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Internet Draft                                                     A. Li
draft-ietf-avt-ulp-04.txt                                         F. Liu
February 15, 2002                                          J. Villasenor
Expires: August 15, 2002                             Univ. of Calif., LA
                                                               J.H. Park
                                                               D.S. Park
                                                                Y.L. Lee
                                                     Samsung Electronics

   An RTP Payload Format for Generic FEC with Uneven Level Protection


STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   anytime. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as work in progress.

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


ABSTRACT

   This document specifies a payload format for generic forward error
   correction to achieve uneven level protection (ULP) for media data
   encapsulated in RTP. It is an extension of the forward error
   correction scheme specified in RFC 2733 [1], and it is based on the
   same exclusive-or (parity) operation. This payload format allows end
   systems to apply protection using arbitrary protection lengths and
   levels, in addition to using arbitrary protection group sizes. It
   also enables both complete recovery or partial recovery of the
   critical payload and RTP header fields depending on the packet loss
   situation. This scheme is completely backward compatible with non-FEC
   capable hosts and with hosts that are only capable of handling the
   FEC schemes specified in RFC 2733 [1]. Those receivers that do not
   know about ULP forward error correction can simply ignore the
   extensions.




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Table of Contents

   1. Introduction ................................................... 3
   1.1. General Overview ............................................. 4
   1.2. Application Statement ........................................ 4
   2. Terminology .................................................... 6
   3. Basic Operation ................................................ 6
   4. RTP Media Packet Structure ..................................... 7
   5. ULP FEC Packet Structure ....................................... 7
   5.1. RTP Header of ULP FEC Packets ................................ 8
   5.2. FEC Header ................................................... 9
   5.3. ULP Level Header ............................................. 9
   6. Protection Operation .......................................... 10
   6.1. Protection Level 0 .......................................... 10
   6.2. Protection Level 1 and Higher ............................... 12
   7. Recovery Procedure ............................................ 12
   7.1. Reconstruction of Level 0 ................................... 12
   7.2. Reconstruction of Level 1 and Higher ........................ 13
   8. Examples ...................................................... 14
   8.1. An Example With Only Protection Level 0 ..................... 14
   8.2. An Example That Generates Idential Protection as in RFC 2733  16
   8.3. An Example With Two Protection Levels (0 and 1) ............. 17
   9. Security and Congestion Considerations ........................ 20
   10. Indication ULP FEC Usage in SDP .............................. 21
   10.1. ULP FEC as a Separate Stream ............................... 21
   10.2. Use with Redundant Encoding ................................ 22
   10.3. Usage with RTSP ............................................ 23
   11. MIME Registrations ........................................... 24
   11.1. Registration of audio/ulpfec ............................... 24
   11.2. Registration of video/ulpfec ............................... 25
   11.3. Registration of text/ulpfec ................................ 26
   11.4. Registration of application/ulpfec ......................... 27
   12. Acknowledgements ............................................. 28
   13. Bibliography ................................................. 28
   14. Authors' Address ............................................. 29


















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1. Introduction

1.1. General Overview

   Because of the real-time nature of many applications, they 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 has been one of the main methods used to
   protect against packet loss over packet switched networks [2].

   In many cases, the bandwidth of the network connections is a very
   limited resource. However, most of traditional FEC schemes are not
   designed for optimal utilization of the limited bandwidth resource. A
   more efficient way to utilize the limited bandwidth would be to use
   unequal error protection to provide different levels of protection
   for different parts of the data stream which vary in importance.
   These unequal error protection schemes can make more efficient use of
   the bandwidth to provide better overall protection of the data stream
   against the loss. Proper protocol support is essential for realizing
   these unequal error protection mechanisms. However, the application
   of most of the unequal error protection schemes requires the
   knowledge of the importance for different parts of the data stream.
   Most of such schemes are designed for a particular type of media
   according to the structure of the media protected, and as a result,
   are not generic.

   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 for most codecs 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. Also, 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, the motion vector data, and DCT coefficient
   data are separated into their individual partitions. 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.



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   For audio streams, the bitstreams generated by many of the new audio
   codecs also contain data with different classes of importance. These
   different classes are then transmitted in order of descending
   importance. Thus, applying more protection to the beginning of the
   packet would also be beneficial in these cases. Even for uniform-
   significance audio streams, special stretching techniques can be
   applied to the partially recovered audio data packets. In cases where
   audio redundancy coding is used, more protection should be applied to
   the original data located in the first half of the packet. The rest
   of the packet containing the redundant copies of the data, does not
   need the same level of protection.

