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rfc:rfc2733

Network Working Group J. Rosenberg Request for Comments: 2733 dynamicsoft Category: Standards Track H. Schulzrinne

                                                   Columbia University
                                                         December 1999
     An RTP Payload Format for Generic Forward Error Correction

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 This document specifies a payload format for generic forward error
 correction of media encapsulated in RTP. It is engineered for FEC
 algorithms based on the exclusive-or (parity) operation. The payload
 format allows end systems to transmit using arbitrary block lengths
 and parity schemes. It also allows for the recovery of both the
 payload and critical RTP header fields. Since FEC is sent as a
 separate stream, it is backwards compatible with non-FEC capable
 hosts, so that receivers which do not wish to implement FEC can just
 ignore the extensions.

Table of Contents

 1     Introduction ...........................................    2
 2     Terminology ............................................    2
 3     Basic Operation ........................................    3
 4     Parity Codes ...........................................    5
 5     RTP Media Packet Structure .............................    6
 6     FEC Packet Structure ...................................    7
 6.1   RTP Header of FEC Packets ..............................    7
 6.2   FEC Header .............................................    7
 7     Protection Operation ...................................    9
 8     Recovery Procedures ....................................   10
 8.1   Reconstruction .........................................   10
 8.2   Determination of When to Recover .......................   12

Rosenberg & Schulzrinne Standards Track [Page 1] RFC 2733 Generic FEC December 1999

 9     Example ................................................   16
 10    Use with Redundant Encodings ...........................   17
 11    Indicating FEC Usage in SDP ............................   20
 11.1  FEC as a Separate Stream ...............................   20
 11.2  Use with Redundant Encodings ...........................   21
 11.3  Usage with RTSP ........................................   22
 12    Security Considerations ................................   23
 13    Acknowledgments ........................................   24
 14    Authors' Addresses .....................................   24
 15    Bibliography ...........................................   25
 16    Full Copyright Statement ...............................   26

1 Introduction

 The quality of packet voice on the Internet has been mediocre due, in
 part, to high packet loss rates. This is especially true on wide-area
 connections. Unfortunately, the strict delay requirements of real-
 time multimedia usually eliminate the possibility of retransmissions.
 It is for this reason that forward error correction (FEC) has been
 proposed to compensate for packet loss in the Internet [1] [2]. In
 particular, the use of traditional error correcting codes, such as
 parity, Reed-Solomon, and Hamming codes, has attracted attention. To
 support these mechanisms, protocol support is required.
 This document defines a payload format for RTP [3] which 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 mechanisms, (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.

2 Terminology

 The following terms are used throughout this document:
     Media Payload: is a piece of raw, un-protected user data which
          is to be transmitted from the sender. The media payload is
          placed inside of an RTP packet.
     Media Header: is 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.

Rosenberg & Schulzrinne Standards Track [Page 2] RFC 2733 Generic FEC December 1999

     FEC Packet: The forward error correction algorithms at the
          transmitter take the media packets as an input. They output
          both the media packets that they are passed, and new
          packets called FEC packets. The FEC packets are formatted
          according to the rules specified in this document.
     FEC Header: The FEC header is the header information contained
          in an FEC packet.
     FEC Payload: The FEC payload is the payload in an FEC packet.
     Associated: An FEC packet is said to be "associated" with one or
          more media packets when those media packets are used to
          generate the FEC packet (by use of the exclusive or
          operation).
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [4].

3 Basic Operation

 The payload format described here is used whenever a participant in
 an RTP session would like to protect a media stream it is sending
 with forward error correction (FEC). The FEC supported by the format
 are those codes based on simple exclusive or (xor) parities. The
 sender takes some set of packets from the media stream, and applies
 an xor operation across the payloads. The sender also applies the xor
 operation over components of the RTP headers. Based on the procedures
 defined here, the result is an RTP packet containing FEC information.
 This packet can be used at the receiver to recover any one of the
 packets used to generate the FEC packet. This document does not
 mandate the particular set of media packets combined to generate an
 FEC packet (such a set [is] referred to as a code). Use of differing
 sets results in a tradeoff between overhead, delay, and
 recoverability.  Section 4 outlines some possible combinations.
 The payload format contains information that allows the sender to
 tell the receiver exactly which media packets have been used to
 generate the FEC. Specifically, each FEC packet contains a bitmask,
 called the offset mask, containing 24 bits. If bit i in the mask is
 set to 1, the media packet with sequence number N + i was used to
 generate this FEC packet. N is called the sequence number base, and
 is sent in the FEC packet as well. The offset mask and payload type
 are sufficient to signal arbitrary parity based forward error
 correction schemes with little overhead.

