RTP Payload Format for Essential Video Coding (EVC)
draft-ietf-avtcore-rtp-evc-07
| Document | Type | Active Internet-Draft (avtcore WG) | |
|---|---|---|---|
| Authors | Shuai Zhao , Stephan Wenger , Youngkwon Lim | ||
| Last updated | 2024-02-07 (Latest revision 2023-12-20) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
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by Marc Blanchet
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Dr. Bernard D. Aboba | ||
| Shepherd write-up | Show Last changed 2023-07-30 | ||
| IESG | IESG state | RFC Ed Queue | |
| Action Holders |
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| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Murray Kucherawy | ||
| Send notices to | bernard.aboba@gmail.com | ||
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| Details |
draft-ietf-avtcore-rtp-evc-07
avtcore S. Zhao
Internet-Draft Intel
Intended status: Standards Track S. Wenger
Expires: 21 June 2024 Tencent
Y. Lim
Samsung Electronics
19 December 2023
RTP Payload Format for Essential Video Coding (EVC)
draft-ietf-avtcore-rtp-evc-07
Abstract
This document describes an RTP payload format for the Essential Video
Coding (EVC) standard, published as ISO/IEC International Standard
23094-1. EVC was developed by the Moving Picture Experts Group
(MPEG). The RTP payload format allows for the packetization of one
or more Network Abstraction Layer (NAL) units in each RTP packet
payload and the fragmentation of a NAL unit into multiple RTP
packets. The payload format has broad applicability in
videoconferencing, Internet video streaming, and high-bitrate
entertainment-quality video, among other applications.
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
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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 21 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview of the EVC Codec . . . . . . . . . . . . . . . . 3
1.1.1. Coding-Tool Features (informative) . . . . . . . . . 4
1.1.2. Systems and Transport Interfaces . . . . . . . . . . 6
1.1.3. Parallel Processing Support (informative) . . . . . . 9
1.1.4. NAL Unit Header . . . . . . . . . . . . . . . . . . . 9
1.2. Overview of the Payload Format . . . . . . . . . . . . . 10
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 11
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1. Definitions from the EVC Standard . . . . . . . . . . 11
3.1.2. Definitions Specific to This Document . . . . . . . . 13
3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 14
4. RTP Payload Format . . . . . . . . . . . . . . . . . . . . . 16
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 16
4.2. Payload Header Usage . . . . . . . . . . . . . . . . . . 17
4.3. Payload Structures . . . . . . . . . . . . . . . . . . . 17
4.3.1. Single NAL Unit Packets . . . . . . . . . . . . . . . 18
4.3.2. Aggregation Packets (APs) . . . . . . . . . . . . . . 19
4.3.3. Fragmentation Units . . . . . . . . . . . . . . . . . 23
4.4. Decoding Order Number . . . . . . . . . . . . . . . . . . 26
5. Packetization Rules . . . . . . . . . . . . . . . . . . . . . 27
6. De-packetization Process . . . . . . . . . . . . . . . . . . 28
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 30
7.1. Media Type Registration . . . . . . . . . . . . . . . . . 30
7.2. Optional Parameters Definition . . . . . . . . . . . . . 31
7.3. SDP Parameters . . . . . . . . . . . . . . . . . . . . . 35
7.3.1. Mapping of Payload Type Parameters to SDP . . . . . . 35
7.3.2. Usage with SDP Offer/Answer Model . . . . . . . . . . 37
7.3.3. Multicast . . . . . . . . . . . . . . . . . . . . . . 41
7.3.4. Usage in Declarative Session Descriptions . . . . . . 42
7.3.5. Considerations for Parameter Sets . . . . . . . . . . 43
8. Use with Feedback Messages . . . . . . . . . . . . . . . . . 43
8.1. Picture Loss Indication (PLI) . . . . . . . . . . . . . . 43
8.2. Full Intra Request (FIR) . . . . . . . . . . . . . . . . 44
9. Security Considerations . . . . . . . . . . . . . . . . . . . 44
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 46
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11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. Normative References . . . . . . . . . . . . . . . . . . 47
13.2. Informative References . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
The Essential Video Coding [EVC] standard, which is formally
designated as ISO/IEC International Standard 23094-1 [ISO23094-1] has
been published in 2020. One goal of MPEG is to keep [EVC]'s Baseline
profile essentially royalty-free by using technologies published more
than 20 years ago or otherwise known to be available for use without
a requirement for paying royalties, whereas more advanced profiles
follow a reasonable and non-discriminatory licensing terms policy.
Both the Baseline profile and higher profiles of [EVC] are reported
to provide coding efficiency gains over High Efficiency Video Coding
[HEVC] and Advanced Video Coding [AVC] under certain configurations.
This document describes an RTP payload format for EVC. It shares its
basic design with the NAL unit-based RTP payload formats of H.264
Video Coding [RFC6184], Scalable Video Coding (SVC) [RFC6190], High
Efficiency Video Coding (HEVC) [RFC7798], and Versatile Video Coding
(VVC)[RFC9328]. With respect to design philosophy, security,
congestion control, and overall implementation complexity, it has
similar properties to those earlier payload format specifications.
This is a conscious choice, as at least [RFC6184] is widely deployed
and generally known in the relevant implementer communities. Certain
mechanisms known from [RFC6190] were incorporated as EVC supports
temporal scalability. EVC currently does not offer higher forms of
scalability.
