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rfc:rfc8450

Internet Engineering Task Force (IETF) J. Weaver Request for Comments: 8450 BBC Category: Standards Track October 2018 ISSN: 2070-1721

       RTP Payload Format for VC-2 High Quality (HQ) Profile

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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8450.

Weaver Standards Track [Page 1] RFC 8450 VC-2 HQ RTP Payload October 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
 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions, Definitions, and Acronyms  . . . . . . . . . . .   3
 3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
 4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .  10
   4.2.  Payload Header  . . . . . . . . . . . . . . . . . . . . .  11
   4.3.  The Choice of Parse Codes (Informative) . . . . . . . . .  13
   4.4.  Stream Constraints  . . . . . . . . . . . . . . . . . . .  14
   4.5.  Payload Data  . . . . . . . . . . . . . . . . . . . . . .  15
     4.5.1.  Reassembling the Data . . . . . . . . . . . . . . . .  16
 5.  Forward Error Correction (FEC) Considerations . . . . . . . .  18
 6.  Congestion Control Considerations . . . . . . . . . . . . . .  18
 7.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  19
   7.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  19
   7.2.  Mapping to the Session Description Protocol (SDP) . . . .  21
   7.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  21
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
   10.2.  Informative References . . . . . . . . . . . . . . . . .  23
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

Weaver Standards Track [Page 2] RFC 8450 VC-2 HQ RTP Payload October 2018

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 have a low level of complexity
 and potentially 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
 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, and the type of a
 Data Unit is signaled in the preceding Parse Info Header.  The most
 important types are 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.

Weaver Standards Track [Page 3] RFC 8450 VC-2 HQ RTP Payload October 2018

 The first Data Unit in a Sequence as produced by an encoder is always
 a Sequence Header; however, Sequences can be joined in the middle, so
 it should not be assumed that the first Data Unit received will
 always be a Sequence Header.
 The High Quality (HQ) profile for VC-2 restricts the types of Parse
 Info Headers that may appear in the Sequence (and hence also the
 types of Data Unit) to only:
 o  Sequence Headers (which are always followed by a Data Unit),
 o  High Quality Pictures (which are always followed by a Data Unit),
 o  High Quality Fragments (which are always followed by a Data Unit),
 o  Auxiliary Data (which are always followed by a Data Unit),
 o  Padding Data (which are always followed by a Data Unit), and
 o  End of Sequence (which are never followed by a Data Unit).
 At the time of writing, there is 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 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

 In this specification, each RTP packet is used to carry data
 corresponding to a single Parse Info Header and its following Data
 Unit (if it has one).  A single packet MAY NOT contain data from more
 than one Parse Info Header or Data Unit.  A single Parse Info Header
 and Data Unit pair MUST NOT be split across more than one packet.
 The sole exception to this rule is that an Auxiliary Data Unit MAY be
 split between multiple packets, using the B and E flags to indicate
 start and end.

Weaver Standards Track [Page 4] RFC 8450 VC-2 HQ RTP Payload October 2018

 This specification only covers the transport of Sequence Headers
 (together with their accompanying Data Unit), High Quality Fragments
 (together with their accompanying Data Unit), Auxiliary Data
 (together with their accompanying Data Unit), and (optionally) End
 Sequence Headers and Padding Data (for which no Data Unit it
 carried).
 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) (see Section 11 of the VC-2
    specification [VC2]),
 o  a Picture Fragment containing the VC-2 Transform Parameters for a
    Picture (Figure 2) (see Section 14 of the VC-2 specification
    [VC2]),
 o  a Picture Fragment containing VC-2 Coded Slices (Figure 3) for a
    picture (see Section 14 of the VC-2 specification [VC2]),
 o  the end of a VC-2 Sequence (Figure 4) (see Section 10.5.2 of the
    VC-2 specification [VC2]),
 o  the contents of an Auxiliary Data Unit (Figure 5) (see
    Section 10.4.4 of the VC-2 specification [VC2]), and
 o  an indication of the presence of a padding Data Unit (Figure 6)
    (see Section 10.4.5 of the VC-2 specification [VC2]).
 These six packet types can be distinguished by the fact that they use
 different codes in the Parse Code ("PC") field, except for the two
 types of Picture Fragment that use the same value in PC but have
 different values in the "No. of Slices" field.
 The options for PC codes are explained in more detail in Section 4.3.

Weaver Standards Track [Page 5] RFC 8450 VC-2 HQ RTP Payload October 2018

  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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            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

Weaver Standards Track [Page 6] RFC 8450 VC-2 HQ RTP Payload October 2018

  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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            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

Weaver Standards Track [Page 7] RFC 8450 VC-2 HQ RTP Payload October 2018

  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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            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

Weaver Standards Track [Page 8] RFC 8450 VC-2 HQ RTP Payload October 2018

  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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            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

Weaver Standards Track [Page 9] RFC 8450 VC-2 HQ RTP Payload October 2018

  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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            contributing source (CSRC) identifiers             |
 |                             ....                              |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                 Optional Extension Header                     |
 |                             ....                              |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |   Extended Sequence Number    |B|E|  Reserved |   PC = 0x30   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Data Length                         |
 +---------------------------------------------------------------+
             Figure 6: RTP Payload Format for Padding Data
 All fields in the headers longer than a single bit are interpreted 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 usage:
 Marker Bit (M): 1 bit.  The marker bit MUST be set on any packet that
       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.

