GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc5109

Network Working Group A. Li, Ed. Request for Comments: 5109 December 2007 Obsoletes: 2733, 3009 Category: Standards Track

      RTP Payload Format for Generic Forward Error Correction

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document specifies a payload format for generic Forward Error
 Correction (FEC) for media data encapsulated in RTP.  It is based on
 the exclusive-or (parity) operation.  The payload format described in
 this document allows end systems to apply protection using various
 protection lengths and levels, in addition to using various
 protection group sizes to adapt to different media and channel
 characteristics.  It enables complete recovery of the protected
 packets or partial recovery of the critical parts of the payload
 depending on the packet loss situation.  This scheme is completely
 compatible with non-FEC-capable hosts, so the receivers in a
 multicast group that do not implement FEC can still work by simply
 ignoring the protection data.  This specification obsoletes RFC 2733
 and RFC 3009.  The FEC specified in this document is not backward
 compatible with RFC 2733 and RFC 3009.

Li Standards Track [Page 1] RFC 5109 RTP Payload Format for Generic FEC December 2007

Table of Contents

 1. Introduction ....................................................2
 2. Terminology .....................................................5
 3. Basic Operation .................................................6
 4. Parity Codes ....................................................7
 5. Uneven Level Protection (ULP) ...................................7
 6. RTP Media Packet Structure ......................................9
 7. FEC Packet Structure ............................................9
    7.1. Packet Structure ...........................................9
    7.2. RTP Header for FEC Packets ................................10
    7.3. FEC Header for FEC Packets ................................11
    7.4. FEC Level Header for FEC Packets ..........................12
 8. Protection Operation ...........................................15
    8.1. Generation of the FEC Header ..............................15
    8.2. Generation of the FEC Payload .............................16
 9. Recovery Procedures ............................................16
    9.1. Reconstruction of the RTP Header ..........................16
    9.2. Reconstruction of the RTP Payload .........................18
 10. Examples ......................................................19
    10.1. An Example Offers Similar Protection as RFC 2733 .........19
    10.2. An Example with Two Protection Levels ....................21
    10.3. An Example with FEC as Redundant Coding ..................26
 11. Security Considerations .......................................29
 12. Congestion Considerations .....................................30
 13. IANA Considerations ...........................................31
    13.1. Registration of audio/ulpfec .............................31
    13.2. Registration of video/ulpfec .............................32
    13.3. Registration of text/ulpfec ..............................34
    13.4. Registration of application/ulpfec .......................35
 14. Multiplexing of FEC ...........................................36
    14.1. FEC as a Separate Stream .................................36
    14.2. FEC as Redundant Encoding ................................38
    14.3. Offer / Answer Consideration .............................39
 15. Application Statement .........................................40
 16. Acknowledgments ...............................................42
 17. References ....................................................42
    17.1. Normative References .....................................42
    17.2. Informative References ...................................43

1. Introduction

 The nature of real-time applications implies that they usually have
 more stringent delay requirements than normal data transmissions.  As
 a result, retransmission of the lost packets is generally not a valid
 option for such applications.  In these cases, a better method to
 attempt recovery of information from packet loss is through Forward
 Error Correction (FEC).  FEC is one of the main methods used to

Li Standards Track [Page 2] RFC 5109 RTP Payload Format for Generic FEC December 2007

 protect against packet loss over packet-switched networks
 [9, 10].  In particular, the use of traditional error correcting
 codes, such as parity, Reed-Solomon, and Hamming codes, has seen much
 application.  To apply these mechanisms, protocol support is
 required.  RFC 2733 [9] and RFC 3009 [11] defined one of such FEC
 protocols.  However, in these two RFCs a few fields (the P, X, and CC
 fields) in the RTP header are specified in ways that are not
 consistent as they are designed in RTP [1].  This prevents the
 payload-independent validity check of the RTP packets.
 This document extends the FEC defined in RFC 2733 and RFC 3009 to
 include unequal error protection on the payload data.  It specifies a
 general algorithm with the two previous RFCs as its special cases.
 This specification also fixes the above-mentioned inconsistency with
 RFC 2733 and RFC 3009, and will obsolete those two previous RFCs.
 Please note that the payload specified in this document is not
 backward compatible with RFC 2733 and RFC 3009.  Because the payload
 specified in this document is signaled by different MIMEs from those
 of RFC 3009, there is no concern of misidentification of different
 parity FEC versions in capacity exchange.  For parity FECs specified
 here and in RFC 2733 and RFC 3009, the payload data are unaltered and
 additional FEC data are sent along to protect the payload data.
 Hence, the communication of the payload data would flow without
 problem between hosts of different parity FEC versions and hosts that
 did not implement parity FEC.  The receiving hosts with incompatible
 FEC from the sending host would not be able to benefit from the
 additional FEC data, so it is recommended that existing host
 implementing RFC 2733 and RFC 3009 should be updated to follow this
 specification when possible.
 This document defines a payload format for RTP [1] that allows for
 generic forward error correction of real-time media.  In this
 context, generic means that the FEC protocol is (1) independent of
 the nature of the media being protected, be it audio, video, or
 otherwise; (2) flexible enough to support a wide variety of FEC
 configurations; (3) designed for adaptivity so that the FEC technique
 can be modified easily without out-of-band signaling; and (4)
 supportive of a number of different mechanisms for transporting the
 FEC packets.
 Furthermore, in many scenarios the bandwidth of the network
 connections is a very limited resource.  On the other hand, most of
 the traditional FEC schemes are not designed for optimal utilization
 of the limited bandwidth resource.  An often used improvement is
 unequal error protection that provides different levels of protection
 for different parts of the data stream, which vary in importance.
 The unequal error protection schemes can usually make more efficient
 use of bandwidth to provide better overall protection of the data

Li Standards Track [Page 3] RFC 5109 RTP Payload Format for Generic FEC December 2007

 stream against the loss.  Proper protocol support is essential for
 realizing these unequal error protection mechanisms.  The application
 of most of the unequal error protection schemes requires having the
 knowledge of the importance for different parts of the data stream.
 For that reason, most of such schemes are designed for particular
 types of media according to the structure of the media protected, and
 as a result, are not generic.
 The FEC algorithm and protocol are defined in this document for
 generic forward error correction with unequal error protection for
 real-time media.  The particular algorithm defined here is called the
 Uneven Level Protection (ULP).  The payload data are protected by one
 or more protection levels.  Lower protection levels can provide
 greater protection by using smaller group sizes (compared to higher
 protection levels) for generating the FEC packet.  As we will discuss
 below, audio/video applications would generally benefit from unequal
 error protection schemes that give more protection to the beginning
 part of each packet such as ULP.  The data that are closer to the
 beginning of the packet are in general more important and tend to
 carry more information than the data farther behind in the packet.
 It is well known that in many multimedia streams the more important
 parts of the data are always at the beginning of the data packet.
 This is the common practice in codec design since the beginning of
 the packet is closer to the re-synchronization marker at the header
 and thus is more likely to be correctly decoded.  In addition, almost
 all media formats have the frame headers at the beginning of the
 packet, which is the most vital part of the packet.
 For video streams, most modern formats have optional data
 partitioning modes to improve error resilience in which the video
 macroblock header data, motion vector data, and Discrete Cosine
 Transform (DCT) coefficient data are separated into their individual
 partitions.  For example, in ITU-T H.263 version 3, there is the
 optional data partitioned syntax of Annex V.  In MPEG-4 Visual Simple
 Profile, there is the optional data partitioning mode.  When these
 modes are enabled, the video macroblock (MB) header and motion vector
 partitions (which are much more important to the quality of the video
 reconstruction) are transmitted in the partition(s) at the beginning
 of the video packet while residue DCT coefficient partitions (which
 are less important) are transmitted in the partition close to the end
 of the packet.  Because the data is arranged in descending order of
 importance, it would be beneficial to provide more protection to the
 beginning part of the packet in transmission.
 For audio streams, the bitstreams generated by many of the new audio
 codecs also contain data with different classes of importance.  These
 different classes are then transmitted in order of descending

Li Standards Track [Page 4] RFC 5109 RTP Payload Format for Generic FEC December 2007

 importance.  Applying more protection to the beginning of the packet
 would also be beneficial in these cases.  Even for uniform-
 significance audio streams, various time shifting and stretching
 techniques can be applied to the partially recovered audio data
 packets.
 Audio/video applications would generally benefit from the FEC
 algorithms specified in this document.  With ULP, the efficiency of
 the protection of the media payload can potentially be further
 improved.  This document specifies the protocol and algorithm for
 applying the generic FEC to the RTP media payloads.

