Internet DRAFT - draft-ietf-payload-rtp-vc2hq
draft-ietf-payload-rtp-vc2hq
Payload Working Group J. Weaver
Internet-Draft BBC
Intended status: Standards Track August 29, 2018
Expires: March 2, 2019
RTP Payload Format for VC-2 HQ Profile Video
draft-ietf-payload-rtp-vc2hq-08
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 March 2, 2019.
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
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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 . . . . . . . . . . . . . . . . . . . . 10
4.2. Payload Header . . . . . . . . . . . . . . . . . . . . . 11
4.3. The Choice of Parse Codes (Informative) . . . . . . . . . 13
4.4. Stream Constraints . . . . . . . . . . . . . . . . . . . 13
4.5. Payload Data . . . . . . . . . . . . . . . . . . . . . . 15
4.5.1. Reassembling the Data . . . . . . . . . . . . . . . . 15
5. FEC Considerations . . . . . . . . . . . . . . . . . . . . . 17
6. Congestion Control Considerations . . . . . . . . . . . . . . 18
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 18
7.1. Media Type Definition . . . . . . . . . . . . . . . . . . 18
7.2. Mapping to SDP . . . . . . . . . . . . . . . . . . . . . 20
7.3. Offer/Answer Considerations . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
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
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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 proceding 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.
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 (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),
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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 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
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 accross 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.
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:
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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 "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).
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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 = 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 |
+---------------------------------------------------------------+
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 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.
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
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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. 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 Unit, and otherwise MUST be 0. If the
recommendations of the Stream Contraints Section of this
specification (Section 4.4) 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 of the Stream Contraints Section of this
specification (Section 4.4) 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
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.
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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 of the Stream Contraints
Section of this specification (Section 4.4) 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 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.
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, the latter will result
in dropped frames. Such an occurance is an indication either
of packet loss, joining a stream mid-picture, or of a non-
compliant transmitter.
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
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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.
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
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(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.
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 the above requirements via this
mechanism is not possible unless the stream is re-encoded by a VC-2
Encoder so as to meet them.
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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 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.
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 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
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
identical Sequence Header has already been included, and MAY be
repeated (with apropriate modfifications to the next and previous
header oddsets) 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
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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. The next parse offset of
the End of Sequence header MUST be set t 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
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.
5. 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
RFC5109 [RFC5109]. If making use of this mechanism to provide multi-
level protection then the packets SHOULD be assigned to layers based
upon their packet type, with the packet types in order of importance
being:
1. Sequence Headers
2. Fragments constaining Transform Parameters
3. Fragments containing coded slices
4. Auxiliary Data and end of Sequence
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5. Padding
It is RECOMMENDED that if multi-level 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 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.
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] and 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.
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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
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:
RFC XXXX
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.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.
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Author:
Change controller:
IETF Payload working group delegated from the IESG.
Provisional registration? (standards tree only):
No
7.2. 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.
7.3. Offer/Answer Considerations
All parameters are declarative.
8. IANA Considerations
This memo requests that IANA registers video/vc2 as specified in
Section 7.1. The media type is also requested to be added to the
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IANA registry for "RTP Payload Format MIME types"
(http://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 in any applicable RTP profile such as
RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711] or RTP/
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.
10. 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.
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11. References
11.1. 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>.
[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/>.
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11.2. 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>.
[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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