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Versions: (draft-weaver-payload-rtp-vc2hq) 00 01 02 03 04 06

Payload Working Group                                          J. Weaver
Internet-Draft                                                       BBC
Intended status: Standards Track                            May 22, 2018
Expires: November 23, 2018


              RTP Payload Format for VC-2 HQ Profile Video
                    draft-ietf-payload-rtp-vc2hq-06

Abstract

   This memo describes an RTP Payload format for the High Quality (HQ)
   profile of Society of Motion Picture and Television Engineers
   Standard ST 2042-1 known as VC-2.  This document describes the
   transport of HQ Profile VC-2 in RTP packets and has applications for
   low-complexity, high-bandwidth streaming of both lossless and lossy
   compressed video.

   The HQ profile of VC-2 is intended for low latency video compression
   (with latency potentially on the order of lines of video) at high
   data rates (with compression ratios on the order of 2:1 or 4:1).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on November 23, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions and Acronyms . . . . . . . . . . . .   3
   3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
   4.  Payload format  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Payload Header  . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  The Choice of Parse Codes (Informative) . . . . . . . . .  12
     4.4.  Stream Constraints  . . . . . . . . . . . . . . . . . . .  12
     4.5.  Payload Data  . . . . . . . . . . . . . . . . . . . . . .  13
       4.5.1.  Reassembling the Data . . . . . . . . . . . . . . . .  14
   5.  Congestion Control Considerations . . . . . . . . . . . . . .  16
   6.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  16
     6.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  16
     6.2.  Mapping to SDP  . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  RFC Editor Considerations . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This memo specifies an RTP payload format for the video coding
   standard Society of Motion Picture and Television Engineers ST
   2042-1:2017 [VC2] also known as VC-2

   The VC-2 codec is a wavelet-based codec intended primarily for
   professional video use with high bit-rates and only low levels of
   compression.  It has been designed to be low-complexity, and
   potentially have a very low latency through both encoder and decoder:
   with some choices of parameters this latency may be as low as a few
   lines of video.

   The low level of complexity in the VC-2 codec allows for this low
   latency operation but also means that it lacks many of the more
   powerful compression techniques used in other codecs.  As such it is
   suitable for low compression ratios that produce coded data rates



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   around half to a quarter of that of uncompressed video, at a similar
   visual quality.

   The primary use for VC-2 is likely to be in professional video
   production environments.

2.  Conventions, Definitions and Acronyms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Media Format Description

   The VC-2 specification defines a VC-2 stream as being composed of one
   or more Sequences.  Each Sequence is independently decodable,
   containing all of the needed parameters and metadata for configuring
   the decoder.

   Each Sequence consists of a series of 13-octet Parse Info headers and
   variable length Data Units.  The Sequence begins and ends with a
   Parse Info header and each Data Unit is preceded by a Parse Info
   Header.  Data Units come in a variety of types, the most important
   being the Sequence Header, which contains configuration data needed
   by the decoder, and several types of Coded Picture, which contain the
   coded data for the pictures themselves.  Each picture represents a
   frame in a progressively scanned video Sequence or a field in an
   interlaced video Sequence.

   The first Data Unit in a Sequence as produced by an encoder is always
   a Sequence Header, but Sequences can be joined in the middle, so this
   should not be assumed.

   The High Quality (HQ) profile for VC-2 restricts the types of Parse
   Info Headers which may appear in the Sequence to only:

   o  Sequence Headers,

   o  High Quality Pictures,

   o  High Quality Fragments,

   o  Auxiliary Data,

   o  Padding Data, and




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   o  End of Sequence.

   At time of writing there is currently no definition for the use of
   Auxiliary Data in VC-2, and Padding Data is required to be ignored by
   all receivers.

   Each High Quality Picture data unit contains a set of parameters for
   the picture followed by a series of coded Slices, each representing a
   rectangular region of the transformed picture.  Slices within a
   picture may vary in coded length, but all represent the same shape
   and size of rectangle in the picture.

   Each High Quality Fragment data unit contains either a set of
   parameters for a picture or it contains a series of coded Slices.
   Fragments carry the same data as pictures, but broken up into smaller
   units to facilitate transmission via packet-based protocols such as
   RTP.

   This payload format only makes use of fragments, not pictures.