2. Terminology

 The following terms are used throughout this document:
 Media Payload: The raw, unprotected user data that are transmitted
 from the sender.  The media payload is placed inside of an RTP
 packet.
 Media Header: The RTP header for the packet containing the media
 payload.
 Media Packet: The combination of a media payload and media header is
 called a media packet.
 FEC Packet: The FEC algorithms at the transmitter take the media
 packets as an input.  They output both the media packets that they
 are passed, and newly generated packets called FEC packets, which
 contain redundant media data used for error correction.  The FEC
 packets are formatted according to the rules specified in this
 document.
 FEC Header: The header information contained in an FEC packet.
 FEC Level Header: The header information contained in an FEC packet
 for each level.
 FEC Payload: The payload of an FEC packet.  It may be divided into
 multiple levels.
 Associated: A FEC packet is said to be "associated" with one or more
 media packets (or vice versa) when those media packets are used to
 generate the FEC packet (by use of the exclusive-or operation).  It
 refers to only those packets used to generate the level 0 FEC
 payload, if not explicitly stated otherwise.

Li Standards Track [Page 5] RFC 5109 RTP Payload Format for Generic FEC December 2007

 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 RFC 2119 [2].

3. Basic Operation

 The payload format described here is used when the sender in an RTP
 session would like to protect the media stream it is sending with
 generic parity FEC.  The FEC supported by this format is based on
 simple exclusive-or (XOR) parities operation.  The sender takes the
 packets from the media stream requiring protection and determines the
 protection levels for these packets and the protection length for
 each level.  The data are grouped together as described below in
 Section 7.  The XOR operation is applied across the payload to
 generate the FEC information.  The results following the procedures
 defined here are RTP packets containing FEC information.  These
 packets can be used at the receiver to recover the packets or parts
 of the packets used to generate the FEC information.
 The payload format for FEC contains information that allows the
 sender to tell the receiver exactly which media packets are protected
 by the FEC packet, and the protection levels and lengths for each of
 the levels.  Specifically, each FEC packet contains an offset mask
 m(k) for each protection level k.  If the bit i in the mask m(k) is
 set to 1, then media packet number N + i is protected by this FEC
 packet at level k.  N is called the sequence number base, and is sent
 in the FEC packet as well.  The amount of data that is protected at
 level k is indicated by L(k), which is also sent in the FEC packet.
 The protection length, offset mask, payload type, and sequence number
 base fully identify the parity code applied to generate the FEC
 packet with little overhead.  A set of rules is described in Section
 7.4 that defines how the mask should be set for different protection
 levels, with examples in Section 10.
 This document also describes procedures on transmitting all the
 protection operation parameters in-band.  This allows the sender
 great flexibility; the sender can adapt the protection to current
 network conditions and be certain the receivers can still make use of
 the FEC for recovery.
 At the receiver, both the FEC and original media are received.  If no
 media packets are lost, the FEC packets can be ignored.  In the event
 of a loss, the FEC packets can be combined with other received media
 to recover all or part of the missing media packets.

Li Standards Track [Page 6] RFC 5109 RTP Payload Format for Generic FEC December 2007

4. Parity Codes

 For brevity, we define the function f(x,y,..) to be the XOR (parity)
 operator applied to the data blocks x,y,...  The output of this
 function is another block, called the parity block.  For simplicity,
 we assume here that the parity block is computed as the bitwise XOR
 of the input blocks.  The exact procedure is specified in Section 8.
 Protection of data blocks using parity codes is accomplished by
 generating one or more parity blocks over a group of data blocks.  To
 be most effective, the parity blocks must be generated by linearly
 independent combinations of data blocks.  The particular combination
 is called a parity code.  The payload format uses XOR parity codes.
 For example, consider a parity code that generates a single parity
 block over two data blocks.  If the original media packets are
 a,b,c,d, the packets generated by the sender are:
    a        b        c        d               <-- media stream
               f(a,b)            f(c,d)        <-- FEC stream
 where time increases to the right.  In this example, the error
 correction scheme (we use the terms scheme and code interchangeably)
 introduces a 50% overhead.  But if b is lost, a and f(a,b) can be
 used to recover b.
 It may be useful to point out that there are many other types of
 forward error correction codes that can also be used to protect the
 payload besides the XOR parity code.  One notable example is Reed-
 Solomon code, and there are many others [12].  However, XOR parity
 code is used here because of its effectiveness and simplicity in both
 protocol design and implementation.  This is particularly important
 for implementation in nodes with limited resources.

5. Uneven Level Protection (ULP)

 As we can see from the simple example above, the protection on the
 data depends on the size of the group.  In the above example, the
 group size is 2.  So if any one of the three packets (two payload
 packets and one FEC packet) is lost, the original payload data can
 still be recovered.
 In general, the FEC protection operation is a trade-off between the
 bandwidth and the protection strength.  The more FEC packets that are
 generated as a fraction of the source media packets, the stronger the
 protection against loss but the greater the bandwidth consumed by the
 combined stream.

Li Standards Track [Page 7] RFC 5109 RTP Payload Format for Generic FEC December 2007

 As is the common case in most of the media payload, not all the parts
 of the packets are of the same importance.  Using this property, one
 can potentially achieve more efficient use of the channel bandwidth
 using unequal error protection, i.e., applying different protection
 for different parts of the packet.  More bandwidth is spent on
 protecting the more important parts, while less bandwidth on the less
 important parts.
 The packets are separated into sections of decreasing importance, and
 protection of different strength is applied to each portion - the
 sections are known as "levels".  The protection operation is applied
 independently at each level.  A single FEC packet can carry parity
 data for multiple levels.  This algorithm is called uneven level
 protection, or ULP.
 The protection of ULP is illustrated in Figure 1 below.  In this
 example, two ULP FEC packets are protecting four payload packets.
 ULP FEC packet #1 has only one level, which protects packets A and B.
 Instead of applying parity operation to the entire packets of A and
 B, it only protects a length of data of both packets.  The length,
 which can be chosen and changed dynamically during a session, is
 called the protection length.
 ULP FEC packet #2 has two protection levels.  The level 0 protection
 is the same as for ULP FEC packet #1 except that it is operating on
 packets C and D.  The level 1 protection is using parity operation
 applied on data from packets A, B, C, and D.  Note that level 1
 protection operates on a different set of packets from level 0 and
 has a different protection length from level 0, so are any other
 levels.  Information is all conveyed in-band through the protocols
 specified in this document.
       Packet A          #####################
                                :        :
       Packet B          ############### :
                                :        :
       ULP FEC Packet #1 @@@@@@@@        :
                                :        :
       Packet C          ###########     :
                                :        :
       Packet D          ###################################
                                :        :
       ULP FEC Packet #2 @@@@@@@@@@@@@@@@@
                         :      :        :
                         :<-L0->:<--L1-->:
             Figure 1: Unequal Level Protection

Li Standards Track [Page 8] RFC 5109 RTP Payload Format for Generic FEC December 2007

 As we have discussed in the introduction, media streams usually have
 the more important parts at the beginning of the packet.  It is
 usually useful to have the stronger protection in the levels closer
 to the beginning of the packet, and weaker protection in the levels
 farther back.  ULP algorithm provides such FEC protection.
 ULP FEC not only provides more protection to the beginning of the
 packet (which is more important), it also avoids as much as possible
 the less efficient scenarios that an earlier section of a packet is
 unrecoverable while a later section can be recovered (and often has
 to be discarded).

6. RTP Media Packet Structure

 The formatting of the media packets is unaffected by FEC.  If the FEC
 is sent as a separate stream, the media packets are sent as if there
 was no FEC.
 This approach has the advantage that media packets can be interpreted
 by receivers that do not support FEC.  This compatibility with
 non-FEC capable receivers is particularly useful in the multicast
 scenarios.  The overhead for using the FEC scheme is only present in
 FEC packets, and can be easily monitored and adjusted by tracking the
 amount of FEC in use.

7. FEC Packet Structure

7.1. Packet Structure

 A FEC packet is constructed by placing an FEC header and one or more
 levels of FEC header and payload into the RTP payload, as shown in
 Figure 2:

Li Standards Track [Page 9] RFC 5109 RTP Payload Format for Generic FEC December 2007

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                RTP Header (12 octets or more)                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    FEC Header (10 octets)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      FEC Level 0 Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     FEC Level 0 Payload                       |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      FEC Level 1 Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     FEC Level 1 Payload                       |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Cont.                              |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 2: FEC Packet Structure

7.2. RTP Header for FEC Packets

 The RTP header for FEC packets is only used when the FEC are sent in
 a separate stream from the protected payload stream (as defined in
 Section 14).  Hence, much of the discussion below applies only to
 that scenario.  All the fields in the RTP header of FEC packets are
 used according to RFC 3550 [1], with some of them further clarified
 below.
 Marker: This field is not used for this payload type, and SHALL be
 set to 0.
 Synchronization Source (SSRC): The SSRC value SHALL be the same as
 the SSRC value of the media stream it protects.
 Sequence Number (SN): The sequence number has the standard definition
 - it MUST be one higher than the sequence number in the previously
 transmitted FEC packet.
 Timestamp (TS): The timestamp MUST be set to the value of the media
 RTP clock at the instant the FEC packet is transmitted.  Thus, the TS
 value in FEC packets is always monotonically increasing.
 Payload type: The payload type for the FEC packets is determined
 through dynamic, out-of-band means.  According to RFC 3550 [1], RTP
 participants that cannot recognize a payload type must discard it.
 This provides backward compatibility.  The FEC mechanisms can then be

Li Standards Track [Page 10] RFC 5109 RTP Payload Format for Generic FEC December 2007

 used in a multicast group with mixed FEC-capable and FEC-incapable
 receivers, particularly when the FEC protection is sent as redundant
 encoding (see Section 14).  In such cases, the FEC protection will
 have a payload type that is not recognized by the FEC-incapable
 receivers, and will thus be disregarded.

