    Network Working Group                                          S. Wenger
    Request for Comments: 3984                               M.M. Hannuksela
    Category: Standards Track                                 T. Stockhammer
                                                               M. Westerlund
                                                                   D. Singer
                                                               February 2005

                       RTP Payload Format for H.264 Video

    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 Internet Society (2005).

    Abstract

       This memo describes an RTP Payload format for the ITU-T
       Recommendation H.264 video codec and the technically identical
       ISO/IEC International Standard 14496-10 video codec.  The RTP payload
       format allows for packetization of one or more Network Abstraction
       Layer Units (NALUs), produced by an H.264 video encoder, in each RTP
       payload.  The payload format has wide applicability, as it supports
       applications from simple low bit-rate conversational usage, to
       Internet video streaming with interleaved transmission, to high bit-
       rate video-on-demand.

    Table of Contents

       1.  Introduction..................................................  3
           1.1.  The H.264 Codec.........................................  3
           1.2.  Parameter Set Concept...................................  4
           1.3.  Network Abstraction Layer Unit Types....................  5
       2.  Conventions...................................................  6
       3.  Scope.........................................................  6
       4.  Definitions and Abbreviations.................................  6
           4.1.  Definitions.............................................  6
       5.  RTP Payload Format............................................  8
           5.1.  RTP Header Usage........................................  8
           5.2.  Common Structure of the RTP Payload Format.............. 11
           5.3.  NAL Unit Octet Usage.................................... 12

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           5.4.  Packetization Modes..................................... 14
           5.5.  Decoding Order Number (DON)............................. 15
           5.6.  Single NAL Unit Packet.................................. 18
           5.7.  Aggregation Packets..................................... 18
           5.8.  Fragmentation Units (FUs)............................... 27
       6.  Packetization Rules........................................... 31
           6.1.  Common Packetization Rules.............................. 31
           6.2.  Single NAL Unit Mode.................................... 32
           6.3.  Non-Interleaved Mode.................................... 32
           6.4.  Interleaved Mode........................................ 33
       7.  De-Packetization Process (Informative)........................ 33
           7.1.  Single NAL Unit and Non-Interleaved Mode................ 33
           7.2.  Interleaved Mode........................................ 34
           7.3.  Additional De-Packetization Guidelines.................. 36
       8.  Payload Format Parameters..................................... 37
           8.1.  MIME Registration....................................... 37
           8.2.  SDP Parameters.......................................... 52
           8.3.  Examples................................................ 58
           8.4.  Parameter Set Considerations............................ 60
       9.  Security Considerations....................................... 62
       10. Congestion Control............................................ 63
       11. IANA Considerations........................................... 64
       12. Informative Appendix: Application Examples.................... 65
           12.1. Video Telephony according to ITU-T Recommendation H.241
                 Annex A................................................. 65
           12.2. Video Telephony, No Slice Data Partitioning, No NAL
                 Unit Aggregation........................................ 65
           12.3. Video Telephony, Interleaved Packetization Using NAL
                 Unit Aggregation........................................ 66
           12.4. Video Telephony with Data Partitioning.................. 66
           12.5. Video Telephony or Streaming with FUs and Forward
                 Error Correction........................................ 67
           12.6. Low Bit-Rate Streaming.................................. 69
           12.7. Robust Packet Scheduling in Video Streaming............. 70
       13. Informative Appendix: Rationale for Decoding Order Number..... 71
           13.1. Introduction............................................ 71
           13.2. Example of Multi-Picture Slice Interleaving............. 71
           13.3. Example of Robust Packet Scheduling..................... 73
           13.4. Robust Transmission Scheduling of Redundant Coded
                 Slices.................................................. 77
           13.5. Remarks on Other Design Possibilities................... 77
       14. Acknowledgements.............................................. 78
       15. References.................................................... 78
           15.1. Normative References.................................... 78
           15.2. Informative References.................................. 79
       Authors' Addresses................................................ 81
       Full Copyright Statement.......................................... 83

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

    1.  Introduction

    1.1.  The H.264 Codec

       This memo specifies an RTP payload specification for the video coding
       standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
       International Standard 14496 Part 10 [2] (both also known as Advanced
       Video Coding, or AVC).  Recommendation H.264 was approved by ITU-T on
       May 2003, and the approved draft specification is available for
       public review [8].  In this memo the H.264 acronym is used for the
       codec and the standard, but the memo is equally applicable to the
       ISO/IEC counterpart of the coding standard.

       The H.264 video codec has a very broad application range that covers
       all forms of digital compressed video from, low bit-rate Internet
       streaming applications to HDTV broadcast and Digital Cinema
       applications with nearly lossless coding.  Compared to the current
       state of technology, the overall performance of H.264 is such that
       bit rate savings of 50% or more are reported.  Digital Satellite TV
       quality, for example, was reported to be achievable at 1.5 Mbit/s,
       compared to the current operation point of MPEG 2 video at around 3.5
       Mbit/s [9].

       The codec specification [1] itself distinguishes conceptually between
       a video coding layer (VCL) and a network abstraction layer (NAL).
       The VCL contains the signal processing functionality of the codec;
       mechanisms such as transform, quantization, and motion compensated
       prediction; and a loop filter.  It follows the general concept of
       most of today's video codecs, a macroblock-based coder that uses
       inter picture prediction with motion compensation and transform
       coding of the residual signal.  The VCL encoder outputs slices: a bit
       string that contains the macroblock data of an integer number of
       macroblocks, and the information of the slice header (containing the
       spatial address of the first macroblock in the slice, the initial
       quantization parameter, and similar information).  Macroblocks in
       slices are arranged in scan order unless a different macroblock
       allocation is specified, by using the so-called Flexible Macroblock
       Ordering syntax.  In-picture prediction is used only within a slice.
       More information is provided in [9].

       The Network Abstraction Layer (NAL) encoder encapsulates the slice
       output of the VCL encoder into Network Abstraction Layer Units (NAL
       units), which are suitable for transmission over packet networks or
       use in packet oriented multiplex environments.  Annex B of H.264
       defines an encapsulation process to transmit such NAL units over
       byte-stream oriented networks.  In the scope of this memo, Annex B is
       not relevant.

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       Internally, the NAL uses NAL units.  A NAL unit consists of a one-
       byte header and the payload byte string.  The header indicates the
       type of the NAL unit, the (potential) presence of bit errors or
       syntax violations in the NAL unit payload, and information regarding
       the relative importance of the NAL unit for the decoding process.
       This RTP payload specification is designed to be unaware of the bit
       string in the NAL unit payload.

       One of the main properties of H.264 is the complete decoupling of the
       transmission time, the decoding time, and the sampling or
       presentation time of slices and pictures.  The decoding process
       specified in H.264 is unaware of time, and the H.264 syntax does not
       carry information such as the number of skipped frames (as is common
       in the form of the Temporal Reference in earlier video compression
       standards).  Also, there are NAL units that affect many pictures and
       that are, therefore, inherently timeless.  For this reason, the
       handling of the RTP timestamp requires some special considerations
       for NAL units for which the sampling or presentation time is not
       defined or, at transmission time, unknown.

    1.2.  Parameter Set Concept

       One very fundamental design concept of H.264 is to generate self-
       contained packets, to make mechanisms such as the header duplication
       of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11]
       unnecessary.  This was achieved by decoupling information relevant to
       more than one slice from the media stream.  This higher layer meta
       information should be sent reliably, asynchronously, and in advance
       from the RTP packet stream that contains the slice packets.
       (Provisions for sending this information in-band are also available
       for applications that do not have an out-of-band transport channel
       appropriate for the purpose.)  The combination of the higher-level
       parameters is called a parameter set.  The H.264 specification
       includes two types of parameter sets: sequence parameter set and
       picture parameter set.  An active sequence parameter set remains
       unchanged throughout a coded video sequence, and an active picture
       parameter set remains unchanged within a coded picture.  The sequence
       and picture parameter set structures contain information such as
       picture size, optional coding modes employed, and macroblock to slice
       group map.

       To be able to change picture parameters (such as the picture size)
       without having to transmit parameter set updates synchronously to the
       slice packet stream, the encoder and decoder can maintain a list of
       more than one sequence and picture parameter set.  Each slice header
       contains a codeword that indicates the sequence and picture parameter
       set to be used.

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       This mechanism allows the decoupling of the transmission of parameter
       sets from the packet stream, and the transmission of them by external
       means (e.g., as a side effect of the capability exchange), or through
       a (reliable or unreliable) control protocol.  It may even be possible
       that they are never transmitted but are fixed by an application
       design specification.

    1.3.  Network Abstraction Layer Unit Types

       Tutorial information on the NAL design can be found in [12], [13],
       and [14].

       All NAL units consist of a single NAL unit type octet, which also
       co-serves as the payload header of this RTP payload format.  The
       payload of a NAL unit follows immediately.

       The syntax and semantics of the NAL unit type octet are specified in
       [1], but the essential properties of the NAL unit type octet are
       summarized below.  The NAL unit type octet has the following format:

          +---------------+
          |0|1|2|3|4|5|6|7|
          +-+-+-+-+-+-+-+-+
          |F|NRI|  Type   |
          +---------------+

       The semantics of the components of the NAL unit type octet, as
       specified in the H.264 specification, are described briefly below.

       F: 1 bit
          forbidden_zero_bit.  The H.264 specification declares a value of
          1 as a syntax violation.

       NRI: 2 bits
          nal_ref_idc.  A value of 00 indicates that the content of the NAL
          unit is not used to reconstruct reference pictures for inter
          picture prediction.  Such NAL units can be discarded without
          risking the integrity of the reference pictures.  Values greater
          than 00 indicate that the decoding of the NAL unit is required to
          maintain the integrity of the reference pictures.

       Type: 5 bits
          nal_unit_type.  This component specifies the NAL unit payload type
          as defined in table 7-1 of [1], and later within this memo.  For a
          reference of all currently defined NAL unit types and their
          semantics, please refer to section 7.4.1 in [1].

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       This memo introduces new NAL unit types, which are presented in
       section 5.2.  The NAL unit types defined in this memo are marked as
       unspecified in [1].  Moreover, this specification extends the
       semantics of F and NRI as described in section 5.3.

    2.  Conventions

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in BCP 14, RFC 2119 [3].

       This specification uses the notion of setting and clearing a bit when
       bit fields are handled.  Setting a bit is the same as assigning that
       bit the value of 1 (On).  Clearing a bit is the same as assigning
       that bit the value of 0 (Off).

    3.  Scope

       This payload specification can only be used to carry the "naked"
       H.264 NAL unit stream over RTP, and not the bitstream format
       discussed in Annex B of H.264.  Likely, the first applications of
       this specification will be in the conversational multimedia field,
       video telephony or video conferencing, but the payload format also
       covers other applications, such as Internet streaming and TV over IP.

    4.  Definitions and Abbreviations

    4.1.  Definitions

       This document uses the definitions of [1].  The following terms,
       defined in [1], are summed up for convenience:

          access unit: A set of NAL units always containing a primary coded
          picture.  In addition to the primary coded picture, an access unit
          may also contain one or more redundant coded pictures or other NAL
          units not containing slices or slice data partitions of a coded
          picture.  The decoding of an access unit always results in a
          decoded picture.

          coded video sequence: A sequence of access units that consists, in
          decoding order, of an instantaneous decoding refresh (IDR) access
          unit followed by zero or more non-IDR access units including all
          subsequent access units up to but not including any subsequent IDR
          access unit.

          IDR access unit: An access unit in which the primary coded picture
          is an IDR picture.

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          IDR picture: A coded picture containing only slices with I or SI
          slice types that causes a "reset" in the decoding process.  After
          the decoding of an IDR picture, all following coded pictures in
          decoding order can be decoded without inter prediction from any
          picture decoded prior to the IDR picture.

          primary coded picture: The coded representation of a picture to be
          used by the decoding process for a bitstream conforming to H.264.
          The primary coded picture contains all macroblocks of the picture.

          redundant coded picture: A coded representation of a picture or a
          part of a picture.  The content of a redundant coded picture shall
          not be used by the decoding process for a bitstream conforming to
          H.264.  The content of a redundant coded picture may be used by
          the decoding process for a bitstream that contains errors or
          losses.

          VCL NAL unit: A collective term used to refer to coded slice and
          coded data partition NAL units.

       In addition, the following definitions apply:

          decoding order number (DON): A field in the payload structure, or
          a derived variable indicating NAL unit decoding order.  Values of
          DON are in the range of 0 to 65535, inclusive.  After reaching the
          maximum value, the value of DON wraps around to 0.

          NAL unit decoding order: A NAL unit order that conforms to the
          constraints on NAL unit order given in section 7.4.1.2 in [1].

          transmission order: The order of packets in ascending RTP sequence
          number order (in modulo arithmetic).  Within an aggregation
          packet, the NAL unit transmission order is the same as the order
          of appearance of NAL units in the packet.

          media aware network element (MANE): A network element, such as a
          middlebox or application layer gateway that is capable of parsing
          certain aspects of the RTP payload headers or the RTP payload and
          reacting to the contents.

             Informative note: The concept of a MANE goes beyond normal
             routers or gateways in that a MANE has to be aware of the
             signaling (e.g., to learn about the payload type mappings of
             the media streams), and in that it has to be trusted when
             working with SRTP.  The advantage of using MANEs is that they
             allow packets to be dropped according to the needs of the media
             coding.  For example, if a MANE has to drop packets due to
             congestion on a certain link, it can identify those packets

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             whose dropping has the smallest negative impact on the user
             experience and remove them in order to remove the congestion
             and/or keep the delay low.

       Abbreviations

          DON:        Decoding Order Number
          DONB:       Decoding Order Number Base
          DOND:       Decoding Order Number Difference
          FEC:        Forward Error Correction
          FU:         Fragmentation Unit
          IDR:        Instantaneous Decoding Refresh
          IEC:        International Electrotechnical Commission
          ISO:        International Organization for Standardization
          ITU-T:      International Telecommunication Union,
                      Telecommunication Standardization Sector
          MANE:       Media Aware Network Element
          MTAP:       Multi-Time Aggregation Packet
          MTAP16:     MTAP with 16-bit timestamp offset
          MTAP24:     MTAP with 24-bit timestamp offset
          NAL:        Network Abstraction Layer
          NALU:       NAL Unit
          SEI:        Supplemental Enhancement Information
          STAP:       Single-Time Aggregation Packet
          STAP-A:     STAP type A
          STAP-B:     STAP type B
          TS:         Timestamp
          VCL:        Video Coding Layer

    5.  RTP Payload Format

    5.1.  RTP Header Usage

       The format of the RTP header is specified in RFC 3550 [4] and
       reprinted in Figure 1 for convenience.  This payload format uses the
       fields of the header in a manner consistent with that specification.

       When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP
       payload format is specified in section 5.6.  The RTP payload (and the
       settings for some RTP header bits) for aggregation packets and
       fragmentation units are specified in sections 5.7 and 5.8,
       respectively.

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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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |V=2|P|X|  CC   |M|     PT      |       sequence number         |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           timestamp                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |           synchronization source (SSRC) identifier            |
          +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
          |            contributing source (CSRC) identifiers             |
          |                             ....                              |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 1.  RTP header according to RFC 3550

       The RTP header information to be set according to this RTP payload
       format is set as follows:

       Marker bit (M): 1 bit
          Set for the very last packet of the access unit indicated by the
          RTP timestamp, in line with the normal use of the M bit in video
          formats, to allow an efficient playout buffer handling.  For
          aggregation packets (STAP and MTAP), the marker bit in the RTP
          header MUST be set to the value that the marker bit of the last
          NAL unit of the aggregation packet would have been if it were
          transported in its own RTP packet.  Decoders MAY use this bit as
          an early indication of the last packet of an access unit, but MUST
          NOT rely on this property.

             Informative note: Only one M bit is associated with an
             aggregation packet carrying multiple NAL units.  Thus, if a
             gateway has re-packetized an aggregation packet into several
             packets, it cannot reliably set the M bit of those packets.

       Payload type (PT): 7 bits
          The assignment of an RTP payload type for this new packet format
          is outside the scope of this document and will not be specified
          here.  The assignment of a payload type has to be performed either
          through the profile used or in a dynamic way.

       Sequence number (SN): 16 bits
          Set and used in accordance with RFC 3550.  For the single NALU and
          non-interleaved packetization mode, the sequence number is used to
          determine decoding order for the NALU.

       Timestamp: 32 bits
          The RTP timestamp is set to the sampling timestamp of the content.
          A 90 kHz clock rate MUST be used.

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          If the NAL unit has no timing properties of its own (e.g.,
          parameter set and SEI NAL units), the RTP timestamp is set to the
          RTP timestamp of the primary coded picture of the access unit in
          which the NAL unit is included, according to section 7.4.1.2 of
          [1].

          The setting of the RTP Timestamp for MTAPs is defined in section
          5.7.2.

          Receivers SHOULD ignore any picture timing SEI messages included
          in access units that have only one display timestamp.  Instead,
          receivers SHOULD use the RTP timestamp for synchronizing the
          display process.

          RTP senders SHOULD NOT transmit picture timing SEI messages for
          pictures that are not supposed to be displayed as multiple fields.

          If one access unit has more than one display timestamp carried in
          a picture timing SEI message, then the information in the SEI
          message SHOULD be treated as relative to the RTP timestamp, with
          the earliest event occurring at the time given by the RTP
          timestamp, and subsequent events later, as given by the difference
          in SEI message picture timing values.  Let tSEI1, tSEI2, ...,
          tSEIn be the display timestamps carried in the SEI message of an
          access unit, where tSEI1 is the earliest of all such timestamps.
          Let tmadjst() be a function that adjusts the SEI messages time
          scale to a 90-kHz time scale.  Let TS be the RTP timestamp.  Then,
          the display time for the event associated with tSEI1 is TS.  The
          display time for the event with tSEIx, where x is [2..n] is TS +
          tmadjst (tSEIx - tSEI1).