Weaver Standards Track [Page 10] RFC 8450 VC-2 HQ RTP Payload October 2018

 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
       picture that will follow it in the Stream.  An End of Sequence
       packet SHOULD have the same timestamp as the previous picture
       that 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.  This is needed
       since the high data rates of VC-2 Sequences mean that it is
       highly likely that the 16-bit sequence number will roll over
       too frequently to be of use for Stream synchronization.
 B: 1 bit.  MUST be set to 1 if the packet contains the first byte of
       an Auxiliary Data Unit and otherwise MUST be 0.  If the
       recommendations in Section 4.4 ("Stream Constraints") are
       followed, then every Auxiliary Data Unit will be small enough
       to fit in a single packet, and so this bit (where present) will
       always be 1.
 E: 1 bit.  MUST be set to 1 if the packet contains the final byte of
       an Auxiliary Data Unit and otherwise MUST be 0.  If the
       recommendations in Section 4.4 ("Stream Constraints") are
       followed, then every Auxiliary Data Unit will be small enough
       to fit in a single packet, and so this bit (where present) will
       always be 1.
 I: 1 bit.  MUST be set to 1 if the packet contains coded picture
       parameters or slice data from a field in an interlaced frame.
       MUST be set to 0 if the packet contains data from any part of a
       progressive frame.

Weaver Standards Track [Page 11] RFC 8450 VC-2 HQ RTP Payload October 2018

 F: 1 bit.  MUST be set to 1 if the packet contains coded picture
       parameters or slice data from the second field of an interlaced
       frame.  MUST be set 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 that 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 payload section of
       this packet.  If the recommendations in Section 4.4 ("Stream
       Constraints") are followed, then no Auxiliary Data Unit will be
       large enough to cause a packet to exceed the MTU of the
       network.
 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 receives Coded Slices packets for a picture but
       does not receive a Transform Parameters packet for that
       picture, then this is an indication of either packet loss,
       joining a Stream mid-picture, or a non-compliant transmitter.
       In this case, the receiver MAY assume that the parameters are
       unchanged since the last picture, or it MAY discard the
       picture.  Choosing between these two options is left up to the
       implementation as it will be dependent on intended use.  The
       former may result in malformed pictures, while the latter will
       result in dropped frames.
 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.

Weaver Standards Track [Page 12] RFC 8450 VC-2 HQ RTP Payload October 2018

 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.
 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.
 Figure 7 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.

Weaver Standards Track [Page 13] RFC 8450 VC-2 HQ RTP Payload October 2018

 +----------+-----------+---------------------+---------------+
 | 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

 A Sequence needs to conform to certain constraints in order to be
 transmissible with this specification.
 o  The Sequence MUST NOT contain Parse Info Headers with a Parse Code
    other than 0x00 (Sequence Header), 0x10 (End of Sequence), 0x20
    (Auxiliary Data), 0x30 (Padding Data), or 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.
 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.

Weaver Standards Track [Page 14] RFC 8450 VC-2 HQ RTP Payload October 2018

 If a Sequence intended for transmission 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
    other than 0x00 (Sequence Header), 0x10 (End of Sequence), 0x20
    (Auxiliary Data), 0x30 (Padding Data), 0xE8 (High Quality
    Picture), or 0xEC (High Quality Picture Fragment).
 o  None of the High Quality Pictures or High Quality Picture
    Fragments contain slices that 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  None of the High Quality Pictures or High Quality Picture
    Fragments contain slices that are individually so large that an
    RTP packet carrying a Fragment containing that single slice will
    fit within the network MTU size.
 It is not possible to send a Stream that does not meet the above
 requirements via this mechanism unless the Stream is re-encoded by a
 VC-2 Encoder so as to meet them.
 In addition, every Auxiliary Data Unit SHOULD be small enough that a
 single RTP packet carrying it will fit within the network MTU size.
 Since there is currently no specification for the format of Auxiliary
 Data in VC-2, the mechanism for ensuring this with an encoder
 implementation that includes Auxiliary Data Units will be dependent
 upon the implementation's use for them.
 When encoding VC-2 video intended to be transported via RTP, a VC-2
 profile and level that 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.
 For the Transform Parameters packet type (PC = 0xEC and No. of 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).

Weaver Standards Track [Page 15] RFC 8450 VC-2 HQ RTP Payload October 2018

 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 transmitted.  The B flag MUST be set on the packet
 that contains the first byte, the E flag MUST be set on the packet
 that contains the last byte, the bytes MUST be included in order, and
 the packets MUST have contiguous sequence numbers.
 For the Picture Fragment packet type (PC = 0xEC and No. of 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:
 o  The receiver SHOULD 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 (0x00).  The resulting Sequence Header
    Parse Info Header and Data Unit MUST be included in the output
    stream before any coded pictures that followed the packet being
    processed in the RTP stream, unless an identical Sequence Header
    has already been included, and they MAY be repeated (with
    appropriate modifications to the next and previous header offsets)
    at any point that results in a valid VC-2 Stream.