Rosenberg & Schulzrinne Standards Track [Page 3] RFC 2733 Generic FEC December 1999

 This document also describes procedures that allow the receiver to
 make use of the FEC without having to know the details of specific
 codes. This allows the sender much flexibility; it can adapt the code
 in use based on network conditions, and be certain the receivers can
 still make use of the FEC for recovery.
 As the sender generates FEC packets, they are sent to the receivers.
 The sender still usually sends the original media stream, as if there
 were no FEC. This allows the media stream to still be used by
 receivers who are not FEC capable. However, some FEC codes do not
 require the original media to be sent; the FEC stream is sufficient
 for recovery. These codes have the drawback that all receivers must
 be FEC capable. However, they are supported by this format.
 The FEC packets are not sent in the same RTP stream as the media
 packets. They can be sent as a separate stream, or as a secondary
 codec in the redundant codec payload format [5]. When sent as a
 separate stream, the FEC packets have their own sequence number
 space. Although the timestamps for the FEC packets are derived from
 the media packets, they increment monotonically. FEC packet streams
 thus work well with any header compression mechanism which requires
 fixed deltas between fields in the packet header.
 This document does not prescribe the definition of "separate
 streams", but leaves this to applications and higher level protocols
 to define. For multicast, the separate stream may be implemented by
 separate multicast groups, different ports in the same group, or by a
 different SSRC within the same group/port. For unicast, different
 ports or different SSRC may be used. Each of these approaches has
 drawbacks and benefits which depend on the application.
 At the receiver, the FEC and original media are received. If no media
 packets are lost, the FEC can be ignored. In the event of loss, the
 FEC packets can be combined with other media and FEC packets that
 have been received, resulting in recovery of missing media packets.
 The recovery is exact; the payload is perfectly reconstructed, along
 with most components of the header.
 RTP packets which contain data formatted according to this
 specification (i.e., FEC packets) are signaled using dynamic RTP
 payload types.

Rosenberg & Schulzrinne Standards Track [Page 4] RFC 2733 Generic FEC December 1999

4 Parity Codes

 For brevity, we define the function f(x,y,..) to be the XOR (parity)
 operator applied to the packets x,y,... The output of this function
 is another packet, called the parity packet. For simplicity, we
 assume here that the parity packet is computed as the bitwise XOR of
 the input packets. The exact procedure is specified in section 6.
 Recovery of data packets using parity codes is accomplished by
 generating one or more parity packets over a group of data packets.
 To be effective, the parity packets must be generated by linearly
 independent combinations of data packets. The particular combination
 is called a parity code. One class of codes takes a group of k data
 packets, and generates n-k parity packets. There are a large number
 of possible parity codes for a given n,k. The payload format does not
 mandate a particular code.
 For example, consider a parity code which generates a single parity
 packet over two data packets. 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.
 Some additional codes are listed below. In each, the original media
 stream consists of packets a,b,c,d and so on.
 Scheme 1
 --------
 This scheme is the similar to the one in the example above. However,
 instead of sending b, followed by f(a,b), f(a,b) is sent before b.
 Doing this clearly requires additional delay at the sender. However,
 if allows some bursts of two consecutive packet losses to be
 recovered. The packets generated by the sender look like:
 a        b        c        d        e        <-- media stream
   f(a,b)   f(b,c)   f(c,d)   f(d,e)          <-- FEC stream

Rosenberg & Schulzrinne Standards Track [Page 5] RFC 2733 Generic FEC December 1999

 Scheme 2
 --------
 It is not strictly necessary for the original media stream to be
 transmitted. In this scheme, only FEC packets are transmitted.  This
 scheme allows for recovery of all single packet losses and some
 consecutive packet losses, but with slightly less overhead than
 scheme 1. The packets generated by the sender look like:
 f(a,b)  f(a,c)  f(a,b,c)  f(c,d)  f(c,e)  f(c,d,e)  <-- FEC stream
 Scheme 3
 --------
 This scheme requires the receiver to wait an additional four packet
 intervals to recover the original media packets. However, it can
 recover from one, two or three consecutive packet losses. The packets
 generated by the sender look like:
 a         b          c                    d     <-- media stream
             f(a,b,c)    f(a,c,d) f(a,b,d)       <-- FEC stream

5 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. If the FEC is being sent as a redundant codec, the media
 packets are sent as the main codec as defined in RFC 2198 [5].
 This lends to a very efficient encoding. When little (or no) FEC is
 used, there are mostly media packets being sent. This means that the
 overhead (present in FEC packets only) tracks the amount of FEC in
 use.

