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PAYLOAD                                                        M. Zanaty
Internet-Draft                                                     Cisco
Intended status: Standards Track                                V. Singh
Expires: January 17, 2019                                   callstats.io
                                                                A. Begen
                                                         Networked Media
                                                              G. Mandyam
                                              Qualcomm Innovation Center
                                                           July 16, 2018


     RTP Payload Format for Flexible Forward Error Correction (FEC)
               draft-ietf-payload-flexible-fec-scheme-08

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, 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
   which are defined in this specification offer a good protection
   against random and bursty packet losses, respectively, at a cost of
   decent complexity.  The RTP payload formats that are defined in this
   document address the scalability issues experienced with the earlier
   specifications including RFC 2733, RFC 5109 and SMPTE 2022-1, and
   offer several improvements.  Due to these changes, the new payload
   formats are not backward compatible with the earlier specifications,
   but endpoints that do not implement this specification can still work
   by simply ignoring the FEC repair packets.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2019.

Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . .   3
       1.1.1.  1-D Non-interleaved (Row) FEC Protection  . . . . . .   5
       1.1.2.  1-D Interleaved (Column) FEC Protection . . . . . . .   5
       1.1.3.  Use Cases for 1-D FEC Protection  . . . . . . . . . .   6
       1.1.4.  2-D (Row and Column) FEC Protection . . . . . . . . .   8
       1.1.5.  FEC Overhead Considerations . . . . . . . . . . . . .   9
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   9
   3.  Definitions and Notations . . . . . . . . . . . . . . . . . .  10
     3.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Notations . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Source Packets  . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  FEC Repair Packets  . . . . . . . . . . . . . . . . . . .  11
       4.2.1.  RTP Header of FEC Repair Packets  . . . . . . . . . .  12
       4.2.2.  FEC Header of FEC Repair Packets  . . . . . . . . . .  13
   5.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  18
     5.1.  Media Type Registration - Parity Codes  . . . . . . . . .  18
       5.1.1.  Registration of audio/flexfec . . . . . . . . . . . .  18
       5.1.2.  Registration of video/flexfec . . . . . . . . . . . .  20
       5.1.3.  Registration of text/flexfec  . . . . . . . . . . . .  21
       5.1.4.  Registration of application/flexfec . . . . . . . . .  22
     5.2.  Mapping to SDP Parameters . . . . . . . . . . . . . . . .  24
       5.2.1.  Offer-Answer Model Considerations . . . . . . . . . .  24
       5.2.2.  Declarative Considerations  . . . . . . . . . . . . .  25
   6.  Protection and Recovery Procedures - Parity Codes . . . . . .  25



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     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.2.  Repair Packet Construction  . . . . . . . . . . . . . . .  26
     6.3.  Source Packet Reconstruction  . . . . . . . . . . . . . .  27
       6.3.1.  Associating the Source and Repair Packets . . . . . .  28
       6.3.2.  Recovering the RTP Header . . . . . . . . . . . . . .  29
       6.3.3.  Recovering the RTP Payload  . . . . . . . . . . . . .  31
       6.3.4.  Iterative Decoding Algorithm for the 2-D Parity FEC
               Protection  . . . . . . . . . . . . . . . . . . . . .  31
   7.  SDP Examples  . . . . . . . . . . . . . . . . . . . . . . . .  34
     7.1.  Example SDP for Flexible FEC Protection with in-band SSRC
           mapping . . . . . . . . . . . . . . . . . . . . . . . . .  34
     7.2.  Example SDP for Flex FEC Protection with explicit
           signalling in the SDP . . . . . . . . . . . . . . . . . .  34
   8.  Congestion Control Considerations . . . . . . . . . . . . . .  35
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  36
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     12.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

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 SDP).  Furthermore, the
   associations or relationships between the source and repair RTP
   streams may be communicated in-band 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.

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

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:





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   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.
   Figure 1 and Figure 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

                              +------------+
   +--+    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



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   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.  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, 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.


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

   If 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.











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       +-------------+ +-------------+ +-------------+     +-------+
       | 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

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
   non-consecutive 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 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.

















