rfc2032.txt

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Network Working Group                                        T. Turletti
Request for Comments: 2032                                           MIT
Category: Standards Track                                     C. Huitema
                                                                Bellcore
                                                            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 [e-mail] and/or the authors.




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RFC 2032           RTP Payload Format for H.261 Video       October 1996


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
   hierarchy:

   -    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



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RFC 2032           RTP Payload Format for H.261 Video       October 1996


   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
   overhead.

   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



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RFC 2032           RTP Payload Format for H.261 Video       October 1996


   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



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RFC 2032           RTP Payload Format for H.261 Video       October 1996


        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
        displayed.

   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.



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RFC 2032           RTP Payload Format for H.261 Video       October 1996


   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