4.  Payload format

   This specification only covers the transport of Sequence Headers,
   High Quality Fragments, Auxiliary Data, and (optionally) End of
   Sequence Headers and Padding Data.

   High Quality Pictures can be transported by converting them into an
   equivalent set of High Quality Fragments.  The size of fragments
   should be chosen so as to fit within the MTU of the network in use.

   For this reason this document defines six types of RTP packets in a
   VC-2 media stream:

   o  A VC-2 Sequence Header (Figure 1),

   o  A Picture Fragment containing the VC-2 Transform Parameters for a
      Picture (Figure 2),

   o  A Picture Fragment containing VC-2 Coded Slices (Figure 3) for a
      picture,

   o  The end of a VC-2 Sequence (Figure 4)

   o  The contents of an auxiliary data unit (Figure 5), and

   o  An indication of the presence of a padding data unit (Figure 6).





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   These six packet-types can be distinguished by the fact that they use
   different codes in the "PC (Parse Code)" field, except for the two
   types of picture fragment which both use the same value in PC but
   have different values in the "No. of slices" field.

   The choices of PC codes is explained in more detail in a following
   informative section (Section 4.3).

    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |    Reserved   |   PC = 0x00   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   .                                                               .
   .               Variable Length Coded Sequence Header           .
   .                                                               .
   +---------------------------------------------------------------+

             Figure 1: RTP Payload Format For Sequence Header




















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    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |  Reserved |I|F|   PC = 0xEC   |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                       Picture Number                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |       Slice Prefix Bytes      |        Slice Size Scaler      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |       Fragment Length         |         No. of Slices = 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   .                                                               .
   .         Variable Length Coded Transform Parameters            .
   .                                                               .
   +---------------------------------------------------------------+

      Figure 2: RTP Payload Format For Transform Parameters Fragment






















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    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |  Reserved |I|F|   PC = 0xEC   |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                       Picture Number                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |       Slice Prefix Bytes      |        Slice Size Scaler      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |       Fragment Length         |          No. of Slices        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |        Slice Offset X         |         Slice Offset Y        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   .                                                               .
   .                          Coded Slices                         .
   .                                                               .
   +---------------------------------------------------------------+

        Figure 3: RTP Payload Format For Fragment Containing Slices




















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    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |    Reserved   |   PC = 0x10   |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

             Figure 4: RTP Payload Format For End of Sequence

    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |B|E|  Reserved |   PC = 0x20   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Data Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                      Uncoded Payload Data                     .
   .                                                               .
   +---------------------------------------------------------------+

              Figure 5: RTP Payload Format For Auxiliary Data






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    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 Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             SSRC                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 Optional Extension Header                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |   Extended Sequence Number    |B|E|  Reserved |   PC = 0x30   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Data Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                      Optional Payload Data                     .
   .                                                               .
   +---------------------------------------------------------------+

               Figure 6: RTP Payload Format For Padding Data

   All fields in the headers longer than a single bit are interprted as
   unsigned integers in network byte order.

4.1.  RTP Header Usage

   The fields of the RTP header have the following additional notes on
   their useage:

   Marker Bit (M): 1 bit  The marker bit MUST be set on any packet which
         contains the final slice in a coded picture and MUST NOT be set
         otherwise.

   Payload Type (PT): 7 bits  A dynamically allocated payload type field
         that designates the payload as VC-2 coded video.

   Sequence Number: 16 bits  Because the data rate of VC-2 coded streams
         can often be very high, in the order of gigabits rather than
         megabits per second, the standard 16-bit RTP sequence number
         can cycle very quickly.  For this reason the sequence number is
         extended to 32-bits, and this field MUST hold the low-order
         16-bits of this value.




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   Timestamp: 32 bits  If the packet contains transform parameters or
         coded slice data for a coded picture then the timestamp
         corresponds to the sampling instant of the coded picture.  A
         90kHz clock SHOULD be used.  A single RTP packet MUST NOT
         contain coded data for more than one coded picture, so there is
         no ambiguity here.

         A Sequence Header packet SHOULD have the same timestamp as the
         next picture which will follow it in the stream.  An End of
         Sequence packet SHOULD have the same timestamp as the previous
         picture which appeared in the stream.

   The remaining RTP header fields are used as specified in RTP
   [RFC3550].

