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RFC 3497 - RTP Payload Format for Society of Motion Picture and

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Network Working Group                                          L. Gharai
Request for Comments: 3497                                    C. Perkins
Category: Standards Track                                        USC/ISI
                                                              G. Goncher
                                                               A. Mankin
                                           Bell Labs, Lucent Corporation
                                                              March 2003

                        RTP Payload Format for
 Society of Motion Picture and Television Engineers (SMPTE) 292M Video

Status 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 (2003).  All Rights Reserved.


   This memo specifies an RTP payload format for encapsulating
   uncompressed High Definition Television (HDTV) as defined by the
   Society of Motion Picture and Television Engineers (SMPTE) standard,
   SMPTE 292M.  SMPTE is the main standardizing body in the motion
   imaging industry and the SMPTE 292M standard defines a bit-serial
   digital interface for local area HDTV transport.

1.  Introduction

   The serial digital interface, SMPTE 292M [1], defines a universal
   medium of interchange for uncompressed High Definition Television
   (HDTV) between various types of video equipment (cameras, encoders,
   VTRs, etc.).  SMPTE 292M stipulates that the source data be in 10 bit
   words and the total data rate be either 1.485 Gbps or 1.485/1.001

   The use of a dedicated serial interconnect is appropriate in a studio
   environment, but it is desirable to leverage the widespread
   availability of high bandwidth IP connectivity to allow efficient
   wide area delivery of SMPTE 292M content.  Accordingly, this memo
   defines an RTP payload format for SMPTE 292M format video.

   It is to be noted that SMPTE 292M streams have a constant high bit
   rate and are not congestion controlled.  Accordingly, use of this
   payload format should be tightly controlled and limited to private
   networks or those networks that provide resource reservation and
   enhanced quality of service.  This is discussed further in section 9.

   This memo only addresses the transfer of uncompressed HDTV.
   Compressed HDTV is a subset of MPEG-2 [9], which is fully described
   in document A/53 [10] of the Advanced Television Standards Committee.
   The ATSC has also adopted the MPEG-2 transport system (ISO/IEC
   13818-1) [11].  Therefore RFC 2250 [12] sufficiently describes
   transport for compressed HDTV over RTP.

2.  Overview of SMPTE 292M

   A SMPTE 292M television line comprises two interleaved streams, one
   containing the luminance (Y) samples, the other chrominance (CrCb)
   values.  Since chrominance is horizontally sub-sampled (4:2:2 coding)
   the lengths of the two streams match (see Figure 3 of SMPTE 292M
   [1]).  In addition to being the same length the streams also have
   identical structures: each stream is divided into four parts, (figure
   1): (1) start of active video timing reference (SAV); (2) digital
   active line; (3) end of active video timing reference (EAV); and (4)
   digital line blanking.  A SMPTE 292M line may also carry horizontal
   ancillary data (H-ANC) or vertical ancillary data (V-ANC) instead of
   the blanking level; Likewise, ancillary data may be transported
   instead of a digital active line.

   The EAV and SAV are made up of three 10 bit words, with constant
   values of 0x3FF 0x000 0x000 and an additional word (designated as XYZ
   in figure 2), carrying a number of flags.  This includes an F flag
   which designates which field (1 or 2) the line is transporting and
   also a V flag which indicates field blanking.  Table 1, further
   displays the code values in SAV and EAV.  After EAV, are two words,
   LN0 and LN1 (Table 2), that carry the 11 bit line number for the
   SMPTE 292M line.  The Cyclic Redundancy Check, CRC, is also a two
   word value, shown as CR0 and CR1 in figure 2.

      |            | Digital Line Blanking |     | Digital Active Line |
      | EAV+LN+CRC | (Blanking level or    | SAV |  (Active Picture or |
      |            |  Ancillary Data)      |     |   Ancillary Data)   |

                     Figure 1. The SMPTE 292M line format.

