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rfc:rfc2190

Network Working Group C. Zhu Request for Comments: 2190 Intel Corp. Category: Standards Track September 1997

             RTP Payload Format for H.263 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.

Abstract

 This document specifies the payload format for encapsulating an H.263
 bitstream in the Real-Time Transport Protocol (RTP). Three modes are
 defined for the H.263 payload header. An RTP packet can use one of
 the three modes for H.263 video streams depending on the desired
 network packet size and H.263 encoding options employed. The shortest
 H.263 payload header (mode A) supports fragmentation at Group of
 Block (GOB) boundaries. The long H.263 payload headers (mode B and C)
 support fragmentation at Macroblock (MB) boundaries.

1. Introduction

 This document describes a scheme to packetize an H.263 video stream
 for transport using RTP [1]. H.263 video stream is defined by ITU-T
 Recommendation H.263 (referred to as H.263 in this document) [4] for
 video coding at very low data rates. RTP is defined by the Internet
 Engineering Task Force (IETF) to provide end-to-end network transport
 functions suitable for applications transmitting real-time data over
 multicast or unicast network services.

2. Definitions

 The following definitions apply in this document:
 CIF: Common Intermediate Format. For H.263, a CIF picture has 352 x
 288 pixels for luminance, and 176 x 144 pixels for chrominance.
 QCIF: Quarter CIF source format with 176 x 144 pixels for luminance
 and 88 x 72 pixels for chrominance.
 Sub-QCIF:  picture source format with 128 x 96 pixels for luminance
 and 64 x 48 pixels for chrominance.

Zhu Standards Track [Page 1] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 4CIF: Picture source format with 704 x 576 pixels for luminance and
 352 x 288 pixels for chrominance.
 16CIF: Picture source format with 1408 x 1152 pixels for luminance
 and 704 x 576 pixels for chrominance.
 GOB: For H.263, a Group of Blocks (GOB) consists of  k*16 lines,
 where k depends on the picture format (k=1 for QCIF, CIF and sub-
 QCIF; k=2 for 4CIF and k=4 for 16CIF).
 MB: A macroblock (MB) contains four blocks of luminance and the
 spatially corresponding two blocks of chrominance. Each block
 consists of 8x8 pixels. For example, there are eleven MBs in a GOB in
 QCIF format and twenty two MBs in a GOB in CIF format.

3. Design Issues for Packetizing H.263 Bitstreams

 H.263 is based on the ITU-T Recommendation H.261 [2] (referred to as
 H.261 in this document). Compared to H.261, H.263 employs similar
 techniques to reduce both temporal and spatial redundancy, but there
 are several major differences between the two algorithms that affect
 the design of packetization schemes significantly. This section
 summarizes those differences.

3.1 Optional Features of H.263

 In addition to the basic source coding algorithms, H.263 supports
 four negotiable coding options to improve performance: Advanced
 Prediction, PB-frames, Syntax-based Arithmetic Coding, and
 Unrestricted Motion Vectors. They can be used in any combination.
 Advanced Prediction(AP): One or four motion vectors can be used for
 some macroblocks in a frame. This feature makes recovery from packet
 loss difficult, because more redundant information has to be
 preserved at the beginning of a packet when fragmenting at a
 macroblock boundary.
 PB-frames:  Two frames (a P frame and a B frame) are coded into one
 bitstream with macroblocks from the two frames interleaved. From a
 packetization point of view, a MB from the P frame and a MB from the
 B frame must be treated together because each MB for the B frame is
 coded based on the corresponding MB for the P frame. A means must be
 provided to ensure proper rendering of two frames in the right order.
 Also, if part of this combined bitstream is lost, it will affect both
 frames, and possibly more.

Zhu Standards Track [Page 2] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 Syntax-based Arithmetic Coding (SAC): When the SAC option is used,
 the resultant run-value pair after quantization of Discrete Cosine
 Transform (DCT) coefficients will be coded differently from Huffman
 codes, but the macroblock hierarchy will be preserved. Since context
 variables are only synchronized after fixed length codes in the
 bitstream, any fragmentation starting at variable length codes will
 result in difficulty in decoding in the presence of packet loss
 without carrying the values of all the context variables in each
 H.263 payload header.
 The Unrestricted motion vectors feature allows large range of motion
 vectors to improve performance of motion compensation for inter-coded
 pictures. This option also affects packetization because it uses
 larger range of motion vectors than normal.
 To enable proper decoding of packets received, without dependency on
 previous packets, the use of these optional features is signaled in
 the H.263 payload header, as described in Section 5.