7.3. FEC Header for FEC Packets

 The FEC header is 10 octets.  The format of the header is shown in
 Figure 3 and consists of extension flag (E bit), long-mask flag (L
 bit), P recovery field, X recovery field, CC recovery field, M
 recovery field, PT recovery field, SN base field, TS recovery field,
 and length recovery field.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E|L|P|X|  CC   |M| PT recovery |            SN base            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          TS recovery                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        length recovery        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 3: FEC Header Format
 The E bit is the extension flag reserved to indicate any future
 extension to this specification.  It SHALL be set to 0, and SHOULD be
 ignored by the receiver.
 The L bit indicates whether the long mask is used.  When the L bit is
 not set, the mask is 16 bits long.  When the L bit is set, the mask
 is then 48 bits long.
 The P recovery field, the X recovery field, the CC recovery field,
 the M recovery field, and the PT recovery field are obtained via the
 protection operation applied to the corresponding P, X, CC, M, and PT
 values from the RTP header of the media packets associated with the
 FEC packet.
 The SN base field MUST be set to the lowest sequence number, taking
 wrap around into account, of those media packets protected by FEC (at
 all levels).  This allows for the FEC operation to extend over any
 string of at most 16 packets when the L field is set to 0, or 48
 packets when the L field is set to 1, and so on.

Li Standards Track [Page 11] RFC 5109 RTP Payload Format for Generic FEC December 2007

 The TS recovery field is computed via the protection operation
 applied to the timestamps of the media packets associated with this
 FEC packet.  This allows the timestamp to be completely recovered.
 The length recovery field is used to determine the length of any
 recovered packets.  It is computed via the protection operation
 applied to the unsigned network-ordered 16-bit representation of the
 sums of the lengths (in bytes) of the media payload, CSRC list,
 extension and padding of each of the media packets associated with
 this FEC packet (in other words, the CSRC list, RTP extension, and
 padding of the media payload packets, if present, are "counted" as
 part of the payload).  This allows the FEC procedure to be applied
 even when the lengths of the protected media packets are not
 identical.  For example, assume that an FEC packet is being generated
 by xor'ing two media packets together.  The length of the payload of
 two media packets is 3 (0b011) and 5 (0b101) bytes, respectively.
 The length recovery field is then encoded as 0b011 xor 0b101 = 0b110.

7.4. FEC Level Header for FEC Packets

 The FEC level header is 4 or 8 octets (depending on the L bit in the
 FEC header).  The formats of the headers are shown in Figure 4.
 The FEC level headers consist of a protection length field and a mask
 field.  The protection length field is 16 bits long.  The mask field
 is 16 bits long (when the L bit is not set) or 48 bits long (when the
 L bit is set).
 The mask field in the FEC level header indicates which packets are
 associated with the FEC packet at the current level.  It is either 16
 or 48 bits depending on the value of the L bit.  If bit i in the mask
 is set to 1, then the media packet with sequence number N + i is
 associated with this FEC packet, where N is the SN Base field in the
 FEC packet header.  The most significant bit of the mask corresponds
 to i=0, and the least significant to i=15 when the L bit is set to 0,
 or i=47 when the L bit is set to 1.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Protection Length       |             mask              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              mask cont. (present only when L = 1)             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 4: ULP Level Header Format
 The setting of the mask field shall follow the following rules:

Li Standards Track [Page 12] RFC 5109 RTP Payload Format for Generic FEC December 2007

 a. A media packet SHALL be protected only once at each protection
    level higher than level 0.  A media packet MAY be protected more
    than once at level 0 by different packets, providing the
    protection lengths of level 0 of these packets are equal.
 b. For a media packet to be protected at level p, it MUST also be
    protected at level p-1 in any FEC packets.  Please note that the
    protection level p for a media packet can be in an FEC packet that
    is different from the one that contains protection level p-1 for
    the same media packet.
 c. If a ULP FEC packet contains protection at level p, it MUST also
    contain protection at level p-1.  Note that the combination of
    payload packets that are protected in level p may be different
    from those of level p-1.
 The rationale for rule (a) is that multiple protection increases the
 complexity of the recovery implementation.  At higher levels, the
 multiple protection offers diminishing benefit, so its application is
 restricted to level 0 for simpler implementation.  The rationale for
 rule (b) is that the protection offset (for each associated packet)
 is not explicitly signaled in the protocol.  With this restriction,
 the offset can be easily deducted from protection lengths of the
 levels.  The rationale of rule (c) is that the level of protection is
 not explicitly indicated.  This rule is set to implicitly specify the
 levels.
 One example of the protection combinations is illustrated in Figure 5
 below.  It is the same example as shown in Figure 1.  This same
 example is also shown in more detail in Section 10.2 to illustrate
 how the fields in the headers are set.

Li Standards Track [Page 13] RFC 5109 RTP Payload Format for Generic FEC December 2007

       Packet A          #####################
                                :        :
       Packet B          ############### :
                                :        :
       ULP FEC Packet #1 @@@@@@@@        :
                                :        :
       Packet C          ###########     :
                                :        :
       Packet D          ###################################
                                :        :
       ULP FEC Packet #2 @@@@@@@@@@@@@@@@@
                         :      :        :
                         :<-L0->:<--L1-->:
       Payload packet #  |  ULP FEC packet that protects at level
                         |          L0             L1
    ---------------------+---------------------------------------
              A          |          #1             #2
              B          |          #1             #2
              C          |          #2             #2
              D          |          #2             #2
         Figure 5: An Example of Protection Combination
 In this example, ULP FEC packet #1 only has protection level 0.  ULP
 FEC packet #2 has protection levels 0 and 1.  Read across the table,
 it is shown that payload packet A is protected by ULP FEC packet #1
 at level 0, by ULP FEC packet #2 at level 1, and so on.  Also, it can
 be easily seen from the table that ULP FEC packet #2 protects at
 level 0 payload packets C and D, at level 1 payload packets A-D, and
 so on.  For additional examples with more details, please refer to
 Section 10, "Examples".
 The payload of the ULP FEC packets of each level is the protection
 operation (XOR) applied to the media payload and padding of the media
 packets associated with the ULP FEC packet at that level.  Details
 are described in Section 8 on the protection operation.
 The size of the ULP FEC packets is determined by the protection
 lengths chosen for the protection operation.  In the above example,
 ULP FEC packet #1 has length L0 (plus the header overhead).  ULP FEC
 packet #2 with two levels has length L0+L1 (plus the header
 overhead).  It is longer than some of the packets it protects
 (packets B and C in this example), and is shorter than some of the
 packets it protects (packets A and D in this example).

Li Standards Track [Page 14] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Note that it's possible for the FEC packet (non-ULP and ULP) to be
 larger than the longest media packets it protects because of the
 overhead from the headers and/or if a large protection length is
 chosen for ULP.  This could cause difficulties if this results in the
 FEC packet exceeding the Maximum Transmission Unit size for the path
 along which it is sent.

8. Protection Operation

 FEC packets are formed from an "FEC bit string" that is generated
 from the data of the protected media RTP packets.  More specifically,
 the FEC bit string is the bitwise exclusive OR of the "protected bit
 strings" of the protected media RTP packets.
 The following procedure MAY be followed for the protection operation.
 Other procedures MAY be used, but the end result MUST be identical to
 the one described here.

8.1. Generation of the FEC Header

 In the case of the FEC header, the protected bit strings (80 bits in
 length) are generated for each media packet to be protected at FEC
 level 0.  It is formed by concatenating the following fields together
 in the order specified:
    o The first 64 bits of the RTP header (64 bits)
    o Unsigned network-ordered 16-bit representation of the media
      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)
 After the FEC bit string is formed by applying parity operation on
 the protected bit strings, the FEC header is generated from the FEC
 bit string as follows:
 The first (most significant) 2 bits in the FEC bit string are
 skipped.  The next bit in the FEC bit string is written into the P
 recovery bit of the FEC header in the FEC packet.  The next bit in
 the FEC bit string is written into the E recovery bit of the FEC
 header.  The next 4 bits of the FEC bit string are written into the
 CC recovery field of the FEC header.  The next bit is written into
 the M recovery bit of the FEC header.  The next 7 bits of the FEC bit
 string are written into the PT recovery field in the FEC header.  The
 next 16 bits are skipped.  The next 32 bits of the FEC bit string are
 written into the TS recovery field in the FEC header.  The next 16
 bits are written into the length recovery field in the packet header.