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

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

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


    Figure 5: Example scenario where 1-D non-interleaved FEC protection
                     fails error recovery (Burst Loss)

   The sender may generate interleaved FEC repair packets to combat with
   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 may occur
   back-to-back, in which case interleaved FEC repair packets may still
   fail to recover the lost data.


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

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

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

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

   Figure 6: Example scenario where 1-D interleaved FEC protection fails
                      error recovery (Periodic Loss)








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1.1.4.  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.


                     +---+                +---+  +===+
                     | 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 where 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.












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                     +---+  +---+         +---+
                     | 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 where 2-D parity FEC protection fails
                              error recovery

1.1.5.  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 compared to 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 carried by a source packet, the overhead for
   different FEC protection methods can be computed as follows:

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

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

   o  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.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].






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3.  Definitions and Notations

3.1.  Definitions

   This document uses a number of 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.

      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 which 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 signficant 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.








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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 additional field(s).  The
   primary advantage of not modifying the source packets in any way is
   that it provides backward compatibility for the receivers that do not
   support FEC at all.  In multicast scenarios, this backward
   compatibility becomes quite useful as it allows the non-FEC-capable
   and FEC-capable receivers to receive and interpret the same source
   packets sent in the same multicast session.

   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.

   The source packets are full RTP packets with optional 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 MUST 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.










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

                  Figure 9: Format of FEC repair packets

   Repair "Payload", which follows the FEC Header, includes repair of
   everything following the fixed 12-byte RTP header of the source
   packet, including any CSRC 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.  The reason for this restriction is
      the first 2 bits of the FEC header contain other information (R
      and F bits) rather than recovering the RTP version field.

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

      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 SSRC(s) of the source RTP stream(s) protected by
      this FEC repair packet.

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




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      Payload Type: The (dynamic) payload type for the FEC repair
      packets is determined through out-of-band means.  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, and hence, will discard the repair packet.

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

      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 suggested by [RFC3550].  This
      allows the sender to multiplex the source and repair RTP streams
      on the same port, or multiplex multiple repair streams on a single
      port.  The repair streams SHOULD use the RTCP CNAME field to
      associate themselves with the source stream.

      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 MUST 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 3 variants, depending on the values
   in the first 2 bits (R and F bits) as shown in Figure 10.






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

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


   +---+---+----------------------------------------------------------+
   | R | F | FEC Header variant                                       |
   +---+---+----------------------------------------------------------+
   | 0 | 0 | Flexible FEC Mask fields indicate source packets         |
   | 0 | 1 | Fixed FEC M/N (cols/rows) fields indicate source packets |
   | 1 | 0 | Retransmission of a single source packet                 |
   | 1 | 1 | Invalid, MUST NOT send, MUST ignore if received          |
   +---+---+----------------------------------------------------------+


           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 (M) and
   rows (N) 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 uses R=1 and F=1, which is invalid, and MUST NOT be sent
   by senders, and MUST be ignored by receivers.

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





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   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.

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 options 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



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      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.

   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  Repair "Payload", which follows the FEC Header, includes repair of
      everything following the fixed 12-byte RTP header of the source
      packet, including any CSRC 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.


      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 M/N for CSRC_i in CSRC list ...       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :               Repair "Payload" follows FEC Header             :
     :                                                               :



                       Figure 13: FEC Header for F=1




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


   If L=0, D=0, use the optional payload format parameters for L and D.

   If L>0, D=0, indicates Row FEC, and no column FEC will follow.
                Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L.

   If L>0, D=1, indicates Row FEC, and column FEC will follow.
                Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L.
                And more FEC to come.

   If L>0, D>1, indicates column FEC of every L packet
                in a group of D packets starting at SN base.
                Hence, FEC = SN+(Lx0), SN+(Lx1), ... , SN+(LxD).


             Figure 14: Interpreting the L and D field values

   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 included in the
   FEC header for retransmissions, not in the RTP header CSRC list as in
   the FEC header variants with R=0.

   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 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.







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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 [RFC3555].

