    Internet Engineering Task Force (IETF)                          A. Begen
    Request for Comments: 6015                                         Cisco
    Category: Standards Track                                   October 2010
    ISSN: 2070-1721

                 RTP Payload Format for 1-D Interleaved Parity
                         Forward Error Correction (FEC)

    Abstract

       This document defines a new RTP payload format for the Forward Error
       Correction (FEC) that is generated by the 1-D interleaved parity code
       from a source media encapsulated in RTP.  The 1-D interleaved parity
       code is a systematic code, where a number of repair symbols are
       generated from a set of source symbols and sent in a repair flow
       separate from the source flow that carries the source symbols.  The
       1-D interleaved parity code offers a good protection against bursty
       packet losses at a cost of reasonable complexity.  The new payload
       format defined in this document should only be used (with some
       exceptions) as a part of the Digital Video Broadcasting-IPTV (DVB-
       IPTV) Application-layer FEC specification.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6015.

    Begen                        Standards Track                    [Page 1]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

    Copyright Notice

       Copyright (c) 2010 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Begen                        Standards Track                    [Page 2]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

    Table of Contents

       1. Introduction ....................................................4
          1.1. Use Cases ..................................................6
          1.2. Overhead Computation .......................................8
          1.3. Relation to Existing Specifications ........................8
               1.3.1. RFCs 2733 and 3009 ..................................8
               1.3.2. SMPTE 2022-1 ........................................8
               1.3.3. ETSI TS 102 034 .....................................9
          1.4. Scope of the Payload Format ...............................10
       2. Requirements Notation ..........................................10
       3. Definitions, Notations, and Abbreviations ......................10
          3.1. Definitions ...............................................10
          3.2. Notations .................................................11
       4. Packet Formats .................................................11
          4.1. Source Packets ............................................11
          4.2. Repair Packets ............................................11
       5. Payload Format Parameters ......................................15
          5.1. Media Type Registration ...................................15
               5.1.1. Registration of audio/1d-interleaved-parityfec .....15
               5.1.2. Registration of video/1d-interleaved-parityfec .....16
               5.1.3. Registration of text/1d-interleaved-parityfec ......18
               5.1.4. Registration of
                      application/1d-interleaved-parityfec ...............19
          5.2. Mapping to SDP Parameters .................................20
               5.2.1. Offer-Answer Model Considerations ..................21
               5.2.2. Declarative Considerations .........................22
       6. Protection and Recovery Procedures .............................22
          6.1. Overview ..................................................22
          6.2. Repair Packet Construction ................................22
          6.3. Source Packet Reconstruction ..............................24
               6.3.1. Associating the Source and Repair Packets ..........25
               6.3.2. Recovering the RTP Header and Payload ..............25
       7. Session Description Protocol (SDP) Signaling ...................27
       8. Congestion Control Considerations ..............................27
       9. Security Considerations ........................................28
       10. IANA Considerations ...........................................29
       11. Acknowledgments ...............................................29
       12. References ....................................................29
          12.1. Normative References .....................................29
          12.2. Informative References ...................................30

    Begen                        Standards Track                    [Page 3]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

    1.  Introduction

       This document extends the Forward Error Correction (FEC) header
       defined in [RFC2733] and uses this new FEC header for the FEC that is
       generated by the 1-D interleaved parity code from a source media
       encapsulated in RTP [RFC3550].  The resulting new RTP payload format
       is registered by this document.

       The type of the source media protected by the 1-D interleaved parity
       code can be audio, video, text, or application.  The FEC data are
       generated according to the media type parameters that are
       communicated through out-of-band means.  The associations/
       relationships between the source and repair flows are also
       communicated through out-of-band means.

       The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation
       to generate the repair symbols.  In a nutshell, the following steps
       take place:

       1.  The sender determines a set of source packets to be protected
           together based on the media type parameters.

       2.  The sender applies the XOR operation on the source symbols to
           generate the required number of repair symbols.

       3.  The sender packetizes the repair symbols and sends the repair
           packet(s) along with the source packets to the receiver(s) (in
           different flows).  The repair packets may be sent proactively or
           on demand.

       Note that the source and repair packets belong to different source
       and repair flows, and the sender needs to provide a way for the
       receivers to demultiplex them, even in the case in which they are
       sent in the same transport flow (i.e., same source/destination
       address/port with UDP).  This is required to offer backward
       compatibility (see Section 4).  At the receiver side, if all of the
       source packets are successfully received, there is no need for FEC
       recovery and the repair packets are discarded.  However, if there are
       missing source packets, the repair packets can be used to recover the
       missing information.  Block diagrams for the systematic parity FEC
       encoder and decoder are sketched in Figures 1 and 2, respectively.

    Begen                        Standards Track                    [Page 4]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

                                   +------------+
        +--+  +--+  +--+  +--+ --> | Systematic | --> +--+  +--+  +--+  +--+
        +--+  +--+  +--+  +--+     | Parity FEC |     +--+  +--+  +--+  +--+
                                   |  Encoder   |
                                   |  (Sender)  | --> +==+  +==+
                                   +------------+     +==+  +==+

        Source Packet: +--+    Repair Packet: +==+
                       +--+                   +==+

             Figure 1: Block diagram for systematic parity FEC encoder

                                   +------------+
        +--+    X    X    +--+ --> | Systematic | --> +--+  +--+  +--+  +--+
        +--+              +--+     | Parity FEC |     +--+  +--+  +--+  +--+
                                   |  Decoder   |
                    +==+  +==+ --> | (Receiver) |
                    +==+  +==+     +------------+

        Source Packet: +--+    Repair Packet: +==+    Lost Packet: X
                       +--+                   +==+

             Figure 2: Block diagram for systematic parity FEC decoder

       Suppose that we have a group of D x L source packets that have
       sequence numbers starting from 1 running to D x L.  If we apply the
       XOR operation to the group of the source packets whose sequence
       numbers are L apart from each other as sketched in Figure 3, we
       generate L repair packets.  This process is referred to as 1-D
       interleaved FEC protection, and the resulting L repair packets are
       referred to as interleaved (or column) FEC packets.

