<pre>Internet Engineering Task Force (IETF)                         M. Zanaty
Request for Comments: 8627                                         Cisco
Category: Standards Track                                       V. Singh
ISSN: 2070-1721                                             callstats.io
                                                                A. Begen
                                                         Networked Media
                                                              G. Mandyam
                                                           Qualcomm Inc.
                                                               July 2019


     <span class="h1">RTP Payload Format for Flexible Forward Error Correction (FEC)</span>

Abstract

   This document defines new RTP payload formats for the Forward Error
   Correction (FEC) packets that are generated by the non-interleaved
   and interleaved parity codes from source media encapsulated in RTP.
   These parity codes are systematic codes (Flexible FEC, or "FLEX
   FEC"), where a number of FEC repair packets are generated from a set
   of source packets from one or more source RTP streams.  These FEC
   repair packets are sent in a redundancy RTP stream separate from the
   source RTP stream(s) that carries the source packets.  RTP source
   packets that were lost in transmission can be reconstructed using the
   source and repair packets that were received.  The non-interleaved
   and interleaved parity codes that are defined in this specification
   offer a good protection against random and bursty packet losses,
   respectively, at a cost of complexity.  The RTP payload formats that
   are defined in this document address scalability issues experienced
   with the earlier specifications and offer several improvements.  Due
   to these changes, the new payload formats are not backward compatible
   with earlier specifications; however, endpoints that do not implement
   this specification can still work by simply ignoring the FEC repair
   packets.

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 <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc8627">https://www.rfc-editor.org/info/rfc8627</a>.



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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) 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.

Table of Contents

   <a href="#section-1">1</a>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
     <a href="#section-1.1">1.1</a>.  Parity Codes  . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
       1.1.1.  One-Dimensional (1-D) Non-interleaved (Row) FEC
               Protection  . . . . . . . . . . . . . . . . . . . . .   <a href="#page-5">5</a>
       <a href="#section-1.1.2">1.1.2</a>.  1-D Interleaved (Column) FEC Protection . . . . . . .   <a href="#page-6">6</a>
       <a href="#section-1.1.3">1.1.3</a>.  Use Cases for 1-D FEC Protection  . . . . . . . . . .   <a href="#page-7">7</a>
       1.1.4.  Two-Dimensional (2-D) (Row and Column) FEC Protection   8
       <a href="#section-1.1.5">1.1.5</a>.  FEC Protection with Flexible Mask . . . . . . . . . .  <a href="#page-10">10</a>
       <a href="#section-1.1.6">1.1.6</a>.  FEC Overhead Considerations . . . . . . . . . . . . .  <a href="#page-10">10</a>
       <a href="#section-1.1.7">1.1.7</a>.  FEC Protection with Retransmission  . . . . . . . . .  <a href="#page-10">10</a>
       <a href="#section-1.1.8">1.1.8</a>.  Repair Window Considerations  . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-2">2</a>.  Requirements Notation . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3">3</a>.  Definitions and Notations . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-3.1">3.1</a>.  Definitions . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-3.2">3.2</a>.  Notations . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-4">4</a>.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
     <a href="#section-4.1">4.1</a>.  Source Packets  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
     <a href="#section-4.2">4.2</a>.  FEC Repair Packets  . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
       <a href="#section-4.2.1">4.2.1</a>.  RTP Header of FEC Repair Packets  . . . . . . . . . .  <a href="#page-13">13</a>
       <a href="#section-4.2.2">4.2.2</a>.  FEC Header of FEC Repair Packets  . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-5">5</a>.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
     <a href="#section-5.1">5.1</a>.  Media Type Registration -- Parity Codes . . . . . . . . .  <a href="#page-20">20</a>
       <a href="#section-5.1.1">5.1.1</a>.  Registration of audio/flexfec . . . . . . . . . . . .  <a href="#page-21">21</a>
       <a href="#section-5.1.2">5.1.2</a>.  Registration of video/flexfec . . . . . . . . . . . .  <a href="#page-22">22</a>
       <a href="#section-5.1.3">5.1.3</a>.  Registration of text/flexfec  . . . . . . . . . . . .  <a href="#page-23">23</a>
       <a href="#section-5.1.4">5.1.4</a>.  Registration of application/flexfec . . . . . . . . .  <a href="#page-24">24</a>
     <a href="#section-5.2">5.2</a>.  Mapping to SDP Parameters . . . . . . . . . . . . . . . .  <a href="#page-25">25</a>
       <a href="#section-5.2.1">5.2.1</a>.  Offer/Answer Model Considerations . . . . . . . . . .  <a href="#page-25">25</a>
       <a href="#section-5.2.2">5.2.2</a>.  Declarative Considerations  . . . . . . . . . . . . .  <a href="#page-26">26</a>





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   <a href="#section-6">6</a>.  Protection and Recovery Procedures -- Parity Codes  . . . . .  <a href="#page-26">26</a>
     <a href="#section-6.1">6.1</a>.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
     <a href="#section-6.2">6.2</a>.  Repair Packet Construction  . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
     <a href="#section-6.3">6.3</a>.  Source Packet Reconstruction  . . . . . . . . . . . . . .  <a href="#page-28">28</a>
       <a href="#section-6.3.1">6.3.1</a>.  Associating the Source and Repair Packets . . . . . .  <a href="#page-28">28</a>
       <a href="#section-6.3.2">6.3.2</a>.  Recovering the RTP Header . . . . . . . . . . . . . .  <a href="#page-30">30</a>
       <a href="#section-6.3.3">6.3.3</a>.  Recovering the RTP Payload  . . . . . . . . . . . . .  <a href="#page-31">31</a>
       6.3.4.  Iterative Decoding Algorithm for the 2-D Parity FEC
               Protection  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-31">31</a>
   <a href="#section-7">7</a>.  Signaling Requirements  . . . . . . . . . . . . . . . . . . .  <a href="#page-34">34</a>
     <a href="#section-7.1">7.1</a>.  SDP Examples  . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-35">35</a>
       7.1.1.  Example SDP for Flexible FEC Protection with In-Band
               SSRC Mapping  . . . . . . . . . . . . . . . . . . . .  <a href="#page-35">35</a>
       7.1.2.  Example SDP for Flexible FEC Protection with Explicit
               Signaling in the SDP  . . . . . . . . . . . . . . . .  <a href="#page-35">35</a>
     7.2.  On the Use of the RTP Stream Identifier Source
           Description . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-36">36</a>
   <a href="#section-8">8</a>.  Congestion Control Considerations . . . . . . . . . . . . . .  <a href="#page-36">36</a>
   <a href="#section-9">9</a>.  Security Considerations . . . . . . . . . . . . . . . . . . .  <a href="#page-37">37</a>
   <a href="#section-10">10</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  <a href="#page-37">37</a>
   <a href="#section-11">11</a>. References  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-38">38</a>
     <a href="#section-11.1">11.1</a>.  Normative References . . . . . . . . . . . . . . . . . .  <a href="#page-38">38</a>
     <a href="#section-11.2">11.2</a>.  Informative References . . . . . . . . . . . . . . . . .  <a href="#page-39">39</a>
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-40">40</a>
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-41">41</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This document defines new RTP payload formats for the Forward Error
   Correction (FEC) that is generated by the non-interleaved and
   interleaved parity codes from a source media encapsulated in RTP
   [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].  The type of the source media protected by these parity
   codes can be audio, video, text, or application.  The FEC data are
   generated according to the media type parameters, which are
   communicated out of band (e.g., in the Session Description Protocol
   (SDP)).  Furthermore, the associations or relationships between the
   source and repair RTP streams may be communicated in or out of band.
   The in-band mechanism is advantageous when the endpoint is adapting
   the FEC parameters.  The out-of-band mechanism may be preferable when
   the FEC parameters are fixed.  While this document fully defines the
   use of FEC to protect RTP streams, it also leverages several
   definitions along with the basic source/repair header description
   from [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>] in their application to the parity codes defined here.

   The Redundancy RTP Stream [<a href="./rfc7656" title="&quot;A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources&quot;">RFC7656</a>] repair packets proposed in this
   document protect the Source RTP Stream packets that belong to the
   same RTP session.




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   The RTP payload formats that are defined in this document address the
   scalability issues experienced with the formats defined in earlier
   specifications including [<a href="./rfc2733" title="&quot;An RTP Payload Format for Generic Forward Error Correction&quot;">RFC2733</a>], [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>], and [<a href="#ref-SMPTE2022-1">SMPTE2022-1</a>].