         0       20      40      60     80       0      20      40
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       +-+-+-+-+-+-+-+-+
         |3FF| 0 | 0 |XYZ|LN1|LN2|CR0|CR1|       |3FF| 0 | 0 |XYZ|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       +-+-+-+-+-+-+-+-+
         <---- EAV -----> <- LN-> <- CRC->       <----- SAV ----->

                        Figure 2. Timing reference format.

         |      (MSB)                                        (LSB) |
         | Word    9    8    7    6    5    4    3    2    1    0  |
         | 3FF     1    1    1    1    1    1    1    1    1    1  |
         | 000     0    0    0    0    0    0    0    0    0    0  |
         | 000     0    0    0    0    0    0    0    0    0    0  |
         | XYZ     1    F    V    H    P    P    P    P    P    P  |
         | NOTES:                                                  |
         |     F=0 during field 1; F=1 during field 2.             |
         |     V=0 elsewhere; V=1 during field blanking.           |
         |     H=0 in SAV; H=1 in EAV.                             |
         |     MSB=most significant bit; LSB=least significant bit.|
         |     P= protected bits defined in Table 2 of SMPTE 292M  |

                      Table 1: Timing reference codes.

         |      (MSB)                                        (LSB) |
         | Word    9    8    7    6    5    4    3    2    1    0  |
         |  LN0    R    L6   L5   L4   L3   L2   L1   L0   R    R  |
         |  LN1    R     R    R    R   L10  L9   L8   L7   R    R  |
         | NOTES:                                                  |
         |    LN0 - L10 - line number in binary code.              |
         |    R = reserved, set to "0".                            |

                      Table 2: Line number data.

   The number of words and the format for active lines and line blanking
   is defined by source format documents.  Currently, source video
   formats transfered by SMPTE 292M include SMPTE 260M, 295M, 274M and
   296M [5-8].  In this memo, we specify how to transfer SMPTE 292M over
   RTP, irrespective of the source format.

3.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

4.  Payload Design

   Each SMPTE 292M data line is packetized into one or more RTP packets.
   This includes all timing signals, blanking levels, active lines
   and/or ancillary data.  Start of active video (SAV) and end of active
   video (EAV+LN+CRC) signals MUST NOT be fragmented across packets, as
   the SMPTE 292M decoder uses them to detect the start of scan lines.

   The standard RTP header is followed by a 4 octet payload header.  All
   information in the payload header pertains to the first data sample
   in the packet.  The end of a video frame (the packet containing the
   last sample before the EAV) is marked by the M bit in the RTP header.

   The payload header contains a 16 bit extension to the standard 16 bit
   RTP sequence number, thereby extending the sequence number to 32 bits
   and enabling RTP to accommodate HDTV's high data rates.  At 1.485
   Gbps, with packet sizes of at least one thousand octets, 32 bits
   allows for an approximate 6 hour period before the sequence number
   wraps around.  Given the same assumptions, the standard 16 bit RTP
   sequence number wraps around in less than a second (336
   milliseconds), which is clearly not sufficient for the purpose of
   detecting loss and out of order packets.

   A 148.5 MHz (or 148.5/1.001 MHz) time-stamp is used as the RTP
   timestamp.  This allows the receiver to reconstruct the timing of the
   SMPTE 292M stream, without knowledge of the exact type of source
   format (e.g., SMPTE 274M or SMPTE 296M).  With this timestamp, the
   location of the first sample of each packet can be uniquely
   identified in the SMPTE 292M stream.  At 148.5 MHz, the 32 bit
   timestamp wraps around in 21 seconds.

   The payload header also carries the 11 bit line number from the SMPTE
   292M timing signals.  This provides more information at the
   application level and adds a level of resiliency, in case the packet
   containing the EAV is lost.

   The bit length of both timing signals, SAV and EAV+LN+CRC, are
   multiples of 8 bits, 40 bits and 80 bits, respectively, and therefore
   are naturally octet aligned.

   For the video content, it is desirable for the video to both octet
   align when packetized and also adhere to the principles of
   application level framing, also known as ALF [13].  For YCrCb video,
   the ALF principle translates into not fragmenting related luminance
   and chrominance values across packets.  For example, with the 4:2:0
   color subsampling, a 4 pixel group is represented by 6 values, Y1 Y2
   Y3 Y4 Cr Cb, and video content should be packetized such that these
   values are not fragmented across 2 packets.  However, with 10 bit
   words, this is a 60 bit value which is not octet aligned.  To be both
   octet aligned, and adhere to ALF, an ALF unit must represent 2 groups
   of 4 Pixels, thereby becoming octet aligned on a 15 octet boundary.
   This length is referred to as the pixel group or pgroup, and it is
   conveyed in the SDP parameters.  Table 3 displays the pgroup value
   for various color samplings.  Typical source formats use 4:2:2
   sampling, and require a pgroup of 5 octets, other values are included
   for completeness.