   It is clear that audio/video applications would generally benefit
   from an unequal error protection scheme that gives more protection to
   the beginning part of each packet. This document defines a payload
   format for RTP [3] that allows for generic forward error correction
   with unequal error protection for real-time media. The payload data
   are protected by one or more protection levels. Lower protection
   levels provide greater protection by using smaller group sizes
   (compared to higher protection levels) for generating the FEC packet.
   The data that are closer to the beginning of the packet are protected
   by lower protection levels because these data are in general more
   important, and they tend to carry more information than the data
   further behind in the packet.

   This document specifies an RTP payload format that extends the
   generic forward error correction schemes as specified in RFC 2733
   [1]. This extension enables different levels of protection to be
   applied to different parts of the packet. While the whole packet can
   always be treated as a single level (same as in RFC 2733), this
   multiple Uneven Level Protection (ULP) can potentially achieve more
   efficient protection of the media payload.

1.2. Application Statement

   The ULP algorithm specified in this document is designed to deal with
   any type of packet loss occurring in transmission, just as does RFC
   2733, which it extends. The ULP algorithm is designed to be fully
   interoperable between the hosts that are ULP-capable and those that
   are not. Since the media payload is not altered and the protection is
   sent as additional information, the receivers that are unaware of ULP
   can simply ignore the additional ULP information and process the main
   media payload. This interoperability is particularly important for
   backward 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 ULP algorithm 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, (3) it is designed for adaptivity, so that the FEC
   technique can be modified easily without resorting to out of band

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   signaling, and (4) it is supportive of a number of different
   mechanisms for transporting the FEC packets.

   An Unequal Erasure Protection (UXP) scheme has also 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 un-altered 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 un-recoverable 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, any
   errors in the bitstream usually occur in contiguous pieces at the end
   of the packets. Depending on the encoding of the media payload
   stream, many applications may find it easier to decode more data from
   a stream with a contiguous piece missing at the end than from a
   stream with multiple corrupted holes.

   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.


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2. Terminology

   The following terms are used throughout this document:

   Media Payload: The raw, un-protected 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.

   ULP FEC Packet: The uneven level protection 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 ULP FEC packets, which contain redundant media data used for
   error correction. The ULP FEC packets are formatted according to the
   rules specified in this document.

   FEC Header: The header information contained in an FEC packet.

   FEC Payload: The payload of an FEC packet.

   Associated: A ULP FEC packet is said to be "associated" with one or
   more media packets when those media packets are used to generate the
   ULP 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  the sender in an
   RTP session would like to protect the media stream it is sending with
   uneven level protection (ULP) FEC. The ULP FEC supported by the
   format is based on the same simple exclusive-or (XOR) parities used
   in RFC 2733 [1]. 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 of each
   level are grouped as described below in Section 6 to provide each
   level with a different degree of error resilience. An XOR operation
   is applied across the payload to generate the ULP FEC information for
   each level. The lower protection levels (which provide higher
   protection, or greater error resilience) are applied to the data that
   are closer to the beginning of the packet to ensure more protection.
   The result based on the procedures defined here is an RTP packet
   containing ULP FEC information. This packet can be used at the
   receiver to recover the packets or parts of the packets used to
   generate the ULP FEC packet. By using uneven error protection, this

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   scheme can make more efficient use of the channel bandwidth, and
   provide more efficient error resilience for transmission over error
   prone channels.

   The payload format contains information that allows the sender to
   tell the receiver exactly which media packets are protected by the
   ULP FEC packet, and the protection levels and lengths for each of the
   levels. Specifically, each ULP FEC packet contains a protection
   length L(k) and a bitmask m(k), called the offset mask, for each
   protection level k. If the bit i in the mask m(k) is set to 1, then
   data of length L(k) in media packet number N + i is protected by this
   ULP FEC packet at level k. N is called the sequence number base, and
   is sent in the ULP FEC packet as well. The protection length, offset
   mask and payload type are sufficient to signal ULP forward error
   correction schemes based on arbitrarily defined parity protection
   with little overhead. A set of rules is described in Section 5.3 that
   defines how the mask should be set for different protection levels.

