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Network Working Group                                           D. TynanRequest for Comments: 2431                                Claddagh FilmsCategory: Standards Track                                   October 1998RTP Payload Format for BT.656 Video EncodingStatus 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.Abstract   This document specifies the RTP payload format for encapsulating ITU   Recommendation BT.656-3 video streams in the Real-Time Transport   Protocol (RTP).  Each RTP packet contains all or a portion of one   scan line as defined by ITU Recommendation BT.601-5, and includes   fragmentation, decoding and positioning information.1. Introduction   This document describes a scheme to packetize uncompressed, studio-   quality video streams as defined by BT.656 for transport using RTP   [1].  A BT.656 video stream is defined by ITU-R Recommendation   BT.656-3 [2], as a means of interconnecting digital television   equipment operating on the 525-line or 625-line standards, and   complying with the 4:2:2 encoding parameters as defined in ITU-R   Recommendation BT.601-5 (formerly CCIR-601) [3], Part A.   RTP is defined by the Internet Engineering Task Force (IETF) to   provide end-to-end network transport functions suitable for   applications transmitting real-time data over multicast or unicast   network services.  The complete specification of RTP for a particular   application requires the RTP protocol document [1], a profile   specification document [4], and a payload format specification.  This   document is intended to serve as the payload format specification for   studio-quality video streams.Tynan                       Standards Track                     [Page 1]

RFC 2431             RTP Payload Format for BT.656          October 1998   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [5].2. Definitions   For the purposes of this document, the following definitions apply:   Y: An 8-bit or 10-bit coded "luminance" sample.  Luminance in this   context refers to the BT.601-5 [3] definition which is not the same   as a true CIE luminance value.  The value of "luminance" refers   specifically to video luma. However, in order to avoid confusion with   the BT.656 and BT.601 standards, the video luma value is referenced   in this document as luminance.  Each value has 220 quantization   levels with the black level corresponding to level 16 and the peak   white level corresponding to 235.   Cb, Cr: An 8-bit or 10-bit coded color-difference sample (as per   BT.601-5).  Each color-difference value has 225 quantization levels   in the centre part of the quantization scale with a color-difference   of zero having an encoded value of 128.   True Black: BT.601-5 defines a true black level as the quad-sample   sequence 0x80, 0x10, 0x80, 0x10, representing color-difference values   of 128 (0x80) and a luminance value of 16 (0x10).   SAV, EAV: Video timing reference codes which appear at the start and   end of a BT.656 scan line.3. Payload Design   ITU Recommendation BT.656-3 defines a schema for the digital   interconnection of television video signals in conjunction with   BT.601-5 which defines the digital representation of the original   analog signal.  While BT.601-5 refers to images with or without color   subsampling, the interconnection standard (BT.656-3) specifically   requires 4:2:2 subsampling. This specification also requires 4:2:2   subsampling such that the luminance stream occupies twice the   bandwidth of each of the two color-difference streams.  For normal   4:3 aspect ratio images, this results in 720 luminance samples per   scan line, and 360 samples of each of the two chrominance channels.   The total number of samples per scan line in this case is 1440.   While this payload format specification can accomodate various image   sizes and frame rates, only those in accordance with BT.601-5 are   currently supported.Tynan                       Standards Track                     [Page 2]

RFC 2431             RTP Payload Format for BT.656          October 1998   Due to the lack of any form of video compression within the payload   and sampling-rate compliance with BT.601-5, the resultant video   stream can be considered "studio quality".  However, such a stream   can require approximately 20 megabytes per second of network   bandwidth.  In order to maximize packet size within a given MTU, and   to optimize scan line decoding, each video scan line is encoded   within one or more RTP packets.   To allow for scan line synchronization, each packet includes certain   flag bits (as defined in BT.656-3) and a unique scan line number.   The SAV and EAV timing reference codes are removed. Furthermore, no   line blanking samples are included, so no ancillary data can be   included in the line blanking period.  It is the responsibility of   the receiver to generate the timing reference codes, and to insert   the correct number of line blanking samples.   Similarly, there is no requirement that the frame blanking samples be   provided.  However, it is possible to include frame blanking samples   if such samples contain relevant information, such as a vertical-   interlace time code (VITC), or teletext data.  In the absence of   frame blanking samples, the receiver MUST generate true black levels   as defined above, to complete the correct number of scan lines per   field.  If frame blanking samples are provided, they MUST be copied   without modification into the resultant BT.656-3 stream.   Scan lines MUST be sent in sequential order.  Error concealment for   missing scan lines or fragments of scan lines is at the discretion of   the receiver.   Both 8-bit and 10-bit quantization types as defined by BT.601-5 are   supported.  10-bit samples are considered to have two extra bits of   fixed-point precision such that a binary value of 10111110.11   represents a sample value of 190.75.  Using 8-bit quantization, this   would give a sample value of 190.  An application receiving 8-bit   samples for a 10-bit device MUST consider the sample as reflecting   the most-significant 8 bits.  The two least-significant bits SHOULD   be set to zero.  Similarly, an application sending 8-bit samples from   a 10-bit device MUST drop the two least-significant bits.  For a 10-   bit quantization payload, each pair of samples MUST be encoded into a   40-bit word (five octets) prior to transmission, as specified inSection 6.   To allow for scan lines with octet lengths larger than the path   maximum transmission unit (MTU), a scan offset field is included in   the packet header.  Applications SHOULD attempt path MTU discovery   [6] and fragment scan lines into multiple packets no larger than the   MTU.Tynan                       Standards Track                     [Page 3]