             Informative note: Displaying coded frames as fields is needed
             commonly in an operation known as 3:2 pulldown, in which film
             content that consists of coded frames is displayed on a display
             using interlaced scanning.  The picture timing SEI message
             enables carriage of multiple timestamps for the same coded
             picture, and therefore the 3:2 pulldown process is perfectly
             controlled.  The picture timing SEI message mechanism is
             necessary because only one timestamp per coded frame can be
             conveyed in the RTP timestamp.

             Informative note: Because H.264 allows the decoding order to be
             different from the display order, values of RTP timestamps may
             not be monotonically non-decreasing as a function of RTP
             sequence numbers.  Furthermore, the value for interarrival
             jitter reported in the RTCP reports may not be a trustworthy
             indication of the network performance, as the calculation rules

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             for interarrival jitter (section 6.4.1 of RFC 3550) assume that
             the RTP timestamp of a packet is directly proportional to its
             transmission time.

    5.2.  Common Structure of the RTP Payload Format

       The payload format defines three different basic payload structures.
       A receiver can identify the payload structure by the first byte of
       the RTP payload, which co-serves as the RTP payload header and, in
       some cases, as the first byte of the payload.  This byte is always
       structured as a NAL unit header.  The NAL unit type field indicates
       which structure is present.  The possible structures are as follows:

       Single NAL Unit Packet: Contains only a single NAL unit in the
       payload.  The NAL header type field will be equal to the original NAL
       unit type; i.e., in the range of 1 to 23, inclusive.  Specified in
       section 5.6.

       Aggregation packet: Packet type used to aggregate multiple NAL units
       into a single RTP payload.  This packet exists in four versions, the
       Single-Time Aggregation Packet type A (STAP-A), the Single-Time
       Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet
       (MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet
       (MTAP) with 24-bit offset (MTAP24).  The NAL unit type numbers
       assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and
       27, respectively.  Specified in section 5.7.

       Fragmentation unit: Used to fragment a single NAL unit over multiple
       RTP packets.  Exists with two versions, FU-A and FU-B, identified
       with the NAL unit type numbers 28 and 29, respectively.  Specified in
       section 5.8.

       Table 1.  Summary of NAL unit types and their payload structures

          Type   Packet    Type name                        Section
          ---------------------------------------------------------
          0      undefined                                    -
          1-23   NAL unit  Single NAL unit packet per H.264   5.6
          24     STAP-A    Single-time aggregation packet     5.7.1
          25     STAP-B    Single-time aggregation packet     5.7.1
          26     MTAP16    Multi-time aggregation packet      5.7.2
          27     MTAP24    Multi-time aggregation packet      5.7.2
          28     FU-A      Fragmentation unit                 5.8
          29     FU-B      Fragmentation unit                 5.8
          30-31  undefined                                    -

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          Informative note: This specification does not limit the size of
          NAL units encapsulated in single NAL unit packets and
          fragmentation units.  The maximum size of a NAL unit encapsulated
          in any aggregation packet is 65535 bytes.

    5.3.  NAL Unit Octet Usage

       The structure and semantics of the NAL unit octet were introduced in
       section 1.3.  For convenience, the format of the NAL unit type octet
       is reprinted below:

          +---------------+
          |0|1|2|3|4|5|6|7|
          +-+-+-+-+-+-+-+-+
          |F|NRI|  Type   |
          +---------------+

       This section specifies the semantics of F and NRI according to this
       specification.

       F: 1 bit
          forbidden_zero_bit.  A value of 0 indicates that the NAL unit type
          octet and payload should not contain bit errors or other syntax
          violations.  A value of 1 indicates that the NAL unit type octet
          and payload may contain bit errors or other syntax violations.

          MANEs SHOULD set the F bit to indicate detected bit errors in the
          NAL unit.  The H.264 specification requires that the F bit is
          equal to 0.  When the F bit is set, the decoder is advised that
          bit errors or any other syntax violations may be present in the
          payload or in the NAL unit type octet.  The simplest decoder
          reaction to a NAL unit in which the F bit is equal to 1 is to
          discard such a NAL unit and to conceal the lost data in the
          discarded NAL unit.

       NRI: 2 bits
          nal_ref_idc.  The semantics of value 00 and a non-zero value
          remain unchanged from the H.264 specification.  In other words, a
          value of 00 indicates that the content of the NAL unit is not used
          to reconstruct reference pictures for inter picture prediction.
          Such NAL units can be discarded without risking the integrity of
          the reference pictures.  Values greater than 00 indicate that the
          decoding of the NAL unit is required to maintain the integrity of
          the reference pictures.

          In addition to the specification above, according to this RTP
          payload specification, values of NRI greater than 00 indicate the
          relative transport priority, as determined by the encoder.  MANEs

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          can use this information to protect more important NAL units
          better than they do less important NAL units.  The highest
          transport priority is 11, followed by 10, and then by 01; finally,
          00 is the lowest.

             Informative note: Any non-zero value of NRI is handled
             identically in H.264 decoders.  Therefore, receivers need not
             manipulate the value of NRI when passing NAL units to the
             decoder.

          An H.264 encoder MUST set the value of NRI according to the H.264
          specification (subclause 7.4.1) when the value of nal_unit_type is
          in the range of 1 to 12, inclusive.  In particular, the H.264
          specification requires that the value of NRI SHALL be equal to 0
          for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or
          12.

          For NAL units having nal_unit_type equal to 7 or 8 (indicating a
          sequence parameter set or a picture parameter set, respectively),
          an H.264 encoder SHOULD set the value of NRI to 11 (in binary
          format).  For coded slice NAL units of a primary coded picture
          having nal_unit_type equal to 5 (indicating a coded slice
          belonging to an IDR picture), an H.264 encoder SHOULD set the
          value of NRI to 11 (in binary format).

          For a mapping of the remaining nal_unit_types to NRI values, the
          following example MAY be used and has been shown to be efficient
          in a certain environment [13].  Other mappings MAY also be
          desirable, depending on the application and the H.264/AVC Annex A
          profile in use.

             Informative note: Data Partitioning is not available in certain
             profiles; e.g., in the Main or Baseline profiles.
             Consequently, the nal unit types 2, 3, and 4 can occur only if
             the video bitstream conforms to a profile in which data
             partitioning is allowed and not in streams that conform to the
             Main or Baseline profiles.

          Table 2.  Example of NRI values for coded slices and coded slice
          data partitions of primary coded reference pictures

          NAL Unit Type     Content of NAL unit              NRI (binary)
          ----------------------------------------------------------------
           1              non-IDR coded slice                         10
           2              Coded slice data partition A                10
           3              Coded slice data partition B                01
           4              Coded slice data partition C                01

    Wenger, et al.              Standards Track                    [Page 13]

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             Informative note: As mentioned before, the NRI value of non-
             reference pictures is 00 as mandated by H.264/AVC.

          An H.264 encoder SHOULD set the value of NRI for coded slice and
          coded slice data partition NAL units of redundant coded reference
          pictures equal to 01 (in binary format).

          Definitions of the values for NRI for NAL unit types 24 to 29,
          inclusive, are given in sections 5.7 and 5.8 of this memo.

          No recommendation for the value of NRI is given for NAL units
          having nal_unit_type in the range of 13 to 23, inclusive, because
          these values are reserved for ITU-T and ISO/IEC.  No
          recommendation for the value of NRI is given for NAL units having
          nal_unit_type equal to 0 or in the range of 30 to 31, inclusive,
          as the semantics of these values are not specified in this memo.

    5.4.  Packetization Modes

       This memo specifies three cases of packetization modes:

          o Single NAL unit mode
          o Non-interleaved mode
          o Interleaved mode

       The single NAL unit mode is targeted for conversational systems that
       comply with ITU-T Recommendation H.241 [15] (see section 12.1).  The
       non-interleaved mode is targeted for conversational systems that may
       not comply with ITU-T Recommendation H.241.  In the non-interleaved
       mode, NAL units are transmitted in NAL unit decoding order.  The
       interleaved mode is targeted for systems that do not require very low
       end-to-end latency.  The interleaved mode allows transmission of NAL
       units out of NAL unit decoding order.

       The packetization mode in use MAY be signaled by the value of the
       OPTIONAL packetization-mode MIME parameter or by external means.  The
       used packetization mode governs which NAL unit types are allowed in
       RTP payloads.  Table 3 summarizes the allowed NAL unit types for each
       packetization mode.  Some NAL unit type values (indicated as
       undefined in Table 3) are reserved for future extensions.  NAL units
       of those types SHOULD NOT be sent by a sender and MUST be ignored by
       a receiver.  For example, the Types 1-23, with the associated packet
       type "NAL unit", are allowed in "Single NAL Unit Mode" and in "Non-
       Interleaved Mode", but disallowed in "Interleaved Mode".
       Packetization modes are explained in more detail in section 6.

    Wenger, et al.              Standards Track                    [Page 14]

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       Table 3.  Summary of allowed NAL unit types for each packetization
       mode (yes = allowed, no = disallowed, ig = ignore)

          Type   Packet    Single NAL    Non-Interleaved    Interleaved
                           Unit Mode           Mode             Mode
          -------------------------------------------------------------

          0      undefined     ig               ig               ig
          1-23   NAL unit     yes              yes               no
          24     STAP-A        no              yes               no
          25     STAP-B        no               no              yes
          26     MTAP16        no               no              yes
          27     MTAP24        no               no              yes
          28     FU-A          no              yes              yes
          29     FU-B          no               no              yes
          30-31  undefined     ig               ig               ig

    5.5.  Decoding Order Number (DON)

       In the interleaved packetization mode, the transmission order of NAL
       units is allowed to differ from the decoding order of the NAL units.
       Decoding order number (DON) is a field in the payload structure or a
       derived variable that indicates the NAL unit decoding order.
       Rationale and examples of use cases for transmission out of decoding
       order and for the use of DON are given in section 13.

       The coupling of transmission and decoding order is controlled by the
       OPTIONAL sprop-interleaving-depth MIME parameter as follows.  When
       the value of the OPTIONAL sprop-interleaving-depth MIME parameter is
       equal to 0 (explicitly or per default) or transmission of NAL units
       out of their decoding order is disallowed by external means, the
       transmission order of NAL units MUST conform to the NAL unit decoding
       order.  When the value of the OPTIONAL sprop-interleaving-depth MIME
       parameter is greater than 0 or transmission of NAL units out of their
       decoding order is allowed by external means,

       o  the order of NAL units in an MTAP16 and an MTAP24 is NOT REQUIRED
          to be the NAL unit decoding order, and

       o  the order of NAL units generated by decapsulating STAP-Bs, MTAPs,
          and FUs in two consecutive packets is NOT REQUIRED to be the NAL
          unit decoding order.

       The RTP payload structures for a single NAL unit packet, an STAP-A,
       and an FU-A do not include DON.  STAP-B and FU-B structures include
       DON, and the structure of MTAPs enables derivation of DON as
       specified in section 5.7.2.

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          Informative note: When an FU-A occurs in interleaved mode, it
          always follows an FU-B, which sets its DON.

          Informative note: If a transmitter wants to encapsulate a single
          NAL unit per packet and transmit packets out of their decoding
          order, STAP-B packet type can be used.

       In the single NAL unit packetization mode, the transmission order of
       NAL units, determined by the RTP sequence number, MUST be the same as
       their NAL unit decoding order.  In the non-interleaved packetization
       mode, the transmission order of NAL units in single NAL unit packets,
       STAP-As, and FU-As MUST be the same as their NAL unit decoding order.
       The NAL units within an STAP MUST appear in the NAL unit decoding
       order.  Thus, the decoding order is first provided through the
       implicit order within a STAP, and second provided through the RTP
       sequence number for the order between STAPs, FUs, and single NAL unit
       packets.

       Signaling of the value of DON for NAL units carried in STAP-B, MTAP,
       and a series of fragmentation units starting with an FU-B is
       specified in sections 5.7.1, 5.7.2, and 5.8, respectively.  The DON
       value of the first NAL unit in transmission order MAY be set to any
       value.  Values of DON are in the range of 0 to 65535, inclusive.
       After reaching the maximum value, the value of DON wraps around to 0.

       The decoding order of two NAL units contained in any STAP-B, MTAP, or
       a series of fragmentation units starting with an FU-B is determined
       as follows.  Let DON(i) be the decoding order number of the NAL unit
       having index i in the transmission order.  Function don_diff(m,n) is
       specified as follows:

          If DON(m) == DON(n), don_diff(m,n) = 0

          If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
          don_diff(m,n) = DON(n) - DON(m)

          If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
          don_diff(m,n) = 65536 - DON(m) + DON(n)

          If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
          don_diff(m,n) = - (DON(m) + 65536 - DON(n))

          If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
          don_diff(m,n) = - (DON(m) - DON(n))

       A positive value of don_diff(m,n) indicates that the NAL unit having
       transmission order index n follows, in decoding order, the NAL unit
       having transmission order index m.  When don_diff(m,n) is equal to 0,

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       then the NAL unit decoding order of the two NAL units can be in
       either order.  A negative value of don_diff(m,n) indicates that the
       NAL unit having transmission order index n precedes, in decoding
       order, the NAL unit having transmission order index m.

       Values of DON related fields (DON, DONB, and DOND; see section 5.7)
       MUST be such that the decoding order determined by the values of DON,
       as specified above, conforms to the NAL unit decoding order.  If the
       order of two NAL units in NAL unit decoding order is switched and the
       new order does not conform to the NAL unit decoding order, the NAL
       units MUST NOT have the same value of DON.  If the order of two
       consecutive NAL units in the NAL unit stream is switched and the new
       order still conforms to the NAL unit decoding order, the NAL units
       MAY have the same value of DON.  For example, when arbitrary slice
       order is allowed by the video coding profile in use, all the coded
       slice NAL units of a coded picture are allowed to have the same value
       of DON.  Consequently, NAL units having the same value of DON can be
       decoded in any order, and two NAL units having a different value of
       DON should be passed to the decoder in the order specified above.
       When two consecutive NAL units in the NAL unit decoding order have a
       different value of DON, the value of DON for the second NAL unit in
       decoding order SHOULD be the value of DON for the first, incremented
       by one.

       An example of the decapsulation process to recover the NAL unit
       decoding order is given in section 7.

          Informative note: Receivers should not expect that the absolute
          difference of values of DON for two consecutive NAL units in the
          NAL unit decoding order will be equal to one, even in error-free
          transmission.  An increment by one is not required, as at the time
          of associating values of DON to NAL units, it may not be known
          whether all NAL units are delivered to the receiver.  For example,
          a gateway may not forward coded slice NAL units of non-reference
          pictures or SEI NAL units when there is a shortage of bit rate in
          the network to which the packets are forwarded.  In another
          example, a live broadcast is interrupted by pre-encoded content,
          such as commercials, from time to time.  The first intra picture
          of a pre-encoded clip is transmitted in advance to ensure that it
          is readily available in the receiver.  When transmitting the first
          intra picture, the originator does not exactly know how many NAL
          units will be encoded before the first intra picture of the pre-
          encoded clip follows in decoding order.  Thus, the values of DON
          for the NAL units of the first intra picture of the pre-encoded
          clip have to be estimated when they are transmitted, and gaps in
          values of DON may occur.

    Wenger, et al.              Standards Track                    [Page 17]

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    5.6.  Single NAL Unit Packet

       The single NAL unit packet defined here MUST contain only one NAL
       unit, of the types defined in [1].  This means that neither an
       aggregation packet nor a fragmentation unit can be used within a
       single NAL unit packet.  A NAL unit stream composed by decapsulating
       single NAL unit packets in RTP sequence number order MUST conform to
       the NAL unit decoding order.  The structure of the single NAL unit
       packet is shown in Figure 2.

          Informative note: The first byte of a NAL unit co-serves as the
          RTP payload header.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |F|NRI|  type   |                                               |
          +-+-+-+-+-+-+-+-+                                               |
          |                                                               |
          |               Bytes 2..n of a Single NAL unit                 |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 2.  RTP payload format for single NAL unit packet

    5.7.  Aggregation Packets

       Aggregation packets are the NAL unit aggregation scheme of this
       payload specification.  The scheme is introduced to reflect the
       dramatically different MTU sizes of two key target networks:
       wireline IP networks (with an MTU size that is often limited by the
       Ethernet MTU size; roughly 1500 bytes), and IP or non-IP (e.g., ITU-T
       H.324/M) based wireless communication systems with preferred
       transmission unit sizes of 254 bytes or less.  To prevent media
       transcoding between the two worlds, and to avoid undesirable
       packetization overhead, a NAL unit aggregation scheme is introduced.

       Two types of aggregation packets are defined by this specification:

       o  Single-time aggregation packet (STAP): aggregates NAL units with
          identical NALU-time.  Two types of STAPs are defined, one without
          DON (STAP-A) and another including DON (STAP-B).

       o  Multi-time aggregation packet (MTAP): aggregates NAL units with
          potentially differing NALU-time.  Two different MTAPs are defined,
          differing in the length of the NAL unit timestamp offset.

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       The term NALU-time is defined as the value that the RTP timestamp
       would have if that NAL unit would be transported in its own RTP
       packet.

       Each NAL unit to be carried in an aggregation packet is encapsulated
       in an aggregation unit.  Please see below for the four different
       aggregation units and their characteristics.