1.1. Overview of the EVC Codec
[EVC], [AVC], [HEVC] and [VVC] share a similar hybrid video codec
design. In this document, we provide a very brief overview of those
features of EVC that are, in some form, addressed by the payload
format specified herein. Implementers have to read, understand, and
apply the ISO/IEC standard pertaining to EVC to arrive at
interoperable, well-performing implementations. The EVC standard has
a Baseline profile and a Main profile, the latter being a superset of
the Baseline profile but including more advanced features. EVC also
includes still image variants of both Baseline and Main profiles, in
each of which the bitstream is restricted to a single IDR picture.
EVC facilitates certain walled-garden implementations under
commercial constraints imposed by intellectual property rights by
including syntax elements that allow encoders to mark a bitstream as
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to what of the many independent coding tools are exercised in the
bitstream, in a spirit similar to the general_constraint_flags of
[VVC].
Conceptually, all EVC, AVC, HEVC and VVC include a Video Coding Layer
(VCL); a term that is often used to refer to the coding-tool
features, and a Network Abstraction Layer (NAL), which usually refers
to the systems and transport interface aspects of the codecs.
1.1.1. Coding-Tool Features (informative)
Coding blocks and transform structure
EVC uses a traditional block-based coding structure, which divides
the encoded image into blocks of up to 64x64 luma samples for the
Baseline profile and 128x128 luma samples for the Main profile that
can be recursively divided into smaller blocks. The baseline
profiles utilize an HEVC-like quad-tree blocks partitioning that
allows to divide a block horizontally and vertically onto four
smaller square blocks. The Main profile adds two advanced coding
structure tools: 1) Binary Ternary Tree (BTT) partitioning that
allows non-square coding units; and 2) Split Unit Coding Order
segmentation that changes the processing order of the blocks from
traditional left-to-right and top-to-bottom scanning order processing
to an alternative right-to-left and bottom-to-top scanning order. In
the Main profile, the picture can be divided into slices and tiles,
which can be independently encoded and/or decoded in parallel.
EVC also uses a traditional video codecs prediction model assuming
two general types of predictions: Intra (spatial) and Inter
(temporal) predictions. A residue block is calculated by subtracting
predicted data from the original (encoded) one. The Baseline profile
allows only discrete cosine transform (DCT-2) and scalar quantization
to transform and quantize residue data, wherein the Main profile
additionally has options to use discrete sine transform (DST-7) and
another type of discrete cosine transform (DCT-8). In addition, for
the Main profile, Improved Quantization and Transform (IQT) uses a
different mapping/clipping function for quantization. An inverse
zig-zag scanning order is used for coefficient coding. Advanced
Coefficient Coding (ADCC) in the Main profile can code coefficient
values more efficiently, for example, indicated by the last non-zero
coefficient. The Baseline profile uses a straightforward run-length
encoding (RLE) based approach to encode the quantized coefficients.
Entropy coding
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EVC uses a similar binary arithmetic coding mechanism as HEVC CABAC
and VVC. The mechanism includes a binarization step and a
probability update defined by a lookup table. In the Main profile,
the derivation process of syntax elements based on adjacent blocks
makes the context modeling and initialization process more efficient.
In-loop filtering
The Baseline profile of EVC uses the deblocking filter defined in
H.263 Annex J. In the Main profile, an Advanced Deblocking Filter
(ADDB) can be used as an alternative, which can further reduce
undesirable compression artifacts. The Main profile also defines two
additional in-loop filters that can be used to improve the quality of
decoded pictures before output and/or for inter-prediction. A
Hadamard Transform Domain Filter (HTDF) is applied to the luma
samples before deblocking, and a lookup table is used to determine
four adjacent samples for filtering. An adaptive Loop Filter (ALF)
allows to send signals of up to 25 different filters for the luma
components, and the best filter can be selected through the
classification process for each 4x4 block. Similarly to VVC, the
filter parameters of ALF are signaled in the Adaptation Parameter Set
(APS).
Inter-prediction
The basis of EVC's inter-prediction is motion compensation using
interpolation filters with a quarter sample resolution. In the
Baseline profile, a motion vector is transmitted using one of three
spatially neighboring motion vectors and a temporally collocated
motion vector as a predictor. A motion vector difference may be
signaled relative to the selected predictor, but there is a case
where no motion vector difference is signaled, and there is no
remaining data in the block. This mode is called a skip mode. The
Main profile includes six additional tools to provide improved inter-
prediction. With Advanced Motion Vectors Prediction (ADMVP),
adjacent blocks can be conceptually merged to indicate that they use
the same motion, but more advanced schemes can also be used to create
predictions from the basic model list of candidate predictors. The
Merge with Motion Vector Difference (MMVD) tool uses a process
similar to the concept of merging neighboring blocks but also allows
the use of expressions that include a starting point, motion
amplitude, and direction of motion to send a motion vector signal.
Using Advanced Motion Vector Prediction (AMVP), candidate motion
vector predictions for the block can be derived from its neighboring
blocks in the same picture and collocated blocks in the reference
picture. The Adaptive Motion Vector Resolution (AMVR) tool provides
a way to reduce the accuracy of a motion vector from a quarter sample
to half sample, full sample, double sample, or quad sample, which
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provides an efficiency advantage, such as when sending large motion
vector differences. The Main profile also includes the Decoder-side
Motion Vector Refinement (DMVR), which uses a bilateral template
matching process to refine the motion vectors without additional
signaling.
Intra prediction and intra-coding
Intra prediction in EVC is performed on adjacent samples of coding
units in a partitioned structure. For the Baseline profile, when all
coding units are square, there are five different prediction modes:
DC (mean value of the neighborhood), horizontal, vertical, and two
different diagonal directions. In the Main profile, intra prediction
can be applied to any rectangular coding unit, and 28 additional
direction modes are available in the so-called Enhanced Intra
Prediction Directions (EIPD). In the Main profile, an encoder can
also use Intra Block Copy (IBC), where previously decoded sample
blocks of the same picture are used as a predictor. A displacement
vector in integer sample precision is signaled to indicate where the
prediction block in the current picture is used for this mode.