3.2 GOB Numbering

 In H.263, each picture is divided into groups of blocks (GOB). GOBs
 are numbered according to a vertical scan of a picture, starting with
 the top GOB and ending with the bottom GOB. In contrast, a GOB in
 H.261 is composed of three rows of 16x16 MB for QCIF, and three
 half-rows of MBs for CIF. A GOB is divided into macroblocks in H.263
 and the definition of the macroblocks are the same as in H.261.
 Each GOB in H.263 can have a fixed GOB header, but the use of the
 header is optional. If the GOB header is present, it may or may not
 start on a byte boundary. Byte alignment can be achieved by proper
 bit stuffing by the encoder, but it is not required by the H.263
 bitstream specification [4].
 In summary, a GOB in H.263 is defined and coded with finer
 granularity but with the same source format, resulting in more
 flexibility for packetization than with H.261.

3.3 Motion Vector Encoding

 Differential coding is used to code motion vectors as variable length
 codes. Unlike in H.261, where each motion vector is predicted from
 the previous MB in the GOB, H.263 employs a more flexible prediction
 scheme, where one or three candidate predictors could be used
 depending on the presence of GOB headers.

Zhu Standards Track [Page 3] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 If the GOB header is present in a GOB, motion vectors are coded with
 reference to MBs in the current GOB only. If a GOB header is not
 present in the current GOB, three motion vectors must be available to
 decode one macroblock, where two of them might come from the previous
 GOB. To correctly decode a whole inter-coded GOB, all the motion
 vectors for MBs in the previous GOB  must be available to compute the
 predictors or the predictors themselves must be present. The optional
 use of three motion vector predictors can be a major problem for a
 packetization scheme like the one defined for H.261 when packetizing
 at MB boundaries [5].
 Consider the case that a packet starts with a MB but the GOB header
 is not present. If the previous packet is lost, then all the motion
 vectors needed to predict the motion vectors for the MBs in the
 current GOB are not available. In order to decode the received MBs
 correctly, all the motion vectors for the previous GOB or the motion
 vector predictors would have to be duplicated at the beginning of the
 packet. This kind of duplication would be very expensive and
 unacceptable in terms of bandwidth overhead.
 The encoding strategy of each H.263 CODEC (CODer and DECoder)
 implementation is beyond the scope of this document, even though it
 has significant effect on visual quality in the presence of packet
 loss. However, we strongly recommend use of the GOB header for every
 GOB at the beginning of a packet to address this problem.
 Similar problems exist because of cross-GOB data dependency related
 to motion vectors, but they can not be addressed by using the GOB
 header. For 16CIF and 4CIF pictures, a GOB contains more than one row
 of MBs. If a GOB can not fit in one RTP packet, and the first packet
 containing the GOB header is lost, then MBs in the second packet can
 not compute motion vectors correctly, because they are coded relative
 to data in the lost packet. Similarly,  when OBMC (Overlapped Block
 Motion Compensation) [4] in Advanced Prediction mode is used, motion
 compensation for some MBs in one GOB could use motion vectors of MBs
 in previous GOB regardless of the presence of GOB header. When MBs
 that are used to decode received MBs are lost, those received MBs can
 not be decoded correctly. Each implementation of the method described
 in this document should take these limitations into account.

Zhu Standards Track [Page 4] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

3.4 Macroblock Address

 As specified by H.261, a macroblock address (MBA) is encoded with a
 variable length code to indicate the position of a macroblock within
 a group of MBs in H.261 bitstreams. H.263 does not code the MBA
 explicitly, but the macroblock address within a GOB is necessary to
 recover from packet loss when fragmenting at MB boundaries.
 Therefore, this information must be included in the H.263 payload
 header for modes (mode B and mode C as described in Section 5) that
 allow packetization at MB boundaries.

