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 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
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

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

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 RFC1890).

- 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:

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 IV 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=2P 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
(192).

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=2P 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
FRANCE

EMail: turletti@sophia.inria.fr

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

EMail: huitema@bellcore.com

Acknowledgements

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.

References

[1] Schulzrinne, H., Casner, S., Frederick, R., and
V. Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC1889, 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>.

[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.