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Parity Codes</span>

   Both the non-interleaved and interleaved parity codes use the
   eXclusive OR (XOR) operation to generate the repair packets.  The
   following steps take place:

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

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

   3.  The sender sends the repair packet(s) along with the source
       packets, in different RTP streams, to the receiver(s).  The
       repair packets may be sent proactively or on demand based on RTCP
       feedback messages such as NACK [<a href="./rfc4585" title="&quot;Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)&quot;">RFC4585</a>].

   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.
   Figures 1 and 2 describe example block diagrams for the systematic
   parity FEC encoder and decoder, respectively.

                              +------------+
   +--+  +--+  +--+  +--+ --&gt; | Systematic | --&gt; +--+  +--+  +--+  +--+
   +--+  +--+  +--+  +--+     | Parity FEC |     +--+  +--+  +--+  +--+
                              |  Encoder   |
                              |  (Sender)  | --&gt; +==+  +==+
                              +------------+     +==+  +==+

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

         Figure 1: Block Diagram for Systematic Parity FEC Encoder












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                              +------------+
   +--+    X    X    +--+ --&gt; | Systematic | --&gt; +--+  +--+  +--+  +--+
   +--+              +--+     | Parity FEC |     +--+  +--+  +--+  +--+
                              |  Decoder   |
               +==+  +==+ --&gt; | (Receiver) |
               +==+  +==+     +------------+

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

         Figure 2: Block Diagram for Systematic Parity FEC Decoder

   In Figure 2, it is clear that the FEC repair packets have to be
   received by the endpoint within a certain amount of time for the FEC
   recovery process to be useful.  The repair window is defined as the
   time that spans a FEC block, which consists of the source packets and
   the corresponding repair packets.  At the receiver side, the FEC
   decoder SHOULD buffer source and repair packets at least for the
   duration of the repair window to allow all the repair packets to
   arrive.  The FEC decoder can start decoding the already-received
   packets sooner; however, it should not register a FEC decoding
   failure until it waits at least for the duration of the repair
   window.

<span class="h4"><a class="selflink" id="section-1.1.1" href="#section-1.1.1">1.1.1</a>.  One-Dimensional (1-D) Non-interleaved (Row) FEC Protection</span>

   Consider a group of D x L source packets that have Sequence Numbers
   starting from 1 running to D x L (where D and L are as defined in
   <a href="#section-3.2">Section 3.2</a>) and a repair packet is generated by applying the XOR
   operation to every L consecutive packets as sketched in Figure 3.
   This process is referred to as "1-D non-interleaved FEC protection".
   As a result of this process, D repair packets are generated, which
   are referred to as non-interleaved (or row) FEC repair packets.  In
   general, D and L represent values that describe how packets are
   grouped together from a depth and length perspective (respectively)
   when interleaving all D x L source packets.















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   +--------------------------------------------------+    ---    +===+
   | S_1          S_2          S3          ...  S_L   | + |XOR| = |R_1|
   +--------------------------------------------------+    ---    +===+
   +--------------------------------------------------+    ---    +===+
   | S_L+1        S_L+2        S_L+3       ...  S_2xL | + |XOR| = |R_2|
   +--------------------------------------------------+    ---    +===+
     .            .            .                .           .       .
     .            .            .                .           .       .
     .            .            .                .           .       .
   +--------------------------------------------------+    ---    +===+
   | S_(D-1)xL+1  S_(D-1)xL+2  S_(D-1)xL+3 ...  S_DxL | + |XOR| = |R_D|
   +--------------------------------------------------+    ---    +===+

       Figure 3: Generating Non-interleaved (Row) FEC Repair Packets

<span class="h4"><a class="selflink" id="section-1.1.2" href="#section-1.1.2">1.1.2</a>.  1-D Interleaved (Column) FEC Protection</span>

   Consider the case where the XOR operation is applied to the group of
   the source packets whose Sequence Numbers are L apart from each
   other, as sketched in Figure 4.  In this case, the endpoint generates
   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 repair packets".

       +-------------+ +-------------+ +-------------+     +-------+
       | 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 4: Generating Interleaved (Column) FEC Repair Packets









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<span class="h4"><a class="selflink" id="section-1.1.3" href="#section-1.1.3">1.1.3</a>.  Use Cases for 1-D FEC Protection</span>

   A sender may generate one non-interleaved repair packet out of L
   consecutive source packets or one interleaved repair packet out of D
   nonconsecutive source packets.  Regardless of whether the repair
   packet is a non-interleaved or an interleaved one, it 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 non-interleaved FEC protection performs better when the source
   packets are randomly lost.  However, if the packet losses occur in
   bursts, 1-D interleaved FEC protection performs better provided that
   L is chosen to be large enough, i.e., L-packet duration is not
   shorter than the observed burst duration.  If the sender generates
   non-interleaved FEC repair packets and a burst loss hits the source
   packets, the repair operation fails.  This is illustrated in
   Figure 5.

                     +---+                +---+  +===+
                     | 1 |    X      X    | 4 |  |R_1|
                     +---+                +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 9 |  | 10|  | 11|  | 12|  |R_3|
                     +---+  +---+  +---+  +---+  +===+

                        Figure 5: Example Scenario:
   1-D Non-interleaved FEC Protection Fails Error Recovery (Burst Loss)

   The sender may generate interleaved FEC repair 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 as well.  This is illustrated in Figure 6.
   Note that it is possible that two burst losses occur back-to-back, in
   which case, interleaved FEC repair packets may still fail to recover
   the lost data.












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

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

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

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

                        Figure 6: Example Scenario:
    1-D Interleaved FEC Protection Fails Error Recovery (Periodic Loss)

<span class="h4"><a class="selflink" id="section-1.1.4" href="#section-1.1.4">1.1.4</a>.  Two-Dimensional (2-D) (Row and Column) FEC Protection</span>

   In networks where the source packets are lost both randomly and in
   bursts, the sender ought to generate both non-interleaved and
   interleaved FEC repair packets.  This type of FEC protection is known
   as "2-D parity FEC protection".  At the expense of generating more
   FEC repair packets, thus increasing the FEC overhead, 2-D FEC
   provides superior protection against mixed loss patterns.  However,
   it is still possible for 2-D parity FEC protection to fail to recover
   all of the lost source packets if a particular loss pattern occurs.
   An example scenario is illustrated in Figure 7.





















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                     +---+                +---+  +===+
                     | 1 |    X      X    | 4 |  |R_1|
                     +---+                +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+                +---+  +===+
                     | 9 |    X      X    | 12|  |R_3|
                     +---+                +---+  +===+

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

                      Figure 7: Example Scenario #1:
              2-D Parity FEC Protection Fails Error Recovery

   2-D parity FEC protection also fails when at least two rows are
   missing a source and the FEC packet and the missing source packets
   (in at least two rows) are aligned in the same column.  An example
   loss pattern is sketched in Figure 8.  Similarly, 2-D parity FEC
   protection cannot repair all missing source packets when at least two
   columns are missing a source and the FEC packet and the missing
   source packets (in at least two columns) are aligned in the same row.

                     +---+  +---+         +---+
                     | 1 |  | 2 |    X    | 4 |    X
                     +---+  +---+         +---+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+  +---+         +---+
                     | 9 |  | 10|    X    | 12|    X
                     +---+  +---+         +---+

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

                      Figure 8: Example Scenario #2:
              2-D Parity FEC Protection Fails Error Recovery






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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


<span class="h4"><a class="selflink" id="section-1.1.5" href="#section-1.1.5">1.1.5</a>.  FEC Protection with Flexible Mask</span>

   It is possible to define FEC protection for selected packets in the
   source stream.  This would enable differential protection, i.e.,
   application of FEC selectively to packets that require a higher level
   of reliability than the other packets in the source stream.  The
   sender will be required to send a bitmap indicating the packets to be
   protected, i.e., a "mask", to the receiver.  Since the mask can be
   modified during an RTP session ("flexible mask"), this kind of FEC
   protection can also be used to implement FEC dynamically (e.g., for
   adaptation to different types of traffic during the RTP session).

<span class="h4"><a class="selflink" id="section-1.1.6" href="#section-1.1.6">1.1.6</a>.  FEC Overhead Considerations</span>

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

   Generally, repair packets are larger in size than the source packets.
   Also, not all the source packets are necessarily equal in size.
   However, assuming that each repair packet carries an equal number of
   bytes as carried by a source packet, the overhead for different FEC
   protection methods can be computed as follows:

      1-D Non-interleaved FEC Protection: Overhead = 1/L

      1-D Interleaved FEC Protection: Overhead = 1/D

      2-D Parity FEC Protection: Overhead = 1/L + 1/D

   where L and D are the number of columns and rows in the source block,
   respectively.