Li Standards Track [Page 15] RFC 5109 RTP Payload Format for Generic FEC December 2007

8.2. Generation of the FEC Payload

 For generation of the FEC payload, the protected bit strings are
 simply the protected RTP packets.  The FEC bit string is thus the
 bitwise exclusive OR of these protected media RTP packets.  Such FEC
 bit strings need to be generated for each level, as the group of
 protected payload packets may be different for each level.  If the
 lengths of the protected RTP packets are not equal, each shorter
 packet MUST be padded to the length of the longest packet by adding
 octet 0 at the end.
 For protection level n (n = 0, 1, ...), only Ln octets of data are
 set as the FEC level n payload data after the level n ULP header.
 The data is the Ln octets of data starting with the (Sn + 13)th octet
 in the FEC bit string, where:
 Sn = sum(Li : 0 <= i < n).
 Li is the protection length of level i, and S0 is defined to be 0.
 The reason for omitting the first 12 octets is that that information
 is protected by the FEC header already.

9. Recovery Procedures

 The FEC packets allow end systems to recover from the loss of media
 packets.  This section describes the procedure for performing this
 recovery.
 Recovery requires two distinct operations.  The first determines
 which packets (media and FEC) must be combined in order to recover a
 missing packet.  Once this is done, the second step is to actually
 reconstruct the data.  The second step MUST be performed as described
 below.  The first step MAY be based on any algorithm chosen by the
 implementer.  Different algorithms result in a trade-off between
 complexity and the ability to recover missing packets, if possible.
 The lost payload packets may be recovered in full or in parts
 depending on the data-loss situation due to the nature of unequal
 error protection (when it is used).  The partial recovery of the
 packet can be detected by checking the recovery length of the packet
 retrieved from the FEC header against the actual length of the
 recovered payload data.

9.1. Reconstruction of the RTP Header

 Let T be the list of packets (FEC and media) that can be combined to
 recover some media packet xi at level 0.  The procedure is as
 follows:

Li Standards Track [Page 16] RFC 5109 RTP Payload Format for Generic FEC December 2007

    1.  For the media packets in T, compute the first 80 bits of the
        protected bit string following the procedure as described for
        generating the FEC header in the previous section.
    2.  For the FEC packet in T, the FEC bit string is the 80-bit FEC
        header.
    3.  Calculate the recovery bit string as the bitwise exclusive OR
        of the protected bit string generated from all the media
        packets in T and the FEC bit string generated from all the FEC
        packets 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 recovery bit string.
    6.  Set the Padding bit in the new packet to the next bit in the
        recovery bit string.
    7.  Set the Extension bit in the new packet to the next bit in the
        recovery bit string.
    8.  Set the CC field to the next 4 bits in the recovery bit
        string.
    9.  Set the marker bit in the new packet to the next bit in the
        recovery bit string.
    10. Set the payload type in the new packet to the next 7 bits in
        the recovery bit string.
    11. Set the SN field in the new packet to xi.  Skip the next 16
        bits in the recovery bit string.
    12. Set the TS field in the new packet to the next 32 bits in the
        recovery bit string.
    13. Take the next 16 bits of the recovery bit string.  Whatever
        unsigned integer this represents (assuming network-order),
        take that many bytes from the recovery bit string and append
        them to the new packet.  This represents the CSRC list,
        extension, payload, and the padding of the RTP payload.
    14. Set the SSRC of the new packet to the SSRC of the media stream
        it's protecting, i.e., the SSRC of the media stream to which
        the FEC stream is associated.

Li Standards Track [Page 17] RFC 5109 RTP Payload Format for Generic FEC December 2007

 This procedure will recover the header of an RTP packet up to the
 SSRC field.

9.2. Reconstruction of the RTP Payload

 Let T be the list of packets (FEC and media) that can be combined to
 recover some media packet xi at a certain protection level.  The
 procedure is as follows:
    1.  Assume that we are reconstructing the data for level n, the
        first step is to get the protection length of level n (Ln)
        from the ULP header of level n.
    2.  For the FEC packets in T, the FEC bit string of level n is FEC
        level n payload, i.e., the Ln octets of data following the ULP
        header of level n.
    3.  For the media packets in T, the protected bit string of level
        n is Ln octets of data starting with the (Sn + 13)th octet of
        the packet.  Sn is the same as defined in Section 8.2.  Note
        that the protection of level 0 starts from the 13th octet of
        the media packet after the SSRC field.  The information of the
        first 12 octets are protected by the FEC header.
    4.  If any of the protected bit strings of level n generated from
        the media packets are shorter than the protection length of
        the current level, pad them to that length.  The padding of
        octet 0 MUST be added at the end of the bit string.
    5.  Calculate the recovery bit string as the bitwise exclusive OR
        of the protected bit string of level n generated from all the
        media packets in T and the FEC bit string of level n generated
        from all the FEC packets in T.
    6.  The recovery bit string of the current protection level as
        generated above is combined through concatenation with the
        recovery bit string of all the other levels to form the (fully
        or partially) recovered media packet.  Note that the recovery
        bit string of each protection level MUST be placed at the
        correct location in the recovered media packet for that level
        based on protection length settings.
    7.  The total length of the recovered media packet is recovered
        from the recovery operation at protection level 0 of the
        recovered media packet.  This information can be used to check
        if the complete recovery operation (of all levels) has
        recovered the packet to its full length.

Li Standards Track [Page 18] RFC 5109 RTP Payload Format for Generic FEC December 2007

 The data protected at the lower protection level is recoverable in a
 majority of the cases if the higher-level protected data is
 recoverable.  This procedure (together with the procedure for the
 lower protection levels) will usually recover both the header and
 payload of an RTP packet up to the protection length of the current
 level.

10. Examples

 In the first two examples considered below (Sections 10.1 and 10.2),
 we assume that the FEC streams are sent through a separate RTP
 session as described in Section 14.1.  For these examples, we assume
 that four media packets are to be sent, A, B, C, and D, from SSRC 2.
 Their sequence numbers are 8, 9, 10, and 11, respectively, and have
 timestamps of 3, 5, 7, and 9, respectively.  Packets A and C use
 payload type 11, and packets B and D use payload type 18.  Packet A
 has 200 bytes of payload, packet B 140, packet C 100, and packet D
 340.  Packets A and C have their marker bit set.
 The third example (Section 10.3) is to illustrate when the FEC data
 is sent as redundant data with the payload packets.

10.1. An Example Offers Similar Protection as RFC 2733

 We can protect the four payload packets to their full length in one
 single level with one FEC packet.  This offers similar protection as
 RFC 2733.  The scheme is as shown in Figure 6.
                  +-------------------+             :
       Packet A   |                   |             :
                  +-------------+-----+             :
       Packet B   |             |                   :
                  +---------+---+                   :
       Packet C   |         |                       :
                  +---------+-----------------------+
       Packet D   |                                 |
                  +---------------------------------+
                                                    :
                  +---------------------------------+
       Packet FEC |                                 |
                  +---------------------------------+
                  :                                 :
                  :<------------- L0 -------------->:
       Figure 6: FEC Scheme with Single-Level Protection

Li Standards Track [Page 19] RFC 5109 RTP Payload Format for Generic FEC December 2007

 An FEC packet is generated from these four packets.  We assume that
 payload type 127 is used to indicate an FEC packet.  The resulting
 RTP header is shown in Figure 7.
 The FEC header in the FEC packet is shown in Figure 8.
 The FEC level header for level 0 is shown in Figure 9.
  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|0|0|0 0 0 0|0|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Version:   2
    Padding:   0
    Extension: 0
    Marker:    0
    PT:        127
    SN:        1
    TS:        9
    SSRC:      2
                Figure 7: RTP Header of FEC Packet

Li Standards Track [Page 20] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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|0|0|0 0 0 0|0|0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 1 0 1 1 1 0 1 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    E:         0     [this specification]
    L:         0     [short 16-bit mask]
    P rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    X rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    CC rec.:   0     [0 XOR 0 XOR 0 XOR 0]
    M rec.:    0     [1 XOR 0 XOR 1 XOR 0]
    PT rec.:   0     [11 XOR 18 XOR 11 XOR 18]
    SN base:   8     [min(8,9,10,11)]
    TS rec.:   8     [3 XOR 5 XOR 7 XOR 9]
    len. rec.: 372   [200 XOR 140 XOR 100 XOR 340]
             Figure 8: FEC Header of FEC Packet
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 0|1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L0:        340   [the longest of 200, 140, 100, and 340]
    mask:      61440 [with Bits 1, 2, 3, and 4 marked accordingly for
                      Packets 8, 9, 10, and 11]
    The payload length for level 0 is 340 bytes.
             Figure 9: FEC Level Header (Level 0)

10.2. An Example with Two Protection Levels

 A more complex example is to use FEC at two levels.  The level 0 FEC
 will provide greater protection to the beginning part of the payload
 packets.  The level 1 FEC will apply additional protection to the
 rest of the packets.  This is illustrated in Figure 10.  In this
 example, L0 = 70 and L1 = 90.

