Network Working Group R. Even
Request for Comments: 4587 Polycom
Obsoletes: 2032 August 2006
Category: Standards Track
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.
Copyright (C) The Internet Society (2006).
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.
The memo also describes the syntax and semantics of the Session
Description Protocol (SDP) parameters needed to support the H.261
video codec. A media type registration is included for this payload
This specification obsoletes RFC 2032.
Table of Contents
1. Introduction ....................................................32. Terminology .....................................................33. Structure of the Packet Stream ..................................33.1. Overview of the ITU-T Recommendation H.261 .................33.2. Considerations for Packetization ...........................44. Specification of the Packetization Scheme .......................54.1. Usage of RTP ...............................................54.2. Recommendations for Operation with Hardware Codecs .........85. Packet Loss Issues ..............................................96. IANA Considerations ............................................106.1. Media Type Registrations ..................................106.1.1. Registration of MIME Media Type video/H261 .........106.2. SDP Parameters ............................................126.2.1. Usage with the SDP Offer Answer Model ..............127. Backward Compatibility to RFC 2032 .............................137.1. Optional H.261-Specific Control Packets ...................137.2. New SDP Optional Parameters ...............................138. Security Considerations ........................................149. Acknowledgements ...............................................1410. Changes from RFC 2032 .........................................1411. References ....................................................1511.1. Normative References .....................................1511.2. Informative References ...................................15
ITU-T Recommendation H.261 [H261] specifies the encoding used by
ITU-T-compliant video-conference codecs. Although this encoding was
originally specified for fixed-data rate Integrated Services Digital
Network (ISDN) circuits, experiments [INRIA], [MICE] have shown that
they can also be used over packet-switched networks, such as the
The purpose of this memo is to specify the RTP payload format for
encapsulating H.261 video streams in RTP [RFC3550].
This document obsoletes RFC 2032 and updates the "video/h261" media
type that was registered in RFC 3555.
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 [RFC2119] and
indicate requirement levels for compliant RTP implementations.
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 to 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, whereas 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 ITU-R
Recommendation 601 [BT601].
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 that 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,
which is a word of 16 bits, 0000 0000 0000 0001. 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 zero bits followed by four bits with a value of one; 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 px 64 kbps circuits according to specification
For 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
suitable 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 hierarchical structure
of the 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 decode its components properly.
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 an MB boundary; that is, an 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
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 macroblock
address (MBA) encoded in the previous packet), the quantizer value in
effect prior to the start of this packet (GQUANT, MQUANT, or zero in
the 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
Moreover, since the compressed MB may not fill an integer number of
octets, the data header contains two 3-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
Each RTP packet starts with a fixed RTP header, as explained in RFC
3550 [RFC3550]. The following fields of the RTP fixed header used
for H.261 video streams are further emphasized here:
- Payload type. The assignment of an RTP payload type for this
packet format is outside the scope of this document and will not be
specified here. It is expected that the RTP profile for a
particular class of applications will assign a payload type for
this encoding, or, if that is not done, then a payload type in the
dynamic range shall be chosen.
- 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 SHALL be the same on all of
those packets. Packets from different video images MUST have a
different timestamp so that frames may be distinguished by the
timestamp. For H.261 video streams, the RTP timestamp is based on
a 90-kHz clock. This clock rate is a multiple of the natural H.261
frame rate (i.e., 30000/1001 or approximately 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 MUST be random
(unpredictable) to make known-plaintext attacks on encryption more
difficult; see RTP [RFC3550]. Note that if multiple frames are
encoded in a packet (e.g., when there are very few 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 MUST be set to one in the last
packet of a video frame; otherwise, it 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 SHALL follow the RTP header, as in the 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
. RTP header .
| H.261 header |
| H.261 stream ... .
The H.261 header is defined as follows:
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 meaning of this bit should not be changed 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
meaning of this bit should not be changed during the course of the
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 shall not 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 motion
compensation (MC). HMVD is encoded as a 2s 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 2s complement number, and '10000' corresponding to
the value -16 SHALL not be used (motion vector fields range from
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 the bit stream was decoded. Therefore, these bits
cannot change for the duration of the stream. A conforming
implementation can always set V=1 and I=0.
The H.261 stream SHALL be used without BCH error correction and
without error correction framing.