    Begen                        Standards Track                    [Page 5]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

           +-------------+ +-------------+ +-------------+     +-------+
           | S_1         | | S_2         | | S3          | ... | S_L   |
           | S_L+1       | | S_L+2       | | S_L+3       | ... | S_2xL |
           | .           | | .           | |             |     |       |
           | .           | | .           | |             |     |       |
           | .           | | .           | |             |     |       |
           | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL |
           +-------------+ +-------------+ +-------------+     +-------+
                  +               +               +                +
            -------------   -------------   -------------       -------
           |     XOR     | |     XOR     | |     XOR     | ... |  XOR  |
            -------------   -------------   -------------       -------
                  =               =               =                =
                +===+           +===+           +===+            +===+
                |C_1|           |C_2|           |C_3|      ...   |C_L|
                +===+           +===+           +===+            +===+

               Figure 3: Generating interleaved (column) FEC packets

       In Figure 3, S_n and C_m denote the source packet with a sequence
       number n and the interleaved (column) FEC packet with a sequence
       number m, respectively.

    1.1.  Use Cases

       We generate one interleaved FEC packet out of D non-consecutive
       source packets.  This repair packet can provide a full recovery of
       the missing information if there is only one packet missing among the
       corresponding source packets.  This implies that 1-D interleaved FEC
       protection performs well under bursty loss conditions provided that a
       large enough value is chosen for L, i.e., L packet duration should
       not be shorter than the duration of the burst that is intended to be
       repaired.

       For example, consider the scenario depicted in Figure 4 in which the
       sender generates interleaved FEC packets and a bursty loss hits the
       source packets.  Since the number of columns is larger than the
       number of packets lost due to the bursty loss, the repair operation
       succeeds.

    Begen                        Standards Track                    [Page 6]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

                             +---+
                             | 1 |    X      X      X
                             +---+

                             +---+  +---+  +---+  +---+
                             | 5 |  | 6 |  | 7 |  | 8 |
                             +---+  +---+  +---+  +---+

                             +---+  +---+  +---+  +---+
                             | 9 |  | 10|  | 11|  | 12|
                             +---+  +---+  +---+  +---+

                             +===+  +===+  +===+  +===+
                             |C_1|  |C_2|  |C_3|  |C_4|
                             +===+  +===+  +===+  +===+

          Figure 4: Example scenario where 1-D interleaved FEC protection
                              succeeds error recovery

       The sender may generate interleaved FEC packets to combat the bursty
       packet losses.  However, two or more random packet losses may hit the
       source and repair packets in the same column.  In that case, the
       repair operation fails.  This is illustrated in Figure 5.  Note that
       it is possible that two or more bursty losses may occur in the same
       source block, in which case interleaved FEC packets may still fail to
       recover the lost data.

                             +---+         +---+  +---+
                             | 1 |    X    | 3 |  | 4 |
                             +---+         +---+  +---+

                             +---+         +---+  +---+
                             | 5 |    X    | 7 |  | 8 |
                             +---+         +---+  +---+

                             +---+  +---+  +---+  +---+
                             | 9 |  | 10|  | 11|  | 12|
                             +---+  +---+  +---+  +---+

                             +===+  +===+  +===+  +===+
                             |C_1|  |C_2|  |C_3|  |C_4|
                             +===+  +===+  +===+  +===+

       Figure 5: Example scenario where 1-D interleaved FEC protection fails
                                  error recovery

    Begen                        Standards Track                    [Page 7]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

    1.2.  Overhead Computation

       The overhead is defined as the ratio of the number of bytes that
       belong to the repair packets to the number of bytes that belong to
       the protected source packets.

       Assuming that each repair packet carries an equal number of bytes
       carried by a source packet and ignoring the size of the FEC header,
       we can compute the overhead as follows:

            Overhead = 1/D

       where D is the number of rows in the source block.

    1.3.  Relation to Existing Specifications

       This section discusses the relation of the current specification to
       other existing specifications.

    1.3.1.  RFCs 2733 and 3009

       The current specification extends the FEC header defined in [RFC2733]
       and registers a new RTP payload format.  This new payload format is
       not backward compatible with the payload format that was registered
       by [RFC3009].

    1.3.2.  SMPTE 2022-1

       In 2007, the Society of Motion Picture and Television Engineers
       (SMPTE) - Technology Committee N26 on File Management and Networking
       Technology - decided to revise the Pro-MPEG Code of Practice (CoP) #3
       Release 2 specification (initially produced by the Pro-MPEG Forum in
       2004), which discussed several aspects of the transmission of MPEG-2
       transport streams over IP networks.  The new SMPTE specification is
       referred to as [SMPTE2022-1].

       The Pro-MPEG CoP #3 Release 2 document was originally based on
       [RFC2733].  SMPTE revised the document by extending the FEC header
       proposed in [RFC2733] (by setting the E bit).  This extended header
       offers some improvements.

       For example, instead of utilizing the bitmap field used in [RFC2733],
       [SMPTE2022-1] introduces separate fields to convey the number of rows
       (D) and columns (L) of the source block as well as the type of the
       repair packet (i.e., whether the repair packet is an interleaved FEC
       packet computed over a column or a non-interleaved FEC packet
       computed over a row).  These fields, plus the base sequence number,
       allow the receiver side to establish associations between the source

    Begen                        Standards Track                    [Page 8]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       and repair packets.  Note that although the bitmap field is not
       utilized, the FEC header of [SMPTE2022-1] inherently carries over the
       bitmap field from [RFC2733].

       On the other hand, some parts of [SMPTE2022-1] are not in compliance
       with RTP [RFC3550].  For example, [SMPTE2022-1] sets the
       Synchronization Source (SSRC) field to zero and does not use the
       timestamp field in the RTP headers of the repair packets (receivers
       ignore the timestamps of the repair packets).  Furthermore,
       [SMPTE2022-1] also sets the CSRC Count (CC) field in the RTP header
       to zero and does not allow any Contributing Source (CSRC) entry in
       the RTP header.

       The current document adopts the extended FEC header of [SMPTE2022-1]
       and registers a new RTP payload format.  At the same time, this
       document fixes the parts of [SMPTE2022-1] that are not compliant with
       RTP [RFC3550], except the one discussed below.