Rosenberg & Schulzrinne Standards Track [Page 6] RFC 2733 Generic FEC December 1999

6 FEC Packet Structure

 An FEC packet is constructed by placing an FEC header and FEC payload
 in the RTP payload, as shown in Figure 1:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         RTP Header                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         FEC Header                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         FEC Payload                           |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 1: FEC Packet Structure

6.1 RTP Header of FEC Packets

 The version field is set to 2. The padding bit is computed via the
 protection operation, defined below. The extension bit is also
 computed via the protection operation. The SSRC value will generally
 be the same as the SSRC value of the media stream it protects. It MAY
 be different if the FEC stream is being demultiplexed via the SSRC
 value. The CC value is computed via the protection operation. The
 CSRC list is never present, independent of the value of the CC field.
 The extension is never present, independent of the value of the X
 bit. The marker bit is computed via the protection operation.
 The sequence number has the standard definition: it MUST be one
 higher than the sequence number in the previously transmitted FEC
 packet. The timestamp MUST be set to the value of the media RTP clock
 at the instant the FEC packet is transmitted. This results in the TS
 value in FEC packets to be monotonically increasing, independent of
 the FEC scheme.
 The payload type for the FEC packet is determined through dynamic,
 out of band means. According to RFC 1889 [3], RTP participants which
 cannot recognize a payload type must discard it. This provides
 backwards compatibility. The FEC mechanisms can then be used in a
 multicast group with mixed FEC-capable and FEC-incapable receivers.

6.2 FEC Header

 This header is 12 bytes. The format of the header is shown in Figure
 2, and consists of an SN base field, length recovery field, E field,
 PT recovery field, mask field and TS recovery field.

Rosenberg & Schulzrinne Standards Track [Page 7] RFC 2733 Generic FEC December 1999

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      SN base                  |        length recovery        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| PT recovery |                 mask                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          TS recovery                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 2: Parity Header Format
 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 media packets associated with this FEC
 packet (in other words, the CSRC list, extension, and padding, if
 present, are "counted" as part of the payload). This allows the FEC
 procedure to be applied even when the lengths of the media packets
 are not identical. For example, assume an FEC packet is being
 generated by xor'ing two media packets together. The length of the
 two media packets are 3 (0b011) and 5 (0b101) bytes, respectively.
 The length recovery field is then encoded as 0b011 xor 0b101 = 0b110.
 The E bit indicates a header extension. Implementations conforming to
 this version of the specification MUST set this bit to zero.
 The PT recovery field is obtained via the protection operation
 applied to the payload type values of the media packets associated
 with the FEC packet.
 The mask field is 24 bits. 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
 least significant bit corresponds to i=0, and the most significant to
 i=23.
 The SN base field MUST be set to the minimum sequence number of those
 media packets protected by FEC. This allows for the FEC operation to
 extend over any string of at most 24 packets.
 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 payload of the FEC packet is the protection operation applied to
 the concatenation of the CSRC list, RTP extension, media payload, and
 padding of the media packets associated with the FEC packet.

Rosenberg & Schulzrinne Standards Track [Page 8] RFC 2733 Generic FEC December 1999

 Note that it's possible for the FEC packet to be slightly larger than
 the media packets it protects (due to the presence of the FEC
 header). 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.

7 Protection Operation

 The protection operation involves concatenating specific fields from
 the RTP header of the media packet, appending the payload, padding
 with zeroes, and then computing the xor across the resulting bit
 strings. The resulting bit string is used to generate the FEC packet.
 The following procedure MAY be followed for the protection operation.
 Other procedures MAY be followed, but the end result MUST be
 identical to the one described here. For each media packet to be
 protected, a bit string is generated by concatenating the following
 fields together in the order specifed:
    o Padding Bit (1 bit)
    o Extension Bit (1 bit)
    o CC bits (4 bits)
    o Marker bit (1 bit)
    o Payload Type (7 bits)
    o Timestamp (32 bits)
    o Unsigned network-ordered 16 bit representation of the sum of
      the lengths (in bytes) of the CSRC List, length of the padding,
      length of the extension, and length of the media payload (16
      bits)
    o if CC is nonzero, the CSRC List (variable length)
    o if X is 1, the Header Extension (variable length)
    o the payload (variable length)
    o Padding, if present (variable length)
 Note that the Padding Bit (first entry above) forms the most
 significant bit of the bit string.