Reference frames management
In EVC, decoded pictures can be stored in a decoded picture buffer
(DPB) for predicting pictures that follow them in the decoding order.
In the Baseline profile, the management of the DPB (i.e., the process
of adding and deleting reference pictures) is controlled by a
straightforward AVC-like sliding window approach with very few
parameters from the SPS. For the Main profile, DPB management can be
handled much more flexibly using explicitly signaled reference
Picture Lists (RPL) in the SPS or slice level.
1.1.2. Systems and Transport Interfaces
EVC inherits the basic systems and transport interface designs from
AVC and HEVC. These include the NAL-unit-based syntax, hierarchical
syntax and data unit structure, and Supplemental Enhancement
Information (SEI) message mechanism. The hierarchical syntax and
data unit structure consists of a sequence-level parameter set (SPS),
two picture-level parameter sets (PPS and APS, each of which can
apply to one or more pictures), slice-level header parameters, and
lower-level parameters.
A number of key components that influenced the Network Abstraction
Layer design of EVC as well as this document, are described below:
Sequence parameter set
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The Sequence Parameter Set (SPS) contains syntax elements
pertaining to a Coded Video Sequence (CVS), which is a group of
pictures, starting with a random access point picture and followed
by zero or more pictures that may depend on each other and the
random access point picture. In MPEG-2, the equivalent of a CVS
is a Group of Pictures (GOP), which generally started with an I
frame and is followed by P and B frames. While more complex in
its options of random access points, EVC retains this basic
concept. In many TV-like applications, a CVS contains a few
hundred milliseconds to a few seconds of video. In video
conferencing (without switching MCUs involved), a CVS can be as
long in duration as the whole session.
Picture and adaptation parameter set
The Picture Parameter Set and the Adaptation Parameter Set (PPS
and APS, respectively) carry information pertaining to a single
picture. The PPS contains information that is likely to stay
constant from picture to picture, at least for pictures of a
certain type whereas the APS contains information, such as
adaptive loop filter coefficients, that are likely to change from
picture to picture.
Profile, level, and toolsets
Profiles and levels follow the same design considerations known
from AVC, HEVC, and video codecs as old as MPEG-1 Video. The
profile defines a set of tools (not to confuse with the "toolset"
discussed below) that a decoder compliant with this profile has to
support. In EVC, profiles are defined in Annex A. Formally, they
are defined as a set of constraints that a bitstream needs to
conform to. In EVC, the Baseline profile is much more severely
constraint than the Main profile, reducing implementation
complexity. Levels relate to bitstream complexity in dimensions
such as maximum sample decoding rate, maximum picture size, and
similar parameters directly related to computational complexity
and/or memory demands.
Profiles and levels are signaled in the highest parameter set
available, the SPS.
EVC contains another mechanism related to the use of coding tools,
known as the toolset syntax element. This syntax element,
toolset_idc_h and toolset_idc_l located in the SPS, is a bitmask
that allows encoders to indicate which coding tools they are using
within the menu of profiles offered by the profile that is also
signaled. No decoder conformance point is associated with the
toolset, but a bitstream that was using a coding tool that is
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indicated as not being used in the toolset syntax element would be
non-compliant. While MPEG specifically rules out the use of the
toolset syntax element as a conformance point, walled garden
implementations could do so without incurring the interoperability
problems MPEG fears and create bitstreams and decoders that do not
support one or more given tools. That, in turn, may be useful to
mitigate certain intellectual property-related risks.
Bitstream and elementary stream
Above the Coded Video Sequence (CVS), EVC defines a video
bitstream that can be used as an elementary stream in the MPEG
systems context. For this document, the video bitstream syntax
level is not relevant.
Random access support
EVC supports random access mechanisms based on IDR and CRA access
units.
Temporal scalability support
EVC supports temporal scalability through the generalized
reference picture selection approach known since AVC/SVC. Up to
six temporal layers are supported. The temporal layer is signaled
in the NAL unit header (which co-serves as the payload header in
this document), in the nuh_temporal_id field.
Reference picture management
EVC's reference picture management is POC-based (Picture Order
Count), similar to HEVC. In the Main profile, substantially all
reference picture list manipulations available in HEVC are
available, including explicit transmissions/updates of reference
picture lists, although for reference pictures management
purposes, EVC uses a modern VVC-like RPL approach, which is
conceptually simpler than the HEVC one. In the Baseline profile,
reference picture management is more restricted, allowing for a
comparatively simple group of picture structures only.
SEI Message
EVC inherits many of HEVC's SEI Messages, occasionally with syntax
and/or semantics changes, making them applicable to EVC. In
addition, some of the codec-agnostic SEI Messages of the VSEI
specification are also mapped.
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1.1.3. Parallel Processing Support (informative)
EVC's Baseline profile includes no tools specifically addressing
parallel processing support. The Main profile includes
independently decodable slices for parallel processing. The
slices are defined as any rectangular region within a picture and
can be encoded to have no coding dependencies with other slices in
the same picture but with other slices from the previous picture.
No specific support for parallel processing is specified in this
RTP payload format.
1.1.4. NAL Unit Header
EVC maintains the NAL unit concept of [VVC] with different parameter
options. EVC also uses a two-byte NAL unit header, as shown in
Figure 1. The payload of a NAL unit refers to the NAL unit excluding
the NAL unit header.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| Type | TID | Reserve |E|
+-------------+-----------------+
The Structure of the EVC NAL Unit Header
Figure 1
The semantics of the fields in the NAL unit header are as specified
in EVC and described briefly below for convenience. In addition to
the name and size of each field, the corresponding syntax element
name in EVC is also provided.
