6.1 and referenced in Section 6.2.

         o  MIDI command timestamp semantics.  The tsmode, octpos, mperiod,
            and linerate parameters customize the semantics of timestamps in
            the MIDI command section.  These parameters let RTP MIDI
            accurately encode the implicit time coding of MIDI 1.0 DIN
            cables.  These parameters are described in Appendix C.3 and
            override default stream behavior 3, listed in Section 6.1 and
            referenced in Section 6.2

         o  Media time.  The rtp_ptime and rtp_maxptime parameters define
            the temporal duration ("media time") of an RTP MIDI packet.  The
            guardtime parameter sets the minimum sending rate of stream
            packets.  These parameters are described in Appendix C.4 and
            override default stream behavior 4, listed in Section 6.1 and
            referenced in Section 6.2.

         o  Stream description.  The musicport parameter labels the MIDI
            name space of RTP streams in a multimedia session.  Musicport is
            described in Appendix C.5.  The musicport parameter overrides
            default stream behavior 5, in Sections 6.1 and 6.2.

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         o  MIDI rendering.  Several parameters specify the MIDI rendering
            method of a stream.  These parameters are described in Appendix
            C.6 and override default stream behavior 6, in Sections 6.1 and
            6.2.

       In Appendix C.7, we specify interoperability guidelines for two RTP
       MIDI application areas: content-streaming using RTSP (Appendix C.7.1)
       and network musical performance using SIP (Appendix C.7.2).

    7.  Extensibility

       The payload format defined in this memo exclusively encodes all
       commands that may legally appear on a MIDI 1.0 DIN cable.

       Many worthy uses of MIDI over RTP do not fall within the narrow scope
       of the payload format.  For example, the payload format does not
       support the direct transport of Standard MIDI File (SMF) meta-event
       and metric timing data.  As a second example, the payload format does
       not define transport tools for user-defined commands (apart from
       tools to support System Exclusive commands [MIDI]).

       The payload format does not provide an extension mechanism to support
       new features of this nature, by design.  Instead, we encourage the
       development of new payload formats for specialized musical
       applications.  The IETF session management tools [RFC3264] [RFC2326]
       support codec negotiation, to facilitate the use of new payload
       formats in a backward-compatible way.

       However, the payload format does provide several extensibility tools,
       which we list below:

         o  Journalling.  As described in Appendix C.2, new token values for
            the j_sec and j_update parameters may be defined in IETF
            standards-track documents.  This mechanism supports the design
            of new journal formats and the definition of new journal sending
            policies.

         o  Rendering.  The payload format may be extended to support new
            MIDI renderers (Appendix C.6.2).  Certain general aspects of the
            RTP MIDI rendering process may also be extended, via the
            definition of new token values for the render (Appendix C.6) and
            smf_info (Appendix C.6.4.1) parameters.

         o  Undefined commands.  [MIDI] reserves 4 MIDI System commands for
            future use (0xF4, 0xF5, 0xF9, 0xFD).  If updates to [MIDI]
            define the reserved commands, IETF standards-track documents may
            be defined to provide resiliency support for the commands.

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            Opaque LEGAL fields appear in System Chapter D for this purpose
            (Appendix B.1.1).

       A final form of extensibility involves the inclusion of the payload
       format in framework documents.  Framework documents describe how to
       combine protocols to form a platform for interoperable applications.
       For example, a stage and studio framework might define how to use SIP
       [RFC3261], RTSP [RFC2326], SDP [RFC4566], and RTP [RFC3550] to
       support media networking for professional audio equipment and
       electronic musical instruments.

    8.  Congestion Control

       The RTP congestion control requirements defined in [RFC3550] apply to
       RTP MIDI sessions, and implementors should carefully read the
       congestion control section in [RFC3550].  As noted in [RFC3550], all
       transport protocols used on the Internet need to address congestion
       control in some way, and RTP is not an exception.

       In addition, the congestion control requirements defined in [RFC3551]
       applies to RTP MIDI sessions run under applicable profiles.  The
       basic congestion control requirement defined in [RFC3551] is that RTP
       sessions that use UDP transport should monitor packet loss (via RTCP
       or other means) to ensure that the RTP stream competes fairly with
       TCP flows that share the network.

       Finally, RTP MIDI has congestion control issues that are unique for
       an audio RTP payload format.  In applications such as network musical
       performance [NMP], the packet rate is linked to the gestural rate of
       a human performer.  Senders MUST monitor the MIDI command source for
       patterns that result in excessive packet rates and take actions
       during RTP transcoding to reduce the RTP packet rate.  [RFC4696]
       offers implementation guidance on this issue.

    9.  Security Considerations

       Implementors should carefully read the Security Considerations
       sections of the RTP [RFC3550], AVP [RFC3551], and other RTP profile
       documents, as the issues discussed in these sections directly apply
       to RTP MIDI streams.  Implementors should also review the Secure
       Real-time Transport Protocol (SRTP, [RFC3711]), an RTP profile that
       addresses the security issues discussed in [RFC3550] and [RFC3551].

       Here, we discuss security issues that are unique to the RTP MIDI
       payload format.

       When using RTP MIDI, authentication of incoming RTP and RTCP packets
       is RECOMMENDED.  Per-packet authentication may be provided by SRTP or

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       by other means.  Without the use of authentication, attackers could
       forge MIDI commands into an ongoing stream, damaging speakers and
       eardrums.  An attacker could also craft RTP and RTCP packets to
       exploit known bugs in the client and take effective control of a
       client machine.

       Session management tools (such as SIP [RFC3261]) SHOULD use
       authentication during the transport of all session descriptions
       containing RTP MIDI media streams.  For SIP, the Security
       Considerations section in [RFC3261] provides an overview of possible
       authentication mechanisms.  RTP MIDI session descriptions should use
       authentication because the session descriptions may code
       initialization data using the parameters described in Appendix C.  If
       an attacker inserts bogus initialization data into a session
       description, he can corrupt the session or forge an client attack.

       Session descriptions may also code renderer initialization data by
       reference, via the url (Appendix C.6.3) and smf_url (Appendix
       C.6.4.2) parameters.  If the coded URL is spoofed, both session and
       client are open to attack, even if the session description itself is
       authenticated.  Therefore, URLs specified in url and smf_url
       parameters SHOULD use [RFC2818].

       Section 2.1 allows streams sent by a party in two RTP sessions to
       have the same SSRC value and the same RTP timestamp initialization
       value, under certain circumstances.  Normally, these values are
       randomly chosen for each stream in a session, to make plaintext
       guessing harder to do if the payloads are encrypted.  Thus, Section
       2.1 weakens this aspect of RTP security.

    10.  Acknowledgements

       We thank the networking, media compression, and computer music
       community members who have commented or contributed to the effort,
       including Kurt B, Cynthia Bruyns, Steve Casner, Paul Davis, Robin
       Davies, Joanne Dow, Tobias Erichsen, Nicolas Falquet, Dominique
       Fober, Philippe Gentric, Michael Godfrey, Chris Grigg, Todd Hager,
       Michel Jullian, Phil Kerr, Young-Kwon Lim, Jessica Little, Jan van
       der Meer, Colin Perkins, Charlie Richmond, Herbie Robinson, Larry
       Rowe, Eric Scheirer, Dave Singer, Martijn Sipkema, William Stewart,
       Kent Terry, Magnus Westerlund, Tom White, Jim Wright, Doug Wyatt, and
       Giorgio Zoia.  We also thank the members of the San Francisco Bay
       Area music and audio community for creating the context for the work,
       including Don Buchla, Chris Chafe, Richard Duda, Dan Ellis, Adrian
       Freed, Ben Gold, Jaron Lanier, Roger Linn, Richard Lyon, Dana Massie,
       Max Mathews, Keith McMillen, Carver Mead, Nelson Morgan, Tom
       Oberheim, Malcolm Slaney, Dave Smith, Julius Smith, David Wessel, and
       Matt Wright.

    Lazzaro & Wawrzynek         Standards Track                    [Page 36]

    RFC 4695              RTP Payload Format for MIDI          November 2006

    11.  IANA Considerations

       This section makes a series of requests to IANA.  The IANA has
       completed registration/assignments of the below requests.

       The sub-sections that follow hold the actual, detailed requests.  All
       registrations in this section are in the IETF tree and follow the
       rules of [RFC4288] and [RFC3555], as appropriate.

       In Section 11.1, we request the registration of a new media type:
       "audio/rtp-midi".  Paired with this request is a request for a
       repository for new values for several parameters associated with
       "audio/rtp-midi".  We request this repository in Section 11.1.1.

       In Section 11.2, we request the registration of a new value ("rtp-
       midi") for the "mode" parameter of the "mpeg4-generic" media type.
       The "mpeg4-generic" media type is defined in [RFC3640], and [RFC3640]
       defines a repository for the "mode" parameter.  However, we believe
       we are the first to request the registration of a "mode" value, so we
       believe the registry for "mode" has not yet been created by IANA.

       Paired with our "mode" parameter value request for "mpeg4-generic" is
       a request for a repository for new values for several parameters we
       have defined for use with the "rtp-midi" mode value.  We request this
       repository in Section 11.2.1.

       In Section 11.3, we request the registration of a new media type:
       "audio/asc".  No repository request is associated with this request.

    11.1.  rtp-midi Media Type Registration

       This section requests the registration of the "rtp-midi" subtype for
       the "audio" media type.  We request the registration of the
       parameters listed in the "optional parameters" section below (both
       the "non-extensible parameters" and the "extensible parameters"
       lists).  We also request the creation of repositories for the
       "extensible parameters"; the details of this request appear in
       Section 11.1.1, below.

       Media type name:

           audio

       Subtype name:

           rtp-midi

    Lazzaro & Wawrzynek         Standards Track                    [Page 37]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Required parameters:

           rate: The RTP timestamp clock rate.  See Sections 2.1 and 6.1
           for usage details.

       Optional parameters:

           Non-extensible parameters:

              ch_anchor:    See Appendix C.2.3 for usage details.
              ch_default:   See Appendix C.2.3 for usage details.
              ch_never:     See Appendix C.2.3 for usage details.
              cm_unused:    See Appendix C.1 for usage details.
              cm_used:      See Appendix C.1 for usage details.
              chanmask:     See Appendix C.6.4.3 for usage details.
              cid:          See Appendix C.6.3 for usage details.
              guardtime:    See Appendix C.4.2 for usage details.
              inline:       See Appendix C.6.3 for usage details.
              linerate:     See Appendix C.3 for usage details.
              mperiod:      See Appendix C.3 for usage details.
              multimode:    See Appendix C.6.1 for usage details.
              musicport:    See Appendix C.5 for usage details.
              octpos:       See Appendix C.3 for usage details.
              rinit:        See Appendix C.6.3 for usage details.
              rtp_maxptime: See Appendix C.4.1 for usage details.
              rtp_ptime:    See Appendix C.4.1 for usage details.
              smf_cid:      See Appendix C.6.4.2 for usage details.
              smf_inline:   See Appendix C.6.4.2 for usage details.
              smf_url:      See Appendix C.6.4.2 for usage details.
              tsmode:       See Appendix C.3 for usage details.
              url:          See Appendix C.6.3 for usage details.

           Extensible parameters:

              j_sec:        See Appendix C.2.1 for usage details.  See
                            Section 11.1.1 for repository details.
              j_update:     See Appendix C.2.2 for usage details.  See
                            Section 11.1.1 for repository details.
              render:       See Appendix C.6 for usage details.  See
                            Section 11.1.1 for repository details.
              subrender:    See Appendix C.6.2 for usage details.  See
                            Section 11.1.1 for repository details.
              smf_info:     See Appendix C.6.4.1 for usage details.  See
                            Section 11.1.1 for repository details.

       Encoding considerations:

           The format for this type is framed and binary.

    Lazzaro & Wawrzynek         Standards Track                    [Page 38]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Restrictions on usage:

           This type is only defined for real-time transfers of MIDI
           streams via RTP.  Stored-file semantics for rtp-midi may
           be defined in the future.

       Security considerations:

           See Section 9 of this memo.

       Interoperability considerations:

           None.

       Published specification:

           This memo and [MIDI] serve as the normative specification.  In
           addition, references [NMP], [GRAME], and [RFC4696] provide
           non-normative implementation guidance.

       Applications that use this media type:

           Audio content-creation hardware, such as MIDI controller piano
           keyboards and MIDI audio synthesizers.  Audio content-creation
           software, such as music sequencers, digital audio workstations,
           and soft synthesizers.  Computer operating systems, for network
           support of MIDI Application Programmer Interfaces.  Content
           distribution servers and terminals may use this media type for
           low bit-rate music coding.

       Additional information:

           None.

       Person & email address to contact for further information:

           John Lazzaro <lazzaro@cs.berkeley.edu>

       Intended usage:

           COMMON.

       Author:

           John Lazzaro <lazzaro@cs.berkeley.edu>

    Lazzaro & Wawrzynek         Standards Track                    [Page 39]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Change controller:

           IETF Audio/Video Transport Working Group delegated
           from the IESG.

    11.1.1.  Repository Request for "audio/rtp-midi"

       For the "rtp-midi" subtype, we request the creation of repositories
       for extensions to the following parameters (which are those listed as
       "extensible parameters" in Section 11.1).

          j_sec:

             Registrations for this repository may only occur
             via an IETF standards-track document.  Appendix C.2.1
             of this memo describes appropriate registrations for this
             repository.

             Initial values for this repository appear below:

             "none":  Defined in Appendix C.2.1 of this memo.
             "recj":  Defined in Appendix C.2.1 of this memo.

          j_update:

             Registrations for this repository may only occur
             via an IETF standards-track document.  Appendix C.2.2
             of this memo describes appropriate registrations for this
             repository.

             Initial values for this repository appear below:

             "anchor":  Defined in Appendix C.2.2 of this memo.
             "open-loop":  Defined in Appendix C.2.2 of this memo.
             "closed-loop":  Defined in Appendix C.2.2 of this memo.

          render:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text in the preamble of Appendix C.6 for details
             (the paragraph that begins "Other render token ...").

    Lazzaro & Wawrzynek         Standards Track                    [Page 40]

    RFC 4695              RTP Payload Format for MIDI          November 2006

             Initial values for this repository appear below:

             "unknown":  Defined in Appendix C.6 of this memo.
             "synthetic":  Defined in Appendix C.6 of this memo.
             "api":  Defined in Appendix C.6 of this memo.
             "null":  Defined in Appendix C.6 of this memo.

          subrender:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text Appendix C.6.2 for details (the paragraph
             that begins "Other subrender token ...").

             Initial values for this repository appear below:

             "default":  Defined in Appendix C.6.2 of this memo.

          smf_info:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text in Appendix C.6.4.1 for details (the
             paragraph that begins "Other smf_info token ...").

             Initial values for this repository appear below:

             "ignore":  Defined in Appendix C.6.4.1 of this memo.
             "sdp_start":  Defined in Appendix C.6.4.1 of this memo.
             "identity":  Defined in Appendix C.6.4.1 of this memo.

    11.2.  mpeg4-generic Media Type Registration

       This section requests the registration of the "rtp-midi" value for
       the "mode" parameter of the "mpeg4-generic" media type.  The "mpeg4-
       generic" media type is defined in [RFC3640], and [RFC3640] defines a
       repository for the "mode" parameter.  We are registering mode rtp-
       midi to support the MPEG Audio codecs [MPEGSA] that use MIDI.

       In conjunction with this registration request, we request the
       registration of the parameters listed in the "optional parameters"
       section below (both the "non-extensible parameters" and the
       "extensible parameters" lists).  We also request the creation of
       repositories for the "extensible parameters"; the details of this
       request appear in Appendix 11.2.1, below.

    Lazzaro & Wawrzynek         Standards Track                    [Page 41]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Media type name:

           audio

       Subtype name:

           mpeg4-generic

       Required parameters:

           The "mode" parameter is required by [RFC3640].  [RFC3640]
           requests a repository for "mode", so that new values for mode
           may be added.  We request that the value "rtp-midi" be
           added to the "mode" repository.

           In mode rtp-midi, the mpeg4-generic parameter rate is
           a required parameter.  Rate specifies the RTP timestamp
           clock rate.  See Sections 2.1 and 6.2 for usage details
           of rate in mode rtp-midi.

       Optional parameters:

           We request registration of the following parameters
           for use in mode rtp-midi for mpeg4-generic.

           Non-extensible parameters:

              ch_anchor:    See Appendix C.2.3 for usage details.
              ch_default:   See Appendix C.2.3 for usage details.
              ch_never:     See Appendix C.2.3 for usage details.
              cm_unused:    See Appendix C.1 for usage details.
              cm_used:      See Appendix C.1 for usage details.
              chanmask:     See Appendix C.6.4.3 for usage details.
              cid:          See Appendix C.6.3 for usage details.
              guardtime:    See Appendix C.4.2 for usage details.
              inline:       See Appendix C.6.3 for usage details.
              linerate:     See Appendix C.3 for usage details.
              mperiod:      See Appendix C.3 for usage details.
              multimode:    See Appendix C.6.1 for usage details.
              musicport:    See Appendix C.5 for usage details.
              octpos:       See Appendix C.3 for usage details.
              rinit:        See Appendix C.6.3 for usage details.
              rtp_maxptime: See Appendix C.4.1 for usage details.
              rtp_ptime:    See Appendix C.4.1 for usage details.
              smf_cid:      See Appendix C.6.4.2 for usage details.
              smf_inline:   See Appendix C.6.4.2 for usage details.

    Lazzaro & Wawrzynek         Standards Track                    [Page 42]

    RFC 4695              RTP Payload Format for MIDI          November 2006

              smf_url:      See Appendix C.6.4.2 for usage details.
              tsmode:       See Appendix C.3 for usage details.
              url:          See Appendix C.6.3 for usage details.

           Extensible parameters:

              j_sec:        See Appendix C.2.1 for usage details.  See
                            Section 11.2.1 for repository details.
              j_update:     See Appendix C.2.2 for usage details.  See
                            Section 11.2.1 for repository details.
              render:       See Appendix C.6 for usage details.  See
                            Section 11.2.1 for repository details.
              subrender:    See Appendix C.6.2 for usage details.  See
                            Section 11.2.1 for repository details.
              smf_info:     See Appendix C.6.4.1 for usage details.  See
                            Section 11.2.1 for repository details.

       Encoding considerations:

           The format for this type is framed and binary.

       Restrictions on usage:

           Only defined for real-time transfers of audio/mpeg4-generic
           RTP streams with mode=rtp-midi.

       Security considerations:

           See Section 9 of this memo.

       Interoperability considerations:

           Except for the marker bit (Section 2.1), the packet formats
           for audio/rtp-midi and audio/mpeg4-generic (mode rtp-midi)
           are identical.  The formats differ in use: audio/mpeg4-generic
           is for MPEG work, and audio/rtp-midi is for all other work.

       Published specification:

           This memo, [MIDI], and [MPEGSA] are the normative references.
           In addition, references [NMP], [GRAME], and [RFC4696] provide
           non-normative implementation guidance.

       Applications that use this media type:

           MPEG 4 servers and terminals that support [MPEGSA].

    Lazzaro & Wawrzynek         Standards Track                    [Page 43]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Additional information:

           None.

       Person & email address to contact for further information:

           John Lazzaro <lazzaro@cs.berkeley.edu>

       Intended usage:

           COMMON.

       Author:

           John Lazzaro <lazzaro@cs.berkeley.edu>

       Change controller:

           IETF Audio/Video Transport Working Group delegated
           from the IESG.

    11.2.1.  Repository Request for Mode rtp-midi for mpeg4-generic

       For mode rtp-midi of the mpeg4-generic subtype, we request the
       creation of repositories for extensions to the following parameters
       (which are those listed as "extensible parameters" in Section 11.2).

          j_sec:

             Registrations for this repository may only occur
             via an IETF standards-track document.  Appendix C.2.1
             of this memo describes appropriate registrations for this
             repository.

             Initial values for this repository appear below:

             "none":  Defined in Appendix C.2.1 of this memo.
             "recj":  Defined in Appendix C.2.1 of this memo.

          j_update:

             Registrations for this repository may only occur
             via an IETF standards-track document.  Appendix C.2.2
             of this memo describes appropriate registrations for this
             repository.

    Lazzaro & Wawrzynek         Standards Track                    [Page 44]

    RFC 4695              RTP Payload Format for MIDI          November 2006

             Initial values for this repository appear below:

             "anchor":  Defined in Appendix C.2.2 of this memo.
             "open-loop":  Defined in Appendix C.2.2 of this memo.
             "closed-loop":  Defined in Appendix C.2.2 of this memo.

          render:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text in the preamble of Appendix C.6 for details
             (the paragraph that begins "Other render token ...").

             Initial values for this repository appear below:

             "unknown":  Defined in Appendix C.6 of this memo.
             "synthetic":  Defined in Appendix C.6 of this memo.
             "null":  Defined in Appendix C.6 of this memo.

          subrender:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text Appendix C.6.2 for details (the paragraph
             that begins "Other subrender token ..." and
             subsequent paragraphs).  Note that the text in
             Appendix C.6.2 contains restrictions on subrender
             registrations for mpeg4-generic ("Registrations
             for mpeg4-generic subrender values ...").

             Initial values for this repository appear below:

             "default":  Defined in Appendix C.6.2 of this memo.

          smf_info:

             Registrations for this repository MUST include a
             specification of the usage of the proposed value.
             See text in Appendix C.6.4.1 for details (the
             paragraph that begins "Other smf_info token ...").

             Initial values for this repository appear below:

             "ignore":  Defined in Appendix C.6.4.1 of this memo.
             "sdp_start":  Defined in Appendix C.6.4.1 of this memo.
             "identity":  Defined in Appendix C.6.4.1 of this memo.

    Lazzaro & Wawrzynek         Standards Track                    [Page 45]

    RFC 4695              RTP Payload Format for MIDI          November 2006

    11.3.  asc Media Type Registration

       This section registers "asc" as a subtype for the "audio" media type.
       We register this subtype to support the remote transfer of the
       "config" parameter of the mpeg4-generic media type [RFC3640] when it
       is used with mpeg4-generic mode rtp-midi (registered in Appendix 11.2
       above).  We explain the mechanics of using "audio/asc" to set the
       config parameter in Section 6.2 and Appendix C.6.5 of this document.

       Note that this registration is a new subtype registration and is not
       an addition to a repository defined by MPEG-related memos (such as
       [RFC3640]).  Also note that this request for "audio/asc" does not
       register parameters, and does not request the creation of a
       repository.

       Media type name:

           audio

       Subtype name:

           asc

       Required parameters:

           None.

       Optional parameters:

           None.

       Encoding considerations:

           The native form of the data object is binary data,
           zero-padded to an octet boundary.

       Restrictions on usage:

           This type is only defined for data object (stored file)
           transfer.  The most common transports for the type are
           HTTP and SMTP.

       Security considerations:

           See Section 9 of this memo.

    Lazzaro & Wawrzynek         Standards Track                    [Page 46]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       Interoperability considerations:

           None.

       Published specification:

           The audio/asc data object is the AudioSpecificConfig
           binary data structure, which is normatively defined in
           [MPEGAUDIO].

       Applications that use this media type:

           MPEG 4 Audio servers and terminals that support
           audio/mpeg4-generic RTP streams for mode rtp-midi.

       Additional information:

           None.

       Person & email address to contact for further information:

           John Lazzaro <lazzaro@cs.berkeley.edu>

       Intended usage:

           COMMON.

       Author:

           John Lazzaro <lazzaro@cs.berkeley.edu>

       Change controller:

           IETF Audio/Video Transport Working Group delegated
           from the IESG.

    Lazzaro & Wawrzynek         Standards Track                    [Page 47]

    RFC 4695              RTP Payload Format for MIDI          November 2006

    A.  The Recovery Journal Channel Chapters

    A.1.  Recovery Journal Definitions

       This appendix defines the terminology and the coding idioms that are
       used in the recovery journal bitfield descriptions in Section 5
       (journal header structure), Appendices A.2 to A.9 (channel journal
       chapters) and Appendices B.1 to B.5 (system journal chapters).

       We assume that the recovery journal resides in the journal section of
       an RTP packet with sequence number I ("packet I") and that the
       Checkpoint Packet Seqnum field in the top-level recovery journal
       header refers to a previous packet with sequence number C (an
       exception is the self-referential C = I case).  Unless stated
       otherwise, algorithms are assumed to use modulo 2^16 arithmetic for
       calculations on 16-bit sequence numbers and modulo 2^32 arithmetic
       for calculations on 32-bit extended sequence numbers.

       Several bitfield coding idioms appear throughout the recovery journal
       system, with consistent semantics.  Most recovery journal elements
       begin with an "S" (Single-packet loss) bit.  S bits are designed to
       help receivers efficiently parse through the recovery journal
       hierarchy in the common case of the loss of a single packet.

       As a rule, S bits MUST be set to 1.  However, an exception applies if
       a recovery journal element in packet I encodes data about a command
       stored in the MIDI command section of packet I - 1.  In this case,
       the S bit of the recovery journal element MUST be set to 0.  If a
       recovery journal element has its S bit set to 0, all higher-level
       recovery journal elements that contain it MUST also have S bits that
       are set to 0, including the top-level recovery journal header.

       Other consistent bitfield coding idioms are described below:

         o R flag bit.  R flag bits are reserved for future use.  Senders
           MUST set R bits to 0.  Receivers MUST ignore R bit values.

         o LENGTH field.  All fields named LENGTH (as distinct from LEN)
           code the number of octets in the structure that contains it,
           including the header it resides in and all hierarchical levels
           below it.  If a structure contains a LENGTH field, a receiver
           MUST use the LENGTH field value to advance past the structure
           during parsing, rather than use knowledge about the internal
           format of the structure.

    Lazzaro & Wawrzynek         Standards Track                    [Page 48]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       We now define normative terms used to describe recovery journal
       semantics.

         o Checkpoint history.  The checkpoint history of a recovery journal
           is the concatenation of the MIDI command sections of packets C
           through I - 1.  The final command in the MIDI command section for
           packet I - 1 is considered the most recent command; the first
           command in the MIDI command section for packet C is the oldest
           command.  If command X is less recent than command Y, X is
           considered to be "before Y".  A checkpoint history with no
           commands is considered to be empty.  The checkpoint history never
           contains the MIDI command section of packet I (the packet
           containing the recovery journal), so if C == I, the checkpoint
           history is empty by definition.

         o Session history.  The session history of a recovery journal is
           the concatenation of MIDI command sections from the first packet
           of the session up to packet I - 1.  The definitions of command
           recency and history emptiness follow those in the checkpoint
           history.  The session history never contains the MIDI command
           section of packet I, and so the session history of the first
           packet in the session is empty by definition.

         o Finished/unfinished commands.  If all octets of a MIDI command
           appear in the session history, the command is defined as being
           finished.  If some but not all octets of a command appear in the
           session history, the command is defined as being unfinished.
           Unfinished commands occur if segments of a SysEx command appear
           in several RTP packets.  For example, if a SysEx command is coded
           as 3 segments, with segment 1 in packet K, segment 2 in packet K
           + 1, and segment 3 in packet K + 2, the session histories for
           packets K + 1 and K + 2 contain unfinished versions of the
           command.  A session history contains a finished version of a
           cancelled SysEx command if the history contains the cancel
           sublist for the command.

         o Reset State commands.  Reset State (RS) commands reset renderers
           to an initialized "powerup" condition.  The RS commands are:
           System Reset (0xFF), General MIDI System Enable (0xF0 0x7E 0xcc
           0x09 0x01 0xF7), General MIDI 2 System Enable (0xF0 0x7E 0xcc
           0x09 0x03 0xF7), General MIDI System Disable (0xF0 0x7E 0xcc 0x09
           0x00 0xF7), Turn DLS On (0xF0 0x7E 0xcc 0x0A 0x01 0xF7), and Turn
           DLS Off (0xF0 0x7E 0xcc 0x0A 0x02 0xF7).  Registrations of
           subrender parameter token values (Appendix C.6.2) and IETF
           standards-track documents MAY specify additional RS commands.

         o Active commands.  Active command are MIDI commands that do not
           appear before a Reset State command in the session history.

    Lazzaro & Wawrzynek         Standards Track                    [Page 49]

    RFC 4695              RTP Payload Format for MIDI          November 2006

         o N-active commands.  N-active commands are MIDI commands that do
           not appear before one of the following commands in the session
           history:  MIDI Control Change numbers 123-127 (numbers with All
           Notes Off semantics) or 120 (All Sound Off), and any Reset State
           command.

         o C-active commands.  C-active commands are MIDI commands that do
           not appear before one of the following commands in the session
           history:  MIDI Control Change number 121 (Reset All Controllers)
           and any Reset State command.

         o Oldest-first ordering rule.  Several recovery journal chapters
           contain a list of elements, where each element is associated with
           a MIDI command that appears in the session history.  In most
           cases, the chapter definition requires that list elements be
           ordered in accordance with the "oldest-first ordering rule".
           Below, we normatively define this rule:

           Elements associated with the most recent command in the session
           history coded in the list MUST appear at the end of the list.

           Elements associated with the oldest command in the session
           history coded in the list MUST appear at the start of the list.

           All other list elements MUST be arranged with respect to these
           boundary elements, to produce a list ordering that strictly
           reflects the relative session history recency of the commands
           coded by the elements in the list.

         o Parameter system.  A MIDI feature that provides two sets of
           16,384 parameters to expand the 0-127 controller number space.
           The Registered Parameter Names (RPN) system and the Non-
           Registered Parameter Names (NRPN) system each provides 16,384
           parameters.

         o Parameter system transaction.  The value of RPNs and NRPNs are
           changed by a series of Control Change commands that form a
           parameter system transaction.  A canonical transaction begins
           with two Control Change commands to set the parameter number
           (controller numbers 99 and 98 for NRPNs, controller numbers 101
           and 100 for RPNs).  The transaction continues with an arbitrary
           number of Data Entry (controller numbers 6 and 38), Data
           Increment (controller number 96), and Data Decrement (controller
           number 97) Control Change commands to set the parameter value.
           The transaction ends with a second pair of (99, 98) or (101, 100)
           Control Change commands that specify the null parameter (MSB
           value 0x7F, LSB value 0x7F).

    Lazzaro & Wawrzynek         Standards Track                    [Page 50]

    RFC 4695              RTP Payload Format for MIDI          November 2006

           Several variants of the canonical transaction sequence are
           possible.  Most commonly, the terminal pair of (99, 98) or (101,
           100) Control Change commands may specify a parameter other than
           the null parameter.  In this case, the command pair terminates
           the first transaction and starts a second transaction.  The
           command pair is considered to be a part of both transactions.
           This variant is legal and recommended in [MIDI].  We refer to
           this variant as a "type 1 variant".

           Less commonly, the MSB (99 or 101) or LSB (98 or 100) command of
           a (99, 98) or (101, 100) Control Change pair may be omitted.