Rosenberg & Schulzrinne Standards Track [Page 9] RFC 2733 Generic FEC December 1999

 If the lengths of the bit strings are not equal, each bit string that
 is shorter than the length of the longest, MUST be padded to the
 length of the longest. Any value for the pad may be used. The pad
 MUST be added at the end of the bit string.
 The parity operation is then applied across the bit strings. The
 result is the bit string used to build the FEC packet. Call this the
 FEC bit string.
 The first (most significant) bit in the FEC bit string is written
 into the Padding Bit of the FEC packet. The second bit in the FEC bit
 string is written into the Extension bit of the FEC packet. The next
 four bits of the FEC bit string are written into the CC field of the
 FEC packet. The next bit of the FEC bit string is written into the
 marker bit of the FEC packet. The next 7 bits of the FEC bit string
 are written into the PT recovery field in the FEC packet header. The
 next 32 bits of the FEC bit string are written into the TS recovery
 field in the packet header. The next 16 bits are written into the
 length recovery field in the FEC packet header. The remaining bits
 are set to be the payload of the FEC packet.

8 Recovery Procedures

 The FEC packets allow end systems to recover from the loss of media
 packets. All of the header fields of the missing packets, including
 CSRC lists, extensions, padding bits, marker and payload type, are
 recoverable.  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 tradeoff between
 complexity and the ability to recover missing packets if at all
 possible.

8.1 Reconstruction

 Let T be the list of packets (FEC and media) which can be combined to
 recover some media packet xi. The procedure is as follows:
     1.   For the media packets in T, compute the bit string as
          described in the protection operation of the previous
          section.

Rosenberg & Schulzrinne Standards Track [Page 10] RFC 2733 Generic FEC December 1999

     2.   For the FEC packet in T, compute the bit string in the same
          fashion, except use the PT Recovery instead of Payload Type,
          TS Recovery instead of Timestamp,  and always set the CSRC
          list, extension, and padding to null.
     3.   If any of the bit strings generated from the media packets
          are shorter than the bit string generated from the FEC
          packet, pad them to be the same length as the bit string
          generated from the FEC. The padding MUST be added at the
          end of the bit string, and MAY be of any value.
     4.   Perform the exclusive or (parity) operation across the bit
          strings, resulting in a recovery bit string.
     5.   Create a new packet with the standard 12 byte RTP header
          and no payload.
     6.   Set the version of the new packet to 2.
     7.   Set the Padding bit in the new packet to the first bit in
          the recovery bit string.
     8.   Set the Extension bit in the new packet to the second bit
          in the recovery bit string.
     9.   Set the CC field to the next four bits in the recovery bit
          string.
     10.  Set the marker bit in the new packet to the next bit in the
          recovery bit string.
     11.  Set the payload type in the new packet to the next 7 bits
          in the recovery bit string.
     12.  Set the SN field in the new packet to xi.
     13.  Set the TS field in the new packet to the next 32 bits in
          the recovery bit string.
     14.  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 padding.

Rosenberg & Schulzrinne Standards Track [Page 11] RFC 2733 Generic FEC December 1999

     15.  Set the SSRC of the new packet to the SSRC of the media
          stream it's protecting.
 This procedure will completely recover both the header and payload of
 an RTP packet.

8.2 Determination of When to Recover

 The previous section discussed how to recover a media packet with
 sequence number xi when all of the packets needed to recover it were
 available. The decision about whether to attempt recovery of some
 media packet xi, and how to determine if sufficient data is available
 to recover it, is left to the implementer. However, this section
 provides a simple algorithm which MAY be used for this purpose.
 The algorithm is described below in C code. The code assumes that
 several functions exist. recover_packet() takes the sequence number
 of a packet, and an FEC packet. Using the FEC packet and data packets
 received previously, the data packet with the given sequence number
 is recovered. add_fec_to_pending_list() adds the given FEC packet to
 a linked list of FEC packets which have not yet been used for
 recovery. wait_for_packet() waits for a packet, FEC or data, from the
 network. remove_from_pending_list() removes the FEC packet from the
 pending list. The structure packet contains a boolean variable fec
 which is true when the packet is FEC, false if it's media. When its
 an FEC packet, the mask and snbase field contain those values from
 the FEC packet header. When it's a media packet, the sn variable
 contains the sequence number of the packet. The global array A
 indicates which media packets have been received, and which have not.
 It is indexed by the sequence number of the packet.
 The function fec_recovery implements the algorithm. It waits for
 packets, and when it receives an FEC packet, calls recover_with_fec()
 to attempt to use it to recover. If no recovery is possible, the FEC
 packet is stored for later attempts. If the received packet was a
 media packet, its presence is noted, and any old FEC packets are
 checked to see if recovery is now possible. Recovered packets are
 treated as if they were received, triggering further attempts at
 recovery.
 A real implementation will need to use a circular buffer instead of
 the simple array (A in the code) in order to avoid running off the
 end of the buffer. In addition, the code below does not attempt to
 free up FEC packets that are old and were never used. Normally, such
 discarding is done based on time constraints introduced by the
 playout buffer. If an FEC data protects packets whose play time has
 elapsed, the FEC is no longer needed.