Weaver Standards Track [Page 16] RFC 8450 VC-2 HQ RTP Payload October 2018

 o  The receiver SHOULD take the data from each packet with a Parse
    Code of 0xEC and No. of Slices set to 0 (which together indicate
    that this packet contains the Transform Parameters for a coded
    picture) and prepend with the same Parse Code a valid VC-2 Parse
    Info Header (Figure 8) followed by the picture number, Fragment
    data length, and slice count (0).
 o  The receiver SHOULD take the data from each packet with a Parse
    Code of 0xEC and No. of Slices not set to 0 (which together
    indicate that this packet contains Coded Slices) and prepend with
    the same Parse Code 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.
 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 specification, then this MUST be
    done.
 o  The receiver SHOULD 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 its 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 header.
 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.  The next parse offset of
    the End of Sequence header MUST be set to 0, and the previous
    parse offset SHOULD be filled with the offset from the start of
    the previous Parse Info Header in the Stream.
 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

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    length of the accompanying Data Unit MAY have any value, and its
    contents MUST be set to a series of zero bytes.  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 header.

5. Forward Error Correction (FEC) Considerations

 VC-2 provides no underlying protection against data loss, so it may
 be useful to employ Forward Error Correction to the Stream.  A
 mechanism for doing this to a generic RTP stream is specified in RFC
 5109 [RFC5109].  If making use of this mechanism to provide
 multilevel protection, then the packets SHOULD be assigned to layers
 based upon their packet type, with the packet types being, in order
 of importance:
 1.  Sequence Headers
 2.  Fragments containing Transform Parameters
 3.  Fragments containing Coded Slices
 4.  Auxiliary Data and end of Sequence
 5.  Padding
 It is RECOMMENDED that if multilevel protection is to be used, then
 one layer will protect all Sequence Header packets, and a second will
 protect Sequence Headers and all Fragments.  If desired, a third
 layer MAY protect Auxiliary Data and End of Sequence packets.
 Padding data SHOULD NOT be protected by FEC.

6. Congestion Control Considerations

 Congestion control for RTP SHALL be used in accordance with RFC 3550
 [RFC3550] and 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] are 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.

Weaver Standards Track [Page 18] RFC 8450 VC-2 HQ RTP Payload October 2018

 In particular, it should be noted that the expected data rate for RTP
 sessions that use this profile is likely to be in the range of
 gigabits per second.  If used on a closed network that 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.

7. Payload Format Parameters

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

7.1. 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 value currently
    allowed is "HQ".
 Optional parameters:
    version: the VC-2 specification version in use.  The only
    currently allowed value is "3" since all Sequences transported
    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 RFC 6838
    [RFC6838].
 Security considerations:
    Please see the security considerations in RFC 8450.

Weaver Standards Track [Page 19] RFC 8450 VC-2 HQ RTP Payload October 2018

 Interoperability considerations: N/A
 Published specification:
    RFC 8450
 Applications that use this media type:
    Video Communication.
 Fragment identifier considerations: N/A
 Additional information: N/A
 Person & email address to contact for further information:
    James P. Weaver <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:
    James P. Weaver <james.barrett@bbc.co.uk>
 Change controller:
    IETF PAYLOAD Working Group delegated from the IESG.
 Provisional registration? (standards tree only):
    No

Weaver Standards Track [Page 20] RFC 8450 VC-2 HQ RTP Payload October 2018

7.2. Mapping to the Session Description Protocol (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.

7.3. Offer/Answer Considerations

 All parameters are declarative.

8. IANA Considerations

 IANA has registered 'video/vc2' as specified in Section 7.1.  The
 media type has been added to the IANA registry for "RTP Payload
 Format Media Types"
 <https://www.iana.org/assignments/rtp-parameters>.

9. 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 any applicable RTP profile such as
 RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or
 RTP/SAVPF [RFC5124].  However, as "Securing the RTP Framework: Why
 RTP Does Not Mandate a Single Media Security Solution" [RFC7202]

Weaver Standards Track [Page 21] RFC 8450 VC-2 HQ RTP Payload October 2018

 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 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.

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>.
 [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>.

Weaver Standards Track [Page 22] RFC 8450 VC-2 HQ RTP Payload October 2018

 [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]      SMPTE, "SMPTE Standard - VC-2 Video Compression",
            ST 2042-1:2017, DOI 10.5594/SMPTE.ST2042-1.2017, June
            2017, <https://ieeexplore.ieee.org/servlet/
            opac?punumber=7967894>.

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>.
 [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>.
 [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
            Correction", RFC 5109, DOI 10.17487/RFC5109, December
            2007, <https://www.rfc-editor.org/info/rfc5109>.
 [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>.

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 [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

Weaver Standards Track [Page 24]

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