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RFC 2032 - RTP Payload Format for H.261 Video Streams

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Network Working Group                                        T. Turletti
Request for Comments: 2032                                           MIT
Category: Standards Track                                     C. Huitema
                                                            October 1996

               RTP Payload Format for H.261 Video Streams

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.

Table of Contents

   1. Abstract .............................................    1
   2. Purpose of this document .............................    2
   3. Structure of the packet stream .......................    2
   3.1 Overview of the ITU-T recommendation H.261 ..........    2
   3.2 Considerations for packetization ....................    3
   4. Specification of the packetization scheme ............    4
   4.1 Usage of RTP ........................................    4
   4.2 Recommendations for operation with hardware codecs ..    6
   5. Packet loss issues ...................................    7
   5.1 Use of optional H.261-specific control packets ......    8
   5.2 H.261 control packets definition ....................    9
   5.2.1 Full INTRA-frame Request (FIR) packet .............    9
   5.2.2 Negative ACKnowledgements (NACK) packet ...........    9
   6. Security Considerations ..............................   10
    Authors' Addresses .....................................   10
    Acknowledgements .......................................   10
    References .............................................   11

1.  Abstract

   This memo describes a scheme to packetize an H.261 video stream for
   transport using the Real-time Transport Protocol, RTP, with any of
   the underlying protocols that carry RTP.

   This specification is a product of the Audio/Video Transport working
   group within the Internet Engineering Task Force.  Comments are
   solicited and should be addressed to the working group's mailing list
   at rem-conf@es.net and/or the authors.

2.  Purpose of this document

   The ITU-T recommendation H.261 [6] specifies the encodings used by
   ITU-T compliant video-conference codecs. Although these encodings
   were originally specified for fixed data rate ISDN circuits,
   experiments [3],[8] have shown that they can also be used over
   packet-switched networks such as the Internet.

   The purpose of this memo is to specify the RTP payload format for
   encapsulating H.261 video streams in RTP [1].

3.  Structure of the packet stream

3.1.  Overview of the ITU-T recommendation H.261

   The H.261 coding is organized as a hierarchy of groupings.  The video
   stream is composed of a sequence of images, or frames, which are
   themselves organized as a set of Groups of Blocks (GOB). Note that
   H.261 "pictures" are referred as "frames" in this document.  Each GOB
   holds a set of 3 lines of 11 macro blocks (MB). Each MB carries
   information on a group of 16x16 pixels: luminance information is
   specified for 4 blocks of 8x8 pixels, while chrominance information
   is given by two "red" and "blue" color difference components at a
   resolution of only 8x8 pixels.  These components and the codes
   representing their sampled values are as defined in the ITU-R
   Recommendation 601 [7].

   This grouping is used to specify information at each level of the

   -    At the frame level, one specifies information such as the
        delay from the previous frame, the image format, and
        various indicators.

   -    At the GOB level, one specifies the GOB number and the
        default quantifier that will be used for the MBs.

   -    At the MB level, one specifies which blocks are present
        and which did not change, and optionally a quantifier and
        motion vectors.

   Blocks which have changed are encoded by computing the discrete
   cosine transform (DCT) of their coefficients, which are then
   quantized and Huffman encoded (Variable Length Codes).

   The H.261 Huffman encoding includes a special "GOB start" pattern,
   composed of 15 zeroes followed by a single 1, that cannot be imitated
   by any other code words. This pattern is included at the beginning of

   each GOB header (and also at the beginning of each frame header) to
   mark the separation between two GOBs, and is in fact used as an
   indicator that the current GOB is terminated. The encoding also
   includes a stuffing pattern, composed of seven zeroes followed by
   four ones; that stuffing pattern can only be entered between the
   encoding of MBs, or just before the GOB separator.

3.2.  Considerations for packetization

   H.261 codecs designed for operation over ISDN circuits produce a bit
   stream composed of several levels of encoding specified by H.261 and
   companion recommendations.  The bits resulting from the Huffman
   encoding are arranged in 512-bit frames, containing 2 bits of
   synchronization, 492 bits of data and 18 bits of error correcting
   code.  The 512-bit frames are then interlaced with an audio stream
   and transmitted over px64 kbps circuits according to specification
   H.221 [5].