4. Usage of RTP

 When transmitting H.263 video streams over the Internet, the output
 of the encoder can be packetized directly. For every video frame, the
 H.263 bitstream itself is carried in the RTP payload without
 alteration, including the picture start code, the entire picture
 header, in addition to any fixed length codes and variable length
 codes.  In addition, the output of the encoder is packetized without
 adding the framing information specified by H.223 [6]. Therefore
 multiplexing audio and video signals in the same packet is not
 accommodated, as UDP and RTP provide a much more efficient way to
 achieve multiplexing.
 RTP does not guarantee a reliable and orderly data delivery service,
 so a packet might get lost in the network. To achieve a best-effort
 recovery from packet loss, the decoder needs assistance to proceed
 with decoding of other packets that are received. Thus it is
 desirable to be able to process each packet independent of other
 packets. Some frame level information is included in each packet,
 such as source format and flags for optional features to assist the
 decoder in operating correctly and efficiently in presence of packet
 loss. The flags for H.263 optional features also provide information
 about coding options used in H.263 video bitstreams that can be used
 by session management tools.
 H.263 video bitstreams will be carried as payload data within RTP
 packets. A new H.263 payload header is defined in section 5 on the
 H.263 payload header. This section defines the usage of RTP fixed
 header and H.263 video packet structure.

4.1 RTP Header Usage

 Each RTP packet starts with a fixed RTP header [1]. The following
 fields of the RTP fixed header are used for H.263 video streams:

Zhu Standards Track [Page 5] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 Marker bit (M bit): The Marker bit of the RTP fixed header is set to
 1 when the current packet carries the end of current frame; set to 0
 otherwise.
 Payload Type (PT): The Payload Type shall specify H.263 video payload
 format using the value specified by the RTP profile in use, for
 example RFC 1890 [3].
 Timestamp: The RTP timestamp encodes the sampling instant of the
 video frame contained in the RTP data packet. The RTP timestamp may
 be the same  on successive packets if a video frame occupies more
 than one packet. For H.263 video streams, the RTP timestamp is based
 on a 90 kHz clock, the same as the RTP timestamp for H.261 video
 streams [5].

4.2 Video Packet Structure

 For each RTP packet, the RTP fixed header is followed by the H.263
 payload header, which is followed by the standard H.263 compressed
 bitstream [4].
 The size of the H.263 payload header is variable depending on modes
 used as detailed in the next section. The layout of an RTP H.263
 video packet is shown as:
  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.263 payload header                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 H.263 bitstream                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5. H.263 Payload Header

 For H.263 video streams, each RTP packet carries only one H.263 video
 packet. The H.263 payload header is always present for each H.263
 video packet.
 Three formats (mode A, mode B and mode C) are defined for H.263
 payload header. In mode A, an H.263 payload header of four bytes is
 present before actual compressed H.263 video bitstream in a packet.
 It allows fragmentation at GOB boundaries. In mode B, an eight byte
 H.263 payload header is used and each packet starts at MB boundaries
 without the PB-frames option. Finally, a twelve byte H.263 payload

Zhu Standards Track [Page 6] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 header is defined in mode C to support fragmentation at MB boundaries
 for frames that are coded with the PB-frames option.
 The mode of each H.263 payload header is indicated by the F and P
 fields in the header. Packets of different modes can be intermixed.
 All client application are required to be able to receive packets in
 any mode, but decoding of mode C packets is optional because the PB-
 frames feature is optional.
 In this section, the H.263 payload format is shown as rows of 32-bit
 words. Each word is transmitted in network byte order. Whenever a
 field represents a numeric value, the most significant bit is at the
 left of the field.

5.1 Mode A

 In this mode, an H.263 bitstream will be packetized on a GOB boundary
 or a picture boundary. Mode A packets always start with the H.263
 picture start code [4] or a GOB, but do not necessarily contain
 complete GOBs. Four bytes are used for the mode A H.263 payload
 header. The H.263 payload header definition for mode A is shown as
 follows with F=0. Mode A packets are allowed to start at a GOB
 boundary even if no GOB header is present in the bitstream for the
 GOB.  However, such use is discouraged due to the dependencies it
 creates across GOB boundaries, as described in Section 3.3.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |F|P|SBIT |EBIT | SRC |I|U|S|A|R      |DBQ| TRB |    TR         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 F: 1 bit
 The flag bit indicates the mode of the payload header. F=0, mode A;
 F=1, mode B or mode C depending on P bit defined below.
 P: 1 bit
 Optional PB-frames mode as defined by the H.263 [4]. "0" implies
 normal I or P frame, "1" PB-frames. When F=1, P also indicates modes:
 mode B if P=0, mode C if P=1.
 SBIT: 3 bits
 Start bit position specifies number of most significant bits that
 shall be ignored in the first data byte.
 EBIT: 3 bits
 End bit position specifies number of least significant bits that
 shall be ignored in the last data byte.