Rosenberg & Schulzrinne Standards Track [Page 12] RFC 2733 Generic FEC December 1999

typedef struct packet_s {

BOOLEAN fec;               /* FEC or media */
int sn;                    /* SN of the packet, for media only */
BOOLEAN mask[24];          /* Mask, FEC only */
int snbase;                /* SN Base, FEC only */
struct packet_s *next;

} packet;

BOOLEAN A[65535]; packet *pending_list;

packet *recover_with_fec(packet *fec_pkt) {

packet *data_pkt;
int pkts_present,  /* number of packets from the mask that are
                      present */
  pkts_needed,    /* number of packets needed is the number of ones
                      in the mask minus 1 */
  pkt_to_recover, /* sn of the packet we are recovering */
  i;
pkts_present = 0;
/* The number of packets needed is the number of ones in the mask
   minus 1.  The code below increments pkts_needed by the number
   of ones in the mask, so we initialize this to -1 so that the
   final count is correct */
pkts_needed = -1;
/* Go through all 24 bits in the mask, and check if we have
   all but one of the media packets */
for(i = 0; i < 24; i++) {
   /* If the packet is here and in the mask, increment counter */
   if(A[i+fec_pkt->snbase] && fec_pkt->mask[i]) pkts_present++;
   /* Count the number of packets needed as well */
   if(fec_pkt->mask[i]) pkts_needed++;

Rosenberg & Schulzrinne Standards Track [Page 13] RFC 2733 Generic FEC December 1999

   /* The packet to recover is the one with a bit in the
      mask that's not here yet */
   if(!A[i+fec_pkt->snbase] && fec_pkt->mask[i])
     pkt_to_recover = i+fec_pkt->snbase;
 }
 /* If we can recover, do so. Otherwise, return NULL */
 if(pkts_present == pkts_needed) {
   data_pkt = recover_packet(pkt_to_recover, fec_pkt);
 }  else {
   data_pkt = NULL;
 }
 return(data_pkt);

}

void fec_recovery() {

 packet *p,    /* packet received or regenerated */
     *fecp,    /* fec packet from pending list */
     *pnew;    /* new packets recovered */
 while(1) {
   p = wait_for_packet();    /* get packet from network */
   while(p) {
     /* if it's an FEC packet, try to recover with it. If we can't,
        store it for later potential use. If we can recover, act as
        if the recovered packet is received and try to recover some
        more.  Otherwise, if it's a data packet, mark it as received,
        and check if we can now recover a data packet with the list
        of pending FEC packets */
     if(p->fec == TRUE) {
        pnew = recover_with_fec(p);
        if(pnew)
          A[pnew->sn] = TRUE;
        else
          add_fec_to_pending_list(p);
        /* We assign pnew to p since the while loop will continue
           to recover based on p not being NULL */

Rosenberg & Schulzrinne Standards Track [Page 14] RFC 2733 Generic FEC December 1999

        p = pnew;
     } else {
       /* Mark this data packet as here */
       A[p->sn] = TRUE;
       free(p);
       p = NULL;
       /* Go through pending list. Try and recover a packet using
          each FEC. If we are successful, add the data packet to
          the list of received packets, remove the FEC packet from
          the pending list, since we've used it, and then try to
          recover some more */
       for(fecp = pending_list; fecp != NULL; fecp = fecp->next) {
         pnew = recover_with_fec(fecp);
         if(pnew) {
           /* The packet is now here, as we've recovered it */
           A[pnew->sn] = TRUE;
           /* One FEC packet can only be used once to recover,
              so remove it from the pending list */
           remove_fec_from_pending_list(fecp);
           p = pnew;
           break;
         }
       } /*for*/
     } /*p->fec was false */
   } /* while p*/
 } /* while 1 */

}

Rosenberg & Schulzrinne Standards Track [Page 15] RFC 2733 Generic FEC December 1999

9 Example

 Consider 2 media packets to be sent, x and y, from SSRC 2. Their
 sequence numbers are 8 and 9, respectively, with timestamps of 3 and
 5, respectively. Packet x uses payload type 11, and packet y uses
 payload type 18. Packet x is has 10 bytes of payload, and packet y
 11. Packet y has its marker bit set. The RTP headers for packets x
 and y are shown in Figures 3 and 4 respectively.

Media Packet x

 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|0 0 0 1 0 1 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 0 1 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
    PTI:       11
    SN:        8
    TS:        3
    SSRC:      2
 Figure 3: RTP Header for Media Packet X
 An FEC packet is generated from these two. We assume that payload
 type 127 is used to indicate an FEC packet. The resulting RTP header
 is shown in Figure 5.
 The FEC header in the FEC packet is shown in Figure 6.

Rosenberg & Schulzrinne Standards Track [Page 16] RFC 2733 Generic FEC December 1999

11 Use with Redundant Encodings

 One can consider an FEC packet as a "redundant coding" of the media.

Media Packet y

 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|0 0 1 0 0 1 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 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
    PTI:       18
    SN:        9
    TS:        5
    SSRC:      2
 Figure 4: RTP Header for Media Packet Y
 Because of this, the payload format for encoding of redundant audio
 data [5] can be used to carry the FEC data along with the media. The
 procedure for this is as follows.
 The FEC operation defined above acts on a stream of RTP media
 packets. The stream which is operated on is the stream before the
 encapsulation defined in RFC 2198 [5]. In other words, the media
 stream to be protected is encapsulated in standard RTP media packets.
 The FEC operation above is performed (with one minor change),
 generating a stream of FEC packets. The change to the procedure above
 is that if the RTP packets being protected contain an RTP extension,
 padding, or a CSRC list, these MUST be removed from the packets, and
 the CC field, Padding Bit, and Extension but MUST be set to zero,
 before the FEC operation is applied. These modified packets are used
 in the procedure above. Note that the sender MUST still send the
 original packets (with the CSRC list, padding, and extension in tact)
 as the primary encoding in RFC 2198. The removal of these fields only
 applies to the protection operation.

Rosenberg & Schulzrinne Standards Track [Page 17] RFC 2733 Generic FEC December 1999

 Once the FEC packets have been generated, the media payload is
 extracted from the media packets. This payload is used as the primary
 encoding as defined in RFC 2198. Then, the FEC header and payload of
 the FEC packets is extracted, and treated as a redundant encoding.
 Additional redundant encodings, besides FEC, MAY be added to the
 packet as well. These encodings will not be protected by FEC,
 however.
 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
    PTI:       127
    SN:        1
    TS:        5
    SSRC:      2
 Figure 5: RTP Header of FEC for Packets X and Y
 The redundant encodings header for the primary codec is set as
 defined in RFC 2198. The redundant encodings header for the FEC data
 is set as follows. The block PT is set to the dynamic PT associated
 with the FEC format. The block length is set to the sum of the
 lengths of the FEC header and payload. The timestamp offset SHOULD be
 set to zero. The secondary coder payload includes the FEC header and
 FEC payload.
 At the receiver, the primary codec and all secondary codecs are
 extracted as separate RTP packets. This is done by copying the
 sequence number, SSRC, marker bit, CC field, RTP version, and
 extension bit from the RTP header of the redundant encodings packet
 to the RTP header of each extracted packet. If the secondary codec
 contains FEC, the CC field, Extension Bit, and Padding Bit in the RTP
 header of the FEC packet MUST be set to zero instead. The payload
 type identifier in the extracted packet is copied from the block PT
 of the redundant encodings header. The timestamp of the extracted
 packet is the difference between the timestamp in the RTP header and

