| Rfc | 8450 |
|---|
| Title | RTP Payload Format for VC-2 High Quality (HQ) Profile |
|---|
| Author | J. Weaver |
|---|
| Date | October 2018 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
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.
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
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.
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.
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.
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
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
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
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
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.
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.
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.
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.
+----------+-----------+---------------------+---------------+
| 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.
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).
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.
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
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.
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.
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
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]
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>.
[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>.
[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