4.2.  Payload Header

   The fields of the extended headers are defined as follows:

   Extended Sequence Number: 16 bits  MUST Contain the high-order
         16-bits of the 32-bit packet sequence number, a number which
         increments with each packet.  This is needed since the high
         data rates of VC2 Sequences mean that it is highly likely that
         the 16-bit sequence number will roll-over too frequently to be
         of use for stream synchronisation.

   B: 1 bit  MUST be set to 1 if the packet contains the first byte of
         an Auxiliary Data or Padded Data Unit.

   E: 1 bit  MUST be set to 1 if the packet contains the final byte of
         an Auxiliary Data or Padded Data Unit.

   I: 1 bit  MUST be set to 1 if the packet contains coded picture
         paramaters or slice data from a field in an interlaced frame,
         and to 0 if the packet contains data from any part of a
         progressive frame.

   F: 1 bit  MUST be set to 1 if the packet contains coded picture
         paramaters or slice data from the second field of an interlaced
         frame, and to 0 if the packet contains data from the first
         field of an interlaced frame or any part of a progressive
         frame.

   Parse Code (PC): 8 bits  Contains a Parse Code which MUST be the
         value indicated for the type of data in the packet.

   Data Length: 32 bits  For an auxiliary data unit this contains the
         number of bytes of data contained in the uncoded payload



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         section of this packet.  For a Padding Data Unit this field may
         have any value and simply indicates the size of the recommended
         padding.

   Picture Number: 32 bits  MUST contain the Picture Number for the
         coded picture this packet contains data for, as described in
         Section 12.1 of the VC-2 specification [VC2].

         The sender MUST send at least one transform parameters packet
         for each coded picture and MAY include more than one as long as
         they contain identical data.  The sender MUST NOT send a packet
         from a new picture until all the coded data from the current
         picture has been sent.

         If the receiver does not receive a transform parameters packet
         for a picture then it MAY assume that the parameters are
         unchanged since the last picture, or MAY discard the picture.

   Slice Prefix Bytes: 16 bits  MUST contain the Slice Prefix Bytes
         value for the coded picture this packet contains data for, as
         described in Section 12.3.4 of the VC-2 specification [VC2].

         In the VC-2 specification this value is not restricted to 16
         bits, but the constraints on streams specified in this document
         (Section 4.4) do require this.

   Slice Size Scaler: 16 bits  MUST contain the Slice Size Scaler value
         for the coded picture this packet contains data for, as
         described in Section 12.3.4 of the VC-2 specification [VC2].

         In the VC-2 specification this value is not restricted to 16
         bits, but the constraints on streams specified in this document
         (Section 4.4) do require this.

   Fragment Length: 16 bits  MUST contain the number of bytes of data
         contained in the coded payload section of this packet.

   No. of Slices: 16 bits  MUST contain the number of coded slices
         contained in this packet, which MUST be 0 for a packet
         containing transform parameters.  In a packet containing coded
         slices this number MUST be the number of whole slices contained
         in the packet, and the packet MUST NOT contain any partial
         slices.

   Slice Offset X: 16 bits  MUST contain the X coordinate of the first
         slice in this packet, in slices, starting from the top left
         corner of the picture.




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   Slice Offset Y: 16 bits  MUST contain the Y coordinate of the first
         slice in this packet, in slices, starting from the top left
         corner of the picture.

4.3.  The Choice of Parse Codes (Informative)

   The "PC" field in the packets is used to carry the Parse Code which
   identifies the type of content in the packet.  This code matches the
   value of the Parse Code used to identify each data unit in a VC-2
   stream, as defined in the VC-2 specification, and each packet
   contains the entire data unit.

   The table below lists all of the parse codes currently allowed in a
   VC-2 Sequence.  The final column indicates whether the code in
   question can be present in a stream transmitted using this
   specification.