Zhu Standards Track [Page 7] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 SRC : 3 bits
 Source format, bit 6,7 and 8 in PTYPE defined by H.263 [4], specifies
 the resolution of the current picture.
 I:  1 bit.
 Picture coding type, bit 9 in PTYPE defined by H.263[4], "0" is
 intra-coded, "1" is inter-coded.
 U: 1 bit
 Set to 1 if the Unrestricted Motion Vector option, bit 10 in PTYPE
 defined by H.263 [4] was set to 1 in the current picture header,
 otherwise 0.
 S: 1 bit
 Set to 1 if the Syntax-based Arithmetic Coding option, bit 11 in
 PTYPE defined by the H.263 [4] was set to 1 for current picture
 header, otherwise 0.
 A: 1 bit
 Set to 1 if the Advanced Prediction option, bit 12 in PTYPE defined
 by H.263 [4] was set to 1 for current picutre header, otherwise 0.
 R: 4 bits
 Reserved, must be set to zero.
 DBQ: 2 bits
 Differential quantization parameter used to calculate quantizer for
 the B frame based on quantizer for the P frame, when PB-frames option
 is used. The value should be the same as DBQUANT defined by H.263
 [4].  Set to zero if PB-frames option is not used.
 TRB: 3 bits
 Temporal Reference for the B frame as defined by H.263 [4]. Set to
 zero if PB-frames option is not used.
 TR: 8 bits
 Temporal Reference for the P frame as defined by H.263 [4]. Set to
 zero if the PB-frames option is not used.

5.2 Mode B

 In this mode, an H.263 bitstream can be fragmented at MB boundaries.
 Whenever a packet starts at a MB boundary, this mode shall be used
 without PB-frames option. Mode B packets are intended for a GOB whose
 size is larger than the maximum packet size allowed in the underlying
 protocol, thus making it impossible to fit one or more complete GOBs
 in a packet. This mode can only be used without the PB-frames option.
 Mode C as defined in the next section can be used to fragment H.263

Zhu Standards Track [Page 8] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 bitstreams at MB boundaries with the PB-frames option.  The H.263
 payload header definition for mode B is shown as follows with F=1 and
 P=0:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |F|P|SBIT |EBIT | SRC | QUANT   |  GOBN   |   MBA           |R  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |I|U|S|A| HMV1        | VMV1        | HMV2        | VMV2        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The following fields are defined the same as in mode A: F, P, SBIT,
 EBIT, SRC, I, U, S and A. Other fields are defined as follows:
 QUANT: 5 bits
 Quantization value for the first MB coded at the starting of the
 packet.  Set to 0 if the packet begins with a GOB header. This is the
 equivalent of GQUANT defined by the H.263 [4].
 GOBN: 5 bits
 GOB number in effect at the start of the packet. GOB number is
 specified differently for different resolutions. See H.263 [4] for
 details.
 MBA: 9 bits
 The address within the GOB of the first MB in the packet, counting
 from zero in scan order. For example, the third MB in any GOB is
 given MBA = 2.
 HMV1, VMV1: 7 bits each.
 Horizontal and vertical motion vector predictors for the first MB in
 this packet [4]. When four motion vectors are used for current MB
 with advanced prediction option, these would be the motion vector
 predictors for block number 1 in the MB. Each 7 bits field encodes a
 motion vector predictor in half pixel resolution as a 2's complement
 number.
 HMV2, VMV2: 7 bits each.
 Horizontal and vertical motion vector predictors for block number 3
 in the first MB in this packet when four motion vectors are used with
 the advanced prediction option. This is needed because block number 3
 in the MB needs different motion vector predictors from other blocks
 in the MB. These two fields are not used when the MB only has one
 motion vector. See the H.263 [4] for block organization in a
 macroblock.  Each 7 bits field encodes a motion vector predictor in
 half pixel resolution as a 2's complement number.