   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 code to current network
   conditions and be certain the receivers can still make use of the ULP
   FEC for recovery.

   At the receiver, the ULP FEC and original media are received. If no
   media packets are lost, the ULP FEC can be ignored. In the event of a
   loss, the ULP FEC packets can be combined with other received media
   and ULP FEC packets to recover all or part of the missing media
   packets.

   RTP packets that contain data formatted according to this
   specification (i.e., ULP FEC packets) use dynamic RTP payload types.


4. RTP Media Packet Structure

   The formatting of the media packets is unaffected by ULP FEC. If the
   ULP FEC is sent as a separate stream, the media packets are sent as
   if there was no FEC.

   This scheme leads to a very efficient encoding. When little or no ULP
   FEC is used, the transmitted stream contains mostly media packets.
   The overhead for using the ULP FEC scheme is only present in ULP FEC
   packets, and can be easily monitored and adjusted by tracking the
   amount of FEC in use.


5. ULP FEC Packet Structure

   A ULP FEC packet is constructed by placing an FEC header and the ULP
   FEC payload into the RTP payload, as shown in Figure 1:



Adam H. Li, et al.                                              [Page 7]

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         RTP Header                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         FEC Header                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ULP Level 0 Header                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ULP Level 0 Payload                       |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ULP Level 1 Header                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ULP Level 1 Payload                       |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Cont.                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: ULP FEC Packet Structure


5.1. RTP Header of ULP 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, though
   it MAY be different if the FEC stream is being demultiplexed via the
   SSRC value. The CC value is computed from 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 ULP FEC packet is transmitted. Thus, the TS value
   in FEC packets is always monotonically increasing.

   The payload type for the ULP FEC packet is determined through
   dynamic, out of band means. According to RFC 1889 [3], RTP
   participants that cannot recognize a payload type must discard it.
   This provides backwards compatibility. The ULP FEC mechanisms can
   then be used in a multicast group with mixed ULP-FEC-capable and ULP-
   FEC-incapable receivers. In such a case, the ULP stream will have a
   payload type which is not recognized by the ULP-FEC-incapable
   receivers, and will thus be disregarded.




5.2. FEC Header

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   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.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           SN base             |        length recovery        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |E| PT recovery |                     mask                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          TS recovery                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: FEC Header Format


   This is exactly the same as the FEC header used in RFC 2733 [1]. The
   usage will also be exactly the same as specified as in RFC 2733,
   except that the E bit MUST set to one for this version.

5.3. ULP Level Header

   The ULP Level Header is 2 bytes for ULP level 0, and 5 bytes for ULP
   level 1 and higher. The formats of the headers are shown in Figure 3
   and Figure 4, and consist of a Protection Length field and a mask
   field (for level 1 and higher headers).


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

   Figure 3: ULP Level Header Format (Level 0)


    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.) |
   +-+-+-+-+-+-+-+-+

   Figure 4: ULP Level Header Format (Level 1 and higher)


   The Protection Length field is 16 bits. It indicates the protection
   length provided by the ULP FEC for the current protection level

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   (i.e., the payload length for the current protection level after the
   header).

   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 ULP
   FEC packet of current protection level, where N is the SN Base field
   in the ULP FEC packet header. The least significant bit corresponds
   to i=0, and the most significant to i=23.

   The SN base field in the FEC header MUST be set to the minimum
   sequence number of those media packets protected by ULP FEC. This
   allows for the ULP FEC operation to extend over any string of at most
   24 packets.

   The setting of mask field shall follow the following rules:

      a. A media packet can only be protected at each protection level
         once.

      b. For a media packet to be protected at level p, it must also be
         protect at level p-1.

      c. If an ULP FEC packet contains protection at level p, it must
         also contain protection at level p-1.

   The payload of the ULP FEC packet of each level 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 ULP FEC packet. Details are described in the next section on
   the protection operation.


6. Protection Operation

   The protection operation involves copying the payload, padding it
   with zeroes, and computing the parity (XOR) across the resulting bit
   strings. In addition, for protection of level 0, it also involves
   concatenating specific fields from the RTP header of the media packet
   before the payload data. The resulting bit string is used to generate
   the ULP FEC packet.

   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.

6.1. Protection Level 0

   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)


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      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 of the CSRC List, length of the padding, length of the
        extension, and length of the media packet (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.