F: 1 bit
forbidden_zero_bit. Required to be zero in EVC. Note that the
inclusion of this bit in the NAL unit header was included to
enable transport of EVC video over MPEG-2 transport systems
(avoidance of start code emulations) [MPEG2S]. In this document,
the value 1 may be used to indicate a syntax violation, e.g., for
a NAL unit resulting from aggregating a number of fragmented units
of a NAL unit but missing the last fragment, as described in
Section 4.3.3.
Type: 6 bits
nal_unit_type_plus1. This field allows the NAL Unit Type to be
computed. The NAL Unit Type (NalUnitType) is equal to the value
found in this field, minus 1; in other words:
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NalUnitType = nal_unit_type_plus1 - 1.
The NAL unit type is detailed in Table 4 of [EVC]. If the value
of NalUnitType is less than or equal to 23, the NAL unit is a VCL
NAL unit. Otherwise, the NAL unit is a non-VCL NAL unit. For a
reference of all currently defined NAL unit types and their
semantics, please refer to Section 7.4.2.2 in [EVC]. Note that
nal_unit_type_plus1 MUST NOT be zero.
TID: 3 bits
nuh_temporal_id. This field specifies the temporal identifier of
the NAL unit. The value of TemporalId is equal to TID.
TemporalId shall be equal to 0 if it is an IDR NAL unit type (NAL
unit type 1).
Reserve: 5 bits
nuh_reserved_zero_5bits. This field shall be equal to the version
of the EVC standard. Values of nuh_reserved_zero_5bits greater
than 0 are reserved for future use by ISO/IEC. Decoders
conforming to a profile specified in [EVC]'s Annex A shall ignore
(i.e., remove from the bitstream and discard) all NAL units with
values of nuh_reserved_zero_5bits greater than 0.
E: 1 bit
nuh_extension_flag. This field shall be equal to the version of
the EVC standard. The value of nuh_extension_flag equal to 1 is
reserved for future use by ISO/IEC. Decoders conforming to a
profile specified in [EVC]'s Annex A shall ignore (i.e., remove
from the bitstream and discard) all NAL units with values of
nuh_extension_flag equal to 1.
1.2. Overview of the Payload Format
This payload format defines the following processes required for
transport of EVC-coded data over RTP [RFC3550]:
* usage of RTP header with this payload format
* packetization of EVC-coded NAL units into RTP packets using three
types of payload structures: a single NAL unit, aggregation, and
fragment unit
* transmission of EVC NAL units of the same bitstream within a
single RTP stream.
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* media type parameters to be used with the Session Description
Protocol (SDP) [RFC8866]
* usage of RTCP feedback messages
2. Conventions
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 above.
3. Definitions and Abbreviations
3.1. Definitions
This document uses the terms and definitions of EVC. Section 3.1.1
lists relevant definitions from [EVC] for convenience. Section 3.1.2
provides definitions specific to this document.
3.1.1. Definitions from the EVC Standard
Access Unit: A set of NAL units that are associated with each other
according to a specified classification rule, are consecutive in
decoding order, and contain exactly one coded picture.
Adaptation parameter set (APS): A syntax structure containing syntax
elements that apply to zero or more slices as determined by zero or
more syntax elements found in slice headers.
Bitstream: A sequence of bits, in the form of a NAL unit stream or a
byte stream, that forms the representation of coded pictures and
associated data forming one or more coded video sequences (CVSs).
Coded Picture: A coded representation of a picture containing all
CTUs of the picture.
Coded Video Sequence (CVS): A sequence of access units that consists,
in decoding order, of an IDR access unit, followed by zero or more
access units that are not IDR access units, including all subsequent
access units up to but not including any subsequent access unit that
is an IDR access unit.
Coding Tree Block (CTB): An NxN block of samples for some value of N
such that the division of a component into CTBs is a partitioning.
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Coding Tree Unit (CTU): A CTB of luma samples, two corresponding CTBs
of chroma samples of a picture that has three sample arrays, or a CTB
of samples of a monochrome picture or a picture that is coded using
three separate colour planes and syntax structures used to code the
samples.
Decoded Picture: A decoded picture is derived by decoding a coded
picture.
Decoded Picture Buffer (DPB): A buffer holding decoded pictures for
reference, output reordering, or output delay specified for the
hypothetical reference decoder in Annex C of [EVC] standard.
Dynamic Range Adjustment (DRA): A mapping process that is applied to
decoded picture prior to cropping and output as part of the decoding
process and is controlled by parameters conveyed in an Adaptation
Parameter Set (APS).
Hypothetical Reference Decoder (HRD): A hypothetical decoder model
that specifies constraints on the variability of conforming NAL unit
streams or conforming byte streams that an encoding process may
produce.
IDR access unit: access unit in which the coded picture is an IDR
picture.
IDR picture: coded picture for which each VCL NAL unit has
NalUnitType equal to IDR_NUT.
Level: A defined set of constraints on the values that may be taken
by the syntax elements and variables of this document, or the value
of a transform coefficient prior to scaling.
Network Abstraction Layer (NAL) unit: A syntax structure containing
an indication of the type of data to follow and bytes containing that
data in the form of an RBSP interspersed as necessary.
Network Abstraction Layer (NAL) Unit Stream: A sequence of NAL units.
Non-IDR Picture: A coded picture that is not an IDR picture.
Non-VCL NAL Unit: A NAL unit that is not a VCL NAL unit.
