INTERNET-DRAFT Ladan Gharai <draft-ietf-avt-smpte292-video-00.txt> USC/ISI Gary Goncher Tektronix David Richardson University of Washington Allison Mankin USC/ISI July 13, 2000 RTP Payload Format for SMPTE 292M <draft-ietf-avt-smpte292-video-00.txt> Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document specifies a packetization scheme for encapsulating uncompressed HDTV as defined by SMPTE 292M [1] into a payload format for the Real-Time Transport Protocol (RTP). The RTP packet counter is extended to 26 bits to accommodate SMPTE 292M's 1.485Gb/s data rate, and additional positioning information is added to the payload header. draft-ietf-avt-smpte292-video-00.txt [Page 1]
INTERNET-DRAFT July 13, 2000 1. Introduction The serial digital interface, SMPTE 292M, defines a universal medium of interchange for uncompressed HDTV between various types of video equipment (camera's, encoders, VTRs, ...) at data rates of 1.485Gb/s (and 1.485/1.001 Gb/s). Source data are 10-bit words, sampled at 4:2:2. In this draft we specify how to transfer SMPTE 292M over RTP. 2. A Note on Compressed HDTV over RTP HDTV is compressed using a subset of MPEG-2 [3] as its compression scheme. This subset is fully described in document A/53 [4] of the Advanced Television Standards Committee. The ATSC has also adopted the MPEG-2 transport system (ISO/IEC 13818-1) [5]. Therefore: 1. The HDTV transport system is a compatible subset of the MPEG-2 transport system. Section 2 of RFC 2250 [7] describes the RTP payload for MPEG-2's transport system, where multiple fixed length (188 bytes) MTS packets are aggregated into a single RTP packet. 2. Compressed HDTV is a subset of MPEG-2 MP@HL with some additional restrictions. Section 3 of RFC 2250 describes a packetization scheme for MPEG-2 elementary streams. The additional restrictions of HDTV do not have any implications for RTP packetization. 3. Payload Design Each video frame of SMPTE292M in packetized into a number of variable size RTP packets. All active, vertical blanking and timing information is packetized. The end of a frame is marked by the M bit in the RTP header. A single packet may contain data for two consecutive scan lines. The SMPTE292M decoder uses the sync info in the scan lines to detect the start of scan lines. A single packet may also contain information from adjacent scan lines in two consecutive frames, or by agreement between sender and receiver the last packet of a video frame may be padded and the the new frame start in a new packet. The standard 16 bit RTP sequence counter is extended to 26 bits to accommodate HDTV's high data rates. At 1.485Gb/s, with packet sizes of draft-ietf-avt-smpte292-video-00.txt [Page 2]
INTERNET-DRAFT July 13, 2000 at least 1k, 26bits allows for 5minute period before the sequence counter wraps around. The payload header includes the offset of the payload data in the video frame. The offset is for 20-bit video words and accounts for active and inactive samples. Given SMPTE292M's 4:2:2 color subsampling, scan line fragmentation must occur on sample-pair boundaries, such that Y and Cb and Cr values are not split across packets. 4. RTP Packetization The standard RTP header is followed by a 4 byte payload header, and the payload data. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | V |P|X| CC |M| PT | sequence# (low bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time stamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |squence#(high bits)| offset in frame | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.1. The RTP Header The following fields of the RTP fixed header are used for SMPTE 292M encapsulation: Payload Type (PT): 7bits A dynamically allocated payload type field which designates the payload as SMPTE 292M. Timestamp: 32 bits The timestamp field shall be defined from a counter at 10 MHz. The timestamp shall be defined as the arrival time of the first 20-bit video sample to be transmitted in the current packet. At an arrival rate of 74.25 MHz for 20-bit 292M video samples with 24/30/60 Hz frame rates, the timestamp will be unique for packets with more draft-ietf-avt-smpte292-video-00.txt [Page 3]