Li Standards Track [Page 21] RFC 5109 RTP Payload Format for Generic FEC December 2007

            +------:--------:---+
 Packet A   |      :        :   |
            +------:------+-:---+
 Packet B   |      :      | :
            +------:--+---+ :
                   :        :
            +------+        :
 ULP #1     |      |        :
            +------+        :
                   :        :
            +------:--+     :
 Packet C   |      :  |     :
            +------:--+-----:-----------------+
 Packet D   |      :        :                 |
            +------:--------:-----------------+
                   :        :
            +------:--------+
 ULP #2     |      :        |
            +------:--------+
            :      :        :
            :<-L0->:<--L1-->:
 Figure 10: ULP FEC Scheme with Protection Level 0 and Level 1
 This will result in two FEC packets - #1 and #2.
 The resulting ULP FEC packet #1 will have the RTP header as shown in
 Figure 11.  The FEC header for ULP FEC packet #1 will be as shown in
 Figure 12.  The level 0 ULP header for #1 will be as shown in Figure
 13.

Li Standards Track [Page 22] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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|0|0|0 0 0 0|1|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Version:   2
    Padding:   0
    Extension: 0
    Marker:    1
    PT:        127
    SN:        1
    TS:        5
    SSRC:      2
             Figure 11: RTP Header of FEC Packet #1
  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|0|0|0 0 0 0|0|0 0 1 1 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    E:         0     [this specification]
    L:         0     [short 16-bit mask]
    P rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    X rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    CC rec.:   0     [0 XOR 0 XOR 0 XOR 0]
    M rec.:    0     [1 XOR 0 XOR 1 XOR 0]
    PT rec.:   25    [11 XOR 18]
    SN base:   8     [min(8,9)]
    TS rec.:   6     [3 XOR 5]
    len. rec.: 68    [200 XOR 140]
             Figure 12: FEC Header of ULP FEC Packet #1

Li Standards Track [Page 23] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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 0 0 0 0 0 0 0 1 0 0 0 1 1 0|1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L0:        70
    mask:      49152 [with Bits 1 and 2 marked accordingly for
                      Packets 8 and 9]
    The payload length for level 0 is 70 bytes.
     Figure 13: FEC Level Header (Level 0) for FEC Packet #1
 The resulting FEC packet #2 will have the RTP header as shown in
 Figure 14.  The FEC header for FEC packet #2 will be as shown in
 Figure 15.  The level 0 ULP header for #2 will be as shown in Figure
 16.  The level 1 ULP header for #2 will be as shown in Figure 17.
  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|0|0|0 0 0 0|1|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Version:   2
    Padding:   0
    Extension: 0
    Marker:    1
    PT:        127
    SN:        2
    TS:        9
    SSRC:      2
              Figure 14: RTP Header of FEC Packet #2

Li Standards Track [Page 24] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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|0|0|0 0 0 0|0|0 0 1 1 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 1 0 0 1 1 0 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    E:         0     [this specification]
    L:         0     [short 16-bit mask]
    P rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    X rec.:    0     [0 XOR 0 XOR 0 XOR 0]
    CC rec.:   0     [0 XOR 0 XOR 0 XOR 0]
    M rec.:    0     [1 XOR 0 XOR 1 XOR 0]
    PT rec.:   25    [11 XOR 18]
    SN base:   8     [min(8,9,10,11)]
    TS rec.:   14    [7 XOR 9]
    len. rec.: 304   [100 XOR 340]
             Figure 15: FEC Header of FEC Packet #2
  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 0 0 0 0 0 0 0 1 0 0 0 1 1 0|0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L0:        70
    mask:      12288 [with Bits 3 and 4 marked accordingly for
                      Packets 10 and 11]
    The payload length for level 0 is 70 bytes.
    Figure 16: FEC Level Header (Level 0) for FEC Packet #2

Li Standards Track [Page 25] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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 0 0 0 0 0 0 0 1 0 1 1 0 1 0|1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L1:        90
    mask:      61440 [with Bits 1, 2, 3, and 4 marked accordingly for
                      Packets 8, 9, 10, and 11]
    The payload length for level 1 is 90 bytes.
     Figure 17: FEC Level Header (Level 1) for FEC Packet #2

10.3. An Example with FEC as Redundant Coding

 This example illustrates FEC sent as redundant coding in the same
 stream as the payload.  We assume that five media packets are to be
 sent, A, B, C, D, and E, from SSRC 2.  Their sequence numbers are 8,
 9, 10, 11, and 12, respectively, and have timestamps of 3, 5, 7, 9,
 and 11, respectively.  All the media data is coded with primary
 coding (and FEC as redundant coding only protects the primary coding)
 and uses payload type 11.  Packet A has 200 bytes of payload, packet
 B 140, packet C 100, packet D 340, and packet E 160.  Packets A and C
 have their marker bit set.
 The FEC scheme we use will be with one level as illustrated by Figure
 6 in Section 10.1.  The protection length L0 = 340 octets.
 A redundant coding packetization is used with payload type 100.  The
 payload type of the FEC is assumed to be 127.  The first four RED
 packets, RED #1 through RED #4, each contains an individual media
 packet, A, B, C, or D, respectively.  The FEC data protecting the
 media data in the first four media packets is generated.  The fifth
 packet, RED #5, contains this FEC data as redundant coding along with
 media packet E.
 RED Packet #1:    Media Packet A
 RED Packet #2:    Media Packet B
 RED Packet #3:    Media Packet C
 RED Packet #4:    Media Packet D
 RED Packet #5:    FEC Packet, Media Packet E
 RED packets #1 through #4 will have the structure as shown in Figure
 18.  The RTP header of the RED packet #1 is as shown in Figure 19,
 with all the other RED packets in similar format with corresponding
 sequence numbers and timestamps.  The primary encoding block header
 of the RED packets is as shown in Figure 20.

Li Standards Track [Page 26] RFC 5109 RTP Payload Format for Generic FEC December 2007

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 RTP Header (RED) - 6 octets                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Primary Encoding Block Header (RED) - 1 octet          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Media Packet Data                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure 18: RED Packet Structure - Media Data Only
  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|0|0|0 0 0 0|0|1 1 0 0 1 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Version:   2
    Padding:   0
    Extension: 0
    Marker:    0     [Even though media packet A has marker set]
    PT:        100   [Payload type for RED]
    SN:        1
    TS:        5
    SSRC:      2
             Figure 19: RTP Header of RED Packet #1
  0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |0|0 0 0 1 0 1 1|
 +-+-+-+-+-+-+-+-+
    F bit:     0     [This is the primary coding data]
    Block PT:  11    [The payload type of media]
      Figure 20: Primary Encoding Block Header
 The FEC data is generated not directly from the RED packets, but from
 the virtual RTP packets containing the media packet data.  Those
 virtual RTP packets can be very easily generated from the RED packets
 both with and without redundant coding included.  The conversion from
 RED packets to virtual RTP packets is simply done by (1) removing any
 RED block headers and redundant coding data, and (2) replacing the PT
 in the RTP header with the PT of the primary coding.

Li Standards Track [Page 27] RFC 5109 RTP Payload Format for Generic FEC December 2007

    Note: In the payload format for redundant coding as specified by
    RFC 2198, the marker bit is lost as soon as the primary coding is
    carried in the RED packets.  So the marker bit cannot be recovered
    regardless of whether or not the FEC is used.
 As mentioned above, RED packet #5 will contain the FEC data (that
 protects media packets A, B, C, and D) as well as the data of media
 packet E.  The structure of RED packet #5 is as illustrated in Figure
 21.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 RTP Header (RED) - 6 octets                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Redundant Encoding Block Header (RED) - 4 octets        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        FEC Packet Data                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Primary Encoding Block Header (RED) - 1 octet          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Media Packet Data                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure 21: RED Packet Structure - With FEC Data
 The RTP header of the RED packets with FEC included is the same as
 shown in Figure 19, with their corresponding sequence numbers and
 timestamps.
 In RED packet #5, the redundant encoding block header for the FEC
 packet data block is as shown below in Figure 22.  It will be
 followed by the FEC packet data, which, in this case, includes an FEC
 header (10 octets as shown in Figure 8), ULP level 0 header (4 octets
 as shown in Figure 9), and the ULP level 0 data (340 octets as set
 for level 0).  These are followed by the primary encoding block that
 contains the data of media packet E.