Picture Parameter Set (PPS): A syntax structure containing syntax
elements that apply to zero or more entire coded pictures as
determined by a syntax element found in each slice header.
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Picture Order Count (POC): A variable that is associated with each
picture, uniquely identifies the associated picture among all
pictures in the CVS, and, when the associated picture is to be output
from the decoded picture buffer, indicates the position of the
associated picture in output order relative to the output order
positions of the other pictures in the same CVS that are to be output
from the decoded picture buffer.
Raw Byte Sequence Payload (RBSP): A syntax structure containing an
integer number of bytes that is encapsulated in a NAL unit and that
is either empty or has the form of a string of data bits containing
syntax elements followed by an RBSP stop bit and zero or more
subsequent bits equal to 0.
Sequence Parameter Set (SPS): A syntax structure containing syntax
elements that apply to zero or more entire CVSs as determined by the
content of a syntax element found in the PPS referred to by a syntax
element found in each slice header.
Slice: integer number of tiles of a picture in the tile scan of the
picture and that are exclusively contained in a single NAL unit.
Tile: rectangular region of CTUs within a particular tile column and
a particular tile row in a picture.
Tile column: rectangular region of CTUs having a height equal to the
height of the picture and width specified by syntax elements in the
PPS.
Tile row: A rectangular region of CTUs having a height specified by
syntax elements in the PPS and a width equal to the width of the
picture.
Tile scan: A specific sequential ordering of CTUs partitioning a
picture in which the CTUs are ordered consecutively in CTU raster
scan in a tile whereas tiles in a picture are ordered consecutively
in a raster scan of the tiles of the picture.
Video coding layer (VCL) NAL unit: A collective term for coded slice
NAL units and the subset of NAL units that have reserved values of
NalUnitType that are classified as VCL NAL units in this document.
3.1.2. Definitions Specific to This Document
Media-Aware Network Element (MANE): A network element, such as a
middlebox, selective forwarding unit, or application-layer gateway
that is capable of parsing certain aspects of the RTP payload headers
or the RTP payload and reacting to their contents.
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Informative note: The concept of a MANE goes beyond normal routers
or gateways in that a MANE has to be aware of the signaling (e.g.,
to learn about the payload type mappings of the media streams),
and in that it has to be trusted when working with Secure RTP
(SRTP). The advantage of using MANEs is that they allow packets
to be dropped according to the needs of the media coding. For
example, if a MANE has to drop packets due to congestion on a
certain link, it can identify and remove those packets whose
elimination produces the least adverse effect on the user
experience. After dropping packets, MANEs must rewrite RTCP
packets to match the changes to the RTP stream, as specified in
Section 7 of [RFC3550].
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in Section 7.4.2.3 in [EVC],
follow the order of NAL units in the bitstream.
NALU-time: The value that the RTP timestamp would have if the NAL
unit would be transported in its own RTP packet.
NAL unit output order: A NAL unit order in which NAL units of
different access units are in the output order of the decoded
pictures corresponding to the access units, as specified in [EVC],
and in which NAL units within an access unit are in their decoding
order.
RTP stream: See [RFC7656]. Within the scope of this document, one
RTP stream is utilized to transport a EVC bitstream, which may
contain one or more temporal sub-layers.
Transmission order: The order of packets in ascending RTP sequence
number order (in modulo arithmetic). Within an aggregation packet,
the NAL unit transmission order is the same as the order of
appearance of NAL units in the packet.
3.2. Abbreviations
AU Access Unit
AP Aggregation Packet
APS Adaptation Parameter Set
ATS Adaptive Transform Selection
B Bi-predictive
CBR Constant Bit Rate
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CPB Coded Picture Buffer
CTB Coding Tree Block
CTU Coding Tree Unit
CVS Coded Video Sequence
DPB Decoded Picture Buffer
HRD Hypothetical Reference Decoder
HSS Hypothetical Stream Scheduler
I Intra
IDR Instantaneous Decoding Refresh
LSB Least Significant Bit
LTRP Long-Term Reference Picture
MMVD Merge with Motion Vector Difference
MSB Most Significant Bit
NAL Network Abstraction Layer
P Predictive
POC Picture Order Count
PPS Picture Parameter Set
QP Quantization Parameter
RBSP Raw Byte Sequence Payload
RGB Same as GBR
SAR Sample Aspect Ratio
SEI Supplemental Enhancement Information
SODB String Of Data Bits
SPS Sequence Parameter Set
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STRP Short-Term Reference Picture
VBR Variable Bit Rate
VCL Video Coding Layer
4. RTP Payload Format
4.1. RTP Header Usage
The format of the RTP header is specified in [RFC3550] (reprinted as
Figure 2 for convenience). This payload format uses the fields of
the header in a manner consistent with that specification.
The RTP payload (and the settings for some RTP header bits) for
aggregation packets and fragmentation units are specified in
Section 4.3.2 and Section 4.3.3, respectively.
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=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP Header According to [RFC3550]
Figure 2
The RTP header information to be set according to this RTP payload
format is set as follows:
Marker bit (M): 1 bit
Set for the last packet of the access unit, carried in the current
RTP stream. This is in line with the normal use of the M bit in
video formats to allow an efficient playout buffer handling.
Payload Type (PT): 7 bits
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The assignment of an RTP payload type for this new payload format
is outside the scope of this document and will not be specified
here. The assignment of a payload type has to be performed either
through the profile used or in a dynamic way.
Sequence Number (SN): 16 bits
Set and used in accordance with [RFC3550].
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the content.