           If the MSB command is omitted, the transaction uses the MSB value
           of the most recent C-active Control Change command for controller
           number 99 or 101 that appears in the session history.  We refer
           to this variant as a "type 2 variant".

           If the LSB command is omitted, the LSB value 0x00 is assumed.  We
           refer to this variant as a "type 3 variant".  The type 2 and type
           3 variants are defined as legal, but are not recommended, in
           [MIDI].

           System real-time commands may appear at any point during a
           transaction (even between octets of individual commands in the
           transaction).  More generally, [MIDI] does not forbid the
           appearance of unrelated MIDI commands during an open transaction.
           As a rule, these commands are considered to be "outside" the
           transaction and do not affect the status of the transaction in
           any way.  Exceptions to this rule are commands whose semantics
           act to terminate transactions:  Reset State commands, and Control
           Change (0xB) for controller number 121 (Reset All Controllers)
           [RP015].

         o Initiated parameter system transaction.  A canonical parameter
           system transaction whose (99, 98) or (101, 100) initial Control
           Change command pair appears in the session history is considered
           to be an initiated parameter system transaction.  This definition
           also holds for type 1 variants.  For type 2 variants (dropped
           MSB), a transaction whose initial LSB Control Change command
           appears in the session history is an initiated transaction.  For
           type 3 variants (dropped LSB), a transaction is considered to be
           initiated if at least one transaction command follows the initial
           MSB (99 or 101) Control Change command in the session history.
           The completion of a transaction does not nullify its "initiated"
           status.

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         o Session history reference counts.  Several recovery journal
           chapters include a reference count field, which codes the total
           number of commands of a type that appear in the session history.
           Examples include the Reset and Tune Request command logs (Chapter
           D, Appendix B.1) and the Active Sense command (Chapter V,
           Appendix B.2).  Upon the detection of a loss event, reference
           count fields let a receiver deduce if any instances of the
           command have been lost, by comparing the journal reference count
           with its own reference count.  Thus, a reference count field
           makes sense, even for command types in which knowing the NUMBER
           of lost commands is irrelevant (as is true with all of the
           example commands mentioned above).

       The chapter definitions in Appendices A.2 to A.9 and B.1 to B.5
       reflect the default recovery journal behavior.  The ch_default,
       ch_never, and ch_anchor parameters modify these definitions, as
       described in Appendix C.2.3.

       The chapter definitions specify if data MUST be present in the
       journal.  Senders MAY also include non-required data in the journal.
       This optional data MUST comply with the normative chapter definition.
       For example, if a chapter definition states that a field codes data
       from the most recent active command in the session history, the
       sender MUST NOT code inactive commands or older commands in the
       field.

       Finally, we note that a channel journal only encodes information
       about MIDI commands appearing on the MIDI channel the journal
       protects.  All references to MIDI commands in Appendices A.2 to A.9
       should be read as "MIDI commands appearing on this channel."

    A.2.  Chapter P: MIDI Program Change

       A channel journal MUST contain Chapter P if an active Program Change
       (0xC) command appears in the checkpoint history.  Figure A.2.1 shows
       the format for Chapter P.

                    0                   1                   2
                    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |S|   PROGRAM   |B|   BANK-MSB  |X|  BANK-LSB   |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.2.1 -- Chapter P format

       The chapter has a fixed size of 24 bits.  The PROGRAM field indicates
       the data value of the most recent active Program Change command in
       the session history.  By default, the B, BANK-MSB, X, and BANK-LSB

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       fields MUST be set to 0.  Below, we define exceptions to this default
       condition.

       If an active Control Change (0xB) command for controller number 0
       (Bank Select MSB) appears before the Program Change command in the
       session history, the B bit MUST be set to 1, and the BANK-MSB field
       MUST code the data value of the Control Change command.

       If B is set to 1, the BANK-LSB field MUST code the data value of the
       most recent Control Change command for controller number 32 (Bank
       Select LSB) that preceded the Program Change command coded in the
       PROGRAM field and followed the Control Change command coded in the
       BANK-MSB field.  If no such Control Change command exists, the BANK-
       LSB field MUST be set to 0.

       If B is set to 1, and if a Control Change command for controller
       number 121 (Reset All Controllers) appears in the MIDI stream between
       the Control Change command coded by the BANK-MSB field and the
       Program Change command coded by the PROGRAM field, the X bit MUST be
       set to 1.

       Note that [RP015] specifies that Reset All Controllers does not reset
       the values of controller numbers 0 (Bank Select MSB) and 32 (Bank
       Select LSB).  Thus, the X bit does not effect how receivers will use
       the BANK-LSB and BANK-MSB values when recovering from a lost Program
       Change command.  The X bit serves to aid recovery in MIDI
       applications where controller numbers 0 and 32 are used in a non-
       standard way.

    A.3.  Chapter C: MIDI Control Change

       Figure A.3.1 shows the format for Chapter C.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|     LEN     |S|   NUMBER    |A|  VALUE/ALT  |S|   NUMBER    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |A|  VALUE/ALT  |  ....                                         |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure A.3.1 -- Chapter C format

       The chapter consists of a 1-octet header, followed by a variable
       length list of 2-octet controller logs.  The list MUST contain at
       least one controller log.  The 7-bit LEN field codes the number of
       controller logs in the list, minus one.  We define the semantics of
       the controller log fields in Appendix A.3.2.

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       A channel journal MUST contain Chapter C if the rules defined in this
       appendix require that one or more controller logs appear in the list.

    A.3.1.  Log Inclusion Rules

       A controller log encodes information about a particular Control
       Change command in the session history.

       In the default use of the payload format, list logs MUST encode
       information about the most recent active command in the session
       history for a controller number.  Logs encoding earlier commands MUST
       NOT appear in the list.

       Also, as a rule, the list MUST contain a log for the most recent
       active command for a controller number that appears in the checkpoint
       history.  Below, we define exceptions to this rule:

         o  MIDI streams may transmit 14-bit controller values using paired
            Most Significant Byte (MSB, controller numbers 0-31, 99, 101)
            and Least Significant Byte (LSB, controller numbers 32-63, 98,
            100) Control Change commands [MIDI].

            If the most recent active Control Change command in the session
            history for a 14-bit controller pair uses the MSB number,
            Chapter C MAY omit the controller log for the most recent active
            Control Change command for the associated LSB number, as the
            command ordering makes this LSB value irrelevant.  However, this
            exception MUST NOT be applied if the sender is not certain that
            the MIDI source uses 14-bit semantics for the controller number
            pair.  Note that some MIDI sources ignore 14-bit controller
            semantics and use the LSB controller numbers as independent 7-
            bit controllers.

         o  If active Control Change commands for controller numbers 0 (Bank
            Select MSB) or 32 (Bank Select LSB) appear in the checkpoint
            history, and if the command instances are also coded in the
            BANK-MSB and BANK-LSB fields of the Chapter P (Appendix A.2),
            Chapter C MAY omit the controller logs for the commands.

         o  Several controller number pairs are defined to be mutually
            exclusive.  Controller numbers 124 (Omni Off) and 125 (Omni On)
            form a mutually exclusive pair, as do controller numbers 126
            (Mono) and 127 (Poly).

            If active Control Change commands for one or both members of a
            mutually exclusive pair appear in the checkpoint history, a log
            for the controller number of the most recent command for the
            pair in the checkpoint history MUST appear in the controller

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            list.  However, the list MAY omit the controller log for the
            most recent active command for the other number in the pair.

            If active Control Change commands for one or both members of a
            mutually exclusive pair appear in the session history, and if a
            log for the controller number of the most recent command for the
            pair does not appear in the controller list, a log for the most
            recent command for the other number of the pair MUST NOT appear
            in the controller list.

         o  If an active Control Change command for controller number 121
            (Reset All Controllers) appears in the session history, the
            controller list MAY omit logs for Control Change commands that
            precede the Reset All Controllers command in the session
            history, under certain conditions.

            Namely, a log MAY be omitted if the sender is certain that a
            command stream follows the Reset All Controllers semantics
            defined in [RP015], and if the log codes a controller number for
            which [RP015] specifies a reset value.

            For example, [RP015] specifies that controller number 1
            (Modulation Wheel) is reset to the value 0, and thus a
            controller log for Modulation Wheel MAY be omitted from the
            controller log list.  In contrast, [RP015] specifies that
            controller number 7 (Channel Volume) is not reset, and thus a
            controller log for Channel Volume MUST NOT be omitted from the
            controller log list.

         o  Appendix A.3.4 defines exception rules for the MIDI Parameter
            System controller numbers 6, 38, and 96-101.

    A.3.2.  Controller Log Format

       Figure A.3.2 shows the controller log structure of Chapter C.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|    NUMBER   |A|  VALUE/ALT  |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure A.3.2 -- Chapter C controller log

       The 7-bit NUMBER field identifies the controller number of the coded
       command.  The 7-bit VALUE/ALT field codes recovery information for
       the command.  The A bit sets the format of the VALUE/ALT field.

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       A log encodes recovery information using one of the following tools:
       the value tool, the toggle tool, or the count tool.

       A log uses the value tool if the A bit is set to 0.  The value tool
       codes the 7-bit data value of a command in the VALUE/ALT field.  The
       value tool works best for controllers that code a continuous
       quantity, such as number 1 (Modulation Wheel).

       The A bit is set to 1 to code the toggle or count tool.  These tools
       work best for controllers that code discrete actions.  Figure A.3.3
       shows the controller log for these tools.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|    NUMBER   |1|T|    ALT    |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure A.3.3 -- Controller log for ALT tools

       A log uses the toggle tool if the T bit is set to 0.  A log uses the
       count tool if the T bit is set to 1.  Both methods use the 6-bit ALT
       field as an unsigned integer.

       The toggle tool works best for controllers that act as on/off
       switches, such as 64 (Damper Pedal (Sustain)).  These controllers
       code the "off" state with control values 0-63 and the "on" state with
       64-127.

       For the toggle tool, the ALT field codes the total number of toggles
       (off->on and on->off) due to Control Change commands in the session
       history, up to and including a toggle caused by the command coded by
       the log.  The toggle count includes toggles caused by Control Change
       commands for controller number 121 (Reset All Controllers).

       Toggle counting is performed modulo 64.  The toggle count is reset at
       the start of a session, and whenever a Reset State command (Appendix
       A.1) appears in the session history.  When these reset events occur,
       the toggle count for a controller is set to 0 (for controllers whose
       default value is 0-63) or 1 (for controllers whose default value is
       64-127).

       The Damper Pedal (Sustain) controller illustrates the benefits of the
       toggle tool over the value tool for switch controllers.  As often
       used in piano applications, the "on" state of the controller lets
       notes resonate, while the "off" state immediately damps notes to
       silence.  The loss of the "off" command in an "on->off->on" sequence
       results in ringing notes that should have been damped silent.  The

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       toggle tool lets receivers detect this lost "off" command, but the
       value tool does not.

       The count tool is conceptually similar to the toggle tool.  For the
       count tool, the ALT field codes the total number of Control Change
       commands in the session history, up to and including the command
       coded by the log.  Command counting is performed modulo 64.  The
       command count is set to 0 at the start of the session and is reset to
       0 whenever a Reset State command (Appendix A.1) appears in the
       session history.

       Because the count tool ignores the data value, it is a good match for
       controllers whose controller value is ignored, such as number 123
       (All Notes Off).  More generally, the count tool may be used to code
       a (modulo 64) identification number for a command.

    A.3.3.  Log List Coding Rules

       In this section, we describe the organization of controller logs in
       the Chapter C log list.

       A log encodes information about a particular Control Change command
       in the session history.  In most cases, a command SHOULD be coded by
       a single tool (and, thus, a single log).  If a number is coded with a
       single tool and this tool is the count tool, recovery Control Change
       commands generated by a receiver SHOULD use the default control value
       for the controller.

       However, a command MAY be coded by several tool types (and, thus,
       several logs, each using a different tool).  This technique may
       improve recovery performance for controllers with complex semantics,
       such as controller number 84 (Portamento Control) or controller
       number 121 (Reset All Controllers) when used with a non-zero data
       octet (with the semantics described in [DLS2]).

       If a command is encoded by multiple tools, the logs MUST be placed in
       the list in the following order: count tool log (if any), followed by
       value tool log (if any), followed by toggle tool log (if any).

       The Chapter C log list MUST obey the oldest-first ordering rule
       (defined in Appendix A.1).  Note that this ordering preserves the
       information necessary for the recovery of 14-bit controller values,
       without precluding the use of MSB and LSB controller pairs as
       independent 7-bit controllers.

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       In the default use of the payload format, all logs that appear in the
       list for a controller number encode information about one Control
       Change command -- namely, the most recent active Control Change
       command in the session history for the number.

       This coding scheme provides good recovery performance for the
       standard uses of Control Change commands defined in [MIDI].  However,
       not all MIDI applications restrict the use of Control Change commands
       to those defined in [MIDI].

       For example, consider the common MIDI encoding of rotary encoders
       ("infinite" rotation knobs).  The mixing console MIDI convention
       defined in [LCP] codes the position of rotary encoders as a series of
       Control Change commands.  Each command encodes a relative change of
       knob position from the last update (expressed as a clockwise or
       counter-clockwise knob turning angle).

       As the knob position is encoded incrementally over a series of
       Control Change commands, the best recovery performance is obtained if
       the log list encodes all Control Change commands for encoder
       controller numbers that appear in the checkpoint history, not only
       the most recent command.

       To support application areas that use Control Change commands in this
       way, Chapter C may be configured to encode information about several
       Control Change commands for a controller number.  We use the term
       "enhanced" to describe this encoding method, which we describe below.

       In Appendix C.2.3, we show how to configure a stream to use enhanced
       Chapter C encoding for specific controller numbers.  In Section 5 in
       the main text, we show how the H bits in the recovery journal header
       (Figure 8) and in the channel journal header (Figure 9) indicate the
       use of enhanced Chapter C encoding.

       Here, we define how to encode a Chapter C log list that uses the
       enhanced encoding method.

       Senders that use the enhanced encoding method for a controller number
       MUST obey the rules below.  These rules let a receiver determine
       which logs in the list correspond to lost commands.  Note that these
       rules override the exceptions listed in Appendix A.3.1.

         o  If N commands for a controller number are encoded in the list,
            the commands MUST be the N most recent commands for the
            controller number in the session history.  For example, for N =
            2, the sender MUST encode the most recent command and the second
            most recent command, not the most recent command and the third
            most recent command.

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         o  If a controller number uses enhanced encoding, the encoding of
            the least-recent command for the controller number in the log
            list MUST include a count tool log.  In addition, if commands
            are encoded for the controller number whose logs have S bits set
            to 0, the encoding of the least-recent command with S = 0 logs
            MUST include a count tool log.

            The count tool is OPTIONAL for the other commands for the
            controller number encoded in the list, as a receiver is able to
            efficiently deduce the count tool value for these commands, for
            both single-packet and multi-packet loss events.

         o  The use of the value and toggle tools MUST be identical for all
            commands for a controller number encoded in the list.  For
            example, a value tool log either MUST appear for all commands
            for the controller number coded in the list, or alternatively,
            value tool logs for the controller number MUST NOT appear in the
            list.  Likewise, a toggle tool log either MUST appear for all
            commands for the controller number coded in the list, or
            alternatively, toggle tool logs for the controller number MUST
            NOT appear in the list.

         o  If a command is encoded by multiple tools, the logs MUST be
            placed in the list in the following order: count tool log (if
            any), followed by value tool log (if any), followed by toggle
            tool log (if any).

       These rules permit a receiver recovering from a packet loss to use
       the count tool log to match the commands encoded in the list with its
       own history of the stream, as we describe below.  Note that the text
       below describes a non-normative algorithm; receivers are free to use
       any algorithm to match its history with the log list.

       In a typical implementation of the enhanced encoding method, a
       receiver computes and stores count, value, and toggle tool data field
       values for the most recent Control Change command it has received for
       a controller number.

       After a loss event, a receiver parses the Chapter C list and
       processes list logs for a controller number that uses enhanced
       encoding as follows.

       The receiver compares the count tool ALT field for the least-recent
       command for the controller number in the list against its stored
       count data for the controller number, to determine if recovery is
       necessary for the command coded in the list.  The value and toggle
       tool logs (if any) that directly follow the count tool log are
       associated with this least-recent command.

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       To check more-recent commands for the controller, the receiver
       detects additional value and/or toggle tool logs for the controller
       number in the list and infers count tool data for the command coded
       by these logs.  This inferred data is used to determine if recovery
       is necessary for the command coded by the value and/or toggle tool
       logs.

       In this way, a receiver is able to execute only lost commands,
       without executing a command twice.  While recovering from a single
       packet loss, a receiver may skip through S = 1 logs in the list, as
       the first S = 0 log for an enhanced controller number is always a
       count tool log.

       Note that the requirements in Appendix C.2.2.2 for protective sender
       and receiver actions during session startup for multicast operation
       are of particular importance for enhanced encoding, as receivers need
       to initialize its count tool data structures with recovery journal
       data in order to match commands correctly after a loss event.

       Finally, we note in passing that in some applications of rotary
       encoders, a good user experience may be possible without the use of
       enhanced encoding.  These applications are distinguished by visual
       feedback of encoding position that is driven by the post-recovery
       rotary encoding stream, and relatively low packet loss.  In these
       domains, recovery performance may be acceptable for rotary encoders
       if the log list encodes only the most recent command, if both count
       and value logs appear for the command.

    A.3.4.  The Parameter System

       Readers may wish to review the Appendix A.1 definitions of "parameter
       system", "parameter system transaction", and "initiated parameter
       system transaction" before reading this section.

       Parameter system transactions update a MIDI Registered Parameter
       Number (RPN) or Non-Registered Parameter Number (NRPN) value.  A
       parameter system transaction is a sequence of Control Change commands
       that may use the following controllers numbers:

         o  Data Entry MSB (6)
         o  Data Entry LSB (38)
         o  Data Increment (96)
         o  Data Decrement (97)
         o  Non-Registered Parameter Number (NRPN) LSB (98)
         o  Non-Registered Parameter Number (NRPN) MSB (99)
         o  Registered Parameter Number (RPN) LSB (100)
         o  Registered Parameter Number (RPN) MSB (101)

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       Control Change commands that are a part of a parameter system
       transaction MUST NOT be coded in Chapter C controller logs.  Instead,
       these commands are coded in Chapter M, the MIDI Parameter chapter
       defined in Appendix A.4.

       However, Control Change commands that use the listed controllers as
       general-purpose controllers (i.e., outside of a parameter system
       transaction) MUST NOT be coded in Chapter M.

       Instead, the controllers are coded in Chapter C controller logs.  The
       controller logs follow the coding rules stated in Appendix A.3.2 and
       A.3.3.  The rules for coding paired LSB and MSB controllers, as
       defined in Appendix A.3.1, apply to the pairs (6, 38), (99, 98), and
       (101, 100) when coded in Chapter C.

       If active Control Change commands for controller numbers 6, 38, or
       96-101 appear in the checkpoint history, and these commands are used
       as general-purpose controllers, the most recent general-purpose
       command instance for these controller numbers MUST appear as entries
       in the Chapter C controller list.

       MIDI syntax permits a source to use controllers 6, 38, 96, and 97 as
       parameter-system controllers and general-purpose controllers in the
       same stream.  An RTP MIDI sender MUST deduce the role of each Control
       Change command for these controller numbers by noting the placement
       of the command in the stream and MUST use this information to code
       the command in Chapter C or Chapter M, as appropriate.

       Specifically, active Control Change commands for controllers 6, 38,
       96, and 97 act in a general-purpose way when

         o  no active Control Change commands that set an RPN or NRPN
            parameter number appear in the session history, or

         o  the most recent active Control Change commands in the session
            history that set an RPN or NRPN parameter number code the null
            parameter (MSB value 0x7F, LSB value 0x7F), or

         o  a Control Change command for controller number 121 (Reset All
            Controllers) appears more recently in the session history than
            all active Control Change commands that set an RPN or NRPN
            parameter number (see [RP015] for details).

       Finally, we note that a MIDI source that follows the recommendations
       of [MIDI] exclusively uses numbers 98-101 as parameter system
       controllers.  Alternatively, a MIDI source may exclusively use 98-101
       as general-purpose controllers and lose the ability perform parameter
       system transactions in a stream.

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       In the language of [MIDI], the general-purpose use of controllers
       98-101 constitutes a non-standard controller assignment.  As most
       real-world MIDI sources use the standard controller assignment for
       controller numbers 98-101, an RTP MIDI sender SHOULD assume these
       controllers act as parameter system controllers, unless it knows that
       a MIDI source uses controller numbers 98-101 in a general-purpose
       way.

    A.4.  Chapter M: MIDI Parameter System

       Readers may wish to review the Appendix A.1 definitions for
       "C-active", "parameter system", "parameter system transaction", and
       "initiated parameter system transaction" before reading this
       appendix.

       Chapter M protects parameter system transactions for Registered
       Parameter Number (RPN) and Non-Registered Parameter Number (NRPN)
       values.  Figure A.4.1 shows the format for Chapter M.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|P|E|U|W|Z|      LENGTH       |Q|  PENDING    |  Log list ... |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure A.4.1 -- Top-level Chapter M format

       Chapter M begins with a 2-octet header.  If the P header bit is set
       to 1, a 1-octet field follows the header, coding the 7-bit PENDING
       value and its associated Q bit.

       The 10-bit LENGTH field codes the size of Chapter M and conforms to
       semantics described in Appendix A.1.

       Chapter M ends with a list of zero or more variable-length parameter
       logs.  Appendix A.4.2 defines the bitfield format of a parameter log.
       Appendix A.4.1 defines the inclusion semantics of the log list.

       A channel journal MUST contain Chapter M if the rules defined in
       Appendix A.4.1 require that one or more parameter logs appear in the
       list.

       A channel journal also MUST contain Chapter M if the most recent
       C-active Control Change command involved in a parameter system
       transaction in the checkpoint history is

         o  an RPN MSB (101) or NRPN MSB (99) controller, or

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         o  an RPN LSB (100) or NRPN LSB (98) controller that completes the
            coding of the null parameter (MSB value 0x7F, LSB value 0x7F).

       This rule provides loss protection for partially transmitted
       parameter numbers and for the null parameter numbers.

       If the most recent C-active Control Change command involved in a
       parameter system transaction in the session history is for the RPN
       MSB or NRPN MSB controller, the P header bit MUST be set to 1, and
       the PENDING field (and its associated Q bit) MUST follow the Chapter
       M header.  Otherwise, the P header bit MUST be set to 0, and the
       PENDING field and Q bit MUST NOT appear in Chapter M.

       If PENDING codes an NRPN MSB, the Q bit MUST be set to 1.  If PENDING
       codes an RPN MSB, the Q bit MUST be set to 0.

       The E header bit codes the current transaction state of the MIDI
       stream.  If E = 1, an initiated transaction is in progress.  Below,
       we define the rules for setting the E header bit:

         o  If no C-active parameter system transaction Control Change
            commands appear in the session history, the E bit MUST be set to
            0.

         o  If the P header bit is set to 1, the E bit MUST be set to 0.

         o  If the most recent C-active parameter system transaction Control
            Change command in the session history is for the NRPN LSB or RPN
            LSB controller number, and if this command acts to complete the
            coding of the null parameter (MSB value 0x7F, LSB value 0x7F),
            the E bit MUST be set to 0.

         o  Otherwise, an initiated transaction is in progress, and the E
            bit MUST be set to 1.

       The U, W, and Z header bits code properties that are shared by all
       parameter logs in the list.  If these properties are set, parameter
       logs may be coded with improved efficiency (we explain how in A.4.1).

       By default, the U, W, and Z bits MUST be set to 0.  If all parameter
       logs in the list code RPN parameters, the U bit MAY be set to 1.  If
       all parameter logs in the list code NRPN parameters, the W bit MAY be
       set to 1.  If the parameter numbers of all RPN and NRPN logs in the
       list lie in the range 0-127 (and thus have an MSB value of 0), the Z
       bit MAY be set to 1.

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       Note that C-active semantics appear in the preceding paragraphs
       because [RP015] specifies that pending Parameter System transactions
       are closed by a Control Change command for controller number 121
       (Reset All Controllers).

    A.4.1.  Log Inclusion Rules

       Parameter logs code recovery information for a specific RPN or NRPN
       parameter.

       A parameter log MUST appear in the list if an active Control Change
       command that forms a part of an initiated transaction for the
       parameter appears in the checkpoint history.

       An exception to this rule applies if the checkpoint history only
       contains transaction Control Change commands for controller numbers
       98-101 that act to terminate the transaction.  In this case, a log
       for the parameter MAY be omitted from the list.

       A log MAY appear in the list if an active Control Change command that
       forms a part of an initiated transaction for the parameter appears in
       the session history.  Otherwise, a log for the parameter MUST NOT
       appear in the list.

       Multiple logs for the same RPN or NRPN parameter MUST NOT appear in
       the log list.

       The parameter log list MUST obey the oldest-first ordering rule
       (defined in Appendix A.1), with the phrase "parameter transaction"
       replacing the word "command" in the rule definition.

       Parameter logs associated with the RPN or NRPN null parameter (LSB =
       0x7F, MSB = 0x7F) MUST NOT appear in the log list.  Chapter M uses
       the E header bit (Figure A.4.1) and the log list ordering rules to
       code null parameter semantics.

       Note that "active" semantics (rather than "C-active" semantics)
       appear in the preceding paragraphs because [RP015] specifies that
       pending Parameter System transactions are not reset by a Control
       Change command for controller number 121 (Reset All Controllers).
       However, the rule that follows uses C-active semantics, because it
       concerns the protection of the transaction system itself, and [RP015]
       specifies that Reset All Controllers acts to close a transaction in
       progress.

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       In most cases, parameter logs for RPN and NRPN parameters that are
       assigned to the ch_never parameter (Appendix C.2.3) MAY be omitted
       from the list.  An exception applies if

         o  the log codes the most recent initiated transaction in the
            session history, and

         o  a C-active command that forms a part of the transaction appears
            in the checkpoint history, and

         o  the E header bit for the top-level Chapter M header (Figure
            A.4.1) is set to 1.

       In this case, a log for the parameter MUST appear in the list.  This
       log informs receivers recovering from a loss that a transaction is in
       progress, so that the receiver is able to correctly interpret RPN or
       NRPN Control Change commands that follow the loss event.

    A.4.2.  Log Coding Rules

       Figure A.4.2 shows the parameter log structure of Chapter M.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|  PNUM-LSB   |Q|  PNUM-MSB   |J|K|L|M|N|T|V|R|   Fields ...  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.4.2 -- Parameter log format

       The log begins with a header, whose default size (as shown in Figure
       A.4.2) is 3 octets.  If the Q header bit is set to 0, the log encodes
       an RPN parameter.  If Q = 1, the log encodes an NRPN parameter.  The
       7-bit PNUM-MSB and PNUM-LSB fields code the parameter number and
       reflect the Control Change command data values for controllers 99 and
       98 (for NRPNs) or 101 and 100 (for RPNs).

       The J, K, L, M, and N header bits form a Table of Contents (TOC) for
       the log and signal the presence of fixed-sized fields that follow the
       header.  A header bit that is set to 1 codes the presence of a field
       in the log.  The ordering of fields in the log follows the ordering
       of the header bits in the TOC.  Appendices A.4.2.1-2 define the
       fields associated with each TOC header bit.

       The T and V header bits code information about the parameter log but
       are not part of the TOC.  A set T or V bit does not signal the
       presence of any parameter log field.

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       If the rules in Appendix A.4.1 state that a log for a given parameter
       MUST appear in Chapter M, the log MUST code sufficient information to
       protect the parameter from the loss of active parameter transaction
       Control Change commands in the checkpoint history.

       This rule does not apply if the parameter coded by the log is
       assigned to the ch_never parameter (Appendix C.2.3).  In this case,
       senders MAY choose to set the J, K, L, M, and N TOC bits to 0, coding
       a parameter log with no fields.

       Note that logs to protect parameters that are assigned to ch_never
       are REQUIRED under certain conditions (see Appendix A.4.1).  The
       purpose of the log is to inform receivers recovering from a loss that
       a transaction is in progress, so that the receiver is able to
       correctly interpret RPN or NRPN Control Change commands that follow
       the loss event.

       Parameter logs provide two tools for parameter protection: the value
       tool and the count tool.  Depending on the semantics of the
       parameter, senders may use either tool, both tools, or neither tool
       to protect a given parameter.

       The value tool codes information a receiver may use to determine the
       current value of an RPN or NRPN parameter.  If a parameter log uses
       the value tool, the V header bit MUST be set to 1, and the semantics
       defined in Appendices A.4.2.1 for setting the J, K, L, and M TOC bits
       MUST be followed.  If a parameter log does not use the value tool,
       the V bit MUST be set to 0, and the J, K, L, and M TOC bits MUST also
       be set to 0.

       The count tool codes the number of transactions for an RPN or NRPN
       parameter.  If a parameter log uses the count tool, the T header bit
       MUST be set to 1, and the semantics defined in Appendices A.4.2.2 for
       setting the N TOC bit MUST be followed.  If a parameter log does not
       use the count tool, the T bit and the N TOC bit MUST be set to 0.

       Note that V and T are set if the sender uses value (V) or count (T)
       tool for the log on an ongoing basis.  Thus, V may be set even if J =
       K = L = M = 0, and T may be set even if N = 0.

       In many cases, all parameters coded in the log list are of one type
       (RPN and NRPN), and all parameter numbers lie in the range 0-127.  As
       described in Appendix A.4.1, senders MAY signal this condition by
       setting the top-level Chapter M header bit Z to 1 (to code the
       restricted range) and by setting the U or W bit to 1 (to code the
       parameter type).

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       If the top-level Chapter M header codes Z = 1 and either U = 1 or
       W = 1, all logs in the parameter log list MUST use a modified header
       format.  This modification deletes bits 8-15 of the bitfield shown in
       Figure A.4.2, to yield a 2-octet header.  The values of the deleted
       PNUM-MSB and Q fields may be inferred from the U, W, and Z bit
       values.

    A.4.2.1.  The Value Tool

       The value tool uses several fields to track the value of an RPN or
       NRPN parameter.

       The J TOC bit codes the presence of the octet shown in Figure A.4.3
       in the field list.

                                 0
                                 0 1 2 3 4 5 6 7
                                +-+-+-+-+-+-+-+-+
                                |X|  ENTRY-MSB  |
                                +-+-+-+-+-+-+-+-+

                          Figure A.4.3 -- ENTRY-MSB field

       The 7-bit ENTRY-MSB field codes the data value of the most recent
       active Control Change command for controller number 6 (Data Entry
       MSB) in the session history that appears in a transaction for the log
       parameter.