   When transmitting over the Internet, we will directly consider the
   output of the Huffman encoding. All the bits produced by the Huffman
   encoding stage will be included in the packet. We will not carry the
   512-bit frames, as protection against bit errors can be obtained by
   other means. Similarly, we will not attempt to multiplex audio and
   video signals in the same packets, as UDP and RTP provide a much more
   efficient way to achieve multiplexing.

   Directly transmitting the result of the Huffman encoding over an
   unreliable stream of UDP datagrams would, however, have poor error
   resistance characteristics. The result of the hierachical structure
   of H.261 bit stream is that one needs to receive the information
   present in the frame header to decode the GOBs, as well as the
   information present in the GOB header to decode the MBs.  Without
   precautions, this would mean that one has to receive all the packets
   that carry an image in order to properly decode its components.

   If each image could be carried in a single packet, this requirement
   would not create a problem. However, a video image or even one GOB by
   itself can sometimes be too large to fit in a single packet.
   Therefore, the MB is taken as the unit of fragmentation.  Packets
   must start and end on a MB boundary, i.e. a MB cannot be split across
   multiple packets.  Multiple MBs may be carried in a single packet
   when they will fit within the maximal packet size allowed. This
   practice is recommended to reduce the packet send rate and packet

   To allow each packet to be processed independently for efficient
   resynchronization in the presence of packet losses, some state
   information from the frame header and GOB header is carried with each

   packet to allow the MBs in that packet to be decoded.  This state
   information includes the GOB number in effect at the start of the
   packet, the macroblock address predictor (i.e. the last MBA encoded
   in the previous packet), the quantizer value in effect prior to the
   start of this packet (GQUANT, MQUANT or zero in case of a beginning
   of GOB) and the reference motion vector data (MVD) for computing the
   true MVDs contained within this packet. The bit stream cannot be
   fragmented between a GOB header and MB 1 of that GOB.

   Moreover, since the compressed MB may not fill an integer number of
   octets, the data header contains two three-bit integers, SBIT and
   EBIT, to indicate the number of unused bits in the first and last
   octets of the H.261 data, respectively.

4.  Specification of the packetization scheme

4.1.  Usage of RTP

   The H.261 information is carried as payload data within the RTP
   protocol. The following fields of the RTP header are specified:

   -    The payload type should specify H.261 payload format (see
        the companion RTP profile document RFC 1890).

   -    The RTP timestamp encodes the sampling instant of the
        first video image contained in the RTP data packet. If a
        video image occupies more than one packet, the timestamp
        will be the same on all of those packets. Packets from
        different video images must have different timestamps so
        that frames may be distinguished by the timestamp. For
        H.261 video streams, the RTP timestamp is based on a
        90kHz clock. This clock rate is a multiple of the natural
        H.261 frame rate (i.e. 30000/1001 or approx. 29.97 Hz).
        That way, for each frame time, the clock is just
        incremented by the multiple and this removes inaccuracy
        in calculating the timestamp. Furthermore, the initial
        value of the timestamp is random (unpredictable) to make
        known-plaintext attacks on encryption more difficult, see
        RTP [1]. Note that if multiple frames are encoded in a
        packet (e.g. when there are very little changes between
        two images), it is necessary to calculate display times
        for the frames after the first using the timing
        information in the H.261 frame header. This is required
        because the RTP timestamp only gives the display time of
        the first frame in the packet.

   -    The marker bit of the RTP header is set to one in the
        last packet of a video frame, and otherwise, must be

        zero. Thus, it is not necessary to wait for a following
        packet (which contains the start code that terminates the
        current frame) to detect that a new frame should be

   The H.261 data will follow the RTP header, as in:

     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
    .                                                               .
    .                          RTP header                           .
    .                                                               .
    |                          H.261  header                        |
    |                          H.261 stream ...                     .

