rfc2429.txt
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Network Working Group
Request for Comments: 2429 C. Bormann
Category: Standards Track Univ. Bremen
L. Cline
G. Deisher
T. Gardos
C. Maciocco
D. Newell
Intel
J. Ott
Univ. Bremen
G. Sullivan
PictureTel
S. Wenger
TU Berlin
C. Zhu
Intel
October 1998
RTP Payload Format for the 1998 Version of
ITU-T Rec. H.263 Video (H.263+)
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 Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
1. Introduction
This document specifies an RTP payload header format applicable to
the transmission of video streams generated based on the 1998 version
of ITU-T Recommendation H.263 [4]. Because the 1998 version of H.263
is a superset of the 1996 syntax, this format can also be used with
the 1996 version of H.263 [3], and is recommended for this use by new
implementations. This format does not replace RFC 2190, which
continues to be used by existing implementations, and may be required
for backward compatibility in new implementations. Implementations
using the new features of the 1998 version of H.263 shall use the
format described in this document.
Bormann, et. al. Standards Track [Page 1]
RFC 2429 H.263+ October 1998
The 1998 version of ITU-T Recommendation H.263 added numerous coding
options to improve codec performance over the 1996 version. The 1998
version is referred to as H.263+ in this document. Among the new
options, the ones with the biggest impact on the RTP payload
specification and the error resilience of the video content are the
slice structured mode, the independent segment decoding mode, the
reference picture selection mode, and the scalability mode. This
section summarizes the impact of these new coding options on
packetization. Refer to [4] for more information on coding options.
The slice structured mode was added to H.263+ for three purposes: to
provide enhanced error resilience capability, to make the bitstream
more amenable to use with an underlying packet transport such as RTP,
and to minimize video delay. The slice structured mode supports
fragmentation at macroblock boundaries.
With the independent segment decoding (ISD) option, a video picture
frame is broken into segments and encoded in such a way that each
segment is independently decodable. Utilizing ISD in a lossy network
environment helps to prevent the propagation of errors from one
segment of the picture to others.
The reference picture selection mode allows the use of an older
reference picture rather than the one immediately preceding the
current picture. Usually, the last transmitted frame is implicitly
used as the reference picture for inter-frame prediction. If the
reference picture selection mode is used, the data stream carries
information on what reference frame should be used, indicated by the
temporal reference as an ID for that reference frame. The reference
picture selection mode can be used with or without a back channel,
which provides information to the encoder about the internal status
of the decoder. However, no special provision is made herein for
carrying back channel information.
H.263+ also includes bitstream scalability as an optional coding
mode. Three kinds of scalability are defined: temporal, signal-to-
noise ratio (SNR), and spatial scalability. Temporal scalability is
achieved via the disposable nature of bi-directionally predicted
frames, or B-frames. (A low-delay form of temporal scalability known
as P-picture temporal scalability can also be achieved by using the
reference picture selection mode described in the previous
paragraph.) SNR scalability permits refinement of encoded video
frames, thereby improving the quality (or SNR). Spatial scalability
is similar to SNR scalability except the refinement layer is twice
the size of the base layer in the horizontal dimension, vertical
dimension, or both.
Bormann, et. al. Standards Track [Page 2]
RFC 2429 H.263+ October 1998
2. Usage of RTP
When transmitting H.263+ video streams over the Internet, the output
of the encoder can be packetized directly. All the bits resulting
from the bitstream including the fixed length codes and variable
length codes will be included in the packet, with the only exception
being that when the payload of a packet begins with a Picture, GOB,
Slice, EOS, or EOSBS start code, the first two (all-zero) bytes of
the start code are removed and replaced by setting an indicator bit
in the payload header.
For H.263+ bitstreams coded with temporal, spatial, or SNR
scalability, each layer may be transported to a different network
address. More specifically, each layer may use a unique IP address
and port number combination. The temporal relations between layers
shall be expressed using the RTP timestamp so that they can be
synchronized at the receiving ends in multicast or unicast
applications.
The H.263+ video stream will be carried as payload data within RTP
packets. A new H.263+ payload header is defined in section 4. This
section defines the usage of the RTP fixed header and H.263+ video
packet structure.
2.1 RTP Header Usage
Each RTP packet starts with a fixed RTP header. The following fields
of the RTP fixed header are used for H.263+ video streams:
Marker bit (M bit): The Marker bit of the RTP header is set to 1 when
the current packet carries the end of current frame, and is 0
otherwise.
Payload Type (PT): The Payload Type shall specify the H.263+ video
payload format.
Timestamp: The RTP Timestamp encodes the sampling instance of the
first video frame data contained in the RTP data packet. The RTP
timestamp shall be the same on successive packets if a video frame
occupies more than one packet. In a multilayer scenario, all
pictures corresponding to the same temporal reference should use the
same timestamp. If temporal scalability is used (if B-frames are
present), the timestamp may not be monotonically increasing in the
RTP stream. If B-frames are transmitted on a separate layer and
address, they must be synchronized properly with the reference
frames. Refer to the 1998 ITU-T Recommendation H.263 [4] for
information on required transmission order to a decoder. For an
H.263+ video stream, the RTP timestamp is based on a 90 kHz clock,
Bormann, et. al. Standards Track [Page 3]
RFC 2429 H.263+ October 1998
the same as that of the RTP payload for H.261 stream [5]. Since both
the H.263+ data and the RTP header contain time information, it is
required that those timing information run synchronously. That is,
both the RTP timestamp and the temporal reference (TR in the picture
header of H.263) should carry the same relative timing information.
Any H.263+ picture clock frequency can be expressed as
1800000/(cd*cf) source pictures per second, in which cd is an integer
from 1 to 127 and cf is either 1000 or 1001. Using the 90 kHz clock
of the RTP timestamp, the time increment between each coded H.263+
picture should therefore be a integer multiple of (cd*cf)/20. This
will always be an integer for any "reasonable" picture clock
frequency (for example, it is 3003 for 29.97 Hz NTSC, 3600 for 25 Hz
PAL, 3750 for 24 Hz film, and 1500, 1250 and 1200 for the computer
display update rates of 60, 72 and 75 Hz, respectively). For RTP
packetization of hypothetical H.263+ bitstreams using "unreasonable"
custom picture clock frequencies, mathematical rounding could become
necessary for generating the RTP timestamps.
2.2 Video Packet Structure
A section of an H.263+ compressed bitstream is carried as a payload
within each RTP packet. For each RTP packet, the RTP header is
followed by an H.263+ payload header, which is followed by a number
of bytes of a standard H.263+ compressed bitstream. The size of the
H.263+ payload header is variable depending on the payload involved
as detailed in the section 4. The layout of the 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+ Compressed Data Stream ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Any H.263+ start codes can be byte aligned by an encoder by using the
stuffing mechanisms of H.263+. As specified in H.263+, picture,
slice, and EOSBS starts codes shall always be byte aligned, and GOB
and EOS start codes may be byte aligned. For packetization purposes,
GOB start codes should be byte aligned; however, since this is not
required in H.263+, there may be some cases where GOB start codes are
not aligned, such as when transmitting existing content, or when
using H.263 encoders that do not support GOB start code alignment.
In this case, follow-on packets (see section 5.2) should be used for
packetization.
Bormann, et. al. Standards Track [Page 4]
RFC 2429 H.263+ October 1998
All H.263+ start codes (Picture, GOB, Slice, EOS, and EOSBS) begin
with 16 zero-valued bits. If a start code is byte aligned and it
occurs at the beginning of a packet, these two bytes shall be removed
from the H.263+ compressed data stream in the packetization process
and shall instead be represented by setting a bit (the P bit) in the
payload header.
3. Design Considerations
The goals of this payload format are to specify an efficient way of
encapsulating an H.263+ standard compliant bitstream and to enhance
the resiliency towards packet losses. Due to the large number of
different possible coding schemes in H.263+, a copy of the picture
header with configuration information is inserted into the payload
header when appropriate. The use of that copy of the picture header
along with the payload data can allow decoding of a received packet
even in such cases in which another packet containing the original
picture header becomes lost.
There are a few assumptions and constraints associated with this
H.263+ payload header design. The purpose of this section is to
point out various design issues and also to discuss several coding
options provided by H.263+ that may impact the performance of
network-based H.263+ video.
o The optional slice structured mode described in Annex K of H.263+
[4] enables more flexibility for packetization. Similar to a
picture segment that begins with a GOB header, the motion vector
predictors in a slice are restricted to reside within its
boundaries. However, slices provide much greater freedom in the
selection of the size and shape of the area which is represented as
a distinct decodable region. In particular, slices can have a size
which is dynamically selected to allow the data for each slice to
fit into a chosen packet size. Slices can also be chosen to have a
rectangular shape which is conducive for minimizing the impact of
errors and packet losses on motion compensated prediction. For
these reasons, the use of the slice structured mode is strongly
recommended for any applications used in environments where
significant packet loss occurs.
o In non-rectangular slice structured mode, only complete slices
should be included in a packet. In other words, slices should not
be fragmented across packet boundaries. The only reasonable need
for a slice to be fragmented across packet boundaries is when the
encoder which generated the H.263+ data stream could not be
influenced by an awareness of the packetization process (such as
when sending H.263+ data through a network other than the one to
which the encoder is attached, as in network gateway
Bormann, et. al. Standards Track [Page 5]
RFC 2429 H.263+ October 1998
implementations). Optimally, each packet will contain only one
slice.
o The independent segment decoding (ISD) described in Annex R of [4]
prevents any data dependency across slice or GOB boundaries in the
reference picture. It can be utilized to further improve
resiliency in high loss conditions.
o If ISD is used in conjunction with the slice structure, the
rectangular slice submode shall be enabled and the dimensions and
quantity of the slices present in a frame shall remain the same
between each two intra-coded frames (I-frames), as required in
H.263+. The individual ISD segments may also be entirely intra
coded from time to time to realize quick error recovery without
adding the latency time associated with sending complete INTRA-
pictures.
o When the slice structure is not applied, the insertion of a
(preferably byte-aligned) GOB header can be used to provide resync
boundaries in the bitstream, as the presence of a GOB header
eliminates the dependency of motion vector prediction across GOB
boundaries. These resync boundaries provide natural locations for
packet payload boundaries.
o H.263+ allows picture headers to be sent in an abbreviated form in
order to prevent repetition of overhead information that does not
change from picture to picture. For resiliency, sending a complete
picture header for every frame is often advisable. This means that
(especially in cases with high packet loss probability in which
picture header contents are not expected to be highly predictable),
the sender may find it advisable to always set the subfield UFEP in
PLUSPTYPE to '001' in the H.263+ video bitstream. (See [4] for the
definition of the UFEP and PLUSPTYPE fields).
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