       The X bit MUST be set to 1 if the command coded by ENTRY-MSB precedes
       the most recent Control Change command for controller 121 (Reset All
       Controllers) in the session history.  Otherwise, the X bit MUST be
       set to 0.

       A parameter log that uses the value tool MUST include the ENTRY-MSB
       field if an active Control Change command for controller number 6
       appears in the checkpoint history.

       Note that [RP015] specifies that Control Change commands for
       controller 121 (Reset All Controllers) do not reset RPN and NRPN
       values, and thus the X bit would not play a recovery role for MIDI
       systems that comply with [RP015].

       However, certain renderers (such as DLS 2 [DLS2]) specify that
       certain RPN values are reset for some uses of Reset All Controllers.
       The X bit (and other bitfield features of this nature in this
       appendix) plays a role in recovery for renderers of this type.

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       The K TOC bit codes the presence of the octet shown in Figure A.4.4
       in the field list.

                                 0
                                 0 1 2 3 4 5 6 7
                                +-+-+-+-+-+-+-+-+
                                |X|  ENTRY-LSB  |
                                +-+-+-+-+-+-+-+-+

                          Figure A.4.4 -- ENTRY-LSB field

       The 7-bit ENTRY-LSB field codes the data value of the most recent
       active Control Change command for controller number 38 (Data Entry
       LSB) in the session history that appears in a transaction for the log
       parameter.

       The X bit MUST be set to 1 if the command coded by ENTRY-LSB precedes
       the most recent Control Change command for controller 121 (Reset All
       Controllers) in the session history.  Otherwise, the X bit MUST be
       set to 0.

       As a rule, a parameter log that uses the value tool MUST include the
       ENTRY-LSB field if an active Control Change command for controller
       number 38 appears in the checkpoint history.  However, the ENTRY-LSB
       field MUST NOT appear in a parameter log if the Control Change
       command associated with the ENTRY-LSB precedes a Control Change
       command for controller number 6 (Data Entry MSB) that appears in a
       transaction for the log parameter in the session history.

       The L TOC bit codes the presence of the octets shown in Figure A.4.5
       in the field list.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |G|X|       A-BUTTON            |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure A.4.5 -- A-BUTTON field

       The 14-bit A-BUTTON field codes a count of the number of active
       Control Change commands for controller numbers 96 and 97 (Data
       Increment and Data Decrement) in the session history that appear in a
       transaction for the log parameter.

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       The M TOC bit codes the presence of the octets shown in Figure A.4.6
       in the field list.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |G|R|       C-BUTTON            |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure A.4.6 -- C-BUTTON field

       The 14-bit C-BUTTON field has semantics identical to A-BUTTON, except
       that Data Increment and Data Decrement Control Change commands that
       precede the most recent Control Change command for controller 121
       (Reset All Controllers) in the session history are not counted.

       For both A-BUTTON and C-BUTTON, Data Increment and Data Decrement
       Control Change commands are not counted if they precede Control
       Changes commands for controller numbers 6 (Data Entry MSB) or 38
       (Data Entry LSB) that appear in a transaction for the log parameter
       in the session history.

       The A-BUTTON and C-BUTTON fields are interpreted as unsigned
       integers, and the G bit associated the field codes the sign of the
       integer (G = 0 for positive or zero, G = 1 for negative).

       To compute and code the count value, initialize the count value to 0,
       add 1 for each qualifying Data Increment command, and subtract 1 for
       each qualifying Data Decrement command.  After each add or subtract,
       limit the count magnitude to 16383.  The G bit codes the sign of the
       count, and the A-BUTTON or C-BUTTON field codes the count magnitude.

       For the A-BUTTON field, if the most recent qualified Data Increment
       or Data Decrement command precedes the most recent Control Change
       command for controller 121 (Reset All Controllers) in the session
       history, the X bit associated with A-BUTTON field MUST be set to 1.
       Otherwise, the X bit MUST be set to 0.

       A parameter log that uses the value tool MUST include the A-BUTTON
       and C-BUTTON fields if an active Control Change command for
       controller numbers 96 or 97 appears in the checkpoint history.
       However, to improve coding efficiency, this rule has several
       exceptions:

         o  If the log includes the A-BUTTON field, and if the X bit of the
            A-BUTTON field is set to 1, the C-BUTTON field (and its
            associated R and G bits) MAY be omitted from the log.

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         o  If the log includes the A-BUTTON field, and if the A-BUTTON and
            C-BUTTON fields (and their associated G bits) code identical
            values, the C-BUTTON field (and its associated R and G bits) MAY
            be omitted from the log.

    A.4.2.2.  The Count Tool

       The count tool tracks the number of transactions for an RPN or NRPN
       parameter.  The N TOC bit codes the presence of the octet shown in
       Figure A.4.7 in the field list.

                              0
                              0 1 2 3 4 5 6 7
                             +-+-+-+-+-+-+-+-+
                             |X|    COUNT    |
                             +-+-+-+-+-+-+-+-+

                         Figure A.4.7 -- COUNT field

       The 7-bit COUNT codes the number of initiated transactions for the
       log parameter that appear in the session history.  Initiated
       transactions are counted if they contain one or more active Control
       Change commands, including commands for controllers 98-101 that
       initiate the parameter transaction.

       If the most recent counted transaction precedes the most recent
       Control Change command for controller 121 (Reset All Controllers) in
       the session history, the X bit associated with the COUNT field MUST
       be set to 1.  Otherwise, the X bit MUST be set to 0.

       Transaction counting is performed modulo 128.  The transaction count
       is set to 0 at the start of a session and is reset to 0 whenever a
       Reset State command (Appendix A.1) appears in the session history.

       A parameter log that uses the count tool MUST include the COUNT field
       if an active command that increments the transaction count (modulo
       128) appears in the checkpoint history.

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    A.5.  Chapter W: MIDI Pitch Wheel

       A channel journal MUST contain Chapter W if a C-active MIDI Pitch
       Wheel (0xE) command appears in the checkpoint history.  Figure A.5.1
       shows the format for Chapter W.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|     FIRST   |R|    SECOND   |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.5.1 -- Chapter W format

       The chapter has a fixed size of 16 bits.  The FIRST and SECOND fields
       are the 7-bit values of the first and second data octets of the most
       recent active Pitch Wheel command in the session history.

       Note that Chapter W encodes C-active commands and thus does not
       encode active commands that are not C-active (see the second-to-last
       paragraph of Appendix A.1 for an explanation of chapter inclusion
       text in this regard).

       Chapter W does not encode "active but not C-active" commands because
       [RP015] declares that Control Change commands for controller number
       121 (Reset All Controllers) act to reset the Pitch Wheel value to 0.
       If Chapter W encoded "active but not C-active" commands, a repair
       operation following a Reset All Controllers command could incorrectly
       repair the stream with a stale Pitch Wheel value.

    A.6.  Chapter N: MIDI NoteOff and NoteOn

       In this appendix, we consider NoteOn commands with zero velocity to
       be NoteOff commands.  Readers may wish to review the Appendix A.1
       definition of "N-active commands" before reading this appendix.

       Chapter N completely protects note commands in streams that alternate
       between NoteOn and NoteOff commands for a particular note number.
       However, in rare applications, multiple overlapping NoteOn commands
       may appear for a note number.  Chapter E, described in Appendix A.7,
       augments Chapter N to completely protect these streams.

       A channel journal MUST contain Chapter N if an N-active MIDI NoteOn
       (0x9) or NoteOff (0x8) command appears in the checkpoint history.
       Figure A.6.1 shows the format for Chapter N.

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           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |B|     LEN     |  LOW  | HIGH  |S|   NOTENUM   |Y|  VELOCITY   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|   NOTENUM   |Y|  VELOCITY   |             ....              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |    OFFBITS    |    OFFBITS    |     ....      |    OFFBITS    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.6.1 -- Chapter N format

       Chapter N consists of a 2-octet header, followed by at least one of
       the following data structures:

          o A list of note logs to code NoteOn commands.
          o A NoteOff bitfield structure to code NoteOff commands.

       We define the header bitfield semantics in Appendix A.6.1.  We define
       the note log semantics and the NoteOff bitfield semantics in Appendix
       A.6.2.

       If one or more N-active NoteOn or NoteOff commands in the checkpoint
       history reference a note number, the note number MUST be coded in
       either the note log list or the NoteOff bitfield structure.

       The note log list MUST contain an entry for all note numbers whose
       most recent checkpoint history appearance is in an N-active NoteOn
       command.  The NoteOff bitfield structure MUST contain a set bit for
       all note numbers whose most recent checkpoint history appearance is
       in an N-active NoteOff command.

       A note number MUST NOT be coded in both structures.

       All note logs and NoteOff bitfield set bits MUST code the most recent
       N-active NoteOn or NoteOff reference to a note number in the session
       history.

       The note log list MUST obey the oldest-first ordering rule (defined
       in Appendix A.1).

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    A.6.1.  Header Structure

       The header for Chapter N, shown in Figure A.6.2, codes the size of
       the note list and bitfield structures.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |B|     LEN     |  LOW  | HIGH  |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.6.2 -- Chapter N header

       The LEN field, a 7-bit integer value, codes the number of 2-octet
       note logs in the note list.  Zero is a valid value for LEN and codes
       an empty note list.

       The 4-bit LOW and HIGH fields code the number of OFFBITS octets that
       follow the note log list.  LOW and HIGH are unsigned integer values.
       If LOW <= HIGH, there are (HIGH - LOW + 1) OFFBITS octets in the
       chapter.  The value pairs (LOW = 15, HIGH = 0) and (LOW = 15, HIGH =
       1) code an empty NoteOff bitfield structure (i.e., no OFFBITS
       octets).  Other (LOW > HIGH) value pairs MUST NOT appear in the
       header.

       The B bit provides S-bit functionality (Appendix A.1) for the NoteOff
       bitfield structure.  By default, the B bit MUST be set to 1.
       However, if the MIDI command section of the previous packet (packet I
       - 1, with I as defined in Appendix A.1) includes a NoteOff command
       for the channel, the B bit MUST be set to 0.  If the B bit is set to
       0, the higher-level recovery journal elements that contain Chapter N
       MUST have S bits that are set to 0, including the top-level journal
       header.

       The LEN value of 127 codes a note list length of 127 or 128 note
       logs, depending on the values of LOW and HIGH.  If LEN = 127, LOW =
       15, and HIGH = 0, the note list holds 128 note logs, and the NoteOff
       bitfield structure is empty.  For other values of LOW and HIGH, LEN =
       127 codes that the note list contains 127 note logs.  In this case,
       the chapter has (HIGH - LOW + 1) NoteOff OFFBITS octets if LOW <=
       HIGH and has no OFFBITS octets if LOW = 15 and HIGH = 1.

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    A.6.2.  Note Structures

       Figure A.6.3 shows the 2-octet note log structure.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|   NOTENUM   |Y|  VELOCITY   |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure A.6.3 -- Chapter N note log

       The 7-bit NOTENUM field codes the note number for the log.  A note
       number MUST NOT be represented by multiple note logs in the note
       list.

       The 7-bit VELOCITY field codes the velocity value for the most recent
       N-active NoteOn command for the note number in the session history.
       Multiple overlapping NoteOns for a given note number may be coded
       using Chapter E, as discussed in Appendix A.7.

       VELOCITY is never zero; NoteOn commands with zero velocity are coded
       as NoteOff commands in the NoteOff bitfield structure.

       The note log does not code the execution time of the NoteOn command.
       However, the Y bit codes a hint from the sender about the NoteOn
       execution time.  The Y bit codes a recommendation to play (Y = 1) or
       skip (Y = 0) the NoteOn command recovered from the note log.  See
       Section 4.2 of [RFC4696] for non-normative guidance on the use of the
       Y bit.

       Figure A.6.1 shows the NoteOff bitfield structure, as the list of
       OFFBITS octets at the end of the chapter.  A NoteOff OFFBITS octet
       codes NoteOff information for eight consecutive MIDI note numbers,
       with the most-significant bit representing the lowest note number.
       The most-significant bit of the first OFFBITS octet codes the note
       number 8*LOW; the most-significant bit of the last OFFBITS octet
       codes the note number 8*HIGH.

       A set bit codes a NoteOff command for the note number.  In the most
       efficient coding for the NoteOff bitfield structure, the first and
       last octets of the structure contain at least one set bit.  Note that
       Chapter N does not code NoteOff velocity data.

       Note that in the general case, the recovery journal does not code the
       relative placement of a NoteOff command and a Change Control command
       for controller 64 (Damper Pedal (Sustain)).  In many cases, a
       receiver processing a loss event may deduce this relative placement

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       from the history of the stream and thus determine if a NoteOff note
       is sustained by the pedal.  If such a determination is not possible,
       receivers SHOULD err on the side of silencing pedal sustains, as
       erroneously sustained notes may produce unpleasant (albeit transient)
       artifacts.

    A.7.  Chapter E: MIDI Note Command Extras

       Readers may wish to review the Appendix A.1 definition of "N-active
       commands" before reading this appendix.  In this appendix, a NoteOn
       command with a velocity of 0 is considered to be a NoteOff command
       with a release velocity value of 64.

       Chapter E encodes recovery information about MIDI NoteOn (0x9) and
       NoteOff (0x8) command features that rarely appear in MIDI streams.
       Receivers use Chapter E to reduce transient artifacts for streams
       where several NoteOn commands appear for a note number without an
       intervening NoteOff.  Receivers also use Chapter E to reduce
       transient artifacts for streams that use NoteOff release velocity.
       Chapter E supplements the note information coded in Chapter N
       (Appendix A.6).

       Figure A.7.1 shows the format for Chapter E.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|     LEN     |S|   NOTENUM   |V|  COUNT/VEL  |S|  NOTENUM    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |V|  COUNT/VEL  |  ....                                         |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure A.7.1 -- Chapter E format

       The chapter consists of a 1-octet header, followed by a variable-
       length list of 2-octet note logs.  Appendix A.7.1 defines the
       bitfield format for a note log.

       The log list MUST contain at least one note log.  The 7-bit LEN
       header field codes the number of note logs in the list, minus one.  A
       channel journal MUST contain Chapter E if the rules defined in this
       appendix require that one or more note logs appear in the list.  The
       note log list MUST obey the oldest-first ordering rule (defined in
       Appendix A.1).

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    A.7.1.  Note Log Format

       Figure A.7.2 reproduces the note log structure of Chapter E.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|   NOTENUM   |V|  COUNT/VEL  |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.7.2 -- Chapter E note log

       A note log codes information about the MIDI note number coded by the
       7-bit NOTENUM field.  The nature of the information depends on the
       value of the V flag bit.

       If the V bit is set to 1, the COUNT/VEL field codes the release
       velocity value for the most recent N-active NoteOff command for the
       note number that appears in the session history.

       If the V bit is set to 0, the COUNT/VEL field codes a reference count
       of the number of NoteOn and NoteOff commands for the note number that
       appear in the session history.

       The reference count is set to 0 at the start of the session.  NoteOn
       commands increment the count by 1.  NoteOff commands decrement the
       count by 1.  However, a decrement that generates a negative count
       value is not performed.

       If the reference count is in the range 0-126, the 7-bit COUNT/VEL
       field codes an unsigned integer representation of the count.  If the
       count is greater than or equal to 127, COUNT/VEL is set to 127.

       By default, the count is reset to 0 whenever a Reset State command
       (Appendix A.1) appears in the session history, and whenever MIDI
       Control Change commands for controller numbers 123-127 (numbers with
       All Notes Off semantics) or 120 (All Sound Off) appear in the session
       history.

    A.7.2.  Log Inclusion Rules

       If the most recent N-active NoteOn or NoteOff command for a note
       number in the checkpoint history is a NoteOff command with a release
       velocity value other than 64, a note log whose V bit is set to 1 MUST
       appear in Chapter E for the note number.

       If the most recent N-active NoteOn or NoteOff command for a note
       number in the checkpoint history is a NoteOff command, and if the

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       reference count for the note number is greater than 0, a note log
       whose V bit is set to 0 MUST appear in Chapter E for the note number.

       If the most recent N-active NoteOn or NoteOff command for a note
       number in the checkpoint history is a NoteOn command, and if the
       reference count for the note number is greater than 1, a note log
       whose V bit is set to 0 MUST appear in Chapter E for the note number.

       At most, two note logs MAY appear in Chapter E for a note number: one
       log whose V bit is set to 0, and one log whose V bit is set to 1.

       Chapter E codes a maximum of 128 note logs.  If the log inclusion
       rules yield more than 128 REQUIRED logs, note logs whose V bit is set
       to 1 MUST be dropped from Chapter E in order to reach the 128-log
       limit.  Note logs whose V bit is set to 0 MUST NOT be dropped.

       Most MIDI streams do not use NoteOn and NoteOff commands in ways that
       would trigger the log inclusion rules.  For these streams, Chapter E
       would never be REQUIRED to appear in a channel journal.

       The ch_never parameter (Appendix C.2.3) may be used to configure the
       log inclusion rules for Chapter E.

    A.8.  Chapter T: MIDI Channel Aftertouch

       A channel journal MUST contain Chapter T if an N-active and C-active
       MIDI Channel Aftertouch (0xD) command appears in the checkpoint
       history.  Figure A.8.1 shows the format for Chapter T.

                                 0
                                 0 1 2 3 4 5 6 7
                                +-+-+-+-+-+-+-+-+
                                |S|   PRESSURE  |
                                +-+-+-+-+-+-+-+-+

                          Figure A.8.1 -- Chapter T format

       The chapter has a fixed size of 8 bits.  The 7-bit PRESSURE field
       holds the pressure value of the most recent N-active and C-active
       Channel Aftertouch command in the session history.

       Chapter T only encodes commands that are C-active and N-active.  We
       define a C-active restriction because [RP015] declares that a Control
       Change command for controller 121 (Reset All Controllers) acts to
       reset the channel pressure to 0 (see the discussion at the end of
       Appendix A.5 for a more complete rationale).

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       We define an N-active restriction on the assumption that aftertouch
       commands are linked to note activity, and thus Channel Aftertouch
       commands that are not N-active are stale and should not be used to
       repair a stream.

    A.9.  Chapter A: MIDI Poly Aftertouch

       A channel journal MUST contain Chapter A if a C-active Poly
       Aftertouch (0xA) command appears in the checkpoint history.  Figure
       A.9.1 shows the format for Chapter A.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|    LEN      |S|   NOTENUM   |X|  PRESSURE   |S|   NOTENUM   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |X|  PRESSURE   |  ....                                         |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure A.9.1 -- Chapter A format

       The chapter consists of a 1-octet header, followed by a variable-
       length list of 2-octet note logs.  A note log MUST appear for a note
       number if a C-active Poly Aftertouch command for the note number
       appears in the checkpoint history.  A note number MUST NOT be
       represented by multiple note logs in the note list.  The note log
       list MUST obey the oldest-first ordering rule (defined in Appendix
       A.1).

       The 7-bit LEN field codes the number of note logs in the list, minus
       one.  Figure A.9.2 reproduces the note log structure of Chapter A.

                           0                   1
                           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |S|   NOTENUM   |X|  PRESSURE   |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure A.9.2 -- Chapter A note log

       The 7-bit PRESSURE field codes the pressure value of the most recent
       C-active Poly Aftertouch command in the session history for the MIDI
       note number coded in the 7-bit NOTENUM field.

       As a rule, the X bit MUST be set to 0.  However, the X bit MUST be
       set to 1 if the command coded by the log appears before one of the
       following commands in the session history: MIDI Control Change

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       numbers 123-127 (numbers with All Notes Off semantics) or 120 (All
       Sound Off).

       We define C-active restrictions for Chapter A because [RP015]
       declares that a Control Change command for controller 121 (Reset All
       Controllers) acts to reset the polyphonic pressure to 0 (see the
       discussion at the end of Appendix A.5 for a more complete rationale).

    B.  The Recovery Journal System Chapters

    B.1.  System Chapter D: Simple System Commands

       The system journal MUST contain Chapter D if an active MIDI Reset
       (0xFF), MIDI Tune Request (0xF6), MIDI Song Select (0xF3), undefined
       MIDI System Common (0xF4 and 0xF5), or undefined MIDI System Real-
       time (0xF9 and 0xFD) command appears in the checkpoint history.

       Figure B.1.1 shows the variable-length format for Chapter D.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|B|G|H|J|K|Y|Z|  Command logs ...                             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure B.1.1 -- System Chapter D format

       The chapter consists of a 1-octet header, followed by one or more
       command logs.  Header flag bits indicate the presence of command logs
       for the Reset (B = 1), Tune Request (G = 1), Song Select (H = 1),
       undefined System Common 0xF4 (J = 1), undefined System Common 0xF5 (K
       = 1), undefined System Real-time 0xF9 (Y = 1), or undefined System
       Real-time 0xFD (Z = 1) commands.

       Command logs appear in a list following the header, in the order that
       the flag bits appear in the header.

       Figure B.1.2 shows the 1-octet command log format for the Reset and
       Tune Request commands.

                                0
                                0 1 2 3 4 5 6 7
                               +-+-+-+-+-+-+-+-+
                               |S|    COUNT    |
                               +-+-+-+-+-+-+-+-+

                 Figure B.1.2 -- Command log for Reset and Tune Request

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       Chapter D MUST contain the Reset command log if an active Reset
       command appears in the checkpoint history.  The 7-bit COUNT field
       codes the total number of Reset commands (modulo 128) present in the
       session history.

       Chapter D MUST contain the Tune Request command log if an active Tune
       Request command appears in the checkpoint history.  The 7-bit COUNT
       field codes the total number of Tune Request commands (modulo 128)
       present in the session history.

       For these commands, the COUNT field acts as a reference count.  See
       the definition of "session history reference counts" in Appendix A.1
       for more information.

       Figure B.1.3 shows the 1-octet command log format for the Song Select
       command.

                                   0
                                   0 1 2 3 4 5 6 7
                                  +-+-+-+-+-+-+-+-+
                                  |S|    VALUE    |
                                  +-+-+-+-+-+-+-+-+

                     Figure B.1.3 -- Song Select command log format

       Chapter D MUST contain the Song Select command log if an active Song
       Select command appears in the checkpoint history.  The 7-bit VALUE
       field codes the song number of the most recent active Song Select
       command in the session history.

    B.1.1.  Undefined System Commands

       In this section, we define the Chapter D command logs for the
       undefined System commands.  [MIDI] reserves the undefined System
       commands 0xF4, 0xF5, 0xF9, and 0xFD for future use.  At the time of
       this writing, any MIDI command stream that uses these commands is
       non-compliant with [MIDI].  However, future versions of [MIDI] may
       define these commands, and a few products do use these commands in a
       non-compliant manner.

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       Figure B.1.4 shows the variable-length command log format for the
       undefined System Common commands (0xF4 and 0xF5).

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|C|V|L|DSZ|      LENGTH       |    COUNT      |  VALUE ...    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  LEGAL ...                                                    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure B.1.4 -- Undefined System Common command log format

       The command log codes a single command type (0xF4 or 0xF5, not both).
       Chapter D MUST contain a command log if an active 0xF4 command
       appears in the checkpoint history and MUST contain an independent
       command log if an active 0xF5 command appears in the checkpoint
       history.

       Chapter D consists of a two-octet header followed by a variable
       number of data fields.  Header flag bits indicate the presence of the
       COUNT field (C = 1), the VALUE field (V = 1), and the LEGAL field (L
       = 1).  The 10-bit LENGTH field codes the size of the command log and
       conforms to semantics described in Appendix A.1.

       The 2-bit DSZ field codes the number of data octets in the command
       instance that appears most recently in the session history.  If DSZ =
       0-2, the command has 0-2 data octets.  If DSZ = 3, the command has 3
       or more command data octets.

       We now define the default rules for the use of the COUNT, VALUE, and
       LEGAL fields.  The session configuration tools defined in Appendix
       C.2.3 may be used to override this behavior.

       By default, if the DSZ field is set to 0, the command log MUST
       include the COUNT field.  The 8-bit COUNT field codes the total
       number of commands of the type coded by the log (0xF4 or 0xF5)
       present in the session history, modulo 256.

       By default, if the DSZ field is set to 1-3, the command log MUST
       include the VALUE field.  The variable-length VALUE field codes a
       verbatim copy the data octets for the most recent use of the command
       type coded by the log (0xF4 or 0xF5) in the session history.  The
       most-significant bit of the final data octet MUST be set to 1, and
       the most-significant bit of all other data octets MUST be set to 0.

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       The LEGAL field is reserved for future use.  If an update to [MIDI]
       defines the 0xF4 or 0xF5 command, an IETF standards-track document
       may define the LEGAL field.  Until such a document appears, senders
       MUST NOT use the LEGAL field, and receivers MUST use the LENGTH field
       to skip over the LEGAL field.  The LEGAL field would be defined by
       the IETF if the semantics of the new 0xF4 or 0xF5 command could not
       be protected from packet loss via the use of the COUNT and VALUE
       fields.

       Figure B.1.5 shows the variable-length command log format for the
       undefined System Real-time commands (0xF9 and 0xFD).

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|C|L| LENGTH  |     COUNT     |  LEGAL ...                    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure B.1.5 -- Undefined System Real-time command log format

       The command log codes a single command type (0xF9 or 0xFD, not both).
       Chapter D MUST contain a command log if an active 0xF9 command
       appears in the checkpoint history and MUST contain an independent
       command log if an active 0xFD command appears in the checkpoint
       history.

       Chapter D consists of a one-octet header followed by a variable
       number of data fields.  Header flag bits indicate the presence of the
       COUNT field (C = 1) and the LEGAL field (L = 1).  The 5-bit LENGTH
       field codes the size of the command log and conforms to semantics
       described in Appendix A.1.

       We now define the default rules for the use of the COUNT and LEGAL
       fields.  The session configuration tools defined in Appendix C.2.3
       may be used to override this behavior.

       The 8-bit COUNT field codes the total number of commands of the type
       coded by the log present in the session history, modulo 256.  By
       default, the COUNT field MUST be present in the command log.

       The LEGAL field is reserved for future use.  If an update to [MIDI]
       defines the 0xF9 or 0xFD command, an IETF standards-track document
       may define the LEGAL field to protect the command.  Until such a
       document appears, senders MUST NOT use the LEGAL field, and receivers
       MUST use the LENGTH field to skip over the LEGAL field.  The LEGAL
       field would be defined by the IETF if the semantics of the new 0xF9
       or 0xFD command could not be protected from packet loss via the use
       of the COUNT field.

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       Finally, we note that some non-standard uses of the undefined System
       Real-time commands act to implement non-compliant variants of the
       MIDI sequencer system.  In Appendix B.3.1, we describe resiliency
       tools for the MIDI sequencer system that provide some protection in
       this case.

    B.2.  System Chapter V: Active Sense Command

       The system journal MUST contain Chapter V if an active MIDI Active
       Sense (0xFE) command appears in the checkpoint history.  Figure B.2.1
       shows the format for Chapter V.

                                   0
                                   0 1 2 3 4 5 6 7
                                  +-+-+-+-+-+-+-+-+
                                  |S|    COUNT    |
                                  +-+-+-+-+-+-+-+-+

                         Figure B.2.1 -- System Chapter V format

       The 7-bit COUNT field codes the total number of Active Sense commands
       (modulo 128) present in the session history.  The COUNT field acts as
       a reference count.  See the definition of "session history reference
       counts" in Appendix A.1 for more information.

    B.3.  System Chapter Q: Sequencer State Commands

       This appendix describes Chapter Q, the system chapter for the MIDI
       sequencer commands.

       The system journal MUST contain Chapter Q if an active MIDI Song
       Position Pointer (0xF2), MIDI Clock (0xF8), MIDI Start (0xFA), MIDI
       Continue (0xFB), or MIDI Stop (0xFC) command appears in the
       checkpoint history, and if the rules defined in this appendix require
       a change in the Chapter Q bitfield contents because of the command
       appearance.

       Figure B.3.1 shows the variable-length format for Chapter Q.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|N|D|C|T| TOP |            CLOCK              | TIMETOOLS ... |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |              ...              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure B.3.1 -- System Chapter Q format

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       Chapter Q consists of a 1-octet header followed by several optional
       fields, in the order shown in Figure B.3.1.

       Header flag bits signal the presence of the 16-bit CLOCK field (C =
       1) and the 24-bit TIMETOOLS field (T = 1).  The 3-bit TOP header
       field is interpreted as an unsigned integer, as are CLOCK and
       TIMETOOLS.  We describe the TIMETOOLS field in Appendix B.3.1.

       Chapter Q encodes the most recent state of the sequencer system.
       Receivers use the chapter to re-synchronize the sequencer after a
       packet loss episode.  Chapter fields encode the on/off state of the
       sequencer, the current position in the song, and the downbeat.

       The N header bit encodes the relative occurrence of the Start, Stop,
       and Continue commands in the session history.  If an active Start or
       Continue command appears most recently, the N bit MUST be set to 1.
       If an active Stop appears most recently, or if no active Start, Stop,
       or Continue commands appear in the session history, the N bit MUST be
       set to 0.

       The C header flag, the TOP header field, and the CLOCK field act to
       code the current position in the sequence:

         o  If C = 1, the 3-bit TOP header field and the 16-bit CLOCK field
            are combined to form the 19-bit unsigned quantity 65536*TOP +
            CLOCK.  This value encodes the song position in units of MIDI
            Clocks (24 clocks per quarter note), modulo 524288.  Note that
            the maximum song position value that may be coded by the Song
            Position Pointer command is 98303 clocks (which may be coded
            with 17 bits), and that MIDI-coded songs are generally
            constructed to avoid durations longer than this value.  However,
            the 19-bit size may be useful for real-time applications, such
            as a drum machine MIDI output that is sending clock commands for
            long periods of time.

         o  If C = 0, the song position is the start of the song.  The C = 0
            position is identical to the position coded by C = 1, TOP = 0,
            and CLOCK = 0, for the case where the song position is less than
            524288 MIDI clocks.  In certain situations (defined later in
            this section), normative text may require the C = 0 or the C =
            1, TOP = 0, CLOCK = 0 encoding of the start of the song.

       The C, TOP, and CLOCK fields MUST be set to code the current song
       position, for both N = 0 and N = 1 conditions.  If C = 0, the TOP
       field MUST be set to 0.  See [MIDI] for a precise definition of a
       song position.

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       The D header bit encodes information about the downbeat and acts to
       qualify the song position coded by the C, TOP, and CLOCK fields.

       If the D bit is set to 1, the song position represents the most
       recent position in the sequence that has played.  If D = 1, the next
       Clock command (if N = 1) or the next (Continue, Clock) pair (if
       N = 0) acts to increment the song position by one clock, and to play
       the updated position.

       If the D bit is set to 0, the song position represents a position in
       the sequence that has not yet been played.  If D = 0, the next Clock
       command (if N = 1) or the next (Continue, Clock) pair (if N = 0) acts
       to play the point in the song coded by the song position.  The song
       position is not incremented.