INTERNET-DRAFT July 13, 2000 than 8 video samples (20 bytes). Timestamps shall increase monotonically until they roll over at 32 bits. The 10 MHz timestamp clock may be obtained from a GPS (Global Positioning System) board. These boards have a disciplined oscillator that is synchronized to GPS time. The disciplined oscillator can be as accurate as 1 in 10-12, but is more typically 1 in 10-8. Thus clocks at widely separate locations can be synchronized with an accuracy of 100 ns for video timing recovery. Marker bit (M): 1bit The Marker bit denotes the end of a video frame, and is set to 1 for the last packet of the video frame and is otherwise set to 0 for all other packets. Sequence Number (low bits): 16 bits The low order bits for RTP sequence counter. The standard 16 bit RTP sequence number is augmented by 10 bits in the payload header in order to accommodate the 1.485Gb/s data rate of SMPTE292M. 4.2. Payload Header Sequence Number (high bits): 10bits The high order bits for the 26bit RTP sequence counter. Offset in Frame: 22bits An offset for the position of 20-bit video words in the video frame. The offset includes all information in the video frame and scan lines. Range of values are: 1. 0 to 1125*2750-1 for 1080/24p. 2. 0 to 1125*2200-1 for 1080/30i. 3. 0 to 750*1650-1 for 720/60p. 5. Payload Format For 4:2:2 color subsampling Cb and Cr values are subsampled by a factor of two horizontally and are co-sited with even numbered Y samples. Therefore, Cb, Cr and Y samples must be arranged and transmitted in the following order: Cb, Y, Cr, Y, Cb, Y, Cr, ... where the first Cb, Y, Cr sequence refers to co-sited luminance and color-difference samples, and the next Y belongs to the next luminance sample. draft-ietf-avt-smpte292-video-00.txt [Page 4]
INTERNET-DRAFT July 13, 2000 Therefore, as set forth in RFC2431, for 10-bit words, each group of four samples must be encoded into a 40-bit word (five octets) prior to transmission. The following is a representation of a 720 sample packet with 10-bit quantization: 0 1 2 3 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 +---------+---------+---------+---------+ | Cb0 | Y0 | Cr0 | Y1 | +---------+---------+---------+---------+ | Cb1 | Y2 | Cr1 | Y3 | +---------+---------+---------+---------+ . . . +---------+---------+---------+---------+ | Cb359 | Y718 | Cr359 | Y719 | +---------+---------+---------+---------+ (Note that the word width is 40 bits) +-------+-------+-------+-------+-------+ Octets: | 0 | 1 | 2 | 3 | 4 | +-------+-------+-------+-------+-------+ The octets shown in these diagrams are transmitted in network bit and byte order, that is, left-to-right as shown. 6. Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [4], and any appropriate RTP profile. This implies that confidentiality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to- end, encryption may be performed after compression so there is no conflict between the two operations. This payload type does not exhibit any significant non-uniformity in the receiver side computational complexity for packet processing to cause a potential denial-of-service threat. It is perhaps to be noted that the bandwidth of this payload is high enough (1.5 Gbps without the RTP overhead) to cause potential for denial-of-service if transmitted onto most currently available Internet paths. In the absence from the standards track of a suitable congestion control mechanism for flows of this sort, use of the payload should be narrowly limited to suitably connected network endpoints and great care draft-ietf-avt-smpte292-video-00.txt [Page 5]
INTERNET-DRAFT July 13, 2000 taken with the scope of multicast transmissions. This potential threat is common to all high bit rate applications. 7. IANA Considerations [To be done] 8. Full Copyright Statement Copyright (C) The Internet Society (1999). 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 Soci- ety 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 fol- lowed, 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 MER- CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 9. Authors' Addresses Ladan Gharai ladan@isi.edu Gary Goncher ggoncher@tek.com draft-ietf-avt-smpte292-video-00.txt [Page 6]
INTERNET-DRAFT July 13, 2000 Allison Mankin mankin@isi.edu David Richardson drr@u.washington.edu 10. Bibliography [1] Society of Motion Picture and Television Engineers, Bit-Serial Digital Interface for High-Definition Television Systems, SMPTE292M, 1998. [2] Society of Motion Picture and Television Engineers, 1280*720 Scanning, Analog and Digital Representation and Analog Interfaces, SMPTE 296M, 1998. [3] ISO/IEC International Standard 13818-2; "Generic coding of moving pictures and associated audio information: Video", 1996. [4] ATSC Digital Television Standard Document A/53, September 1995, http://www.atsc.org [5] ISO/IEC International Standard 13818-1; "Generic coding of moving pictures and associated audio information: Systems",1996. [6] Schulzrinne, Casner, Frederick, Jacobson, "RTP: A transport protocol for real time Applications", RFC 1889, IETF, January 1996. [7] Hoffman, Fernando, Goyal, Civanlar, "RTP Payload Format for MPEG1/MPEG2 Video", RFC 2250, IETF, January 1998. [8] Schulzrinne, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, IETF, January 1996. draft-ietf-avt-smpte292-video-00.txt [Page 7]
