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

      - 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

   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.

   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

   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.

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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