       An example of a stream that uses D = 0 coding is one whose most
       recent sequence command is a Start or Song Position Pointer command
       (both N = 1 conditions).  However, it is also possible to construct
       examples where D = 0 and N = 0.  A Start command immediately followed
       by a Stop command is coded in Chapter Q by setting C = 0, D = 0,
       N = 0, TOP = 0.

       If N = 1 (coding Start or Continue), D = 0 (coding that the downbeat
       has yet to be played), and the song position is at the start of the
       song, the C = 0 song position encoding MUST be used if a Start
       command occurs more recently than a Continue command in the session
       history, and the C = 1, TOP = 0, CLOCK = 0 song position encoding
       MUST be used if a Continue command occurs more recently than a Start
       command in the session history.

    B.3.1.  Non-compliant Sequencers

       The Chapter Q description in this appendix assumes that the sequencer
       system counts off time with Clock commands, as mandated in [MIDI].
       However, a few non-compliant products do not use Clock commands to
       count off time, but instead use non-standard methods.

       Chapter Q uses the TIMETOOLS field to provide resiliency support for
       these non-standard products.  By default, the TIMETOOLS field MUST
       NOT appear in Chapter Q, and the T header bit MUST be set to 0.  The
       session configuration tools described in Appendix C.2.3 may be used
       to select TIMETOOLS coding.

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       Figure B.3.2 shows the format of the 24-bit TIMETOOLS field.

                    0                   1                   2
                    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |                   TIME                        |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure B.3.2 -- TIMETOOLS format

       The TIME field is a 24-bit unsigned integer quantity, with units of
       milliseconds.  TIME codes an additive correction term for the song
       position coded by the TOP, CLOCK, and C fields.  TIME is coded in
       network byte order (big-endian).

       A receiver computes the correct song position by converting TIME into
       units of MIDI clocks and adding it to 65536*TOP + CLOCK (assuming
       C = 1).  Alternatively, a receiver may convert 65536*TOP + CLOCK into
       milliseconds (assuming C = 1) and add it to TIME.  The downbeat (D
       header bit) semantics defined in Appendix B.3 apply to the corrected
       song position.

    B.4.  System Chapter F: MIDI Time Code Tape Position

       This appendix describes Chapter F, the system chapter for the MIDI
       Time Code (MTC) commands.  Readers may wish to review the Appendix
       A.1 definition of "finished/unfinished commands" before reading this
       appendix.

       The system journal MUST contain Chapter F if an active System Common
       Quarter Frame command (0xF1) or an active finished System Exclusive
       (Universal Real Time) MTC Full Frame command (F0 7F cc 01 01 hr mn sc
       fr F7) appears in the checkpoint history.  Otherwise, the system
       journal MUST NOT contain Chapter F.

       Figure B.4.1 shows the variable-length format for Chapter F.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|C|P|Q|D|POINT|  COMPLETE ...                                 |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     ...       |  PARTIAL  ...                                 |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     ...       |
          +-+-+-+-+-+-+-+-+

                        Figure B.4.1 -- System Chapter F format

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       Chapter F holds information about recent MTC tape positions coded in
       the session history.  Receivers use Chapter F to re-synchronize the
       MTC system after a packet loss episode.

       Chapter F consists of a 1-octet header followed by several optional
       fields, in the order shown in Figure B.4.1.  The C and P header bits
       form a Table of Contents (TOC) and signal the presence of the 32-bit
       COMPLETE field (C = 1) and the 32-bit PARTIAL field (P = 1).

       The Q header bit codes information about the COMPLETE field format.
       If Chapter F does not contain a COMPLETE field, Q MUST be set to 0.

       The D header bit codes the tape movement direction.  If the tape is
       moving forward, or if the tape direction is indeterminate, the D bit
       MUST be set to 0.  If the tape is moving in the reverse direction,
       the D bit MUST be set to 1.  In most cases, the ordering of commands
       in the session history clearly defines the tape direction.  However,
       a few command sequences have an indeterminate direction (such as a
       session history consisting of one Full Frame command).

       The 3-bit POINT header field is interpreted as an unsigned integer.
       Appendix B.4.1 defines how the POINT field codes information about
       the contents of the PARTIAL field.  If Chapter F does not contain a
       PARTIAL field, POINT MUST be set to 7 (if D = 0) or 0 (if D = 1).

       Chapter F MUST include the COMPLETE field if an active finished Full
       Frame command appears in the checkpoint history, or if an active
       Quarter Frame command that completes the encoding of a frame value
       appears in the checkpoint history.

       The COMPLETE field encodes the most recent active complete MTC frame
       value that appears in the session history.  This frame value may take
       the form of a series of 8 active Quarter Frame commands (0xF1 0x0n
       through 0xF1 0x7n for forward tape movement, 0xF1 0x7n through 0xF1
       0x0n for reverse tape movement) or may take the form of an active
       finished Full Frame command.

       If the COMPLETE field encodes a Quarter Frame command series, the Q
       header bit MUST be set to 1, and the COMPLETE field MUST have the
       format shown in Figure B.4.2.  The 4-bit fields MT0 through MT7 code
       the data (lower) nibble for the Quarter Frame commands for Message
       Type 0 through Message Type 7 [MIDI].  These nibbles encode a
       complete frame value, in addition to fields reserved for future use
       by [MIDI].

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           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  MT0  |  MT1  |  MT2  |  MT3  |  MT4  |  MT5  |  MT6  |  MT7  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure B.4.2 -- COMPLETE field format, Q = 1

       In this usage, the frame value encoded in the COMPLETE field MUST be
       offset by 2 frames (relative to the frame value encoded in the
       Quarter Frame commands) if the frame value codes a 0xF1 0x0n through
       0xF1 0x7n command sequence.  This offset compensates for the two-
       frame latency of the Quarter Frame encoding for forward tape
       movement.  No offset is applied if the frame value codes a 0xF1 0x7n
       through 0xF1 0x0n Quarter Frame command sequence.

       The most recent active complete MTC frame value may alternatively be
       encoded by an active finished Full Frame command.  In this case, the
       Q header bit MUST be set to 0, and the COMPLETE field MUST have
       format shown in Figure B.4.3.  The HR, MN, SC, and FR fields
       correspond to the hr, mn, sc, and fr data octets of the Full Frame
       command.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |      HR       |      MN       |      SC       |      FR       |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure B.4.3 -- COMPLETE field format, Q = 0

    B.4.1.  Partial Frames

       The most recent active session history command that encodes MTC frame
       value data may be a Quarter Frame command other than a forward-moving
       0xF1 0x7n command (which completes a frame value for forward tape
       movement) or a reverse-moving 0xF1 0x1n command (which completes a
       frame value for reverse tape movement).

       We consider this type of Quarter Frame command to be associated with
       a partial frame value.  The Quarter Frame sequence that defines a
       partial frame value MUST either start at Message Type 0 and increment
       contiguously to an intermediate Message Type less than 7, or start at
       Message Type 7 and decrement contiguously to an intermediate Message
       type greater than 0.  A Quarter Frame command sequence that does not
       follow this pattern is not associated with a partial frame value.

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       Chapter F MUST include a PARTIAL field if the most recent active
       command in the checkpoint history that encodes MTC frame value data
       is a Quarter Frame command that is associated with a partial frame
       value.  Otherwise, Chapter F MUST NOT include a PARTIAL field.

       The partial frame value consists of the data (lower) nibbles of the
       Quarter Frame command sequence.  The PARTIAL field codes the partial
       frame value, using the format shown in Figure B.4.2.  Message Type
       fields that are not associated with a Quarter Frame command MUST be
       set to 0.

       The POINT header field indicates the Message Type fields in the
       PARTIAL field code valid data.  If P = 1, the POINT field MUST encode
       the unsigned integer value formed by the lower 3 bits of the upper
       nibble of the data value of the most recent active Quarter Frame
       command in the session history.  If D = 0 and P = 1, POINT MUST take
       on a value in the range 0-6.  If D = 1 and P = 1, POINT MUST take on
       a value in the range 1-7.

       If D = 0, MT fields (Figure B.4.2) in the inclusive range from 0 up
       to and including the POINT value encode the partial frame value.  If
       D = 1, MT fields in the inclusive range from 7 down to and including
       the POINT value encode the partial frame value.  Note that, unlike
       the COMPLETE field encoding, senders MUST NOT add a 2-frame offset to
       the partial frame value encoded in PARTIAL.

       For the default semantics, if a recovery journal contains Chapter F,
       and if the session history codes a legal [MIDI] series of Quarter
       Frame and Full Frame commands, the chapter always contains a COMPLETE
       or a PARTIAL field (and may contain both fields).  Thus, a one-octet
       Chapter F (C = P = 0) always codes the presence of an illegal command
       sequence in the session history (under some conditions, the C = 1,
       P = 0 condition may also code the presence of an illegal command
       sequence).  The illegal command sequence conditions are transient in
       nature and usually indicate that a Quarter Frame command sequence
       began with an intermediate Message Type.

    B.5.  System Chapter X: System Exclusive

       This appendix describes Chapter X, the system chapter for MIDI System
       Exclusive (SysEx) commands (0xF0).  Readers may wish to review the
       Appendix A.1 definition of "finished/unfinished commands" before
       reading this appendix.

       Chapter X consists of a list of one or more command logs.  Each log
       in the list codes information about a specific finished or unfinished
       SysEx command that appears in the session history.  The system

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       journal MUST contain Chapter X if the rules defined in Appendix B.5.2
       require that one or more logs appear in the list.

       The log list is not preceded by a header.  Instead, each log
       implicitly encodes its own length.  Given the length of the N'th list
       log, the presence of the (N+1)'th list log may be inferred from the
       LENGTH field of the system journal header (Figure 10 in Section 5 of
       the main text).  The log list MUST obey the oldest-first ordering
       rule (defined in Appendix A.1).

    B.5.1.  Chapter Format

       Figure B.5.1 shows the bitfield format for the Chapter X command log.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |S|T|C|F|D|L|STA|    TCOUNT     |     COUNT     |  FIRST ...    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  DATA ...                                                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure B.5.1 -- Chapter X command log format

       A Chapter X command log consists of a 1-octet header, followed by the
       optional TCOUNT, COUNT, FIRST, and DATA fields.

       The T, C, F, and D header bits act as a Table of Contents (TOC) for
       the log.  If T is set to 1, the 1-octet TCOUNT field appears in the
       log.  If C is set to 1, the 1-octet COUNT field appears in the log.
       If F is set to 1, the variable-length FIRST field appears in the log.
       If D is set to 1, the variable-length DATA field appears in the log.

       The L header bit sets the coding tool for the log.  We define the log
       coding tools in Appendix B.5.2.

       The STA field codes the status of the command coded by the log.  The
       2-bit STA value is interpreted as an unsigned integer.  If STA is 0,
       the log codes an unfinished command.  Non-zero STA values code
       different classes of finished commands.  An STA value of 1 codes a
       cancelled command, an STA value of 2 codes a command that uses the
       "dropped F7" construction, and an STA value of 3 codes all other
       finished commands.  Section 3.2 in the main text describes cancelled
       and "dropped F7" commands.

       The S bit (Appendix A.1) of the first log in the list acts as the S
       bit for Chapter X.  For the other logs in the list, the S bit refers

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       to the log itself.  The value of the "phantom" S bit associated with
       the first log is defined by the following rules:

         o  If the list codes one log, the phantom S-bit value is the same
            as the Chapter X S-bit value.

         o  If the list codes multiple logs, the phantom S-bit value is the
            logical OR of the S-bit value of the first and second command
            logs in the list.

       In all other respects, the S bit follows the semantics defined in
       Appendix A.1.

       The FIRST field (present if F = 1) encodes a variable-length unsigned
       integer value that sets the coverage of the DATA field.

       The FIRST field (present if F = 1) encodes a variable-length unsigned
       integer value that specifies which SysEx data bytes are encoded in
       the DATA field of the log.  The FIRST field consists of an octet
       whose most-significant bit is set to 0, optionally preceded by one or
       more octets whose most-significant bit is set to 1.  The algorithm
       shown in Figure B.5.2 decodes this format into an unsigned integer,
       to yield the value dec(FIRST).  FIRST uses a variable-length encoding
       because dec(FIRST) references a data octet in a SysEx command, and a
       SysEx command may contain an arbitrary number of data octets.

            One-Octet FIRST value:

               Encoded form: 0ddddddd
               Decoded form: 00000000 00000000 00000000 0ddddddd

            Two-Octet FIRST value:

               Encoded form: 1ccccccc 0ddddddd
               Decoded form: 00000000 00000000 00cccccc cddddddd

            Three-Octet FIRST value:

               Encoded form: 1bbbbbbb 1ccccccc 0ddddddd
               Decoded form: 00000000 000bbbbb bbcccccc cddddddd

            Four-Octet FIRST value:

               Encoded form: 1aaaaaaa 1bbbbbbb 1ccccccc 0ddddddd
               Decoded form: 0000aaaa aaabbbbb bbcccccc cddddddd

                    Figure B.5.2 -- Decoding FIRST field formats

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       The DATA field (present if D = 1) encodes a modified version of the
       data octets of the SysEx command coded by the log.  Status octets
       MUST NOT be coded in the DATA field.

       If F = 0, the DATA field begins with the first data octet of the
       SysEx command and includes all subsequent data octets for the command
       that appear in the session history.  If F = 1, the DATA field begins
       with the (dec(FIRST) + 1)'th data octet of the SysEx command and
       includes all subsequent data octets for the command that appear in
       the session history.  Note that the word "command" in the
       descriptions above refers to the original SysEx command as it appears
       in the source MIDI data stream, not to a particular MIDI list SysEx
       command segment.

       The length of the DATA field is coded implicitly, using the most-
       significant bit of each octet.  The most-significant bit of the final
       octet of the DATA field MUST be set to 1.  The most-significant bit
       of all other DATA octets MUST be set to 0.  This coding method relies
       on the fact that the most-significant bit of a MIDI data octet is 0
       by definition.  Apart from this length-coding modification, the DATA
       field encodes a verbatim copy of all data octets it encodes.

    B.5.2.  Log Inclusion Semantics

       Chapter X offers two tools to protect SysEx commands: the "recency"
       tool and the "list" tool.  The tool definitions use the concept of
       the "SysEx type" of a command, which we now define.

       Each SysEx command instance in a session, excepting MTC Full Frame
       commands, is said to have a "SysEx type".  Types are used in equality
       comparisons: two SysEx commands in a session are said to have "the
       same SysEx type" or "different SysEx types".

       If efficiency is not a concern, a sender may follow a simple typing
       rule: every SysEx command in the session history has a different
       SysEx type, and thus no two commands in the session have the same
       type.

       To improve efficiency, senders MAY implement exceptions to this rule.
       These exceptions declare that certain sets of SysEx command instances
       have the same SysEx type.  Any command not covered by an exception
       follows the simple rule.  We list exceptions below:

         o  All commands with identical data octet fields (same number of
            data octets, same value for each data octet) have the same type.
            This rule MUST be applied to all SysEx commands in the session,
            or not at all.  Note that the implementation of this exception
            requires no sender knowledge of the format and semantics of the

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            SysEx commands in the stream, merely the ability to count and
            compare octets.

         o  Two instances of the same command whose semantics set or report
            the value of the same "parameter" have the same type.  The
            implementation of this exception requires specific knowledge of
            the format and semantics of SysEx commands.  In practice, a
            sender implementation chooses to support this exception for
            certain classes of commands (such as the Universal System
            Exclusive commands defined in [MIDI]).  If a sender supports
            this exception for a particular command in a class (for example,
            the Universal Real Time System Exclusive message for Master
            Volume, F0 F7 cc 04 01 vv vv F7, defined in [MIDI]), it MUST
            support the exception to all instances of this particular
            command in the session.

       We now use this definition of "SysEx type" to define the "recency"
       tool and the "list" tool for Chapter X.

       By default, the Chapter X log list MUST code sufficient information
       to protect the rendered MIDI performance from indefinite artifacts
       caused by the loss of all finished or unfinished active SysEx
       commands that appear in the checkpoint history (excluding finished
       MTC Full Frame commands, which are coded in Chapter F (Appendix
       B.4)).

       To protect a command of a specific SysEx type with the recency tool,
       senders MUST code a log in the log list for the most recent finished
       active instance of the SysEx type that appears in the checkpoint
       history.  Additionally, if an unfinished active instance of the SysEx
       type appears in the checkpoint history, senders MUST code a log in
       the log list for the unfinished command instance.  The L header bit
       of both command logs MUST be set to 0.

       To protect a command of a specific SysEx type with the list tool,
       senders MUST code a log in the Chapter X log list for each finished
       or unfinished active instance of the SysEx type that appears in the
       checkpoint history.  The L header bit of list tool command logs MUST
       be set to 1.

       As a rule, a log REQUIRED by the list or recency tool MUST include a
       DATA field that codes all data octets that appear in the checkpoint
       history for the SysEx command instance associated with the log.  The
       FIRST field MAY be used to configure a DATA field that minimally
       meets this requirement.

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       An exception to this rule applies to cancelled commands (defined in
       Section 3.2).  REQUIRED command logs associated with cancelled
       commands MAY be coded with no DATA field.  However, if DATA appears
       in the log, DATA MUST code all data octets that appear in the
       checkpoint history for the command associated with the log.

       As defined by the preceding text in this section, by default all
       finished or unfinished active SysEx commands that appear in the
       checkpoint history (excluding finished MTC Full Frame commands) MUST
       be protected by the list tool or the recency tool.

       For some MIDI source streams, this default yields a Chapter X whose
       size is too large.  For example, imagine that a sender begins to
       transcode a SysEx command with 10,000 data octets onto a UDP RTP
       stream "on the fly", by sending SysEx command segments as soon as
       data octets are delivered by the MIDI source.  After 1000 octets have
       been sent, the expansion of Chapter X yields an RTP packet that is
       too large to fit in the Maximum Transmission Unit (MTU) for the
       stream.

       In this situation, if a sender uses the closed-loop sending policy
       for SysEx commands, the RTP packet size may always be capped by
       stalling the stream.  In a stream stall, once the packet reaches a
       maximum size, the sender refrains from sending new packets with non-
       empty MIDI Command Sections until receiver feedback permits the
       trimming of Chapter X.  If the stream permits arbitrary commands to
       appear between SysEx segments (selectable during configuration using
       the tools defined in Appendix C.1), the sender may stall the SysEx
       segment stream but continue to code other commands in the MIDI list.

       Stalls are a workable but sub-optimal solution to Chapter X size
       issues.  As an alternative to stalls, senders SHOULD take preemptive
       action during session configuration to reduce the anticipated size of
       Chapter X, using the methods described below:

         o  Partitioned transport.  Appendix C.5 provides tools for sending
            a MIDI name space over several RTP streams.  Senders may use
            these tools to map a MIDI source into a low-latency UDP RTP
            stream (for channel commands and short SysEx commands) and a
            reliable [RFC4571] TCP stream (for bulk-data SysEx commands).
            The cm_unused and cm_used parameters (Appendix C.1) may be used
            to communicate the nature of the SysEx command partition.  As
            TCP is reliable, the RTP MIDI TCP stream would not use the
            recovery journal.  To minimize transmission latency for short
            SysEx commands, senders may begin segmental transmission for all
            SysEx commands over the UDP stream and then cancel the UDP
            transmission of long commands (using tools described in Section
            3.2) and resend the commands over the TCP stream.

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         o  Selective protection.  Journal protection may not be necessary
            for all SysEx commands in a stream.  The ch_never parameter
            (Appendix C.2) may be used to communicate which SysEx commands
            are excluded from Chapter X.

    B.5.3.  TCOUNT and COUNT Fields

       If the T header bit is set to 1, the 8-bit TCOUNT field appears in
       the command log.  If the C header bit is set to 1, the 8-bit COUNT
       field appears in the command log.  TCOUNT and COUNT are interpreted
       as unsigned integers.

       The TCOUNT field codes the total number of SysEx commands of the
       SysEx type coded by the log that appear in the session history, at
       the moment after the (finished or unfinished) command coded by the
       log enters the session history.

       The COUNT field codes the total number of SysEx commands that appear
       in the session history, excluding commands that are excluded from
       Chapter X via the ch_never parameter (Appendix C.2), at the moment
       after the (finished or unfinished) command coded by the log enters
       the session history.

       Command counting for TCOUNT and COUNT uses modulo-256 arithmetic.
       MTC Full Frame command instances (Appendix B.4) are included in
       command counting if the TCOUNT and COUNT definitions warrant their
       inclusion, as are cancelled commands (Section 3.2).

       Senders use the TCOUNT and COUNT fields to track the identity and
       (for TCOUNT) the sequence position of a command instance.  Senders
       MUST use the TCOUNT or COUNT fields if identity or sequence
       information is necessary to protect the command type coded by the
       log.

       If a sender uses the COUNT field in a session, the final command log
       in every Chapter X in the stream MUST code the COUNT field.  This
       rule lets receivers resynchronize the COUNT value after a packet
       loss.

    C.  Session Configuration Tools

       In Sections 6.1-2 of the main text, we show session descriptions for
       minimal native and mpeg4-generic RTP MIDI streams.  Minimal streams
       lack the flexibility to support some applications.  In this appendix,
       we describe how to customize stream behavior through the use of the
       payload format parameters.

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       The appendix begins with 6 sections, each devoted to parameters that
       affect a particular aspect of stream behavior:

         o  Appendix C.1 describes the stream subsetting system (cm_unused
            and cm_used).

         o  Appendix C.2 describes the journalling system (ch_anchor,
            ch_default, ch_never, j_sec, j_update).

         o  Appendix C.3 describes MIDI command timestamp semantics
            (linerate, mperiod, octpos, tsmode).

         o  Appendix C.4 describes the temporal duration ("media time") of
            an RTP MIDI packet (guardtime, rtp_maxptime, rtp_ptime).

         o  Appendix C.5 concerns stream description (musicport).

         o  Appendix C.6 describes MIDI rendering (chanmask, cid, inline,
            multimode, render, rinit, subrender, smf_cid, smf_info,
            smf_inline, smf_url, url).

       The parameters listed above may optionally appear in session
       descriptions of RTP MIDI streams.  If these parameters are used in an
       SDP session description, the parameters appear on an fmtp attribute
       line.  This attribute line applies to the payload type associated
       with the fmtp line.

       The parameters listed above add extra functionality ("features") to
       minimal RTP MIDI streams.  In Appendix C.7, we show how to use these
       features to support two classes of applications: content-streaming
       using RTSP (Appendix C.7.1) and network musical performance using SIP
       (Appendix C.7.2).

       The participants in a multimedia session MUST share a common view of
       all of the RTP MIDI streams that appear in an RTP session, as defined
       by a single media (m=) line.  In some RTP MIDI applications, the
       "common view" restriction makes it difficult to use sendrecv streams
       (all parties send and receive), as each party has its own
       requirements.  For example, a two-party network musical performance
       application may wish to customize the renderer on each host to match
       the CPU performance of the host [NMP].

       We solve this problem by using two RTP MIDI streams -- one sendonly,
       one recvonly -- in lieu of one sendrecv stream.  The data flows in
       the two streams travel in opposite directions, to control receivers
       configured to use different renderers.  In the third example in
       Appendix C.5, we show how the musicport parameter may be used to
       define virtual sendrecv streams.

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       As a general rule, the RTP MIDI protocol does not handle parameter
       changes during a session well, because the parameters describe
       heavyweight or stateful configuration that is not easily changed once
       a session has begun.  Thus, parties SHOULD NOT expect that parameter
       change requests during a session will be accepted by other parties.
       However, implementors SHOULD support in-session parameter changes
       that are easy to handle (for example, the guardtime parameter defined
       in Appendix C.4) and SHOULD be capable of accepting requests for
       changes of those parameters, as received by its session management
       protocol (for example, re-offers in SIP [RFC3264]).

       Appendix D defines the Augmented Backus-Naur Form (ABNF, [RFC4234])
       syntax for the payload parameters.  Section 11 provides information
       to the Internet Assigned Numbers Authority (IANA) on the media types
       and parameters defined in this document.

       Appendix C.6.5 defines the media type "audio/asc", a stored object
       for initializing mpeg4-generic renderers.  As described in Appendix
       C.6, the audio/asc media type is assigned to the "rinit" parameter to
       specify an initialization data object for the default mpeg4-generic
       renderer.  Note that RTP stream semantics are not defined for
       "audio/asc".  Therefore, the "asc" subtype MUST NOT appear on the
       rtpmap line of a session description.

    C.1.  Configuration Tools: Stream Subsetting

       As defined in Section 3.2 in the main text, the MIDI list of an RTP
       MIDI packet may encode any MIDI command that may legally appear on a
       MIDI 1.0 DIN cable.

       In this appendix, we define two parameters (cm_unused and cm_used)
       that modify this default condition, by excluding certain types of
       MIDI commands from the MIDI list of all packets in a stream.  For
       example, if a multimedia session partitions a MIDI name space into
       two RTP MIDI streams, the parameters may be used to define which
       commands appear in each stream.

       In this appendix, we define a simple language for specifying MIDI
       command types.  If a command type is assigned to cm_unused, the
       commands coded by the string MUST NOT appear in the MIDI list.  If a
       command type is assigned to cm_used, the commands coded by the string
       MAY appear in the MIDI list.

       The parameter list may code multiple assignments to cm_used and
       cm_unused.  Assignments have a cumulative effect and are applied in
       the order of appearance in the parameter list.  A later assignment of
       a command type to the same parameter expands the scope of the earlier
       assignment.  A later assignment of a command type to the opposite

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       parameter cancels (partially or completely) the effect of an earlier
       assignment.

       To initialize the stream subsetting system, "implicit" assignments to
       cm_unused and cm_used are processed before processing the actual
       assignments that appear in the parameter list.  The System Common
       undefined commands (0xF4, 0xF5) and the System Real-Time Undefined
       commands (0xF9, 0xFD) are implicitly assigned to cm_unused.  All
       other command types are implicitly assigned to cm_used.

       Note that the implicit assignments code the default behavior of an
       RTP MIDI stream as defined in Section 3.2 in the main text (namely,
       that all commands that may legally appear on a MIDI 1.0 DIN cable may
       appear in the stream).  Also note that assignments of the System
       Common undefined commands (0xF4, 0xF5) apply to the use of these
       commands in the MIDI source command stream, not the special use of
       0xF4 and 0xF5 in SysEx segment encoding defined in Section 3.2 in the
       main text.

       As a rule, parameter assignments obey the following syntax (see
       Appendix D for ABNF):

         <parameter> = [channel list]<command-type list>[field list]

       The command-type list is mandatory; the channel and field lists are
       optional.

       The command-type list specifies the MIDI command types for which the
       parameter applies.  The command-type list is a concatenated sequence
       of one or more of the letters (ABCFGHJKMNPQTVWXYZ).  The letters code
       the following command types:

          o  A: Poly Aftertouch (0xA)
          o  B: System Reset (0xFF)
          o  C: Control Change (0xB)
          o  F: System Time Code (0xF1)
          o  G: System Tune Request (0xF6)
          o  H: System Song Select (0xF3)
          o  J: System Common Undefined (0xF4)
          o  K: System Common Undefined (0xF5)
          o  N: NoteOff (0x8), NoteOn (0x9)
          o  P: Program Change (0xC)
          o  Q: System Sequencer (0xF2, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC)
          o  T: Channel Aftertouch (0xD)
          o  V: System Active Sense (0xFE)
          o  W: Pitch Wheel (0xE)

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          o  X: SysEx (0xF0)
          o  Y: System Real-Time Undefined (0xF9)
          o  Z: System Real-Time Undefined (0xFD)

       In addition to the letters above, the letter M may also appear in the
       command-type list.  The letter M refers to the MIDI parameter system
       (see definition in Appendix A.1 and in [MIDI]).  An assignment of M
       to cm_unused codes that no RPN or NRPN transactions may appear in the
       MIDI list.

       Note that if cm_unused is assigned the letter M, Control Change (0xB)
       commands for the controller numbers in the standard controller
       assignment might still appear in the MIDI list.  For an explanation,
       see Appendix A.3.4 for a discussion of the "general-purpose" use of
       parameter system controller numbers.

       In the text below, rules that apply to "MIDI voice channel commands"
       also apply to the letter M.

       The letters in the command-type list MUST be uppercase and MUST
       appear in alphabetical order.  Letters other than
       (ABCFGHJKMNPQTVWXYZ) that appear in the list MUST be ignored.

       For MIDI voice channel commands, the channel list specifies the MIDI
       channels for which the parameter applies.  If no channel list is
       provided, the parameter applies to all MIDI channels (0-15).  The
       channel list takes the form of a list of channel numbers (0 through
       15) and dash-separated channel number ranges (i.e., 0-5, 8-12, etc).
       Dots (i.e., "." characters) separate elements in the channel list.

       Recall that System commands do not have a MIDI channel associated
       with them.  Thus, for most command-type letters that code System
       commands (B, F, G, H, J, K, Q, V, Y, and Z), the channel list is
       ignored.

       For the command-type letter X, the appearance of certain numbers in
       the channel list codes special semantics.

         o  The digit 0 codes that SysEx "cancel" sublists (Section 3.2 in
            the main text) MUST NOT appear in the MIDI list.

         o  The digit 1 codes that cancel sublists MAY appear in the MIDI
            list (the default condition).

         o  The digit 2 codes that commands other than System Real-time MIDI
            commands MUST NOT appear between SysEx command segments in the
            MIDI list (the default condition).

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         o  The digit 3 codes that any MIDI command type may appear between
            SysEx command segments in the MIDI list, with the exception of
            the segmented encoding of a second SysEx command (verbatim SysEx
            commands are OK).

       For command-type X, the channel list MUST NOT contain both digits 0
       and 1, and it MUST NOT contain both digits 2 and 3.  For command-type
       X, channel list numbers other than the numbers defined above are
       ignored.  If X does not have a channel list, the semantics marked
       "the default condition" in the list above apply.

       The syntax for field lists in a parameter assignment follows the
       syntax for channel lists.  If no field list is provided, the
       parameter applies to all controller or note numbers.

       For command-type C (Control Change), the field list codes the
       controller numbers (0-255) for which the parameter applies.

       For command-type M (Parameter System), the field list codes the
       Registered Parameter Numbers (RPNs) and Non-Registered Parameter
       Numbers (NRPNs) for which the parameter applies.  The number range
       0-16383 specifies RPNs, the number range 16384-32767 specifies NRPNs
       (16384 corresponds to NRPN 0, 32767 corresponds to NRPN 16383).

       For command-types N (NoteOn and NoteOff) and A (Poly Aftertouch), the
       field list codes the note numbers for which the parameter applies.