       The baseline header format first proposed in [RFC2733] does not have
       fields to protect the P and X bits and the CC fields of the source
       packets associated with a repair packet.  Rather, the P bit, X bit,
       and CC field in the RTP header of the repair packet are used to
       protect those bits and fields.  This, however, may sometimes result
       in failures when doing the RTP header validity checks as specified in
       [RFC3550].  While this behavior has been fixed in [RFC5109], which
       obsoleted [RFC2733], the RTP payload format defined in this document
       still allows this behavior for legacy purposes.  Implementations
       following this specification must be aware of this potential issue
       when RTP header validity checks are applied.

    1.3.3.  ETSI TS 102 034

       In 2009, the Digital Video Broadcasting (DVB) consortium published a
       technical specification [ETSI-TS-102-034] through the European
       Telecommunications Standards Institute (ETSI).  This specification
       covers several areas related to the transmission of MPEG-2 transport
       stream-based services over IP networks.

       Annex E of [ETSI-TS-102-034] defines an optional protocol for
       Application-layer FEC (AL-FEC) protection of streaming media for
       DVB-IP services carried over RTP [RFC3550] transport.  The DVB-IPTV
       AL-FEC protocol uses two layers for protection: a base layer that is
       produced by a packet-based interleaved parity code, and an
       enhancement layer that is produced by a Raptor code [DVB-AL-FEC].
       While the use of the enhancement layer is optional, the use of the
       base layer is mandatory wherever AL-FEC is used.  The DVB-IPTV AL-FEC
       protocol is also described in [DVB-AL-FEC].

    Begen                        Standards Track                    [Page 9]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       The interleaved parity code that is used in the base layer is a
       subset of [SMPTE2022-1].  In particular, the AL-FEC base layer uses
       only the 1-D interleaved FEC protection from [SMPTE2022-1].  The new
       RTP payload format that is defined and registered in this document
       (with some exceptions listed in [DVB-AL-FEC]) is used as the AL-FEC
       base layer.

    1.4.  Scope of the Payload Format

       The payload format specified in this document must only be used in
       legacy applications where the limitations explained in Section 1.3.2
       are known not to impact any system components or other RTP elements.
       Whenever possible, a payload format that is fully compliant with
       [RFC3550], such as [RFC5109] or other newer payload formats, must be
       used.

    2.  Requirements Notation

       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 [RFC2119].

    3.  Definitions, Notations, and Abbreviations

       The definitions and notations commonly used in this document are
       summarized in this section.

    3.1.  Definitions

       This document uses the following definitions:

       Source Flow: The packet flow(s) carrying the source data to which FEC
       protection is to be applied.

       Repair Flow: The packet flow(s) carrying the repair data.

       Symbol: A unit of data.  Its size, in bytes, is referred to as the
       symbol size.

       Source Symbol: The smallest unit of data used during the encoding
       process.

       Repair Symbol: Repair symbols are generated from the source symbols.

       Source Packet: Data packets that contain only source symbols.

       Repair Packet: Data packets that contain only repair symbols.

    Begen                        Standards Track                   [Page 10]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       Source Block: A block of source symbols that are considered together
       in the encoding process.

    3.2.  Notations

       o  L: Number of columns of the source block.

       o  D: Number of rows of the source block.

    4.  Packet Formats

       This section defines the formats of the source and repair packets.

    4.1.  Source Packets

       The source packets need to contain information that identifies the
       source block and the position within the source block occupied by the
       packet.  Since the source packets that are carried within an RTP
       stream already contain unique sequence numbers in their RTP headers
       [RFC3550], we can identify the source packets in a straightforward
       manner, and there is no need to append additional field(s).  The
       primary advantage of not modifying the source packets in any way is
       that it provides backward compatibility for the receivers that do not
       support FEC at all.  In multicast scenarios, this backward
       compatibility becomes quite useful as it allows the non-FEC-capable
       and FEC-capable receivers to receive and interpret the same source
       packets sent in the same multicast session.

    4.2.  Repair Packets

       The repair packets MUST contain information that identifies the
       source block to which they pertain and the relationship between the
       contained repair symbols and the original source block.  For this
       purpose, we use the RTP header of the repair packets as well as
       another header within the RTP payload, which we refer to as the FEC
       header, as shown in Figure 6.

    Begen                        Standards Track                   [Page 11]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

                 +------------------------------+
                 |          IP Header           |
                 +------------------------------+
                 |       Transport Header       |
                 +------------------------------+
                 |          RTP Header          | __
                 +------------------------------+   |
                 |          FEC Header          |    \
                 +------------------------------+     > RTP Payload
                 |        Repair Symbols        |    /
                 +------------------------------+ __|

                        Figure 6: Format of repair packets

       The RTP header is formatted according to [RFC3550] with some further
       clarifications listed below:

       o  Version: The version field is set to 2.

       o  Padding (P) Bit: This bit is equal to the XOR sum of the
          corresponding P bits from the RTP headers of the source packets
          protected by this repair packet.  However, padding octets are
          never present in a repair packet, independent of the value of the
          P bit.

       o  Extension (X) Bit: This bit is equal to the XOR sum of the
          corresponding X bits from the RTP headers of the source packets
          protected by this repair packet.  However, an RTP header extension
          is never present in a repair packet, independent of the value of
          the X bit.

       o  CSRC Count (CC): This field is equal to the XOR sum of the
          corresponding CC values from the RTP headers of the source packets
          protected by this repair packet.  However, a CSRC list is never
          present in a repair packet, independent of the value of the CC
          field.

       o  Marker (M) Bit: This bit is equal to the XOR sum of the
          corresponding M bits from the RTP headers of the source packets
          protected by this repair packet.

       o  Payload Type: The (dynamic) payload type for the repair packets is
          determined through out-of-band means.  Note that this document
          registers a new payload format for the repair packets (refer to
          Section 5 for details).  According to [RFC3550], an RTP receiver
          that cannot recognize a payload type must discard it.  This action
          provides backward compatibility.  The FEC mechanisms can then be
          used in a multicast group with mixed FEC-capable and non-FEC-

    Begen                        Standards Track                   [Page 12]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

          capable receivers.  If a non-FEC-capable receiver receives a
          repair packet, it will not recognize the payload type, and hence,
          discards the repair packet.

       o  Sequence Number (SN): The sequence number has the standard
          definition.  It MUST be one higher than the sequence number in the
          previously transmitted repair packet.  The initial value of the
          sequence number SHOULD be random (unpredictable) [RFC3550].

       o  Timestamp (TS): The timestamp SHALL be set to a time corresponding
          to the repair packet's transmission time.  Note that the timestamp
          value has no use in the actual FEC protection process and is
          usually useful for jitter calculations.

       o  Synchronization Source (SSRC): The SSRC value SHALL be randomly
          assigned as suggested by [RFC3550].  This allows the sender to
          multiplex the source and repair flows on the same port or
          multiplex multiple repair flows on a single port.  The repair
          flows SHOULD use the RTP Control Protocol (RTCP) CNAME field to
          associate themselves with the source flow.