<span class="h4"><a class="selflink" id="section-1.1.7" href="#section-1.1.7">1.1.7</a>.  FEC Protection with Retransmission</span>

   This specification supports both forward error correction, i.e.,
   before any loss is reported, as well as retransmission of source
   packets after the loss is reported.  The retransmission includes the
   RTP header of the source packet in addition to the payload.  If a
   peer supporting both FLEX FEC and other RTP retransmission methods
   (see [<a href="./rfc4588" title="&quot;RTP Retransmission Payload Format&quot;">RFC4588</a>]) receives an Offer including both FLEX FEC and another
   RTP retransmission method, it MUST respond with an Answer containing
   only FLEX FEC.








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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


<span class="h4"><a class="selflink" id="section-1.1.8" href="#section-1.1.8">1.1.8</a>.  Repair Window Considerations</span>

   The value for the repair window duration is related to the maximum L
   and D values that are expected during a FLEX FEC session; therefore,
   it cannot be chosen arbitrarily.  Repair packets that include L and D
   values larger than the repair window MUST NOT be sent.  The rate of
   the source streams should also be considered, as the repair window
   duration should ideally span several packetization intervals in order
   to leverage the error correction capabilities of the parity code.

   Because the FEC configuration can change with each repair packet (see
   <a href="#section-4.2.2">Section 4.2.2</a>), for any given repair packet, the FLEX FEC receiver
   MUST support all possible L and D combinations (both 1-D and 2-D
   interleaved over all source flows) and all flexible mask
   configurations (over all source flows) within the repair window to
   which it has agreed (e.g., through SDP or out-of-band signaling) for
   a FLEX FEC RTP session.  In addition, the FLEX FEC receiver MUST
   support receipt of a retransmission of any source flow packet within
   the repair window to which it has agreed.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Requirements Notation</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] [<a href="./rfc8174" title="&quot;Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words&quot;">RFC8174</a>] when, and only when, they appear in all
   capitals, as shown here.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Definitions and Notations</span>

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Definitions</span>

   This document uses a number of definitions from [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>].
   Additionally, it defines the following and/or updates their
   definitions from [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>].

   1-D Non-interleaved Row FEC:  A protection scheme that operates on
      consecutive source packets in the source block, able to recover a
      single lost source packet per row of the source block.

   1-D Interleaved Column FEC:  A protection scheme that operates on
      interleaved source packets in the source block, able to recover a
      single lost source packet per column of the source block.

   2-D FEC:  A protection scheme that combines row and column FEC.

   Source Block:  A set of source packets that are protected by a set of
      1-D or 2-D FEC repair packets.



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   FEC Block:  A source block and its corresponding FEC repair packets.

   Repair Window:  The time that spans a FEC block, which consists of
      the source packets and the corresponding FEC repair packets.

   XOR Parity Codes:  A FEC code that uses the eXclusive OR (XOR) parity
      operation to encode a set of source packets to form a FEC repair
      packet.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Notations</span>

   L: Number of columns of the source block (length of each row).

   D: Number of rows of the source block (depth of each column).

   bitmask:  A 15-bit, 46-bit, or 110-bit mask indicating which source
      packets are protected by a FEC repair packet.  If the bit i in the
      mask is set to 1, the source packet number N + i is protected by
      this FEC repair packet, where N is the Sequence Number base
      indicated in the FEC repair packet.  The most significant bit of
      the mask corresponds to i=0.  The least significant bit of the
      mask corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit
      mask, or i=109 in the 110-bit mask.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Packet Formats</span>

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

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Source Packets</span>

   The source packets contain the 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
   [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>], the source packets can be identified in a straightforward
   manner and there is no need to append any additional fields.  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.

   The source packets are transmitted as usual without altering them.
   They are used along with the FEC repair packets to recover any
   missing source packets, making this scheme a systematic code.




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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


   The source packets are full RTP packets with optional contributing
   source (CSRC) list, RTP header extension, and padding.  If any of
   these optional elements are present in the source RTP packet, and
   that source packet is lost, they are recovered by the FEC repair
   operation, which recovers the full source RTP packet including these
   optional elements.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  FEC Repair Packets</span>

   The FEC repair packets will contain information that identifies the
   source block they pertain to and the relationship between the
   contained repair packets and the original source block.  For this
   purpose, the RTP header of the repair packets is used, as well as
   another header within the RTP payload, called the "FEC header", as
   shown in Figure 9.

   Note that all the source stream packets that are protected by a
   particular FEC packet need to be in the same RTP session.

             +------------------------------+
             |          IP Header           |
             +------------------------------+
             |       Transport Header       |
             +------------------------------+
             |          RTP Header          |
             +------------------------------+ ---+
             |          FEC Header          |    |
             +------------------------------+    | RTP Payload
             |         Repair Payload       |    |
             +------------------------------+ ---+

                  Figure 9: Format of FEC Repair Packets

   The Repair Payload, which follows the FEC header, includes repair of
   everything following the fixed 12-byte RTP header of each source
   packet, including any CSRC identifier list and header extensions if
   present.

<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  RTP Header of FEC Repair Packets</span>

   The RTP header is formatted according to [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>] with some further
   clarifications listed below:

   Version (V) 2 bits:  This MUST be set to 2 (binary 10), as this
      specification requires all source RTP packets and all FEC repair
      packets to use RTP version 2.





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   Padding (P) bit:  Source packets can have optional RTP padding, which
      can be recovered.  FEC repair packets can have optional RTP
      padding, which is independent of the RTP padding of the source
      packets.

   Extension (X) bit:  Source packets can have optional RTP header
      extensions, which can be recovered.  FEC repair packets can have
      optional RTP header extensions, which are independent of the RTP
      header extensions of the source packets.

   CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each:  Source
      packets can have an optional CSRC list and count, which can be
      recovered.  FEC repair packets MUST use the CSRC list and count to
      specify the synchronization sources (SSRCs) of the source RTP
      stream(s) protected by this FEC repair packet.

   Marker (M) bit:  This bit is not used for this payload type, SHALL be
      set to 0 by senders, and SHALL be ignored by receivers.

   Payload Type:  The (dynamic) payload type for the FEC repair packets
      is determined through out-of-band means (e.g., SDP).  Note that
      this document registers new payload formats for the repair packets
      (refer to <a href="#section-5">Section 5</a> for details).  According to [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>], an RTP
      receiver that cannot recognize a payload type must discard it.
      This provides backward compatibility.  If a non-FEC-capable
      receiver receives a repair packet, it will not recognize the
      payload type; hence, it will discard the repair packet.

   Sequence Number (SN):  The Sequence Number follows the standard
      definition provided in [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].  Therefore, it must be one
      higher than the Sequence Number in the previously transmitted
      repair packet, and the initial value of the Sequence Number should
      be random (i.e., unpredictable).

   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.

   Synchronization Source (SSRC):  The SSRC value for each repair stream
      SHALL be randomly assigned as per the guidelines provided in
      <a href="./rfc3550#section-8">Section&nbsp;8 of [RFC3550]</a>.  This allows the sender to multiplex the
      source and repair RTP streams in the same RTP session, or
      multiplex multiple repair streams in an RTP session.  The repair
      stream's SSRC's CNAME SHOULD be identical to the CNAME of the
      source RTP stream(s) that this repair stream protects.  A FEC
      stream that protects multiple source RTP streams with different
      CNAME's uses the CNAME associated with the entity generating the



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      FEC stream or the CNAME of the entity on whose behalf it performs
      the protection operation.  In cases when the repair stream covers
      packets from multiple source RTP streams with different CNAME
      values and none of these CNAME values can be associated with the
      entity generating the FEC stream, any of these CNAME values MAY be
      used.

      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 RTP
      streams means that the RTP Source and the FEC Source will share
      the same CNAME (for this specific source-repair stream
      association).  A common CNAME may be produced based on an
      algorithm that is known both to the RTP and FEC Source [<a href="./rfc7022" title="&quot;Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)&quot;">RFC7022</a>].
      This usage is compliant with [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

      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 [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  FEC Header of FEC Repair Packets</span>

   The format of the FEC header has three variants, depending on the
   values in the first two bits (R and F bits) as shown in Figure 10.
   Note that R and F stand for "retransmit" and "fixed block",
   respectively.  Two of these variants are meant to describe different
   methods for deriving the source data from a source packet for a
   repair packet.  This allows for customizing the FEC method to allow
   for robustness against different levels of burst errors and random
   packet losses.  The third variant is for a straight retransmission of
   the source packet.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R|F|P|X|  CC   |M| PT recovery | ...varies depending on R/F... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 ...varies depending on R/F...                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                Repair Payload follows FEC header              :
     :                                                               :

                           Figure 10: FEC header





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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


   The Repair Payload, which follows the FEC header, includes repair of
   everything following the fixed 12-byte RTP header of each source
   packet, including any CSRC identifier list and header extensions if
   present.  An overview on how the Repair Payload can be used to
   recover source packets is provided in <a href="#section-6">Section 6</a>.