   +----------+-----------+---------------------+---------------+
   | PC (hex) | Binary    | Description         | Valid         |
   +----------+-----------+---------------------+---------------+
   | 0x00     | 0000 0000 | Sequence Header     | Yes           |
   | 0x10     | 0001 0000 | End of Sequence     | Yes           |
   | 0x20     | 0010 0000 | Auxiliary Data      | Yes           |
   | 0x30     | 0011 0000 | Padding Data        | Yes           |
   +----------+-----------+---------------------+---------------+
   | 0xC8     | 1100 1000 | LD Picture          | No            |
   | 0xE8     | 1110 1000 | HQ Picture          | No            |
   | 0xEC     | 1110 1100 | HQ Picture Fragment | Yes           |
   +----------+-----------+---------------------+---------------+

                    Figure 7: Parse Codes and Meanings

4.4.  Stream Constraints

   There are some constraints which a Sequence needs to conform to in
   order to be transmissible with this specification.

   o  The sequence MUST NOT contain Parse Info Headers with a Parse Code
      which is not 0x00 (Sequence Header), 0x10 (End of Sequence), 0x20
      (Auxiliary Data), 0x30 (Padding Data) and 0xEC (High Quality
      Picture Fragment).  Some other streams MAY be convertible to meet
      this restriction (see below).

   o  Every High Quality Picture Fragment MUST be no longer than 65535
      bytes.  This can usually be ensured by splitting large fragments
      into several smaller fragments, except in the case where an
      individual slice is too large, in which case see the notes below
      on conversion.



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   o  Informative note: this requirement ensures that every High Quality
      Picture Fragment will always contain no more than 65535 slices.

   o  Every High Quality Picture Fragment SHOULD be small enough that
      the RTP packet carrying it will fit within the network MTU size.
      This can usually be ensured by splitting large fragments into
      several smaller fragments, except in the case where an individual
      slice is too large, in which case see the notes below on
      conversion.

   o  Every High Quality Picture Fragment MUST be encoded using values
      for Slice Prefix Bytes and Slice Size Scaler no greater than
      65535.

   If a Sequence intended for tranmission does not conform to these
   restrictions then it MAY be possible to simply convert it into a form
   that does by splitting pictures and/or large fragments into suitably
   sized fragments.  This can be done provided that the following
   (weaker) constraints are met:

   o  The sequence does not contain Parse Info Headers with a Parse Code
      which is not 0x00 (Sequence Header), 0x10 (End of Sequence), 0x20
      (Auxiliary Data), 0x30 (Padding Data), 0xE8 (High Quality
      Picture), and 0xEC (High Quality Picture Fragment).

   o  Every High Quality Picture or High Quality Picture Fragment
      contains no slices which are individually longer than 65535 bytes.
      Note: When this is the case the values of Slice Prefix Bytes and
      Slice Size Scaler will necessarily also be smaller than 65535.

   o  Every High Quality Picture or High Quality Picture Fragment
      contains no slices which are individually so large that an RTP
      packet carrying a Fragment containing that single slice will fit
      within the network MTU size.

   Sending a Stream which does not meet these requirements via this
   mechanism is not possible unless the stream is re-encoded by a VC-2
   Encoder so as to meet them.

   When encoding VC-2 video intended to be transported via RTP a VC-2
   profile and level which ensures these requirements are met SHOULD be
   used.

4.5.  Payload Data

   For the Sequence Header packet type (PC = 0x00) the payload data MUST
   be the coded Sequence Header exactly as it appears in the VC-2
   Sequence.



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   For the Transform Parameters packet type (PC = 0xEC and No.  Slices =
   0) the payload data MUST be the variable length coded transform
   parameters.  This MUST NOT include the fragment header (since all
   data in the picture header is already included in the packet header).

   For the Auxiliary Data packet type (PC = 0x20) the payload data MUST
   be a portion of the auxiliary data bytes contained in the Auxiliary
   data unit being being transmitted.  The B flag MUST be set on the
   packet which contains the first byte, the E flag MUST be set on the
   packet which contains the last byte, the bytes MUST be included in
   order, and the packets MUST have contiguous sequence numbers.

   For the Padding Data packet type (PC = 0x30) the payload data is
   OPTIONAL, and if present MUST be a series of 0x00 values.

   For the Picture Fragment packet type (PC = 0xEC and No.  Slices > 0)
   the payload data MUST be a specified number of coded slices in the
   same order that they appear in the VC-2 stream.  Which slices appear
   in the packet is identified using the Slice Offset X and Slice Offset
   Y fields in the payload header.

   For the End of Sequence packet type (PC = 0x10) there is no payload
   data.