   If the lengths of the bit strings are not equal, each bit string that
   is shorter than the Protection Length 0 plus 96 bits, MUST be padded
   to that length. Any value may be used for padding. 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 ULP FEC packet. We will
   call this the ULP FEC bit string (level 0).

   The first (most significant) bit in the ULP FEC bit string is written
   into the Padding Bit of the ULP FEC packet. The second bit in the ULP
   FEC bit string is written into the Extension bit of the ULP FEC
   packet. The next four bits of the ULP FEC bit string are written into
   the CC field of the ULP FEC packet. The next bit of the ULP FEC bit
   string is written into the marker bit of the ULP FEC packet. The next
   7 bits of the ULP FEC bit string are written into the PT recovery
   field in the ULP FEC packet header. The next 32 bits of the ULP 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 ULP FEC packet header. This is exactly the same as in RFC 2733
   [1].

   The remaining bits (of length Protection Length 0) are set to be the
   payload of the ULP FEC packet.


6.2. Protection Level 1 and Higher


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   The protected data of the corresponding packets are copied into the
   bit strings. If the packet ends before the Protection Length of the
   current level is reached, the string is padded to that length. Any
   value may be used for the padding. The padding MUST be added at the
   end of the bit string.

   The parity operation is applied across the protected data of the
   corresponding packets. The generated ULP FEC bit of that level is
   then appended to the payload of the ULP FEC packet.


7. Recovery Procedures

   The ULP 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 possible.

7.1. Reconstruction of Level 0

   Let T be the list of packets (ULP 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.

      2.  For the ULP FEC packet in T, compute the bit string in the
          same fashion, except always set the CSRC list, extension, and
          padding to null. Read the Protection Length 0. Read string of
          that length from that ULP FEC packet.

      3.  If any of the bit strings generated from the media packets are
          shorter than the bit string generated from the ULP FEC packet,
          pad them to be the same length as the bit string generated
          from the ULP 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.


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      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.

      15. Set the SSRC of the new packet to the SSRC of the media stream
          it's protecting.

   This procedure will recover both the header and payload of an RTP
   packet up to the Protection Length of level 0.

7.2. Reconstruction of Level 1 and Higher

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

      1.  For the media packet in T, get the protection length of that
          level. Copy the data of the that protection level (data of the
          length read following the level header) to the bit strings.

      2.  If any of the bit strings generated from the media packets are
          shorter than the Protection Length of the current level, pad
          them to that length. The padding MUST be added at the end of
          the bit string, and MUST be of the same value as used in the
          process of generating the ULP FEC packets.

      3.  Perform the exclusive-or (parity) operation across the bit
          strings, resulting in a recovery bit string.


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   The data protected at lower protection level is almost always
   recoverable 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.


8. Examples

   Consider 4 media packets 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. Packet A and C uses
   payload type 11, and packet B and D uses payload type 18. Packet A is
   has 200 bytes of payload, packet B 140, packet C 100 and packet D
   340. Packet A and C have their marker bit set.

8.1. An Example With Only Protection Level 0

   Suppose we want to protect the data of length L0 = 70 bytes of them
   at the beginning of these packets, as illustrated in Figure 5 below.


              +------:------------+
   Packet A   |      :            |
              +------:------+-----+
   Packet B   |      :      |
              +------:--+---+
   Packet C   |      :  |
              +------:--+-----------------------+
   Packet D   |      :                          |
              +------:--------------------------+
                     :
              +------+
   Packet FEC |      |
              +------+
              :      :
              :<-L0->:

   Figure 5 ULP FEC scheme with only protection level 0


   An ULP 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 6.

   The FEC header in the ULP FEC packet is shown in Figure 7.

   The ULP header for level 0 in the ULP FEC packet is shown in Figure
   8.


    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

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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 6: RTP Header of ULP FEC for Packets A, B, C and D (one level)


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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 1 1 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 1 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      SN base:   8    [min(8,9,10,11)]
      len. rec.: 372  [200 XOR 140 XOR 100 XOR 340]
      E:         1    [ULP FEC]
      PT rec.:   0    [11 XOR 18 XOR 11 XOR 18]
      mask:      15
      TS rec.:   8    [3 XOR 5 XOR 7 XOR 9]

   Figure 7: FEC Header of ULP Packet (one level)


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

      L0:        70

      The payload length for level 0 is 70 bytes.