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 decompress the stream fully, so this requires relatively
little processing; examples of implementations can be found in some
public H.261 codecs, such as IVS [IVS] and VIC [VIC].) 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 to transmission errors. Using UDP, no mechanism is
available at the sender to know whether 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
sequence number field that 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 an MB be 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). 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 H.261-specific control packets FIR and NACK, as described in RFC
2032, SHALL NOT be used to request image refreshment. Old
implementations are encouraged to use the methods described in this
section. Image refreshment may be needed due to packet loss or due
to application requirements. An example of application requirement
may be the change of the speaker in a voice-activated multipoint
video switching conference. There are two methods that can be used
for requesting image refreshment. The first method is by using the
Extended RTP Profile for RTCP-based Feedback and sending RTCP generic
control packets, as described in RFC 4585 [RFC4585]. The second
method is by using application protocol-specific commands, such as
H.245 [ITU.H245] FastUpdateRequest.
6. IANA Considerations
This section updates the H.261 media type described in RFC 3555
This section specifies optional parameters that MAY be used to select
optional features of the payload format. The parameters are
specified here as part of the MIME subtype registration for the ITU-T
H.261 codec. A mapping of the parameters into the Session
Description Protocol (SDP) [RFC4566] is also provided for those
applications that use SDP. Multiple parameters SHOULD be expressed
as a media type string, in the form of a semicolon-separated list of
6.1. Media Type Registrations
This section describes the media types and names associated with this
payload format. The section updates the previous registered version
in RFC 3555 [RFC3555]. This registration uses the template defined
in RFC 4288 [RFC4288]
6.1.1. Registration of MIME Media Type video/H261
MIME media type name: video
MIME subtype name: H261
Required parameters: None
CIF. This parameter has the format of parameter=value. It
describes the maximum supported frame rate for CIF resolution.
Permissible values are integer values 1 to 4, and it means that
the maximum rate is 29.97/specified value.
QCIF. This parameter has the format of parameter=value. It
describes the maximum supported frame rate for QCIF resolution.
Permissible values are integer values 1 to 4, and it means that
the maximum rate is 29.97/specified value.
D. Specifies support for still image graphics according to H.261,
annex D. If supported, the parameter value SHALL be "1". If not
supported, the parameter SHOULD NOT be used or SHALL have the
This media type is framed and binary, see Section 4.8 in
Security considerations: See Section 8
These are receiver options; current implementations will not send
any optional parameters in their SDP. They will ignore the
optional parameters and will encode the H.261 stream without annex
D. Most decoders support at least QCIF resolutions, and they are
expected to be available in almost every H.261-based video
Published specification: RFC 4587
Applications that use this media type:
Audio and video streaming and conferencing applications.
Additional information: None
Person and email address to contact for further information:
Roni Even: firstname.lastname@example.org
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing and thus is only defined
for transfer via RTP [RFC3550]. Transport within other framing
protocols is not defined at this time.
Author: Roni Even
IETF Audio/Video Transport working group, delegated from the IESG.
6.2. SDP Parameters
The MIME media type video/H261 string is mapped to fields in the
Session Description Protocol (SDP) as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be H261 (the
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The optional parameters "CIF", "QCIF", and "D", if any, SHALL be
included in the "a=fmtp" line of SDP. These parameters are
expressed as a MIME media type string, in the form of as a
semicolon-separated list of parameters
6.2.1. Usage with the SDP Offer Answer Model
When H.261 is offered over RTP using SDP in an Offer/Answer model
[RFC3264] the following considerations are necessary.
Codec options: (D) This option MUST NOT appear unless the sender of
this SDP message is able to decode this option. This option SHALL be
considered a receiver's capability even when it is sent in a
Picture sizes and MPI:
Supported picture sizes and their corresponding minimum picture
interval (MPI) information for H.261 can be combined. All picture
sizes may be advertised to the other party, or only a subset of it.
Using the recvonly or sendrev direction attribute, a terminal SHOULD
announce those picture sizes (with their MPIs) that it is willing to
receive. For example, CIF=2 means that receiver can receive a CIF
picture and that the frame rate SHALL be less then 15 frames per
When the direction attribute is sendonly, the parameters describe the
capabilities of the stream that the sender can produce.
Implementations following this specification SHALL specify at least
one supported picture size.
If the receiver does not specify the picture size/MPI parameter, then
it is safe to assume that it is an implementation that follows RFC
2032. In that case, it is RECOMMENDED to assume that such a receiver
is able to support reception of QCIF resolution with MPI=1.
Parameters offered first are the most preferred picture mode to be
An example of media representation in SDP is as follows CIF at 15
frames per second, QCIF at 30 frames per second and annex D
m=video 49170/2 RTP/AVP 31
This means that the sender of this message can decode an H.261 bit
stream with the following options and parameters: preferred
resolution is CIF (its MPI is 2), but if that is not possible, then
QCIF size is also supported. Still image using annex D MAY be used.