       For command-types J and K (System Common Undefined), the field list
       consists of a single digit, which specifies the number of data octets
       that follow the command octet.

       For command-type X (SysEx), the field list codes the number of data
       octets that may appear in a SysEx command.  Thus, the field list
       0-255 specifies SysEx commands with 255 or fewer data octets, the
       field list 256-4294967295 specifies SysEx commands with more than 255
       data octets but excludes commands with 255 or fewer data octets, and
       the field list 0 excludes all commands.

       A secondary parameter assignment syntax customizes command-type X
       (see Appendix D for complete ABNF):

         <parameter> = "__" <h-list> ["_" <h-list>] "__"

       The assignment defines the class of SysEx commands that obeys the
       semantics of the assigned parameter.  The command class is specified
       by listing the permitted values of the first N data octets that
       follow the SysEx 0xF0 command octet.  Any SysEx command whose first N
       data octets match the list is a member of the class.

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       Each <h-list> defines a data octet of the command, as a dot-separated
       (".") list of one or more hexadecimal constants (such as "7F") or
       dash-separated hexadecimal ranges (such as "01-1F").  Underscores
       ("_") separate each <h-list>.  Double-underscores ("__") delineate
       the data octet list.

       Using this syntax, each assignment specifies a single SysEx command
       class.  Session descriptions may use several assignments to cm_used
       and cm_unused to specify complex behaviors.

       The example session description below illustrates the use of the
       stream subsetting parameters:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 rtp-midi/44100
       a=fmtp:96 cm_unused=ACGHJKNMPTVWXYZ; cm_used=__7F_00-7F_01_01__

       The session description configures the stream for use in clock
       applications.  All voice channels are unused, as are all System
       Commands except those used for MIDI Time Code (command-type F, and
       the Full Frame SysEx command that is matched by the string assigned
       to cm_used), the System Sequencer commands (command-type Q), and
       System Reset (command-type B).

    C.2.  Configuration Tools: The Journalling System

       In this appendix, we define the payload format parameters that
       configure stream journalling and the recovery journal system.

       The j_sec parameter (Appendix C.2.1) sets the journalling method for
       the stream.  The j_update parameter (Appendix C.2.2) sets the
       recovery journal sending policy for the stream.  Appendix C.2.2 also
       defines the sending policies of the recovery journal system.

       Appendix C.2.3 defines several parameters that modify the recovery
       journal semantics.  These parameters change the default recovery
       journal semantics as defined in Section 5 and Appendices A-B.

       The journalling method for a stream is set at the start of a session
       and MUST NOT be changed thereafter.  This requirement forbids changes
       to the j_sec parameter once a session has begun.

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       A related requirement, defined in the appendix sections below,
       forbids the acceptance of parameter values that would violate the
       recovery journal mandate.  In many cases, a change in one of the
       parameters defined in this appendix during an ongoing session would
       result in a violation of the recovery journal mandate for an
       implementation; in this case, the parameter change MUST NOT be
       accepted.

    C.2.1.  The j_sec Parameter

       Section 2.2 defines the default journalling method for a stream.
       Streams that use unreliable transport (such as UDP) default to using
       the recovery journal.  Streams that use reliable transport (such as
       TCP) default to not using a journal.

       The parameter j_sec may be used to override this default.  This memo
       defines two symbolic values for j_sec: "none", to indicate that all
       stream payloads MUST NOT contain a journal section, and "recj", to
       indicate that all stream payloads MUST contain a journal section that
       uses the recovery journal format.

       For example, the j_sec parameter might be set to "none" for a UDP
       stream that travels between two hosts on a local network that is
       known to provide reliable datagram delivery.

       The session description below configures a UDP stream that does not
       use the recovery journal:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 rtp-midi/44100
       a=fmtp:96 j_sec=none

       Other IETF standards-track documents may define alternative journal
       formats.  These documents MUST define new symbolic values for the
       j_sec parameter to signal the use of the format.

       Parties MUST NOT accept a j_sec value that violates the recovery
       journal mandate (see Section 4 for details).  If a session
       description uses a j_sec value unknown to the recipient, the
       recipient MUST NOT accept the description.

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       Special j_sec issues arise when sessions are managed by session
       management tools (like RTSP, [RFC2326]) that use SDP for "declarative
       usage" purposes (see the preamble of Section 6 for details).  For
       these session management tools, SDP does not code transport details
       (such as UDP or TCP) for the session.  Instead, server and client
       negotiate transport details via other means (for RTSP, the SETUP
       method).

       In this scenario, the use of the j_sec parameter may be ill-advised,
       as the creator of the session description may not yet know the
       transport type for the session.  In this case, the session
       description SHOULD configure the journalling system using the
       parameters defined in the remainder of Appendix C.2, but it SHOULD
       NOT use j_sec to set the journalling status.  Recall that if j_sec
       does not appear in the session description, the default method for
       choosing the journalling method is in effect (no journal for reliable
       transport, recovery journal for unreliable transport).

       However, in declarative usage situations where the creator of the
       session description knows that journalling is always required or
       never required, the session description SHOULD use the j_sec
       parameter.

    C.2.2.  The j_update Parameter

       In Section 4, we use the term "sending policy" to describe the method
       a sender uses to choose the checkpoint packet identity for each
       recovery journal in a stream.  In the sub-sections that follow, we
       normatively define three sending policies: anchor, closed-loop, and
       open-loop.

       As stated in Section 4, the default sending policy for a stream is
       the closed-loop policy.  The j_update parameter may be used to
       override this default.

       We define three symbolic values for j_update: "anchor", to indicate
       that the stream uses the anchor sending policy, "open-loop", to
       indicate that the stream uses the open-loop sending policy, and
       "closed-loop", to indicate that the stream uses the closed-loop
       sending policy.  See Appendix C.2.3 for examples session descriptions
       that use the j_update parameter.

       Parties MUST NOT accept a j_update value that violates the recovery
       journal mandate (Section 4).

       Other IETF standards-track documents may define additional sending
       policies for the recovery journal system.  These documents MUST
       define new symbolic values for the j_update parameter to signal the

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       use of the new policy.  If a session description uses a j_update
       value unknown to the recipient, the recipient MUST NOT accept the
       description.

    C.2.2.1.  The anchor Sending Policy

       In the anchor policy, the sender uses the first packet in the stream
       as the checkpoint packet for all packets in the stream.  The anchor
       policy satisfies the recovery journal mandate (Section 4), as the
       checkpoint history always covers the entire stream.

       The anchor policy does not require the use of the RTP control
       protocol (RTCP, [RFC3550]) or other feedback from receiver to sender.
       Senders do not need to take special actions to ensure that received
       streams start up free of artifacts, as the recovery journal always
       covers the entire history of the stream.  Receivers are relieved of
       the responsibility of tracking the changing identity of the
       checkpoint packet, because the checkpoint packet never changes.

       The main drawback of the anchor policy is bandwidth efficiency.
       Because the checkpoint history covers the entire stream, the size of
       the recovery journals produced by this policy usually exceeds the
       journal size of alternative policies.  For single-channel MIDI data
       streams, the bandwidth overhead of the anchor policy is often
       acceptable (see Appendix A.4 of [NMP]).  For dense streams, the
       closed-loop or open-loop policies may be more appropriate.

    C.2.2.2.  The closed-loop Sending Policy

       The closed-loop policy is the default policy of the recovery journal
       system.  For each packet in the stream, the policy lets senders
       choose the smallest possible checkpoint history that satisfies the
       recovery journal mandate.  As smaller checkpoint histories generally
       yield smaller recovery journals, the closed-loop policy reduces the
       bandwidth of a stream, relative to the anchor policy.

       The closed-loop policy relies on feedback from receiver to sender.
       The policy assumes that a receiver periodically informs the sender of
       the highest sequence number it has seen so far in the stream, coded
       in the 32-bit extension format defined in [RFC3550].  For RTCP,
       receivers transmit this information in the Extended Highest Sequence
       Number Received (EHSNR) field of Receiver Reports.  RTCP Sender or
       Receiver Reports MUST be sent by any participant in a session with
       closed loop sending policy, unless another feedback mechanism has
       been agreed upon.

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       The sender may safely use receiver sequence number feedback to guide
       checkpoint history management, because Section 4 requires that
       receivers repair indefinite artifacts whenever a packet loss event
       occur.

       We now normatively define the closed-loop policy.  At the moment a
       sender prepares an RTP packet for transmission, the sender is aware
       of R >= 0 receivers for the stream.  Senders may become aware of a
       receiver via RTCP traffic from the receiver, via RTP packets from a
       paired stream sent by the receiver to the sender, via messages from a
       session management tool, or by other means.  As receivers join and
       leave a session, the value of R changes.

       Each known receiver k (1 <= k <= R) is associated with a 32-bit
       extended packet sequence number M(k), where the extension reflects
       the sequence number rollover count of the sender.

       If the sender has received at least one feedback report from receiver
       k, M(k) is the most recent report of the highest RTP packet sequence
       number seen by the receiver, normalized to reflect the rollover count
       of the sender.

       If the sender has not received a feedback report from the receiver,
       M(k) is the extended sequence number of the last packet the sender
       transmitted before it became aware of the receiver.  If the sender
       became aware of this receiver before it sent the first packet in the
       stream, M(k) is the extended sequence number of the first packet in
       the stream.

       Given this definition of M(), we now state the closed-loop policy.
       When preparing a new packet for transmission, a sender MUST choose a
       checkpoint packet with extended sequence number N, such that M(k) >=
       (N - 1) for all k, 1 <= k <= R, where R >= 1.  The policy does not
       restrict sender behavior in the R == 0 (no known receivers) case.

       Under the closed-loop policy as defined above, a sender may transmit
       packets whose checkpoint history is shorter than the session history
       (as defined in Appendix A.1).  In this event, a new receiver that
       joins the stream may experience indefinite artifacts.

       For example, if a Control Change (0xB) command for Channel Volume
       (controller number 7) was sent early in a stream, and later a new
       receiver joins the session, the closed-loop policy may permit all
       packets sent to the new receiver to use a checkpoint history that
       does not include the Channel Volume Control Change command.  As a
       result, the new receiver experiences an indefinite artifact, and
       plays all notes on a channel too loudly or too softly.

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       To address this issue, the closed-loop policy states that whenever a
       sender becomes aware of a new receiver, the sender MUST determine if
       the receiver would be subject to indefinite artifacts under the
       closed-loop policy.  If so, the sender MUST ensure that the receiver
       starts the session free of indefinite artifacts.

       For example, to solve the Channel Volume issue described above, the
       sender may code the current state of the Channel Volume controller
       numbers in the recovery journal Chapter C, until it receives the
       first RTCP RR report that signals that a packet containing this
       Chapter C has been received.

       In satisfying this requirement, senders MAY infer the initial MIDI
       state of the receiver from the session description.  For example, the
       stream example in Section 6.2 has the initial state defined in [MIDI]
       for General MIDI.

       In a unicast RTP session, a receiver may safely assume that the
       sender is aware of its presence of a receiver from the first packet
       sent in the RTP stream.  However, in other types of RTP sessions
       (multicast, conference focus, RTP translator/mixer), a receiver is
       often not able to determine if the sender is initially aware of its
       presence as a receiver.

       To address this issue, the closed-loop policy states that if a
       receiver participates in a session where it may have access to a
       stream whose sender is not aware of the receiver, the receiver MUST
       take actions to ensure that its rendered MIDI performance does not
       contain indefinite artifacts.  These protections will be necessarily
       incomplete.  For example, a receiver may monitor the Checkpoint
       Packet Seqnum for uncovered loss events, and "err on the side of
       caution" with respect to handling stuck notes due to lost MIDI
       NoteOff commands, but the receiver is not able to compensate for the
       lack of Channel Volume initialization data in the recovery journal.

       The receiver MUST NOT discontinue these protective actions until it
       is certain that the sender is aware of its presence.  If a receiver
       is not able to ascertain sender awareness, the receiver MUST continue
       these protective actions for the duration of the session.

       Note that in a multicast session where all parties are expected to
       send and receive, the reception of RTCP receiver reports from the
       sender about the RTP stream a receiver is multicasting is evidence of
       the sender's awareness that the RTP stream multicast by the sender is
       being monitored by the receiver.  Receivers may also obtain sender
       awareness evidence from session management tools, or by other means.
       In practice, ongoing observation of the Checkpoint Packet Seqnum to
       determine if the sender is taking actions to prevent loss events for

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       a receiver is a good indication of sender awareness, as is the sudden
       appearance of recovery journal chapters with numerous Control Change
       controller data that was not foreshadowed by recent commands coded in
       the MIDI list shortly after sending an RTCP RR.

       The final set of normative closed-loop policy requirements concern
       how senders and receivers handle unplanned disruptions of RTCP
       feedback from a receiver to a sender.  By "unplanned", we refer to
       disruptions that are not due to the signalled termination of an RTP
       stream, via an RTCP BYE or via session management tools.

       As defined earlier in this section, the closed-loop policy states
       that a sender MUST choose a checkpoint packet with extended sequence
       number N, such that M(k) >= (N - 1) for all k, 1 <= k <= R, where R
       >= 1.  If the sender has received at least one feedback report from
       receiver k, M(k) is the most recent report of the highest RTP packet
       sequence number seen by the receiver, normalized to reflect the
       rollover count of the sender.

       If this receiver k stops sending feedback to the sender, the M(k)
       value used by the sender reflects the last feedback report from the
       receiver.  As time progresses without feedback from receiver k, this
       fixed M(k) value forces the sender to increase the size of the
       checkpoint history, and thus increases the bandwidth of the stream.

       At some point, the sender may need to take action in order to limit
       the bandwidth of the stream.  In most envisioned uses of RTP MIDI,
       long before this point is reached, the SSRC time-out mechanism
       defined in [RFC3550] will remove the uncooperative receiver from the
       session (note that the closed-loop policy does not suggest or require
       any special sender behavior upon an SSRC time-out, other than the
       sender actions related to changing R, described earlier in this
       section).

       However, in rare situations, the bandwidth of the stream (due to a
       lack of feedback reports from the sender) may become too large to
       continue sending the stream to the receiver before the SSRC time-out
       occurs for the receiver.  In this case, the closed-loop policy states
       that the sender should invoke the SSRC time-out for the receiver
       early.

       We now discuss receiver responsibilities in the case of unplanned
       disruptions of RTCP feedback from receiver to sender.

       In the unicast case, if a sender invokes the SSRC time-out mechanism
       for a receiver, the receiver stops receiving packets from the sender.
       The sender behavior imposed by the guardtime parameter (Appendix

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       C.4.2) lets the receiver conclude that an SSRC time-out has occurred
       in a reasonable time period.

       In this case of a time-out, a receiver MUST keep sending RTCP
       feedback, in order to re-establish the RTP flow from the sender.
       Unless the receiver expects a prompt recovery of the RTP flow, the
       receiver MUST take actions to ensure that the rendered MIDI
       performance does not exhibit "very long transient artifacts" (for
       example, by silencing NoteOns to prevent stuck notes) while awaiting
       reconnection of the flow.

       In the multicast case, if a sender invokes the SSRC time-out
       mechanism for a receiver, the receiver may continue to receive
       packets, but the sender will no longer be using the M(k) feedback
       from the receiver to choose each checkpoint packet.  If the receiver
       does not have additional information that precludes an SSRC time-out
       (such as RTCP Receiver Reports from the sender about an RTP stream
       the receiver is multicasting back to the sender), the receiver MUST
       monitor the Checkpoint Packet Seqnum to detect an SSRC time-out.  If
       an SSRC time-out is detected, the receiver MUST follow the
       instructions for SSRC time-outs described for the unicast case above.

       Finally, we note that the closed-loop policy is suitable for use in
       RTP/RTCP sessions that use multicast transport.  However, aspects of
       the closed-loop policy do not scale well to sessions with large
       numbers of participants.  The sender state scales linearly with the
       number of receivers, as the sender needs to track the identity and
       M(k) value for each receiver k.  The average recovery journal size is
       not independent of the number of receivers, as the RTCP reporting
       interval backoff slows down the rate of a full update of M(k) values.
       The backoff algorithm may also increase the amount of ancillary state
       used by implementations of the normative sender and receiver
       behaviors defined in Section 4.

    C.2.2.3.  The open-loop Sending Policy

       The open-loop policy is suitable for sessions that are not able to
       implement the receiver-to-sender feedback required by the closed-loop
       policy, and that are also not able to use the anchor policy because
       of bandwidth constraints.

       The open-loop policy does not place constraints on how a sender
       chooses the checkpoint packet for each packet in the stream.  In the
       absence of such constraints, a receiver may find that the recovery
       journal in the packet that ends a loss event has a checkpoint history
       that does not cover the entire loss event.  We refer to loss events
       of this type as uncovered loss events.

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       To ensure that uncovered loss events do not compromise the recovery
       journal mandate, the open-loop policy assigns specific recovery tasks
       to senders, receivers, and the creators of session descriptions.  The
       underlying premise of the open-loop policy is that the indefinite
       artifacts produced during uncovered loss events fall into two
       classes.

       One class of artifacts is recoverable indefinite artifacts.
       Receivers are able to repair recoverable artifacts that occur during
       an uncovered loss event without intervention from the sender, at the
       potential cost of unpleasant transient artifacts.

       For example, after an uncovered loss event, receivers are able to
       repair indefinite artifacts due to NoteOff (0x8) commands that may
       have occurred during the loss event, by executing NoteOff commands
       for all active NoteOns commands.  This action causes a transient
       artifact (a sudden silent period in the performance), but ensures
       that no stuck notes sound indefinitely.  We refer to MIDI commands
       that are amenable to repair in this fashion as recoverable MIDI
       commands.

       A second class of artifacts is unrecoverable indefinite artifacts.
       If this class of artifact occurs during an uncovered loss event, the
       receiver is not able to repair the stream.

       For example, after an uncovered loss event, receivers are not able to
       repair indefinite artifacts due to Control Change (0xB) Channel
       Volume (controller number 7) commands that have occurred during the
       loss event.  A repair is impossible because the receiver has no way
       of determining the data value of a lost Channel Volume command.  We
       refer to MIDI commands that are fragile in this way as unrecoverable
       MIDI commands.

       The open-loop policy does not specify how to partition the MIDI
       command set into recoverable and unrecoverable commands.  Instead, it
       assumes that the creators of the session descriptions are able to
       come to agreement on a suitable recoverable/unrecoverable MIDI
       command partition for an application.

       Given these definitions, we now state the normative requirements for
       the open-loop policy.

       In the open-loop policy, the creators of the session description MUST
       use the ch_anchor parameter (defined in Appendix C.2.3) to protect
       all unrecoverable MIDI command types from indefinite artifacts, or
       alternatively MUST use the cm_unused parameter (defined in Appendix

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       C.1) to exclude the command types from the stream.  These options act
       to shield command types from artifacts during an uncovered loss
       event.

       In the open-loop policy, receivers MUST examine the Checkpoint Packet
       Seqnum field of the recovery journal header after every loss event,
       to check if the loss event is an uncovered loss event.  Section 5
       shows how to perform this check.  If an uncovered loss event has
       occurred, a receiver MUST perform indefinite artifact recovery for
       all MIDI command types that are not shielded by ch_anchor and
       cm_unused parameter assignments in the session description.

       The open-loop policy does not place specific constraints on the
       sender.  However, the open-loop policy works best if the sender
       manages the size of the checkpoint history to ensure that uncovered
       losses occur infrequently, by taking into account the delay and loss
       characteristics of the network.  Also, as each checkpoint packet
       change incurs the risk of an uncovered loss, senders should only move
       the checkpoint if it reduces the size of the journal.

    C.2.3.  Recovery Journal Chapter Inclusion Parameters

       The recovery journal chapter definitions (Appendices A-B) specify
       under what conditions a chapter MUST appear in the recovery journal.
       In most cases, the definition states that if a certain command
       appears in the checkpoint history, a certain chapter type MUST appear
       in the recovery journal to protect the command.

       In this section, we describe the chapter inclusion parameters.  These
       parameters modify the conditions under which a chapter appears the
       journal.  These parameters are essential to the use of the open-loop
       policy (Appendix C.2.2.3) and may also be used to simplify
       implementations of the closed-loop (Appendix C.2.2.2) and anchor
       (Appendix C.2.2.1) policies.

       Each parameter represents a type of chapter inclusion semantics.  An
       assignment to a parameter declares which chapters (or chapter
       subsets) obey the inclusion semantics.  We describe the assignment
       syntax for these parameters later in this section.

       A party MUST NOT accept chapter inclusion parameter values that
       violate the recovery journal mandate (Section 4).  All assignments of
       the subsetting parameters (cm_used and cm_unused) MUST precede the
       first assignment of a chapter inclusion parameter in the parameter
       list.

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       Below, we normatively define the semantics of the chapter inclusion
       parameters.  For clarity, we define the action of parameters on
       complete chapters.  If a parameter is assigned a subset of a chapter,
       the definition applies only to the chapter subset.

         o  ch_never.  A chapter assigned to the ch_never parameter MUST NOT
            appear in the recovery journal (Appendix A.4.1-2 defines
            exceptions to this rule for Chapter M).  To signal the exclusion
            of a chapter from the journal, an assignment to ch_never MUST be
            made, even if the commands coded by the chapter are assigned to
            cm_unused.  This rule simplifies the handling of commands types
            that may be coded in several chapters.

         o  ch_default.  A chapter assigned to the ch_default parameter MUST
            follow the default semantics for the chapter, as defined in
            Appendices A-B.

         o  ch_anchor.  A chapter assigned to the ch_anchor MUST obey a
            modified version of the default chapter semantics.  In the
            modified semantics, all references to the checkpoint history are
            replaced with references to the session history, and all
            references to the checkpoint packet are replaced with references
            to the first packet sent in the stream.

       Parameter assignments obey the following syntax (see Appendix D for
       ABNF):

         <parameter> = [channel list]<chapter list>[field list]

       The chapter list is mandatory; the channel and field lists are
       optional.  Multiple assignments to parameters have a cumulative
       effect and are applied in the order of parameter appearance in a
       media description.

       To determine the semantics of a list of chapter inclusion parameter
       assignments, we begin by assuming an implicit assignment of all
       channel and system chapters to the ch_default parameter, with the
       default values for the channel list and field list for each chapter
       that are defined below.

       We then interpret the semantics of the actual parameter assignments,
       using the rules below.

       A later assignment of a chapter to the same parameter expands the
       scope of the earlier assignment.  In most cases, a later assignment
       of a chapter to a different parameter cancels (partially or
       completely) the effect of an earlier assignment.

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       The chapter list specifies the channel or system chapters for which
       the parameter applies.  The chapter list is a concatenated sequence
       of one or more of the letters corresponding to the chapter types
       (ACDEFMNPQTVWX).  In addition, the list may contain one or more of
       the letters for the sub-chapter types (BGHJKYZ) of System Chapter D.

       The letters in a chapter list MUST be uppercase and MUST appear in
       alphabetical order.  Letters other than (ABCDEFGHJKMNPQTVWXYZ) that
       appear in the chapter list MUST be ignored.

       The channel list specifies the channel journals for which this
       parameter applies; if no channel list is provided, the parameter
       applies to all channel journals.  The channel list takes the form of
       a list of channel numbers (0 through 15) and dash-separated channel
       number ranges (i.e., 0-5, 8-12, etc.).  Dots (i.e., "." characters)
       separate elements in the channel list.

       Several of the systems chapters may be configured to have special
       semantics.  Configuration occurs by specifying a channel list for the
       systems channel, using the coding described below (note that MIDI
       Systems commands do not have a "channel", and thus the original
       purpose of the channel list does not apply to systems chapters).  The
       expression "the digit N" in the text below refers to the inclusion of
       N as a "channel" in the channel list for a systems chapter.

       For the J and K Chapter D sub-chapters (undefined System Common), the
       digit 0 codes that the parameter applies to the LEGAL field of the
       associated command log (Figure B.1.4 of Appendix B.1), the digit 1
       codes that the parameter applies to the VALUE field of the command
       log, and the digit 2 codes that the parameter applies to the COUNT
       field of the command log.

       For the Y and Z Chapter D sub-chapters (undefined System Real-time),
       the digit 0 codes that the parameter applies to the LEGAL field of
       the associated command log (Figure B.1.5 of Appendix B.1) and the
       digit 1 codes that the parameter applies to the COUNT field of the
       command log.

       For Chapter Q (Sequencer State Commands), the digit 0 codes that the
       parameter applies to the default Chapter Q definition, which forbids
       the TIME field.  The digit 1 codes that the parameter applies to the
       optional Chapter Q definition, which supports the TIME field.

       The syntax for field lists follows the syntax for channel lists.  If
       no field list is provided, the parameter applies to all controller or
       note numbers.  For Chapter C, if no field list is provided, the
       controller numbers do not use enhanced Chapter C encoding (Appendix
       A.3.3).

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       For Chapter C, the field list may take on values in the range 0 to
       255.  A field value X in the range 0-127 refers to a controller
       number X, and indicates that the controller number does not use
       enhanced Chapter C encoding.  A field value X in the range 128-255
       refers to a controller number "X minus 128" and indicates the
       controller number does use the enhanced Chapter C encoding.

       Assignments made to configure the Chapter C encoding method for a
       controller number MUST be made to the ch_default or ch_anchor
       parameters, as assignments to ch_never act to exclude the number from
       the recovery journal (and thus the indicated encoding method is
       irrelevant).

       A Chapter C field list MUST NOT encode conflicting information about
       the enhanced encoding status of a particular controller number.  For
       example, values 0 and 128 MUST NOT both be coded by a field list.

       For Chapter M, the field list codes the Registered Parameter Numbers
       (RPNs) and Non-Registered Parameter Numbers (NRPNs) for which the
       parameter applies.  The number range 0-16383 specifies RPNs, the
       number range 16384-32767 specifies NRPNs (16384 corresponds to NRPN
       0, 32767 corresponds to NRPN 16383).

       For Chapters N and A, the field list codes the note numbers for which
       the parameter applies.  The note number range specified for Chapter N
       also applies to Chapter E.

       For Chapter E, the digit 0 codes that the parameter applies to
       Chapter E note logs whose V bit is set to 0, and the digit 1 codes
       that the parameter applies to note logs whose V bit is set to 1.

       For Chapter X, the field list codes the number of data octets that
       may appear in a SysEx command that is coded in the chapter.  Thus,
       the field list 0-255 specifies SysEx commands with 255 or fewer data
       octets, the field list 256-4294967295 specifies SysEx commands with
       more than 255 data octets but excludes commands with 255 or fewer
       data octets, and the field list 0 excludes all commands.

       A secondary parameter assignment syntax customizes Chapter X (see
       Appendix D for complete ABNF):

         <parameter> = "__" <h-list> ["_" <h-list>] "__"

       The assignment defines a class of SysEx commands whose Chapter X
       coding obeys the semantics of the assigned parameter.  The command
       class is specified by listing the permitted values of the first N

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       data octets that follow the SysEx 0xF0 command octet.  Any SysEx
       command whose first N data octets match the list is a member of the
       class.

       Each <h-list> defines a data octet of the command, as a dot-separated
       (".") list of one or more hexadecimal constants (such as "7F") or
       dash-separated hexadecimal ranges (such as "01-1F").  Underscores
       ("_") separate each <h-list>.  Double-underscores ("__") delineate
       the data octet list.

       Using this syntax, each assignment specifies a single SysEx command
       class.  Session descriptions may use several assignments to the same
       (or different) parameters to specify complex Chapter X behaviors.
       The ordering behavior of multiple assignments follows the guidelines
       for chapter parameter assignments described earlier in this section.

       The example session description below illustrates the use of the
       chapter inclusion parameters:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 rtp-midi/44100
       a=fmtp:96 j_update=open-loop; cm_unused=ABCFGHJKMQTVWXYZ;
       cm_used=__7E_00-7F_09_01.02.03__;
       cm_used=__7F_00-7F_04_01.02__; cm_used=C7.64;
       ch_never=ABCDEFGHJKMQTVWXYZ; ch_never=4.11-13N;
       ch_anchor=P; ch_anchor=C7.64;
       ch_anchor=__7E_00-7F_09_01.02.03__;
       ch_anchor=__7F_00-7F_04_01.02__

       (The a=fmtp line has been wrapped to fit the page to accommodate
        memo formatting restrictions; it comprises a single line in SDP.)

       The j_update parameter codes that the stream uses the open-loop
       policy.  Most MIDI command-types are assigned to cm_unused and thus
       do not appear in the stream.  As a consequence, the assignments to
       the first ch_never parameter reflect that most chapters are not in
       use.

       Chapter N for several MIDI channels is assigned to ch_never.  Chapter
       N for MIDI channels other than 4, 11, 12, and 13 may appear in the
       recovery journal, using the (default) ch_default semantics.  In
       practice, this assignment pattern would reflect knowledge about a
       resilient rendering method in use for the excluded channels.

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       The MIDI Program Change command and several MIDI Control Change
       controller numbers are assigned to ch_anchor.  Note that the ordering
       of the ch_anchor chapter C assignment after the ch_never command acts
       to override the ch_never assignment for the listed controller numbers
       (7 and 64).

       The assignment of command-type X to cm_unused excludes most SysEx
       commands from the stream.  Exceptions are made for General MIDI
       System On/Off commands and for the Master Volume and Balance
       commands, via the use of the secondary assignment syntax.  The
       cm_used assignment codes the exception, and the ch_anchor assignment
       codes how these commands are protected in Chapter X.

    C.3.  Configuration Tools: Timestamp Semantics

       The MIDI command section of the payload format consists of a list of
       commands, each with an associated timestamp.  The semantics of
       command timestamps may be set during session configuration, using the
       parameters we describe in this section

       The parameter "tsmode" specifies the timestamp semantics for a
       stream.  The parameter takes on one of three token values: "comex",
       "async", or "buffer".

       The default "comex" value specifies that timestamps code the
       execution time for a command (Appendix C.3.1) and supports the
       accurate transcoding Standard MIDI Files (SMFs, [MIDI]).  The "comex"
       value is also RECOMMENDED for new MIDI user-interface controller
       designs.  The "async" value specifies an asynchronous timestamp
       sampling algorithm for time-of-arrival sources (Appendix C.3.2).  The
       "buffer" value specifies a synchronous timestamp sampling algorithm
       (Appendix C.3.3) for time-of-arrival sources.

       Ancillary parameters MAY follow tsmode in a media description.  We
       define these parameters in Appendices C.3.2-3 below.

    C.3.1.  The comex Algorithm

       The default "comex" (COMmand EXecution) tsmode value specifies the
       execution time for the command.  With comex, the difference between
       two timestamps indicates the time delay between the execution of the
       commands.  This difference may be zero, coding simultaneous
       execution.