      +---+---+-----------------------------------------------------+
      | R | F | FEC header variant                                  |
      +---+---+-----------------------------------------------------+
      | 0 | 0 | Flexible FEC Mask fields indicate source packets    |
      | 0 | 1 | Fixed FEC L/D (cols/rows) indicate source packets   |
      | 1 | 0 | Retransmission of a single source packet            |
      | 1 | 1 | Reserved for future use, MUST NOT send, MUST ignore |
      +---+---+-----------------------------------------------------+

           Figure 11: R and F Bit Values for FEC Header Variants

   The first variant, when R=0 and F=0, has a mask to signal protected
   source packets, as shown in Figure 12.

   The second variant, when R=0 and F=1, has a number of columns (L) and
   rows (D) to signal protected source packets, as shown in Figure 13.

   The final variant, when R=1 and F=0, is a retransmission format as
   shown in Figure 15.

   No variant presently uses R=1 and F=1, which is reserved for future
   use.  Current FLEX FEC implementations MUST NOT send packets with
   this variant, and receivers MUST ignore these packets.  Future FLEX
   FEC implementations may use this by updating the media type
   registration.

   The FEC header for all variants consists of the following common
   fields:

   o  The R bit MUST be set to 1 to indicate a retransmission packet,
      and MUST be set to 0 for FEC repair packets.

   o  The F bit indicates the type of FEC repair packets, as shown in
      Figure 11, when the R bit is 0.  The F bit MUST be set to 0 when
      the R bit is 1 for retransmission packets.

   o  The P, X, CC, M, and PT recovery fields are used to determine the
      corresponding fields of the recovered packets (see also
      <a href="#section-6.3.2">Section 6.3.2</a>).






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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


<span class="h5"><a class="selflink" id="section-4.2.2.1" href="#section-4.2.2.1">4.2.2.1</a>.  FEC Header with Flexible Mask</span>

   When R=0 and F=0, the FEC header includes flexible Mask fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0|0|P|X|  CC   |M| PT recovery |        length recovery        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          TS recovery                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           SN base_i           |k|          Mask [0-14]        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |k|                   Mask [15-45] (optional)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Mask [46-109] (optional)                  |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   ... next SN base and Mask for CSRC_i in CSRC list ...       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                Repair Payload follows FEC header              :
     :                                                               :

                       Figure 12: FEC Header for F=0

   o  The Length recovery (16 bits) field is used to determine the
      length of the recovered packets.  This length includes all octets
      following the fixed 12-byte RTP header of source packets,
      including CSRC list and optional header extension(s) if present.
      It excludes the fixed 12-byte RTP header of source packets.

   o  The TS recovery (32 bits) field is used to determine the timestamp
      of the recovered packets.

   o  The CSRC_i (32 bits) field in the RTP header (not FEC header)
      describes the SSRC of the source packets protected by this
      particular FEC packet.  If a FEC packet protects multiple SSRCs
      (indicated by the CSRC Count &gt; 1 in the RTP header), there will be
      multiple blocks of data containing the SN base and Mask fields.

   o  The SN base_i (16 bits) field indicates the lowest sequence
      number, taking wrap around into account, of the source packets for
      a particular SSRC (indicated in CSRC_i) protected by this repair
      packet.







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   o  The Mask fields indicate a bitmask of which source packets are
      protected by this FEC repair packet, where bit j of the mask set
      to 1 indicates that the source packet with Sequence Number (SN
      base_i + j) is protected by this FEC repair packet, where j=0 is
      the most significant bit in the mask.

   o  The k-bit in the bitmasks indicates if the mask is 15, 46, or 110
      bits.  k=1 denotes that another mask follows, and k=0 denotes that
      it is the last block of mask.

   o  The Repair Payload, which follows the FEC header, includes repair
      of everything following the fixed 12-byte RTP header of each
      source packet, including any CSRC identifier list and header
      extensions if present.

<span class="h5"><a class="selflink" id="section-4.2.2.2" href="#section-4.2.2.2">4.2.2.2</a>.  FEC Header with Fixed L Columns and D Rows</span>

   When R=0 and F=1, the FEC header includes L and D fields for fixed
   columns and rows.  The other fields are the same as the prior
   section.  As in the previous section, the CSRC_i (32 bits) field in
   the RTP header (not FEC Header) describes the SSRC of the source
   packets protected by this particular FEC packet.  If there are
   multiple SSRC's protected by the FEC packet, then there will be
   multiple blocks of data containing an SN base along with L and D
   fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0|1|P|X|  CC   |M| PT recovery |         length recovery       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          TS recovery                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           SN base_i           |  L (columns)  |    D (rows)   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    ... next SN base and L/D for CSRC_i in CSRC list ...       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                Repair Payload follows FEC header              :
     :                                                               :

                       Figure 13: FEC Header for F=1










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   Consequently, the following conditions occur for L and D values:

   If L=0, D=0, reserved for future use,
                MUST NOT send, MUST ignore if received.

   If L&gt;0, D=0, indicates row FEC, and no column FEC will follow (1D).
                Source packets for each row: SN, SN+1, ..., SN+(L-1)

   If L&gt;0, D=1, indicates row FEC, and column FEC will follow (2D).
                Source packets for each row: SN, SN+1, ..., SN+(L-1)
                Source packets for each col: SN, SN+L, ..., SN+(D-1)*L
                After all row FEC packets have been sent,
                the column FEC packets will be sent.

   If L&gt;0, D&gt;1, indicates column FEC of every L packet, D times.
                Source packets for each col: SN, SN+L, ..., SN+(D-1)*L

             Figure 14: Interpreting the L and D Field Values

   Given the 8-bit limit on L and D (as depicted in Figure 13), the
   maximum value of either parameter is 255.  If L=0 and D=0 are in a
   packet, then the repair packet MUST be ignored by the receiver.  In
   addition, when L=1 and D=0, the repair packet becomes a
   retransmission of a corresponding source packet.

   The values of L and D for a given block of recovery data will
   correspond to the type of recovery in use for that block of data.  In
   particular, for 2-D repair, the (L,D) values may not be constant
   across all packets for a given SSRC being repaired.  Similarly, the L
   and D values can differ across different blocks of repair data
   (repairing different SSRCs) in a single packet.  If the values of L
   and D result in a repair packet that exceed the repair window of the
   FLEX FEC session, then the repair packet MUST be ignored.

   It should be noted that the flexible mask-based approach may be
   inefficient for protecting a large number of source packets, or
   impossible to signal if larger than the largest mask size.  In such
   cases, the fixed columns and rows variant may be more useful.

<span class="h5"><a class="selflink" id="section-4.2.2.3" href="#section-4.2.2.3">4.2.2.3</a>.  FEC Header for Retransmissions</span>

   When R=1 and F=0, the FEC packet is a retransmission of a single
   source packet.  Note that the layout of this retransmission packet is
   different from other FEC repair packets.  The Sequence Number (SN
   base_i) replaces the length recovery in the FEC header, since the
   length is already known for a single packet.  There are no L, D, or
   Mask fields, since only a single packet is retransmitted, identified
   by the Sequence Number in the FEC header.  The source packet SSRC is



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   included in the FEC header for retransmissions, not in the RTP header
   CSRC list as in the FEC header variants with R=0.  When performing
   retransmissions, a single repair packet stream (SSRC) MAY be used for
   retransmitting packets from multiple source packet streams (SSRCs),
   as well as transmitting FEC repair packets that protect multiple
   source packet streams (SSRCs).