Li Standards Track [Page 28] RFC 5109 RTP Payload Format for Generic FEC December 2007

  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|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 1 0 1 1 0 0 0 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    F bit:     1     [This is the redundant coding data]
    Block PT:  127   [The dynamic payload type for FEC]
    TS Offset: 0     [The instance at which the FEC data is
                      transmitted]
    Block Len: 354   [FEC header (10 octets) plus ULP level 0 header
                      (4 octets) and ULP level 0 data (340 octets)]
        Figure 22: Redundant Encoding Block Header

11. Security Considerations

 There are two ways to use FEC with encryption in secure
 communications: one way is to apply the FEC on already encrypted
 payloads, and the other way is to apply the FEC before the
 encryption.  The first case is encountered when FEC is needed by a
 not trusted node during transmission after the media data is
 encrypted.  The second case is encountered when media data is
 protected by FEC before it is transmitted through a secured
 transport.
 Since the protected payload of this FEC is RTP packets, applying FEC
 on encrypted payloads is primarily applicable in the case of secure
 RTP (SRTP) [13].  Because the FEC applies XOR across the payload, the
 FEC packets should be cryptographically as secure as the original
 payload.  In such cases, additional encryption of the FEC packets is
 not necessary.
 In the following discussion, it is assumed that the FEC is applied to
 the payload before the encryption.  The use of FEC has implications
 on the usage and changing of keys for encryption.  As the FEC packets
 do consist of a separate stream, there are a number of combinations
 on the usage of encryption.  These include:
    o The FEC stream may be encrypted, while the media stream is not.
    o The media stream may be encrypted, while the FEC stream is not.
    o The media stream and FEC stream are both encrypted, but using
      the same key.
    o The media stream and FEC stream are both encrypted, but using
      different keys.

Li Standards Track [Page 29] RFC 5109 RTP Payload Format for Generic FEC December 2007

 The first three of these would require all application-level
 signaling protocols used to be aware of the usage of FEC, and to thus
 exchange keys and negotiate encryption usage on the media and FEC
 streams separately.  In the final case, no such additional mechanisms
 are needed.  The first two cases present a layering violation, as ULP
 FEC packets should be treated no differently than other RTP packets.
 Encrypting just one stream may also make certain known-plaintext
 attacks possible.  For these reasons, applications utilizing
 encryption SHOULD encrypt both streams, i.e., the last two options.
 Furthermore, because the encryption may potentially be weakened by
 the known relationship between the media payload and FEC data for
 certain ciphers, different encryption keys MUST be used for each
 stream when the media payload and the FEC data are sent in separate
 streams.  Note that when SRTP [13] is used for security of the RTP
 sessions, different keys for each RTP session are required by the
 SRTP specification.
 The changing of encryption keys is another crucial issue that needs
 to be addressed.  Consider the case where two packets a and b are
 sent along with the FEC packet that protects them.  The keys used to
 encrypt a and b are different, so which key should be used to decode
 the FEC packet?  In general, old keys need to be cached, so that when
 the keys change for the media stream, the old key can be used until
 it is determined that the key has changed for the ULP FEC packets as
 well.  Furthermore, the new key SHOULD be used to encrypt the FEC
 packets that are generated from a combination of payload packets
 encrypted by the old and new keys.  The sender and the receiver need
 to define how the encryption is performed and how the keys are used.
 Altering the FEC data and packets can have a big impact on the
 reconstruction operation.  An attack by changing some bits in the FEC
 data can have a significant effect on the calculation and the
 recovery of the payload packets.  For example, changing the length
 recovery field can result in the recovery of a packet that is too
 long.  Also, the computational complexity of the recovery can easily
 be affected for up to at least one order of magnitude.  Depending on
 the application scenario, it may be helpful to perform a sanity check
 on the received payload and FEC data before performing the recovery
 operation and to determine the validity of the recovered data from
 the recovery operation before using them.

12. Congestion Considerations

 Another issue with the use of FEC is its impact on network
 congestion.  In many situations, the packet loss in the network is
 induced by congestions.  In such scenarios, adding FEC when
 encountering increasing network losses should be avoided.  If it is

Li Standards Track [Page 30] RFC 5109 RTP Payload Format for Generic FEC December 2007

 used on a widespread basis, this can result in increased congestion
 and eventual congestion collapse.  The applications may include
 stronger protections while at the same time reduce the bandwidth for
 the payload packets.  In any event, implementations MUST NOT
 substantially increase the total amount of bandwidth in use
 (including the payload and the FEC) as network losses increase.
 The general congestion control considerations for transporting RTP
 data apply; see RTP [1] and any applicable RTP profile (e.g., RTP/AVP
 [14]).  An additional requirement if best-effort service is being
 used is that users of this payload format MUST monitor packet loss to
 ensure that the packet loss rate is within acceptable parameters.
 Packet loss is considered acceptable if a TCP flow across the same
 network path, and experiencing the same network conditions, would
 achieve an average throughput, measured on a reasonable timescale,
 that is not less than the RTP flow is achieving.  This condition can
 be satisfied by implementing congestion control mechanisms to adapt
 the transmission rate (or the number of layers subscribed for a
 layered multicast session), or by arranging for a receiver to leave
 the session if the loss rate is unacceptably high.

13. IANA Considerations

 Four new media subtypes have been registered with IANA, as described
 in this section.  This registration is done using the registration
 template [3] and following RFC 3555 [4].

13.1. Registration of audio/ulpfec

 Type name: audio
 Subtype name: ulpfec
 Required parameters:
 rate: The RTP timestamp rate that is used to mark the time of
    transmission of the FEC packet in a separate stream.  In cases in
    which it is sent as redundant data to another stream, the rate
    SHALL be the same as the primary encoding it is used to protect.
    When used in a separate stream, the rate SHALL be larger than 1000
    Hz, to provide sufficient resolution to RTCP operations.  The
    selected rate MAY be any value above 1000 Hz but is RECOMMENDED to
    match the rate of the media this stream protects.

Li Standards Track [Page 31] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Optional parameters:
 onelevelonly: This specifies whether only one level of FEC protection
    is used.  The permissible values are 0 and 1.  If 1 is signaled,
    only one level of FEC protection SHALL be used in the stream.  If
    0 is signaled, more than one level of FEC protection MAY be used.
    If omitted, it has the default value of 0.
 Encoding considerations: This format is framed (see Section 4.8 in
 the template document [3]) and contains binary data.
 Security considerations: The same security considerations apply to
 these media type registrations as to the payloads for them, as
 detailed in RFC 5109.
 Interoperability considerations: none
 Published specification: RFC 5109
 Applications that use this media type: Multimedia applications that
 seek to improve resiliency to loss by sending additional data with
 the media stream.
 Additional information: none
 Person & email address to contact for further information:
    Adam Li adamli@hyervision.com
    IETF Audio/Video Transport Working Group
 Intended usage: COMMON
 Restrictions on usage: This media, type depends on RTP framing, and
 hence is only defined for transfer via RTP [1].  Transport within
 other framing protocols SHALL NOT be defined as this is a robustness
 mechanism for RTP.
 Author:
    Adam Li adamli@hyervision.com
 Change controller:
    IETF Audio/Video Transport Working Group delegated from the IESG.

13.2. Registration of video/ulpfec

 Type name: video
 Subtype name: ulpfec

Li Standards Track [Page 32] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Required parameters:
 rate: The RTP timestamp rate that is used to mark the time of
    transmission of the FEC packet in a separate stream.  In cases in
    which it is sent as redundant data to another stream, the rate
    SHALL be the same as the primary encoding it is used to protect.
    When used in a separate stream, the rate SHALL be larger than 1000
    Hz to provide sufficient resolution to RTCP operations.  The
    selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
    to match the rate of the media this stream protects.
 Optional parameters:
 onelevelonly: This specifies whether only one level of FEC protection
    is used.  The permissible values are 0 and 1.  If 1 is signaled,
    only one level of FEC protection SHALL be used in the stream.  If
    0 is signaled, more than one level of FEC protection MAY be used.
    If omitted, it has the default value of 0.
 Encoding considerations: This format is framed (see Section 4.8 in
 the template document [3]) and contains binary data.
 Security considerations: The same security considerations apply to
 these media type registrations as to the payloads for them, as
 detailed in RFC 5109.
 Interoperability considerations: none
 Published specification: RFC 5109
 Applications that use this media type: Multimedia applications that
 seek to improve resiliency to loss by sending additional data with
 the media stream.
 Additional information: none
 Person & email address to contact for further information:
    Adam Li adamli@hyervision.com
    IETF Audio/Video Transport Working Group
 Intended usage: COMMON
 Restrictions on usage: This media type depends on RTP framing, and
 hence is only defined for transfer via RTP [1].  Transport within
 other framing protocols SHALL NOT be defined as this is a robustness
 mechanism for RTP.

Li Standards Track [Page 33] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Author:
    Adam Li adamli@hyervision.com
 Change controller:  IETF Audio/Video Transport Working Group
    delegated from the IESG.