       The comex interpretation of timestamps works well for transcoding a
       Standard MIDI File (SMF, [MIDI]) into an RTP MIDI stream, as SMFs
       code a timestamp for each MIDI command stored in the file.  To
       transcode an SMF that uses metric time markers, use the SMF tempo map

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       (encoded in the SMF as meta-events) to convert metric SMF timestamp
       units into seconds-based RTP timestamp units.

       New MIDI controller designs (piano keyboard, drum pads, etc.) that
       support RTP MIDI and that have direct access to sensor data SHOULD
       use comex interpretation for timestamps, so that simultaneous
       gestural events may be accurately coded by RTP MIDI.

       Comex is a poor choice for transcoding MIDI 1.0 DIN cables [MIDI],
       for a reason that we will now explain.  A MIDI DIN cable is an
       asynchronous serial protocol (320 microseconds per MIDI byte).  MIDI
       commands on a DIN cable are not tagged with timestamps.  Instead,
       MIDI DIN receivers infer command timing from the time of arrival of
       the bytes.  Thus, two two-byte MIDI commands that occur at a source
       simultaneously are encoded on a MIDI 1.0 DIN cable with a 640
       microsecond time offset.  A MIDI DIN receiver is unable to tell if
       this time offset existed in the source performance or is an artifact
       of the serial speed of the cable.  However, the RTP MIDI comex
       interpretation of timestamps declares that a timestamp offset between
       two commands reflects the timing of the source performance.

       This semantic mismatch is the reason that comex is a poor choice for
       transcoding MIDI DIN cables.  Note that the choice of the RTP
       timestamp rate (Section 6.1-2 in the main text) cannot fix this
       inaccuracy issue.  In the sections that follow, we describe two
       alternative timestamp interpretations ("async" and "buffer") that are
       a better match to MIDI 1.0 DIN cable timing, and to other MIDI time-
       of-arrival sources.

       The "octpos", "linerate", and "mperiod" ancillary parameters (defined
       below) SHOULD NOT be used with comex.

    C.3.2.  The async Algorithm

       The "async" tsmode value specifies the asynchronous sampling of a
       MIDI time-of-arrival source.  In asynchronous sampling, the moment an
       octet is received from a source, it is labelled with a wall-clock
       time value.  The time value has RTP timestamp units.

       The "octpos" ancillary parameter defines how RTP command timestamps
       are derived from octet time values.  If octpos has the token value
       "first", a timestamp codes the time value of the first octet of the
       command.  If octpos has the token value "last", a timestamp codes the
       time value of the last octet of the command.  If the octpos parameter
       does not appear in the media description, the sender does not know
       which octet of the command the timestamp references (for example, the
       sender may be relying on an operating system service that does not
       specify this information).

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       The octpos semantics refer to the first or last octet of a command as
       it appears on a time-of-arrival MIDI source, not as it appears in an
       RTP MIDI packet.  This distinction is significant because the RTP
       coding may contain octets that are not present in the source.  For
       example, the status octet of the first MIDI command in a packet may
       have been added to the MIDI stream during transcoding, to comply with
       the RTP MIDI running status requirements (Section 3.2).

       The "linerate" ancillary parameter defines the timespan of one MIDI
       octet on the transmission medium of the MIDI source to be sampled
       (such as a MIDI 1.0 DIN cable).  The parameter has units of
       nanoseconds, and takes on integral values.  For MIDI 1.0 DIN cables,
       the correct linerate value is 320000 (this value is also the default
       value for the parameter).

       We now show a session description example for the async algorithm.
       Consider a sender that is transcoding a MIDI 1.0 DIN cable source
       into RTP.  The sender runs on a computing platform that assigns time
       values to every incoming octet of the source, and the sender uses the
       time values to label the first octet of each command in the RTP
       packet.  This session description describes the transcoding:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 rtp-midi/44100
       a=sendonly
       a=fmtp:96 tsmode=async; linerate=320000; octpos=first

    C.3.3.  The buffer Algorithm

       The "buffer" tsmode value specifies the synchronous sampling of a
       MIDI time-of-arrival source.

       In synchronous sampling, octets received from a source are placed in
       a holding buffer upon arrival.  At periodic intervals, the RTP sender
       examines the buffer.  The sender removes complete commands from the
       buffer and codes those commands in an RTP packet.  The command
       timestamp codes the moment of buffer examination, expressed in RTP
       timestamp units.  Note that several commands may have the same
       timestamp value.

       The "mperiod" ancillary parameter defines the nominal periodic
       sampling interval.  The parameter takes on positive integral values
       and has RTP timestamp units.

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       The "octpos" ancillary parameter, defined in Appendix C.3.1 for
       asynchronous sampling, plays a different role in synchronous
       sampling.  In synchronous sampling, the parameter specifies the
       timestamp semantics of a command whose octets span several sampling
       periods.

       If octpos has the token value "first", the timestamp reflects the
       arrival period of the first octet of the command.  If octpos has the
       token value "last", the timestamp reflects the arrival period of the
       last octet of the command.  The octpos semantics refer to the first
       or last octet of the command as it appears on a time-of-arrival
       source, not as it appears in the RTP packet.

       If the octpos parameter does not appear in the media description, the
       timestamp MAY reflect the arrival period of any octet of the command;
       senders use this option to signal a lack of knowledge about the
       timing details of the buffering process at sub-command granularity.

       We now show a session description example for the buffer algorithm.
       Consider a sender that is transcoding a MIDI 1.0 DIN cable source
       into RTP.  The sender runs on a computing platform that places source
       data into a buffer upon receipt.  The sender polls the buffer 1000
       times a second, extracts all complete commands from the buffer, and
       places the commands in an RTP packet.  This session description
       describes the transcoding:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 rtp-midi/44100
       a=sendonly
       a=fmtp:96 tsmode=buffer; linerate=320000; octpos=last; mperiod=44

       The mperiod value of 44 is derived by dividing the clock rate
       specified by the rtpmap attribute (44100 Hz) by the 1000 Hz buffer
       sampling rate and rounding to the nearest integer.  Command
       timestamps might not increment by exact multiples of 44, as the
       actual sampling period might not precisely match the nominal mperiod
       value.

    C.4.  Configuration Tools: Packet Timing Tools

       In this appendix, we describe session configuration tools for
       customizing the temporal behavior of MIDI stream packets.

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    C.4.1.  Packet Duration Tools

       Senders control the granularity of a stream by setting the temporal
       duration ("media time") of the packets in the stream.  Short media
       times (20 ms or less) often imply an interactive session.  Longer
       media times (100 ms or more) usually indicate a content streaming
       session.  The RTP AVP profile [RFC3551] recommends audio packet media
       times in a range from 0 to 200 ms.

       By default, an RTP receiver dynamically senses the media time of
       packets in a stream and chooses the length of its playout buffer to
       match the stream.  A receiver typically sizes its playout buffer to
       fit several audio packets and adjusts the buffer length to reflect
       the network jitter and the sender timing fidelity.

       Alternatively, the packet media time may be statically set during
       session configuration.  Session descriptions MAY use the RTP MIDI
       parameter "rtp_ptime" to set the recommended media time for a packet.
       Session descriptions MAY also use the RTP MIDI parameter
       "rtp_maxptime" to set the maximum media time for a packet permitted
       in a stream.  Both parameters MAY be used together to configure a
       stream.

       The values assigned to the rtp_ptime and rtp_maxptime parameters have
       the units of the RTP timestamp for the stream, as set by the rtpmap
       attribute (see Section 6.1).  Thus, if rtpmap sets the clock rate of
       a stream to 44100 Hz, a maximum packet media time of 10 ms is coded
       by setting rtp_maxptime=441.  As stated in the Appendix C preamble,
       the senders and receivers of a stream MUST agree on common values for
       rtp_ptime and rtp_maxptime if the parameters appear in the media
       description for the stream.

       0 ms is a reasonable media time value for MIDI packets and is often
       used in low-latency interactive applications.  In a packet with a 0
       ms media time, all commands execute at the instant they are coded by
       the packet timestamp.  The session description below configures all
       packets in the stream to have 0 ms media time:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 rtp-midi/44100
       a=fmtp:96 rtp_ptime=0; rtp_maxptime=0

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       The session attributes ptime and maxptime [RFC4566] MUST NOT be used
       to configure an RTP MIDI stream.  Sessions MUST use rtp_ptime in lieu
       of ptime and MUST use rtp_maxptime in lieu of maxptime.  RTP MIDI
       defines its own parameters for media time configuration because 0 ms
       values for ptime and maxptime are forbidden by [RFC3264] but are
       essential for certain applications of RTP MIDI.

       See the Appendix C.7 examples for additional discussion about using
       rtp_ptime and rtp_maxptime for session configuration.

    C.4.2.  The guardtime Parameter

       RTP permits a sender to stop sending audio packets for an arbitrary
       period of time during a session.  When sending resumes, the RTP
       sequence number series continues unbroken, and the RTP timestamp
       value reflects the media time silence gap.

       This RTP feature has its roots in telephony, but it is also well
       matched to interactive MIDI sessions, as players may fall silent for
       several seconds during (or between) songs.

       Certain MIDI applications benefit from a slight enhancement to this
       RTP feature.  In interactive applications, receivers may use on-line
       network models to guide heuristics for handling lost and late RTP
       packets.  These models may work poorly if a sender ceases packet
       transmission for long periods of time.

       Session descriptions may use the parameter "guardtime" to set a
       minimum sending rate for a media session.  The value assigned to
       guardtime codes the maximum separation time between two sequential
       packets, as expressed in RTP timestamp units.

       Typical guardtime values are 500-2000 ms.  This value range is not a
       normative bound, and parties SHOULD be prepared to process values
       outside this range.

       The congestion control requirements for sender implementations
       (described in Section 8 and [RFC3550]) take precedence over the
       guardtime parameter.  Thus, if the guardtime parameter requests a
       minimum sending rate, but sending at this rate would violate the
       congestion control requirements, senders MUST ignore the guardtime
       parameter value.  In this case, senders SHOULD use the lowest minimum
       sending rate that satisfies the congestion control requirements.

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       Below, we show a session description that uses the guardtime
       parameter.

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 rtp-midi/44100
       a=fmtp:96 guardtime=44100; rtp_ptime=0; rtp_maxptime=0

    C.5.  Configuration Tools: Stream Description

       As we discussed in Section 2.1, a party may send several RTP MIDI
       streams in the same RTP session, and several RTP sessions that carry
       MIDI may appear in a multimedia session.

       By default, the MIDI name space (16 channels + systems) of each RTP
       stream sent by a party in a multimedia session is independent.  By
       independent, we mean three distinct things:

         o  If a party sends two RTP MIDI streams (A and B), MIDI voice
            channel 0 in stream A is a different "channel 0" than MIDI voice
            channel 0 in stream B.

         o  MIDI voice channel 0 in stream B is not considered to be
            "channel 16" of a 32-channel MIDI voice channel space whose
            "channel 0" is channel 0 of stream A.

         o  Streams sent by different parties over different RTP sessions,
            or over the same RTP session but with different payload type
            numbers, do not share the association that is shared by a MIDI
            cable pair that cross-connects two devices in a MIDI 1.0 DIN
            network.  By default, this association is only held by streams
            sent by different parties in the same RTP session that use the
            same payload type number.

       In this appendix, we show how to express that specific RTP MIDI
       streams in a multimedia session are not independent but instead are
       related in one of the three ways defined above.  We use two tools to
       express these relations:

         o  The musicport parameter.  This parameter is assigned a non-
            negative integer value between 0 and 4294967295.  It appears in
            the fmtp lines of payload types.

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         o  The FID grouping attribute [RFC3388] signals that several RTP
            sessions in a multimedia session are using the musicport
            parameter to express an inter-session relationship.

       If a multimedia session has several payload types whose musicport
       parameters are assigned the same integer value, streams using these
       payload types share an "identity relationship" (including streams
       that use the same payload type).  Streams in an identity relationship
       share two properties:

         o  Identity relationship streams sent by the same party target the
            same MIDI name space.  Thus, if streams A and B share an
            identity relationship, voice channel 0 in stream A is the same
            "channel 0" as voice channel 0 in stream B.

         o  Pairs of identity relationship streams that are sent by
            different parties share the association that is shared by a MIDI
            cable pair that cross-connects two devices in a MIDI 1.0 DIN
            network.

       A party MUST NOT send two RTP MIDI streams that share an identity
       relationship in the same RTP session.  Instead, each stream MUST be
       in a separate RTP session.  As explained in Section 2.1, this
       restriction is necessary to support the RTP MIDI method for the
       synchronization of streams that share a MIDI name space.

       If a multimedia session has several payload types whose musicport
       parameters are assigned sequential values (i.e., i, i+1, ... i+k),
       the streams using the payload types share an "ordered relationship".
       For example, if payload type A assigns 2 to musicport and payload
       type B assigns 3 to musicport, A and B are in an ordered
       relationship.

       Streams in an ordered relationship that are sent by the same party
       are considered by renderers to form a single larger MIDI space.  For
       example, if stream A has a musicport value of 2 and stream B has a
       musicport value of 3, MIDI voice channel 0 in stream B is considered
       to be voice channel 16 in the larger MIDI space formed by the
       relationship.  Note that it is possible for streams to participate in
       both an identity relationship and an ordered relationship.

       We now state several rules for using musicport:

         o  If streams from several RTP sessions in a multimedia session use
            the musicport parameter, the RTP sessions MUST be grouped using
            the FID grouping attribute defined in [RFC3388].

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         o  An ordered or identity relationship MUST NOT contain both native
            RTP MIDI streams and mpeg4-generic RTP MIDI streams.  An
            exception applies if a relationship consists of sendonly and
            recvonly (but not sendrecv) streams.  In this case, the sendonly
            streams MUST NOT contain both types of streams, and the recvonly
            streams MUST NOT contain both types of streams.

         o  It is possible to construct identity relationships that violate
            the recovery journal mandate (for example, sending NoteOns for a
            voice channel on stream A and NoteOffs for the same voice
            channel on stream B).  Parties MUST NOT generate (or accept)
            session descriptions that exhibit this flaw.

         o  Other payload formats MAY define musicport media type
            parameters.  Formats would define these parameters so that their
            sessions could be bundled into RTP MIDI name spaces.  The
            parameter definitions MUST be compatible with the musicport
            semantics defined in this appendix.

       As a rule, at most one payload type in a relationship may specify a
       MIDI renderer.  An exception to the rule applies to relationships
       that contain sendonly and recvonly streams but no sendrecv streams.
       In this case, one sendonly session and one recvonly session may each
       define a renderer.

       Renderer specification in a relationship may be done using the tools
       described in Appendix C.6.  These tools work for both native streams
       and mpeg4-generic streams.  An mpeg4-generic stream that uses the
       Appendix C.6 tools MUST set all "config" parameters to the empty
       string ("").

       Alternatively, for mpeg4-generic streams, renderer specification may
       be done by setting one "config" parameter in the relationship to the
       renderer configuration string, and all other config parameters to the
       empty string ("").

       We now define sender and receiver rules that apply when a party sends
       several streams that target the same MIDI name space.

       Senders MAY use the subsetting parameters (Appendix C.1) to predefine
       the partitioning of commands between streams, or they MAY use a
       dynamic partitioning strategy.

       Receivers that merge identity relationship streams into a single MIDI
       command stream MUST maintain the structural integrity of the MIDI
       commands coded in each stream during the merging process, in the same
       way that software that merges traditional MIDI 1.0 DIN cable flows is

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    RFC 4695              RTP Payload Format for MIDI          November 2006

       responsible for creating a merged command flow compatible with
       [MIDI].

       Senders MUST partition the name space so that the rendered MIDI
       performance does not contain indefinite artifacts (as defined in
       Section 4).  This responsibility holds even if all streams are sent
       over reliable transport, as different stream latencies may yield
       indefinite artifacts.  For example, stuck notes may occur in a
       performance split over two TCP streams, if NoteOn commands are sent
       on one stream and NoteOff commands are sent on the other.

       Senders MUST NOT split a Registered Parameter Name (RPN) or Non-
       Registered Parameter Name (NRPN) transaction appearing on a MIDI
       channel across multiple identity relationship sessions.  Receivers
       MUST assume that the RPN/NRPN transactions that appear on different
       identity relationship sessions are independent and MUST preserve
       transactional integrity during the MIDI merge.

       A simple way to safely partition voice channel commands is to place
       all MIDI commands for a particular voice channel into the same
       session.  Safe partitioning of MIDI Systems commands may be more
       complicated for sessions that extensively use System Exclusive.

       We now show several session description examples that use the
       musicport parameter.

       Our first session description example shows two RTP MIDI streams that
       drive the same General MIDI decoder.  The sender partitions MIDI
       commands between the streams dynamically.  The musicport values
       indicate that the streams share an identity relationship.

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       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       a=group:FID 1 2
       c=IN IP4 192.0.2.94
       m=audio 5004 RTP/AVP 96
       a=rtpmap:96 mpeg4-generic/44100
       a=mid:1
       a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
       config=7A0A0000001A4D546864000000060000000100604D54726B0
       000000600FF2F000; musicport=12
       m=audio 5006 RTP/AVP 96
       a=rtpmap:96 mpeg4-generic/44100
       a=mid:2
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; musicport=12

       (The a=fmtp lines have been wrapped to fit the page to accommodate
        memo formatting restrictions; they comprise single lines in SDP.)

       Recall that Section 2.1 defines rules for streams that target the
       same MIDI name space.  Those rules, implemented in the example above,
       require that each stream resides in a separate RTP session, and that
       the grouping mechanisms defined in [RFC3388] signal an inter-session
       relationship.  The "group" and "mid" attribute lines implement this
       grouping mechanism.

       A variant on this example, whose session description is not shown,
       would use two streams in an identity relationship driving the same
       MIDI renderer, each with a different transport type.  One stream
       would use UDP and would be dedicated to real-time messages.  A second
       stream would use TCP [RFC4571] and would be used for SysEx bulk data
       messages.

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       In the next example, two mpeg4-generic streams form an ordered
       relationship to drive a Structured Audio decoder with 32 MIDI voice
       channels.  Both streams reside in the same RTP session.

       v=0
       o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
       s=Example
       t=0 0
       m=audio 5006 RTP/AVP 96 97
       c=IN IP6 2001:DB80::7F2E:172A:1E24
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=13; musicport=5
       a=rtpmap:97 mpeg4-generic/44100
       a=fmtp:97 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=13; musicport=6; render=synthetic;
       rinit="audio/asc";
       url="http://example.com/cardinal.asc";
       cid="azsldkaslkdjqpwojdkmsldkfpe"

       (The a=fmtp lines have been wrapped to fit the page to accommodate
        memo formatting restrictions; they comprise single lines in SDP.)

       The sequential musicport values for the two sessions establish the
       ordered relationship.  The musicport=5 session maps to Structured
       Audio extended channels range 0-15, the musicport=6 session maps to
       Structured Audio extended channels range 16-31.

       Both config strings are empty.  The configuration data is specified
       by parameters that appear in the fmtp line of the second media
       description.  We define this configuration method in Appendix C.6.

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       The next example shows two RTP MIDI streams (one recvonly, one
       sendonly) that form a "virtual sendrecv" session.  Each stream
       resides in a different RTP session (a requirement because sendonly
       and recvonly are RTP session attributes).

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       a=group:FID 1 2
       c=IN IP4 192.0.2.94
       m=audio 5004 RTP/AVP 96
       a=sendonly
       a=rtpmap:96 mpeg4-generic/44100
       a=mid:1
       a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
       config=7A0A0000001A4D546864000000060000000100604D54726B0
       000000600FF2F000; musicport=12
       m=audio 5006 RTP/AVP 96
       a=recvonly
       a=rtpmap:96 mpeg4-generic/44100
       a=mid:2
       a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
       config=7A0A0000001A4D546864000000060000000100604D54726B0
       000000600FF2F000; musicport=12

       (The a=fmtp lines have been wrapped to fit the page to accommodate
        memo formatting restrictions; they comprise single lines in SDP.)

       To signal the "virtual sendrecv" semantics, the two streams assign
       musicport to the same value (12).  As defined earlier in this
       section, pairs of identity relationship streams that are sent by
       different parties share the association that is shared by a MIDI
       cable pair that cross-connects two devices in a MIDI 1.0 network.  We
       use the term "virtual sendrecv" because streams sent by different
       parties in a true sendrecv session also have this property.

       As discussed in the preamble to Appendix C, the primary advantage of
       the virtual sendrecv configuration is that each party can customize
       the property of the stream it receives.  In the example above, each
       stream defines its own "config" string that could customize the
       rendering algorithm for each party (in fact, the particular strings
       shown in this example are identical, because General MIDI is not a
       configurable MPEG 4 renderer).

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    C.6.  Configuration Tools: MIDI Rendering

       This appendix defines the session configuration tools for rendering.

       The "render" parameter specifies a rendering method for a stream.
       The parameter is assigned a token value that signals the top-level
       rendering class.  This memo defines four token values for render:
       "unknown", "synthetic", "api", and "null":

         o  An "unknown" renderer is a renderer whose nature is unspecified.
            It is the default renderer for native RTP MIDI streams.

         o  A "synthetic" renderer transforms the MIDI stream into audio
            output (or sometimes into stage lighting changes or other
            actions).  It is the default renderer for mpeg4-generic RTP MIDI
            streams.

         o  An "api" renderer presents the command stream to applications
            via an Application Programmer Interface (API).

         o  The "null" renderer discards the MIDI stream.

       The "null" render value plays special roles during Offer/Answer
       negotiations [RFC3264].  A party uses the "null" value in an answer
       to reject an offered renderer.  Note that rejecting a renderer is
       independent from rejecting a payload type (coded by removing the
       payload type from a media line) and rejecting a media stream (coded
       by zeroing the port of a media line that uses the renderer).

       Other render token values MAY be registered with IANA.  The token
       value MUST adhere to the ABNF for render tokens defined in Appendix
       D.  Registrations MUST include a complete specification of parameter
       value usage, similar in depth to the specifications that appear
       throughout Appendix C.6 for "synthetic" and "api" render values.  If
       a party is offered a session description that uses a render token
       value that is not known to the party, the party MUST NOT accept the
       renderer.  Options include rejecting the renderer (using the "null"
       value), the payload type, the media stream, or the session
       description.

       Other parameters MAY follow a render parameter in a parameter list.
       The additional parameters act to define the exact nature of the
       renderer.  For example, the "subrender" parameter (defined in
       Appendix C.6.2) specifies the exact nature of the renderer.

       Special rules apply to using the render parameter in an mpeg4-generic
       stream.  We define these rules in Appendix C.6.5.

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    C.6.1.  The multimode Parameter

       A media description MAY contain several render parameters.  By
       default, if a parameter list includes several render parameters, a
       receiver MUST choose exactly one renderer from the list to render the
       stream.  The "multimode" parameter may be used to override this
       default.  We define two token values for multimode: "one" and "all":

         o  The default "one" value requests rendering by exactly one of the
            listed renderers.

         o  The "all" value requests the synchronized rendering of the RTP
            MIDI stream by all listed renderers, if possible.

       If the multimode parameter appears in a parameter list, it MUST
       appear before the first render parameter assignment.

       Render parameters appear in the parameter list in order of decreasing
       priority.  A receiver MAY use the priority ordering to decide which
       renderer(s) to retain in a session.

       If the "offer" in an Offer/Answer-style negotiation [RFC3264]
       contains a parameter list with one or more render parameters, the
       "answer" MUST set the render parameters of all unchosen renderers to
       "null".

    C.6.2.  Renderer Specification

       The render parameter (Appendix C.6 preamble) specifies, in a broad
       sense, what a renderer does with a MIDI stream.  In this appendix, we
       describe the "subrender" parameter.  The token value assigned to
       subrender defines the exact nature of the renderer.  Thus, "render"
       and "subrender" combine to define a renderer, in the same way as MIME
       types and MIME subtypes combine to define a type of media [RFC2045].

       If the subrender parameter is used for a renderer definition, it MUST
       appear immediately after the render parameter in the parameter list.
       At most one subrender parameter may appear in a renderer definition.

       This document defines one value for subrender: the value "default".
       The "default" token specifies the use of the default renderer for the
       stream type (native or mpeg4-generic).  The default renderer for
       native RTP MIDI streams is a renderer whose nature is unspecified
       (see point 6 in Section 6.1 for details).  The default renderer for
       mpeg4-generic RTP MIDI streams is an MPEG 4 Audio Object Type whose
       ID number is 13, 14, or 15 (see Section 6.2 for details).

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       If a renderer definition does not use the subrender parameter, the
       value "default" is assumed for subrender.

       Other subrender token values may be registered with IANA.  We now
       discuss guidelines for registering subrender values.

       A subrender value is registered for a specific stream type (native or
       mpeg4-generic) and a specific render value (excluding "null" and
       "unknown").  Registrations for mpeg4-generic subrender values are
       restricted to new MPEG 4 Audio Object Types that accept MIDI input.
       The syntax of the token MUST adhere to the token definition in
       Appendix D.

       For "render=synthetic" renderers, a subrender value registration
       specifies an exact method for transforming the MIDI stream into audio
       (or sometimes into video or control actions, such as stage lighting).
       For standardized renderers, this specification is usually a pointer
       to a standards document, perhaps supplemented by RTP-MIDI-specific
       information.  For commercial products and open-source projects, this
       specification usually takes the form of instructions for interfacing
       the RTP MIDI stream with the product or project software.  A
       "render=synthetic" registration MAY specify additional Reset State
       commands for the renderer (Appendix A.1).

       A "render=api" subrender value registration specifies how an RTP MIDI
       stream interfaces with an API (Application Programmers Interface).
       This specification is usually a pointer to programmer's documentation
       for the API, perhaps supplemented by RTP-MIDI-specific information.

       A subrender registration MAY specify an initialization file (referred
       to in this document as an initialization data object) for the stream.
       The initialization data object MAY be encoded in the parameter list
       (verbatim or by reference) using the coding tools defined in Appendix
       C.6.3.  An initialization data object MUST have a registered
       [RFC4288] media type and subtype [RFC2045].

       For "render=synthetic" renderers, the data object usually encodes
       initialization data for the renderer (sample files, synthesis patch
       parameters, reverberation room impulse responses, etc.).

       For "render=api" renderers, the data object usually encodes data
       about the stream used by the API (for example, for an RTP MIDI stream
       generated by a piano keyboard controller, the manufacturer and model
       number of the keyboard, for use in GUI presentation).

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       Usually, only one initialization object is encoded for a renderer.
       If a renderer uses multiple data objects, the correct receiver
       interpretation of multiple data objects MUST be defined in the
       subrender registration.

       A subrender value registration may also specify additional
       parameters, to appear in the parameter list immediately after
       subrender.  These parameter names MUST begin with the subrender
       value, followed by an underscore ("_"), to avoid name space
       collisions with future RTP MIDI parameter names (for example, a
       parameter "foo_bar" defined for subrender value "foo").

       We now specify guidelines for interpreting the subrender parameter
       during session configuration.

       If a party is offered a session description that uses a renderer
       whose subrender value is not known to the party, the party MUST NOT
       accept the renderer.  Options include rejecting the renderer (using
       the "null" value), the payload type, the media stream, or the session
       description.

       Receivers MUST be aware of the Reset State commands (Appendix A.1)
       for the renderer specified by the subrender parameter and MUST insure
       that the renderer does not experience indefinite artifacts due to the
       presence (or the loss) of a Reset State command.

    C.6.3.  Renderer Initialization

       If the renderer for a stream uses an initialization data object, an
       "rinit" parameter MUST appear in the parameter list immediately after
       the "subrender" parameter.  If the renderer parameter list does not
       include a subrender parameter (recall the semantics for "default" in
       Appendix C.6.2), the "rinit" parameter MUST appear immediately after
       the "render" parameter.

       The value assigned to the rinit parameter MUST be the media
       type/subtype [RFC2045] for the initialization data object.  If an
       initialization object type is registered with several media types,
       including audio, the assignment to rinit MUST use the audio media
       type.

       RTP MIDI supports several parameters for encoding initialization data
       objects for renderers in the parameter list: "inline", "url", and
       "cid".

       If the "inline", "url", and/or "cid" parameters are used by a
       renderer, these parameters MUST immediately follow the "rinit"
       parameter.

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       If a "url" parameter appears for a renderer, an "inline" parameter
       MUST NOT appear.  If an "inline" parameter appears for a renderer, a
       "url" parameter MUST NOT appear.  However, neither "url" or "inline"
       is required to appear.  If neither "url" or "inline" parameters
       follow "rinit", the "cid" parameter MUST follow "rinit".

       The "inline" parameter supports the inline encoding of the data
       object.  The parameter is assigned a double-quoted Base64 [RFC2045]
       encoding of the binary data object, with no line breaks.  Appendix
       E.4 shows an example that constructs an inline parameter value.

       The "url" parameter is assigned a double-quoted string representation
       of a Uniform Resource Locator (URL) for the data object.  The string
       MUST specify a HyperText Transport Protocol URL (HTTP, [RFC2616]).
       HTTP MAY be used over TCP or MAY be used over a secure network
       transport, such as the method described in [RFC2818].  The media
       type/subtype for the data object SHOULD be specified in the
       appropriate HTTP transport header.

       The "cid" parameter supports data object caching.  The parameter is
       assigned a double-quoted string value that encodes a globally unique
       identifier for the data object.

       A cid parameter MAY immediately follow an inline parameter, in which
       case the cid identifier value MUST be associated with the inline data
       object.

       If a url parameter is present, and if the data object for the URL is
       expected to be unchanged for the life of the URL, a cid parameter MAY
       immediately follow the url parameter.  The cid identifier value MUST
       be associated with the data object for the URL.  A cid parameter
       assigned to the same identifier value SHOULD be specified following
       the data object type/subtype in the appropriate HTTP transport
       header.

       If a url parameter is present, and if the data object for the URL is
       expected to change during the life of the URL, a cid parameter MUST
       NOT follow the url parameter.  A receiver interprets the presence of
       a cid parameter as an indication that it is safe to use a cached copy
       of the url data object; the absence of a cid parameter is an
       indication that it is not safe to use a cached copy, as it may
       change.

       Finally, the cid parameter MAY be used without the inline and url
       parameters.  In this case, the identifier references a local or
       distributed catalog of data objects.

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       In most cases, only one data object is coded in the parameter list
       for each renderer.  For example, the default renderer for mpeg4-
       generic streams uses a single data object (see Appendix C.6.5 for
       example usage).

       However, a subrender registration MAY permit the use of multiple data
       objects for a renderer.  If multiple data objects are encoded for a
       renderer, each object encoding begins with an "rinit" parameter,
       followed by "inline", "url", and/or "cid" parameters.

       Initialization data objects MAY encapsulate a Standard MIDI File
       (SMF).  By default, the SMFs that are encapsulated in a data object
       MUST be ignored by an RTP MIDI receiver.  We define parameters to
       override this default in Appendix C.6.4.