   Note to the RFC Editor: In the following sections, please replace
   "XXXX" with the number of this document prior to publication as an
   RFC.

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.






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   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.

   Optional parameters:

   o  L: indicates the number of columns of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  L is a positive integer.

   o  D: indicates the number of rows of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  D is a positive integer.

   o  ToP: indicates the type of protection applied by the sender: 0 for
      1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC
      protection, and 2 for 2-D parity FEC protection.  The ToP value of
      3 is reserved for future uses.

   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 [RFCXXXX].

   Interoperability considerations: None.

   Published specification: [RFCXXXX].

   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: Varun
   Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads
   Working Group.

   Intended usage: COMMON.

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

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





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

   Provisional registration? (standards tree only): Yes.

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.

   Optional parameters:

   o  L: indicates the number of columns of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  L is a positive integer.

   o  D: indicates the number of rows of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  D is a positive integer.

   o  ToP: indicates the type of protection applied by the sender: 0 for
      1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC
      protection, and 2 for 2-D parity FEC protection.  The ToP value of
      3 is reserved for future uses.

   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 [RFCXXXX].

   Interoperability considerations: None.

   Published specification: [RFCXXXX].






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   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: Varun
   Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads
   Working Group.

   Intended usage: COMMON.

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

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

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

   Provisional registration? (standards tree only): Yes.

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.

   Optional parameters:

   o  L: indicates the number of columns of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  L is a positive integer.





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   o  D: indicates the number of rows of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  D is a positive integer.

   o  ToP: indicates the type of protection applied by the sender: 0 for
      1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC
      protection, and 2 for 2-D parity FEC protection.  The ToP value of
      3 is reserved for future uses.

   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 [RFCXXXX].

   Interoperability considerations: None.

   Published specification: [RFCXXXX].

   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: Varun
   Singh <vvarun@callstats.io> and IETF Audio/Video Transport Payloads
   Working Group.

   Intended usage: COMMON.

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

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

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

   Provisional registration? (standards tree only): Yes.

5.1.4.  Registration of application/flexfec

   Type name: application

   Subtype name: flexfec




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   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.

   Optional parameters:

   o  L: indicates the number of columns of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  L is a positive integer.

   o  D: indicates the number of rows of the source block that are
      protected by this FEC block and it applies to all the source
      SSRCs.  D is a positive integer.

   o  ToP: indicates the type of protection applied by the sender: 0 for
      1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC
      protection, and 2 for 2-D parity FEC protection.  The ToP value of
      3 is reserved for future uses.

   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 [RFCXXXX].

   Interoperability considerations: None.

   Published specification: [RFCXXXX].

   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: Varun
   Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads
   Working Group.

   Intended usage: COMMON.



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

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

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

   Provisional registration? (standards tree only): Yes.

5.2.  Mapping to SDP Parameters

   Applications that are using RTP transport commonly use Session
   Description Protocol (SDP) [RFC4566] to describe their RTP sessions.
   The information that is used to specify the media types in an RTP
   session has specific mappings to the fields in an SDP description.
   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 "non-interleaved-
   parityfec" and "interleaved-parityfec" and their 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.

5.2.1.  Offer-Answer Model Considerations

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

   o  Each combination of the L and D parameters produces a different
      FEC data and is not compatible with any other combination.  A
      sender application may desire to offer multiple offers with
      different sets of L and D values as long as the parameter values



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      are valid.  The receiver SHOULD normally choose the offer that has
      a sufficient amount of interleaving.  If multiple such offers
      exist, the receiver may choose the offer that has the lowest
      overhead or the one that requires the smallest amount of
      buffering.  The selection depends on the application requirements.

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

   o  Although combinations with the same L and D values but with
      different repair-window sizes produce the same FEC data, such
      combinations are still considered different offers.  The size of
      the repair-window is related to the maximum delay between the
      transmission of a source packet and the associated repair packet.
      This directly impacts the buffering requirement on the receiver
      side and the receiver must consider this when choosing an offer.

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

5.2.2.  Declarative Considerations

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

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

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







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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).

   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.