   The H.261 header is defined as following:

     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
    |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |

   The fields in the H.261 header have the following meanings:

   Start bit position (SBIT): 3 bits
     Number of most significant bits that should be ignored
     in the first data octet.

   End bit position (EBIT): 3 bits
     Number of least significant bits that should be ignored
     in the last data octet.

   INTRA-frame encoded data (I): 1 bit
     Set to 1 if this stream contains only INTRA-frame coded
     blocks. Set to 0 if this stream may or may not contain
     INTRA-frame coded blocks. The sense of this bit may not
     change during the course of the RTP session.

   Motion Vector flag (V): 1 bit
     Set to 0 if motion vectors are not used in this stream.
     Set to 1 if motion vectors may or may not be used in
     this stream. The sense of this bit may not change during
     the course of the session.

   GOB number (GOBN): 4 bits
     Encodes the GOB number in effect at the start of the
     packet. Set to 0 if the packet begins with a GOB header.

   Macroblock address predictor (MBAP): 5 bits
     Encodes the macroblock address predictor (i.e. the last
     MBA encoded in the previous packet). This predictor ranges
     from 0-32 (to predict the valid MBAs 1-33), but because
     the bit stream cannot be fragmented between a GOB header
     and MB 1, the predictor at the start of the packet can
     never be 0. Therefore, the range is 1-32, which is biased
     by -1 to fit in 5 bits. For example, if MBAP is 0, the
     value of the MBA predictor is 1. Set to 0 if the packet
     begins with a GOB header.

   Quantizer (QUANT): 5 bits
     Quantizer value (MQUANT or GQUANT) in effect prior to the
     start of this packet. Set to 0 if the packet begins with
     a GOB header.

   Horizontal motion vector data (HMVD): 5 bits
     Reference horizontal motion vector data (MVD). Set to 0
     if V flag is 0 or if the packet begins with a GOB header,
     or when the MTYPE of the last MB encoded in the previous
     packet was not MC. HMVD is encoded as a 2's complement
     number, and `10000' corresponding to the value -16 is
     forbidden (motion vector fields range from +/-15).

   Vertical motion vector data (VMVD): 5 bits
     Reference vertical motion vector data (MVD). Set to 0 if
     V flag is 0 or if the packet begins with a GOB header, or
     when the MTYPE of the last MB encoded in the previous
     packet was not MC. VMVD is encoded as a 2's complement
     number, and `10000' corresponding to the value -16 is
     forbidden (motion vector fields range from +/-15).

   Note that the I and V flags are hint flags, i.e. they can be inferred
   from the bit stream. They are included to allow decoders to make
   optimizations that would not be possible if these hints were not
   provided before bit stream was decoded.  Therefore, these bits cannot
   change for the duration of the stream. A conformant implementation
   can always set V=1 and I=0.

4.2.  Recommendations for operation with hardware codecs

   Packetizers for hardware codecs can trivially figure out GOB
   boundaries using the GOB-start pattern included in the H.261 data.
   (Note that software encoders already know the boundaries.) The

   cheapest packetization implementation is to packetize at the GOB
   level all the GOBs that fit in a packet.  But when a GOB is too
   large, the packetizer has to parse it to do MB fragmentation. (Note
   that only the Huffman encoding must be parsed and that it is not
   necessary to fully decompress the stream, so this requires relatively
   little processing; example implementations can be found in some
   public H.261 codecs such as IVS [4] and VIC [9].) It is recommended
   that MB level fragmentation be used when feasible in order to obtain
   more efficient packetization. Using this fragmentation scheme reduces
   the output packet rate and therefore reduces the overhead.

   At the receiver, the data stream can be depacketized and directed to
   a hardware codec's input.  If the hardware decoder operates at a
   fixed bit rate, synchronization may be maintained by inserting the
   stuffing pattern between MBs (i.e., between packets) when the packet
   arrival rate is slower than the bit rate.