   This FEC header layout is identical to the source RTP (version 2)
   packet, starting with its RTP header, where the retransmission
   "payload" is everything following the fixed 12-byte RTP header of the
   source packet, including the CSRC list and extensions if present.
   Therefore, the only operation needed for sending retransmissions is
   to prepend a new RTP header to the source packet.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|P|X|  CC   |M| Payload Type|        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :           Retransmission Payload follows FEC header           :
   :                                                               :

                 Figure 15: FEC Header for Retransmission

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Payload Format Parameters</span>

   This section provides the media subtype registration for the non-
   interleaved and interleaved parity FEC.  The parameters that are
   required to configure the FEC encoding and decoding operations are
   also defined in this section.  If no specific FEC code is specified
   in the subtype, then the FEC code defaults to the parity code defined
   in this specification.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Media Type Registration -- Parity Codes</span>

   This registration is done using the template defined in [<a href="./rfc6838" title="&quot;Media Type Specifications and Registration Procedures&quot;">RFC6838</a>] and
   following the guidance provided in [<a href="./rfc4855" title="&quot;Media Type Registration of RTP Payload Formats&quot;">RFC4855</a>] along with [<a href="./rfc4856" title="&quot;Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences&quot;">RFC4856</a>].










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<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Registration of audio/flexfec</span>

   Type name: audio

   Subtype name: flexfec

   Required parameters:

   o  rate: The RTP timestamp (clock) rate.  The rate SHALL be larger
      than 1000 Hz 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  repair-window: The time that spans the source packets and the
      corresponding repair packets.  The size of the repair window is
      specified in microseconds.

   Encoding considerations: This media type is framed (see <a href="#section-4.8">Section 4.8</a>
   in the template document [<a href="./rfc6838" title="&quot;Media Type Specifications and Registration Procedures&quot;">RFC6838</a>]) and contains binary data.

   Security considerations: See <a href="./rfc8627#section-9">Section&nbsp;9 of [RFC8627]</a>.

   Interoperability considerations: None.

   Published specification: [<a href="./rfc8627">RFC8627</a>].

   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.

   Fragment identifier considerations: None.

   Additional information: None.

   Person &amp; email address to contact for further information:
   IESG &lt;iesg@ietf.org&gt; and IETF Audio/Video Transport Payloads Working
   Group (or its successor as delegated by the IESG).

   Intended usage: COMMON.

   Restrictions on usage: This media type depends on RTP framing; hence,
   it is only defined for transport via RTP [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

   Author: Varun Singh &lt;varun@callstats.io&gt;.

   Change controller: IETF Audio/Video Transport Payloads Working Group
   delegated from the IESG (or its successor as delegated by the IESG).




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<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  Registration of video/flexfec</span>

   Type name: video

   Subtype name: flexfec

   Required parameters:

   o  rate: The RTP timestamp (clock) rate.  The rate SHALL be larger
      than 1000 Hz 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  repair-window: The time that spans the source packets and the
      corresponding repair packets.  The size of the repair window is
      specified in microseconds.

   Encoding considerations: This media type is framed (see <a href="#section-4.8">Section 4.8</a>
   in the template document [<a href="./rfc6838" title="&quot;Media Type Specifications and Registration Procedures&quot;">RFC6838</a>]) and contains binary data.

   Security considerations: See <a href="./rfc8627#section-9">Section&nbsp;9 of [RFC8627]</a>.

   Interoperability considerations: None.

   Published specification: [<a href="./rfc8627">RFC8627</a>].

   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.

   Fragment identifier considerations: None.

   Additional information: None.

   Person &amp; email address to contact for further information:
   IESG &lt;iesg@ietf.org&gt; and IETF Audio/Video Transport Payloads Working
   Group (or its successor as delegated by the IESG).

   Intended usage: COMMON.

   Restrictions on usage: This media type depends on RTP framing; hence,
   it is only defined for transport via RTP [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

   Author: Varun Singh &lt;varun@callstats.io&gt;.

   Change controller: IETF Audio/Video Transport Payloads Working Group
   delegated from the IESG (or its successor as delegated by the IESG).




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<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  Registration of text/flexfec</span>

   Type name: text

   Subtype name: flexfec

   Required parameters:

   o  rate: The RTP timestamp (clock) rate.  The rate SHALL be larger
      than 1000 Hz 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  repair-window: The time that spans the source packets and the
      corresponding repair packets.  The size of the repair window is
      specified in microseconds.

   Encoding considerations: This media type is framed (see <a href="#section-4.8">Section 4.8</a>
   in the template document [<a href="./rfc6838" title="&quot;Media Type Specifications and Registration Procedures&quot;">RFC6838</a>]) and contains binary data.

   Security considerations: See <a href="./rfc8627#section-9">Section&nbsp;9 of [RFC8627]</a>.

   Interoperability considerations: None.

   Published specification: [<a href="./rfc8627">RFC8627</a>].

   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.

   Fragment identifier considerations: None.

   Additional information: None.

   Person &amp; email address to contact for further information:
   IESG &lt;iesg@ietf.org&gt; and IETF Audio/Video Transport Payloads Working
   Group (or its successor as delegated by the IESG).

   Intended usage: COMMON.

   Restrictions on usage: This media type depends on RTP framing; hence,
   it is only defined for transport via RTP [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

   Author: Varun Singh &lt;varun@callstats.io&gt;.

   Change controller: IETF Audio/Video Transport Payloads Working Group
   delegated from the IESG (or its successor as delegated by the IESG).




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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


<span class="h4"><a class="selflink" id="section-5.1.4" href="#section-5.1.4">5.1.4</a>.  Registration of application/flexfec</span>

   Type name: application

   Subtype name: flexfec

   Required parameters:

   o  rate: The RTP timestamp (clock) rate.  The rate SHALL be larger
      than 1000 Hz 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  repair-window: The time that spans the source packets and the
      corresponding repair packets.  The size of the repair window is
      specified in microseconds.

   Encoding considerations: This media type is framed (see <a href="#section-4.8">Section 4.8</a>
   in the template document [<a href="./rfc6838" title="&quot;Media Type Specifications and Registration Procedures&quot;">RFC6838</a>]) and contains binary data.

   Security considerations: See <a href="./rfc8627#section-9">Section&nbsp;9 of [RFC8627]</a>.

   Interoperability considerations: None.

   Published specification: [<a href="./rfc8627">RFC8627</a>].

   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.

   Fragment identifier considerations: None.

   Additional information: None.

   Person &amp; email address to contact for further information:
   IESG &lt;iesg@ietf.org&gt; and IETF Audio/Video Transport Payloads Working
   Group (or its successor as delegated by the IESG).

   Intended usage: COMMON.

   Restrictions on usage: This media type depends on RTP framing; hence,
   it is only defined for transport via RTP [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].

   Author: Varun Singh &lt;varun@callstats.io&gt;.

   Change controller: IETF Audio/Video Transport Payloads Working Group
   delegated from the IESG (or its successor as delegated by the IESG).




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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Mapping to SDP Parameters</span>

   Applications that use the RTP transport commonly use the Session
   Description Protocol (SDP) [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] 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.
   This section provides these mappings for the media subtypes
   registered by this document.  Note that if an application does not
   use SDP to describe the RTP sessions, an 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 "flexfec" and its
   associated 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 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 <a href="#section-7.1">Section 7.1</a>.

<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>.  Offer/Answer Model Considerations</span>

   When offering parity FEC over RTP using SDP in an Offer/Answer model
   [<a href="./rfc3264" title="&quot;An Offer/Answer Model with Session Description Protocol (SDP)&quot;">RFC3264</a>], the following considerations apply:

   o  A sender application will indicate a repair window consistent with
      the desired amount of protection.  Since the sender can change the
      FEC configuration on a packet-by-packet basis, note that the
      receiver must support any valid FLEX FEC configuration within the
      repair window associated with the offer (see <a href="#section-4.2.2">Section 4.2.2</a>).  If
      the receiver cannot support the offered repair window it MUST
      reject the offer.

   o  The size of the repair-window is related to the maximum delay
      between the 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.





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   o  Any unknown option in the offer must be ignored and deleted from
      the answer (see <a href="./rfc3264#section-6">Section&nbsp;6 of [RFC3264]</a>).  If FEC is not desired by
      the receiver, it can be deleted from the answer.

<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>.  Declarative Considerations</span>

   In declarative usage, like SDP in the Real-time Streaming Protocol
   (RTSP, for RTSP 1.0 see [<a href="./rfc2326" title="&quot;Real Time Streaming Protocol (RTSP)&quot;">RFC2326</a>] and for RTSP 2.0 see [<a href="./rfc7826" title="&quot;Real-Time Streaming Protocol Version 2.0&quot;">RFC7826</a>]) or
   the Session Announcement Protocol (SAP) [<a href="./rfc2974" title="&quot;Session Announcement Protocol&quot;">RFC2974</a>], 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 stream that is best for them.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Protection and Recovery Procedures -- Parity Codes</span>

   This section provides a complete specification of the 1-D and 2-D
   parity codes and their RTP payload formats.  It does not apply to the
   single packet retransmission format (R=1 in the FEC header).