RFC 2431             RTP Payload Format for BT.656          October 1998   Fragmentation MUST occur on a sample-pair boundary, such that the   chrominance and luminance values are not split across packets.  For   8-bit quantization this gives a four-octet alignment, and a five-   octet alignment for 10-bit quantization.  As a result, the scan   offset refers not to the byte offset within the payload, but the   sample-pair offset.4. Usage of RTP   Due to the unreliable nature of the RTP protocol, and the lack of an   orderly delivery mechanism, each packet contains enough information   to form a single scan line without reference to prior scan lines or   prior frames.  In addition to the RTP header, a fixed length payload   header is included in each packet.  This header is four octets in   length.       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                          |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                         Payload Header                        |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                          Payload Data                         |      |                                .                              |      |                                .                              |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+4.1. RTP Header usage   Each RTP packet starts with a fixed RTP header.  The following fields   of the RTP fixed header are used for BT.656-3 encapsulation:   Marker bit (M): The Marker bit of the RTP header is set to 1 for the   last packet of a frame (or the last fragment of the last scan line if   fragmented), and set to 0 on all other packets.   Payload Type (PT): The Payload Type indicates the use of the payload   format defined in this document.  A profile MAY assign a payload type   value for this format either statically or dynamically as described   inRFC 1890 [4].   Timestamp: The RTP Timestamp encodes the sampling instant of the   video frame currently being rendered.  All scan line packets within   the same frame will have the same timestamp.  The timestamp SHOULD   refer to the 'Ov' field synchronization point of the first field.   For the payload format defined by this document, the RTP timestamp is   based on a 90kHz clock.Tynan                       Standards Track                     [Page 4]

RFC 2431             RTP Payload Format for BT.656          October 19985. Payload Header   The payload header is a fixed four-octet header broken down as   follows:       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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |F|V| Type  |P| Z |     Scan Line (SL)    |  Scan Offset (SO)   |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   F: 1 bit   When 0, indicates the first field of a frame (line 4 through 265   inclusive for Type=0 or 2, and line 1 through 312 inclusive for Type=1   or 3).  Any other scan line is considered a component of the second   field, and the F bit will be set to 1.  This bit is copied directly   from the BT.656-compliant stream by the transmitter, and inserted into   the stream by the receiver.   V: 1 bit   When 1, indicates that the scan line is part of the vertical interval.   Should always be 0 unless frame blanking data is sent.  In which case,   the V bit SHOULD be set to 1 for scan lines which do not form an   integral part of the image. This bit is copied directly from the   BT.656-compliant stream by the transmitter, and inserted into the   stream by the receiver.  For receivers which do not receive scan lines   during the vertical interval, BT.656 vertical interval data MUST be   generated by repeating the quad-sample sequence 0x80, 0x10, 0x80,   0x10, representing a true black level.   Type: 4 bits   This field indicates the type of frame encoding within the payload.   It MUST remain unchanged for all scan lines within the same frame.   Currently only four types of encoding are defined.  These are as   follows;      0 - NTSC (13.5MHz sample rate; 720 samples per line; 60 fields          per second; 525 lines per frame)      1 - PAL (13.5MHz sample rate; 720 samples per line; 50 fields          per second; 625 lines per frame)      2 - High Definition NTSC (18MHz sample rate; 1144 samples per          line; 60 fields per second; 525 lines per frame)      3 - High Definition PAL (18MHz sample rate; 1152 samples per          line; 50 fields per second; 625 lines per frame)Tynan                       Standards Track                     [Page 5]