   The contents of the Digital Active Line SHOULD NOT be fragmented
   within a pgroup.  A pgroup of 1 indicates that data may be split at
   any octet boundary (this is applicable to instances where the source
   format is not known).  The SAV and EAV+LN+CRC fields MUST NOT be

         |   Color            10  bit                            |
         |Subsampling  Pixels  words    aligned on octet#  pgroup|
         |   4:2:0   |   4   |  6*10  |   2*60/8 = 15     |  15  |
         |   4:2:2   |   2   |  4*10  |     40/8 = 5      |   5  |
         |   4:4:4   |   1   |  3*10  |   4*30/8 = 15     |  15  |

                   Table 3. Color subsampling and pgroups.

5.  RTP Packetization

   The standard RTP header is followed by a 4 octet payload header, and
   the payload data, as shown in Figure 3.

       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                                |
      |                        ssrc                                   |
      |    sequence# (high bits)      |F|V| Z |        line no        |
      |                                                               |
      :                      SMPTE 292M data                          :
      :                                                               :
      |                                                               |

          Figure 3: RTP Packet showing SMPTE 292M headers and payload

5.1.  The RTP Header

   The following fields of the RTP fixed header are used for SMPTE 292M
   encapsulation (the other fields in the RTP header are used in their
   usual manner):

   Payload Type (PT): 7 bits
      A dynamically allocated payload type field that designates the
      payload as SMPTE 292M.

   Timestamp: 32 bits
      For a SMPTE 292M transport stream at 1.485 Gbps (or 1.485/1.001
      Gbps), the timestamp field contains a 148.5 MHz (or 148.5/1.001
      MHz) timestamp, respectively.  This allows for a unique timestamp
      for each 10 bit word.

   Marker bit (M): 1 bit
      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 with another 16 bits in the
      payload header in order to accommodate the 1.485 Gbps data rate of
      SMPTE 292M.

5.2.  Payload Header

   Sequence Number (high bits): 16 bits
      The high order bits for the 32 bit RTP sequence counter, in
      network byte order.

   F: 1 bit
      The F bit as defined in the SMPTE 292M timing signals (see Table
      1).  F=1 identifies field 2 and F=0 identifies field 1.

   V: 1 bit
      The V bit as defined in the SMPTE 292M timing signals (see Table
      1).  V=1 during field blanking, and V=0 else where.

   Z: 2 bits
      SHOULD be set to zero by the sender and MUST be ignored by

   Line No: 11 bits
      The line number of the source data format, extracted from the
      SMPTE 292M stream (see Table 2).  The line number MUST correspond
      to the line number of the first 10 bit word in the packet.

6.  RTCP Considerations

   RFC 1889 should be used as specified in RFC 1889 [3], which specifies
   two limits on the RTCP packet rate: RTCP bandwidth should be limited
   to 5% of the data rate, and the minimum for the average of the
   randomized intervals between RTCP packets should be 5 seconds.
   Considering the high data rate of this payload format, the minimum
   interval is the governing factor in this case.

   It should be noted that the sender's octet count in SR packets wraps
   around in 23 seconds, and that the cumulative  number of packets lost
   wraps around in 93 seconds.  This means these two fields cannot
   accurately represent the octet count and number of packets lost since
   the beginning of transmission, as defined in RFC 1889.  Therefore,
   for network monitoring purposes or any other application that
   requires the sender's octet count and the cumulative number of
   packets lost since the beginning of transmission, the application
   itself must keep track of the number of rollovers of these fields via
   a counter.

7.  IANA Considerations

   This document defines a new RTP payload format and associated MIME
   type, SMPTE292M.  The MIME registration form for SMPTE 292M video is
   enclosed below:

   MIME media type name: video

   MIME subtype name: SMPTE292M

   Required parameters: rate
      The RTP timestamp clock rate.  The clock runs at either 148500000
      Hz or 148500000/1.001 Hz.  If the latter rate is used a timestamp
      of 148351648 MUST be used, and receivers MUST interpret this as
      148500000/1.001 Hz.

   Optional parameters: pgroup
      The RECOMMENDED grouping for aligning 10 bit words and octets.
      Defaults to 1 octet, if not present.

   Encoding considerations: SMPTE292M video can be transmitted with RTP
      as specified in RFC 3497.

   Security considerations: see RFC 3497 section 9.

   Interoperability considerations: NONE

   Published specification: SMPTE292M
                            RFC 3497

   Applications which use this media type:
                            Video communication.

   Additional information: None

   Magic number(s): None

   File extension(s): None

   Macintosh File Type Code(s): None

   Person & email address to contact for further information:
      Ladan Gharai <ladan@isi.edu>
      IETF AVT working group.