5.  Packet loss issues

   On the Internet, most packet losses are due to network congestion
   rather than transmission errors. Using UDP, no mechanism is available
   at the sender to know if a packet has been successfully received. It
   is up to the application, i.e.  coder and decoder, to handle the
   packet loss. Each RTP packet includes a a sequence number field which
   can be used to detect packet loss.

   H.261 uses the temporal redundancy of video to perform compression.
   This differential coding (or INTER-frame coding) is sensitive to
   packet loss. After a packet loss, parts of the image may remain
   corrupt until all corresponding MBs have been encoded in INTRA-frame
   mode (i.e. encoded independently of past frames). There are several
   ways to mitigate packet loss:

   (1)  One way is to use only INTRA-frame encoding and MB level
        conditional replenishment. That is, only MBs that change
        (beyond some threshold) are transmitted.

   (2)  Another way is to adjust the INTRA-frame encoding
        refreshment rate according to the packet loss observed by
        the receivers. The H.261 recommendation specifies that a
        MB is INTRA-frame encoded at least every 132 times it is
        transmitted. However, the INTRA-frame refreshment rate
        can be raised in order to speed the recovery when the
        measured loss rate is significant.

   (3)  The fastest way to repair a corrupted image is to request
        an INTRA-frame coded image refreshment after a packet
        loss is detected. One means to accomplish this is for the

        decoder to send to the coder a list of packets lost. The
        coder can decide to encode every MB of every GOB of the
        following video frame in INTRA-frame mode (i.e. Full
        INTRA-frame encoded), or if the coder can deduce from the
        packet sequence numbers which MBs were affected by the
        loss, it can save bandwidth by sending only those MBs in
        INTRA-frame mode. This mode is particularly efficient in
        point-to-point connection or when the number of decoders
        is low.  The next section specifies how the refresh
        function may be implemented.

   Note that the method (1) is currently implemented in the VIC
   videoconferencing software [9]. Methods (2) and (3) are currently
   implemented in the IVS videoconferencing software [4].

5.1.  Use of optional H.261-specific control packets

   This specification defines two H.261-specific RTCP control packets,
   "Full INTRA-frame Request" and "Negative Acknowledgement", described
   in the next section.  Their purpose is to speed up refreshment of the
   video in those situations where their use is feasible.  Support of
   these H.261-specific control packets by the H.261 sender is optional;
   in particular, early experiments have shown that the usage of this
   feature could have very negative effects when the number of sites is
   very large. Thus, these control packets should be used with caution.

   The H.261-specific control packets differ from normal RTCP packets in
   that they are not transmitted to the normal RTCP destination
   transport address for the RTP session (which is often a multicast
   address).  Instead, these control packets are sent directly via
   unicast from the decoder to the coder.  The destination port for
   these control packets is the same port that the coder uses as a
   source port for transmitting RTP (data) packets.  Therefore, these
   packets may be considered "reverse" control packets.

   As a consequence, these control packets may only be used when no RTP
   mixers or translators intervene in the path from the coder to the
   decoder.  If such intermediate systems do intervene, the address of
   the coder would no longer be present as the network-level source
   address in packets received by the decoder, and in fact, it might not
   be possible for the decoder to send packets directly to the coder.

   Some reliable multicast protocols use similar NACK control packets
   transmitted over the normal multicast distribution channel, but they
   typically use random delays to prevent a NACK implosion problem [2].
   The goal of such protocols is to provide reliable multicast packet
   delivery at the expense of delay, which is appropriate for
   applications such as a shared whiteboard.

   On the other hand, interactive video transmission is more sensitive
   to delay and does not require full reliability.  For video
   applications it is more effective to send the NACK control packets as
   soon as possible, i.e. as soon as a loss is detected, without adding
   any random delays. In this case, multicasting the NACK control
   packets would generate useless traffic between receivers since only
   the coder will use them.  But this method is only effective when the
   number of receivers is small. e.g. in IVS [4] the H.261 specific
   control packets are used only in point-to-point connections or in
   point-to-multipoint connections when there are less than 10
   participants in the conference.