          In some networks, the RTP Source (which produces the source
          packets) and the FEC Source (which generates the repair packets
          from the source packets) may not be the same host.  In such
          scenarios, using the same CNAME for the source and repair flows
          means that the RTP Source and the FEC Source MUST share the same
          CNAME (for this specific source-repair flow association).  A
          common CNAME may be produced based on an algorithm that is known
          both to the RTP and FEC Source.  This usage is compliant with
          [RFC3550].

          Note that due to the randomness of the SSRC assignments, there is
          a possibility of SSRC collision.  In such cases, the collisions
          MUST be resolved as described in [RFC3550].

       Note that the P bit, X bit, CC field, and M bit of the source packets
       are protected by the corresponding bits/fields in the RTP header of
       the repair packet.  On the other hand, the payload of a repair packet
       protects the concatenation of (if present) the CSRC list, RTP
       extension, payload, and padding of the source RTP packets associated
       with this repair packet.

       The FEC header is 16 octets.  The format of the FEC header is shown
       in Figure 7.

    Begen                        Standards Track                   [Page 13]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          SN base low          |        Length recovery        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |E| PT recovery |                     Mask                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                          TS recovery                          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |N|D|Type |Index|     Offset    |       NA      |  SN base ext  |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 7: Format of the FEC header

       The FEC header consists of the following fields:

       o  The SN base low field is used to indicate the lowest sequence
          number, taking wraparound into account, of those source packets
          protected by this repair packet.

       o  The Length recovery field is used to determine the length of any
          recovered packets.

       o  The E bit is the extension flag introduced in [RFC2733] and used
          to extend the [RFC2733] FEC header.

       o  The PT recovery field is used to determine the payload type of the
          recovered packets.

       o  The Mask field is not used.

       o  The TS recovery field is used to determine the timestamp of the
          recovered packets.

       o  The N bit is the extension flag that is reserved for future use.

       o  The D bit is not used.

       o  The Type field indicates the type of the error-correcting code
          used.  This document defines only one error-correcting code.

       o  The Index field is not used.

       o  The Offset and NA fields are used to indicate the number of
          columns (L) and rows (D) of the source block, respectively.

       o  The SN base ext field is not used.

    Begen                        Standards Track                   [Page 14]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       The details on setting the fields in the FEC header are provided in
       Section 6.2.

       It should be noted that a Mask-based approach (similar to the one
       specified in [RFC2733]) may not be very efficient to indicate which
       source packets in the current source block are associated with a
       given repair packet.  In particular, for the applications that would
       like to use large source block sizes, the size of the Mask that is
       required to describe the source-repair packet associations may be
       prohibitively large.  Instead, a systematized approach is inherently
       more efficient.

    5.  Payload Format Parameters

       This section provides the media subtype registration for the 1-D
       interleaved parity FEC.  The parameters that are required to
       configure the FEC encoding and decoding operations are also defined
       in this section.

    5.1.  Media Type Registration

       This registration is done using the template defined in [RFC4288] and
       following the guidance provided in [RFC4855].

    5.1.1.  Registration of audio/1d-interleaved-parityfec

       Type name: audio

       Subtype name: 1d-interleaved-parityfec

       Required parameters:

       o  rate: The RTP timestamp (clock) rate in Hz.  The (integer) rate
          SHALL be larger than 1000 to provide sufficient resolution to RTCP
          operations.  However, it is RECOMMENDED to select the rate that
          matches the rate of the protected source RTP stream.

       o  L: Number of columns of the source block.  L is a positive integer
          that is less than or equal to 255.

       o  D: Number of rows of the source block.  D is a positive integer
          that is less than or equal to 255.

       o  repair-window: The time that spans the FEC block (i.e., source
          packets and the corresponding repair packets).  An FEC encoder
          processes a block of source packets and generates a number of
          repair packets, which are then transmitted within a certain
          duration not larger than the value of the repair window.  At the

    Begen                        Standards Track                   [Page 15]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

          receiver side, the FEC decoder should wait at least for the
          duration of the repair window after getting the first packet in an
          FEC block to allow all the repair packets to arrive (the waiting
          time can be adjusted if there are missing packets at the beginning
          of the FEC block).  The FEC decoder can start decoding the already
          received packets sooner; however, it SHOULD NOT register an FEC
          decoding failure until it waits at least for the repair-window
          duration.  The size of the repair window is specified in
          microseconds.

       Optional parameters: None.

       Encoding considerations: This media type is framed (see Section 4.8
       in the template document [RFC4288]) and contains binary data.

       Security considerations: See Section 9 of [RFC6015].

       Interoperability considerations: None.

       Published specification: [RFC6015].

       Applications that use this media type: Multimedia applications that
       want to improve resiliency against packet loss by sending redundant
       data in addition to the source media.

       Additional information: None.

       Person & email address to contact for further information: Ali Begen
       <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.

       Intended usage: COMMON.

       Restriction on usage: This media type depends on RTP framing, and
       hence, is only defined for transport via RTP [RFC3550].

       Author: Ali Begen <abegen@cisco.com>.