A 90 kHz clock rate MUST be used. If the NAL unit has no timing
properties of its own (e.g., parameter sets or certain SEI NAL
units), the RTP timestamp MUST be set to the RTP timestamp of the
coded picture of the access unit in which the NAL unit is
included. For SEI messages, this information is specified in
Annex D of [EVC]. Receivers MUST use the RTP timestamp for the
display process, even when the bitstream contains picture timing
SEI messages or decoding unit information SEI messages as
specified in [EVC].
Synchronization source (SSRC): 32 bits
Used to identify the source of the RTP packets. According to this
document, a single SSRC is used for all parts of a single
bitstream.
4.2. Payload Header Usage
The first two bytes of the payload of an RTP packet are referred to
as the payload header. The payload header consists of the same
fields (F, TID, Reserve and E) as the NAL unit header as shown in
Section 1.1.4, irrespective of the type of the payload structure.
The TID value indicates (among other things) the relative importance
of an RTP packet, for example, because NAL units with larger TID
value are not used for the decoding of the ones with smaller TID
value. A lower value of TID indicates a higher importance. More-
important NAL units MAY be better protected against transmission
losses than less-important NAL units.
4.3. Payload Structures
Three different types of RTP packet payload structures are specified.
A receiver can identify the type of an RTP packet payload through the
Type field in the payload header.
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The three different payload structures are as follows:
* Single NAL unit packet: Contains a single NAL unit in the payload,
and the NAL unit header of the NAL unit also serves as the payload
header. This payload structure is specified in Section 4.3.1.
* Aggregation Packet (AP): Contains more than one NAL unit within
one access unit. This payload structure is specified in
Section 4.3.2.
* Fragmentation Unit (FU): Contains a subset of a single NAL unit.
This payload structure is specified in Section 4.3.3.
4.3.1. Single NAL Unit Packets
A single NAL unit packet contains exactly one NAL unit, and consists
of a payload header as defined in Table 4 of [EVC] (denoted as
PayloadHdr), followed by a conditional 16-bit DONL field (in network
byte order), and the NAL unit payload data (the NAL unit excluding
its NAL unit header) of the contained NAL unit, as shown in Figure 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr | DONL (conditional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NAL unit payload data |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of a Single NAL Unit Packet
Figure 3
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the contained NAL
unit. If sprop-max-don-diff (defined in Section 7.2 is greater than
0, the DONL field MUST be present, and the variable DON for the
contained NAL unit is derived as equal to the value of the DONL
field. Otherwise (sprop-max-don-diff is equal to 0), the DONL field
MUST NOT be present.
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4.3.2. Aggregation Packets (APs)
Aggregation Packets (APs) enable the reduction of packetization
overhead for small NAL units, such as most of the non-VCL NAL units,
which are often only a few octets in size.
An AP aggregates NAL units of one access unit, and it MUST NOT
contain NAL units from more than one AU. Each NAL unit to be carried
in an AP is encapsulated in an aggregation unit. NAL units
aggregated in one AP are included in NAL-unit-decoding order.
An AP consists of a payload header, as defined in Table 4 of [EVC]
(denoted here as PayloadHdr with Type=56) followed by two or more
aggregation units, as shown in Figure 4.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| two or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Packet
Figure 4
The fields in the payload header of an AP are set as follows. The F
bit MUST be equal to 0 if the F bit of each aggregated NAL unit is
equal to zero; otherwise, it MUST be equal to 1. The Type field MUST
be equal to 56.
The value of TID MUST be the smallest value of TID of all the
aggregated NAL units. The value of Reserve and E MUST be equal to 0
for this specification.
Informative note: All VCL NAL units in an AP have the same TID
value since they belong to the same access unit. However, an AP
may contain non-VCL NAL units for which the TID value in the NAL
unit header may be different from the TID value of the VCL NAL
units in the same AP.
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An AP MUST carry at least two aggregation units and can carry as many
aggregation units as necessary; however, the total amount of data in
an AP obviously MUST fit into an IP packet, and the size SHOULD be
chosen so that the resulting IP packet is smaller than the path MTU
size so to avoid IP layer fragmentation. An AP MUST NOT contain FUs
specified in Section 4.3.3. APs MUST NOT be nested; i.e., an AP can
not contain another AP.
Informative note: If a receiver encounters nested Aggregation
Packets, which is against the aforementioned requirement, it has
several options, listed in order of ease of implementation: 1)
Ignore the nested AP; 2) Ignore the nested AP and report a "packet
loss" to the decoder, if such functionality exists in the API, 3)
Implement support for nested APs and extract the Network
Abstraction Layer (NAL) units from these nested APs.
The first aggregation unit in an AP consists of a conditional 16-bit
DONL field (in network byte order) followed by a 16-bit unsigned size
information (in network byte order) that indicates the size of the
NAL unit in bytes (excluding these two octets but including the NAL
unit header), followed by the NAL unit itself, including its NAL unit
header, as shown in Figure 5.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : DONL (conditional) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU size | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of the First Aggregation Unit in an AP
Figure 5
Informative note: The first octet of Figure 5 (indicated by the
first colon) belongs to a previous aggregation unit. It is
depicted to emphasize that aggregation units are octet aligned
only. Similarly, the NAL unit carried in the aggregation unit can
terminate at the octet boundary.
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the aggregated NAL
unit.