13.3. Registration of text/ulpfec

 Type name: text
 Subtype name: ulpfec
 Required parameters:
 rate: The RTP timestamp rate that is used to mark the time of
    transmission of the FEC packet in a separate stream.  In cases in
    which it is sent as redundant data to another stream, the rate
    SHALL be the same as the primary encoding it is used to protect.
    When used in a separate stream, the rate SHALL be larger than 1000
    Hz to provide sufficient resolution to RTCP operations.  The
    selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
    to match the rate of the media this stream protects.
 Optional parameters:
 onelevelonly: This specifies whether only one level of FEC protection
    is used.  The permissible values are 0 and 1.  If 1 is signaled,
    only one level of FEC protection SHALL be used in the stream.  If
    0 is signaled, more than one level of FEC protection MAY be used.
    If omitted, it has the default value of 0.
 Encoding considerations: This format is framed (see Section 4.8 in
 the template document [3]) and contains binary data.
 Security considerations: The same security considerations apply to
 these media type registrations as to the payloads for them, as
 detailed in RFC 5109.
 Interoperability considerations: none
 Published specification: RFC 5109
 Applications that use this media type: Multimedia applications that
 seek to improve resiliency to loss by sending additional data with
 the media stream.
 Additional information: none

Li Standards Track [Page 34] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Person & email address to contact for further information:
    Adam Li adamli@hyervision.com
    IETF Audio/Video Transport Working Group
 Intended usage: COMMON
 Restrictions on usage: This media type depends on RTP framing, and
 hence is only defined for transfer via RTP [1].  Transport within
 other framing protocols SHALL NOT be defined as this is a robustness
 mechanism for RTP.
 Author:
    Adam Li adamli@hyervision.com
 Change controller:
    IETF Audio/Video Transport Working Group delegated from the IESG.

13.4. Registration of application/ulpfec

 Type name: application
 Subtype name: ulpfec
 Required parameters:
 rate: The RTP timestamp rate that is used to mark the time of
    transmission of the FEC packet in a separate stream.  In cases in
    which it is sent as redundant data to another stream, the rate
    SHALL be the same as the primary encoding it is used to protect.
    When used in a separate stream, the rate SHALL be larger than 1000
    Hz to provide sufficient resolution to RTCP operations.  The
    selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
    to match the rate of the media this stream protects.
 Optional parameters:
 onelevelonly: This specifies whether only one level of FEC protection
    is used.  The permissible values are 0 and 1.  If 1 is signaled,
    only one level of FEC protection SHALL be used in the stream.  If
    0 is signaled, more than one level of FEC protection MAY be used.
    If omitted, it has the default value of 0.
 Encoding considerations: This format is framed (see Section 4.8 in
 the template document [3]) and contains binary data.
 Security considerations: The same security considerations apply to
 these media type registrations as to the payloads for them, as
 detailed in RFC 5109.

Li Standards Track [Page 35] RFC 5109 RTP Payload Format for Generic FEC December 2007

 Interoperability considerations: none
 Published specification: RFC 5109
 Applications that use this media type: Multimedia applications that
 seek to improve resiliency to loss by sending additional data with
 the media stream.
 Additional information: none
 Person & email address to contact for further information:
    Adam Li adamli@hyervision.com
    IETF Audio/Video Transport Working Group
 Intended usage: COMMON
 Restrictions on usage: This media type depends on RTP framing, and
 hence is only defined for transfer via RTP [1].  Transport within
 other framing protocols SHALL NOT be defined as this is a robustness
 mechanism for RTP.
 Author:
    Adam Li adamli@hyervision.com
 Change controller:
    IETF Audio/Video Transport Working Group delegated from the IESG.

14. Multiplexing of FEC

 The FEC packets can be sent to the receiver along with the protected
 payload primarily in one of two ways: as a separate stream, or in the
 same stream as redundant encoding.  The configuration options MUST be
 indicated out of band.  This section also describes how this can be
 accomplished using the Session Description Protocol (SDP), specified
 in RFC 2327 [8].

14.1. FEC as a Separate Stream

 When the FEC packets are sent in a separate stream, several pieces of
 information must be conveyed:
 o The address and port to which the FEC is being sent
 o The payload type number for the FEC
 o Which media stream the FEC is protecting

Li Standards Track [Page 36] RFC 5109 RTP Payload Format for Generic FEC December 2007

 There is no static payload type assignment for FEC, so dynamic
 payload type numbers MUST be used.  The SSRC of the FEC stream MUST
 be set to that of the protected payload stream.  The association of
 the FEC stream with its corresponding stream is done by line grouping
 in SDP [5] with the FEC semantics [6] or other external means.
 Following the principles as discussed in Section 5.2 of RFC 3550 [1],
 multiplexing of the FEC stream and its associated payload stream is
 usually provided by the destination transport address (network
 address and port number), which is different for each RTP session.
 Sending FEC together with the payload in one single RTP session and
 multiplex only by SSRC or payload type precludes: (1) the use of
 different network paths or network resource allocations for the
 payload and the FEC protection data; (2) reception of a subset of the
 media if desired, particularly for the hosts that do not understand
 FEC; and (3) receiver implementations that use separate processes for
 the different media.  In addition, multiplexing FEC with payload data
 streams will affect the timing and sequence number space of the
 original payload stream, which is usually undesirable.  So the FEC
 stream and the payload stream SHOULD be sent through two separate RTP
 session, and multiplexing them by payload type into one single RTP
 session SHOULD be avoided.  In addition, the FEC and the payload MUST
 NOT be multiplexed by SSRC into one single RTP session since they
 always have the same SSRC.
 Just like any media stream, the port number and the payload type
 number for the FEC stream are conveyed in their m line in the SDP.
 There is no static payload type assignment for FEC, so dynamic
 payload type numbers MUST be used.  The binding to the number is
 indicated by an rtpmap attribute.  The name used in this binding is
 "ulpfec".  The address that the FEC stream is on is conveyed in its
 corresponding c line.
 The association relationship between the FEC stream and the payload
 stream it protects is conveyed through media line grouping in SDP
 (RFC 3388) [5] using FEC semantics (RFC 4756) [6].  The FEC stream
 and the protected payload stream form an FEC group.

Li Standards Track [Page 37] RFC 5109 RTP Payload Format for Generic FEC December 2007

 The following is an example SDP for FEC application in a multicast
 session:
     v=0
     o=adam 289083124 289083124 IN IP4 host.example.com
     s=ULP FEC Seminar
     t=0 0
     c=IN IP4 224.2.17.12/127
     a=group:FEC 1 2
     a=group:FEC 3 4
     m=audio 30000 RTP/AVP 0
     a=mid:1
     m=application 30002 RTP/AVP 100
     a=rtpmap:100 ulpfec/8000
     a=mid:2
     m=video 30004 RTP/AVP 31
     a=mid:3
     m=application 30004 RTP/AVP 101
     c=IN IP4 224.2.17.13/127
     a=rtpmap:101 ulpfec/8000
     a=mid:4
 The presence of two a=group lines in this SDP indicates that there
 are two FEC groups.  The first FEC group, as indicated by the
 "a=group:FEC 1 2" line, consists of stream 1 (an audio stream using
 PCM [14]) and stream 2 (the protecting FEC stream).  The FEC stream
 is sent to the same multicast group and has the same Time to Live
 (TTL) as the audio, but on a port number two higher.  The second FEC
 group, as indicated by the "a=group:FEC 3 4" line, consists of stream
 3 (a video stream) and stream 4 (the protecting FEC stream).  The FEC
 stream is sent to a different multicast address, but has the same
 port number (30004) as the payload video stream.

14.2. FEC as Redundant Encoding

 When the FEC stream is being sent as a secondary codec in the
 redundant encoding format, this must be signaled through SDP.  To do
 this, the procedures defined in RFC 2198 [7] are used to signal the
 use of redundant encoding.  The FEC payload type is indicated in the
 same fashion as any other secondary codec.  The FEC MUST protect only
 the main codec, with the payload of FEC engine coming from virtual
 RTP packets created from the main codec data.  The virtual RTP
 packets can be very easily converted from the RFC 2198 packets by
 simply (1) removing all the additional headers and the redundant
 coding data, and (2) replacing the payload type in the RTP header
 with that of the primary codec.