   Figure 8: ULP Level Header (Level 0)


8.2. An Example That Generates Identical Protection as in RFC 2733


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   We can choose to extend the level 0 protection to cover the whole
   length of the packets (as shown in Figure 9). This gives almost
   identical protection as provided in RFC 2733. Please note that when
   using ULP this way, each ULP FEC packet will use two more bytes (for
   the level 0 payload length field) than that of RFC 2733 - a small
   price to pay for the added flexbility.


              +-------------------+             :
   Packet A   |                   |             :
              +-------------+-----+             :
   Packet B   |             |                   :
              +---------+---+                   :
   Packet C   |         |                       :
              +---------+-----------------------+
   Packet D   |                                 |
              +---------------------------------+
                                                :
              +---------------------------------+
   Packet FEC |                                 |
              +---------------------------------+
              :                                 :
              :<------------- L0 -------------->:

   Figure 9 ULP FEC scheme with only protection level 0


   The resulting ULP FEC packet will have the RTP header same as shown
   in Figure 6 and FEC header same as shown in Figure 7. The ULP level
   header is shown in Figure 10.


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

      L0:        340  [max(200,140,100,340)]

      The payload length for level 0 is 340 bytes.

   Figure 10: ULP Level Header (Level 0)








8.3. An Example With Two Protection Levels (0 and 1)


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   A more complete example is to use ULP at two levels. The level 0 ULP
   will provide greater protection to the beginning part of the payload
   packets. The level 1 ULP will apply additional protection to the rest
   of the packets. This is illustrated in Figure 11. In this example, we
   take L0 = 70 and L1 = 90.


              +------:--------:---+
   Packet A   |      :        :   |
              +------:------+-:---+
   Packet B   |      :      | :
              +------:--+---+ :
                     :        :
              +------+        :
   ULP #1     |      |        :
              +------+        :
                     :        :
              +------:--+     :
   Packet C   |      :  |     :
              +------:--+-----:-----------------+
   Packet D   |      :        :                 |
              +------:--------:-----------------+
                     :        :
              +------:--------+
   ULP #2     |      :        |
              +------:--------+
              :      :        :
              :<-L0->:<--L1-->:

   Figure 11 ULP FEC scheme with protection level 0 and level 1


   This will result in two ULP FEC packets - #1 and #2.

   The resulting ULP FEC packet #1 will have the RTP header as shown in
   Figure 12. The FEC header for ULP FEC packet #1 will be as shown in
   Figure 13. The level 0 ULP header for #1 will be shown in Figure 14.













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

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   |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 12: RTP Header of ULP FEC #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 1 0 0 0 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|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.: 68   [200 XOR 140]
      E:         1    [ULP FEC]
      PT rec.:   25   [11 XOR 18]
      mask:      3
      TS rec.:   6    [3 XOR 5]

   Figure 13: FEC Header of ULP FEC Packet #1


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

      L0:        70

      The payload length for level 0 is 70 bytes.

   Figure 14: ULP Level Header (Level 0) for ULP FEC Packet #1


   The resulting ULP FEC packet #2 will have the RTP header as shown in
   Figure 15. The FEC header for ULP FEC packet #2 will be as shown in


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   Figure 16. The level 0 ULP header for #2 will be shown in Figure 17.
   The level 1 ULP header for #2 will be shown in Figure 18.


    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 15: RTP Header of ULP FEC Packet #2


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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 0 1 1 0 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|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 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 0 1 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      SN base:   8    [min(8,9,10,11)]
      len. rec.: 308  [100 XOR 340]
      E:         1    [ULP FEC]
      PT rec.:   25   [11 XOR 18]
      mask:      12
      TS rec.:   6    [7 XOR 9]

   Figure 16: FEC Header of ULP Packet #2









    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      L0:        70

      The payload length for level 0 is 70 bytes.

   Figure 17: ULP Level Header (Level 0) for ULP 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 1 1 0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 1 1 1|
   +-+-+-+-+-+-+-+-+

      L1:        90
      mask:      15

      The payload length for level 1 is 90 bytes.