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If sprop-max-don-diff is greater than 0, the DONL field MUST be
present in an aggregation unit that is the first aggregation unit in
an AP. The variable DON for the aggregated NAL unit is derived as
equal to the value of the DONL field, and the variable DON for an
aggregation unit that is not the first aggregation unit in an AP-
aggregated NAL unit is derived as equal to the DON of the preceding
aggregated NAL unit in the same AP plus 1 modulo 65536. Otherwise
(sprop-max-don-diff is equal to 0), the DONL field MUST NOT be
present in an aggregation unit that is the first aggregation unit in
an AP
An aggregation unit that is not the first aggregation unit in an AP
will be followed immediately by a 16-bit unsigned size information
(in network byte order) that indicates the size of the NAL unit in
bytes (excluding these two octets but including the NAL unit header),
followed by the NAL unit itself, including its NAL unit header, as
shown in Figure 6.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU size | NAL unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Unit That Is Not the First
Aggregation Unit in an AP
Figure 6
Informative note: The first octet of Figure 6 (indicated by the
first colon) belongs to a previous aggregation unit. It is
depicted to emphasize that aggregation units are octet aligned
only. Similarly, the NAL unit carried in the aggregation unit can
terminate at the octet boundary.
Figure 7 presents an example of an AP that contains two aggregation
units, labeled as NALU 1 and NALU 2 in the figure, without the DONL
field being present.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 1 Data |
| . . . |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+ NALU 2 Data |
| . . . |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Packet Containing
Two Aggregation Units without the DONL Field
Figure 7
Figure 8 presents an example of an AP that contains two aggregation
units, labeled as NALU 1 and NALU 2 in the figure, with the DONL
field being present.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | NALU 1 DONL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NALU 1 Data . . . |
| |
+ . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU 2 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 2 Data |
| |
| . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Containing
Two Aggregation Units with the DONL Field
Figure 8
4.3.3. Fragmentation Units
Fragmentation Units (FUs) are introduced to enable fragmenting a
single NAL unit into multiple RTP packets, possibly without
cooperation or knowledge of the EVC encoder. A fragment of a NAL
unit consists of an integer number of consecutive octets of that NAL
unit. Fragments of the same NAL unit MUST be sent in consecutive
order with ascending RTP sequence numbers (with no other RTP packets
within the same RTP stream being sent between the first and last
fragment).
When a NAL unit is fragmented and conveyed within FUs, it is referred
to as a fragmented NAL unit. APs MUST NOT be fragmented. FUs MUST
NOT be nested; i.e., an FU must not contain a subset of another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU-
time of the fragmented NAL unit.
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An FU consists of a payload header as defined in Table 4 of [EVC]
(denoted as PayloadHdr with type=57), an FU header of one octet, a
conditional 16-bit DONL field (in network byte order), and an FU
payload, as shown in Figure 9.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=57) | FU header | DONL (cond) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| DONL (cond) | |
|-+-+-+-+-+-+-+-+ |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an FU
Figure 9
The fields in the payload header are set as follows. The Type field
MUST be equal to 57. The fields F, TID, Reserve and E MUST be equal
to the fields F, TID, Reserve and E, respectively, of the fragmented
NAL unit.
The FU header consists of an S bit, an E bit, and a 6-bit FuType
field, as shown in Figure 10.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|E| FuType |
+---------------+
The Structure of FU Header
Figure 10
The semantics of the FU header fields are as follows:
S: 1 bit
When set to 1, the S bit indicates the start of a fragmented NAL
unit, i.e., the first byte of the FU payload is also the first
byte of the payload of the fragmented NAL unit. When the FU
payload is not the start of the fragmented NAL unit payload, the S
bit MUST be set to 0.
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E: 1 bit
When set to 1, the E bit indicates the end of a fragmented NAL
unit, i.e., the last byte of the payload is also the last byte of
the fragmented NAL unit. When the FU payload is not the last
fragment of a fragmented NAL unit, the E bit MUST be set to 0.
FuType: 6 bits
The field FuType MUST be equal to the field Type of the fragmented
NAL unit.
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the fragmented NAL
unit.
If sprop-max-don-diff is greater than 0, and the S bit is equal to 1,
the DONL field MUST be present in the FU, and the variable DON for
the fragmented NAL unit is derived as equal to the value of the DONL
field. Otherwise (sprop-max-don-diff is equal to 0, or the S bit is
equal to 0), the DONL field MUST NOT be present in the FU.
A non-fragmented NAL unit MUST NOT be transmitted in one FU; i.e.,
the Start bit and End bit MUST NOT both be set to 1 in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that if the FU payloads of consecutive FUs, starting with
an FU with the S bit equal to 1 and ending with an FU with the E bit
equal to 1, are sequentially concatenated, the payload of the
fragmented NAL unit can be reconstructed. The NAL unit header of the
fragmented NAL unit is not included as such in the FU payload, but
rather the information of the NAL unit header of the fragmented NAL
unit is conveyed in F, TID, Reserve and E fields of the FU payload
headers of the FUs and the FuType field of the FU header of the FUs.
An FU payload MUST NOT be empty.
If an FU is lost, the receiver SHOULD discard all following
fragmentation units in transmission order corresponding to the same
fragmented NAL unit unless the decoder in the receiver is known to
gracefully handle incomplete NAL units.
A receiver in an endpoint or a MANE MAY aggregate the first n-1
fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
n of that NAL unit is not received. In this case, the
forbidden_zero_bit of the NAL unit MUST be set to 1 to indicate a
syntax violation.
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4.4. Decoding Order Number
For each NAL unit, the variable AbsDon is derived, representing the
decoding order number that is indicative of the NAL unit decoding
order.
Let NAL unit n be the n-th NAL unit in transmission order within an
RTP stream.
If sprop-max-don-diff is equal to 0, AbsDon[n], the value of AbsDon
for NAL unit n, is derived as equal to n.
Otherwise (sprop-max-don-diff is greater than 0), AbsDon[n] is
derived as follows, where DON[n] is the value of the variable DON for
NAL unit n:
* If n is equal to 0 (i.e., NAL unit n is the very first NAL unit in
transmission order), AbsDon[0] is set equal to DON[0].