7. Backward Compatibility to RFC 2032
The current document replaces RFC 2032. This section will address
the major backward compatibility issues.
7.1. Optional H.261-Specific Control Packets
RFC 2032 defined two H.261-specific RTCP control packets, "Full
INTRA-frame Request" and "Negative Acknowledgement". Support of
these control packets was optional. 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
encoder. The destination port for these control packets is the same
port that the encoder uses as a source port for transmitting RTP
(data) packets. Therefore, these packets may be considered "reverse"
control packets. This memo suggests generic methods to address the
same requirement. The authors of the documents are not aware of
products that support these control packets. Since these are
optional features, new implementations SHALL ignore them, and they
SHALL NOT be used by new implementations.
7.2. New SDP Optional Parameters
The document adds new optional parameters to the H261 payload type.
Since these are optional parameters, we expect that old
implementations ignore these parameters, whereas new implementations
that receive the H261 payload type capabilities with no parameters
will assume that it is an old implementation and will send H.261 at
QCIF resolution and 30 frames per second.
8. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550], and in any appropriate RTP profile (e.g.,
[RFC3551]). This implies that confidentiality of the media streams
is achieved by encryption. SRTP [RFC3711] may be used to provide
both encryption and integrity protection of RTP flow. Because the
data compression used with this payload format is applied end to end,
encryption will be performed after compression, so there is no
conflict between the two operations.
A potential denial-of-service threat exists for data encoding using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream that are complex to decode and cause the receiver to
be overloaded. The usage of authentication of at least the RTP
packet is RECOMMENDED. H.261 is vulnerable to such attacks because
it is possible for an attacker to generate RTP packets containing
frames that affect the decoding process of future frames. Therefore,
the usage of data origin authentication and data integrity protection
of at least the RTP packet is RECOMMENDED; for example, with SRTP.
Note that the appropriate mechanism to ensure confidentiality and
integrity of RTP packets and their payloads is very dependent on the
application and on the transport and signaling protocols employed.
Thus, although SRTP is given as an example above, other possible
This is to acknowledge the authors of RFC 2032, Thierry Turletti and
Christian Huitema. Special thanks for the work done by Petri
Koskelainen from Nokia and Nermeen Ismail from Cisco, who helped with
drafting the text for the new MIME types.
10. Changes from RFC 2032
The changes from the RFC 2032 are:
1. The H.261 MIME type is now in the payload specification.
2. Added optional parameters to the H.261 MIME type
3. Deprecated the H.261 specific control packets
4. Editorial changes to be in line with RFC editing procedures
11.1. Normative References
[H261] International Telecommunications Union, "Video codec for
audiovisual services at px 64 kbit/s", ITU Recommendation
H.261, March 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65,
RFC 3551, July 2003.
[RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", RFC 3555, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
11.2. Informative References
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288,
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.
Rey, "Extended RTP Profile for Real-time Transport
Control Protocol (RTCP)-based Feedback (RTP/AVPF)", RFC
4585, July 2006.
[ITU.H245] International Telecommunications Union, "CONTROL PROTOCOL
FOR MULTIMEDIA COMMUNICATION", ITU Recommendation H.245,
[INRIA] Turletti, T., "H.261 software codec for videoconferencing
over the Internet", INRIA Research Report 1834,
[IVS] Turletti, T., "INRIA Videoconferencing tool (IVS)",
available by anonymous ftp from zenon.inria.fr in the
"rodeo/ivs/last_version" directory. See also URL
[BT601] International Telecommunications Union, "Studio encoding
parameters of digital television for standard 4:3 and
wide-screen 16:9 aspect ratios", ITU-R Recommendation
BT.601-5, October 1995.
[MICE] Sasse, MA., Bilting, U., Schultz, CD., and T. Turletti,
"Remote Seminars through MultiMedia Conferencing:
Experiences from the MICE project", Proc. INET'94/JENC5,
Prague pp. 251/1-251/8, June 1994.
[VIC] MacCanne, S., "VIC Videoconferencing tool, available by
anonymous ftp from ee.lbl.gov in the "conferencing/vic"
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[H221] International Telecommunications Union, "Frame structure
for a 64 to 1920 kbit/s channel in audiovisual
teleservices", ITU Recommendation H.221, May 1999.
94 Derech Em Hamoshavot
Petach Tikva 49130
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