Li Standards Track [Page 38] RFC 5109 RTP Payload Format for Generic FEC December 2007

    Note: In the payload format for redundant coding as specified by
    RFC 2198, the marker bit is lost as soon as the primary coding is
    carried in the RED packets.  So the marker bit cannot be recovered
    regardless of whether or not the FEC is used.
 Because the FEC data (including the ULP header) is sent in the same
 packets as the protected payload, the FEC data is associated with the
 protected payload by being bundled in the same stream.
 When the FEC stream is sent as a secondary codec in the redundant
 encoding format, this can be signaled through SDP.  To do this, the
 procedures defined in RFC 2198 [7] are used to signal the use of
 redundant encoding.  The FEC payload type is indicated in the same
 fashion as any other secondary codec.  An rtpmap attribute MUST be
 used to indicate a dynamic payload type number for the FEC packets.
 The FEC MUST protect only the main codec.
 For example:
    m=audio 12345 RTP/AVP 121 0 5 100
    a=rtpmap:121 red/8000/1
    a=rtpmap:100 ulpfec/8000
    a=fmtp:121 0/5/100
 This SDP indicates that there is a single audio stream, which can
 consist of PCM (media format 0), DVI (media format 5), the redundant
 encodings (indicated by media format 121, which is bound to red
 through the rtpmap attribute), or FEC (media format 100, which is
 bound to ulpfec through the rtpmap attribute).  Although the FEC
 format is specified as a possible coding for this stream, the FEC
 MUST NOT be sent by itself for this stream.  Its presence in the m
 line is required only because non-primary codecs must be listed here
 according to RFC 2198.  The fmtp attribute indicates that the
 redundant encodings format can be used, with DVI as a secondary
 coding and FEC as a tertiary encoding.

14.3. Offer / Answer Consideration

 Some considerations are needed when SDP is used for offer / answer
 [15] exchange.
 The "onelevelonly" parameter is declarative.  For streams declared as
 sendonly, the value indicates whether only one level of FEC will be
 sent.  For streams declared as recvonly or sendrecv, the value
 indicates what the receiver accepts to receive.

Li Standards Track [Page 39] RFC 5109 RTP Payload Format for Generic FEC December 2007

 When the FEC is sent as a separate stream and signaled through media
 line grouping in SDP (RFC 3388) [5] using FEC semantics (RFC 4756)
 [6], the offering side MUST implement both RFC 3388 and RFC 4756.
 The rules for offer / answer in RFC 3388 and RFC 4756 SHALL be
 followed with the below additional consideration.  For all offers
 with FEC, the answerer MAY refuse the separate FEC session by setting
 the port to 0, and remove the "a=group" attribute that groups that
 FEC session with the RTP session being protected.  If the answerer
 accepts the usage of FEC, the answerer simply accepts the FEC RTP
 session and the grouping in the offer by including the same grouping
 in the answer.  Note that the rejection of the FEC RTP session does
 not prevent the media sessions from being accepted and used without
 FEC.
 When the FEC stream is sent as a secondary codec in the redundant
 encoding format (RFC 2198) [7], the offering side can indicate the
 FEC stream as specified in Section 14.2.  The answerer MAY reject the
 FEC stream by removing the payload type for the FEC stream.  To
 accept the usage of FEC, the answerer must in the answer include the
 FEC payload type.  Note that in cases in which the redundancy payload
 format [7] is used with FEC as the only secondary codec, when the FEC
 stream is rejected the redundant encoding payload type SHOULD also be
 removed.

15. Application Statement

 This document describes a generic protocol for Forward Error
 Correction supporting a wide range of short block parity FEC
 algorithms, such as simple and interleaved parity codes.  The scheme
 is limited to interleaving parity codes over a distance of 48
 packets.  This FEC algorithm is fully compatible with hosts that are
 not FEC-capable.  Since the media payload is not altered and the
 protection is sent as additional information, the receivers that are
 unaware of the generic FEC as specified in this document can simply
 ignore the additional FEC information and process the main media
 payload.  This interoperability is particularly important for
 compatibility with existing hosts, and also in the scenario where
 many different hosts need to communicate with each other at the same
 time, such as during multicast.
 The generic FEC algorithm specified in this document is also a
 generic protection algorithm with the following features: (1) it is
 independent of the nature of the media being protected, whether that
 media is audio, video, or otherwise; (2) it is flexible enough to
 support a wide variety of FEC mechanisms and settings; (3) it is

Li Standards Track [Page 40] RFC 5109 RTP Payload Format for Generic FEC December 2007

 designed for adaptivity, so that the FEC parameters can be modified
 easily without resorting to out-of-band signaling; and (4) it
 supports a number of different mechanisms for transporting the FEC
 packets.
 The FEC specified here also provides the user with Unequal Error
 Protection capabilities.  Some other algorithms may also provide the
 Unequal Error Protection capabilities through other means.  For
 example, an Unequal Erasure Protection (UXP) scheme has been proposed
 in the AVT Working Group in "An RTP Payload Format for
 Erasure-Resilient Transmission of Progressive Multimedia Streams".
 The UXP scheme applies unequal error protection to the media payloads
 by interleaving the payload stream to be protected with the
 additional redundancy information obtained using Reed-Solomon
 operations.
 By altering the structure of the protected media payload, the UXP
 scheme sacrifices the backward compatibility with terminals that do
 not support UXP.  This makes it more difficult to apply UXP when
 backward compatibility is desired.  In the case of ULP, however, the
 media payload remains unaltered and can always be used by the
 terminals.  The extra protection can simply be ignored if the
 receiving terminals do not support ULP.
 At the same time, also because the structure of the media payload is
 altered in UXP, UXP offers the unique ability to change packet size
 independent of the original media payload structure and protection
 applied, and is only subject to the protocol overhead constraint.
 This property is useful in scenarios when altering the packet size of
 the media at transport level is desired.
 Because of the interleaving used in UXP, delays will be introduced at
 both the encoding and decoding sides.  For UXP, all data within a
 transmission block need to arrive before encoding can begin, and a
 reasonable number of packets must be received before a transmission
 block can be decoded.  The ULP scheme introduces little delay at the
 encoding side.  On the decoding side, correctly received packets can
 be delivered immediately.  Delay is only introduced in ULP when
 packet losses occur.
 Because UXP is an interleaved scheme, the unrecoverable errors
 occurring in data protected by UXP usually result in a number of
 corrupted holes in the payload stream.  In ULP, on the other hand,
 the unrecoverable errors due to packet loss in the bitstream usually
 appear as contiguous missing pieces at the end of the packets.
 Depending on the encoding of the media payload stream, many
 applications may find it easier to parse and extract data from a

Li Standards Track [Page 41] RFC 5109 RTP Payload Format for Generic FEC December 2007

 packet with only a contiguous piece missing at the end than a packet
 with multiple corrupted holes, especially when the holes are not
 coincident with the independently decodable fragment boundaries.
 The exclusive-or (XOR) parity check operation used by ULP is simpler
 and faster than the more complex operations required by Reed-Solomon
 codes.  This makes ULP more suitable for applications where
 computational cost is a constraint.
 As discussed above, both the ULP and the UXP schemes apply unequal
 error protection to the RTP media stream, but each uses a different
 technique.  Both schemes have their own unique characteristics, and
 each can be applied to scenarios with different requirements.

16. Acknowledgments

 The following authors have made significant contributions to this
 document: Adam H. Li, Fang Liu, John D. Villasenor, Dong-Seek Park,
 Jeong-Hoon Park, Yung-Lyul Lee, Jonathan D. Rosenberg, and Henning
 Schulzrinne.  The authors would also like to acknowledge the
 suggestions from many people, particularly Stephen Casner, Jay
 Fahlen, Cullen Jennings, Colin Perkins, Tao Tian, Matthieu Tisserand,
 Jeffery Tseng, Mark Watson, Stephen Wenger, and Magnus Westerlund.

17. References

17.1. Normative References

 [1]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [3]  Freed, N. and J. Klensin, "Media Type Specifications and
      Registration Procedures", BCP 13, RFC 4288, December 2005.
 [4]  Casner, S., "Media Type Registration of RTP Payload Formats",
      RFC 4855, February 2007.
 [5]  Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
      "Grouping of Media Lines in the Session Description Protocol
      (SDP)", RFC 3388, December 2002.
 [6]  Li, A., "Forward Error Correction Grouping Semantics in Session
      Description Protocol", RFC 4756, November 2006.

Li Standards Track [Page 42] RFC 5109 RTP Payload Format for Generic FEC December 2007

 [7]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,
      Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload
      for Redundant Audio Data", RFC 2198, September 1997.
 [8]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
      Description Protocol", RFC 4566, July 2006.

17.2. Informative References

 [9]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
      Generic Forward Error Correction", RFC 2733, December 1999.
 [10] Perkins, C. and O. Hodson, "Options for Repair of Streaming
      Media", RFC 2354, June 1998.
 [11] Rosenberg, J. and H. Schulzrinne, "Registration of parityfec
      MIME types", RFC 3009, November 2000.
 [12] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
      J. Crowcroft, "Forward Error Correction (FEC) Building Block",
      RFC 3452, December 2002.
 [13] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
      Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
      3711, March 2004.
 [14] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
      Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [15] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
      Session Description Protocol (SDP)", RFC 3264, June 2002.

Editor's Address

 Adam H. Li
 10194 Wateridge Circle #152
 San Diego, CA 92121
 USA
 Phone: +1 858 622 9038
 EMail: adamli@hyervision.com

Li Standards Track [Page 43] RFC 5109 RTP Payload Format for Generic FEC December 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Li Standards Track [Page 44]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5109.txt · Last modified: 2007/12/13 02:35 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki