Network Working Group M. Civanlar
Request for Comments: 2448 G. Cash
Category: Informational B. Haskell
AT&T Labs-Research
November 1998
AT&T's Error Resilient Video Transmission Technique
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document describes a set of techniques for packet loss resilient
transmission of compressed video bitstreams based on reliable
delivery of their vital information-carrying segments. The described
techniques can be used over packet networks without packet
prioritization. These techniques are related to AT&T/Lucent patents
[1, 2].
1. Introduction
It is well known that every bit in a compressed video bitstream is
not equal. Some bits belong to segments defining vital information
such as picture types, quantization values, parameter ranges, average
intensity values for image blocks, etc. When transporting compressed
video bitstreams over packet networks, packet losses from such
segments cause a much longer lasting and severe degradation on the
output of a decoder than that caused by packet losses from other
segments. We will call the vital information-carrying segments "High
Priority (HP)" segments. The rest of the bitstream consists of "Low
Priority (LP)" segments. Clearly, the video outputs resulting from
transport techniques that protect the HP segments against packet
losses are more resilient to packet losses in general.
Protection of the HP segments can be accomplished in many ways. These
include:
- redundant transmission of the HP segments as described
in [3] for MPEG RTP payloads
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- using forward error correction (FEC) techniques
- transmitting HP segments over reserved channels or using
differentiated services.
Both redundant transmission and FEC techniques increase the bandwidth
needed to transmit the compressed video bitstream. FEC techniques
increase the effectiveness of this additional bandwidth for packet
loss protection at the expense of increased processing at the
receiver and the transmitter ends and increased overall delay. Using
channel reservations or differentiated services based approaches may
be the best solutions for protecting the HP segments but, they
require network infrastructure changes.
This document outlines another set of HP segment protection
techniques based on AT&T/Lucent patents [1, 2] that can be used for
reliable video transmission over packet networks without a built-in
prioritization mechanism. These techniques use reliable transport
protocols and "out-of-band" delivery approaches. In this context, the
term "out-of-band" is used to imply information transmission means
other than those used for transmitting the main video stream. The
details of these techniques are discussed in the following sections.
An implementation of these, as applied to MPEG-2 video transmission
over IP networks, is described in [4].
The IESG/IETF take no position regarding the validity or scope of any
intellectual property right or other rights that might be claimed to
pertain to the implementation or use of the technology, or the extent
to which any license under such rights might or might not be
available. See the IETF IPR web page at http://www.ietf.org/ipr.html
for any additional information that has been forwarded to the IETF.
2. Identification of the HP segments
The classification of a part of a video bitstream as an HP segment
depends on two factors. The first one is the encoding algorithm used
in compressing the video data. It is impossible to segment a
compressed video bitstream without knowing the syntax and the
semantics of the encoding algorithm. The second factor is the
determination of a compromise between the HP segment size and the
corresponding loss resilience. As the segment size increases, so does
the loss resilience. On the other hand, it may not be feasible to
deliver large HP segments reliably.
As an example, the "data partitioning" method of the MPEG-2 standard
[5] defines the syntax and semantics for one particular way of
partitioning an MPEG-2 encoded video bitstream into HP and LP
segments. In data partitioning, the smallest useful HP segment can
be selected to contain only the header information, which is usually
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less than two percent of the video data. HP segments defined this way
contain vital information including picture type, quantization
factor, motion vector ranges, etc. without which the rest of the
bitstream is not decodable. As an alternative, the DC coefficients
(the average values) for each picture macroblock may be included in
the HP segment increasing its size to about 40% of the bitstream.
This way HP segments can be made to carry somewhat usable video
information also; however, their reliable transmission may become a
demanding task.
Since it is not possible to formulate a general technique that can be
used for identifying the HP segments in any encoded video bitstream,
we will assume that such segments are identified some way prior to
the transmission. For example, some encoders can generate HP and LP
segments separately, a stored bitstream can be in the partitioned
format, etc. Also, consistent with most of the popular coding
techniques, we assume that the HP segments (HP1, HP2, ...) are
dispersed on the entire bitstream over time as shown in Fig. 1.
+---+----------------+---+----------------------+---+-----
|HP1| LP1 |HP2| LP2 |HP3| ...
+---+----------------+---+----------------------+---+-----
Figure 1
HP segments dispersed on an encoded video bitstream over time
3. Transmission of HP data using a reliable transport protocol [1]
In this approach, one or more of the HP segments are transmitted
using a reliable transport protocol prior to starting the
transmission of the LP segments. For point-to-point applications,
TCP, for multipoint applications, an appropriate reliable multicast
protocol [6] may be used for transporting the HP segments. The number
of HP segments to be sent before starting the transmission of the LP
segments depends on the application's tolerance to the start-up
delay. Depending on the HP segment size and the path-MTU [7], one or
more HP segments can be put in each packet carrying the HP data.
HP segments can be packetized using RTP with the following
definitions for the header fields:
Payload Type: A distinct payload type number, which may be
dynamic, should be assigned to HP segments of each video payload.
M Bit: Set for packets containing HP data for key pictures.
timestamp: Uses the same format as that of the video payload.
Shows the sampling time for the video data following the first HP
segment in the packet.
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The SSRC field may be defined following the rules developed for the
transmission of layered media streams in [8]. That is:
- A single SSRC space is used for the HP segment packets and the
main video stream. Only the latter is used for SSRC allocation and
conflict resolution. When a source discovers that it has collided,
it transmits an RTCP BYE message on only the main video stream.
- A participant sends sender identification (SDES) on only the
main video stream.
Most HP segments are self-identifying and can be packed without any
additional headers. For others, techniques used for packetizing
generic payload types may be used or special payload types may be
defined.
It is possible to send the HP data along with the LP data (i.e., the
original, unpartitioned bitstream) in addition to sending the HP
segments separately. This way, the separately transmitted HP segments
are needed only when packet losses occur.
4. Out-of-band transmission of the HP information [2]
In cases where a certain sequence of HP segments is used periodically
for the entire duration of the video bitstream, this sequence may be
transmitted once before the start of video transmission using a
reliable transport protocol. The receiver can save this information
and use it to recover lost HP segments during the main video
transmission.
In this approach, the timestamps are not meaningful for the HP data
and they may not be included in the transmitted HP segment sequence.
In most cases, the synchronization between the stored HP segments and
the LP data stream can be accomplished using the key-frames because
the HP data sequence usually cover the video segment between two
key-frames (e.g. a group-of-pictures (GOP) in MPEG). If the sequence
of HP segments covers a video sequence with more than one key-frame,
some indicator, e.g. if available the M-bit may be used to indicate a
packet which carries the beginning of LP data that follows the first
stored HP segment.
5. Security Considerations
RTP packets transmitted according to the techniques outlined in this
document are subject to the security considerations discussed in the
RTP specification [9]. This implies that confidentiality of the media
streams is achieved by encryption. Because the data compression used
is applied end-to-end, encryption may be performed after compression
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so there is no conflict between the two operations. For certain
coding techniques and applications, encrypting only the HP segments
may provide sufficent confidentiality.
The described techniques do not introduce any significant additional
non-uniformity in the receiver side computational complexity for
packet processing to cause a potential denial-of-service threat.
References
[1] Glenn L. Cash, Mehmet R. Civanlar, "Method Of And Apparatus For
The Transmission Of High And Low Priority Segments Of A Video
Bitstream Over Packet Networks," United States Patent Number:
5,481,312, Jan. 2, 1996.
[2] Glenn L. Cash, Mehmet R. Civanlar, "Video Bitstream Regeneration
Using Previously Agreed To High Priority Segments," United States
Patent Number: 5,510,844, April 23, 1996.
[3] Hoffman, D., Fernando, G., Goyal, V. and M. Civanlar, "RTP
Payload Format for MPEG1/MPEG2 Video", RFC 2250, April 1997.
[4] M. R. Civanlar, G. L. Cash, "A practical system for MPEG-2 based
video-on-demand over ATM packet networks and the WWW," Signal
Processing: Image Communication, no. 8, pp. 221-227, Elsevier,
1996.
[5] ISO/IEC International Standard 13818; "Generic coding of moving
pictures and associated audio information," November 1994.
[6] Overview of Reliable Multicast Protocols Web Page, URL
http://gaia.cs.umass.edu/sigcomm_mcast/talk1.html.
[7] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[8] M. F. Speer, S. McCanne, "RTP Usage with Layered Multimedia
Streams", Work in Progress.
[9] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
January 1996.
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Authors' Addresses
M. Reha Civanlar
AT&T Labs-Research
100 Schultz Drive
Red Bank, NJ 07701
USA
EMail: civanlar@research.att.com
Glenn L. Cash
AT&T Labs-Research
100 Schultz Drive
Red Bank, NJ 07701
USA
EMail: glenn@research.att.com
Barry G. Haskell
AT&T Labs-Research
100 Schultz Drive
Red Bank, NJ 07701
USA
EMail: bgh@research.att.com
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