       The structure of the RTP payload format for aggregation packets is
       presented in Figure 3.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |F|NRI|  type   |                                               |
          +-+-+-+-+-+-+-+-+                                               |
          |                                                               |
          |             one or more aggregation units                     |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 3.  RTP payload format for aggregation packets

       MTAPs and STAPs share the following packetization rules:  The RTP
       timestamp MUST be set to the earliest of the NALU times of all the
       NAL units to be aggregated.  The type field of the NAL unit type
       octet MUST be set to the appropriate value, as indicated in Table 4.
       The F bit MUST be cleared if all F bits of the aggregated NAL units
       are zero; otherwise, it MUST be set.  The value of NRI MUST be the
       maximum of all the NAL units carried in the aggregation packet.

          Table 4.  Type field for STAPs and MTAPs

          Type   Packet    Timestamp offset   DON related fields
                           field length       (DON, DONB, DOND)
                           (in bits)          present
          --------------------------------------------------------
          24     STAP-A       0                 no
          25     STAP-B       0                 yes
          26     MTAP16      16                 yes
          27     MTAP24      24                 yes

       The marker bit in the RTP header is set to the value that the marker
       bit of the last NAL unit of the aggregated packet would have if it
       were transported in its own RTP packet.

    Wenger, et al.              Standards Track                    [Page 19]

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       The payload of an aggregation packet consists of one or more
       aggregation units.  See sections 5.7.1 and 5.7.2 for the four
       different types of aggregation units.  An aggregation packet can
       carry as many aggregation units as necessary; however, the total
       amount of data in an aggregation packet obviously MUST fit into an IP
       packet, and the size SHOULD be chosen so that the resulting IP packet
       is smaller than the MTU size.  An aggregation packet MUST NOT contain
       fragmentation units specified in section 5.8.  Aggregation packets
       MUST NOT be nested; i.e., an aggregation packet MUST NOT contain
       another aggregation packet.

    5.7.1.  Single-Time Aggregation Packet

       Single-time aggregation packet (STAP) SHOULD be used whenever NAL
       units are aggregated that all share the same NALU-time.  The payload
       of an STAP-A does not include DON and consists of at least one
       single-time aggregation unit, as presented in Figure 4.  The payload
       of an STAP-B consists of a 16-bit unsigned decoding order number
       (DON) (in network byte order) followed by at least one single-time
       aggregation unit, as presented in Figure 5.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          :                                               |
          +-+-+-+-+-+-+-+-+                                               |
          |                                                               |
          |                single-time aggregation units                  |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4.  Payload format for STAP-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 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          :  decoding order number (DON)  |               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
          |                                                               |
          |                single-time aggregation units                  |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 5.  Payload format for STAP-B

    Wenger, et al.              Standards Track                    [Page 20]

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       The DON field specifies the value of DON for the first NAL unit in an
       STAP-B in transmission order.  For each successive NAL unit in
       appearance order in an STAP-B, the value of DON is equal to (the
       value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in
       which '%' stands for the modulo operation.

       A single-time aggregation unit consists of 16-bit unsigned size
       information (in network byte order) that indicates the size of the
       following NAL unit in bytes (excluding these two octets, but
       including the NAL unit type octet of the NAL unit), followed by the
       NAL unit itself, including its NAL unit type byte.  A single-time
       aggregation unit is byte aligned within the RTP payload, but it may
       not be aligned on a 32-bit word boundary.  Figure 6 presents the
       structure of the single-time aggregation unit.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          :        NAL unit size          |               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
          |                                                               |
          |                           NAL unit                            |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 6.  Structure for single-time aggregation unit

    Wenger, et al.              Standards Track                    [Page 21]

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       Figure 7 presents an example of an RTP packet that contains an STAP-
       A.  The STAP contains two single-time aggregation units, labeled as 1
       and 2 in the figure.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                          RTP Header                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |STAP-A NAL HDR |         NALU 1 Size           | NALU 1 HDR    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                         NALU 1 Data                           |
          :                                                               :
          +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |               | NALU 2 Size                   | NALU 2 HDR    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                         NALU 2 Data                           |
          :                                                               :
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 7.  An example of an RTP packet including an STAP-A and two
                     single-time aggregation units

    Wenger, et al.              Standards Track                    [Page 22]

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       Figure 8 presents an example of an RTP packet that contains an STAP-
       B.  The STAP contains two single-time aggregation units, labeled as 1
       and 2 in the figure.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                          RTP Header                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |STAP-B NAL HDR | DON                           | NALU 1 Size   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | NALU 1 Size   | NALU 1 HDR    | NALU 1 Data                   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
          :                                                               :
          +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |               | NALU 2 Size                   | NALU 2 HDR    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                       NALU 2 Data                             |
          :                                                               :
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 8.  An example of an RTP packet including an STAP-B and two
                     single-time aggregation units

    5.7.2.  Multi-Time Aggregation Packets (MTAPs)

       The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding
       order number base (DONB) (in network byte order) and one or more
       multi-time aggregation units, as presented in Figure 9.  DONB MUST
       contain the value of DON for the first NAL unit in the NAL unit
       decoding order among the NAL units of the MTAP.

          Informative note: The first NAL unit in the NAL unit decoding
          order is not necessarily the first NAL unit in the order in which
          the NAL units are encapsulated in an MTAP.

    Wenger, et al.              Standards Track                    [Page 23]

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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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          :  decoding order number base   |               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
          |                                                               |
          |                 multi-time aggregation units                  |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 9.  NAL unit payload format for MTAPs

       Two different multi-time aggregation units are defined in this
       specification.  Both of them consist of 16 bits unsigned size
       information of the following NAL unit (in network byte order), an 8-
       bit unsigned decoding order number difference (DOND), and n bits (in
       network byte order) of timestamp offset (TS offset) for this NAL
       unit, whereby n can be 16 or 24.  The choice between the different
       MTAP types (MTAP16 and MTAP24) is application dependent: the larger
       the timestamp offset is, the higher the flexibility of the MTAP, but
       the overhead is also higher.

       The structure of the multi-time aggregation units for MTAP16 and
       MTAP24 are presented in Figures 10 and 11, respectively.  The
       starting or ending position of an aggregation unit within a packet is
       NOT REQUIRED to be on a 32-bit word boundary.  The DON of the
       following NAL unit is equal to (DONB + DOND) % 65536, in which %
       denotes the modulo operation.  This memo does not specify how the NAL
       units within an MTAP are ordered, but, in most cases, NAL unit
       decoding order SHOULD be used.

       The timestamp offset field MUST be set to a value equal to the value
       of the following formula: If the NALU-time is larger than or equal to
       the RTP timestamp of the packet, then the timestamp offset equals
       (the NALU-time of the NAL unit - the RTP timestamp of the packet).
       If the NALU-time is smaller than the RTP timestamp of the packet,
       then the timestamp offset is equal to the NALU-time + (2^32 - the RTP
       timestamp of the packet).

    Wenger, et al.              Standards Track                    [Page 24]

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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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          :        NAL unit size          |      DOND     |  TS offset    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  TS offset    |                                               |
          +-+-+-+-+-+-+-+-+              NAL unit                         |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 10.  Multi-time aggregation unit for MTAP16

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          :        NALU unit size         |      DOND     |  TS offset    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |         TS offset             |                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
          |                              NAL unit                         |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 11.  Multi-time aggregation unit for MTAP24

       For the "earliest" multi-time aggregation unit in an MTAP the
       timestamp offset MUST be zero.  Hence, the RTP timestamp of the MTAP
       itself is identical to the earliest NALU-time.

          Informative note: The "earliest" multi-time aggregation unit is
          the one that would have the smallest extended RTP timestamp among
          all the aggregation units of an MTAP if the aggregation units were
          encapsulated in single NAL unit packets.  An extended timestamp is
          a timestamp that has more than 32 bits and is capable of counting
          the wraparound of the timestamp field, thus enabling one to
          determine the smallest value if the timestamp wraps.  Such an
          "earliest" aggregation unit may not be the first one in the order
          in which the aggregation units are encapsulated in an MTAP.  The
          "earliest" NAL unit need not be the same as the first NAL unit in
          the NAL unit decoding order either.

    Wenger, et al.              Standards Track                    [Page 25]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

       Figure 12 presents an example of an RTP packet that contains a
       multi-time aggregation packet of type MTAP16 that contains two
       multi-time aggregation units, labeled as 1 and 2 in the figure.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                          RTP Header                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |MTAP16 NAL HDR |  decoding order number base   | NALU 1 Size   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offset        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  NALU 1 HDR   |  NALU 1 DATA                                  |
          +-+-+-+-+-+-+-+-+                                               +
          :                                                               :
          +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |               | NALU 2 SIZE                   |  NALU 2 DOND  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |       NALU 2 TS offset        |  NALU 2 HDR   |  NALU 2 DATA  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
          :                                                               :
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 12.  An RTP packet including a multi-time aggregation
                      packet of type MTAP16 and two multi-time aggregation
                      units

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       Figure 13 presents an example of an RTP packet that contains a
       multi-time aggregation packet of type MTAP24 that contains two
       multi-time aggregation units, labeled as 1 and 2 in the figure.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                          RTP Header                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |MTAP24 NAL HDR |  decoding order number base   | NALU 1 Size   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offs          |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |NALU 1 TS offs |  NALU 1 HDR   |  NALU 1 DATA                  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
          :                                                               :
          +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |               | NALU 2 SIZE                   |  NALU 2 DOND  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |       NALU 2 TS offset                        |  NALU 2 HDR   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  NALU 2 DATA                                                  |
          :                                                               :
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 13.  An RTP packet including a multi-time aggregation
                      packet of type MTAP24 and two multi-time aggregation
                      units

    5.8.  Fragmentation Units (FUs)

       This payload type allows fragmenting a NAL unit into several RTP
       packets.  Doing so on the application layer instead of relying on
       lower layer fragmentation (e.g., by IP) has the following advantages:

       o  The payload format is capable of transporting NAL units bigger
          than 64 kbytes over an IPv4 network that may be present in pre-
          recorded video, particularly in High Definition formats (there is
          a limit of the number of slices per picture, which results in a
          limit of NAL units per picture, which may result in big NAL
          units).

       o  The fragmentation mechanism allows fragmenting a single picture
          and applying generic forward error correction as described in
          section 12.5.

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       Fragmentation is defined only for a single NAL unit and not for any
       aggregation packets.  A fragment of a NAL unit consists of an integer
       number of consecutive octets of that NAL unit.  Each octet of the NAL
       unit MUST be part of exactly one fragment of that NAL unit.
       Fragments of the same NAL unit MUST be sent in consecutive order with
       ascending RTP sequence numbers (with no other RTP packets within the
       same RTP packet stream being sent between the first and last
       fragment).  Similarly, a NAL unit MUST be reassembled in RTP sequence
       number order.

       When a NAL unit is fragmented and conveyed within fragmentation units
       (FUs), it is referred to as a fragmented NAL unit.  STAPs and MTAPs
       MUST NOT be fragmented.  FUs MUST NOT be nested; i.e., an FU MUST NOT
       contain another FU.

       The RTP timestamp of an RTP packet carrying an FU is set to the NALU
       time of the fragmented NAL unit.

       Figure 14 presents the RTP payload format for FU-As.  An FU-A
       consists of a fragmentation unit indicator of one octet, a
       fragmentation unit header of one octet, and a fragmentation unit
       payload.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | FU indicator  |   FU header   |                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
          |                                                               |
          |                         FU payload                            |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 14.  RTP payload format for FU-A

    Wenger, et al.              Standards Track                    [Page 28]

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       Figure 15 presents the RTP payload format for FU-Bs.  An FU-B
       consists of a fragmentation unit indicator of one octet, a
       fragmentation unit header of one octet, a decoding order number (DON)
       (in network byte order), and a fragmentation unit payload.  In other
       words, the structure of FU-B is the same as the structure of FU-A,
       except for the additional DON field.

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | FU indicator  |   FU header   |               DON             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
          |                                                               |
          |                         FU payload                            |
          |                                                               |
          |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               :...OPTIONAL RTP padding        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 15.  RTP payload format for FU-B

       NAL unit type FU-B MUST be used in the interleaved packetization mode
       for the first fragmentation unit of a fragmented NAL unit.  NAL unit
       type FU-B MUST NOT be used in any other case.  In other words, in the
       interleaved packetization mode, each NALU that is fragmented has an
       FU-B as the first fragment, followed by one or more FU-A fragments.

       The FU indicator octet has the following format:

          +---------------+
          |0|1|2|3|4|5|6|7|
          +-+-+-+-+-+-+-+-+
          |F|NRI|  Type   |
          +---------------+

       Values equal to 28 and 29 in the Type field of the FU indicator octet
       identify an FU-A and an FU-B, respectively.  The use of the F bit is
       described in section 5.3.  The value of the NRI field MUST be set
       according to the value of the NRI field in the fragmented NAL unit.

       The FU header has the following format:

          +---------------+
          |0|1|2|3|4|5|6|7|
          +-+-+-+-+-+-+-+-+
          |S|E|R|  Type   |
          +---------------+

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       S: 1 bit
          When set to one, the Start bit indicates the start of a fragmented
          NAL unit.  When the following FU payload is not the start of a
          fragmented NAL unit payload, the Start bit is set to zero.

       E: 1 bit
          When set to one, the End bit indicates the end of a fragmented NAL
          unit, i.e., the last byte of the payload is also the last byte of
          the fragmented NAL unit.  When the following FU payload is not the
          last fragment of a fragmented NAL unit, the End bit is set to
          zero.

       R: 1 bit
          The Reserved bit MUST be equal to 0 and MUST be ignored by the
          receiver.

       Type: 5 bits
          The NAL unit payload type as defined in table 7-1 of [1].

       The value of DON in FU-Bs is selected as described in section 5.5.

          Informative note: The DON field in FU-Bs allows gateways to
          fragment NAL units to FU-Bs without organizing the incoming NAL
          units to the NAL unit decoding order.

       A fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the
       Start bit and End bit MUST NOT both be set to one in the same FU
       header.

       The FU payload consists of fragments of the payload of the fragmented
       NAL unit so that if the fragmentation unit payloads of consecutive
       FUs are sequentially concatenated, the payload of the fragmented NAL
       unit can be reconstructed.  The NAL unit type octet of the fragmented
       NAL unit is not included as such in the fragmentation unit payload,
       but rather the information of the NAL unit type octet of the
       fragmented NAL unit is conveyed in F and NRI fields of the FU
       indicator octet of the fragmentation unit and in the type field of
       the FU header.  A FU payload MAY have any number of octets and MAY be
       empty.

          Informative note: Empty FUs are allowed to reduce the latency of a
          certain class of senders in nearly lossless environments.  These
          senders can be characterized in that they packetize NALU fragments
          before the NALU is completely generated and, hence, before the
          NALU size is known.  If zero-length NALU fragments were not
          allowed, the sender would have to generate at least one bit of
          data of the following fragment before the current fragment could
          be sent.  Due to the characteristics of H.264, where sometimes

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          several macroblocks occupy zero bits, this is undesirable and can
          add delay.  However, the (potential) use of zero-length NALUs
          should be carefully weighed against the increased risk of the loss
          of the NALU because of the additional packets employed for its
          transmission.

       If a fragmentation unit is lost, the receiver SHOULD discard all
       following fragmentation units in transmission order corresponding to
       the same fragmented NAL unit.

       A receiver in an endpoint or in a MANE MAY aggregate the first n-1
       fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
       n of that NAL unit is not received.  In this case, the
       forbidden_zero_bit of the NAL unit MUST be set to one to indicate a
       syntax violation.

    6.  Packetization Rules

       The packetization modes are introduced in section 5.2.  The
       packetization rules common to more than one of the packetization
       modes are specified in section 6.1.  The packetization rules for the
       single NAL unit mode, the non-interleaved mode, and the interleaved
       mode are specified in sections 6.2, 6.3, and 6.4, respectively.

    6.1.  Common Packetization Rules

       All senders MUST enforce the following packetization rules regardless
       of the packetization mode in use:

       o  Coded slice NAL units or coded slice data partition NAL units
          belonging to the same coded picture (and thus sharing the same RTP
          timestamp value) MAY be sent in any order permitted by the
          applicable profile defined in [1]; however, for delay-critical
          systems, they SHOULD be sent in their original coding order to
          minimize the delay.  Note that the coding order is not necessarily
          the scan order, but the order the NAL packets become available to
          the RTP stack.

       o  Parameter sets are handled in accordance with the rules and
          recommendations given in section 8.4.

       o  MANEs MUST NOT duplicate any NAL unit except for sequence or
          picture parameter set NAL units, as neither this memo nor the
          H.264 specification provides means to identify duplicated NAL
          units.  Sequence and picture parameter set NAL units MAY be
          duplicated to make their correct reception more probable, but any
          such duplication MUST NOT affect the contents of any active
          sequence or picture parameter set.  Duplication SHOULD be

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          performed on the application layer and not by duplicating RTP
          packets (with identical sequence numbers).

       Senders using the non-interleaved mode and the interleaved mode MUST
       enforce the following packetization rule:

       o  MANEs MAY convert single NAL unit packets into one aggregation
          packet, convert an aggregation packet into several single NAL unit
          packets, or mix both concepts, in an RTP translator.  The RTP
          translator SHOULD take into account at least the following
          parameters: path MTU size, unequal protection mechanisms (e.g.,
          through packet-based FEC according to RFC 2733 [18], especially
          for sequence and picture parameter set NAL units and coded slice
          data partition A NAL units), bearable latency of the system, and
          buffering capabilities of the receiver.

          Informative note: An RTP translator is required to handle RTCP as
          per RFC 3550.

    6.2.  Single NAL Unit Mode

       This mode is in use when the value of the OPTIONAL packetization-mode
       MIME parameter is equal to 0, the packetization-mode is not present,
       or no other packetization mode is signaled by external means.  All
       receivers MUST support this mode.  It is primarily intended for low-
       delay applications that are compatible with systems using ITU-T
       Recommendation H.241 [15] (see section 12.1).  Only single NAL unit
       packets MAY be used in this mode.  STAPs, MTAPs, and FUs MUST NOT be
       used.  The transmission order of single NAL unit packets MUST comply
       with the NAL unit decoding order.

    6.3.  Non-Interleaved Mode

       This mode is in use when the value of the OPTIONAL packetization-mode
       MIME parameter is equal to 1 or the mode is turned on by external
       means.  This mode SHOULD be supported.  It is primarily intended for
       low-delay applications.  Only single NAL unit packets, STAP-As, and
       FU-As MAY be used in this mode.  STAP-Bs, MTAPs, and FU-Bs MUST NOT
       be used.  The transmission order of NAL units MUST comply with the
       NAL unit decoding order.

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    6.4.  Interleaved Mode

       This mode is in use when the value of the OPTIONAL packetization-mode
       MIME parameter is equal to 2 or the mode is turned on by external
       means.  Some receivers MAY support this mode.  STAP-Bs, MTAPs, FU-As,
       and FU-Bs MAY be used.  STAP-As and single NAL unit packets MUST NOT
       be used.  The transmission order of packets and NAL units is
       constrained as specified in section 5.5.

    7.  De-Packetization Process (Informative)

       The de-packetization process is implementation dependent.  Therefore,
       the following description should be seen as an example of a suitable
       implementation.  Other schemes may be used as well.  Optimizations
       relative to the described algorithms are likely possible.  Section
       7.1 presents the de-packetization process for the single NAL unit and
       non-interleaved packetization modes, whereas section 7.2 describes
       the process for the interleaved mode.  Section 7.3 includes
       additional decapsulation guidelines for intelligent receivers.

       All normal RTP mechanisms related to buffer management apply.  In
       particular, duplicated or outdated RTP packets (as indicated by the
       RTP sequences number and the RTP timestamp) are removed.  To
       determine the exact time for decoding, factors such as a possible
       intentional delay to allow for proper inter-stream synchronization
       must be factored in.

    7.1.  Single NAL Unit and Non-Interleaved Mode

       The receiver includes a receiver buffer to compensate for
       transmission delay jitter.  The receiver stores incoming packets in
       reception order into the receiver buffer.  Packets are decapsulated
       in RTP sequence number order.  If a decapsulated packet is a single
       NAL unit packet, the NAL unit contained in the packet is passed
       directly to the decoder.  If a decapsulated packet is an STAP-A, the
       NAL units contained in the packet are passed to the decoder in the
       order in which they are encapsulated in the packet.  If a
       decapsulated packet is an FU-A, all the fragments of the fragmented
       NAL unit are concatenated and passed to the decoder.

          Informative note: If the decoder supports Arbitrary Slice Order,
          coded slices of a picture can be passed to the decoder in any
          order regardless of their reception and transmission order.

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    7.2.  Interleaved Mode

       The general concept behind these de-packetization rules is to reorder
       NAL units from transmission order to the NAL unit decoding order.

       The receiver includes a receiver buffer, which is used to compensate
       for transmission delay jitter and to reorder packets from
       transmission order to the NAL unit decoding order.  In this section,
       the receiver operation is described under the assumption that there
       is no transmission delay jitter.  To make a difference from a
       practical receiver buffer that is also used for compensation of
       transmission delay jitter, the receiver buffer is here after called
       the deinterleaving buffer in this section.  Receivers SHOULD also
       prepare for transmission delay jitter; i.e., either reserve separate
       buffers for transmission delay jitter buffering and deinterleaving
       buffering or use a receiver buffer for both transmission delay jitter
       and deinterleaving.  Moreover, receivers SHOULD take transmission
       delay jitter into account in the buffering operation; e.g., by
       additional initial buffering before starting of decoding and
       playback.

       This section is organized as follows: subsection 7.2.1 presents how
       to calculate the size of the deinterleaving buffer.  Subsection 7.2.2
       specifies the receiver process how to organize received NAL units to
       the NAL unit decoding order.

    7.2.1.  Size of the Deinterleaving Buffer

       When SDP Offer/Answer model or any other capability exchange
       procedure is used in session setup, the properties of the received
       stream SHOULD be such that the receiver capabilities are not
       exceeded.  In the SDP Offer/Answer model, the receiver can indicate
       its capabilities to allocate a deinterleaving buffer with the deint-
       buf-cap MIME parameter.  The sender indicates the requirement for the
       deinterleaving buffer size with the sprop-deint-buf-req MIME
       parameter.  It is therefore RECOMMENDED to set the deinterleaving
       buffer size, in terms of number of bytes, equal to or greater than
       the value of sprop-deint-buf-req MIME parameter.  See section 8.1 for
       further information on deint-buf-cap and sprop-deint-buf-req MIME
       parameters and section 8.2.2 for further information on their use in
       SDP Offer/Answer model.

       When a declarative session description is used in session setup, the
       sprop-deint-buf-req MIME parameter signals the requirement for the
       deinterleaving buffer size.  It is therefore RECOMMENDED to set the
       deinterleaving buffer size, in terms of number of bytes, equal to or
       greater than the value of sprop-deint-buf-req MIME parameter.

    Wenger, et al.              Standards Track                    [Page 34]

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    7.2.2.  Deinterleaving Process

       There are two buffering states in the receiver: initial buffering and
       buffering while playing.  Initial buffering occurs when the RTP
       session is initialized.  After initial buffering, decoding and
       playback is started, and the buffering-while-playing mode is used.

       Regardless of the buffering state, the receiver stores incoming NAL
       units, in reception order, in the deinterleaving buffer as follows.
       NAL units of aggregation packets are stored in the deinterleaving
       buffer individually.  The value of DON is calculated and stored for
       all NAL units.

       The receiver operation is described below with the help of the
       following functions and constants:

       o  Function AbsDON is specified in section 8.1.

       o  Function don_diff is specified in section 5.5.

       o  Constant N is the value of the OPTIONAL sprop-interleaving-depth
          MIME type parameter (see section 8.1) incremented by 1.

       Initial buffering lasts until one of the following conditions is
       fulfilled:

       o  There are N VCL NAL units in the deinterleaving buffer.

       o  If sprop-max-don-diff is present, don_diff(m,n) is greater than
          the value of sprop-max-don-diff, in which n corresponds to the NAL
          unit having the greatest value of AbsDON among the received NAL
          units and m corresponds to the NAL unit having the smallest value
          of AbsDON among the received NAL units.

       o  Initial buffering has lasted for the duration equal to or greater
          than the value of the OPTIONAL sprop-init-buf-time MIME parameter.

       The NAL units to be removed from the deinterleaving buffer are
       determined as follows:

       o  If the deinterleaving buffer contains at least N VCL NAL units,
          NAL units are removed from the deinterleaving buffer and passed to
          the decoder in the order specified below until the buffer contains
          N-1 VCL NAL units.

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       o  If sprop-max-don-diff is present, all NAL units m for which
          don_diff(m,n) is greater than sprop-max-don-diff are removed from
          the deinterleaving buffer and passed to the decoder in the order
          specified below.  Herein, n corresponds to the NAL unit having the
          greatest value of AbsDON among the received NAL units.

       The order in which NAL units are passed to the decoder is specified
       as follows:

       o  Let PDON be a variable that is initialized to 0 at the beginning
          of the an RTP session.

       o  For each NAL unit associated with a value of DON, a DON distance
          is calculated as follows.  If the value of DON of the NAL unit is
          larger than the value of PDON, the DON distance is equal to DON -
          PDON.  Otherwise, the DON distance is equal to 65535 - PDON + DON
          + 1.

       o  NAL units are delivered to the decoder in ascending order of DON
          distance.  If several NAL units share the same value of DON
          distance, they can be passed to the decoder in any order.

       o  When a desired number of NAL units have been passed to the
          decoder, the value of PDON is set to the value of DON for the last
          NAL unit passed to the decoder.

    7.3.  Additional De-Packetization Guidelines

       The following additional de-packetization rules may be used to
       implement an operational H.264 de-packetizer:

       o  Intelligent RTP receivers (e.g., in gateways) may identify lost
          coded slice data partitions A (DPAs).  If a lost DPA is found, a
          gateway may decide not to send the corresponding coded slice data
          partitions B and C, as their information is meaningless for H.264
          decoders.  In this way a MANE can reduce network load by
          discarding useless packets without parsing a complex bitstream.

       o  Intelligent RTP receivers (e.g., in gateways) may identify lost
          FUs.  If a lost FU is found, a gateway may decide not to send the
          following FUs of the same fragmented NAL unit, as their
          information is meaningless for H.264 decoders.  In this way a MANE
          can reduce network load by discarding useless packets without
          parsing a complex bitstream.

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       o  Intelligent receivers having to discard packets or NALUs should
          first discard all packets/NALUs in which the value of the NRI
          field of the NAL unit type octet is equal to 0.  This will
          minimize the impact on user experience and keep the reference
          pictures intact.  If more packets have to be discarded, then
          packets with a numerically lower NRI value should be discarded
          before packets with a numerically higher NRI value.  However,
          discarding any packets with an NRI bigger than 0 very likely leads
          to decoder drift and SHOULD be avoided.

    8.  Payload Format Parameters

       This section specifies the parameters that MAY be used to select
       optional features of the payload format and certain features of the
       bitstream.  The parameters are specified here as part of the MIME
       subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.  A
       mapping of the parameters into the Session Description Protocol (SDP)
       [5] is also provided for applications that use SDP.  Equivalent
       parameters could be defined elsewhere for use with control protocols
       that do not use MIME or SDP.

       Some parameters provide a receiver with the properties of the stream
       that will be sent.  The name of all these parameters starts with
       "sprop" for stream properties.  Some of these "sprop" parameters are
       limited by other payload or codec configuration parameters.  For
       example, the sprop-parameter-sets parameter is constrained by the
       profile-level-id parameter.  The media sender selects all "sprop"
       parameters rather than the receiver.  This uncommon characteristic of
       the "sprop" parameters may not be compatible with some signaling
       protocol concepts, in which case the use of these parameters SHOULD
       be avoided.

    8.1.  MIME Registration

       The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
       allocated from the IETF tree.

       The receiver MUST ignore any unspecified parameter.

       Media Type name:     video

       Media subtype name:  H264

       Required parameters: none

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       OPTIONAL parameters:
           profile-level-id:
                            A base16 [6] (hexadecimal) representation of
                            the following three bytes in the sequence
                            parameter set NAL unit specified in [1]: 1)
                            profile_idc, 2) a byte herein referred to as
                            profile-iop, composed of the values of
                            constraint_set0_flag, constraint_set1_flag,
                            constraint_set2_flag, and reserved_zero_5bits
                            in bit-significance order, starting from the
                            most significant bit, and 3) level_idc.  Note
                            that reserved_zero_5bits is required to be
                            equal to 0 in [1], but other values for it may
                            be specified in the future by ITU-T or ISO/IEC.

                            If the profile-level-id parameter is used to
                            indicate properties of a NAL unit stream, it
                            indicates the profile and level that a decoder
                            has to support in order to comply with [1] when
                            it decodes the stream.  The profile-iop byte
                            indicates whether the NAL unit stream also
                            obeys all constraints of the indicated profiles
                            as follows.  If bit 7 (the most significant
                            bit), bit 6, or bit 5 of profile-iop is equal
                            to 1, all constraints of the Baseline profile,
                            the Main profile, or the Extended profile,
                            respectively, are obeyed in the NAL unit
                            stream.

                            If the profile-level-id parameter is used for
                            capability exchange or session setup procedure,
                            it indicates the profile that the codec
                            supports and the highest level
                            supported for the signaled profile.  The
                            profile-iop byte indicates whether the codec
                            has additional limitations whereby only the
                            common subset of the algorithmic features and
                            limitations of the profiles signaled with the
                            profile-iop byte and of the profile indicated
                            by profile_idc is supported by the codec.  For
                            example, if a codec supports only the common
                            subset of the coding tools of the Baseline
                            profile and the Main profile at level 2.1 and
                            below, the profile-level-id becomes 42E015, in
                            which 42 stands for the Baseline profile, E0
                            indicates that only the common subset for all
                            profiles is supported, and 15 indicates level
                            2.1.

    Wenger, et al.              Standards Track                    [Page 38]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                                Informative note: Capability exchange and
                                session setup procedures should provide
                                means to list the capabilities for each
                                supported codec profile separately.  For
                                example, the one-of-N codec selection
                                procedure of the SDP Offer/Answer model can
                                be used (section 10.2 of [7]).

                            If no profile-level-id is present, the Baseline
                            Profile without additional constraints at Level
                            1 MUST be implied.

           max-mbps, max-fs, max-cpb, max-dpb, and max-br:
                            These parameters MAY be used to signal the
                            capabilities of a receiver implementation.
                            These parameters MUST NOT be used for any other
                            purpose.  The profile-level-id parameter MUST
                            be present in the same receiver capability
                            description that contains any of these
                            parameters.  The level conveyed in the value of
                            the profile-level-id parameter MUST be such
                            that the receiver is fully capable of
                            supporting.  max-mbps, max-fs, max-cpb, max-
                            dpb, and max-br MAY be used to indicate
                            capabilities of the receiver that extend the
                            required capabilities of the signaled level, as
                            specified below.

                            When more than one parameter from the set (max-
                            mbps, max-fs, max-cpb, max-dpb, max-br) is
                            present, the receiver MUST support all signaled
                            capabilities simultaneously.  For example, if
                            both max-mbps and max-br are present, the
                            signaled level with the extension of both the
                            frame rate and bit rate is supported.  That is,
                            the receiver is able to decode NAL unit
                            streams in which the macroblock processing rate
                            is up to max-mbps (inclusive), the bit rate is
                            up to max-br (inclusive), the coded picture
                            buffer size is derived as specified in the
                            semantics of the max-br parameter below, and
                            other properties comply with the level
                            specified in the value of the profile-level-id
                            parameter.

                            A receiver MUST NOT signal values of max-
                            mbps, max-fs, max-cpb, max-dpb, and max-br that
                            meet the requirements of a higher level,

    Wenger, et al.              Standards Track                    [Page 39]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                            referred to as level A herein, compared to the
                            level specified in the value of the profile-
                            level-id parameter, if the receiver can support
                            all the properties of level A.

                                Informative note: When the OPTIONAL MIME
                                type parameters are used to signal the
                                properties of a NAL unit stream, max-mbps,
                                max-fs, max-cpb, max-dpb, and max-br are
                                not present, and the value of profile-
                                level-id must always be such that the NAL
                                unit stream complies fully with the
                                specified profile and level.

           max-mbps:        The value of max-mbps is an integer indicating
                            the maximum macroblock processing rate in units
                            of macroblocks per second.  The max-mbps
                            parameter signals that the receiver is capable
                            of decoding video at a higher rate than is
                            required by the signaled level conveyed in the
                            value of the profile-level-id parameter.  When
                            max-mbps is signaled, the receiver MUST be able
                            to decode NAL unit streams that conform to the
                            signaled level, with the exception that the
                            MaxMBPS value in Table A-1 of [1] for the
                            signaled level is replaced with the value of
                            max-mbps.  The value of max-mbps MUST be
                            greater than or equal to the value of MaxMBPS
                            for the level given in Table A-1 of [1].
                            Senders MAY use this knowledge to send pictures
                            of a given size at a higher picture rate than
                            is indicated in the signaled level.

           max-fs:          The value of max-fs is an integer indicating
                            the maximum frame size in units of macroblocks.
                            The max-fs parameter signals that the receiver
                            is capable of decoding larger picture sizes
                            than are required by the signaled level conveyed
                            in the value of the profile-level-id parameter.
                            When max-fs is signaled, the receiver MUST be
                            able to decode NAL unit streams that conform to
                            the signaled level, with the exception that the
                            MaxFS value in Table A-1 of [1] for the
                            signaled level is replaced with the value of
                            max-fs.  The value of max-fs MUST be greater
                            than or equal to the value of MaxFS for the
                            level given in Table A-1 of [1].  Senders MAY
                            use this knowledge to send larger pictures at a

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                            proportionally lower frame rate than is
                            indicated in the signaled level.

           max-cpb          The value of max-cpb is an integer indicating
                            the maximum coded picture buffer size in units
                            of 1000 bits for the VCL HRD parameters (see
                            A.3.1 item i of [1]) and in units of 1200 bits
                            for the NAL HRD parameters (see A.3.1 item j of
                            [1]).  The max-cpb parameter signals that the
                            receiver has more memory than the minimum
                            amount of coded picture buffer memory required
                            by the signaled level conveyed in the value of
                            the profile-level-id parameter.  When max-cpb
                            is signaled, the receiver MUST be able to
                            decode NAL unit streams that conform to the
                            signaled level, with the exception that the
                            MaxCPB value in Table A-1 of [1] for the
                            signaled level is replaced with the value of
                            max-cpb.  The value of max-cpb MUST be greater
                            than or equal to the value of MaxCPB for the
                            level given in Table A-1 of [1].  Senders MAY
                            use this knowledge to construct coded video
                            streams with greater variation of bit rate
                            than can be achieved with the
                            MaxCPB value in Table A-1 of [1].

                                Informative note: The coded picture buffer
                                is used in the hypothetical reference
                                decoder (Annex C) of H.264.  The use of the
                                hypothetical reference decoder is
                                recommended in H.264 encoders to verify
                                that the produced bitstream conforms to the
                                standard and to control the output bitrate.
                                Thus, the coded picture buffer is
                                conceptually independent of any other
                                potential buffers in the receiver,
                                including de-interleaving and de-jitter
                                buffers.  The coded picture buffer need not
                                be implemented in decoders as specified in
                                Annex C of H.264, but rather standard-
                                compliant decoders can have any buffering
                                arrangements provided that they can decode
                                standard-compliant bitstreams.  Thus, in
                                practice, the input buffer for video
                                decoder can be integrated with de-
                                interleaving and de-jitter buffers of the
                                receiver.

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

           max-dpb:         The value of max-dpb is an integer indicating
                            the maximum decoded picture buffer size in
                            units of 1024 bytes.  The max-dpb parameter
                            signals that the receiver has more memory than
                            the minimum amount of decoded picture buffer
                            memory required by the signaled level conveyed
                            in the value of the profile-level-id parameter.
                            When max-dpb is signaled, the receiver MUST be
                            able to decode NAL unit streams that conform to
                            the signaled level, with the exception that the
                            MaxDPB value in Table A-1 of [1] for the
                            signaled level is replaced with the value of
                            max-dpb.  Consequently, a receiver that signals
                            max-dpb MUST be capable of storing the
                            following number of decoded frames,
                            complementary field pairs, and non-paired
                            fields in its decoded picture buffer:

                            Min(1024 * max-dpb / ( PicWidthInMbs *
                            FrameHeightInMbs * 256 * ChromaFormatFactor ),
                            16)

                            PicWidthInMbs, FrameHeightInMbs, and
                            ChromaFormatFactor are defined in [1].

                            The value of max-dpb MUST be greater than or
                            equal to the value of MaxDPB for the level
                            given in Table A-1 of [1].  Senders MAY use
                            this knowledge to construct coded video streams
                            with improved compression.

                                Informative note: This parameter was added
                                primarily to complement a similar codepoint
                                in the ITU-T Recommendation H.245, so as to
                                facilitate signaling gateway designs.  The
                                decoded picture buffer stores reconstructed
                                samples and is a property of the video
                                decoder only.  There is no relationship
                                between the size of the decoded picture
                                buffer and the buffers used in RTP,
                                especially de-interleaving and de-jitter
                                buffers.

           max-br:          The value of max-br is an integer indicating
                            the maximum video bit rate in units of 1000
                            bits per second for the VCL HRD parameters (see
                            A.3.1 item i of [1]) and in units of 1200 bits

    Wenger, et al.              Standards Track                    [Page 42]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                            per second for the NAL HRD parameters (see
                            A.3.1 item j of [1]).

                            The max-br parameter signals that the video
                            decoder of the receiver is capable of decoding
                            video at a higher bit rate than is required by
                            the signaled level conveyed in the value of the
                            profile-level-id parameter.  The value of max-
                            br MUST be greater than or equal to the value
                            of MaxBR for the level given in Table A-1 of
                            [1].

                            When max-br is signaled, the video codec of the
                            receiver MUST be able to decode NAL unit
                            streams that conform to the signaled level,
                            conveyed in the profile-level-id parameter,
                            with the following exceptions in the limits
                            specified by the level:
                            o The value of max-br replaces the MaxBR value
                              of the signaled level (in Table A-1 of [1]).
                            o When the max-cpb parameter is not present,
                              the result of the following formula replaces
                              the value of MaxCPB in Table A-1 of [1]:
                              (MaxCPB of the signaled level) * max-br /
                              (MaxBR of the signaled level).

                            For example, if a receiver signals capability
                            for Level 1.2 with max-br equal to 1550, this
                            indicates a maximum video bitrate of 1550
                            kbits/sec for VCL HRD parameters, a maximum
                            video bitrate of 1860 kbits/sec for NAL HRD
                            parameters, and a CPB size of 4036458 bits
                            (1550000 / 384000 * 1000 * 1000).

                            The value of max-br MUST be greater than or
                            equal to the value MaxBR for the signaled level
                            given in Table A-1 of [1].

                            Senders MAY use this knowledge to send higher
                            bitrate video as allowed in the level
                            definition of Annex A of H.264, to achieve
                            improved video quality.

                                Informative note: This parameter was added
                                primarily to complement a similar codepoint
                                in the ITU-T Recommendation H.245, so as to
                                facilitate signaling gateway designs.  No
                                assumption can be made from the value of

    Wenger, et al.              Standards Track                    [Page 43]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                                this parameter that the network is capable
                                of handling such bit rates at any given
                                time.  In particular, no conclusion can be
                                drawn that the signaled bit rate is
                                possible under congestion control
                                constraints.

          redundant-pic-cap:
                            This parameter signals the capabilities of a
                            receiver implementation.  When equal to 0, the
                            parameter indicates that the receiver makes no
                            attempt to use redundant coded pictures to
                            correct incorrectly decoded primary coded
                            pictures.  When equal to 0, the receiver is not
                            capable of using redundant slices; therefore, a
                            sender SHOULD avoid sending redundant slices to
                            save bandwidth.  When equal to 1, the receiver
                            is capable of decoding any such redundant slice
                            that covers a corrupted area in a primary
                            decoded picture (at least partly), and therefore
                            a sender MAY send redundant slices.  When the
                            parameter is not present, then a value of 0
                            MUST be used for redundant-pic-cap.  When
                            present, the value of redundant-pic-cap MUST be
                            either 0 or 1.

                            When the profile-level-id parameter is present
                            in the same capability signaling as the
                            redundant-pic-cap parameter, and the profile
                            indicated in profile-level-id is such that it
                            disallows the use of redundant coded pictures
                            (e.g., Main Profile), the value of redundant-
                            pic-cap MUST be equal to 0.  When a receiver
                            indicates redundant-pic-cap equal to 0, the
                            received stream SHOULD NOT contain redundant
                            coded pictures.

                                Informative note: Even if redundant-pic-cap
                                is equal to 0, the decoder is able to
                                ignore redundant codec pictures provided
                                that the decoder supports such a profile
                                (Baseline, Extended) in which redundant
                                coded pictures are allowed.

                                Informative note: Even if redundant-pic-cap
                                is equal to 1, the receiver may also choose
                                other error concealment strategies to

    Wenger, et al.              Standards Track                    [Page 44]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                                replace or complement decoding of redundant
                                slices.

           sprop-parameter-sets:
                            This parameter MAY be used to convey
                            any sequence and picture parameter set NAL
                            units (herein referred to as the initial
                            parameter set NAL units) that MUST precede any
                            other NAL units in decoding order.  The
                            parameter MUST NOT be used to indicate codec
                            capability in any capability exchange
                            procedure.  The value of the parameter is the
                            base64 [6] representation of the initial
                            parameter set NAL units as specified in
                            sections 7.3.2.1 and 7.3.2.2 of [1].  The
                            parameter sets are conveyed in decoding order,
                            and no framing of the parameter set NAL units
                            takes place.  A comma is used to separate any
                            pair of parameter sets in the list.  Note that
                            the number of bytes in a parameter set NAL unit
                            is typically less than 10, but a picture
                            parameter set NAL unit can contain several
                            hundreds of bytes.

                               Informative note: When several payload
                               types are offered in the SDP Offer/Answer
                               model, each with its own sprop-parameter-
                               sets parameter, then the receiver cannot
                               assume that those parameter sets do not use
                               conflicting storage locations (i.e.,
                               identical values of parameter set
                               identifiers).  Therefore, a receiver should
                               double-buffer all sprop-parameter-sets and
                               make them available to the decoder instance
                               that decodes a certain payload type.

           parameter-add:   This parameter MAY be used to signal whether
                            the receiver of this parameter is allowed to
                            add parameter sets in its signaling response
                            using the sprop-parameter-sets MIME parameter.
                            The value of this parameter is either 0 or 1.
                            0 is equal to false; i.e., it is not allowed to
                            add parameter sets.  1 is equal to true; i.e.,
                            it is allowed to add parameter sets.  If the
                            parameter is not present, its value MUST be 1.

    Wenger, et al.              Standards Track                    [Page 45]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

           packetization-mode:
                            This parameter signals the properties of an
                            RTP payload type or the capabilities of a
                            receiver implementation.  Only a single
                            configuration point can be indicated; thus,
                            when capabilities to support more than one
                            packetization-mode are declared, multiple
                            configuration points (RTP payload types) must
                            be used.

                            When the value of packetization-mode is equal
                            to 0 or packetization-mode is not present, the
                            single NAL mode, as defined in section 6.2 of
                            RFC 3984, MUST be used.  This mode is in use in
                            standards using ITU-T Recommendation H.241 [15]
                            (see section 12.1).  When the value of
                            packetization-mode is equal to 1, the non-
                            interleaved mode, as defined in section 6.3 of
                            RFC 3984, MUST be used.  When the value of
                            packetization-mode is equal to 2, the
                            interleaved mode, as defined in section 6.4 of
                            RFC 3984, MUST be used.  The value of
                            packetization mode MUST be an integer in the
                            range of 0 to 2, inclusive.

           sprop-interleaving-depth:
                            This parameter MUST NOT be present
                            when packetization-mode is not present or the
                            value of packetization-mode is equal to 0 or 1.
                            This parameter MUST be present when the value
                            of packetization-mode is equal to 2.

                            This parameter signals the properties of a NAL
                            unit stream.  It specifies the maximum number
                            of VCL NAL units that precede any VCL NAL unit
                            in the NAL unit stream in transmission order
                            and follow the VCL NAL unit in decoding order.
                            Consequently, it is guaranteed that receivers
                            can reconstruct NAL unit decoding order when
                            the buffer size for NAL unit decoding order
                            recovery is at least the value of sprop-
                            interleaving-depth + 1 in terms of VCL NAL
                            units.

                            The value of sprop-interleaving-depth MUST be
                            an integer in the range of 0 to 32767,
                            inclusive.

    Wenger, et al.              Standards Track                    [Page 46]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

           sprop-deint-buf-req:
                            This parameter MUST NOT be present when
                            packetization-mode is not present or the value
                            of packetization-mode is equal to 0 or 1.  It
                            MUST be present when the value of
                            packetization-mode is equal to 2.

                            sprop-deint-buf-req signals the required size
                            of the deinterleaving buffer for the NAL unit
                            stream.  The value of the parameter MUST be
                            greater than or equal to the maximum buffer
                            occupancy (in units of bytes) required in such
                            a deinterleaving buffer that is specified in
                            section 7.2 of RFC 3984.  It is guaranteed that
                            receivers can perform the deinterleaving of
                            interleaved NAL units into NAL unit decoding
                            order, when the deinterleaving buffer size is
                            at least the value of sprop-deint-buf-req in
                            terms of bytes.

                            The value of sprop-deint-buf-req MUST be an
                            integer in the range of 0 to 4294967295,
                            inclusive.

                                Informative note: sprop-deint-buf-req
                                indicates the required size of the
                                deinterleaving buffer only.  When network
                                jitter can occur, an appropriately sized
                                jitter buffer has to be provisioned for
                                as well.

           deint-buf-cap:   This parameter signals the capabilities of a
                            receiver implementation and indicates the
                            amount of deinterleaving buffer space in units
                            of bytes that the receiver has available for
                            reconstructing the NAL unit decoding order.  A
                            receiver is able to handle any stream for which
                            the value of the sprop-deint-buf-req parameter
                            is smaller than or equal to this parameter.

                            If the parameter is not present, then a value
                            of 0 MUST be used for deint-buf-cap.  The value
                            of deint-buf-cap MUST be an integer in the
                            range of 0 to 4294967295, inclusive.

                                Informative note: deint-buf-cap indicates
                                the maximum possible size of the
                                deinterleaving buffer of the receiver only.

    Wenger, et al.              Standards Track                    [Page 47]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                                When network jitter can occur, an
                                appropriately sized jitter buffer has to
                                be provisioned for as well.

           sprop-init-buf-time:
                            This parameter MAY be used to signal the
                            properties of a NAL unit stream.  The parameter
                            MUST NOT be present, if the value of
                            packetization-mode is equal to 0 or 1.

                            The parameter signals the initial buffering
                            time that a receiver MUST buffer before
                            starting decoding to recover the NAL unit
                            decoding order from the transmission order.
                            The parameter is the maximum value of
                            (transmission time of a NAL unit - decoding
                            time of the NAL unit), assuming reliable and
                            instantaneous transmission, the same
                            timeline for transmission and decoding, and
                            that decoding starts when the first packet
                            arrives.

                            An example of specifying the value of sprop-
                            init-buf-time follows.  A NAL unit stream is
                            sent in the following interleaved order, in
                            which the value corresponds to the decoding
                            time and the transmission order is from left to
                            right:

                            0  2  1  3  5  4  6  8  7 ...

                            Assuming a steady transmission rate of NAL
                            units, the transmission times are:

                            0  1  2  3  4  5  6  7  8 ...

                            Subtracting the decoding time from the
                            transmission time column-wise results in the
                            following series:

                            0 -1  1  0 -1  1  0 -1  1 ...

                            Thus, in terms of intervals of NAL unit
                            transmission times, the value of
                            sprop-init-buf-time in this
                            example is 1.

    Wenger, et al.              Standards Track                    [Page 48]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                            The parameter is coded as a non-negative base10
                            integer representation in clock ticks of a 90-
                            kHz clock.  If the parameter is not present,
                            then no initial buffering time value is
                            defined.  Otherwise the value of sprop-init-
                            buf-time MUST be an integer in the range of 0
                            to 4294967295, inclusive.

                            In addition to the signaled sprop-init-buf-
                            time, receivers SHOULD take into account the
                            transmission delay jitter buffering, including
                            buffering for the delay jitter caused by
                            mixers, translators, gateways, proxies,
                            traffic-shapers, and other network elements.

           sprop-max-don-diff:
                            This parameter MAY be used to signal the
                            properties of a NAL unit stream.  It MUST NOT
                            be used to signal transmitter or receiver or
                            codec capabilities.  The parameter MUST NOT be
                            present if the value of packetization-mode is
                            equal to 0 or 1.  sprop-max-don-diff is an
                            integer in the range of 0 to 32767, inclusive.
                            If sprop-max-don-diff is not present, the value
                            of the parameter is unspecified.  sprop-max-
                            don-diff is calculated as follows:

                            sprop-max-don-diff = max{AbsDON(i) -
                            AbsDON(j)},
                            for any i and any j>i,

                            where i and j indicate the index of the NAL
                            unit in the transmission order and AbsDON
                            denotes a decoding order number of the NAL
                            unit that does not wrap around to 0 after
                            65535.  In other words, AbsDON is calculated as
                            follows: Let m and n be consecutive NAL units
                            in transmission order.  For the very first NAL
                            unit in transmission order (whose index is 0),
                            AbsDON(0) = DON(0).  For other NAL units,
                            AbsDON is calculated as follows:

                            If DON(m) == DON(n), AbsDON(n) = AbsDON(m)

                            If (DON(m) < DON(n) and DON(n) - DON(m) <
                            32768),
                            AbsDON(n) = AbsDON(m) + DON(n) - DON(m)

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                            If (DON(m) > DON(n) and DON(m) - DON(n) >=
                            32768),
                            AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)

                            If (DON(m) < DON(n) and DON(n) - DON(m) >=
                            32768),

                            AbsDON(n) = AbsDON(m) - (DON(m) + 65536 -
                            DON(n))

                            If (DON(m) > DON(n) and DON(m) - DON(n) <
                            32768),
                            AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))

                            where DON(i) is the decoding order number of
                            the NAL unit having index i in the transmission
                            order.  The decoding order number is specified
                            in section 5.5 of RFC 3984.

                                Informative note: Receivers may use sprop-
                                max-don-diff to trigger which NAL units in
                                the receiver buffer can be passed to the
                                decoder.

         max-rcmd-nalu-size:
                            This parameter MAY be used to signal the
                            capabilities of a receiver.  The parameter MUST
                            NOT be used for any other purposes.  The value
                            of the parameter indicates the largest NALU
                            size in bytes that the receiver can handle
                            efficiently.  The parameter value is a
                            recommendation, not a strict upper boundary.
                            The sender MAY create larger NALUs but must be
                            aware that the handling of these may come at a
                            higher cost than NALUs conforming to the
                            limitation.

                            The value of max-rcmd-nalu-size MUST be an
                            integer in the range of 0 to 4294967295,
                            inclusive.  If this parameter is not specified,
                            no known limitation to the NALU size exists.
                            Senders still have to consider the MTU size
                            available between the sender and the receiver
                            and SHOULD run MTU discovery for this purpose.

                            This parameter is motivated by, for example, an
                            IP to H.223 video telephony gateway, where
                            NALUs smaller than the H.223 transport data

    Wenger, et al.              Standards Track                    [Page 50]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

                            unit will be more efficient.  A gateway may
                            terminate IP; thus, MTU discovery will normally
                            not work beyond the gateway.

                                Informative note: Setting this parameter to
                                a lower than necessary value may have a
                                negative impact.

       Encoding considerations:
                            This type is only defined for transfer via RTP
                            (RFC 3550).

                            A file format of H.264/AVC video is defined in
                            [29].  This definition is utilized by other
                            file formats, such as the 3GPP multimedia file
                            format (MIME type video/3gpp) [30] or the MP4
                            file format (MIME type video/mp4).

       Security considerations:
                            See section 9 of RFC 3984.

       Public specification:
                            Please refer to RFC 3984 and its section 15.

       Additional information:
                            None

       File extensions:     none
       Macintosh file type code: none
       Object identifier or OID: none

       Person & email address to contact for further information:
                            stewe@stewe.org

       Intended usage:      COMMON

       Author:
                            stewe@stewe.org
       Change controller:
                            IETF Audio/Video Transport working group
                            delegated from the IESG.

    Wenger, et al.              Standards Track                    [Page 51]

    RFC 3984           RTP Payload Format for H.264 Video      February 2005

    8.2.  SDP Parameters

    8.2.1.  Mapping of MIME Parameters to SDP

       The MIME media type video/H264 string is mapped to fields in the
       Session Description Protocol (SDP) [5] as follows:

       o  The media name in the "m=" line of SDP MUST be video.

       o  The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the
          MIME subtype).

       o  The clock rate in the "a=rtpmap" line MUST be 90000.

       o  The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs",
          "max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
          parameter-sets", "parameter-add", "packetization-mode", "sprop-
          interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req",
          "sprop-init-buf-time", "sprop-max-don-diff", and "max-rcmd-nalu-
          size", when present, MUST be included in the "a=fmtp" line of SDP.
          These parameters are expressed as a MIME media type string, in the
          form of a semicolon separated list of parameter=value pairs.

       An example of media representation in SDP is as follows (Baseline
       Profile, Level 3.0, some of the constraints of the Main profile may
       not be obeyed):

          m=video 49170 RTP/AVP 98
          a=rtpmap:98 H264/90000
          a=fmtp:98 profile-level-id=42A01E;
                    sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==

    8.2.2.  Usage with the SDP Offer/Answer Model

       When H.264 is offered over RTP using SDP in an Offer/Answer model [7]
       for negotiation for unicast usage, the following limitations and
       rules apply:

       o  The parameters identifying a media format configuration for H.264
          are "profile-level-id", "packetization-mode", and, if required by
          "packetization-mode", "sprop-deint-buf-req".  These three
          parameters MUST be used symmetrically; i.e., the answerer MUST
          either maintain all configuration parameters or remove the media
          format (payload type) completely, if one or more of the parameter
          values are not supported.

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             Informative note: The requirement for symmetric use applies
             only for the above three parameters and not for the other
             stream properties and capability parameters.

          To simplify handling and matching of these configurations, the
          same RTP payload type number used in the offer SHOULD also be used
          in the answer, as specified in [7].  An answer MUST NOT contain a
          payload type number used in the offer unless the configuration
          ("profile-level-id", "packetization-mode", and, if present,
          "sprop-deint-buf-req") is the same as in the offer.

             Informative note: An offerer, when receiving the answer, has to
             compare payload types not declared in the offer based on media
             type (i.e., video/h264) and the above three parameters with any
             payload types it has already declared, in order to determine
             whether the configuration in question is new or equivalent to a
             configuration already offered.

       o  The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
          "sprop-interleaving-depth", "sprop-max-don-diff", and "sprop-
          init-buf-time" describe the properties of the NAL unit stream that
          the offerer or answerer is sending for this media format
          configuration.  This differs from the normal usage of the
          Offer/Answer parameters: normally such parameters declare the
          properties of the stream that the offerer or the answerer is able
          to receive.  When dealing with H.264, the offerer assumes that the
          answerer will be able to receive media encoded using the
          configuration being offered.

             Informative note: The above parameters apply for any stream
             sent by the declaring entity with the same configuration; i.e.,
             they are dependent on their source.  Rather then being bound to
             the payload type, the values may have to be applied to another
             payload type when being sent, as they apply for the
             configuration.

       o  The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
          dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY be
          used to declare further capabilities.  Their interpretation
          depends on the direction attribute.  When the direction attribute
          is sendonly, then the parameters describe the limits of the RTP
          packets and the NAL unit stream that the sender is capable of
          producing.  When the direction attribute is sendrecv or recvonly,
          then the parameters describe the limitations of what the receiver
          accepts.

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       o  As specified above, an offerer has to include the size of the
          deinterleaving buffer in the offer for an interleaved H.264
          stream.  To enable the offerer and answerer to inform each other
          about their capabilities for deinterleaving buffering, both
          parties are RECOMMENDED to include "deint-buf-cap".  This
          information MAY be used when the value for "sprop-deint-buf-req"
          is selected in a second round of offer and answer.  For
          interleaved streams, it is also RECOMMENDED to consider offering
          multiple payload types with different buffering requirements when
          the capabilities of the receiver are unknown.

       o  The "sprop-parameter-sets" parameter is used as described above.
          In addition, an answerer MUST maintain all parameter sets received
          in the offer in its answer.  Depending on the value of the
          "parameter-add" parameter, different rules apply: If "parameter-
          add" is false (0), the answer MUST NOT add any additional
          parameter sets.  If "parameter-add" is true (1), the answerer, in
          its answer, MAY add additional parameter sets to the "sprop-
          parameter-sets" parameter.  The answerer MUST also, independent of
          the value of "parameter-add", accept to receive a video stream
          using the sprop-parameter-sets it declared in the answer.

             Informative note: care must be taken when parameter sets are
             added not to cause overwriting of already transmitted parameter
             sets by using conflicting parameter set identifiers.

       For streams being delivered over multicast, the following rules apply
       in addition:

       o  The stream properties parameters ("sprop-parameter-sets", "sprop-
          deint-buf-req", "sprop-interleaving-depth", "sprop-max-don-diff",
          and "sprop-init-buf-time") MUST NOT be changed by the answerer.
          Thus, a payload type can either be accepted unaltered or removed.

       o  The receiver capability parameters "max-mbps", "max-fs", "max-
          cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be
          supported by the answerer for all streams declared as sendrecv or
          recvonly; otherwise, one of the following actions MUST be
          performed: the media format is removed, or the session rejected.

       o  The receiver capability parameter redundant-pic-cap SHOULD be
          supported by the answerer for all streams declared as sendrecv or
          recvonly as follows:  The answerer SHOULD NOT include redundant
          coded pictures in the transmitted stream if the offerer indicated
          redundant-pic-cap equal to 0.  Otherwise (when redundant_pic_cap
          is equal to 1), it is beyond the scope of this memo to recommend
          how the answerer should use redundant coded pictures.

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       Below are the complete lists of how the different parameters shall be
       interpreted in the different combinations of offer or answer and
       direction attribute.

       o  In offers and answers for which "a=sendrecv" or no direction
          attribute is used, or in offers and answers for which "a=recvonly"
          is used, the following interpretation of the parameters MUST be
          used.

          Declaring actual configuration or properties for receiving:

             - profile-level-id
             - packetization-mode

          Declaring actual properties of the stream to be sent (applicable
          only when "a=sendrecv" or no direction attribute is used):

             - sprop-deint-buf-req
             - sprop-interleaving-depth
             - sprop-parameter-sets
             - sprop-max-don-diff
             - sprop-init-buf-time

          Declaring receiver implementation capabilities:

             - max-mbps
             - max-fs
             - max-cpb
             - max-dpb
             - max-br
             - redundant-pic-cap
             - deint-buf-cap
             - max-rcmd-nalu-size

          Declaring how Offer/Answer negotiation shall be performed:

             - parameter-add

       o  In an offer or answer for which the direction attribute
          "a=sendonly" is included for the media stream, the following
          interpretation of the parameters MUST be used:

          Declaring actual configuration and properties of stream proposed
          to be sent:

             - profile-level-id
             - packetization-mode
             - sprop-deint-buf-req

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             - sprop-max-don-diff
             - sprop-init-buf-time
             - sprop-parameter-sets
             - sprop-interleaving-depth

          Declaring the capabilities of the sender when it receives a
          stream:

             - max-mbps
             - max-fs
             - max-cpb
             - max-dpb
             - max-br
             - redundant-pic-cap
             - deint-buf-cap
             - max-rcmd-nalu-size

          Declaring how Offer/Answer negotiation shall be performed:

             - parameter-add

       Furthermore, the following considerations are necessary:

       o  Parameters used for declaring receiver capabilities are in general
          downgradable; i.e., they express the upper limit for a sender's
          possible behavior.  Thus a sender MAY select to set its encoder
          using only lower/lesser or equal values of these parameters.
          "sprop-parameter-sets" MUST NOT be used in a sender's declaration
          of its capabilities, as the limits of the values that are carried
          inside the parameter sets are implicit with the profile and level
          used.

       o  Parameters declaring a configuration point are not downgradable,
          with the exception of the level part of the "profile-level-id"
          parameter.  This expresses values a receiver expects to be used
          and must be used verbatim on the sender side.

       o  When a sender's capabilities are declared, and non-downgradable
          parameters are used in this declaration, then these parameters
          express a configuration that is acceptable.  In order to achieve
          high interoperability levels, it is often advisable to offer
          multiple alternative configurations; e.g., for the packetization
          mode.  It is impossible to offer multiple configurations in a
          single payload type.  Thus, when multiple configuration offers are
          made, each offer requires its own RTP payload type associated with
          the offer.

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       o  A receiver SHOULD understand all MIME parameters, even if it only
          supports a subset of the payload format's functionality.  This
          ensures that a receiver is capable of understanding when an offer
          to receive media can be downgraded to what is supported by the
          receiver of the offer.

       o  An answerer MAY extend the offer with additional media format
          configurations.  However, to enable their usage, in most cases a
          second offer is required from the offerer to provide the stream
          properties parameters that the media sender will use.  This also
          has the effect that the offerer has to be able to receive this
          media format configuration, not only to send it.

       o  If an offerer wishes to have non-symmetric capabilities between
          sending and receiving, the offerer has to offer different RTP
          sessions; i.e., different media lines declared as "recvonly" and
          "sendonly", respectively.  This may have further implications on
          the system.

    8.2.3.  Usage in Declarative Session Descriptions

       When H.264 over RTP is offered with SDP in a declarative style, as in
       RTSP [27] or SAP [28], the following considerations are necessary.

       o  All parameters capable of indicating the properties of both a NAL
          unit stream and a receiver are used to indicate the properties of
          a NAL unit stream.  For example, in this case, the parameter
          "profile-level-id" declares the values used by the stream, instead
          of the capabilities of the sender.  This results in that the
          following interpretation of the parameters MUST be used:

          Declaring actual configuration or properties:

             - profile-level-id
             - sprop-parameter-sets
             - packetization-mode
             - sprop-interleaving-depth
             - sprop-deint-buf-req
             - sprop-max-don-diff
             - sprop-init-buf-time

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          Not usable:

             - max-mbps
             - max-fs
             - max-cpb
             - max-dpb
             - max-br
             - redundant-pic-cap
             - max-rcmd-nalu-size
             - parameter-add
             - deint-buf-cap

       o  A receiver of the SDP is required to support all parameters and
          values of the parameters provided; otherwise, the receiver MUST
          reject (RTSP) or not participate in (SAP) the session.  It falls
          on the creator of the session to use values that are expected to
          be supported by the receiving application.

    8.3.  Examples

       A SIP Offer/Answer exchange wherein both parties are expected to both
       send and receive could look like the following.  Only the media codec
       specific parts of the SDP are shown.  Some lines are wrapped due to
       text constraints.

          Offerer -> Answer SDP message:

          m=video 49170 RTP/AVP 100 99 98
          a=rtpmap:98 H264/90000
          a=fmtp:98 profile-level-id=42A01E; packetization-mode=0;
                    sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
          a=rtpmap:99 H264/90000
          a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
                    sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
          a=rtpmap:100 H264/90000
          a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
                     sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==;
                     sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
                     sprop-init-buf-time=102478; deint-buf-cap=128000

       The above offer presents the same codec configuration in three
       different packetization formats.  PT 98 represents single NALU mode,
       PT 99 non-interleaved mode; PT 100 indicates the interleaved mode.
       In the interleaved mode case, the interleaving parameters that the
       offerer would use if the answer indicates support for PT 100 are also
       included.  In all three cases the parameter "sprop-parameter-sets"
       conveys the initial parameter sets that are required for the answerer
       when receiving a stream from the offerer when this configuration

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       (profile-level-id and packetization mode) is accepted.  Note that the
       value for "sprop-parameter-sets", although identical in the example
       above, could be different for each payload type.

         Answerer -> Offerer SDP message:

         m=video 49170 RTP/AVP 100 99 97
         a=rtpmap:97 H264/90000
         a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
                   sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
                   KyzFGleR
         a=rtpmap:99 H264/90000
         a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
                   sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
                   KyzFGleR; max-rcmd-nalu-size=3980
         a=rtpmap:100 H264/90000
         a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
                   sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
                   KyzFGleR; sprop-interleaving-depth=60;
                   sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
                   deint-buf-cap=128000; max-rcmd-nalu-size=3980

       As the Offer/Answer negotiation covers both sending and receiving
       streams, an offer indicates the exact parameters for what the offerer
       is willing to receive, whereas the answer indicates the same for what
       the answerer accepts to receive.  In this case the offerer declared
       that it is willing to receive payload type 98.  The answerer accepts
       this by declaring a equivalent payload type 97; i.e., it has
       identical values for the three parameters "profile-level-id",
       packetization-mode, and "sprop-deint-buf-req".  This has the
       following implications for both the offerer and the answerer
       concerning the parameters that declare properties.  The offerer
       initially declared a certain value of the "sprop-parameter-sets" in
       the payload definition for PT=98.  However, as the answerer accepted
       this as PT=97, the values of "sprop-parameter-sets" in PT=98 must now
       be used instead when the offerer sends PT=97.  Similarly, when the
       answerer sends PT=98 to the offerer, it has to use the properties
       parameters it declared in PT=97.

       The answerer also accepts the reception of the two configurations
       that payload types 99 and 100 represent.  It provides the initial
       parameter sets for the answerer-to-offerer direction, and for
       buffering related parameters that it will use to send the payload
       types.  It also provides the offerer with its memory limit for
       deinterleaving operations by providing a "deint-buf-cap" parameter.
       This is only useful if the offerer decides on making a second offer,
       where it can take the new value into account.  The "max-rcmd-nalu-
       size" indicates that the answerer can efficiently process NALUs up to

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       the size of 3980 bytes.  However, there is no guarantee that the
       network supports this size.

       Please note that the parameter sets in the above example do not
       represent a legal operation point of an H.264 codec.  The base64
       strings are only used for illustration.

    8.4.  Parameter Set Considerations

       The H.264 parameter sets are a fundamental part of the video codec
       and vital to its operation; see section 1.2.  Due to their
       characteristics and their importance for the decoding process, lost
       or erroneously transmitted parameter sets can hardly be concealed
       locally at the receiver.  A reference to a corrupt parameter set has
       normally fatal results to the decoding process.  Corruption could
       occur, for example, due to the erroneous transmission or loss of a
       parameter set data structure, but also due to the untimely
       transmission of a parameter set update.  Therefore, the following
       recommendations are provided as a guideline for the implementer of
       the RTP sender.

       Parameter set NALUs can be transported using three different
       principles:

       A. Using a session control protocol (out-of-band) prior to the actual
          RTP session.

       B. Using a session control protocol (out-of-band) during an ongoing
          RTP session.

       C. Within the RTP stream in the payload (in-band) during an ongoing
          RTP session.

       It is necessary to implement principles A and B within a session
       control protocol.  SIP and SDP can be used as described in the SDP
       Offer/Answer model and in the previous sections of this memo.  This
       section contains guidelines on how principles A and B must be
       implemented within session control protocols.  It is independent of
       the particular protocol used.  Principle C is supported by the RTP
       payload format defined in this specification.

       The picture and sequence parameter set NALUs SHOULD NOT be
       transmitted in the RTP payload unless reliable transport is provided
       for RTP, as a loss of a parameter set of either type will likely
       prevent decoding of a considerable portion of the corresponding RTP

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       stream.  Thus, the transmission of parameter sets using a reliable
       session control protocol (i.e., usage of principle A or B above) is
       RECOMMENDED.

       In the rest of the section it is assumed that out-of-band signaling
       provides reliable transport of parameter set NALUs and that in-band
       transport does not.  If in-band signaling of parameter sets is used,
       the sender SHOULD take the error characteristics into account and use
       mechanisms to provide a high probability for delivering the parameter
       sets correctly.  Mechanisms that increase the probability for a
       correct reception include packet repetition, FEC, and retransmission.
       The use of an unreliable, out-of-band control protocol has similar
       disadvantages as the in-band signaling (possible loss) and, in
       addition, may also lead to difficulties in the synchronization (see
       below).  Therefore, it is NOT RECOMMENDED.

       Parameter sets MAY be added or updated during the lifetime of a
       session using principles B and C.  It is required that parameter sets
       are present at the decoder prior to the NAL units that refer to them.
       Updating or adding of parameter sets can result in further problems,
       and therefore the following recommendations should be considered.

       -  When parameter sets are added or updated, principle C is
          vulnerable to transmission errors as described above, and
          therefore principle B is RECOMMENDED.

       -  When parameter sets are added or updated, care SHOULD be taken to
          ensure that any parameter set is delivered prior to its usage.  It
          is common that no synchronization is present between out-of-band
          signaling and in-band traffic.  If out-of-band signaling is used,
          it is RECOMMENDED that a sender does not start sending NALUs
          requiring the updated parameter sets prior to acknowledgement of
          delivery from the signaling protocol.

       -  When parameter sets are updated, the following synchronization
          issue should be taken into account.  When overwriting a parameter
          set at the receiver, the sender has to ensure that the parameter
          set in question is not needed by any NALU present in the network
          or receiver buffers.  Otherwise, decoding with a wrong parameter
          set may occur.  To lessen this problem, it is RECOMMENDED either
          to overwrite only those parameter sets that have not been used for
          a sufficiently long time (to ensure that all related NALUs have
          been consumed), or to add a new parameter set instead (which may
          have negative consequences for the efficiency of the video
          coding).

       -  When new parameter sets are added, previously unused parameter set
          identifiers are used.  This avoids the problem identified in the

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          previous paragraph.  However, in a multiparty session, unless a
          synchronized control protocol is used, there is a risk that
          multiple entities try to add different parameter sets for the same
          identifier, which has to be avoided.

       -  Adding or modifying parameter sets by using both principles B and
          C in the same RTP session may lead to inconsistencies of the
          parameter sets because of the lack of synchronization between the
          control and the RTP channel.  Therefore, principles B and C MUST
          NOT both be used in the same session unless sufficient
          synchronization can be provided.

       In some scenarios (e.g., when only the subset of this payload format
       specification corresponding to H.241 is used), it is not possible to
       employ out-of-band parameter set transmission.  In this case,
       parameter sets have to be transmitted in-band.  Here, the
       synchronization with the non-parameter-set-data in the bitstream is
       implicit, but the possibility of a loss has to be taken into account.
       The loss probability should be reduced using the mechanisms discussed
       above.

       -  When parameter sets are initially provided using principle A and
          then later added or updated in-band (principle C), there is a risk
          associated with updating the parameter sets delivered out-of-band.
          If receivers miss some in-band updates (for example, because of a
          loss or a late tune-in), those receivers attempt to decode the
          bitstream using out-dated parameters.  It is RECOMMENDED that
          parameter set IDs be partitioned between the out-of-band and in-
          band parameter sets.

       To allow for maximum flexibility and best performance from the H.264
       coder, it is recommended, if possible, to allow any sender to add its
       own parameter sets to be used in a session.  Setting the "parameter-
       add" parameter to false should only be done in cases where the
       session topology prevents a participant to add its own parameter
       sets.

    9.  Security Considerations

       RTP packets using the payload format defined in this specification
       are subject to the security considerations discussed in the RTP
       specification [4], and in any appropriate RTP profile (for example,
       [16]).  This implies that confidentiality of the media streams is
       achieved by encryption; for example, through the application of SRTP
       [26].  Because the data compression used with this payload format is
       applied end-to-end, any encryption needs to be performed after
       compression.

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       A potential denial-of-service threat exists for data encodings using
       compression techniques that have non-uniform receiver-end
       computational load.  The attacker can inject pathological datagrams
       into the stream that are complex to decode and that cause the
       receiver to be overloaded.  H.264 is particularly vulnerable to such
       attacks, as it is extremely simple to generate datagrams containing
       NAL units that affect the decoding process of many future NAL units.
       Therefore, the usage of data origin authentication and data integrity
       protection of at least the RTP packet is RECOMMENDED; for example,
       with SRTP [26].

       Note that the appropriate mechanism to ensure confidentiality and
       integrity of RTP packets and their payloads is very dependent on the
       application and on the transport and signaling protocols employed.
       Thus, although SRTP is given as an example above, other possible
       choices exist.

       Decoders MUST exercise caution with respect to the handling of user
       data SEI messages, particularly if they contain active elements, and
       MUST restrict their domain of applicability to the presentation
       containing the stream.

       End-to-End security with either authentication, integrity or
       confidentiality protection will prevent a MANE from performing
       media-aware operations other than discarding complete packets.  And
       in the case of confidentiality protection it will even be prevented
       from performing discarding of packets in a media aware way.  To allow
       any MANE to perform its operations, it will be required to be a
       trusted entity which is included in the security context
       establishment.

    10.  Congestion Control

       Congestion control for RTP SHALL be used in accordance with RFC 3550
       [4], and with any applicable RTP profile; e.g., RFC 3551 [16].  An
       additional requirement if best-effort service is being used is:
       users of this payload format MUST monitor packet loss to ensure that
       the packet loss rate is within acceptable parameters.  Packet loss is
       considered acceptable if a TCP flow across the same network path, and
       experiencing the same network conditions, would achieve an average
       throughput, measured on a reasonable timescale, that is not less than
       the RTP flow is achieving.  This condition can be satisfied by
       implementing congestion control mechanisms to adapt the transmission
       rate (or the number of layers subscribed for a layered multicast
       session), or by arranging for a receiver to leave the session if the
       loss rate is unacceptably high.

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       The bit rate adaptation necessary for obeying the congestion control
       principle is easily achievable when real-time encoding is used.
       However, when pre-encoded content is being transmitted, bandwidth
       adaptation requires the availability of more than one coded
       representation of the same content, at different bit rates, or the
       existence of non-reference pictures or sub-sequences [22] in the
       bitstream.  The switching between the different representations can
       normally be performed in the same RTP session; e.g., by employing a
       concept known as SI/SP slices of the Extended Profile, or by
       switching streams at IDR picture boundaries.  Only when non-
       downgradable parameters (such as the profile part of the
       profile/level ID) are required to be changed does it become necessary
       to terminate and re-start the media stream.  This may be accomplished
       by using a different RTP payload type.

       MANEs MAY follow the suggestions outlined in section 7.3 and remove
       certain unusable packets from the packet stream when that stream was
       damaged due to previous packet losses.  This can help reduce the
       network load in certain special cases.

    11.  IANA Consideration

       IANA has registered one new MIME type; see section 8.1.

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    12.  Informative Appendix: Application Examples

       This payload specification is very flexible in its use, in order to
       cover the extremely wide application space anticipated for H.264.
       However, this great flexibility also makes it difficult for an
       implementer to decide on a reasonable packetization scheme.  Some
       information on how to apply this specification to real-world
       scenarios is likely to appear in the form of academic publications
       and a test model software and description in the near future.
       However, some preliminary usage scenarios are described here as well.

    12.1.  Video Telephony according to ITU-T Recommendation H.241
           Annex A

       H.323-based video telephony systems that use H.264 as an optional
       video compression scheme are required to support H.241 Annex A [15]
       as a packetization scheme.  The packetization mechanism defined in
       this Annex is technically identical with a small subset of this
       specification.

       When a system operates according to H.241 Annex A, parameter set NAL
       units are sent in-band.  Only Single NAL unit packets are used.  Many
       such systems are not sending IDR pictures regularly, but only when
       required by user interaction or by control protocol means; e.g., when
       switching between video channels in a Multipoint Control Unit or for
       error recovery requested by feedback.

    12.2.  Video Telephony, No Slice Data Partitioning, No NAL Unit
           Aggregation

       The RTP part of this scheme is implemented and tested (though not the
       control-protocol part; see below).

       In most real-world video telephony applications, picture parameters
       such as picture size or optional modes never change during the
       lifetime of a connection.  Therefore, all necessary parameter sets
       (usually only one) are sent as a side effect of the capability
       exchange/announcement process, e.g., according to the SDP syntax
       specified in section 8.2 of this document.  As all necessary
       parameter set information is established before the RTP session
       starts, there is no need for sending any parameter set NAL units.
       Slice data partitioning is not used, either.  Thus, the RTP packet
       stream basically consists of NAL units that carry single coded
       slices.

       The encoder chooses the size of coded slice NAL units so that they
       offer the best performance.  Often, this is done by adapting the
       coded slice size to the MTU size of the IP network.  For small

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       picture sizes, this may result in a one-picture-per-one-packet
       strategy.  Intra refresh algorithms clean up the loss of packets and
       the resulting drift-related artifacts.

    12.3.  Video Telephony, Interleaved Packetization Using NAL Unit
           Aggregation

       This scheme allows better error concealment and is used in H.263
       based designs using RFC 2429 packetization [10].  It has been
       implemented, and good results were reported [12].

       The VCL encoder codes the source picture so that all macroblocks
       (MBs) of one MB line are assigned to one slice.  All slices with even
       MB row addresses are combined into one STAP, and all slices with odd
       MB row addresses into another.  Those STAPs are transmitted as RTP
       packets.  The establishment of the parameter sets is performed as
       discussed above.

       Note that the use of STAPs is essential here, as the high number of
       individual slices (18 for a CIF picture) would lead to unacceptably
       high IP/UDP/RTP header overhead (unless the source coding tool FMO is
       used, which is not assumed in this scenario).  Furthermore, some
       wireless video transmission systems, such as H.324M and the IP-based
       video telephony specified in 3GPP, are likely to use relatively small
       transport packet size.  For example, a typical MTU size of H.223 AL3
       SDU is around 100 bytes [17].  Coding individual slices according to
       this packetization scheme provides further advantage in communication
       between wired and wireless networks, as individual slices are likely
       to be smaller than the preferred maximum packet size of wireless
       systems.  Consequently, a gateway can convert the STAPs used in a
       wired network into several RTP packets with only one NAL unit, which
       are preferred in a wireless network, and vice versa.

    12.4.  Video Telephony with Data Partitioning

       This scheme has been implemented and has been shown to offer good
       performance, especially at higher packet loss rates [12].

       Data Partitioning is known to be useful only when some form of
       unequal error protection is available.  Normally, in single-session
       RTP environments, even error characteristics are assumed; i.e., the
       packet loss probability of all packets of the session is the same
       statistically.  However, there are means to reduce the packet loss
       probability of individual packets in an RTP session.  A FEC packet
       according to RFC 2733 [18], for example, specifies which media
       packets are associated with the FEC packet.

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       In all cases, the incurred overhead is substantial but is in the same
       order of magnitude as the number of bits that have otherwise been
       spent for intra information.  However, this mechanism does not add
       any delay to the system.

       Again, the complete parameter set establishment is performed through
       control protocol means.

    12.5.  Video Telephony or Streaming with FUs and Forward Error
           Correction

       This scheme has been implemented and has been shown to provide good
       performance, especially at higher packet loss rates [19].

       The most efficient means to combat packet losses for scenarios where
       retransmissions are not applicable is forward error correction (FEC).
       Although application layer, end-to-end use of FEC is often less
       efficient than an FEC-based protection of individual links
       (especially when links of different characteristics are in the
       transmission path), application layer, end-to-end FEC is unavoidable
       in some scenarios.  RFC 2733 [18] provides means to use generic,
       application layer, end-to-end FEC in packet-loss environments.  A
       binary forward error correcting code is generated by applying the XOR
       operation to the bits at the same bit position in different packets.
       The binary code can be specified by the parameters (n,k) in which k
       is the number of information packets used in the connection and n is
       the total number of packets generated for k information packets;
       i.e., n-k parity packets are generated for k information packets.

       When a code is used with parameters (n,k) within the RFC 2733
       framework, the following properties are well known:

       a) If applied over one RTP packet, RFC 2733 provides only packet
          repetition.

       b) RFC 2733 is most bit rate efficient if XOR-connected packets have
          equal length.

       c) At the same packet loss probability p and for a fixed k, the
          greater the value of n is, the smaller the residual error
          probability becomes.  For example, for a packet loss probability
          of 10%, k=1, and n=2, the residual error probability is about 1%,
          whereas for n=3, the residual error probability is about 0.1%.

       d) At the same packet loss probability p and for a fixed code rate
          k/n, the greater the value of n is, the smaller the residual error
          probability becomes.  For example, at a packet loss probability of
          p=10%, k=1 and n=2, the residual error rate is about 1%, whereas

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          for an extended Golay code with k=12 and n=24, the residual error
          rate is about 0.01%.

       For applying RFC 2733 in combination with H.264 baseline coded video
       without using FUs, several options might be considered:

       1) The video encoder produces NAL units for which each video frame is
          coded in a single slice.  Applying FEC, one could use a simple
          code; e.g., (n=2, k=1).  That is, each NAL unit would basically
          just be repeated.  The disadvantage is obviously the bad code
          performance according to d), above, and the low flexibility, as
          only (n, k=1) codes can be used.

       2) The video encoder produces NAL units for which each video frame is
          encoded in one or more consecutive slices.  Applying FEC, one
          could use a better code, e.g., (n=24, k=12), over a sequence of
          NAL units.  Depending on the number of RTP packets per frame, a
          loss may introduce a significant delay, which is reduced when more
          RTP packets are used per frame.  Packets of completely different
          length might also be connected, which decreases bit rate
          efficiency according to b), above.  However, with some care and
          for slices of 1kb or larger, similar length (100-200 bytes
          difference) may be produced, which will not lower the bit
          efficiency catastrophically.

       3) The video encoder produces NAL units, for which a certain frame
          contains k slices of possibly almost equal length.  Then, applying
          FEC, a better code, e.g., (n=24, k=12), can be used over the
          sequence of NAL units for each frame.  The delay compared to that
          of 2), above,  may be reduced, but several disadvantages are
          obvious.  First, the coding efficiency of the encoded video is
          lowered significantly, as slice-structured coding reduces intra-
          frame prediction and additional slice overhead is necessary.
          Second, pre-encoded content or, when operating over a gateway, the
          video is usually not appropriately coded with k slices such that
          FEC can be applied.  Finally, the encoding of video producing k
          slices of equal length is not straightforward and might require
          more than one encoding pass.

       Many of the mentioned disadvantages can be avoided by applying FUs in
       combination with FEC.  Each NAL unit can be split into any number of
       FUs of basically equal length; therefore, FEC with a reasonable k and
       n can be applied, even if the encoder made no effort to produce
       slices of equal length.  For example, a coded slice NAL unit
       containing an entire frame can be split to k FUs, and a parity check
       code (n=k+1, k) can be applied.  However, this has the disadvantage

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       that unless all created fragments can be recovered, the whole slice
       will be lost.  Thus a larger section is lost than would be if the
       frame had been split into several slices.

       The presented technique makes it possible to achieve good
       transmission error tolerance, even if no additional source coding
       layer redundancy (such as periodic intra frames) is present.
       Consequently, the same coded video sequence can be used to achieve
       the maximum compression efficiency and quality over error-free
       transmission and for transmission over error-prone networks.
       Furthermore, the technique allows the application of FEC to pre-
       encoded sequences without adding delay.  In this case, pre-encoded
       sequences that are not encoded for error-prone networks can still be
       transmitted almost reliably without adding extensive delays.  In
       addition, FUs of equal length result in a bit rate efficient use of
       RFC 2733.

       If the error probability depends on the length of the transmitted
       packet (e.g., in case of mobile transmission [14]), the benefits of
       applying FUs with FEC are even more obvious.  Basically, the
       flexibility of the size of FUs allows appropriate FEC to be applied
       for each NAL unit and unequal error protection of NAL units.

       When FUs and FEC are used, the incurred overhead is substantial but
       is in the same order of magnitude as the number of bits that have to
       be spent for intra-coded macroblocks if no FEC is applied.  In [19],
       it was shown that the overall performance of the FEC-based approach
       enhanced quality when using the same error rate and same overall bit
       rate, including the overhead.

    12.6.  Low Bit-Rate Streaming

       This scheme has been implemented with H.263 and non-standard RTP
       packetization and has given good results [20].  There is no technical
       reason why similarly good results could not be achievable with H.264.

       In today's Internet streaming, some of the offered bit rates are
       relatively low in order to allow terminals with dial-up modems to
       access the content.  In wired IP networks, relatively large packets,
       say 500 - 1500 bytes, are preferred to smaller and more frequently
       occurring packets in order to reduce network congestion.  Moreover,
       use of large packets decreases the amount of RTP/UDP/IP header
       overhead.  For low bit-rate video, the use of large packets means
       that sometimes up to few pictures should be encapsulated in one
       packet.

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       However, loss of a packet including many coded pictures would have
       drastic consequences for visual quality, as there is practically no
       other way to conceal a loss of an entire picture than to repeat the
       previous one.  One way to construct relatively large packets and
       maintain possibilities for successful loss concealment is to
       construct MTAPs that contain interleaved slices from several
       pictures.  An MTAP should not contain spatially adjacent slices from
       the same picture or spatially overlapping slices from any picture.
       If a packet is lost, it is likely that a lost slice is surrounded by
       spatially adjacent slices of the same picture and spatially
       corresponding slices of the temporally previous and succeeding
       pictures.  Consequently, concealment of the lost slice is likely to
       be relatively successful.

    12.7.  Robust Packet Scheduling in Video Streaming

       Robust packet scheduling has been implemented with MPEG-4 Part 2 and
       simulated in a wireless streaming environment [21].  There is no
       technical reason why similar or better results could not be
       achievable with H.264.

       Streaming clients typically have a receiver buffer that is capable of
       storing a relatively large amount of data.  Initially, when a
       streaming session is established, a client does not start playing the
       stream back immediately.  Rather, it typically buffers the incoming
       data for a few seconds.  This buffering helps maintain continuous
       playback, as, in case of occasional increased transmission delays or
       network throughput drops, the client can decode and play buffered
       data.  Otherwise, without initial buffering, the client has to freeze
       the display, stop decoding, and wait for incoming data.  The
       buffering is also necessary for either automatic or selective
       retransmission in any protocol level.  If any part of a picture is
       lost, a retransmission mechanism may be used to resend the lost data.
       If the retransmitted data is received before its scheduled decoding
       or playback time, the loss is recovered perfectly.  Coded pictures
       can be ranked according to their importance in the subjective quality
       of the decoded sequence.  For example, non-reference pictures, such
       as conventional B pictures, are subjectively least important, as
       their absence does not affect decoding of any other pictures.  In
       addition to non-reference pictures, the ITU-T H.264 | ISO/IEC
       14496-10 standard includes a temporal scalability method called sub-
       sequences [22].  Subjective ranking can also be made on coded slice
       data partition or slice group basis.  Coded slices and coded slice
       data partitions that are subjectively the most important can be sent
       earlier than their decoding order indicates, whereas coded slices and
       coded slice data partitions that are subjectively the least important
       can be sent later than their natural coding order indicates.
       Consequently, any retransmitted parts of the most important slices

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       and coded slice data partitions are more likely to be received before
       their scheduled decoding or playback time compared to the least
       important slices and slice data partitions.

    13.  Informative Appendix: Rationale for Decoding Order Number

    13.1.  Introduction

       The Decoding Order Number (DON) concept was introduced mainly to
       enable efficient multi-picture slice interleaving (see section 12.6)
       and robust packet scheduling (see section 12.7).  In both of these
       applications, NAL units are transmitted out of decoding order.  DON
       indicates the decoding order of NAL units and should be used in the
       receiver to recover the decoding order.  Example use cases for
       efficient multi-picture slice interleaving and for robust packet
       scheduling are given in sections 13.2 and 13.3, respectively.
       Section 13.4 describes the benefits of the DON concept in error
       resiliency achieved by redundant coded pictures.  Section 13.5
       summarizes considered alternatives to DON and justifies why DON was
       chosen to this RTP payload specification.