Zhu Standards Track [Page 9] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 R : 2 bits
 Reserved, must be set to zero.

5.3 Mode C

 In this mode, an H.263 bitstream is fragmented at MB boundaries of P
 frames with the PB-frames option. It is intended for those GOBs whose
 sizes are larger than the maximum packet size allowed in the
 underlying protocol when PB-frames option is used. The H.263 payload
 header definition for mode C is shown as follows with F=1 and P=1:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |F|P|SBIT |EBIT | SRC | QUANT   |  GOBN   |   MBA           |R  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |I|U|S|A| HMV1        | VMV1        | HMV2        | VMV2        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | RR                                  |DBQ| TRB |    TR         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The following fields are defined the same as in mode B: F, P, SBIT,
 EBIT, SRC, QUANT, GOBN, MBA, R, I, U, S, A, HMV1, VMV1, HMV2, VMV2.
 The rest of the fields (TR, DBQ, TRB) are defined the same as in mode
 A, except field RR. The RR field takes 19 bits, and is currently
 reserved.  It must be set to zero.

5.4 Selection of Modes for the H.263 Payload Header

 Packets carrying H.263 video streams with different modes can be
 intermixed. The modes shall be selected carefully based on network
 packet size, H.263 coding options and underlying network protocols.
 More specifically, mode A shall be used for packets starting with a
 GOB or the H.263 picture start code [4], and mode B or C shall be
 used whenever a packet has to start at a MB boundary. Mode B or C are
 necessary for those GOBs with sizes larger than network packet size.
 We strongly recommend use of mode A whenever possible. The major
 advantage of mode A over mode B and C is its simplicity. The H.263
 payload header is smaller than mode B and C. Transmission overhead is
 reduced and the savings may be very significant when working with
 very low data rates or relatively small packet sizes.
 Another advantage of mode A is that it simplifies error recovery in
 the presence of packet loss. The internal state of a decoder can be
 recovered at GOB boundaries instead of having to synchronize with MBs
 as in mode B and C. The GOB headers and the picture start code are
 easy to identify,  and their presence will normally cause a H.263

Zhu Standards Track [Page 10] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

 decoder to re-synchronize its internal states.
 Finally, we would like to stress that recovery from packet loss
 depends on a decoder's ability to use the information provided in the
 H.263 payload header within RTP packets.

6. Limitations

 The packetization method described in this document applies to the
 1996 version of H.263. It may not be applicable to bitstreams with
 features added after that.

Security Considerations

 Security issues are addressed by RTP [1].  This memo does not bring
 up any additional security issues.

7. Acknowledgments

 The author would like to thank the following people for their
 valuable comments: Linda S. Cline, Christian Maciocco, Mojy
 Mirashrafi, Phillip Lantz, Steve Casner, Gary Sullivan, and Sassan
 Pejhan.

8. References

[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,

  "RTP: A Transport Protocol for Real-Time Applications", RFC 1889,
  January 1996.

[2] International Telecommunication Union.

  Video Codec for Audiovisual Services at  p x 64 kbits/s,
  ITU-T Recommendation H.261, 1993.

[3] Schulzrinne, H.,

  "RTP Profile for Audio and Video Conference with Minimal
  Control", RFC 1890,
  January 1996.

[4] International Telecommunication Union.

  Video Coding for Low Bitrate Communication, ITU-T Recommendation
  H.263, 1996

[5] Turletti, T., and C. Huitema,

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

Zhu Standards Track [Page 11] RFC 2190 RTP Payload Format for H.263 Video Streams September 1997

[6] International Telecommunication Union.

  Multiplexing Protocol for Low Bitrate Multimedia Communication,
  ITU-T Recommendation H.223, 1995.

7. Author's Address

 C. "Chad"  Zhu
 Mail Stop: JF3-202
 Intel Corporation
 2111 N.E. 25th Avenue
 Hillsboro, OR 97124
 USA
 EMail: czhu@ibeam.intel.com
 Phone: (503) 264-6008
 Fax: (503) 264-1805

Zhu Standards Track [Page 12]

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