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Versions: 00 01 02 03 rfc2032                                           
Internet Engineering Task Force       Audio-Video Transport WG
INTERNET-DRAFT                        T. Turletti / C. Huitema
draft-ietf-avt-h261-03.txt                      MIT / Bellcore
                                               October 2, 1996
                                               Expires: 1/1/97

                      RTP payload format
                             for
                     H.261 video streams








1.  Status of this Memo

This document is an Internet-Draft.  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 any time.  It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."

To learn the current status of any Internet-Draft, please
check the "1id-abstracts.txt" listing contained in the
Internet-Drafts Shadow Directories on ftp.is.co.za (Africa),
nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West
Coast).

Distribution of this document is unlimited.

















INTERNET-DRAFT      draft-ietf-avt-h261-03  October 2, 1996


2.  Abstract

This draft 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.

3.  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].


4.  Structure of the packet stream

4.1.  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:







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-    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.

4.2.  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





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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 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.









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5.  Specification of the packetization scheme

5.1.  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
     displayed.

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









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  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.





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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.

5.2.  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





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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.

6.  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





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     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].

6.1.  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.





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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.

6.2.  H.261 control packets definition

6.2.1.  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





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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 (192).

6.2.2.  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.

7.  Security Considerations

Security concerns are not discussed in this memo.









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Addresses of Authors

Thierry Turletti
INRIA - RODEO Project
2004 route des Lucioles
BP 93, 06902 Sophia Antipolis
FRANCE
electronic mail: turletti@sophia.inria.fr

Christian Huitema
MCC 1J236B Bellcore
445 South Street
Morristown, NJ 07960-6438
electronic mail: huitema@bellcore.com

Acknowledgements

This draft 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.

References

[1]  Henning Schulzrinne, Stephen Casner, Ron Frederick, Van
     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
     <http://www.inria.fr/rodeo/ivs.html>.







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[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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Table of Contents


1 Status of this Memo ...................................    1
2 Abstract ..............................................    2
3 Purpose of this document ..............................    2
4 Structure of the packet stream ........................    2
4.1 Overview of the ITU-T recommendation H.261 ..........    2
4.2 Considerations for packetization ....................    3
5 Specification of the packetization scheme .............    5
5.1 Usage of RTP ........................................    5
5.2 Recommendations for operation with hardware codecs
     ....................................................    7
6 Packet loss issues ....................................    8
6.1 Use of optional H.261-specific control packets ......    9
6.2 H.261 control packets definition ....................   10
6.2.1 Full INTRA-frame Request (FIR) packet .............   10
6.2.2 Negative ACKnowledgements (NACK) packet ...........   11
7 Security Considerations ...............................   11
 Addresses of Authors ...................................   12
 Acknowledgements .......................................   12
 References .............................................   12




























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