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Overview</span>

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

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Repair Packet Construction</span>

   The RTP header of a repair packet is formed based on the guidelines
   given in <a href="#section-4.2">Section 4.2</a>.

   The FEC header and Repair Payload of repair packets are formed 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
   <a href="#section-6.3.1">Section 6.3.1</a>.

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

   o  The first 16 bits of the RTP header (16 bits), though the first
      two (version) bits will be ignored by the recovery procedure.



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   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, extension header, RTP payload, and RTP padding (16 bits).

   o  The timestamp of the RTP header (32 bits).

   o  All octets after the fixed 12-byte RTP header.  (Note the SSRC
      field is skipped.)

   The FEC bit string is generated by applying the parity operation on
   the bit strings produced from the source packets.  The FEC header is
   generated from the FEC bit string as follows:

   o  The first (most significant) 2 bits in the FEC bit string, which
      contain the RTP version field, are skipped.  The R and F bits in
      the FEC header are set to the appropriate value, i.e., it depends
      on the chosen format variant.  As a consequence of overwriting the
      RTP version field with the R and F bits, this payload format only
      supports RTP version 2.

   o  The next bit in the FEC bit string is written into the P recovery
      bit in the FEC header.

   o  The next bit in the FEC bit string is written into the X recovery
      bit in the FEC header.

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

   o  The next bit is written into the M recovery bit in the FEC header.

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

   o  The next 16 bits are written into the length 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 lowest Sequence Number of the source packets protected by this
      repair packet is written into the Sequence Number Base field in
      the FEC header.  This needs to be repeated for each SSRC that has
      packets included in the source block.






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   o  Depending on the chosen FEC header variant, the mask(s) is set
      when F=0 or the L and D values are set when F=1.  This needs to be
      repeated for each SSRC that has packets included in the source
      block.

   o  The rest of the FEC bit string, which contains everything after
      the fixed 12-byte RTP header of the source packet, is written into
      the Repair Payload following the FEC header, where "Payload"
      refers to everything after the fixed 12-byte RTP header, including
      extensions, CSRC list, true payloads, and padding.

   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 zeros 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 stream is sent.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Source Packet Reconstruction</span>

   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
   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.

   The following describes 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.

   Note that the same algorithms are used by the 1-D parity codes,
   regardless of whether the FEC protection is applied over a column or
   a row.  The 2-D parity codes, on the other hand, usually require
   multiple iterations of the procedures described here.  This iterative
   decoding algorithm is further explained in <a href="#section-6.3.4">Section 6.3.4</a>.

<span class="h4"><a class="selflink" id="section-6.3.1" href="#section-6.3.1">6.3.1</a>.  Associating the Source and Repair Packets</span>

   Before associating source and repair packets, the receiver must know
   in which RTP sessions the source and repair, respectively, are being
   sent.  After this is established by the receiver, the first step is
   associating the source and repair packets.  This association can be




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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


   via flexible bitmasks or fixed L and D offsets, which can be in the
   FEC header or signaled in SDP in optional payload format parameters
   when L=D=0 in the FEC header.

<span class="h5"><a class="selflink" id="section-6.3.1.1" href="#section-6.3.1.1">6.3.1.1</a>.  Using Bitmasks</span>

   To use flexible bitmasks, the first two FEC header bits MUST have R=0
   and F=0.  A 15-bit, 46-bit, or 110-bit mask indicates which source
   packets are protected by a FEC repair packet.  If the bit i in the
   mask is set to 1, the source packet number N + i is protected by this
   FEC repair packet, where N is the Sequence Number base indicated in
   the FEC header.  The most significant bit of the mask corresponds to
   i=0.  The least significant bit of the mask corresponds to i=14 in
   the 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit
   mask.

   The bitmasks are able to represent arbitrary protection patterns, for
   example, 1-D interleaved, 1-D non-interleaved, 2-D.

<span class="h5"><a class="selflink" id="section-6.3.1.2" href="#section-6.3.1.2">6.3.1.2</a>.  Using L and D Offsets</span>

   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 plus one repair packet for
   the FEC protection applied over a column, and it includes L source
   packets plus one repair packet for the FEC protection applied over a
   row.  Recall that 1-D interleaved and non-interleaved FEC protection
   can fully recover the missing information if there is only one source
   packet missing per column or row in set T.  If more than one source
   packet is missing per column or row in set T, 1-D FEC protection may
   fail to recover all the missing information.

   When the value of L is non-zero, the 8-bit fields indicate the offset
   of packets protected by an interleaved (D&gt;0) or non-interleaved (D=0)
   FEC packet.  Using a combination of interleaved and non-interleaved
   FEC repair packets can form 2-D protection patterns.

   Mathematically, for any received repair packet, p*, the sequence
   numbers of the source packets that are protected by this repair
   packet are determined as follows, where SN is the Sequence Number
   base in the FEC header:

    For each SSRC (in CSRC list):
    When D &lt;= 1: Source packets for each row: SN, SN+1, ..., SN+(L-1)
    When D &gt;  1: Source packets for each col: SN, SN+L, ..., SN+(D-1)*L






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<span class="h4"><a class="selflink" id="section-6.3.2" href="#section-6.3.2">6.3.2</a>.  Recovering the RTP Header</span>

   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:

   1.   For each of the source packets that are successfully received in
        T, compute the 80-bit string by concatenating the first 64 bits
        of their RTP header and the unsigned network-ordered 16-bit
        representation of their length in bytes minus 12.

   2.   For the repair packet in T, extract the FEC bit string as the
        first 80 bits of the FEC header.

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

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

   5.   Set the version of the new packet to 2.  Skip the first 2 bits
        in the recovered bit string.

   6.   Set the Padding bit in the new packet to the next bit in the
        recovered bit string.

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

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

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

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

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

   12.  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.  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.




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

   14.  Set the SSRC of the new packet to the SSRC of the missing source
        RTP stream.

   This procedure recovers the header of an RTP packet up to (and
   including) the SSRC field.

<span class="h4"><a class="selflink" id="section-6.3.3" href="#section-6.3.3">6.3.3</a>.  Recovering the RTP Payload</span>

   Following the recovery of the RTP header, the procedure for the
   recovery of the RTP "payload" is as follows, where "payload" refers
   to everything following the fixed 12-byte RTP header, including
   extensions, CSRC list, true payload, and padding.

   1.  Allocate Y additional bytes for the new packet generated in
       <a href="#section-6.3.2">Section 6.3.2</a>.

   2.  For each of the source packets that are successfully received in
       T, compute the bit string from the Y octets of data starting with
       the 13th octet of the packet.  If any of the bit strings
       generated from the source packets has a length shorter than Y,
       pad them to that length.  The zero-padding octets MUST be added
       at the end of the bit string.  Note that the information of the
       first 8 octets are protected by the FEC header.

   3.  For the repair packet in T, compute the FEC bit string from the
       repair packet payload, i.e., the Y octets of data following the
       FEC header.  Note that the FEC header may be different sizes
       depending on the variant and bitmask size.

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

   5.  Set the last Y octets in the new packet to the recovered bit
       string.

<span class="h4"><a class="selflink" id="section-6.3.4" href="#section-6.3.4">6.3.4</a>.  Iterative Decoding Algorithm for the 2-D Parity FEC Protection</span>

   In 2-D parity FEC protection, the sender generates both non-
   interleaved and interleaved FEC repair packets to combat with the
   mixed loss patterns (random and bursty).  At the receiver side, these
   FEC packets are used iteratively to overcome the shortcomings of the
   1-D non-interleaved/interleaved FEC protection and improve the
   chances of full error recovery.




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   The iterative decoding algorithm runs as follows:

   1.  Set num_recovered_until_this_iteration to zero

   2.  Set num_recovered_so_far to zero

   3.  Recover as many source packets as possible by using the non-
       interleaved FEC repair packets as outlined in Sections <a href="#section-6.3.2">6.3.2</a> and
       6.3.3 and increase the value of num_recovered_so_far by the
       number of recovered source packets.

   4.  Recover as many source packets as possible by using the
       interleaved FEC repair packets as outlined in Sections <a href="#section-6.3.2">6.3.2</a> and
       6.3.3 and increase the value of num_recovered_so_far by the
       number of recovered source packets.

