RTP Payload Format for VC-2 HQ Profile Video
draft-ietf-payload-rtp-vc2hq-03

Abstract

This memo describes an RTP Payload format for the High Quality (HQ) profile of SMPTE 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 http://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 February 11, 2018.

Copyright Notice

Copyright (c) 2017 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 (http://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
• 2. Conventions, Definitions and Acronyms
• 3. Media Format Description
• 4. Payload format
• 4.1. RTP Header Usage
• 4.2. Payload Header
• 4.3. The Choice of Parse Codes (Informative)
• 4.4. Payload Data
• 4.4.1. Reassembling the Data
• 5. Congestion Control Considerations
• 6. Payload Format Parameters
• 6.1. Media Type Definition
• 6.2. Mapping to SDP
• 6.2.1. Offer/Answer Considerations
• 7. IANA Considerations
• 8. Security Considerations
• 9. RFC Editor Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Author's Address

1. Introduction

This memo specifies an RTP payload format for the video coding standard SMPTE ST 2042-1:2017 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 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", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

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:

• Sequence Headers,
• High Quality Pictures,
• High Quality Fragments,
• Auxiliary Data,
• Padding Data, and
• 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 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.

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: one which carries the VC-2 Sequence Header, one which carries the Picture Fragment containing the VC-2 Transform Parameters for a Picture, one which carries a Picture Fragment containing VC-2 Coded Slices for a picture, one which signals the end of a VC-2 Sequence, one which carries the contents of an auxiliary data unit, and one which indicates the presence of a padding data unit.

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.

 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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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

 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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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

 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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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 Slices

 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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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

 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         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Time Stamp                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             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

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 extneded to 32-bits, and this field MUST holds the low-order 16-bits of this value.

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.

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

SHOULD 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

SHOULD 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 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.

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 sendt.

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.

In the VC-2 specification this value is not restricted to 16 bits, but in practice this is unlikely to ever be too large.

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.

In the VC-2 specification this value is not restricted to 16 bits, but in practice this is unlikely to ever be too large.

Fragment Length: 16 bits

Contains the number of bytes of data contained in the coded payload section of this packet.

No. of Slices: 16 bits

Contains 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

Indicates 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

Indicates 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.

If an incoming stream to be transmitted contains High Quality Picture Data Units (Parse Code 0xE8) then these MUST be converted into High Quality Picture Fragments (Parse Code 0xEC) before the data is placed in the RTP packets.

+----------+-----------+---------------------+---------------+
| PC (hex) | Binary    | Description         | Origin        |
+----------+-----------+---------------------+---------------+
| 0x00     | 0000 0000 | Sequence Header     | VC-2 Spec     |
| 0x10     | 0001 0000 | End of Sequence     | VC-2 Spec     |
+----------+-----------+---------------------+---------------+
| 0xEC     | 1110 1100 | HQ Picture Fragment | VC-2 Spec     |
+----------+-----------+---------------------+---------------+
          

Figure 7: Parse Codes and Meanings

4.4. 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. 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.

4.4.1. 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:

• Take the data from each packet with a Parse Code of 0x00 and prepend a valid VC-2 Parse Info header 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 identical sequence header has already been included, and MAY be repeated at any point that results in a valid VC-2 stream.
• 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 followed by the picture number, fragment data length, and slice count (0) to it with the same parse code.
• 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 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.
• 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 version 2 of the VC-2 Spec then this MUST be done.
• 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 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.
• 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.
• 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.
• 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.

5. Congestion Control Considerations

Congestion control for RTP SHALL be used in accordance with RFC 3550, and with any applicable RTP profile; e.g., RFC 3551. 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 is an update to RTP that defines criteria for when one is required to stop sending RTP Packet Streams. The circuit breakers is to be implemented and followed.

6. Payload Format Parameters

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

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: N/A

Encoding considerations:

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

Security considerations:

• Please see security consideration in RFCXXXX

Interoperability considerations: N/A

Published specification:

• "VC-2 Video Compression", SMPTE Standard ST 2042-1

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):

• No

(Any other information that the author deems interesting may be added below this line.)

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.

6.2.1. 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 , and in any applicable RTP profile such as RTP/AVP, RTP/AVPF, RTP/SAVP or RTP/SAVPF. However, as "Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single Media Security Solution" 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 lays on anyone using RTP in an application. They can find guidance on available security mechanisms and important considerations in Options for Securing RTP Sessions. 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.

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

10.2. Informative References

Author's Address