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

    5.1.2.  Registration of video/1d-interleaved-parityfec

       Type name: video

       Subtype name: 1d-interleaved-parityfec

       Required parameters:

    Begen                        Standards Track                   [Page 16]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       o  rate: The RTP timestamp (clock) rate in Hz.  The (integer) rate
          SHALL be larger than 1000 to provide sufficient resolution to RTCP
          operations.  However, it is RECOMMENDED to select the rate that
          matches the rate of the protected source RTP stream.

       o  L: Number of columns of the source block.  L is a positive integer
          that is less than or equal to 255.

       o  D: Number of rows of the source block.  D is a positive integer
          that is less than or equal to 255.

       o  repair-window: The time that spans the FEC block (i.e., source
          packets and the corresponding repair packets).  An FEC encoder
          processes a block of source packets and generates a number of
          repair packets, which are then transmitted within a certain
          duration not larger than the value of the repair window.  At the
          receiver side, the FEC decoder should wait at least for the
          duration of the repair window after getting the first packet in an
          FEC block to allow all the repair packets to arrive (the waiting
          time can be adjusted if there are missing packets at the beginning
          of the FEC block).  The FEC decoder can start decoding the already
          received packets sooner; however, it SHOULD NOT register an FEC
          decoding failure until it waits at least for the repair-window
          duration.  The size of the repair window is specified in
          microseconds.

       Optional parameters: None.

       Encoding considerations: This media type is framed (see Section 4.8
       in the template document [RFC4288]) and contains binary data.

       Security considerations: See Section 9 of [RFC6015].

       Interoperability considerations: None.

       Published specification: [RFC6015].

       Applications that use this media type: Multimedia applications that
       want to improve resiliency against packet loss by sending redundant
       data in addition to the source media.

       Additional information: None.

       Person & email address to contact for further information: Ali Begen
       <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.

       Intended usage: COMMON.

    Begen                        Standards Track                   [Page 17]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       Restriction on usage: This media type depends on RTP framing, and
       hence, is only defined for transport via RTP [RFC3550].

       Author: Ali Begen <abegen@cisco.com>.

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

    5.1.3.  Registration of text/1d-interleaved-parityfec

       Type name: text

       Subtype name: 1d-interleaved-parityfec

       Required parameters:

       o  rate: The RTP timestamp (clock) rate in Hz.  The (integer) rate
          SHALL be larger than 1000 to provide sufficient resolution to RTCP
          operations.  However, it is RECOMMENDED to select the rate that
          matches the rate of the protected source RTP stream.

       o  L: Number of columns of the source block.  L is a positive integer
          that is less than or equal to 255.

       o  D: Number of rows of the source block.  D is a positive integer
          that is less than or equal to 255.

       o  repair-window: The time that spans the FEC block (i.e., source
          packets and the corresponding repair packets).  An FEC encoder
          processes a block of source packets and generates a number of
          repair packets, which are then transmitted within a certain
          duration not larger than the value of the repair window.  At the
          receiver side, the FEC decoder should wait at least for the
          duration of the repair window after getting the first packet in an
          FEC block to allow all the repair packets to arrive (the waiting
          time can be adjusted if there are missing packets at the beginning
          of the FEC block).  The FEC decoder can start decoding the already
          received packets sooner; however, it SHOULD NOT register an FEC
          decoding failure until it waits at least for the repair-window
          duration.  The size of the repair window is specified in
          microseconds.

       Optional parameters: None.

       Encoding considerations: This media type is framed (see Section 4.8
       in the template document [RFC4288]) and contains binary data.

       Security considerations: See Section 9 of [RFC6015].

    Begen                        Standards Track                   [Page 18]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       Interoperability considerations: None.

       Published specification: [RFC6015].

       Applications that use this media type: Multimedia applications that
       want to improve resiliency against packet loss by sending redundant
       data in addition to the source media.

       Additional information: None.

       Person & email address to contact for further information: Ali Begen
       <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.

       Intended usage: COMMON.

       Restriction on usage: This media type depends on RTP framing, and
       hence, is only defined for transport via RTP [RFC3550].

       Author: Ali Begen <abegen@cisco.com>.

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

    5.1.4.  Registration of application/1d-interleaved-parityfec

       Type name: application

       Subtype name: 1d-interleaved-parityfec

       Required parameters:

       o  rate: The RTP timestamp (clock) rate in Hz.  The (integer) rate
          SHALL be larger than 1000 to provide sufficient resolution to RTCP
          operations.  However, it is RECOMMENDED to select the rate that
          matches the rate of the protected source RTP stream.

       o  L: Number of columns of the source block.  L is a positive integer
          that is less than or equal to 255.

       o  D: Number of rows of the source block.  D is a positive integer
          that is less than or equal to 255.

       o  repair-window: The time that spans the FEC block (i.e., source
          packets and the corresponding repair packets).  An FEC encoder
          processes a block of source packets and generates a number of
          repair packets, which are then transmitted within a certain
          duration not larger than the value of the repair window.  At the
          receiver side, the FEC decoder should wait at least for the

    Begen                        Standards Track                   [Page 19]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

          duration of the repair window after getting the first packet in an
          FEC block to allow all the repair packets to arrive (the waiting
          time can be adjusted if there are missing packets at the beginning
          of the FEC block).  The FEC decoder can start decoding the already
          received packets sooner; however, it SHOULD NOT register an FEC
          decoding failure until it waits at least for the repair-window
          duration.  The size of the repair window is specified in
          microseconds.

       Optional parameters: None.

       Encoding considerations: This media type is framed (see Section 4.8
       in the template document [RFC4288]) and contains binary data.

       Security considerations: See Section 9 of [RFC6015].

       Interoperability considerations: None.

       Published specification: [RFC6015].

       Applications that use this media type: Multimedia applications that
       want to improve resiliency against packet loss by sending redundant
       data in addition to the source media.

       Additional information: None.

       Person & email address to contact for further information: Ali Begen
       <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.

       Intended usage: COMMON.

       Restriction on usage: This media type depends on RTP framing, and
       hence, is only defined for transport via RTP [RFC3550].

       Author: Ali Begen <abegen@cisco.com>.

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

    5.2.  Mapping to SDP Parameters

       Applications that use RTP transport commonly use Session Description
       Protocol (SDP) [RFC4566] to describe their RTP sessions.  The
       information that is used to specify the media types in an RTP session
       has specific mappings to the fields in an SDP description.  In this
       section, we provide these mappings for the media subtype registered
       by this document ("1d-interleaved-parityfec").  Note that if an
       application does not use SDP to describe the RTP sessions, an

    Begen                        Standards Track                   [Page 20]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       appropriate mapping must be defined and used to specify the media
       types and their parameters for the control/description protocol
       employed by the application.