   5.  If num_recovered_so_far &gt; num_recovered_until_this_iteration
       ---num_recovered_until_this_iteration = num_recovered_so_far
       ---Go to step 3
       Else
       ---Terminate

   The algorithm terminates either when all missing source packets are
   fully recovered or when there are still remaining missing source
   packets but the FEC repair packets are not able to recover any more
   source packets.  For the example scenarios when the 2-D parity FEC
   protection fails full recovery, refer to <a href="#section-1.1.4">Section 1.1.4</a>.  Upon
   termination, variable num_recovered_so_far has a value equal to the
   total number of recovered source packets.

   Example:

   Suppose that the receiver experienced the loss pattern sketched in
   Figure 16.

















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                                   +---+  +---+  +===+
                       X      X    | 3 |  | 4 |  |R_1|
                                   +---+  +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+                +---+  +===+
                     | 9 |    X      X    | 12|  |R_3|
                     +---+                +---+  +===+

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

   Figure 16: Example: Loss Pattern for the Iterative Decoding Algorithm

   The receiver executes the iterative decoding algorithm and recovers
   source packets #1 and #11 in the first iteration.  The resulting
   pattern is sketched in Figure 17.

                     +---+         +---+  +---+  +===+
                     | 1 |    X    | 3 |  | 4 |  |R_1|
                     +---+         +---+  +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+         +---+  +---+  +===+
                     | 9 |    X    | 11|  | 12|  |R_3|
                     +---+         +---+  +---+  +===+

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

        Figure 17: The Resulting Pattern after the First Iteration

   Since the if condition holds true, the receiver runs a new iteration.
   In the second iteration, source packets #2 and #10 are recovered,
   resulting in a full recovery as sketched in Figure 18.








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                     +---+  +---+  +---+  +---+  +===+
                     | 1 |  | 2 |  | 3 |  | 4 |  |R_1|
                     +---+  +---+  +---+  +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 5 |  | 6 |  | 7 |  | 8 |  |R_2|
                     +---+  +---+  +---+  +---+  +===+

                     +---+  +---+  +---+  +---+  +===+
                     | 9 |  | 10|  | 11|  | 12|  |R_3|
                     +---+  +---+  +---+  +---+  +===+

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

        Figure 18: The Resulting Pattern after the Second Iteration

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Signaling Requirements</span>

   Out-of-band signaling should be designed to enable the receiver to
   identify the RTP streams associated with source packets and repair
   packets, respectively.  At a minimum, the signaling must be designed
   to allow the receiver to:

   o  Determine whether one or more source RTP streams will be sent.

   o  Determine whether one or more repair RTP streams will be sent.

   o  Associate the appropriate SSRC's to both source and repair
      streams.

   o  Clearly identify which SSRC's are associated with each source
      block.

   o  Clearly identify which repair packets correspond to which source
      blocks.

   o  Make use of repair packets to recover source data associated with
      specific SSRC's.

   This section provides several Session Description Protocol (SDP)
   examples to demonstrate how these requirements can be met.








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<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  SDP Examples</span>

   This section provides two SDP [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] examples.  The examples use
   the FEC grouping semantics defined in [<a href="./rfc5956" title="&quot;Forward Error Correction Grouping Semantics in the Session Description Protocol&quot;">RFC5956</a>].

<span class="h4"><a class="selflink" id="section-7.1.1" href="#section-7.1.1">7.1.1</a>.  Example SDP for Flexible FEC Protection with In-Band SSRC</span>
<span class="h4">        Mapping</span>

   In this example, we have one source video stream and one FEC repair
   stream.  The source and repair streams are multiplexed on different
   SSRCs.  The repair window is set to 200 ms.

        v=0
        o=mo 1122334455 1122334466 IN IP4 fec.example.com
        s=FlexFEC minimal SDP signaling Example
        t=0 0
        m=video 30000 RTP/AVP 96 98
        c=IN IP4 233.252.0.1/127
        a=rtpmap:96 VP8/90000
        a=rtpmap:98 flexfec/90000
        a=fmtp:98; repair-window=200000


<span class="h4"><a class="selflink" id="section-7.1.2" href="#section-7.1.2">7.1.2</a>.  Example SDP for Flexible FEC Protection with Explicit Signaling</span>
<span class="h4">        in the SDP</span>

   This example shows one source video stream (ssrc:1234) and one FEC
   repair streams (ssrc:2345).  One FEC group is formed with the
   "a=ssrc-group:FEC-FR 1234 2345" line.  The source and repair streams
   are multiplexed on different SSRCs.  The repair window is set to 200
   ms.

        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=2-D Parity FEC with no in band signaling Example
        t=0 0
        m=video 30000 RTP/AVP 100 110
        c=IN IP4 192.0.2.0/24
        a=rtpmap:100 MP2T/90000
        a=rtpmap:110 flexfec/90000
        a=fmtp:110; repair-window:200000
        a=ssrc:1234
        a=ssrc:2345
        a=ssrc-group:FEC-FR 1234 2345







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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  On the Use of the RTP Stream Identifier Source Description</span>

   The RTP Stream Identifier Source Description [<a href="#ref-RTP-SDES">RTP-SDES</a>] is a format
   that can be used to identify a single RTP source stream along with an
   associated repair stream.  However, this specification already
   defines a method of source and repair stream identification that can
   enable protection of multiple source streams with a single repair
   stream.  Therefore, the RTP Stream Identifier Source Description
   SHOULD NOT be used for the Flexible FEC payload format.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Congestion Control Considerations</span>

   FEC is an effective approach to provide applications 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
   streams into the network.  In particular, in bandwidth-limited
   networks, FEC repair streams may consume a significant part of the
   available bandwidth and, consequently, may congest the network.  In
   such cases, the applications MUST NOT arbitrarily increase the amount
   of FEC 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 avoid
   sending/receiving FEC repair streams if it knows that sending/
   receiving those FEC repair streams would not help at all in
   recovering the missing packets.  Examples of where FEC would not be
   beneficial are (1) if the successful recovery rate as determined by
   RTCP feedback is low (see [<a href="./rfc5725" title="&quot;Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)&quot;">RFC5725</a>] and [<a href="./rfc7509" title="&quot;RTP Control Protocol (RTCP) Extended Report (XR) for Post-Repair Loss Count Metrics&quot;">RFC7509</a>] and (2) the
   application has a smaller latency requirement than the repair window
   adopted by the FEC configuration based on the expected burst loss
   duration and the target FEC overhead.  It is RECOMMENDED that the
   amount and type (row, column, or both) of FEC protection is adjusted
   dynamically based on the packet loss rate and burst loss length
   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 offer multiple repair streams with
   different levels of FEC protection and the receivers join the
   corresponding multicast sessions to receive the repair stream(s) that
   is best for them.







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<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Security Considerations</span>

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>] 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 can be provided by encrypting
   the RTP payload.  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 confidentiality, integrity protection, and at
   least 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 single mechanism is not sufficient; although, if suitable, using
   the Secure Real-time Transport Protocol (SRTP) [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>] is
   recommended.  Other mechanisms that may be used are IPsec [<a href="./rfc4301" title="&quot;Security Architecture for the Internet Protocol&quot;">RFC4301</a>],
   and Datagram Transport Layer Security (DTLS, see [<a href="./rfc6347" title="&quot;Datagram Transport Layer Security Version 1.2&quot;">RFC6347</a>]) when used
   along with RTP-over-UDP; other alternatives may exist.

   Given that FLEX FEC enables the protection of multiple source
   streams, there exists the possibility that multiple source buffers
   may be created that may not be used.  An attacker could leverage
   unused source buffers as a means of occupying memory in a FLEX FEC
   endpoint.  In addition, an attack against the FEC parameters
   themselves (e.g., repair window or D or L values) can result in a
   receiver having to allocate source buffer space that may also lead to
   excessive consumption of resources.  Similarly, a network attacker
   could modify the recovery fields corresponding to packet lengths
   (assuming there are no message integrity mechanisms), which, in turn,
   could force unnecessarily large memory allocations at the receiver.
   Moreover, the application source data may not be perfectly matched
   with FLEX FEC Source partitioning.  If this is the case, there is a
   possibility for unprotected source data if, for instance, the FLEX
   FEC implementation discards data that does not fit perfectly into its
   source processing requirements.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  IANA Considerations</span>

   New media subtypes are subject to IANA registration.  For the
   registration of the payload formats and their parameters introduced
   in this document, refer to <a href="#section-5.1">Section 5.1</a>.




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<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3264">RFC3264</a>]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", <a href="./rfc3264">RFC 3264</a>,
              DOI 10.17487/RFC3264, June 2002,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3264">https://www.rfc-editor.org/info/rfc3264</a>&gt;.