4.5.1.  Reassembling the 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0x42     |      0x42     |      0x43     |      0x44     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Parse Code   |                       Next Parse Offset
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |                       Prev Parse Offset
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |
   +-+-+-+-+-+-+-+-+

                     Figure 8: VC-2 Parse Info Header

   To reassemble the data in the RTP packets into a valid VC-2 Sequence
   the receiver SHOULD:

   o  Take the data from each packet with a Parse Code of 0x00 and
      prepend a valid VC-2 Parse Info Header (Figure 8) with the same
      parse code to it.  The resulting Sequence Header Parse Info Header
      and data unit MUST be included in the output stream before any
      coded pictures which followed it in the RTP stream unless an



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      identical Sequence Header has already been included, and MAY be
      repeated at any point that results in a valid VC-2 stream.

   o  Take the data from each packet with a Parse Code of 0xEC and No.
      of Slices set to 0 (which together indicates that this packet
      contains the transform parameters for a coded picture) and prepend
      a valid VC-2 Parse Info Header (Figure 8) followed by the picture
      number, fragment data length, and slice count (0) to it with the
      same parse code.

   o  Take the data from each packet with a Parse Code of 0xEC and No.
      of Slices not set to 0 (which together indicates that this packet
      contains coded slices) and prepend a valid VC-2 Parse Info Header
      (Figure 8) followed by the picture number, fragment data length,
      slice count, x offset and y offset taken from the packet header to
      it with the same parse code.

   o  A receiver MAY combine all fragment data units (with parse code
      0xEC) and the same picture number into a single picture data unit
      with parse code 0xE8.  If the stream is required to comply with
      major versions 1 or 2 of the VC-2 Spec then this MUST be done.

   o  Take the data from each packet with a Parse Code of 0x20 and the B
      bit set and prepend a valid VC-2 Parse Info Header (Figure 8) with
      the parse code 0x20 and then take each subsequent packet with
      parse code 0x20 without the B bit set and append their payload to
      the growing data unit.  When all packets for a particular data
      unit have been received it SHOULD be included in the output
      stream.  The final packet for a data unit will have the E bit set.

   o  Once a data unit has been assembled, whether a Sequence Header,
      Coded Picture Fragment, Coded Picture, or Auxiliary Data Unit, the
      next parse offset and previous parse offset values in its Parse
      Info Header should be filled with the offset between the start of
      the header and the start of the next or previous.

   o  An End of Sequence Parse Info Header MAY be inserted when a packet
      with parse code set to 0x10 is encountered, or at any other time
      that is allowed in a valid VC-2 stream.  After an End of Sequence
      Parse Info Header is included in the output stream either the
      stream must end or it MUST be followed by a Sequence Header
      indicating the start of a new Sequence.

   o  A Padding Data Parse Info Header MAY be inserted when a packet
      with parse code set to 0x30 and the B bit set is encountered, or
      at any other time that is allowed in a valid VC-2 stream.  The
      length of this padding data MAY have any value, and its contents
      MUST be set to a series of zero bytes.



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5.  Congestion Control Considerations

   Congestion control for RTP SHALL be used in accordance with RFC 3550
   [RFC3550], and with any applicable RTP profile; e.g., RFC 3551
   [RFC3551].  An additional requirement if best-effort service is being
   used is: users of this payload format MUST monitor packet loss to
   ensure that the packet loss rate is within acceptable parameters.
   Circuit Breakers [RFC8083] is an update to RTP [RFC3550] that defines
   criteria for when one is required to stop sending RTP Packet Streams,
   and applications implementing this standard MUST comply with it.  RFC
   8085 [RFC8085] provides additional information on the best practices
   for applying congestion control to UDP streams.

   In particular it should be noted that the expected data rate for RTP
   sessions which use this profile is likely to be in the range of
   gigabits per second.  If used on a closed network which has been
   correctly provisioned for the expected data rates this might not pose
   a problem, but there is always the risk of data getting out onto the
   open internet.

6.  Payload Format Parameters

   This RTP payload format is identified using the video/vc2 media type
   which is registered in accordance with RFC 4855 [RFC4855] and using
   the template of RFC 6838 [RFC6838].

6.1.  Media Type Definition

   Type name:

      video

   Subtype name:

      vc2

   Required parameters:

      rate: The RTP timestamp clock rate.  Applications using this
      payload format SHOULD use a value of 90000.

      profile: The VC-2 profile in use, the only currently allowed value
      is "HQ".

   Optional parameters:

      version: the VC-2 specification version in use.  The only
      currently allowed value is "3" since all Sequences transported



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      using this mechanism will contain HQ Picture Fragment data units,
      which the VC-2 specification [VC2] defines as requiring version 3.

      level: The VC-2 level in use.  Any integer may be used.

