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Network Working Group                                        M. CivanlarRequest for Comments: 2448                                       G. CashCategory: Informational                                       B. Haskell                                                      AT&T Labs-Research                                                           November 1998AT&T's Error Resilient Video Transmission TechniqueStatus 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 payloadsCivanlar, et. al.            Informational                      [Page 1]

RFC 2448                AT&T's Error Resilient             November 1998      - 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 athttp://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 usuallyCivanlar, et. al.            Informational                      [Page 2]

RFC 2448                AT&T's Error Resilient             November 1998   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 time3. 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.Civanlar, et. al.            Informational                      [Page 3]

RFC 2448                AT&T's Error Resilient             November 1998   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 compressionCivanlar, et. al.            Informational                      [Page 4]

RFC 2448                AT&T's Error Resilient             November 1998   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, URLhttp://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.Civanlar, et. al.            Informational                      [Page 5]

RFC 2448                AT&T's Error Resilient             November 1998Authors' 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.comCivanlar, et. al.            Informational                      [Page 6]

RFC 2448                AT&T's Error Resilient             November 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Civanlar, et. al.            Informational                      [Page 7]