    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

         RTP Payload Format for Flexible Forward Error Correction (FEC)

    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 Section 2 of RFC 7841.

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

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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    Copyright Notice

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

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

    Table of Contents

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

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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

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

    1.  Introduction

       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
       [RFC3550].  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 [RFC6363] in their application to the parity codes defined here.

       The Redundancy RTP Stream [RFC7656] repair packets proposed in this
       document protect the Source RTP Stream packets that belong to the
       same RTP session.

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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       The RTP payload formats that are defined in this document address the
       scalability issues experienced with the formats defined in earlier
       specifications including [RFC2733], [RFC5109], and [SMPTE2022-1].

    1.1.  Parity Codes

       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 [RFC4585].

       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.

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

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

             Figure 1: Block Diagram for Systematic Parity FEC Encoder

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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

                                  +------------+
       +--+    X    X    +--+ --> | Systematic | --> +--+  +--+  +--+  +--+
       +--+              +--+     | Parity FEC |     +--+  +--+  +--+  +--+
                                  |  Decoder   |
                   +==+  +==+ --> | (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.

    1.1.1.  One-Dimensional (1-D) Non-interleaved (Row) FEC Protection

       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
       Section 3.2) 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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       +--------------------------------------------------+    ---    +===+
       | 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

    1.1.2.  1-D Interleaved (Column) FEC Protection

       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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    1.1.3.  Use Cases for 1-D FEC Protection

       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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

                            +---+         +---+  +---+
                            | 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)

    1.1.4.  Two-Dimensional (2-D) (Row and Column) FEC Protection

       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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

                         +---+                +---+  +===+
                         | 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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    1.1.5.  FEC Protection with Flexible Mask

       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).

    1.1.6.  FEC Overhead Considerations

       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.

    1.1.7.  FEC Protection with Retransmission

       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 [RFC4588]) 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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    1.1.8.  Repair Window Considerations

       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
       Section 4.2.2), 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.

    2.  Requirements Notation

       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
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    3.  Definitions and Notations

    3.1.  Definitions

       This document uses a number of definitions from [RFC6363].
       Additionally, it defines the following and/or updates their
       definitions from [RFC6363].

       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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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.

    3.2.  Notations

       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.

    4.  Packet Formats

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

    4.1.  Source Packets

       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
       [RFC3550], 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.

    Zanaty, et al.               Standards Track                   [Page 12]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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.

    4.2.  FEC Repair Packets

       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.

    4.2.1.  RTP Header of FEC Repair Packets

       The RTP header is formatted according to [RFC3550] 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.

    Zanaty, et al.               Standards Track                   [Page 13]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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 Section 5 for details).  According to [RFC3550], 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 [RFC3550].  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
          Section 8 of [RFC3550].  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

    Zanaty, et al.               Standards Track                   [Page 14]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

          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 [RFC7022].
          This usage is compliant with [RFC3550].

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

    4.2.2.  FEC Header of FEC Repair Packets

       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

    Zanaty, et al.               Standards Track                   [Page 15]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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 Section 6.

          +---+---+-----------------------------------------------------+
          | 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
          Section 6.3.2).

    Zanaty, et al.               Standards Track                   [Page 16]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    4.2.2.1.  FEC Header with Flexible Mask

       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 > 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.

    Zanaty, et al.               Standards Track                   [Page 17]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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.

    4.2.2.2.  FEC Header with Fixed L Columns and D Rows

       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

    Zanaty, et al.               Standards Track                   [Page 18]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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>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>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>0, D>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.

    4.2.2.3.  FEC Header for Retransmissions

       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

    Zanaty, et al.               Standards Track                   [Page 19]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       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

    5.  Payload Format Parameters

       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.

    5.1.  Media Type Registration -- Parity Codes

       This registration is done using the template defined in [RFC6838] and
       following the guidance provided in [RFC4855] along with [RFC4856].

    Zanaty, et al.               Standards Track                   [Page 20]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    5.1.1.  Registration of audio/flexfec

       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 Section 4.8
       in the template document [RFC6838]) and contains binary data.

       Security considerations: See Section 9 of [RFC8627].

       Interoperability considerations: None.

       Published specification: [RFC8627].

       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 & email address to contact for further information:
       IESG <iesg@ietf.org> 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 [RFC3550].

       Author: Varun Singh <varun@callstats.io>.

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

    Zanaty, et al.               Standards Track                   [Page 21]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    5.1.2.  Registration of video/flexfec

       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 Section 4.8
       in the template document [RFC6838]) and contains binary data.

       Security considerations: See Section 9 of [RFC8627].

       Interoperability considerations: None.

       Published specification: [RFC8627].

       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 & email address to contact for further information:
       IESG <iesg@ietf.org> 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 [RFC3550].

       Author: Varun Singh <varun@callstats.io>.

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

    Zanaty, et al.               Standards Track                   [Page 22]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    5.1.3.  Registration of text/flexfec

       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 Section 4.8
       in the template document [RFC6838]) and contains binary data.

       Security considerations: See Section 9 of [RFC8627].

       Interoperability considerations: None.

       Published specification: [RFC8627].

       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 & email address to contact for further information:
       IESG <iesg@ietf.org> 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 [RFC3550].

       Author: Varun Singh <varun@callstats.io>.

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

    Zanaty, et al.               Standards Track                   [Page 23]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    5.1.4.  Registration of application/flexfec

       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 Section 4.8
       in the template document [RFC6838]) and contains binary data.

       Security considerations: See Section 9 of [RFC8627].

       Interoperability considerations: None.

       Published specification: [RFC8627].