RFC 2431             RTP Payload Format for BT.656          October 1998   Further encodings can only be defined through the Internet Assigned   Numbers Authority (IANA).  For more information refer toSection 8,   "IANA Considerations".   P: 1 bit   Indicates the required sample quantization size.  When 0, the payload   is comprised of 8-bit samples.  Otherwise, it carries 10-bit samples.   This bit MUST remain unchanged for all scan lines within the same   frame.   Z: 2 bits   Reserved for future use.  Must be set to zero by the transmitter and   ignored by the receiver.   Scan Line (SL): 12 bits   Indicates the scan line encapsulated in the payload.  Valid values   range from 1 through 625 inclusive. If no frame blanking data is   being transmitted, only scan lines 23 through 310 inclusive, and   lines 336 through 623 inclusive SHOULD be sent in the case of Type=1   or 3.  For 525/60 encoding (Type=0 or 2), scan lines 10 through 263   inclusive and lines 273 through 525 SHOULD be transmitted.   If a receiver is generating a BT.656-3 data stream directly from this   packet, the F and V bits MUST be copied from the header rather than   being generated implicitly from the scan line number.  In the event   of a conflict, the F and V bits have precedence.   Scan Offset (SO): 11 bits   Indicates the offset within the scan line for application-level   fragmentation.  After doing PMTU discovery, if the path MTU is less   than the required size for one complete scan line, the data SHOULD be   fragmented such that a given RTP packet does not exceed the allowable   MTU.  The offset for the first packet of a scan line MUST be set to   zero.  The scan offset refers to the sample-pair offset within the   scan such that for a scan line width of 720, the maximum scan offset   is 359.6. Payload Format   In keeping with the 4:2:2 color subsampling of BT.656 and BT.601,   each pair of color-difference samples will be intermixed with two   luminance samples.  As per BT.656, the format for transmission SHALL   be Cb, Y, Cr, Y.  The following is a representation of a 720 sample   packet with 8-bit quantization:Tynan                       Standards Track                     [Page 6]

RFC 2431             RTP Payload Format for BT.656          October 1998       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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |      Cb0      |      Y0       |      Cr0      |      Y1       |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |      Cb1      |      Y2       |      Cr1      |      Y3       |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                      .                                      .                                      .      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |     Cb359     |     Y718      |     Cr359     |     Y719      |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   1144 and 1152 sample packets SHOULD increase the packet size   accordingly while maintaining the sample order.   For 10-bit quantization, each group of four samples MUST be encoded   into a 40-bit word (five octets) prior to transmission.  The sample   order is identical to that for 8-bit quantization.  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 byte   order, that is, left-to-right as shown.Tynan                       Standards Track                     [Page 7]

RFC 2431             RTP Payload Format for BT.656          October 19987. Security Considerations   RTP packets using the payload format defined in this specification   are subject to the security considerations discussed in the RTP   specification [1].  This implies that confidentiality of the media   streams is achieved by encryption.  Because the payload format is   arranged end-to-end, encryption MAY be performed after encapsulation   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.8. IANA Considerations   The four encoding types defined by this document relate to specific   schema defined by ITU-R Recommendation BT.656-3.  Future revisions of   the recommendation may create further encoding types which need to be   supported over RTP. The "Type" field is four bits wide allowing for a   total of up to sixteen possible encodings, with twelve currently   reserved for future use.  Due to the small number of possible   encodings and given that it is very unlikely that future revisions of   BT.656 will introduce any new schema, requests to extend the Type   field MUST be vetted by the Internet Assigned Numbers Authority.   Furthermore, implementors SHOULD check the IANA repository for new   definitions of the Type field in order to comply with this document.   Applications for a new Type value MUST be submitted to the IANA and   include the requestors name and contact information, the reason for   requesting a new Type and references to appropriate standards, such   as an updated version of ITU-R Recommendation BT.656.  Furthermore,   in the unlikely event that the new Type will lessen the security of a   compliant implementation, such security risk MUST be detailed in the   application.  The application will be reviewed by a Designated Expert   and if appropriate, a new Type will be assigned.  This type will be   listed in the IANA repository for future implementations.Tynan                       Standards Track                     [Page 8]

RFC 2431             RTP Payload Format for BT.656          October 19989. References   [1]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,         "RTP: A Transport Protocol for Real-Time Applications",RFC1889, January 1996.   [2]   Interfaces for Digital Component Video Signals in 525-Line and         625-Line Television Systems operating at the 4:2:2 Level of         Recommendation ITU-R BT.601 (Part A), ITU-R Recommendation         BT.656-3, 1995.   [3]   Studio Encoding Parameters of Digital Television for Standard         4:3 and Wide-Screen 16:9 Aspect Ratios, ITU-R Recommendation         BT.601-5, 1995.   [4]   Schulzrinne, H., "RTP Profile for Audio and Video Conference         with Minimal Control",RFC 1890, January 1996.   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels",BCP 14,RFC 2119, March 1997.   [6]   Mogul, J., and S. Deering, "Path MTU Discovery",RFC 1191,         November 1990.10. Author's Address   Dermot Tynan   Claddagh Films Limited   3 White Oaks   Clybaun Road   Galway   Ireland   EMail: dtynan@claddagh.ie   Phone: +353 91 529944Tynan                       Standards Track                     [Page 9]

RFC 2431             RTP Payload Format for BT.656          October 199811.  Full 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.Tynan                       Standards Track                    [Page 10]