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   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.

   o  Depending on the chosen FEC header variant, the mask(s) are set
      when F=0, or the L and D values are set when F=1.

   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 0's 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.



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   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

   The first step is associating the source and repair packets.  This
   association can be 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 signficant 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, staircase.

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 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 there are more than one
   source packets missing per column or row in set T, 1-D FEC protection
   may fail to recover all the missing information.

   When 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)




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   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 p*_snb is the sequence number
   base in the FEC header:


          When D = 0:
            p*_snb, p*_snb+1,..., p*_snb+L
          When D > 0:
            p*_snb, p*_snb+(Lx1), p*_snb+(Lx2),..., p*_snb+(LxD)


6.3.1.3.  Signaled in SDP

   If the endpoint relies entirely on out-of-band signaling (R=0, F=1,
   L=0, D=0 in the FEC header), then this information may be inferred
   from the media type parameters specified in the SDP description.
   Furthermore, the payload type field in the RTP header assists the
   receiver to distinguish an interleaved or non-interleaved FEC packet.

   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:


                        p*_snb + i * X_1 (modulo 65536)

   where p*_snb denotes the value in the SN base field of p*'s FEC
   header, X_1 is set to L and 1 for the interleaved and non-interleaved
   FEC repair packets, respectively, and


                                 0 <= i < X_2

   where X_2 is set to D and L for the interleaved and non-interleaved
   FEC repair packets, respectively.

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



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        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, compute the FEC bit string from 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.  Skip the next 16
        bits in the recovered bit string.

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

   13.  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.

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





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   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.  Append Y bytes to the new packet.

   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 padding of octet 0 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.  Append the recovered bit string (Y octets) to the new packet
       generated in Section 6.3.2.

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.

   The iterative decoding algorithm runs as follows:

   1.  Set num_recovered_until_this_iteration to zero

   2.  Set num_recovered_so_far to zero





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   3.  Recover as many source packets as possible by using the non-
       interleaved FEC repair packets as outlined in Section 6.3.2 and
       Section 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 Section 6.3.2 and
       Section 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.


                                   +---+  +---+  +===+
                       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




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   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.


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






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7.  SDP Examples

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

7.1.  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 signalling Example
        t=0 0
        m=video 30000 RTP/AVP 96 98
        c=IN IP4 143.163.151.157
        a=rtpmap:96 VP8/90000
        a=rtpmap:98 flexfec/90000
        a=fmtp:98; repair-window=200ms


7.2.  Example SDP for Flex FEC Protection with explicit signalling 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 signalling Example
        t=0 0
        m=video 30000 RTP/AVP 100 110
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 MP2T/90000
        a=rtpmap:110 flexfec/90000
        a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
        a=ssrc:1234
        a=ssrc:2345
        a=ssrc-group:FEC-FR 1234 2345






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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 MUST 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 NOT send/
   receive FEC repair streams if it knows that sending/receiving those
   FEC repair streams would not help at all in recovering the missing
   packets.  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 offers 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.

9.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] and in any applicable RTP profile.  The main
   security considerations for the RTP packet carrying the RTP payload
   format defined within this memo are confidentiality, integrity and
   source authenticity.  Confidentiality is achieved by encrypting the
   RTP payload.  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



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

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.

11.  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 versions of
   this draft.

12.  References

12.1.  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>.

   [RFC3555]  Casner, S. and P. Hoschka, "MIME Type Registration of RTP
              Payload Formats", RFC 3555, DOI 10.17487/RFC3555, July
              2003, <https://www.rfc-editor.org/info/rfc3555>.






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   [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>.

   [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>.

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,
              <https://www.rfc-editor.org/info/rfc6709>.

   [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>.

   [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>.

12.2.  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>.







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   [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>.

   [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>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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>.

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

Authors' Addresses

   Mo Zanaty
   Cisco
   Raleigh, NC
   USA

   Email: mzanaty@cisco.com









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   Varun Singh
   CALLSTATS I/O Oy
   Runeberginkatu 4c A 4
   Helsinki  00100
   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 Innovation Center
   5775 Morehouse Drive
   San Diego, CA  92121
   USA

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

























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