       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 & email address to contact for further information:
       IESG <iesg@ietf.org> 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 [RFC3550].

       Author: Varun Singh <varun@callstats.io>.

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

    Zanaty, et al.               Standards Track                   [Page 24]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    5.2.  Mapping to SDP Parameters

       Applications that use the RTP transport commonly use the Session
       Description Protocol (SDP) [RFC4566] to describe their RTP sessions.
       The information that is used to specify the media types in an RTP
       session has specific mappings to the fields in an SDP description.
       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 Section 7.1.

    5.2.1.  Offer/Answer Model Considerations

       When offering parity FEC over RTP using SDP in an Offer/Answer model
       [RFC3264], 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 Section 4.2.2).  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.

    Zanaty, et al.               Standards Track                   [Page 25]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

       o  Any unknown option in the offer must be ignored and deleted from
          the answer (see Section 6 of [RFC3264]).  If FEC is not desired by
          the receiver, it can be deleted from the answer.

    5.2.2.  Declarative Considerations

       In declarative usage, like SDP in the Real-time Streaming Protocol
       (RTSP, for RTSP 1.0 see [RFC2326] and for RTSP 2.0 see [RFC7826]) or
       the Session Announcement Protocol (SAP) [RFC2974], the following
       considerations apply:

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

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

    6.  Protection and Recovery Procedures -- Parity Codes

       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).

    6.1.  Overview

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

    6.2.  Repair Packet Construction

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

       The FEC header 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
       Section 6.3.1.

       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.

    6.3.  Source Packet Reconstruction

       This section describes the recovery procedures that are required to
       reconstruct the missing source packets.  The recovery process has two
       steps.  In the first step, the FEC decoder determines which source
       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 Section 6.3.4.

    6.3.1.  Associating the Source and Repair Packets

       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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       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.

    6.3.1.1.  Using Bitmasks

       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.

    6.3.1.2.  Using L and D Offsets

       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>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 <= 1: Source packets for each row: SN, SN+1, ..., SN+(L-1)
        When D >  1: Source packets for each col: SN, SN+L, ..., SN+(D-1)*L

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    6.3.2.  Recovering the RTP Header

       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.

    6.3.3.  Recovering the RTP Payload

       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
           Section 6.3.2.

       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.

    6.3.4.  Iterative Decoding Algorithm for the 2-D Parity FEC Protection

       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 6.3.2 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 6.3.2 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 > 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 Section 1.1.4.  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

    7.  Signaling Requirements

       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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    7.1.  SDP Examples

       This section provides two SDP [RFC4566] examples.  The examples use
       the FEC grouping semantics defined in [RFC5956].

    7.1.1.  Example SDP for Flexible FEC Protection with In-Band SSRC
            Mapping

       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

    7.1.2.  Example SDP for Flexible FEC Protection with Explicit Signaling
            in the SDP

       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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    7.2.  On the Use of the RTP Stream Identifier Source Description

       The RTP Stream Identifier Source Description [RTP-SDES] 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.

    8.  Congestion Control Considerations

       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 [RFC5725] and [RFC7509] 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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    9.  Security Considerations

       RTP packets using the payload format defined in this specification
       are subject to the security considerations discussed in the RTP
       specification [RFC3550] and in any applicable RTP profile.  The main
       security considerations for the RTP packet carrying the RTP payload
       format defined within this memo are confidentiality, integrity, and
       source authenticity.  Confidentiality 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) [RFC3711] is
       recommended.  Other mechanisms that may be used are IPsec [RFC4301],
       and Datagram Transport Layer Security (DTLS, see [RFC6347]) 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.

    10.  IANA Considerations

       New media subtypes are subject to IANA registration.  For the
       registration of the payload formats and their parameters introduced
       in this document, refer to Section 5.1.

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    11.  References

    11.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

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

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

       [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
                  Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
                  July 2006, <https://www.rfc-editor.org/info/rfc4566>.

       [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
                  Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
                  <https://www.rfc-editor.org/info/rfc4855>.

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

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

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

       [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
                  Specifications and Registration Procedures", BCP 13,
                  RFC 6838, DOI 10.17487/RFC6838, January 2013,
                  <https://www.rfc-editor.org/info/rfc6838>.

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

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    11.2.  Informative References

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

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

       [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
                  Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
                  October 2000, <https://www.rfc-editor.org/info/rfc2974>.

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

       [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
                  December 2005, <https://www.rfc-editor.org/info/rfc4301>.

       [RFC4585]  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)", RFC 4585,
                  DOI 10.17487/RFC4585, July 2006,
                  <https://www.rfc-editor.org/info/rfc4585>.

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

       [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
                  Correction", RFC 5109, DOI 10.17487/RFC5109, December
                  2007, <https://www.rfc-editor.org/info/rfc5109>.

    Zanaty, et al.               Standards Track                   [Page 39]

    RFC 8627            RTP Payload Format for Parity FEC          July 2019

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

       [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                  Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
                  January 2012, <https://www.rfc-editor.org/info/rfc6347>.

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

       [RFC7656]  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", RFC 7656,
                  DOI 10.17487/RFC7656, November 2015,
                  <https://www.rfc-editor.org/info/rfc7656>.

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

       [RTP-SDES]
                  Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
                  Identifier Source Description (SDES)", Work in Progress,
                  draft-ietf-avtext-rid-09, October 2016.

       [SMPTE2022-1]
                  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 [RFC5109].  Thus, the
       author would like to thank the editor of [RFC5109] and those who
       contributed to [RFC5109].

       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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    RFC 8627            RTP Payload Format for Parity FEC          July 2019

    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:   http://www.callstats.io/

       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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