5.2.  H.261 control packets definition

5.2.1.  Full INTRA-frame Request (FIR) packet

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |V=2|P|   MBZ   |  PT=RTCP_FIR  |           length              |
    |                              SSRC                             |

   This packet indicates that a receiver requires a full encoded image
   in order to either start decoding with an entire image or to refresh
   its image and speed the recovery after a burst of lost packets. The
   receiver requests the source to force the next image in full "INTRA-
   frame" coding mode, i.e. without using differential coding. The
   various fields are defined in the RTP specification [1]. SSRC is the
   synchronization source identifier for the sender of this packet. The
   value of the packet type (PT) identifier is the constant RTCP_FIR

5.2.2.  Negative ACKnowledgements (NACK) packet

   The format of the NACK packet is as follow:

     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
    |V=2|P|   MBZ   | PT=RTCP_NACK  |           length              |
    |                              SSRC                             |
    |              FSN              |              BLP              |

   The various fields T, P, PT, length and SSRC are defined in the RTP
   specification [1]. The value of the packet type (PT) identifier is
   the constant RTCP_NACK (193). SSRC is the synchronization source
   identifier for the sender of this packet.

   The two remaining fields have the following meanings:

   First Sequence Number (FSN): 16 bits
     Identifies the first sequence number lost.

   Bitmask of following lost packets (BLP): 16 bits
     A bit is set to 1 if the corresponding packet has been lost,
     and set to 0 otherwise. BLP is set to 0 only if no packet
     other than that being NACKed (using the FSN field) has been
     lost. BLP is set to 0x00001 if the packet corresponding to
     the FSN and the following packet have been lost, etc.

6.  Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Thierry Turletti
   INRIA - RODEO Project
   2004 route des Lucioles
   BP 93, 06902 Sophia Antipolis

   EMail: turletti@sophia.inria.fr

   Christian Huitema
   MCC 1J236B Bellcore
   445 South Street
   Morristown, NJ 07960-6438

   EMail: huitema@bellcore.com


   This memo is based on discussion within the AVT working group chaired
   by Stephen Casner. Steve McCanne, Stephen Casner, Ronan Flood, Mark
   Handley, Van Jacobson, Henning G.  Schulzrinne and John Wroclawski
   provided valuable comments.  Stephen Casner and Steve McCanne also
   helped greatly with getting this document into readable form.


   [1]  Schulzrinne, H., Casner, S., Frederick, R., and
        V. Jacobson, "RTP: A Transport Protocol for Real-Time
        Applications", RFC 1889, January 1996.

   [2]  Sridhar Pingali, Don Towsley and James F. Kurose, A
        comparison of sender-initiated and receiver-initiated
        reliable multicast protocols, IEEE GLOBECOM '94.

   [3]  Thierry Turletti, H.261 software codec for
        videoconferencing over the Internet INRIA Research Report
        no 1834, January 1993.

   [4]  Thierry Turletti, INRIA Videoconferencing tool (IVS),
        available by anonymous ftp from zenon.inria.fr in the
        "rodeo/ivs/last_version" directory. See also URL

   [5]  Frame structure for Audiovisual Services for a 64 to 1920
        kbps Channel in Audiovisual Services ITU-T (International
        Telecommunication Union - Telecommunication
        Standardisation Sector) Recommendation H.221, 1990.

   [6]  Video codec for audiovisual services at p x 64 kbit/s
        ITU-T (International Telecommunication Union -
        Telecommunication Standardisation Sector) Recommendation
        H.261, 1993.

   [7]  Digital Methods of Transmitting Television Information
        ITU-R (International Telecommunication Union -
        Radiocommunication Standardisation Sector) Recommendation
        601, 1986.

   [8]  M.A Sasse, U. Bilting, C-D Schulz, T. Turletti, Remote
        Seminars through MultiMedia Conferencing: Experiences
        from the MICE project, Proc. INET'94/JENC5, Prague, June
        1994, pp. 251/1-251/8.

   [9]  Steve MacCanne, Van Jacobson, VIC Videoconferencing tool,
        available by anonymous ftp from ee.lbl.gov in the
        "conferencing/vic" directory.


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