   Intended usage: COMMON

   Author/Change controller:
         Ladan Gharai <ladan@isi.edu>

8.  Mapping to SDP Parameters

   Parameters are mapped to SDP [14] as follows:

      m=video 30000 RTP/AVP 111
      a=rtpmap:111 SMPTE292M/148500000
      a=fmtp:111  pgroup=5

   In this example, a dynamic payload type 111 is used for SMPTE292M.
   The RTP timestamp is 148500000 Hz and the SDP parameter pgroup
   indicates that for video data after the SAV signal, it must be
   packetized in multiples of 5 octets.

9.  Security Considerations

   RTP sessions using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [3] and any appropriate RTP profile (e.g., [4]).

   This payload format does not exhibit any significant non-uniformity
   in the receiver side computational complexity for packet processing
   to cause a potential denial-of-service threat for intended receivers.

   The bandwidth of this payload format is high enough (1.485 Gbps
   without the RTP overhead) to cause potential for denial-of-service if
   transmitted onto most currently available Internet paths.  Since
   congestion control is not possible for SMPTE 292M over RTP flows, use
   of the payload SHOULD be narrowly limited to suitably connected
   network endpoints, or to networks where QoS guarantees are available.

   If QoS enhanced service is used, RTP receivers SHOULD monitor packet
   loss to ensure that the service that was requested is actually being
   delivered.  If it is not, then they SHOULD assume that they are
   receiving best-effort service and behave accordingly.

   If best-effort service is being used, RTP receivers MUST monitor
   packet loss to ensure that the packet loss rate is within acceptable
   parameters and MUST leave the session if the loss rate is too high.
   The loss rate is considered acceptable if a TCP flow across the same
   network path, experiencing the same network conditions, would achieve
   an average throughput, measured on a reasonable timescale, that is
   not less than the RTP flow is achieving.  Since congestion control is
   not possible for SMPTE 292M flows, this condition can only be
   satisfied if receivers leave the session if the loss rate is
   unacceptably high.

10.  Acknowledgments

   We would like to thank David Richardson for his insightful comments
   and contributions to the document.  We would also like to thank Chuck
   Harrison for his input and for explaining the intricacies of SMPTE

11.  Normative References

   [1]  Society of Motion Picture and Television Engineers, Bit-Serial
        Digital Interface for High-Definition Television Systems, SMPTE

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

   [4]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control", RFC 1890, January 1996.

12.  Informative References

   [5]  Society of Motion Picture and Television Engineers, Digital
        Representation and Bit-Parallel Interface - 1125/60 High-
        Definition Production System, SMPTE 260M-1999.

   [6]  Society of Motion Picture and Television Engineers, 1920x1080
        50Hz, Scanning and Interface, SMPTE 295M-1997.

   [7]  Society of Motion Picture and Television Engineers, 1920x1080
        Scanning and Analog and Parallel Digital Interfaces for Multiple
        Picture Rates, SMPTE 274M-1998.

   [8]  Society of Motion Picture and Television Engineers, 1280x720
        Scanning, Analog and Digital Representation and Analog
        Interfaces, SMPTE 296M-1998.

   [9]  ISO/IEC International Standard 13818-2; "Generic coding of
        moving pictures and associated audio information: Video", 1996.

   [10] ATSC Digital Television Standard Document A/53, September 1995,

   [11] ISO/IEC International Standard 13818-1; "Generic coding of
        moving pictures and associated audio information: Systems",1996.

   [12] Hoffman, D., Fernando, G., Goyal, V. and M. Civanlar, "RTP
        Payload Format for MPEG1/MPEG2 Video", RFC 2250, January 1998.

   [13] Clark, D. D., and Tennenhouse, D. L., "Architectural
        Considerations for a New Generation of Protocols", In
        Proceedings of SIGCOMM '90 (Philadelphia, PA, Sept. 1990), ACM.

   [14] Handley, H. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

13.  Authors' Addresses

   Ladan Gharai
   3811 Fairfax Dr.
   Arlington VA 22203

   EMail: ladan@isi.edu

   Colin Perkins
   3811 Fairfax Dr.
   Arlington VA 22203

   EMail: csp@csperkins.org

   Allison Mankin
   Bell Labs, Lucent Corporation

   EMail: mankin@psg.com

   Gary Goncher
   Tektronix, Inc.
   P.O. Box 500, M/S 50-480
   Beaverton, OR  97077

   EMail: Gary.Goncher@tek.com

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