   Figure 18: ULP Level Header (Level 1) for ULP Packet #2


9. Security and Congestion Considerations

   The use of ULP FEC has implications on the usage and changing of keys
   for encryption. As the ULP FEC packets do consist of a separate
   stream, there are a number of combinations on the usage of
   encryption. These include:

      o The ULP FEC stream may be encrypted, while the media stream is
        not.

      o The media stream may be encrypted, while the ULP FEC stream is
        not.

      o The media stream and ULP FEC stream are both encrypted, but
        using different keys.

      o The media stream and ULP FEC stream are both encrypted, but
        using the same key.

   The first three of these would require all application level
   signaling protocols used to be aware of the usage of ULP FEC, and to
   thus exchange keys and negotiate encryption usage on the media and
   ULP 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

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   known-plaintext attacks possible. For these reasons, applications
   utilizing encryption SHOULD encrypt both streams.

   The changing of encryption keys is another crucial issue needs to be
   addressed. Consider the case where two packets a and b are sent along
   with the ULP FEC packet that protects them. The keys used to encrypt
   a and b are different, so which key should be used to decode the ULP
   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.

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


10. Indicating ULP 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 ULP FEC can even be carried in
   packets containing media using the redundant encoding 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].

10.1. ULP FEC as a Separate Stream

   In the first case, the ULP FEC packets are sent as a separate stream.
   This means that they can be 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 ULP FEC is being sent to

   o The payload type number for the ULP FEC

   o Which media stream the ULP FEC is protecting

   The payload type number for the ULP 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 ULP 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".


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   The presence of the payload type number in the m line of the media it
   is protecting does not mean the ULP 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 ULP FEC payload type on
   the m line of the media serves only to indicate which stream the ULP
   FEC is protecting.

   The format for the fmtp line for ULP 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 ULP 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
   lines. Note that since ULP FEC cannot be hierarchically encoded, the
   <number of addresses> parameter MUST NOT appear in the connection
   address.

   The following is an example SDP for ULP FEC:

      v=0
      o=hamming 2890844526 2890842807 IN IP4 128.97.90.168
      s=ULP FEC Seminar
      c=IN IP4 224.2.17.12/127
      t=0 0
      m=audio 49170 RTP/AVP 0 78
      a=rtpmap:78 ulpfec/8000
      a=fmtp:78 49172 IN IP4 224.2.17.12/127
      m=video 51372 RTP/AVP 31 79
      a=rtpmap:79 ulpfec/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 ULP FEC with
   payload type number 78. The ULP 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 ULP FEC with payload type number 79. The
   ULP FEC appears on the same port as the video (51372), but on a
   different multicast address.

10.2. Use with Redundant Encoding

   When the ULP FEC stream is being sent as a secondary codec in the
   redundant encoding format, this must be signaled through SDP. To do
   this, the procedures defined in RFC 2198 [5] are used to signal the
   use of redundant encoding. The ULP 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 ULP


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   FEC packets. The ULP FEC MUST protect only the main codec. In this
   case, the fmtp attribute for the ULP 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 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 read
   through the rtpmap attribute), or ULP FEC (media format 100, which is
   bound to ulpfec through the rtpmap attribute). Although the ULP FEC
   format is specified as a possible coding for this stream, the ULP 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 ULP FEC as a tertiary encoding.

10.3. Usage with RTSP

   RTSP [7] can be used to request ULP 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 ULP
   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 ULP FEC
   packets. The URL may be used in a SETUP command by an RTSP client.

   The format for the fmtp line for ULP 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 ULP 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.


11. MIME Registrations


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   Four new MIME sub-type as described in this section is to be
   registered.

11.1. Registration of audio/ulpfec

   To: ietf-types@iana.org

   Subject: Registration of MIME media type audio/ulpfec

   MIME media type name: audio

   MIME subtype name: ulpfec

   Required parameters: none

   Note that it is mandated that RTP payload formats without a defined
   rate must define a rate parameter as part of their MIME registration.
   The payload format for ULP FEC does not specify a rate parameter.
   However, the rate for ULP FEC data is equal to the rate of the media
   data it protects.

   Optional parameters: none

   Typical optional parameters [8], such as the number of channels, and
   the duration of audio per packet, do not apply to ULP FEC data.  The
   number of channels is effectively the same as the media data it
   protects; the same is true for the duration of audio per packet.

   Encoding considerations: This format is only defined for transport
   within the Real Time Transport protocol (RTP) [3].  Its transport
   within RTP is fully specified with RFC xxxx.

   Security considerations: the same security considerations apply to
   these MIME registrations as to the payloads for them, as detailed in
   RFC xxxx.