Rosenberg & Schulzrinne Standards Track [Page 18] RFC 2733 Generic FEC December 1999

 the offset in the block header. The payload of the extracted packet
 is the data block. This will result in the FEC stream and media
 stream being extracted.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |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 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |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 0 0 0 0 0 1 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 1 0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    SN base:   8    [min(8,9)]
    len. rec.: 1    [8 xor 9]
    E:         0
    PTI rec.:  25   [11 xor 18]
    mask:      3
    TS rec.:   6    [3 xor 5]
    The payload length is 11 bytes.
 Figure 6: FEC Header of Result
 To use the FEC and media packets for recovery, the CSRC list,
 extension, and padding MUST be removed from the media packets, if
 present, and the CC field, Extension Bit, and Padding Bit MUST be set
 to zero. These modified media packets, along with the FEC packets,
 are then used to recover based on the procedures in section 8. The
 recovered media packets will always have no extension, padding, or
 CSRC list. An implementation MAY copy these fields into the recovered
 packet from another media packet, if available.
 Using the redundant encodings payload format also implies that the
 marker bit may not be recovered correctly. Applications MUST set the
 marker bit to zero in media packets reconstructed using FEC
 encapsulated in RFC 2198 redundancy.
 An advantage of this approach is a reduction in the overhead for
 sending FEC packets.

Rosenberg & Schulzrinne Standards Track [Page 19] RFC 2733 Generic FEC December 1999

11 Indicating FEC Usage in SDP

 FEC packets contain RTP packets with dynamic payload type values. In
 addition, the FEC packets can be sent on separate multicast groups or
 separate ports from the media. The FEC can even be carried in packets
 containing media, using the redundant encodings payload format [5].
 These configuration options must be indicated out of band. This
 section describes how this can be accomplished using the Session
 Description Protocol (SDP), specified in RFC 2327 [6].

11.1 FEC as a Separate Stream

 In the first case, the FEC packets are sent as a separate stream.
 This can mean they are sent on a different port and/or multicast
 group from the media. When this is done, several pieces of
 information must be conveyed:
      o The address and port where the FEC is being sent to
      o The payload type number for the FEC
      o Which media stream the FEC is protecting
 The payload type number for the FEC is conveyed in the m line of the
 media it is protecting, listed as if it were another valid encoding
 for the stream. 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 "parityfec".
 The presence of the payload type number in the m line of the media it
 is protecting does not mean the FEC is sent to the same address and
 port as the media. Instead, this information is conveyed through an
 fmtp attribute line. The presence of the FEC payload type on the m
 line of the media serves only to indicate which stream the FEC is
 protecting.
 The format for the fmtp line for FEC is:
 a=fmtp:<number> <port> <network type> <addresss type> <connection
 address>
 where 'number' is the payload type number present in the m line. Port
 is the port number where the FEC is sent to. The remaining three
 items - network type, address type, and connection address - have the
 same syntax and semantics as the c line from SDP. This allows the
 fmtp line to be partially parsed by the same parser used on the c

Rosenberg & Schulzrinne Standards Track [Page 20] RFC 2733 Generic FEC December 1999

 lines. Note that since FEC cannot be hierarchically encoded, the
 <number of addresses> parameter MUST NOT appear in the connection
 address.
 The following is an example SDP for FEC:
 v=0
 o=hamming 2890844526 2890842807 IN IP4 126.16.64.4
 s=FEC Seminar
 c=IN IP4 224.2.17.12/127
 t=0 0
 m=audio 49170 RTP/AVP 0 78
 a=rtpmap:78 parityfec/8000
 a=fmtp:78 49172 IN IP4 224.2.17.12/127
 m=video 51372 RTP/AVP 31 79
 a=rtpmap:79 parityfec/8000
 a=fmtp:79 51372 IN IP4 224.2.17.13/127
 The presence of two m lines in this SDP indicates that there are two
 media streams - one audio and one video. The media format of 0
 indicates that the audio uses PCM, and is protected by FEC with
 payload type number 78. The FEC is sent to the same multicast group
 and TTL as the audio, but on a port number two higher (49172). The
 video is protected by FEC with payload type number 79. The FEC
 appears on the same port as the video (51372), but on a different
 multicast address.

11.2 Use with Redundant Encodings

 When the FEC stream is being sent as a secondary codec in the
 redundant encodings format, this must be signaled through SDP. To do
 this, the procedures defined in RFC 2198 are used to signal the use
 of redundant encodings. 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. In this case, the fmtp
 attribute for the FEC MUST NOT be present.
 For example:
 m=audio 12345 RTP/AVP 121 0 5 100
 a=rtpmap:121 red/8000/1
 a=rtpmap:100 parityfec/8000
 a=fmtp:121 0/5/100

Rosenberg & Schulzrinne Standards Track [Page 21] RFC 2733 Generic FEC December 1999

 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 parityfec through the rtpmap attribute). Although the FEC
 format is specified as a possible coding for this stream, the FEC
 MUST NOT be sent by itself for this stream. Its presence in the m
 line is required only because non-primary codecs must be listed here
 according to RFC 2198. The fmtp attribute indicates that the
 redundant encodings format can be used, with DVI as a secondary
 coding and FEC as a tertiary encoding.