   Encoding considerations:

      This media type is framed and binary, see section 4.8 in RFC6838
      [RFC6838].

   Security considerations:

      Please see security consideration in RFCXXXX

   Interoperability considerations: N/A

   Published specification:

      "VC-2 Video Compression", Society of Motion Picture and Television
      Engineers Standard ST 2042-1 [VC2]

   Applications that use this media type:

      Video Communication.

   Additional information: N/A

   Person & email address to contact for further information:

      james.barrett@bbc.co.uk

   Intended usage:

      COMMON

   Restrictions on usage:

      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [RFC3550].  Transport within other framing
      protocols is not defined at this time.

   Author:

   Change controller:

      IETF Payload working group delegated from the IESG.

   Provisional registration? (standards tree only):



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      No

6.2.  Mapping to SDP

   The mapping of the above defined payload format media type and its
   parameters SHALL be done according to Section 3 of RFC 4855
   [RFC4855].

   o  The type name ("video") goes in SDP "m=" as the media name.

   o  The subtype name ("vc2") goes in SDP "a=rtpmap" as the encoding
      name, followed by a slash ("/") and the rate parameter.

   o  The required parameter profile and the optional parameters version
      and level, when present, are included in the "a=fmtp" attribute
      line of SDP as a semicolon-separated list of parameter=value
      pairs.

   Version and level SHALL be specified in decimal when present.

   For example, a sample SDP mapping for VC-2 could be as follows:

             m=video 30000 RTP/AVP 112
             a=rtpmap:112 vc2/90000
             a=fmtp:112 profile=HQ;version=3;level=0

   In this example, a dynamic payload type 112 is used for vc-2 data.
   The 90 kHz RTP timestamp rate is specified in the "a=rtpmap" line
   after the subtype.  In the "a=fmtp:" line, profile HQ, version 3, and
   level 0 (unknown or non-standard level) are specified.

6.3.  Offer/Answer Considerations

   All parameters are declarative.

7.  IANA Considerations

   This memo requests that IANA registers video/vc2 as specified in
   Section 6.1.  The media type is also requested to be added to the
   IANA registry for "RTP Payload Format MIME types"
   (http://www.iana.org/assignments/rtp-parameters).

8.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] , and in any applicable RTP profile such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711] or RTP/



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   SAVPF [RFC5124].  However, as "Securing the RTP Protocol Framework:
   Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202]
   discusses, it is not an RTP payload format's responsibility to
   discuss or mandate what solutions are used to meet the basic security
   goals like confidentiality, integrity and source authenticity for RTP
   in general.  This responsibility lies with anyone using RTP in an
   application.  They can find guidance on available security mechanisms
   and important considerations in Options for Securing RTP Sessions
   [RFC7201].  Applications SHOULD use one or more appropriate strong
   security mechanisms.  The rest of this security consideration section
   discusses the security impacting properties of the payload format
   itself.

   This RTP payload format and its media decoder do not exhibit any
   significant non-uniformity in the receiver-side computational
   complexity for packet processing, and thus are unlikely to pose a
   denial-of-service threat due to the receipt of pathological data.
   Nor does the RTP payload format contain any active content.

   To avoid buffer overruns when processing these packets the receiver
   MUST consider both the reported fragment length and the actual
   received size of a packet containing slice data.

   In some cases the transmitter may need to decode variable length
   coded headers in order to extract some data from the VC-2 bitstream
   before assembling packets.  This process is potentially subject to
   buffer overruns if not implemented carefully.

9.  RFC Editor Considerations

   Note to RFC Editor: This section may be removed after carrying out
   all the instructions of this section.

   RFCXXXX is to be replaced by the RFC number this specification
   receives when published.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.







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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <https://www.rfc-editor.org/info/rfc8083>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [VC2]      Society of Motion Picture and Television Engineers, "VC-2
              Video Compression", Society of Motion Picture and
              Television Engineers Standard ST 2042-1, 2017,
              <http://ieeexplore.ieee.org/document/7967896/>.

10.2.  Informative References

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.








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   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <https://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <https://www.rfc-editor.org/info/rfc7202>.

Author's Address

   James P. Weaver
   BBC

   Email: james.barrett@bbc.co.uk

























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