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RFC 8627

RTP Payload Format for Flexible Forward Error Correction (FEC)

Pages: 41
Proposed Standard
Errata
Part 2 of 2 – Pages 20 to 41
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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].
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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).
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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).
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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).
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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).
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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.
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   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>.
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   [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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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