   Interoperability considerations: none

   Published specification: RFC xxxx.

   Applications which use this media type: Audio and video streaming
   tools which 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
      Department of Electrical Engineering
      University of California
      Los Angeles, CA 90095
      adamli@icsl.ucla.edu

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   Intended usage: COMMON

   Author/Change controller: This registration is part of the IETF
   registration tree.

   RTP and SDP Issues: Usage of this format within RTP and the Session
   Description Protocol (SDP) [6] are fully specified within Section 10
   of RFC xxxx.

11.2. Registration of video/ulpfec

   To: ietf-types@iana.org

   Subject: Registration of MIME media type video/ulpfec

   MIME media type name: video

   MIME subtype name: ulpfec

   Required parameters: none

   Note that it is mandated that RTP payload formats without a defined
   rate must define a rate parameter as part of their MIME registration.
   The payload format for ULP FEC does not specify a rate parameter.
   However, the rate for ULP FEC data is equal to the rate of the media
   data it protects.

   Optional parameters: none

   Typical optional parameters [8], such as the number of channels, and
   the duration of audio per packet, do not apply to ULP FEC data.  The
   number of channels is effectively the same as the media data it
   protects; the same is true for the duration of video per packet.

   Encoding considerations: This format is only defined for transport
   within the Real Time Transport protocol (RTP) [3].  Its transport
   within RTP is fully specified with RFC xxxx.

   Security considerations: the same security considerations apply to
   these MIME registrations as to the payloads for them, as detailed in
   RFC xxxx.

   Interoperability considerations: none

   Published specification: RFC xxxx.

   Applications which use this media type: Audio and video streaming
   tools which seek to improve resiliency to loss by sending additional
   data with the media stream.

   Additional information: none


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   Person & email address to contact for further information:

      Adam Li
      Department of Electrical Engineering
      University of California
      Los Angeles, CA 90095
      adamli@icsl.ucla.edu

   Intended usage: COMMON

   Author/Change controller: This registration is part of the IETF
   registration tree.

   RTP and SDP Issues: Usage of this format within RTP and the Session
   Description Protocol (SDP) [6] are fully specified within Section 10
   of RFC xxxx.

11.3. Registration of text/ulpfec

   To: ietf-types@iana.org

   Subject: Registration of MIME media type text/ulpfec

   MIME media type name: text

   MIME subtype name: ulpfec

   Required parameters: none

   Note that it is mandated that RTP payload formats without a defined
   rate must define a rate parameter as part of their MIME registration.
   The payload format for ULP FEC does not specify a rate parameter.
   However, the rate for ULP FEC data is equal to the rate of the media
   data it protects.

   Optional parameters: none

   Typical optional parameters [8], such as the number of channels, and
   the duration of audio per packet, do not apply to ULP FEC data.  The
   number of channels is effectively the same as the media data it
   protects; the same is true for the duration of video per packet.

   Encoding considerations: This format is only defined for transport
   within the Real Time Transport protocol (RTP) [3].  Its transport
   within RTP is fully specified with RFC xxxx.

   Security considerations: the same security considerations apply to
   these MIME registrations as to the payloads for them, as detailed in
   RFC xxxx.

   Interoperability considerations: none

   Published specification: RFC xxxx.

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   Applications which use this media type: Audio, video and text
   streaming tools which 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
      Department of Electrical Engineering
      University of California
      Los Angeles, CA 90095
      adamli@icsl.ucla.edu

   Intended usage: COMMON

   Author/Change controller: This registration is part of the IETF
   registration tree.

   RTP and SDP Issues: Usage of this format within RTP and the Session
   Description Protocol (SDP) [6] are fully specified within Section 10
   of RFC xxxx.

11.4. Registration of application/ulpfec

   To: ietf-types@iana.org

   Subject: Registration of MIME media type application/ulpfec

   MIME media type name: application

   MIME subtype name: ulpfec

   Required parameters: none

   Note that it is mandated that RTP payload formats without a defined
   rate must define a rate parameter as part of their MIME registration.
   The payload format for ULP FEC does not specify a rate parameter.
   However, the rate for ULP FEC data is equal to the rate of the media
   data it protects.

   Optional parameters: none

   Typical optional parameters [8], such as the number of channels, and
   the duration of audio per packet, do not apply to ULP FEC data.  The
   number of channels is effectively the same as the media data it
   protects; the same is true for the duration of video per packet.

