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Network Working Group                                         A. Li, Ed.Request for Comments: 5109                                 December 2007Obsoletes:2733,3009Category: Standards TrackRTP Payload Format for Generic Forward Error CorrectionStatus 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 obsoletesRFC 2733   andRFC 3009.  The FEC specified in this document is not backward   compatible withRFC 2733 andRFC 3009.Li                          Standards Track                     [Page 1]

RFC 5109           RTP Payload Format for Generic FEC      December 2007Table of Contents1. Introduction ....................................................22. Terminology .....................................................53. Basic Operation .................................................64. Parity Codes ....................................................75. Uneven Level Protection (ULP) ...................................76. RTP Media Packet Structure ......................................97. FEC Packet Structure ............................................97.1. Packet Structure ...........................................97.2. RTP Header for FEC Packets ................................107.3. FEC Header for FEC Packets ................................117.4. FEC Level Header for FEC Packets ..........................128. Protection Operation ...........................................158.1. Generation of the FEC Header ..............................158.2. Generation of the FEC Payload .............................169. Recovery Procedures ............................................169.1. Reconstruction of the RTP Header ..........................169.2. Reconstruction of the RTP Payload .........................1810. Examples ......................................................1910.1. An Example Offers Similar Protection asRFC 2733 .........1910.2. An Example with Two Protection Levels ....................2110.3. An Example with FEC as Redundant Coding ..................2611. Security Considerations .......................................2912. Congestion Considerations .....................................3013. IANA Considerations ...........................................3113.1. Registration of audio/ulpfec .............................3113.2. Registration of video/ulpfec .............................3213.3. Registration of text/ulpfec ..............................3413.4. Registration of application/ulpfec .......................3514. Multiplexing of FEC ...........................................3614.1. FEC as a Separate Stream .................................3614.2. FEC as Redundant Encoding ................................3814.3. Offer / Answer Consideration .............................3915. Application Statement .........................................4016. Acknowledgments ...............................................4217. References ....................................................4217.1. Normative References .....................................4217.2. Informative References ...................................431.  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 toLi                          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] andRFC 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 inRFC 2733 andRFC 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 withRFC 2733 andRFC 3009, and will obsolete those two previous RFCs.   Please note that the payload specified in this document is not   backward compatible withRFC 2733 andRFC 3009.  Because the payload   specified in this document is signaled by different MIMEs from those   ofRFC 3009, there is no concern of misidentification of different   parity FEC versions in capacity exchange.  For parity FECs specified   here and inRFC 2733 andRFC 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   implementingRFC 2733 andRFC 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 dataLi                          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 descendingLi                          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 inRFC 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 inSection 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 inSection7.4 that defines how the mask should be set for different protection   levels, with examples inSection 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 20074.  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 inSection 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 ProtectionLi                          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 Structure7.1.  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 Structure7.2.  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 inSection 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 toRFC 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 toRFC 3550 [1], RTP   participants that cannot recognize a payload type must discard it.   This provides backward compatibility.  The FEC mechanisms can then beLi                          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 (seeSection 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 inSection 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 toSection 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 inSection 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 20078.2.  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 inSection 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 (Sections10.1 and10.2),   we assume that the FEC streams are sent through a separate RTP   session as described inSection 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 asRFC 2733   We can protect the four payload packets to their full length in one   single level with one FEC packet.  This offers similar protection asRFC 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 ProtectionLi                          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 PacketLi                          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 #1Li                          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 #2Li                          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 #2Li                          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 #210.3.  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 inSection 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 byRFC 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 Header11.  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 isLi                          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 followingRFC 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 (seeSection 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 inRFC 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: ulpfecLi                          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 (seeSection 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 inRFC 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 (seeSection 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 inRFC 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: noneLi                          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 (seeSection 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 inRFC 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   inRFC 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 protectingLi                          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 inSection 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 inRFC 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 theRFC 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 byRFC 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 inRFC 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 toRFC 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 bothRFC 3388 andRFC 4756.   The rules for offer / answer inRFC 3388 andRFC 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 inSection 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 isLi                          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 aLi                          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.  References17.1.  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)",RFC3711, 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.comLi                          Standards Track                    [Page 43]

RFC 5109           RTP Payload Format for Generic FEC      December 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 inBCP 78 andBCP 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 athttp://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]