       The mapping of the media type specification for "1d-interleaved-
       parityfec" and its parameters in SDP is as follows:

       o  The media type (e.g., "application") goes into the "m=" line as
          the media name.

       o  The media subtype ("1d-interleaved-parityfec") goes into the
          "a=rtpmap" line as the encoding name.  The RTP clock rate
          parameter ("rate") also goes into the "a=rtpmap" line as the clock
          rate.

       o  The remaining required payload-format-specific parameters go into
          the "a=fmtp" line by copying them directly from the media type
          string as a semicolon-separated list of parameter=value pairs.

       SDP examples are provided in Section 7.

    5.2.1.  Offer-Answer Model Considerations

       When offering 1-D interleaved parity FEC over RTP using SDP in an
       Offer/Answer model [RFC3264], the following considerations apply:

       o  Each combination of the L and D parameters produces a different
          FEC data and is not compatible with any other combination.  A
          sender application may desire to offer multiple offers with
          different sets of L and D values as long as the parameter values
          are valid.  The receiver SHOULD normally choose the offer that has
          a sufficient amount of interleaving.  If multiple such offers
          exist, the receiver may choose the offer that has the lowest
          overhead or the one that requires the smallest amount of
          buffering.  The selection depends on the application requirements.

       o  The value for the repair-window parameter depends on the L and D
          values and cannot be chosen arbitrarily.  More specifically, L and
          D values determine the lower limit for the repair-window size.
          The upper limit of the repair-window size does not depend on the L
          and D values.

       o  Although combinations with the same L and D values but with
          different repair-window sizes produce the same FEC data, such
          combinations are still considered different offers.  The size of
          the repair-window is related to the maximum delay between the

    Begen                        Standards Track                   [Page 21]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

          transmission of a source packet and the associated repair packet.
          This directly impacts the buffering requirement on the receiver
          side, and the receiver must consider this when choosing an offer.

       o  There are no optional format parameters defined for this payload.
          Any unknown option in the offer MUST be ignored and deleted from
          the answer.  If FEC is not desired by the receiver, it can be
          deleted from the answer.

    5.2.2.  Declarative Considerations

       In declarative usage, like SDP in the Real-time Streaming Protocol
       (RTSP) [RFC2326] or the Session Announcement Protocol (SAP)
       [RFC2974], the following considerations apply:

       o  The payload format configuration parameters are all declarative
          and a participant MUST use the configuration that is provided for
          the session.

       o  More than one configuration may be provided (if desired) by
          declaring multiple RTP payload types.  In that case, the receivers
          should choose the repair flow that is best for them.

    6.  Protection and Recovery Procedures

       This section provides a complete specification of the 1-D interleaved
       parity code and its RTP payload format.

    6.1.  Overview

       The following sections specify the steps involved in generating the
       repair packets and reconstructing the missing source packets from the
       repair packets.

    6.2.  Repair Packet Construction

       The RTP header of a repair packet is formed based on the guidelines
       given in Section 4.2.

       The FEC header includes 16 octets.  It is constructed by applying the
       XOR operation on the bit strings that are generated from the
       individual source packets protected by this particular repair packet.
       The set of the source packets that are associated with a given repair
       packet can be computed by the formula given in Section 6.3.1.

       The bit string is formed for each source packet by concatenating the
       following fields together in the order specified:

    Begen                        Standards Track                   [Page 22]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       o  Padding bit (1 bit) (This is the most significant bit of the bit
          string.)

       o  Extension bit (1 bit)

       o  CC field (4 bits)

       o  Marker bit (1 bit)

       o  PT field (7 bits)

       o  Timestamp (32 bits)

       o  Unsigned network-ordered 16-bit representation of the source
          packet length in bytes minus 12 (for the fixed RTP header), i.e.,
          the sum of the lengths of all the following if present: the CSRC
          list, header extension, RTP payload, and RTP padding (16 bits).

       o  If CC is nonzero, the CSRC list (variable length)

       o  If X is 1, the header extension (variable length)

       o  Payload (variable length)

       o  Padding, if present (variable length)

       Note that if the lengths of the source packets are not equal, each
       shorter packet MUST be padded to the length of the longest packet by
       adding octet(s) of 0 at the end.  Due to this possible padding and
       mandatory FEC header, a repair packet has a larger size than the
       source packets it protects.  This may cause problems if the resulting
       repair packet size exceeds the Maximum Transmission Unit (MTU) size
       of the path over which the repair flow is sent.

       By applying the parity operation on the bit strings produced from the
       source packets, we generate the FEC bit string.  Some parts of the
       RTP header and the FEC header of the repair packet are generated from
       the FEC bit string as follows:

       o  The first (most significant) bit in the FEC bit string is written
          into the Padding bit in the RTP header of the repair packet.

       o  The next bit in the FEC bit string is written into the Extension
          bit in the RTP header of the repair packet.

       o  The next 4 bits of the FEC bit string are written into the CC
          field in the RTP header of the repair packet.

    Begen                        Standards Track                   [Page 23]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       o  The next bit of the FEC bit string is written into the Marker bit
          in the RTP header of the repair packet.

       o  The next 7 bits of the FEC bit string are written into the PT
          recovery field in the FEC header.

       o  The next 32 bits of the FEC bit string are written into the TS
          recovery field in the FEC header.

       o  The next 16 bits are written into the Length recovery field in the
          FEC header.  This allows the FEC procedure to be applied even when
          the lengths of the protected source packets are not identical.

       o  The remaining bits are set to be the payload of the repair packet.

       The remaining parts of the FEC header are set as follows:

       o  The SN base low field MUST be set to the lowest sequence number,
          taking wraparound into account, of those source packets protected
          by this repair packet.

       o  The E bit MUST be set to 1 to extend the [RFC2733] FEC header.

       o  The Mask field SHALL be set to 0 and ignored by the receiver.

       o  The N bit SHALL be set to 0 and ignored by the receiver.

       o  The D bit SHALL be set to 0 and ignored by the receiver.

       o  The Type field MUST be set to 0 and ignored by the receiver.

       o  The Index field SHALL be set to 0 and ignored by the receiver.

       o  The Offset field MUST be set to the number of columns of the
          source block (L).

       o  The NA field MUST be set to the number of rows of the source block
          (D).

       o  The SN base ext field SHALL be set to 0 and ignored by the
          receiver.