   [<a id="ref-RFC3550">RFC3550</a>]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, <a href="./rfc3550">RFC 3550</a>, DOI 10.17487/RFC3550,
              July 2003, &lt;<a href="https://www.rfc-editor.org/info/rfc3550">https://www.rfc-editor.org/info/rfc3550</a>&gt;.

   [<a id="ref-RFC4566">RFC4566</a>]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", <a href="./rfc4566">RFC 4566</a>, DOI 10.17487/RFC4566,
              July 2006, &lt;<a href="https://www.rfc-editor.org/info/rfc4566">https://www.rfc-editor.org/info/rfc4566</a>&gt;.

   [<a id="ref-RFC4855">RFC4855</a>]  Casner, S., "Media Type Registration of RTP Payload
              Formats", <a href="./rfc4855">RFC 4855</a>, DOI 10.17487/RFC4855, February 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4855">https://www.rfc-editor.org/info/rfc4855</a>&gt;.

   [<a id="ref-RFC4856">RFC4856</a>]  Casner, S., "Media Type Registration of Payload Formats in
              the RTP Profile for Audio and Video Conferences",
              <a href="./rfc4856">RFC 4856</a>, DOI 10.17487/RFC4856, February 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4856">https://www.rfc-editor.org/info/rfc4856</a>&gt;.

   [<a id="ref-RFC5956">RFC5956</a>]  Begen, A., "Forward Error Correction Grouping Semantics in
              the Session Description Protocol", <a href="./rfc5956">RFC 5956</a>,
              DOI 10.17487/RFC5956, September 2010,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5956">https://www.rfc-editor.org/info/rfc5956</a>&gt;.

   [<a id="ref-RFC6363">RFC6363</a>]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", <a href="./rfc6363">RFC 6363</a>,
              DOI 10.17487/RFC6363, October 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6363">https://www.rfc-editor.org/info/rfc6363</a>&gt;.

   [<a id="ref-RFC6838">RFC6838</a>]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", <a href="https://www.rfc-editor.org/bcp/bcp13">BCP 13</a>,
              <a href="./rfc6838">RFC 6838</a>, DOI 10.17487/RFC6838, January 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6838">https://www.rfc-editor.org/info/rfc6838</a>&gt;.





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   [<a id="ref-RFC7022">RFC7022</a>]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)
              Canonical Names (CNAMEs)", <a href="./rfc7022">RFC 7022</a>, DOI 10.17487/RFC7022,
              September 2013, &lt;<a href="https://www.rfc-editor.org/info/rfc7022">https://www.rfc-editor.org/info/rfc7022</a>&gt;.

   [<a id="ref-RFC8174">RFC8174</a>]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
              <a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
              May 2017, &lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;.

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-RFC2326">RFC2326</a>]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
              Streaming Protocol (RTSP)", <a href="./rfc2326">RFC 2326</a>,
              DOI 10.17487/RFC2326, April 1998,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2326">https://www.rfc-editor.org/info/rfc2326</a>&gt;.

   [<a id="ref-RFC2733">RFC2733</a>]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
              for Generic Forward Error Correction", <a href="./rfc2733">RFC 2733</a>,
              DOI 10.17487/RFC2733, December 1999,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2733">https://www.rfc-editor.org/info/rfc2733</a>&gt;.

   [<a id="ref-RFC2974">RFC2974</a>]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", <a href="./rfc2974">RFC 2974</a>, DOI 10.17487/RFC2974,
              October 2000, &lt;<a href="https://www.rfc-editor.org/info/rfc2974">https://www.rfc-editor.org/info/rfc2974</a>&gt;.

   [<a id="ref-RFC3711">RFC3711</a>]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              <a href="./rfc3711">RFC 3711</a>, DOI 10.17487/RFC3711, March 2004,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3711">https://www.rfc-editor.org/info/rfc3711</a>&gt;.

   [<a id="ref-RFC4301">RFC4301</a>]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", <a href="./rfc4301">RFC 4301</a>, DOI 10.17487/RFC4301,
              December 2005, &lt;<a href="https://www.rfc-editor.org/info/rfc4301">https://www.rfc-editor.org/info/rfc4301</a>&gt;.

   [<a id="ref-RFC4585">RFC4585</a>]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", <a href="./rfc4585">RFC 4585</a>,
              DOI 10.17487/RFC4585, July 2006,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4585">https://www.rfc-editor.org/info/rfc4585</a>&gt;.

   [<a id="ref-RFC4588">RFC4588</a>]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", <a href="./rfc4588">RFC 4588</a>,
              DOI 10.17487/RFC4588, July 2006,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4588">https://www.rfc-editor.org/info/rfc4588</a>&gt;.

   [<a id="ref-RFC5109">RFC5109</a>]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", <a href="./rfc5109">RFC 5109</a>, DOI 10.17487/RFC5109, December
              2007, &lt;<a href="https://www.rfc-editor.org/info/rfc5109">https://www.rfc-editor.org/info/rfc5109</a>&gt;.



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-40" ></span>
<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


   [<a id="ref-RFC5725">RFC5725</a>]  Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
              Report Block Type for RTP Control Protocol (RTCP) Extended
              Reports (XRs)", <a href="./rfc5725">RFC 5725</a>, DOI 10.17487/RFC5725, February
              2010, &lt;<a href="https://www.rfc-editor.org/info/rfc5725">https://www.rfc-editor.org/info/rfc5725</a>&gt;.

   [<a id="ref-RFC6347">RFC6347</a>]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", <a href="./rfc6347">RFC 6347</a>, DOI 10.17487/RFC6347,
              January 2012, &lt;<a href="https://www.rfc-editor.org/info/rfc6347">https://www.rfc-editor.org/info/rfc6347</a>&gt;.

   [<a id="ref-RFC7509">RFC7509</a>]  Huang, R. and V. Singh, "RTP Control Protocol (RTCP)
              Extended Report (XR) for Post-Repair Loss Count Metrics",
              <a href="./rfc7509">RFC 7509</a>, DOI 10.17487/RFC7509, May 2015,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7509">https://www.rfc-editor.org/info/rfc7509</a>&gt;.

   [<a id="ref-RFC7656">RFC7656</a>]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", <a href="./rfc7656">RFC 7656</a>,
              DOI 10.17487/RFC7656, November 2015,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7656">https://www.rfc-editor.org/info/rfc7656</a>&gt;.

   [<a id="ref-RFC7826">RFC7826</a>]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, Ed., "Real-Time Streaming Protocol
              Version 2.0", <a href="./rfc7826">RFC 7826</a>, DOI 10.17487/RFC7826, December
              2016, &lt;<a href="https://www.rfc-editor.org/info/rfc7826">https://www.rfc-editor.org/info/rfc7826</a>&gt;.

   [<a id="ref-RTP-SDES">RTP-SDES</a>]
              Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
              Identifier Source Description (SDES)", Work in Progress,
              <a href="./draft-ietf-avtext-rid-09">draft-ietf-avtext-rid-09</a>, October 2016.

   [<a id="ref-SMPTE2022-1">SMPTE2022-1</a>]
              SMPTE, "Forward Error Correction for Real-Time Video/Audio
              Transport over IP Networks", ST 2022-1:2007, SMPTE
              Standard, DOI 10.5594/SMPTE.ST2022-1.2007, May 2007.

Acknowledgments

   Some parts of this document are borrowed from [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>].  Thus, the
   author would like to thank the editor of [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>] and those who
   contributed to [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>].

   Thanks to Stephen Botzko, Bernard Aboba, Rasmus Brandt, Brian
   Baldino, Roni Even, Stefan Holmer, Jonathan Lennox, and Magnus
   Westerlund for providing valuable feedback on earlier draft versions
   of this document.






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<span class="grey"><a href="./rfc8627">RFC 8627</a>            RTP Payload Format for Parity FEC          July 2019</span>


Authors' Addresses

   Mo Zanaty
   Cisco
   Raleigh, NC
   United States of America

   Email: mzanaty@cisco.com


   Varun Singh
   CALLSTATS I/O Oy
   Annankatu 31-33 C 42
   Helsinki  00101
   Finland

   Email: varun.singh@iki.fi
   URI:   <a href="http://www.callstats.io/">http://www.callstats.io/</a>


   Ali Begen
   Networked Media
   Konya
   Turkey

   Email: ali.begen@networked.media


   Giridhar Mandyam
   Qualcomm Inc.
   5775 Morehouse Drive
   San Diego, CA  92121
   United States of America

   Phone: +1 858 651 7200
   Email: mandyam@qti.qualcomm.com















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</pre>