   Encoding considerations: This format is only defined for transport
   within the Real Time Transport protocol (RTP) [3].  Its transport
   within RTP is fully specified with RFC xxxx.


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   Security considerations: the same security considerations apply to
   these MIME registrations as to the payloads for them, as detailed in
   RFC xxxx.

   Interoperability considerations: none

   Published specification: RFC xxxx.

   Applications which use this media type: Audio and video streaming
   tools which 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
      Department of Electrical Engineering
      University of California
      Los Angeles, CA 90095
      adamli@icsl.ucla.edu

   Intended usage: COMMON

   Author/Change controller: This registration is part of the IETF
   registration tree.

   RTP and SDP Issues: Usage of this format within RTP and the Session
   Description Protocol (SDP) [6] are fully specified within Section 10
   of RFC xxxx.


12. Acknowledgments

   This text is partially based on an RFC 2733 [1] and RFC 3009 [9] on
   generic RTP FEC payload format by H. Schulzrinne and J. Rosenburg.
   The authors would also like to acknowledge the suggestions from many
   people, particularly Tao Tian, Matthieu Tisserand, Stephen Wenger,
   Jay Fahlen, and Jeffery Tseng.


13. Bibliography

   [1] J. Rosenberg and H. Schulzrine, "An RTP Payload Format for
   Generic Forward Error Correction," Request for Comments (Proposed
   Standard) 2733, Internet Engineering Task Force, December 1999.

   [2] C. Perkins and O. Hodson, "Options for repair of streaming media,
   "Request for Comments (Informational) 2354, Internet Engineering Task
   Force, June 1998.

   [3] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," Request for Comments

Adam H. Li, et al.                                             [Page 28]

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   (Proposed Standard) 1889, Internet Engineering Task Force, January
   1996.

   [4] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments (Best Current Practice) 2119, Internet
   Engineering Task Force, March 1997.

   [5] C. Perkins, I. Kouvelas, O. Hodson, V. Hardman, M. Handley, J.C.
   Bolot, A. Vega-Garcia, and S. Fosse-Parisis, "RTP Payload for
   Redundant Audio Data", RFC 2198, September 1997.

   [6] M. Handley, and V. Jacobson, "SDP: Session Description Protocol",
   RFC 2327, April 1998.

   [7] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming
   Protocol (RTSP)", RFC 2326, April 1998.

   [8] S. Casner, and P. Hoschka, "MIME type registration of RTP payload
   formats", Work in Progress.

   [9] J. Rosenberg and H. Schulzrine, "Registration of parityfec MIME
   types", Request for Comments (Proposed Standard) 3009, Internet
   Engineering Task Force, November 2000.


14. Authors' Addresses

   Adam H. Li
   Electrical Engineering Department
   University of California, Los Angeles
   Los Angeles, CA 90095
   USA
   Phone: +1-310-825-5178
   Fax  : +1-310-825-7928
   EMail: adamli@icsl.ucla.edu

   Fang Liu
   Electrical Engineering Department
   University of California, Los Angeles
   Los Angeles, CA 90095
   USA
   Phone: +1-310-825-5178
   Fax  : +1-310-825-7928
   EMail: fanliu@icsl.ucla.edu



   John D. Villasenor
   Electrical Engineering Department
   University of California, Los Angeles
   Los Angeles, CA 90095
   USA
   Phone: +1-310-825-0228

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   Fax  : +1-310-825-7928
   EMail: villa@icsl.ucla.edu

   Jeong-Hoon Park
   Samsung Electronics
   Suwon City, Kyungki-Do
   Korea
   442-742
   Phone: +82-31-200-3747
   Fax  : +82-31-200-3147
   Email: jeonghoon@samsung.com

   Dong-Seek Park
   Samsung Electronics
   Suwon City, Kyungki-Do
   Korea
   442-742
   Phone: +82-31-200-3674
   Fax  : +82-31-200-3147
   Email: dspark@samsung.com

   Yung-Lyul Lee
   Samsung Electronics
   Suwon City, Kyungki-Do
   Korea
   442-742
   Phone: +82-31-200-3719
   Fax  : +82-31-200-3147
   Email: yllee@samsung.com

























Adam H. Li, et al.                                             [Page 30]


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