    13.2.  Example of Multi-Picture Slice Interleaving

       An example of multi-picture slice interleaving follows.  A subset of
       a coded video sequence is depicted below in output order.  R denotes
       a reference picture, N denotes a non-reference picture, and the
       number indicates a relative output time.

          ... R1 N2 R3 N4 R5 ...

       The decoding order of these pictures from left to right is as
       follows:

          ... R1 R3 N2 R5 N4 ...

       The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
       DON equal to 1, 2, 3, 4, and 5, respectively.

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       Each reference picture consists of three slice groups that are
       scattered as follows (a number denotes the slice group number for
       each macroblock in a QCIF frame):

          0 1 2 0 1 2 0 1 2 0 1
          2 0 1 2 0 1 2 0 1 2 0
          1 2 0 1 2 0 1 2 0 1 2
          0 1 2 0 1 2 0 1 2 0 1
          2 0 1 2 0 1 2 0 1 2 0
          1 2 0 1 2 0 1 2 0 1 2
          0 1 2 0 1 2 0 1 2 0 1
          2 0 1 2 0 1 2 0 1 2 0
          1 2 0 1 2 0 1 2 0 1 2

       For the sake of simplicity, we assume that all the macroblocks of a
       slice group are included in one slice.  Three MTAPs are constructed
       from three consecutive reference pictures so that each MTAP contains
       three aggregation units, each of which contains all the macroblocks
       from one slice group.  The first MTAP contains slice group 0 of
       picture R1, slice group 1 of picture R3, and slice group 2 of
       picture R5.  The second MTAP contains slice group 1 of picture R1,
       slice group 2 of picture R3, and slice group 0 of picture R5.  The
       third MTAP contains slice group 2 of picture R1, slice group 0 of
       picture R3, and slice group 1 of picture R5.  Each non-reference
       picture is encapsulated into an STAP-B.

       Consequently, the transmission order of NAL units is the following:

          R1, slice group 0, DON 1, carried in MTAP,   RTP SN: N
          R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N
          R5, slice group 2, DON 4, carried in MTAP,   RTP SN: N
          R1, slice group 1, DON 1, carried in MTAP,   RTP SN: N+1
          R3, slice group 2, DON 2, carried in MTAP,   RTP SN: N+1
          R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+1
          R1, slice group 2, DON 1, carried in MTAP,   RTP SN: N+2
          R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N+2
          R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+2
          N2,                DON 3, carried in STAP-B, RTP SN: N+3
          N4,                DON 5, carried in STAP-B, RTP SN: N+4

       The receiver is able to organize the NAL units back in decoding order
       based on the value of DON associated with each NAL unit.

       If one of the MTAPs is lost, the spatially adjacent and temporally
       co-located macroblocks are received and can be used to conceal the
       loss efficiently.  If one of the STAPs is lost, the effect of the
       loss does not propagate temporally.

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    13.3.  Example of Robust Packet Scheduling

       An example of robust packet scheduling follows.  The communication
       system used in the example consists of the following components in
       the order that the video is processed from source to sink:

          o camera and capturing
          o pre-encoding buffer
          o encoder
          o encoded picture buffer
          o transmitter
          o transmission channel
          o receiver
          o receiver buffer
          o decoder
          o decoded picture buffer
          o display

       The video communication system used in the example operates as
       follows.  Note that processing of the video stream happens gradually
       and at the same time in all components of the system.  The source
       video sequence is shot and captured to a pre-encoding buffer.  The
       pre-encoding buffer can be used to order pictures from sampling order
       to encoding order or to analyze multiple uncompressed frames for bit
       rate control purposes, for example.  In some cases, the pre-encoding
       buffer may not exist; instead, the sampled pictures are encoded right
       away.  The encoder encodes pictures from the pre-encoding buffer and
       stores the output; i.e., coded pictures, to the encoded picture
       buffer.  The transmitter encapsulates the coded pictures from the
       encoded picture buffer to transmission packets and sends them to a
       receiver through a transmission channel.  The receiver stores the
       received packets to the receiver buffer.  The receiver buffering
       process typically includes buffering for transmission delay jitter.
       The receiver buffer can also be used to recover correct decoding
       order of coded data.  The decoder reads coded data from the receiver
       buffer and produces decoded pictures as output into the decoded
       picture buffer.  The decoded picture buffer is used to recover the
       output (or display) order of pictures.  Finally, pictures are
       displayed.

       In the following example figures, I denotes an IDR picture, R denotes
       a reference picture, N denotes a non-reference picture, and the
       number after I, R, or N indicates the sampling time relative to the
       previous IDR picture in decoding order.  Values below the sequence of
       pictures indicate scaled system clock timestamps.  The system clock
       is initialized arbitrarily in this example, and time runs from left
       to right.  Each I, R, and N picture is mapped into the same timeline
       compared to the previous processing step, if any, assuming that

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       encoding, transmission, and decoding take no time.  Thus, events
       happening at the same time are located in the same column throughout
       all example figures.

       A subset of a sequence of coded pictures is depicted below in
       sampling order.

           ...  N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
           ... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
           ...  58  59  60  61  62  63  64  65  66  ... 128 129 130 131 ...

          Figure 16.  Sequence of pictures in sampling order

       The sampled pictures are buffered in the pre-encoding buffer to
       arrange them in encoding order.  In this example, we assume that the
       non-reference pictures are predicted from both the previous and the
       next reference picture in output order, except for the non-reference
       pictures immediately preceding an IDR picture, which are predicted
       only from the previous reference picture in output order.  Thus, the
       pre-encoding buffer has to contain at least two pictures, and the
       buffering causes a delay of two picture intervals.  The output of the
       pre-encoding buffering process and the encoding (and decoding) order
       of the pictures are as follows:

                    ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
                    ... -|---|---|---|---|---|---|---|---|- ...
                    ... 60  61  62  63  64  65  66  67  68  ...

          Figure 17.  Re-ordered pictures in the pre-encoding buffer

       The encoder or the transmitter can set the value of DON for each
       picture to a value of DON for the previous picture in decoding order
       plus one.

       For the sake of simplicity, let us assume that:

       o  the frame rate of the sequence is constant,
       o  each picture consists of only one slice,
       o  each slice is encapsulated in a single NAL unit packet,
       o  there is no transmission delay, and
       o  pictures are transmitted at constant intervals (that is, 1 / frame
          rate).

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       When pictures are transmitted in decoding order, they are received as
       follows:

                    ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
                    ... -|---|---|---|---|---|---|---|---|- ...
                    ... 60  61  62  63  64  65  66  67  68  ...

          Figure 18.  Received pictures in decoding order

       The OPTIONAL sprop-interleaving-depth MIME type parameter is set to
       0, as the transmission (or reception) order is identical to the
       decoding order.

       The decoder has to buffer for one picture interval initially in its
       decoded picture buffer to organize pictures from decoding order to
       output order as depicted below:

                        ... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
                        ... -|---|---|---|---|---|---|---|---|- ...
                        ... 61  62  63  64  65  66  67  68  69  ...

          Figure 19.  Output order

       The amount of required initial buffering in the decoded picture
       buffer can be signaled in the buffering period SEI message or with
       the num_reorder_frames syntax element of H.264 video usability
       information.  num_reorder_frames indicates the maximum number of
       frames, complementary field pairs, or non-paired fields that precede
       any frame, complementary field pair, or non-paired field in the
       sequence in decoding order and that follow it in output order.  For
       the sake of simplicity, we assume that num_reorder_frames is used to
       indicate the initial buffer in the decoded picture buffer.  In this
       example, num_reorder_frames is equal to 1.

       It can be observed that if the IDR picture I00 is lost during
       transmission and a retransmission request is issued when the value of
       the system clock is 62, there is one picture interval of time (until
       the system clock reaches timestamp 63) to receive the retransmitted
       IDR picture I00.

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       Let us then assume that IDR pictures are transmitted two frame
       intervals earlier than their decoding position; i.e., the pictures
       are transmitted as follows:

                           ...  I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
                           ... --|---|---|---|---|---|---|---|---|- ...
                           ...  62  63  64  65  66  67  68  69  70  ...

          Figure 20.  Interleaving: Early IDR pictures in sending order

       The OPTIONAL sprop-interleaving-depth MIME type parameter is set
       equal to 1 according to its definition.  (The value of sprop-
       interleaving-depth in this example can be derived as follows:
       Picture I00 is the only picture preceding picture N58 or N59 in
       transmission order and following it in decoding order.  Except for
       pictures I00, N58, and N59, the transmission order is the same as the
       decoding order of pictures.  As a coded picture is encapsulated into
       exactly one NAL unit, the value of sprop-interleaving-depth is equal
       to the maximum number of pictures preceding any picture in
       transmission order and following the picture in decoding order.)

       The receiver buffering process contains two pictures at a time
       according to the value of the sprop-interleaving-depth parameter and
       orders pictures from the reception order to the correct decoding
       order based on the value of DON associated with each picture.  The
       output of the receiver buffering process is as follows:

                                ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
                                ... -|---|---|---|---|---|---|---|---|- ...
                                ... 63  64  65  66  67  68  69  70  71  ...

          Figure 21.  Interleaving: Receiver buffer

       Again, an initial buffering delay of one picture interval is needed
       to organize pictures from decoding order to output order, as depicted
       below:

                                    ... N58 N59 I00 N01 N02 R03 N04 N05 ...
                                    ... -|---|---|---|---|---|---|---|- ...
                                    ... 64  65  66  67  68  69  70  71  ...

          Figure 22.  Interleaving: Receiver buffer after reordering

       Note that the maximum delay that IDR pictures can undergo during
       transmission, including possible application, transport, or link
       layer retransmission, is equal to three picture intervals.  Thus, the

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       loss resiliency of IDR pictures is improved in systems supporting
       retransmission compared to the case in which pictures were
       transmitted in their decoding order.

    13.4.  Robust Transmission Scheduling of Redundant Coded Slices

       A redundant coded picture is a coded representation of a picture or a
       part of a picture that is not used in the decoding process if the
       corresponding primary coded picture is correctly decoded.  There
       should be no noticeable difference between any area of the decoded
       primary picture and a corresponding area that would result from
       application of the H.264 decoding process for any redundant picture
       in the same access unit.  A redundant coded slice is a coded slice
       that is a part of a redundant coded picture.

       Redundant coded pictures can be used to provide unequal error
       protection in error-prone video transmission.  If a primary coded
       representation of a picture is decoded incorrectly, a corresponding
       redundant coded picture can be decoded.  Examples of applications and
       coding techniques using the redundant codec picture feature include
       the video redundancy coding [23] and the protection of "key pictures"
       in multicast streaming [24].

       One property of many error-prone video communications systems is that
       transmission errors are often bursty.  Therefore, they may affect
       more than one consecutive transmission packets in transmission order.
       In low bit-rate video communication, it is relatively common that an
       entire coded picture can be encapsulated into one transmission
       packet.  Consequently, a primary coded picture and the corresponding
       redundant coded pictures may be transmitted in consecutive packets in
       transmission order.  To make the transmission scheme more tolerant of
       bursty transmission errors, it is beneficial to transmit the primary
       coded picture and redundant coded picture separated by more than a
       single packet.  The DON concept enables this.

    13.5.  Remarks on Other Design Possibilities

       The slice header syntax structure of the H.264 coding standard
       contains the frame_num syntax element that can indicate the decoding
       order of coded frames.  However, the usage of the frame_num syntax
       element is not feasible or desirable to recover the decoding order,
       due to the following reasons:

       o  The receiver is required to parse at least one slice header per
          coded picture (before passing the coded data to the decoder).

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

       o  Coded slices from multiple coded video sequences cannot be
          interleaved, as the frame number syntax element is reset to 0 in
          each IDR picture.

       o  The coded fields of a complementary field pair share the same
          value of the frame_num syntax element.  Thus, the decoding order
          of the coded fields of a complementary field pair cannot be
          recovered based on the frame_num syntax element or any other
          syntax element of the H.264 coding syntax.

       The RTP payload format for transport of MPEG-4 elementary streams
       [25] enables interleaving of access units and transmission of
       multiple access units in the same RTP packet.  An access unit is
       specified in the H.264 coding standard to comprise all NAL units
       associated with a primary coded picture according to subclause
       7.4.1.2 of [1].  Consequently, slices of different pictures cannot be
       interleaved, and the multi-picture slice interleaving technique (see
       section 12.6) for improved error resilience cannot be used.

    14.  Acknowledgements

       The authors thank Roni Even, Dave Lindbergh, Philippe Gentric,
       Gonzalo Camarillo, Gary Sullivan, Joerg Ott, and Colin Perkins for
       careful review.

    15.  References

    15.1.  Normative References

       [1]  ITU-T Recommendation H.264, "Advanced video coding for generic
            audiovisual services", May 2003.

       [2]  ISO/IEC International Standard 14496-10:2003.

       [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
            Levels", BCP 14, RFC 2119, March 1997.

       [4]  Schulzrinne, H.,  Casner, S., Frederick, R., and V. Jacobson,
            "RTP: A Transport Protocol for Real-Time Applications", STD 64,
            RFC 3550, July 2003.

       [5]  Handley, M. and V. Jacobson, "SDP: Session Description
            Protocol", RFC 2327, April 1998.

       [6]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
            RFC 3548, July 2003.

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

       [7]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
            Session Description Protocol (SDP)", RFC 3264, June 2002.

    15.2.  Informative References

       [8]  "Draft ITU-T Recommendation and Final Draft International
            Standard of Joint Video Specification (ITU-T Rec. H.264 |
            ISO/IEC 14496-10 AVC)", available from http://ftp3.itu.int/av-
            arch/jvt-site/2003_03_Pattaya/JVT-G050r1.zip, May 2003.

       [9]  Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special Issue
            on H.264/AVC. IEEE Transactions on Circuits and Systems on Video
            Technology, July 2003.

       [10] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C.,
            Newell, D., Ott, J., Sullivan, G., Wenger, S., and C. Zhu, "RTP
            Payload Format for the 1998 Version of ITU-T Rec. H.263 Video
            (H.263+)", RFC 2429, October 1998.

       [11] ISO/IEC IS 14496-2.

       [12] Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and
            Systems for Video technology, Vol. 13, No. 7, July 2003.

       [13] Wenger, S., "H.26L over IP: The IP Network Adaptation Layer",
            Proceedings Packet Video Workshop 02, April 2002.

       [14] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
            Coding Network Abstraction Layer and IP-based Transport" in
            Proc. ICIP 2002, Rochester, NY, September 2002.

       [15] ITU-T Recommendation H.241, "Extended video procedures and
            control signals for H.300 series terminals", 2004.

       [16] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
            Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

       [17] ITU-T Recommendation H.223, "Multiplexing protocol for low bit
            rate multimedia communication", July 2001.

       [18] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
            Generic Forward Error Correction", RFC 2733, December 1999.

       [19] Stockhammer, T., Wiegand, T., Oelbaum, T., and F. Obermeier,
            "Video Coding and Transport Layer Techniques for H.264/AVC-Based
            Transmission over Packet-Lossy Networks", IEEE International
            Conference on Image Processing (ICIP 2003), Barcelona, Spain,
            September 2003.

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

       [20] Varsa, V. and M. Karczewicz, "Slice interleaving in compressed
            video packetization", Packet Video Workshop 2000.

       [21] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
            wireless video streaming," International Packet Video Workshop
            2002.

       [22] Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042, available
            http://ftp3.itu.int/av-arch/video-site/0201_Gen/JVT-B042.doc,
            January 2002.

       [23] Wenger, S., "Video Redundancy Coding in H.263+", 1997
            International Workshop on Audio-Visual Services over Packet
            Networks, September 1997.

       [24] Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error Resilient
            Video Coding Using Unequally Protected Key Pictures", in Proc.
            International Workshop VLBV03, September 2003.

       [25] 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.

       [26] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
            Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
            3711, March 2004.

       [27] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
            Protocol (RTSP)", RFC 2326, April 1998.

       [28] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
            Protocol", RFC 2974, October 2000.

       [29] ISO/IEC 14496-15: "Information technology - Coding of audio-
            visual objects - Part 15: Advanced Video Coding (AVC) file
            format".

       [30] Castagno, R. and D. Singer, "MIME Type Registrations for 3rd
            Generation Partnership Project (3GPP) Multimedia files", RFC
            3839, July 2004.

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    Authors' Addresses

       Stephan Wenger
       TU Berlin / Teles AG
       Franklinstr. 28-29
       D-10587 Berlin
       Germany

       Phone: +49-172-300-0813
       EMail: stewe@stewe.org

       Miska M. Hannuksela
       Nokia Corporation
       P.O. Box 100
       33721 Tampere
       Finland

       Phone: +358-7180-73151
       EMail: miska.hannuksela@nokia.com

       Thomas Stockhammer
       Nomor Research
       D-83346 Bergen
       Germany

       Phone: +49-8662-419407
       EMail: stockhammer@nomor.de

       Magnus Westerlund
       Multimedia Technologies
       Ericsson Research EAB/TVA/A
       Ericsson AB
       Torshamsgatan 23
       SE-164 80 Stockholm
       Sweden

       Phone: +46-8-7190000
       EMail: magnus.westerlund@ericsson.com

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

       David Singer
       QuickTime Engineering
       Apple
       1 Infinite Loop MS 302-3MT
       Cupertino
       CA 95014
       USA

       Phone +1 408 974-3162
       EMail: singer@apple.com

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    RFC 3984           RTP Payload Format for H.264 Video      February 2005

    Full Copyright Statement

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