11.3 Usage with RTSP

 RTSP [7] can be used to request FEC packets to be sent as a separate
 stream. When SDP is used with RTSP, the Session Description does not
 include a connection address and port number for each stream.
 Instead, RTSP uses the concept of a "Control URL". Control URLs are
 used in SDP in two distinct ways.
      1.   There is a single control URL for all streams. This is
           referred to as "aggregate control". In this case, the fmtp
           line for the FEC stream is omitted.
      2.   There is a Control URL assigned to each stream. This is
           referred to as "non-aggregate control". In this case, the
           fmtp line specifies the Control URL for the stream of FEC
           packets. The URL may be used in a SETUP command by an RTSP
           client.
 The format for the fmtp line for FEC with RTSP and non-aggregate
 control is:
 a=fmtp:<number> <control URL>
 where 'number' is the payload type number present in the m line.
 Control URL is the URL used to control the stream of FEC packets.
 Note that the Control URL does not need to be an absolute URL. The
 rules for converting a relative Control URL to an absolute URL are
 given in RFC 2326, Section C.1.1.

Rosenberg & Schulzrinne Standards Track [Page 22] RFC 2733 Generic FEC December 1999

12 Security Considerations

 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 permutations on the usage of encryption. In
 particular:
   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
     different keys.
   o The media stream and FEC stream are both encrypted, but using
     the same key.
 The first three of these would require any application level
 signaling protocols to be aware of the usage of FEC, and to thus
 exchange keys for it and negotiate its 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 FEC
 packets should really be treated no differently than other RTP
 packets. Encrypting just one may also make certain known-plaintext
 attacks possible. For these reasons, applications utilizing
 encryption SHOULD encrypt both streams.
 However, the changing of keys becomes problematic. For example, if
 two packets a and b are sent, and FEC packet f(a,b) is sent, and the
 keys used for a and b are different, which key should be used to
 decode f(a,b)? In general, old keys will likely need to be cached, so
 that when the keys change for the media stream, the old key is kept,
 and used, until it is determined that the key has changed on the FEC
 packets as well.
 Another issue with the use of FEC is its impact on network
 congestion. Adding FEC in the face of increasing network losses is a
 bad idea, as it can lead to increased congestion and eventual
 congestion collapse if done on a widespread basis. As a result,
 implementers MUST NOT substantially increase the amount of FEC in use
 as network losses increase.

Rosenberg & Schulzrinne Standards Track [Page 23] RFC 2733 Generic FEC December 1999

13 Acknowledgments

 This work is based on an earlier draft on FEC, submitted by Budge and
 Mackenzie in 1997. We would also like to thank Steve Casner, Mark
 Handley, Orion Hodson and Colin Perkins for their comments. Thanks to
 Anders Klemets who wrote the section on usage with RTSP.

14 Authors' Addresses

 Jonathan Rosenberg
 dynamicsoft
 200 Executive Drive
 Suite 120
 West Orange, NJ 07046
 Email: jdrosen@dynamicsoft.com
 Henning Schulzrinne
 Columbia University
 M/S 0401, 1214 Amsterdam Ave.
 New York, NY 10027-7003
 EMail: schulzrinne@cs.columbia.edu

Rosenberg & Schulzrinne Standards Track [Page 24] RFC 2733 Generic FEC December 1999

15 Bibliography

 [1] J.C. Bolot and A. V. Garcia, "Control mechanisms for packet audio
     in the internet," in Proceedings of the Conference on Computer
     Communications (IEEE Infocom) , (San Francisco, California), Mar.
     1996.
 [2] Perkins, C. and O. Hodson, "Options for Repair of Streaming
     media", RFC 2354, June 1998.
 [3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
     A Transport Protocol for Real-Time Applications", RFC 1889,
     January 1996.
 [4] Bradner, S., "Key words for use in RFCs to indicate requirement
     levels", BCP 14, RFC 2119, March 1997.
 [5] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,
     Bolot, J.C., Vega-Garcia, A. and S. Fosse-Parisis, "RTP Payload
     for Redundant Audio Data", RFC 2198, September 1997.
 [6] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
     RFC 2327, April 1998.
 [7] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
     Protocol (RTSP)", RFC 2326, April 1998.

Rosenberg & Schulzrinne Standards Track [Page 25] RFC 2733 Generic FEC December 1999

16. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Rosenberg & Schulzrinne Standards Track [Page 26]

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