    6.3.  Source Packet Reconstruction

       This section describes the recovery procedures that are required to
       reconstruct the missing source packets.  The recovery process has two
       steps.  In the first step, the FEC decoder determines which source

    Begen                        Standards Track                   [Page 24]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       and repair packets should be used in order to recover a missing
       packet.  In the second step, the decoder recovers the missing packet,
       which consists of an RTP header and RTP payload.

       In the following, we describe the RECOMMENDED algorithms for the
       first and second steps.  Based on the implementation, different
       algorithms MAY be adopted.  However, the end result MUST be identical
       to the one produced by the algorithms described below.

    6.3.1.  Associating the Source and Repair Packets

       The first step is to associate the source and repair packets.  The SN
       base low field in the FEC header shows the lowest sequence number of
       the source packets that form the particular column.  In addition, the
       information of how many source packets are available in each column
       and row is available from the media type parameters specified in the
       SDP description.  This set of information uniquely identifies all of
       the source packets associated with a given repair packet.

       Mathematically, for any received repair packet, p*, we can determine
       the sequence numbers of the source packets that are protected by this
       repair packet as follows:

                           p*_snb + i * L (modulo 65536)

       where p*_snb denotes the value in the SN base low field of the FEC
       header of the p*, L is the number of columns of the source block and

                                     0 <= i < D

       where D is the number of rows of the source block.

       We denote the set of the source packets associated with repair packet
       p* by set T(p*).  Note that in a source block whose size is L columns
       by D rows, set T includes D source packets.  Recall that 1-D
       interleaved FEC protection can fully recover the missing information
       if there is only one source packet missing in set T.  If the repair
       packet that protects the source packets in set T is missing, or the
       repair packet is available but two or more source packets are
       missing, then missing source packets in set T cannot be recovered by
       1-D interleaved FEC protection.

    6.3.2.  Recovering the RTP Header and Payload

       For a given set T, the procedure for the recovery of the RTP header
       of the missing packet, whose sequence number is denoted by SEQNUM, is
       as follows:

    Begen                        Standards Track                   [Page 25]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       1.   For each of the source packets that are successfully received in
            set T, compute the bit string as described in Section 6.2.

       2.   For the repair packet associated with set T, compute the bit
            string in the same fashion except use the PT recovery field
            instead of the PT field and TS recovery field instead of the
            Timestamp field, and set the CSRC list, header extension and
            padding to null regardless of the values of the CC field, X bit,
            and P bit.

       3.   If any of the bit strings generated from the source packets are
            shorter than the bit string generated from the repair packet,
            pad them to be the same length as the bit string generated from
            the repair packet.  For padding, the padding of octet 0 MUST be
            added at the end of the bit string.

       4.   Calculate the recovered bit string as the XOR of the bit strings
            generated from all source packets in set T and the FEC bit
            string generated from the repair packet associated with set T.

       5.   Create a new packet with the standard 12-byte RTP header and no
            payload.

       6.   Set the version of the new packet to 2.

       7.   Set the Padding bit in the new packet to the first bit in the
            recovered bit string.

       8.   Set the Extension bit in the new packet to the next bit in the
            recovered bit string.

       9.   Set the CC field to the next 4 bits in the recovered bit string.

       10.  Set the Marker bit in the new packet to the next bit in the
            recovered bit string.

       11.  Set the Payload type in the new packet to the next 7 bits in the
            recovered bit string.

       12.  Set the SN field in the new packet to SEQNUM.

       13.  Set the TS field in the new packet to the next 32 bits in the
            recovered bit string.

       14.  Take the next 16 bits of the recovered bit string and set the
            new variable Y to whatever unsigned integer this represents
            (assuming network order).  Convert Y to host order and then take
            Y bytes from the recovered bit string and append them to the new

    Begen                        Standards Track                   [Page 26]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

            packet.  Y represents the length of the new packet in bytes
            minus 12 (for the fixed RTP header), i.e., the sum of the
            lengths of all the following if present: the CSRC list, header
            extension, RTP payload, and RTP padding.

       15.  Set the SSRC of the new packet to the SSRC of the source RTP
            stream.

       This procedure completely recovers both the header and payload of an
       RTP packet.

    7.  Session Description Protocol (SDP) Signaling

       This section provides an SDP [RFC4566] example.  The following
       example uses the FEC grouping semantics [RFC5956].

       In this example, we have one source video stream (mid:S1) and one FEC
       repair stream (mid:R1).  We form one FEC group with the "a=group:
       FEC-FR S1 R1" line.  The source and repair streams are sent to the
       same port on different multicast groups.  The repair window is set to
       200 ms.

            v=0
            o=ali 1122334455 1122334466 IN IP4 fec.example.com
            s=Interleaved Parity FEC Example
            t=0 0
            a=group:FEC-FR S1 R1
            m=video 30000 RTP/AVP 100
            c=IN IP4 233.252.0.1/127
            a=rtpmap:100 MP2T/90000
            a=mid:S1
            m=application 30000 RTP/AVP 110
            c=IN IP4 233.252.0.2/127
            a=rtpmap:110 1d-interleaved-parityfec/90000
            a=fmtp:110 L=5; D=10; repair-window=200000
            a=mid:R1

    8.  Congestion Control Considerations

       FEC is an effective approach to provide applications with resiliency
       against packet losses.  However, in networks where the congestion is
       a major contributor to the packet loss, the potential impacts of
       using FEC SHOULD be considered carefully before injecting the repair
       flows into the network.  In particular, in bandwidth-limited
       networks, FEC repair flows may consume most or all of the available
       bandwidth and may consequently congest the network.  In such cases,
       the applications MUST NOT arbitrarily increase the amount of FEC

    Begen                        Standards Track                   [Page 27]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       protection since doing so may lead to a congestion collapse.  If
       desired, stronger FEC protection MAY be applied only after the source
       rate has been reduced.

       In a network-friendly implementation, an application SHOULD NOT send/
       receive FEC repair flows if it knows that sending/receiving those FEC
       repair flows would not help at all in recovering the missing packets.
       Such a practice helps reduce the amount of wasted bandwidth.  It is
       RECOMMENDED that the amount of FEC protection is adjusted dynamically
       based on the packet loss rate observed by the applications.