       To end this section, we offer guidelines for registering media types
       for initialization data objects.  These guidelines are in addition to
       the information in [RFC4288] [RFC4289].

       Some initialization data objects are also capable of encoding MIDI
       note information and thus complete audio performances.  These objects
       SHOULD be registered using the "audio" media type, so that the
       objects may also be used for store-and-forward rendering, and
       "application" media type, to support editing tools.  Initialization
       objects without note storage, or initialization objects for non-audio
       renderers, SHOULD be registered only for an "application" media type.

    C.6.4.  MIDI Channel Mapping

       In this appendix, we specify how to map MIDI name spaces (16 voice
       channels + systems) onto a renderer.

       In the general case:

         o  A session may define an ordered relationship (Appendix C.5) that
            presents more than one MIDI name space to a renderer.

         o  A renderer may accept an arbitrary number of MIDI name spaces,
            or it may expect a specific number of MIDI name spaces.

       A session description SHOULD provide a compatible MIDI name space to
       each renderer in the session.  If a receiver detects that a session
       description has too many or too few MIDI name spaces for a renderer,
       MIDI data from extra stream name spaces MUST be discarded, and extra
       renderer name spaces MUST NOT be driven with MIDI data (except as
       described in Appendix C.6.4.1, below).

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       If a parameter list defines several renderers and assigns the "all"
       token value to the multimode parameter, the same name space is
       presented to each renderer.  However, the "chanmask" parameter may be
       used to mask out selected voice channels to each renderer.  We define
       "chanmask" and other MIDI management parameters in the sub-sections
       below.

    C.6.4.1.  The smf_info Parameter

       The smf_info parameter defines the use of the SMFs encapsulated in
       renderer data objects (if any).  The smf_info parameter also defines
       the use of SMFs coded in the smf_inline, smf_url, and smf_cid
       parameters (defined in Appendix C.6.4.2).

       The smf_info parameter describes the "render" parameter that most
       recently precedes it in the parameter list.  The smf_info parameter
       MUST NOT appear in parameter lists that do not use the "render"
       parameter, and MUST NOT appear before the first use of "render" in
       the parameter list.

       We define three token values for smf_info: "ignore", "sdp_start", and
       "identity":

         o  The "ignore" value indicates that the SMFs MUST be discarded.
            This behavior is the default SMF rendering behavior.

         o  The "sdp_start" value codes that SMFs MUST be rendered, and that
            the rendering MUST begin upon the acceptance of the session
            description.  If a receiver is offered a session description
            with a renderer that uses an smf_info parameter set to
            sdp_start, and if the receiver does not support rendering SMFs,
            the receiver MUST NOT accept the renderer associated with the
            smf_info parameter.  Options include rejecting the renderer (by
            setting the "render" parameter to "null"), the payload type, the
            media stream, or the entire session description.

         o  The "identity" value indicates that the SMFs code the identity
            of the renderer.  The value is meant for use with the "unknown"
            renderer (see Appendix C.6 preamble).  The MIDI commands coded
            in the SMF are informational in nature and MUST NOT be presented
            to a renderer for audio presentation.  In typical use, the SMF
            would use SysEx Identity Reply commands (F0 7E nn 06 02, as
            defined in [MIDI]) to identify devices, and use device-specific
            SysEx commands to describe current state of the devices (patch
            memory contents, etc.).

       Other smf_info token values MAY be registered with IANA.  The token
       value MUST adhere to the ABNF for render tokens defined in Appendix

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       D.  Registrations MUST include a complete specification of parameter
       usage, similar in depth to the specifications that appear in this
       appendix for "sdp_start" and "identity".

       If a party is offered a session description that uses an smf_info
       parameter value that is not known to the party, the party MUST NOT
       accept the renderer associated with the smf_info parameter.  Options
       include rejecting the renderer, the payload type, the media stream,
       or the entire session description.

       We now define the rendering semantics for the "sdp_start" token value
       in detail.

       The SMFs and RTP MIDI streams in a session description share the same
       MIDI name space(s).  In the simple case of a single RTP MIDI stream
       and a single SMF, the SMF MIDI commands and RTP MIDI commands are
       merged into a single name space and presented to the renderer.  The
       indefinite artifact responsibilities for merged MIDI streams defined
       in Appendix C.5 also apply to merging RTP and SMF MIDI data.

       If a payload type codes multiple SMFs, the SMF name spaces are
       presented as an ordered entity to the renderer.  To determine the
       ordering of SMFs for a renderer (which SMF is "first", which is
       "second", etc.), use the following rules:

         o  If the renderer uses a single data object, the order of
            appearance of the SMFs in the object's internal structure
            defines the order of the SMFs (the earliest SMF in the object is
            "first", the next SMF in the object is "second", etc.).

         o  If multiple data objects are encoded for a renderer, the
            appearance of each data object in the parameter list sets the
            relative order of the SMFs encoded in each data object (SMFs
            encoded in parameters that appear earlier in the list are
            ordered before SMFs encoded in parameters that appear later in
            the list).

         o  If SMFs are encoded in data objects parameters and in the
            parameters defined in C.6.4.2, the relative order of the data
            object parameters and C.6.4.2 parameters in the parameter list
            sets the relative order of SMFs (SMFs encoded in parameters that
            appear earlier in the list are ordered before SMFs in parameters
            that appear later in the list).

       Given this ordering of SMFs, we now define the mapping of SMFs to
       renderer name spaces.  The SMF that appears first for a renderer maps
       to the first renderer name space.  The SMF that appears second for a
       renderer maps to the second renderer name space, etc.  If the

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       associated RTP MIDI streams also form an ordered relationship, the
       first SMF is merged with the first name space of the relationship,
       the second SMF is merged to the second name space of the
       relationship, etc.

       Unless the streams and the SMFs both use MIDI Time Code, the time
       offset between SMF and stream data is unspecified.  This restriction
       limits the use of SMFs to applications where synchronization is not
       critical, such as the transport of System Exclusive commands for
       renderer initialization, or human-SMF interactivity.

       Finally, we note that each SMF in the sdp_start discussion above
       encodes exactly one MIDI name space (16 voice channels + systems).
       Thus, the use of the Device Name SMF meta event to specify several
       MIDI name spaces in an SMF is not supported for sdp_start.

    C.6.4.2.  The smf_inline, smf_url, and smf_cid Parameters

       In some applications, the renderer data object may not encapsulate
       SMFs, but an application may wish to use SMFs in the manner defined
       in Appendix C.6.4.1.

       The "smf_inline", "smf_url", and "smf_cid" parameters address this
       situation.  These parameters use the syntax and semantics of the
       inline, url, and cid parameters defined in Appendix C.6.3, except
       that the encoded data object is an SMF.

       The "smf_inline", "smf_url", and "smf_cid" parameters belong to the
       "render" parameter that most recently precedes it in the session
       description.  The "smf_inline", "smf_url", and "smf_cid" parameters
       MUST NOT appear in parameter lists that do not use the "render"
       parameter and MUST NOT appear before the first use of "render" in the
       parameter list.  If several "smf_inline", "smf_url", or "smf_cid"
       parameters appear for a renderer, the order of the parameters defines
       the SMF name space ordering.

    C.6.4.3.  The chanmask Parameter

       The chanmask parameter instructs the renderer to ignore all MIDI
       voice commands for certain channel numbers.  The parameter value is a
       concatenated string of "1" and "0" digits.  Each string position maps
       to a MIDI voice channel number (system channels may not be masked).
       A "1" instructs the renderer to process the voice channel; a "0"
       instructs the renderer to ignore the voice channel.

       The string length of the chanmask parameter value MUST be 16 (for a
       single stream or an identity relationship) or a multiple of 16 (for
       an ordered relationship).

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       The chanmask parameter describes the "render" parameter that most
       recently precedes it in the session description; chanmask MUST NOT
       appear in parameter lists that do not use the "render" parameter and
       MUST NOT appear before the first use of "render" in the parameter
       list.

       The chanmask parameter describes the final MIDI name spaces presented
       to the renderer.  The SMF and stream components of the MIDI name
       spaces may not be independently masked.

       If a receiver is offered a session description with a renderer that
       uses the chanmask parameter, and if the receiver does not implement
       the semantics of the chanmask parameter, the receiver MUST NOT accept
       the renderer unless the chanmask parameter value contains only "1"s.

    C.6.5.  The audio/asc Media Type

       In Appendix 11.3, we register the audio/asc media type.  The data
       object for audio/asc is a binary encoding of the AudioSpecificConfig
       data block used to initialize mpeg4-generic streams (Section 6.2 and
       [MPEGAUDIO]).

       An mpeg4-generic parameter list MAY use the render, subrender, and
       rinit parameters with the audio/asc media type for renderer
       configuration.  Several restrictions apply to the use of these
       parameters in mpeg4-generic parameter lists:

         o  An mpeg4-generic media description that uses the render
            parameter MUST assign the empty string ("") to the mpeg4-generic
            "config" parameter.  The use of the streamtype, mode, and
            profile-level-id parameters MUST follow the normative text in
            Section 6.2.

         o  Sessions that use identity or ordered relationships MUST follow
            the mpeg4-generic configuration restrictions in Appendix C.5.

         o  The render parameter MUST be assigned the value "synthetic",
            "unknown", "null", or a render value that has been added to the
            IANA repository for use with mpeg4-generic RTP MIDI streams.
            The "api" token value for render MUST NOT be used.

         o  If a subrender parameter is present, it MUST immediately follow
            the render parameter, and it MUST be assigned the token value
            "default" or assigned a subrender value added to the IANA
            repository for use with mpeg4-generic RTP MIDI streams.  A
            subrender parameter assignment may be left out of the renderer
            configuration, in which case the implied value of subrender is
            the default value of "default".

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         o  If the render parameter is assigned the value "synthetic" and
            the subrender parameter has the value "default" (assigned or
            implied), the rinit parameter MUST be assigned the value
            "audio/asc", and an AudioSpecificConfig data object MUST be
            encoded using the mechanisms defined in C.6.2-3.  The
            AudioSpecificConfig data MUST encode one of the MPEG 4 Audio
            Object Types defined for use with mpeg4-generic in Section 6.2.
            If the subrender value is other than "default", refer to the
            subrender registration for information on the use of "audio/asc"
            with the renderer.

         o  If the render parameter is assigned the value "null" or
            "unknown", the data object MAY be omitted.

       Several general restrictions apply to the use of the audio/asc media
       type in RTP MIDI:

         o  A native stream MUST NOT assign "audio/asc" to rinit.  The
            audio/asc media type is not intended to be a general-purpose
            container for rendering systems outside of MPEG usage.

         o  The audio/asc media type defines a stored object type; it does
            not define semantics for RTP streams.  Thus, audio/asc MUST NOT
            appear on an rtpmap line of a session description.

       Below, we show session description examples for audio/asc.  The
       session description below uses the inline parameter to code the
       AudioSpecificConfig block for a mpeg4-generic General MIDI stream.
       We derive the value assigned to the inline parameter in Appendix E.4.
       The subrender token value of "default" is implied by the absence of
       the subrender parameter in the parameter list.

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; render=synthetic; rinit="audio/asc";
       inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"

       (The a=fmtp line has been wrapped to fit the page to accommodate
        memo formatting restrictions; it comprises a single line in SDP.)

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       The session description below uses the url parameter to code the
       AudioSpecificConfig block for the same General MIDI stream:

       v=0
       o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       m=audio 5004 RTP/AVP 96
       c=IN IP4 192.0.2.94
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; render=synthetic; rinit="audio/asc";
       url="http://example.net/oski.asc";
       cid="xjflsoeiurvpa09itnvlduihgnvet98pa3w9utnuighbuk"

       (The a=fmtp line has been wrapped to fit the page to accommodate
        memo formatting restrictions; it comprises a single line in SDP.)

    C.7.  Interoperability

       In this appendix, we define interoperability guidelines for two
       application areas:

         o  MIDI content-streaming applications.  RTP MIDI is added to
            RTSP-based content-streaming servers, so that viewers may
            experience MIDI performances (produced by a specified client-
            side renderer) in synchronization with other streams (video,
            audio).

         o  Long-distance network musical performance applications.  RTP
            MIDI is added to SIP-based voice chat or videoconferencing
            programs, as an alternative, or as an addition, to audio and/or
            video RTP streams.

       For each application, we define a core set of functionality that all
       implementations MUST implement.

       The applications we address in this section are not an exhaustive
       list of potential RTP MIDI uses.  We expect framework documents for
       other applications to be developed, within the IETF or within other
       organizations.  We discuss other potential application areas for RTP
       MIDI in Section 1 of the main text of this memo.

    C.7.1.  MIDI Content Streaming Applications

       In content-streaming applications, a user invokes an RTSP client to
       initiate a request to an RTSP server to view a multimedia session.
       For example, clicking on a web page link for an Internet Radio

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       channel launches an RTSP client that uses the link's RTSP URL to
       contact the RTSP server hosting the radio channel.

       The content may be pre-recorded (for example, on-demand replay of
       yesterday's football game) or "live" (for example, football game
       coverage as it occurs), but in either case the user is usually an
       "audience member" as opposed to a "participant" (as the user would be
       in telephony).

       Note that these examples describe the distribution of audio content
       to an audience member.  The interoperability guidelines in this
       appendix address RTP MIDI applications of this nature, not
       applications such as the transmission of raw MIDI command streams for
       use in a professional environment (recording studio, performance
       stage, etc.).

       In an RTSP session, a client accesses a session description that is
       "declared" by the server, either via the RTSP DESCRIBE method, or via
       other means, such as HTTP or email.  The session description defines
       the session from the perspective of the client.  For example, if a
       media line in the session description contains a non-zero port
       number, it encodes the server's preference for the client's port
       numbers for RTP and RTCP reception.  Once media flow begins, the
       server sends an RTP MIDI stream to the client, which renders it for
       presentation, perhaps in synchrony with video or other audio streams.

       We now define the interoperability text for content-streaming RTSP
       applications.

       In most cases, server interoperability responsibilities are described
       in terms of limits on the "reference" session description a server
       provides for a performance if it has no information about the
       capabilities of the client.  The reference session is a "lowest
       common denominator" session that maximizes the odds that a client
       will be able to view the session.  If a server is aware of the
       capabilities of the client, the server is free to provide a session
       description customized for the client in the DESCRIBE reply.

       Clients MUST support unicast UDP RTP MIDI streams that use the
       recovery journal with the closed-loop or the anchor sending policies.
       Clients MUST be able to interpret stream subsetting and chapter
       inclusion parameters in the session description that qualify the
       sending policies.  Client support of enhanced Chapter C encoding is
       OPTIONAL.

       The reference session description offered by a server MUST send all
       RTP MIDI UDP streams as unicast streams that use the recovery journal
       and the closed-loop or anchor sending policies.  Servers SHOULD use

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       the stream subsetting and chapter inclusion parameters in the
       reference session description, to simplify the rendering task of the
       client.  Server support of enhanced Chapter C encoding is OPTIONAL.

       Clients and servers MUST support the use of RTSP interleaved mode (a
       method for interleaving RTP onto the RTSP TCP transport).

       Clients MUST be able to interpret the timestamp semantics signalled
       by the "comex" value of the tsmode parameter (i.e., the timestamp
       semantics of Standard MIDI Files [MIDI]).  Servers MUST use the
       "comex" value for the "tsmode" parameter in the reference session
       description.

       Clients MUST be able to process an RTP MIDI stream whose packets
       encode an arbitrary temporal duration ("media time").  Thus, in
       practice, clients MUST implement a MIDI playout buffer.  Clients MUST
       NOT depend on the presence of rtp_ptime, rtp_maxtime, and guardtime
       parameters in the session description in order to process packets,
       but they SHOULD be able to use these parameters to improve packet
       processing.

       Servers SHOULD strive to send RTP MIDI streams in the same way media
       servers send conventional audio streams: a sequence of packets that
       either all code the same temporal duration (non-normative example: 50
       ms packets) or that code one of an integral number of temporal
       durations (non-normative example: 50 ms, 100 ms, 250 ms, or 500 ms
       packets).  Servers SHOULD encode information about the packetization
       method in the rtp_ptime and rtp_maxtime parameters in the session
       description.

       Clients MUST be able to examine the render and subrender parameter,
       to determine if a multimedia session uses a renderer it supports.
       Clients MUST be able to interpret the default "one" value of the
       "multimode" parameter, to identify supported renderers from a list of
       renderer descriptions.  Clients MUST be able to interpret the
       musicport parameter, to the degree that it is relevant to the
       renderers it supports.  Clients MUST be able to interpret the
       chanmask parameter.

       Clients supporting renderers whose data object (as encoded by a
       parameter value for "inline") could exceed 300 octets in size MUST
       support the url and cid parameters and thus must implement the HTTP
       protocol in addition to RTSP.

       Servers MUST specify complete rendering systems for RTP MIDI streams.
       Note that a minimal RTP MIDI native stream does not meet this
       requirement (Section 6.1), as the rendering method for such streams
       is "not specified".

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       At the time of this memo, the only way for servers to specify a
       complete rendering system is to specify an mpeg4-generic RTP MIDI
       stream in mode rtp-midi (Section 6.2 and C.6.5).  As a consequence,
       the only rendering systems that may be presently used are General
       MIDI [MIDI], DLS 2 [DLS2], or Structured Audio [MPEGSA].  Note that
       the maximum inline value for General MIDI is well under 300 octets
       (and thus clients need not support the "url" parameter), and that the
       maximum inline values for DLS 2 and Structured Audio may be much
       larger than 300 octets (and thus clients MUST support the url
       parameter).

       We anticipate that the owners of rendering systems (both standardized
       and proprietary) will register subrender parameters for their
       renderers.  Once registration occurs, native RTP MIDI sessions may
       use render and subrender (Appendix C.6.2) to specify complete
       rendering systems for RTSP content-streaming multimedia sessions.

       Servers MUST NOT use the sdp_start value for the smf_info parameter
       in the reference session description, as this use would require that
       clients be able to parse and render Standard MIDI Files.

       Clients MUST support mpeg4-generic mode rtp-midi General MIDI (GM)
       sessions, at a polyphony limited by the hardware capabilities of the
       client.  This requirement provides a "lowest common denominator"
       rendering system for content providers to target.  Note that this
       requirement does not force implementors of a non-GM renderer (such as
       DLS 2 or Structured Audio) to add a second rendering engine.
       Instead, a client may satisfy the requirement by including a set of
       voice patches that implement the GM instrument set, and using this
       emulation for mpeg4-generic GM sessions.

       It is RECOMMENDED that servers use General MIDI as the renderer for
       the reference session description, because clients are REQUIRED to
       support it.  We do not require General MIDI as the reference
       renderer, because for normative applications it is an inappropriate
       choice.  Servers using General MIDI as a "lowest common denominator"
       renderer SHOULD use Universal Real-Time SysEx MIP message [SPMIDI] to
       communicate the priority of voices to polyphony-limited clients.

    C.7.2.  MIDI Network Musical Performance Applications

       In Internet telephony and videoconferencing applications, parties
       interact over an IP network as they would face-to-face.  Good user
       experiences require low end-to-end audio latency and tight
       audiovisual synchronization (for "lip-sync").  The Session Initiation
       Protocol (SIP, [RFC3261]) is used for session management.

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       In this appendix section, we define interoperability guidelines for
       using RTP MIDI streams in interactive SIP applications.  Our primary
       interest is supporting Network Musical Performances (NMP), where
       musicians in different locations interact over the network as if they
       were in the same room.  See [NMP] for background information on NMP,
       and see [RFC4696] for a discussion of low-latency RTP MIDI
       implementation techniques for NMP.

       Note that the goal of NMP applications is telepresence: the parties
       should hear audio that is close to what they would hear if they were
       in the same room.  The interoperability guidelines in this appendix
       address RTP MIDI applications of this nature, not applications such
       as the transmission of raw MIDI command streams for use in a
       professional environment (recording studio, performance stage, etc.).

       We focus on session management for two-party unicast sessions that
       specify a renderer for RTP MIDI streams.  Within this limited scope,
       the guidelines defined here are sufficient to let applications
       interoperate.  We define the REQUIRED capabilities of RTP MIDI
       senders and receivers in NMP sessions and define how session
       descriptions exchanged are used to set up network musical performance
       sessions.

       SIP lets parties negotiate details of the session, using the
       Offer/Answer protocol [RFC3264].  However, RTP MIDI has so many
       parameters that "blind" negotiations between two parties using
       different applications might not yield a common session
       configuration.

       Thus, we now define a set of capabilities that NMP parties MUST
       support.  Session description offers whose options lie outside the
       envelope of REQUIRED party behavior risk negotiation failure.  We
       also define session description idioms that the RTP MIDI part of an
       offer MUST follow, in order to structure the offer for simpler
       analysis.

       We use the term "offerer" for the party making a SIP offer, and
       "answerer" for the party answering the offer.  Finally, we note that
       unless it is qualified by the adjective "sender" or "receiver", a
       statement that a party MUST support X implies that it MUST support X
       for both sending and receiving.

       If an offerer wishes to define a "sendrecv" RTP MIDI stream, it may
       use a true sendrecv session or the "virtual sendrecv" construction
       described in the preamble to Appendix C and in Appendix C.5.  A true
       sendrecv session indicates that the offerer wishes to participate in
       a session where both parties use identically configured renderers.  A
       virtual sendrecv session indicates that the offerer is willing to

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       participate in a session where the two parties may be using different
       renderer configurations.  Thus, parties MUST be prepared to see both
       real and virtual sendrecv sessions in an offer.

       Parties MUST support unicast UDP transport of RTP MIDI streams.
       These streams MUST use the recovery journal with the closed-loop or
       anchor sending policies.  These streams MUST use the stream
       subsetting and chapter inclusion parameters to declare the types of
       MIDI commands that will be sent on the stream (for sendonly streams)
       or will be processed (for recvonly streams), including the size
       limits on System Exclusive commands.  Support of enhanced Chapter C
       encoding is OPTIONAL.

       Note that both TCP and multicast UDP support are OPTIONAL.  We make
       TCP OPTIONAL because we expect NMP renderers to rely on data objects
       (signalled by "rinit" and associated parameters) for initialization
       at the start of the session, and only to use System Exclusive
       commands for interactive control during the session.  These
       interactive commands are small enough to be protected via the
       recovery journal mechanism of RTP MIDI UDP streams.

       We now discuss timestamps, packet timing, and packet sending
       algorithms.

       Recall that the tsmode parameter controls the semantics of command
       timestamps in the MIDI list of RTP packets.

       Parties MUST support clock rates of 44.1 kHz, 48 kHz, 88.2 kHz, and
       96 kHz.  Parties MUST support streams using the "comex", "async", and
       "buffer" tsmode values.  Recvonly offers MUST offer the default
       "comex".

       Parties MUST support a wide range of packet temporal durations: from
       rtp_ptime and rtp_maxptime values of 0, to rtp_ptime and rtp_maxptime
       values that code 100 ms.  Thus, receivers MUST be able to implement a
       playout buffer.

       Offers and answers MUST present rtp_ptime, rtp_maxptime, and
       guardtime values that support the latency that users would expect in
       the application, subject to bandwidth constraints.  As senders MUST
       abide by values set for these parameters in a session description, a
       receiver SHOULD use these values to size its playout buffer to
       produce the lowest reliable latency for a session.  Implementers
       should refer to [RFC4696] for information on packet sending
       algorithms for latency-sensitive applications.  Parties MUST be able
       to implement the semantics of the guardtime parameter, for times from
       5 ms to 5000 ms.

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       We now discuss the use of the render parameter.

       Sessions MUST specify complete rendering systems for all RTP MIDI
       streams.  Note that a minimal RTP MIDI native stream does not meet
       this requirement (Section 6.1), as the rendering method for such
       streams is "not specified".

       At the time this writing, the only way for parties to specify a
       complete rendering system is to specify an mpeg4-generic RTP MIDI
       stream in mode rtp-midi (Section 6.2 and C.6.5).  We anticipate that
       the owners of rendering systems (both standardized and proprietary)
       will register subrender values for their renderers.  Once IANA
       registration occurs, native RTP MIDI sessions may use render and
       subrender (Appendix C.6.2) to specify complete rendering systems for
       SIP network musical performance multimedia sessions.

       All parties MUST support General MIDI (GM) sessions, at a polyphony
       limited by the hardware capabilities of the party.  This requirement
       provides a "lowest common denominator" rendering system, without
       which practical interoperability will be quite difficult.  When using
       GM, parties SHOULD use Universal Real-Time SysEx MIP message [SPMIDI]
       to communicate the priority of voices to polyphony-limited clients.

       Note that this requirement does not force implementors of a non-GM
       renderer (for mpeg4-generic sessions, DLS 2, or Structured Audio) to
       add a second rendering engine.  Instead, a client may satisfy the
       requirement by including a set of voice patches that implement the GM
       instrument set, and using this emulation for mpeg4-generic GM
       sessions.  We require GM support so that an offerer that wishes to
       maximize interoperability may do so by offering GM if its preferred
       renderer is not accepted by the answerer.

       Offerers MUST NOT present several renderers as options in a session
       description by listing several payload types on a media line, as
       Section 2.1 uses this construct to let a party send several RTP MIDI
       streams in the same RTP session.

       Instead, an offerer wishing to present rendering options SHOULD offer
       a single payload type that offers several renderers.  In this
       construct, the parameter list codes a list of render parameters (each
       followed by its support parameters).  As discussed in Appendix C.6.1,
       the order of renderers in the list declares the offerer's preference.
       The "unknown" and "null" values MUST NOT appear in the offer.  The
       answer MUST set all render values except the desired renderer to
       "null".  Thus, "unknown" MUST NOT appear in the answer.

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       We use SHOULD instead of MUST in the first sentence in the paragraph
       above, because this technique does not work in all situations
       (example:  an offerer wishes to offer both mpeg4-generic renderers
       and native RTP MIDI renderers as options).  In this case, the offerer
       MUST present a series of session descriptions, each offering a single
       renderer, until the answerer accepts a session description.

       Parties MUST support the musicport, chanmask, subrender, rinit, and
       inline parameters.  Parties supporting renderers whose data object
       (as encoded by a parameter value for "inline") could exceed 300
       octets in size MUST support the url and cid parameters and thus must
       implement HTTP protocol.  Note that in mpeg4-generic, General MIDI
       data objects cannot exceed 300 octets, but DLS 2 and Structured Audio
       data objects may.  Support for the other rendering parameters
       (smf_cif, smf_info, smf_inline, smf_url) is OPTIONAL.

       Thus far in this document, our discussion has assumed that the only
       MIDI flows that drive a renderer are the network flows described in
       the session description.  In NMP applications, this assumption would
       require two rendering engines: one for local use by a party, a second
       for the remote party.

       In practice, applications may wish to have both parties share a
       single rendering engine.  In this case, the session description MUST
       use a virtual sendrecv session and MUST use the stream subsetting and
       chapter inclusion parameters to allocate which MIDI channels are
       intended for use by a party.  If two parties are sharing a MIDI
       channels, the application MUST ensure that appropriate MIDI merging
       occurs at the input to the renderer.

       We now discuss the use of (non-MIDI) audio streams in the session.

       Audio streams may be used for two purposes: as a "talkback" channel
       for parties to converse, or as a way to conduct a performance that
       includes MIDI and audio channels.  In the latter case, offers MUST
       use sample rates and the packet temporal durations for the audio and
       MIDI streams that support low-latency synchronized rendering.

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       We now show an example of an offer/answer exchange in a network
       musical performance application (next page).  Below, we show an offer
       that complies with the interoperability text in this appendix
       section.

       v=0
       o=first 2520644554 2838152170 IN IP4 first.example.net
       s=Example
       t=0 0
       a=group:FID 1 2
       c=IN IP4 192.0.2.94
       m=audio 16112 RTP/AVP 96
       a=recvonly
       a=mid:1
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=2NPTW;
       cm_used=2C0.1.7.10.11.64.121.123; cm_used=2M0.1.2
       cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
       ch_default=2NPTW; ch_default=2C0.1.7.10.11.64.121.123;
       ch_default=2M0.1.2; cm_default=X0-16;
       rtp_ptime=0; rtp_maxptime=0; guardtime=44100;
       musicport=1; render=synthetic; rinit="audio/asc";
       inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
       m=audio 16114 RTP/AVP 96
       a=sendonly
       a=mid:2
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=1NPTW;
       cm_used=1C0.1.7.10.11.64.121.123; cm_used=1M0.1.2
       cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
       ch_default=1NPTW; ch_default=1C0.1.7.10.11.64.121.123;
       ch_default=1M0.1.2; cm_default=X0-16;
       rtp_ptime=0; rtp_maxptime=0; guardtime=44100;
       musicport=1; render=synthetic; rinit="audio/asc";
       inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"

       (The a=fmtp lines have been wrapped to fit the page to accommodate
        memo formatting restrictions; it comprises a single line in SDP.)

       The owner line (o=) identifies the session owner as "first".

       The session description defines two MIDI streams: a recvonly stream
       on which "first" receives a performance, and a sendonly stream that
       "first" uses to send a performance.  The recvonly port number encodes
       the ports on which "first" wishes to receive RTP (16112) and RTCP
       (16113) media at IP4 address 192.0.2.94.  The sendonly port number

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       encodes the port on which "first" wishes to receive RTCP for the
       stream (16115).

       The musicport parameters code that the two streams share and identity
       relationship and thus form a virtual sendrecv stream.

       Both streams are mpeg4-generic RTP MIDI streams that specify a
       General MIDI renderer.  The stream subsetting parameters code that
       the recvonly stream uses MIDI channel 1 exclusively for voice
       commands, and that the sendonly stream uses MIDI channel 2
       exclusively for voice commands.  This mapping permits the application
       software to share a single renderer for local and remote performers.

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       We now show the answer to the offer.

       v=0
       o=second 2520644554 2838152170 IN IP4 second.example.net
       s=Example
       t=0 0
       a=group:FID 1 2
       c=IN IP4 192.0.2.105
       m=audio 5004 RTP/AVP 96
       a=sendonly
       a=mid:1
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=2NPTW;
       cm_used=2C0.1.7.10.11.64.121.123; cm_used=2M0.1.2
       cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
       ch_default=2NPTW; ch_default=2C0.1.7.10.11.64.121.123;
       ch_default=2M0.1.2; cm_default=X0-16;
       rtp_ptime=0; rtp_maxptime=882; guardtime=44100;
       musicport=1; render=synthetic; rinit="audio/asc";
       inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
       m=audio 5006 RTP/AVP 96
       a=recvonly
       a=mid:2
       a=rtpmap:96 mpeg4-generic/44100
       a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
       profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=1NPTW;
       cm_used=1C0.1.7.10.11.64.121.123; cm_used=1M0.1.2
       cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
       ch_default=1NPTW; ch_default=1C0.1.7.10.11.64.121.123;
       ch_default=1M0.1.2; cm_default=X0-16;
       rtp_ptime=0; rtp_maxptime=0; guardtime=88200;
       musicport=1; render=synthetic; rinit="audio/asc";
       inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"

       (The a=fmtp lines have been wrapped to fit the page to accommodate
        memo formatting restrictions; they comprise single lines in SDP.)

       The owner line (o=) identifies the session owner as "second".

       The port numbers for both media streams are non-zero; thus, "second"
       has accepted the session description.  The stream marked "sendonly"
       in the offer is marked "recvonly" in the answer, and vice versa,
       coding the different view of the session held by "session".  The IP4
       number (192.0.2.105) and the RTP (5004 and 5006) and RTCP (5005 and
       5007) have been changed by "second" to match its transport wishes.

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       In addition, "second" has made several parameter changes:
       rtp_maxptime for the sendonly stream has been changed to code 2 ms
       (441 in clock units), and the guardtime for the recvonly stream has
       been doubled.  As these parameter modifications request capabilities
       that are REQUIRED to be implemented by interoperable parties,
       "second" can make these changes with confidence that "first" can
       abide by them.

    D.  Parameter Syntax Definitions

       In this appendix, we define the syntax for the RTP MIDI media type
       parameters in Augmented Backus-Naur Form (ABNF, [RFC4234]).  When
       using these parameters with SDP, all parameters MUST appear on a
       single fmtp attribute line of an RTP MIDI media description.  For
       mpeg4-generic RTP MIDI streams, this line MUST also include any
       mpeg4-generic parameters (usage described in Section 6.2).  An fmtp
       attribute line may be defined (after [RFC3640]) as:

       ;
       ; SDP fmtp line definition
       ;

       fmtp = "a=fmtp:" token SP param-assign 0*(";" SP param-assign) CRLF

       where <token> codes the RTP payload type.  Note that white space MUST
       NOT appear between the "a=fmtp:" and the RTP payload type.

       We now define the syntax of the parameters defined in Appendix C.
       The definition takes the form of the incremental assembly of the
       <param-assign> token.  See [RFC3640] for the syntax of the
       mpeg4-generic parameters discussed in Section 6.2.

       ;
       ;
       ; top-level definition for all parameters
       ;
       ;

       ;
       ; Parameters defined in Appendix C.1

       param-assign =   ("cm_unused="  (([channel-list] command-type
                                         [f-list]) / sysex-data))

       param-assign =/  ("cm_used="    (([channel-list] command-type
                                         [f-list]) / sysex-data))

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       ;
       ; Parameters defined in Appendix C.2

       param-assign =/  ("j_sec="      ("none" / "recj" / *ietf-extension))

       param-assign =/  ("j_update="   ("anchor" / "closed-loop" /
                                        "open-loop" / *ietf-extension))

       param-assign =/  ("ch_default=" (([channel-list] chapter-list
                                         [f-list]) / sysex-data))

       param-assign =/  ("ch_never="   (([channel-list] chapter-list
                                         [f-list]) / sysex-data))

       param-assign =/  ("ch_anchor="  (([channel-list] chapter-list
                                         [f-list]) / sysex-data))

       ;
       ; Parameters defined in Appendix C.3

       param-assign =/  ("tsmode="     ("comex" / "async" / "buffer"))

       param-assign =/  ("linerate="    nonzero-four-octet)

       param-assign =/  ("octpos="      ("first" / "last"))

       param-assign =/  ("mperiod="     nonzero-four-octet)

       ;
       ; Parameter defined in Appendix C.4

       param-assign =/  ("guardtime="     nonzero-four-octet)

       param-assign =/  ("rtp_ptime="     four-octet)

       param-assign =/  ("rtp_maxptime="  four-octet)

       ;
       ; Parameters defined in Appendix C.5

       param-assign =/  ("musicport="     four-octet)

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       ;
       ; Parameters defined in Appendix C.6

       param-assign =/  ("chanmask="     ( 1*( 16( "0" / "1" ) )))

       param-assign =/  ("cid="          double-quote cid-block
                                         double-quote)

       param-assign =/  ("inline="       double-quote base-64-block
                                         double-quote)

       param-assign =/  ("multimode="    ("all" / "one"))

       param-assign =/  ("render="       ("synthetic" / "api" / "null" /
                                          "unknown" / *extension))

       param-assign =/  ("rinit="        mime-type "/" mime-subtype)

       param-assign =/  ("smf_cid="      double-quote cid-block
                                         double-quote)

       param-assign =/  ("smf_info="     ("ignore" / "identity" /
                                         "sdp_start" / *extension))

       param-assign =/  ("smf_inline="   double-quote base-64-block
                                         double-quote)

       param-assign =/  ("smf_url="      double-quote uri-element
                                         double-quote)

       param-assign =/  ("subrender="    ("default" / *extension))

       param-assign =/  ("url="          double-quote uri-element
                                         double-quote)

       ;
       ; list definitions for the cm_ command-type
       ;

       command-type    = command-part1 command-part2 command-part3

       command-part1   = (*1"A") (*1"B") (*1"C") (*1"F") (*1"G") (*1"H")

       command-part2   = (*1"J") (*1"K") (*1"M") (*1"N") (*1"P") (*1"Q")

       command-part3   = (*1"T") (*1"V") (*1"W") (*1"X") (*1"Y") (*1"Z")

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       ;
       ; list definitions for the ch_ chapter-list
       ;

       chapter-list  =  ch-part1 ch-part2 ch-part3

       ch-part1  = (*1"A") (*1"B") (*1"C") (*1"D") (*1"E") (*1"F") (*1"G")

       ch-part2  = (*1"H") (*1"J") (*1"K") (*1"M") (*1"N") (*1"P") (*1"Q")

       ch-part3  = (*1"T") (*1"V") (*1"W") (*1"X") (*1"Y") (*1"Z")

       ;
       ; list definitions for the ch_ channel-list
       ;

       channel-list       = midi-chan-element *("." midi-chan-element)

       midi-chan-element  = midi-chan / midi-chan-range

       midi-chan-range    = midi-chan "-" midi-chan

                          ; decimal value of left midi-chan
                          ; MUST be strictly less than decimal
                          ; value of right midi-chan

       midi-chan          = %d0-15

       ;
       ; list definitions for the ch_ field list (f-list)
       ;

       f-list             = midi-field-element *("." midi-field-element)

       midi-field-element = midi-field / midi-field-range

       midi-field-range   = midi-field "-" midi-field
                          ;
                          ; decimal value of left midi-field
                          ; MUST be strictly less than decimal
                          ; value of right midi-field

       midi-field         = four-octet
                          ;
                          ; large range accommodates Chapter M
                          ; RPN (0-16383) and NRPN (16384-32767)
                          ; parameters, and Chapter X octet sizes.

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       ;
       ; definitions for ch_ sysex-data
       ;

       sysex-data         = "__"  h-list *("_" h-list) "__"

       h-list             = hex-field-element *("." hex-field-element)

       hex-field-element  = hex-octet / hex-field-range

       hex-field-range    = hex-octet "-" hex-octet
                          ;
                          ; hexadecimal value of left hex-octet
                          ; MUST be strictly less than hexadecimal
                          ; value of right hex-octet

       hex-octet          = 2("0" / "1" / "2"/ "3" / "4" /
                              "5" / "6" / "7" / "8" / "9" /
                              "A" / "B" / "C" / "D" / "E" / "F")
                          ;
                          ; rewritten version of hex-octet in [RFC2045]
                          ; (page 23).
                          ; note that a-f are not permitted, only A-F.
                          ; hex-octet values MUST NOT exceed 7F.

       ;
       ; definitions for rinit parameter
       ;

       mime-type          = "audio" / "application"

       mime-subtype       = token
                          ;
                          ; See Appendix C.6.2 for registration
                          ; requirements for rinit type/subtypes.

       ;
       ; definitions for base64 encoding
       ; copied from [RFC4566]

       base-64-block      = *base64-unit [base64-pad]

       base64-unit        =  4base64-char

       base64-pad         =  2base64-char "==" / 3base64-char "="

       base64-char        =  %x41-5A / %x61-7A / %x30-39 / "+" / "/"
                          ;  A-Z, a-z, 0-9, "+" and "/"

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       ;
       ; generic rules
       ;

       ietf-extension     = token
                          ;
                          ; ietf-extension may only be defined in
                          ; standards-track RFCs.

       extension          = token
                          ;
                          ; extension may be defined by filing
                          ; a registration with IANA.

       four-octet         = %d0-4294967295
                          ; unsigned encoding of 32-bits

       nonzero-four-octet = %d1-4294967295
                          ; unsigned encoding of 32-bits, ex-zero

       uri-element        = URI-reference
                          ; as defined in [RFC3986]

       double-quote       = %x22

                          ; the double-quote (") character

       token              =  1*token-char
                          ; copied from [RFC4566]

       token-char         =  %x21 / %x23-27 / %x2A-2B / %x2D-2E /
                             %x30-39 / %x41-5A / %x5E-7E
                          ; copied from [RFC4566]

       cid-block          = 1*cid-char

       cid-char           =  token-char
       cid-char           =/  "@"
       cid-char           =/  ","
       cid-char           =/  ";"
       cid-char           =/  ":"
       cid-char           =/  "\"
       cid-char           =/  "/"
       cid-char           =/  "["
       cid-char           =/  "]"
       cid-char           =/  "?"
       cid-char           =/  "="

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                          ;
                          ; add back in the tspecials [RFC2045], except for
                          ; double-quote and the non-email safe () <>
                          ; note that "cid" defined above ensures that
                          ; cid-block is enclosed with double-quotes

       ; external references
       ; URI-reference: from [RFC3986]

       ;
       ; End of ABNF

       The mpeg4-generic RTP payload [RFC3640] defines a "mode" parameter
       that signals the type of MPEG stream in use.  We add a new mode
       value, "rtp-midi", using the ABNF rule below:

       ;
       ; mpeg4-generic mode parameter extension
       ;

       mode              =/ "rtp-midi"
                         ; as described in Section 6.2 of this memo

    E.  A MIDI Overview for Networking Specialists

       This appendix presents an overview of the MIDI standard, for the
       benefit of networking specialists new to musical applications.
       Implementors should consult [MIDI] for a normative description of
       MIDI.

       Musicians make music by performing a controlled sequence of physical
       movements.  For example, a pianist plays by coordinating a series of
       key presses, key releases, and pedal actions.  MIDI represents a
       musical performance by encoding these physical gestures as a sequence
       of MIDI commands.  This high-level musical representation is compact
       but fragile: one lost command may be catastrophic to the performance.

       MIDI commands have much in common with the machine instructions of a
       microprocessor.  MIDI commands are defined as binary elements.
       Bitfields within a MIDI command have a regular structure and a
       specialized purpose.  For example, the upper nibble of the first
       command octet (the opcode field) codes the command type.  MIDI
       commands may consist of an arbitrary number of complete octets, but
       most MIDI commands are 1, 2, or 3 octets in length.

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           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |     Channel Voice Messages     |      Bitfield Pattern      |
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           | NoteOff (end a note)           | 1000cccc 0nnnnnnn 0vvvvvvv |
           |-------------------------------------------------------------|
           | NoteOn (start a note)          | 1001cccc 0nnnnnnn 0vvvvvvv |
           |-------------------------------------------------------------|
           | PTouch (Polyphonic Aftertouch) | 1010cccc 0nnnnnnn 0aaaaaaa |
           |-------------------------------------------------------------|
           | CControl (Controller Change)   | 1011cccc 0xxxxxxx 0yyyyyyy |
           |-------------------------------------------------------------|
           | PChange (Program Change)       | 1100cccc 0ppppppp          |
           |-------------------------------------------------------------|
           | CTouch (Channel Aftertouch)    | 1101cccc 0aaaaaaa          |
           |-------------------------------------------------------------|
           | PWheel (Pitch Wheel)           | 1110cccc 0xxxxxxx 0yyyyyyy |
            -------------------------------------------------------------

                     Figure E.1 -- MIDI Channel Messages

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           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      System Common Messages    |     Bitfield Pattern       |
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           | System Exclusive               | 11110000, followed by a    |
           |                                | list of 0xxxxxx octets,    |
           |                                | followed by 11110111       |
           |-------------------------------------------------------------|
           | MIDI Time Code Quarter Frame   | 11110001 0xxxxxxx          |
           |-------------------------------------------------------------|
           | Song Position Pointer          | 11110010 0xxxxxxx 0yyyyyyy |
           |-------------------------------------------------------------|
           | Song Select                    | 11110011 0xxxxxxx          |
           |-------------------------------------------------------------|
           | Undefined                      | 11110100                   |
           |-------------------------------------------------------------|
           | Undefined                      | 11110101                   |
           |-------------------------------------------------------------|
           | Tune Request                   | 11110110                   |
           |-------------------------------------------------------------|
           | System Exclusive End Marker    | 11110111                   |
            -------------------------------------------------------------

           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |    System Realtime Messages    |     Bitfield Pattern       |
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           | Clock                          | 11111000                   |
           |-------------------------------------------------------------|
           | Undefined                      | 11111001                   |
           |-------------------------------------------------------------|
           | Start                          | 11111010                   |
           |-------------------------------------------------------------|
           | Continue                       | 11111011                   |
           |-------------------------------------------------------------|
           | Stop                           | 11111100                   |
           |-------------------------------------------------------------|
           | Undefined                      | 11111101                   |
           |-------------------------------------------------------------|
           | Active Sense                   | 11111110                   |
           |-------------------------------------------------------------|
           | System Reset                   | 11111111                   |
            -------------------------------------------------------------

                          Figure E.2 -- MIDI System Messages

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       Figure E.1 and E.2 show the MIDI command family.  There are three
       major classes of commands: voice commands (opcode field values in the
       range 0x8 through 0xE), system common commands (opcode field 0xF,
       commands 0xF0 through 0xF7), and system real-time commands (opcode
       field 0xF, commands 0xF8 through 0xFF).  Voice commands code the
       musical gestures for each timbre in a composition.  Systems commands
       perform functions that usually affect all voice channels, such as
       System Reset (0xFF).

    E.1.  Commands Types

       Voice commands execute on one of 16 MIDI channels, as coded by its
       4-bit channel field (field cccc in Figure E.1).  In most
       applications, notes for different timbres are assigned to different
       channels.  To support applications that require more than 16
       channels, MIDI systems use several MIDI command streams in parallel,
       to yield 32, 48, or 64 MIDI channels.

       As an example of a voice command, consider a NoteOn command (opcode
       0x9), with binary encoding 1001cccc 0nnnnnnn 0aaaaaaa.  This command
       signals the start of a musical note on MIDI channel cccc.  The note
       has a pitch coded by the note number nnnnnnn, and an onset amplitude
       coded by note velocity aaaaaaa.

       Other voice commands signal the end of notes (NoteOff, opcode 0x8),
       map a specific timbre to a MIDI channel (PChange, opcode 0xC), or set
       the value of parameters that modulate the timbral quality (all other
       voice commands).  The exact meaning of most voice channel commands
       depends on the rendering algorithms the MIDI receiver uses to
       generate sound.  In most applications, a MIDI sender has a model (in
       some sense) of the rendering method used by the receiver.

       System commands perform a variety of global tasks in the stream,
       including "sequencer" playback control of pre-recorded MIDI commands
       (the Song Position Pointer, Song Select, Clock, Start, Continue, and
       Stop messages), SMPTE time code (the MIDI Time Code Quarter Frame
       command), and the communication of device-specific data (the System
       Exclusive messages).

    E.2.  Running Status

       All MIDI command bitfields share a special structure: the leading bit
       of the first octet is set to 1, and the leading bit of all subsequent
       octets is set to 0.  This structure supports a data compression
       system, called running status [MIDI], that improves the coding
       efficiency of MIDI.

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       In running status coding, the first octet of a MIDI voice command may
       be dropped if it is identical to the first octet of the previous MIDI
       voice command.  This rule, in combination with a convention to
       consider NoteOn commands with a null third octet as NoteOff commands,
       supports the coding of note sequences using two octets per command.

       Running status coding is only used for voice commands.  The presence
       of a system common message in the stream cancels running status mode
       for the next voice command.  However, system real-time messages do
       not cancel running status mode.

    E.3.  Command Timing

       The bitfield formats in Figures E.1 and E.2 do not encode the
       execution time for a command.  Timing information is not a part of
       the MIDI command syntax itself; different applications of the MIDI
       command language use different methods to encode timing.

       For example, the MIDI command set acts as the transport layer for
       MIDI 1.0 DIN cables [MIDI].  MIDI cables are short asynchronous
       serial lines that facilitate the remote operation of musical
       instruments and audio equipment.  Timestamps are not sent over a MIDI
       1.0 DIN cable.  Instead, the standard uses an implicit "time of
       arrival" code.  Receivers execute MIDI commands at the moment of
       arrival.

       In contrast, Standard MIDI Files (SMFs, [MIDI]), a file format for
       representing complete musical performances, add an explicit timestamp
       to each MIDI command, using a delta encoding scheme that is optimized
       for statistics of musical performance.  SMF timestamps usually code
       timing using the metric notation of a musical score.  SMF meta-events
       are used to add a tempo map to the file, so that score beats may be
       accurately converted into units of seconds during rendering.

    E.4.  AudioSpecificConfig Templates for MMA Renderers

       In Section 6.2 and Appendix C.6.5, we describe how session
       descriptions include an AudioSpecificConfig data block to specify a
       MIDI rendering algorithm for mpeg4-generic RTP MIDI streams.

       The bitfield format of AudioSpecificConfig is defined in [MPEGAUDIO].
       StructuredAudioSpecificConfig, a key data structure coded in
       AudioSpecificConfig, is defined in [MPEGSA].

       For implementors wishing to specify Structured Audio renderers, a
       full understanding of [MPEGSA] and [MPEGAUDIO] is essential.
       However, many implementors will limit their rendering options to the
       two MIDI Manufacturers Association renderers that may be specified in

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       AudioSpecificConfig: General MIDI (GM, [MIDI]) and Downloadable
       Sounds 2 (DLS 2, [DLS2]).

       To aid these implementors, we reproduce the AudioSpecificConfig
       bitfield formats for a GM renderer and a DLS 2 renderer below.  We
       have checked these bitfields carefully and believe they are correct.
       However, we stress that the material below is informative, and that
       [MPEGAUDIO] and [MPEGSA] are the normative definitions for
       AudioSpecificConfig.

       As described in Section 6.2, a minimal mpeg4-generic session
       description encodes the AudioSpecificConfig binary bitfield as a
       hexadecimal string (whose format is defined in [RFC3640]) that is
       assigned to the "config" parameter.  As described in Appendix C.6.3,
       a session description that uses the render parameter encodes the
       AudioSpecificConfig binary bitfield as a Base64-encoded string
       assigned to the "inline" parameter, or in the body of an HTTP URL
       assigned to the "url" parameter.

       Below, we show a simplified binary AudioSpecificConfig bitfield
       format, suitable for sending and receiving GM and DLS 2 data:

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | AOTYPE  |FREQIDX|CHANNEL|SACNK|  FILE_BLK 1 (required) ...    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |1|SACNK|              FILE_BLK 2 (optional) ...                |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  ...  |1|SACNK| FILE_BLK N (optional) ...                     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0|0|        (first "0" bit terminates FILE_BLK list)
          +-+-+

                      Figure E.3 -- Simplified AudioSpecificConfig

       The 5-bit AOTYPE field specifies the Audio Object Type as an unsigned
       integer.  The legal values for use with mpeg4-generic RTP MIDI
       streams are "15" (General MIDI), "14" (DLS 2), and "13" (Structured
       Audio).  Thus, receivers that do not support all three mpeg4-generic
       renderers may parse the first 5 bits of an AudioSpecificConfig coded
       in a session description and reject sessions that specify unsupported
       renderers.

       The 4-bit FREQIDX field specifies the sampling rate of the renderer.
       We show the mapping of FREQIDX values to sampling rates in Figure
       E.4.  Senders MUST specify a sampling frequency that matches the RTP
       clock rate, if possible; if not, senders MUST specify the escape

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       value.  Receivers MUST consult the RTP clock parameter for the true
       sampling rate if the escape value is specified.

                           FREQIDX    Sampling Frequency

                             0x0            96000
                             0x1            88200
                             0x2            64000
                             0x3            48000
                             0x4            44100
                             0x5            32000
                             0x6            24000
                             0x7            22050
                             0x8            16000
                             0x9            12000
                             0xa            11025
                             0xb             8000
                             0xc          reserved
                             0xd          reserved
                             0xe          reserved
                             0xf         escape value

                         Figure E.4 -- FreqIdx encoding

       The 4-bit CHANNEL field specifies the number of audio channels for
       the renderer.  The values 0x1 to 0x5 specify 1 to 5 audio channels;
       the value 0x6 specifies 5+1 surround sound, and the value 0x7
       specifies 7+1 surround sound.  If the rtpmap line in the session
       description specifies one of these formats, CHANNEL MUST be set to
       the corresponding value.  Otherwise, CHANNEL MUST be set to 0x0.

       The CHANNEL field is followed by a list of one or more binary file
       data blocks.  The 3-bit SACNK field (the chunk_type field in class
       StructuredAudioSpecificConfig, defined in [MPEGSA]) specifies the
       type of each data block.

       For General MIDI, only Standard MIDI Files may appear in the list
       (SACNK field value 2).  For DLS 2, only Standard MIDI Files and DLS 2
       RIFF files (SACNK field value 4) may appear.  For both of these file
       types, the FILE_BLK field has the format shown in Figure E.5: a 32-
       bit unsigned integer value (FILE_LEN) coding the number of bytes in
       the SMF or RIFF file, followed by FILE_LEN bytes coding the file
       data.

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           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     FILE_LEN (32-bit, a byte count SMF file or RIFF file)     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  FILE_DATA (file contents, a list of FILE_LEN bytes) ...      |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure E.5 -- The FILE_BLK field format

       Note that several files may follow CHANNEL field.  The "1" constant
       fields in Figure E.3 code the presence of another file; the "0"
       constant field codes the end of the list.  The final "0" bit in
       Figure E.3 codes the absence of special coding tools (see [MPEGAUDIO]
       for details).  Senders not using these tools MUST append this "0"
       bit; receivers that do not understand these coding tools MUST ignore
       all data following a "1" in this position.

       The StructuredAudioSpecificConfig bitfield structure requires the
       presence of one FILE_BLK.  For mpeg4-generic RTP MIDI use of DLS 2,
       FILE_BLKs MUST code RIFF files or SMF files.  For mpeg4-generic RTP
       MIDI use of General MIDI, FILE_BLKs MUST code SMF files.  By default,
       this SMF will be ignored (Appendix C.6.4.1).  In this default case, a
       GM StructuredAudioSpecificConfig bitfield SHOULD code a FILE_BLK
       whose FILE_LEN is 0, and whose FILE_DATA is empty.

       To complete this appendix, we derive the
       StructuredAudioSpecificConfig that we use in the General MIDI session
       examples in this memo.  Referring to Figure E.3, we note that for GM,
       AOTYPE = 15.  Our examples use a 44,100 Hz sample rate (FREQIDX = 4)
       and are in mono (CHANNEL = 1).  For GM, a single SMF is encoded
       (SACNK = 2), using the SMF shown in Figure E.6 (a 26 byte file).

                   --------------------------------------------
                  |  MIDI File = <Header Chunk> <Track Chunk>  |
                   --------------------------------------------

       <Header Chunk> = <chunk type> <length>     <format> <ntrks> <divsn>
                        4D 54 68 64  00 00 00 06  00 00    00 01   00 60

       <Track Chunk> = <chunk type>  <length>     <delta-time> <end-event>
                       4D 54 72 6B   00 00 00 04  00           FF 2F 00

                Figure E.6 -- SMF file encoded in the example

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       Placing these constants in binary format into the data structure
       shown in Figure E.3 yields the constant shown in Figure E.7.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 1 1 1 1|0 1 0 0|0 0 0 1|0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0|0 1 0 0|1 1 0 1|0 1 0 1|0 1 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 1 1 0|1 0 0 0|0 1 1 0|0 1 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0|0 0 0 0|0 0 0 0|0 1 1 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 1|0 0 0 0|0 0 0 0|0 1 1 0|0 0 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 1 0 0|1 1 0 1|0 1 0 1|0 1 0 0|0 1 1 1|0 0 1 0|0 1 1 0|1 0 1 1|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 1 1 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0|0 0 0 0|1 1 1 1|1 1 1 1|0 0 1 0|1 1 1 1|0 0 0 0|0 0 0 0|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0|0|
          +-+-+

                Figure E.7 -- AudioSpecificConfig used in GM examples

       Expressing this bitfield as an ASCII hexadecimal string yields:

          7A0A0000001A4D546864000000060000000100604D54726B0000000600FF2F000

       This string is assigned to the "config" parameter in the minimal
       mpeg4-generic General MIDI examples in this memo (such as the example
       in Section 6.2).  Expressing this string in Base64 [RFC2045] yields:

          egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA

       This string is assigned to the "inline" parameter in the General MIDI
       example shown in Appendix C.6.5.

    Lazzaro & Wawrzynek         Standards Track                   [Page 164]

    RFC 4695              RTP Payload Format for MIDI          November 2006

    References

    Normative References

       [MIDI]      MIDI Manufacturers Association.  "The Complete MIDI 1.0
                   Detailed Specification", 1996.

       [RFC3550]   Schulzrinne, H., Casner, S., Frederick, R., and V.
                   Jacobson, "RTP: A Transport Protocol for Real-Time
                   Applications", STD 64, RFC 3550, July 2003.

       [RFC3551]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
                   Video Conferences with Minimal Control", STD 65, RFC
                   3551, July 2003.

       [RFC3640]   van der Meer, J., Mackie, D., Swaminathan, V., Singer,
                   D., and P. Gentric, "RTP Payload Format for Transport of
                   MPEG-4 Elementary Streams", RFC 3640, November 2003.

       [MPEGSA]    International Standards Organization.  "ISO/IEC 14496
                   MPEG-4", Part 3 (Audio), Subpart 5 (Structured Audio),
                   2001.

       [RFC4566]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
                   Description Protocol", RFC 4566, July 2006.

       [MPEGAUDIO] International Standards Organization.  "ISO 14496 MPEG-
                   4", Part 3 (Audio), 2001.

       [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                   Extensions (MIME) Part One: Format of Internet Message
                   Bodies", RFC 2045, November 1996.

       [DLS2]      MIDI Manufacturers Association.  "The MIDI Downloadable
                   Sounds Specification", v98.2, 1998.

       [RFC4234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
                   Specifications: ABNF", RFC 4234, October 2005.

       [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC3711]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
                   Norrman, "The Secure Real-time Transport Protocol
                   (SRTP)", RFC 3711, March 2004.

    Lazzaro & Wawrzynek         Standards Track                   [Page 165]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       [RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
                   with Session Description Protocol (SDP)", RFC 3264, June
                   2002.

       [RFC3986]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                   Resource Identifier (URI): Generic Syntax", STD 66, RFC
                   3986, January 2005.

       [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                   Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

       [RFC3388]   Camarillo, G., Eriksson, G., Holler, J., and H.
                   Schulzrinne, "Grouping of Media Lines in the Session
                   Description Protocol (SDP)", RFC 3388, December 2002.

       [RP015]     MIDI Manufacturers Association.  "Recommended Practice
                   015 (RP-015): Response to Reset All Controllers", 11/98.

       [RFC4288]   Freed, N. and J. Klensin, "Media Type Specifications and
                   Registration Procedures", BCP 13, RFC 4288, December
                   2005.

       [RFC3555]   Casner, S. and P. Hoschka, "MIME Type Registration of RTP
                   Payload Formats", RFC 3555, July 2003.

    Informative References

       [NMP]       Lazzaro, J. and J. Wawrzynek.  "A Case for Network
                   Musical Performance", 11th International Workshop on
                   Network and Operating Systems Support for Digital Audio
                   and Video (NOSSDAV 2001) June 25-26, 2001, Port
                   Jefferson, New York.

       [GRAME]     Fober, D., Orlarey, Y. and S. Letz.  "Real Time Musical
                   Events Streaming over Internet", Proceedings of the
                   International Conference on WEB Delivering of Music 2001,
                   pages 147-154.

       [RFC3261]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                   A., Peterson, J., Sparks, R., Handley, M., and E.
                   Schooler, "SIP: Session Initiation Protocol", RFC 3261,
                   June 2002.

       [RFC2326]   Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
                   Streaming Protocol (RTSP)", RFC 2326, April 1998.

    Lazzaro & Wawrzynek         Standards Track                   [Page 166]

    RFC 4695              RTP Payload Format for MIDI          November 2006

       [ALF]       Clark, D. D. and D. L. Tennenhouse. "Architectural
                   considerations for a new generation of protocols",
                   SIGCOMM Symposium on Communications Architectures and
                   Protocols , (Philadelphia, Pennsylvania), pp. 200--208,
                   IEEE, Sept. 1990.

       [RFC4696]   Lazzaro, J. and J. Wawrzynek, "An Implementation Guide
                   for RTP MIDI", RFC 4696, November 2006.

       [RFC2205]   Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
                   Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
                   Functional Specification", RFC 2205, September 1997.

       [RFC4288]   Freed, N. and J. Klensin, "Media Type Specifications and
                   Registration Procedures", BCP 13, RFC 4288, December
                   2005.

       [RFC4289]   Freed, N. and J. Klensin, "Multipurpose Internet Mail
                   Extensions (MIME) Part Four: Registration Procedures",
                   BCP 13, RFC 4289, December 2005.

       [RFC4571]   Lazzaro, J. "Framing Real-time Transport Protocol (RTP)
                   and RTP Control Protocol (RTCP) Packets over Connection-
                   Oriented Transport", RFC 4571, July 2006.

       [RFC2818]   Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

       [SPMIDI]    MIDI Manufacturers Association.  "Scalable Polyphony
                   MIDI, Specification and Device Profiles", Document
                   Version 1.0a, 2002.

       [LCP]       Apple Computer. "Logic 7 Dedicated Control Surface
                   Support", Appendix B.  Product manual available from
                   www.apple.com.

    Lazzaro & Wawrzynek         Standards Track                   [Page 167]

    RFC 4695              RTP Payload Format for MIDI          November 2006

    Authors' Addresses

       John Lazzaro (corresponding author)
       UC Berkeley
       CS Division
       315 Soda Hall
       Berkeley CA 94720-1776

       EMail: lazzaro@cs.berkeley.edu

       John Wawrzynek
       UC Berkeley
       CS Division
       631 Soda Hall
       Berkeley CA 94720-1776

       EMail: johnw@cs.berkeley.edu

    Lazzaro & Wawrzynek         Standards Track                   [Page 168]

    RFC 4695              RTP Payload Format for MIDI          November 2006

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    Lazzaro & Wawrzynek         Standards Track                   [Page 169]