       In multicast scenarios, it may be difficult to optimize the FEC
       protection per receiver.  If there is a large variation among the
       levels of FEC protection needed by different receivers, it is
       RECOMMENDED that the sender offers multiple repair flows with
       different levels of FEC protection and the receivers join the
       corresponding multicast sessions to receive the repair flow(s) that
       is best for them.

    9.  Security Considerations

       RTP packets using the payload format defined in this specification
       are subject to the security considerations discussed in the RTP
       specification [RFC3550] and in any applicable RTP profile.

       The main security considerations for the RTP packet carrying the RTP
       payload format defined within this memo are confidentiality,
       integrity, and source authenticity.  Confidentiality is achieved by
       encrypting the RTP payload.  Altering the FEC packets can have a big
       impact on the reconstruction operation.  An attack that changes some
       bits in the FEC packets can have a significant effect on the
       calculation and the recovery of the source packets.  For example,
       changing the length recovery field can result in the recovery of a
       packet that is too long.  Depending on the application, it may be
       helpful to perform a sanity check on the received source and FEC
       packets before performing the recovery operation and to determine the
       validity of the recovered packets before using them.

       The integrity of the RTP packets is achieved through a suitable
       cryptographic integrity protection mechanism.  Such a cryptographic
       system may also allow the authentication of the source of the
       payload.  A suitable security mechanism for this RTP payload format
       should provide source authentication capable of determining if an RTP
       packet is from a member of the RTP session.

       Note that the appropriate mechanism to provide security to RTP and
       payloads following this memo may vary.  It is dependent on the
       application, transport and signaling protocol employed.  Therefore, a

    Begen                        Standards Track                   [Page 28]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       single mechanism is not sufficient, although if suitable, using the
       Secure Real-time Transport Protocol (SRTP) [RFC3711] is RECOMMENDED.
       Other mechanisms that may be used are IPsec [RFC4301] and Transport
       Layer Security (TLS) [RFC5246]; other alternatives may exist.

       If FEC protection is applied on already encrypted source packets,
       there is no need for additional encryption.  However, if the source
       packets are encrypted after FEC protection is applied, the FEC
       packets should be cryptographically as secure as the source packets.
       Failure to provide an equal level of confidentiality, integrity, and
       authentication to the FEC packets can compromise the source packets'
       confidentiality, integrity or authentication since the FEC packets
       are generated by applying XOR operation across the source packets.

    10.  IANA Considerations

       New media subtypes are subject to IANA registration.  For the
       registration of the payload format and its parameters introduced in
       this document, refer to Section 5.

    11.  Acknowledgments

       A major part of this document is borrowed from [RFC2733], [RFC5109],
       and [SMPTE2022-1].  Thus, the author would like to thank the authors
       and editors of these earlier specifications.  The author also thanks
       Colin Perkins for his constructive suggestions for this document.

    12.  References

    12.1.  Normative References

       [RFC2119]          Bradner, S., "Key words for use in RFCs to
                          Indicate Requirement Levels", BCP 14, RFC 2119,
                          March 1997.

       [RFC3550]          Schulzrinne, H., Casner, S., Frederick, R., and V.
                          Jacobson, "RTP: A Transport Protocol for Real-Time
                          Applications", STD 64, RFC 3550, July 2003.

       [RFC4566]          Handley, M., Jacobson, V., and C. Perkins, "SDP:
                          Session Description Protocol", RFC 4566,
                          July 2006.

       [RFC5956]          Begen, A., "Forward Error Correction Grouping
                          Semantics in Session Description Protocol",
                          RFC 5956, September 2010.

    Begen                        Standards Track                   [Page 29]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       [RFC4288]          Freed, N. and J. Klensin, "Media Type
                          Specifications and Registration Procedures",
                          BCP 13, RFC 4288, December 2005.

       [RFC4855]          Casner, S., "Media Type Registration of RTP
                          Payload Formats", RFC 4855, February 2007.

       [RFC3264]          Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                          Model with Session Description Protocol (SDP)",
                          RFC 3264, June 2002.

    12.2.  Informative References

       [DVB-AL-FEC]       Begen, A. and T. Stockhammer, "Guidelines for
                          Implementing DVB-IPTV Application-Layer Hybrid FEC
                          Protection", Work in Progress, December 2009.

       [RFC2733]          Rosenberg, J. and H. Schulzrinne, "An RTP Payload
                          Format for Generic Forward Error Correction",
                          RFC 2733, December 1999.

       [RFC3009]          Rosenberg, J. and H. Schulzrinne, "Registration of
                          parityfec MIME types", RFC 3009, November 2000.

       [RFC5109]          Li, A., "RTP Payload Format for Generic Forward
                          Error Correction", RFC 5109, December 2007.

       [ETSI-TS-102-034]  ETSI TS 102 034 V1.4.1, "Transport of MPEG 2 TS
                          Based DVB Services over IP Based Networks",
                          August 2009.

       [RFC2326]          Schulzrinne, H., Rao, A., and R. Lanphier, "Real
                          Time Streaming Protocol (RTSP)", RFC 2326,
                          April 1998.

       [RFC2974]          Handley, M., Perkins, C., and E. Whelan, "Session
                          Announcement Protocol", RFC 2974, October 2000.

       [SMPTE2022-1]      SMPTE 2022-1-2007, "Forward Error Correction for
                          Real-Time Video/Audio Transport over IP Networks",
                          2007.

       [RFC3711]          Baugher, M., McGrew, D., Naslund, M., Carrara, E.,
                          and K. Norrman, "The Secure Real-time Transport
                          Protocol (SRTP)", RFC 3711, March 2004.

       [RFC4301]          Kent, S. and K. Seo, "Security Architecture for
                          the Internet Protocol", RFC 4301, December 2005.

    Begen                        Standards Track                   [Page 30]

    RFC 6015         RTP Payload Format for Interleaved FEC     October 2010

       [RFC5246]          Dierks, T. and E. Rescorla, "The Transport Layer
                          Security (TLS) Protocol Version 1.2", RFC 5246,
                          August 2008.

    Author's Address

       Ali Begen
       Cisco
       181 Bay Street
       Toronto, ON  M5J 2T3
       Canada

       EMail: abegen@cisco.com

    Begen                        Standards Track                   [Page 31]