* Otherwise (n is greater than 0), the following applies for
derivation of AbsDon[n]:
If DON[n] == DON[n-1],
AbsDon[n] = AbsDon[n-1]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] < 32768),
AbsDon[n] = AbsDon[n-1] + DON[n] - DON[n-1]
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] >= 32768),
AbsDon[n] = AbsDon[n-1] + 65536 - DON[n-1] + DON[n]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] >= 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] + 65536 - DON[n])
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] < 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] - DON[n])
For any two NAL units m and n, the following applies:
* AbsDon[n] greater than AbsDon[m] indicates that NAL unit n follows
NAL unit m in NAL unit decoding order.
* When AbsDon[n] is equal to AbsDon[m], the NAL unit decoding order
of the two NAL units can be in either order.
* AbsDon[n] less than AbsDon[m] indicates that NAL unit n precedes
NAL unit m in decoding order.
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Informative note: When two consecutive NAL units in the NAL
unit decoding order has different values of AbsDon, the the
absolute difference between the two AbsDon values may be
greater than or equal to 1.
Informative note: There are multiple reasons to allow for
the absolute difference of the values of AbsDon for two
consecutive NAL units in the NAL unit decoding order to be
greater than one. An increment by one is not required, as
at the time of associating values of AbsDon to NAL units, it
may not be known whether all NAL units are to be delivered
to the receiver. For example, a gateway might not forward
VCL NAL units of higher sub-layers or some SEI NAL units
when there is congestion in the network. In another
example, the first intra-coded picture of a pre-encoded clip
is transmitted in advance to ensure that it is readily
available in the receiver. When transmitting the first
intra-coded picture, the originator still determines how
many NAL units will be encoded before the first intra-coded
picture of the pre-encoded clip follows in decoding order.
Thus, the values of AbsDon for the NAL units of the first
intra-coded picture of the pre-encoded clip have to be
estimated when they are transmitted, and gaps in the values
of AbsDon may occur.
5. Packetization Rules
The following packetization rules apply:
* If sprop-max-don-diff is greater than 0, the transmission order of
NAL units carried in the RTP stream MAY be different from the NAL
unit decoding order. Otherwise (sprop-max-don-diff equals 0), the
transmission order of NAL units carried in the RTP stream MUST be
the same as the NAL unit decoding order.
* A NAL unit of small size SHOULD be encapsulated in an aggregation
packet together with one or more other NAL units to avoid the
unnecessary packetization overhead for small NAL units. For
example, non-VCL NAL units, such as access unit delimiters,
parameter sets, or SEI NAL units, are typically small and can
often be aggregated with VCL NAL units without violating MTU size
constraints.
* Each non-VCL NAL unit SHOULD, when possible from an MTU size match
viewpoint, be encapsulated in an aggregation packet with its
associated VCL NAL unit, as typically, a non-VCL NAL unit would be
meaningless without the associated VCL NAL unit being available.
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* For carrying precisely one NAL unit in an RTP packet, a single NAL
unit packet MUST be used.
6. De-packetization Process
The general concept behind de-packetization is to get the NAL units
out of the RTP packets in an RTP stream and pass them to the decoder
in the NAL unit decoding order.
The de-packetization process is implementation dependent. Therefore,
the following description should be seen as an example of a suitable
implementation. Other schemes may also be used as long as the output
for the same input is the same as the process described below. The
output is the same when the set of output NAL units and their order
are both identical. Optimizations relative to the described
algorithms are possible.
All normal RTP mechanisms related to buffer management apply. In
particular, duplicated or outdated RTP packets (as indicated by the
RTP sequence number and the RTP timestamp) are removed. To determine
the exact time for decoding, factors such as a possible intentional
delay to allow for proper inter-stream synchronization, MUST be
factored in.
NAL units with NAL unit type values in the range of 0 to 55,
inclusive, may be passed to the decoder. NAL-unit-like structures
with NAL unit type values in the range of 56 to 62, inclusive, MUST
NOT be passed to the decoder.
The receiver includes a receiver buffer, which is used to compensate
for transmission delay jitter within individual RTP streams and to
reorder NAL units from transmission order to the NAL unit decoding
order. In this section, the receiver operation is described under
the assumption that there is no transmission delay jitter within an
RTP stream. To clarify the distinction from a practical receiver
buffer, which is also used to compensate for transmission delay
jitter, the buffer in this section will henceforth be referred to as
the de-packetization buffer. Receivers should also prepare for
transmission delay jitter; that is, either reserve separate buffers
for transmission delay jitter buffering and de-packetization
buffering or use a receiver buffer for both transmission delay jitter
and de-packetization. Moreover, receivers should take transmission
delay jitter into account in the buffering operation, e.g., by
additional initial buffering before starting of decoding and
playback.
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The de-packetization process extracts the NAL units from the RTP
packets in an RTP stream as follows. When an RTP packet carries a
single NAL unit packet, the payload of the RTP packet is extracted as
a single NAL unit, excluding the DONL field, i.e., third and fourth
bytes, when sprop-max-don-diff is greater than 0. When an RTP packet
carries an aggregation packet, several NAL units are extracted from
the payload of the RTP packet. In this case, each NAL unit
corresponds to the part of the payload of each aggregation unit that
follows the NALU size field, as described in Section 4.3.2. When an
RTP packet carries a Fragmentation Unit (FU), all RTP packets from
the first FU (with the S field equal to 1) of the fragmented NAL unit
up to the last FU (with the E field equal to 1) of the fragmented NAL
unit are collected. The NAL unit is extracted from these RTP packets
by concatenating all FU payloads in the same order as the
corresponding RTP packets and appending the NAL unit header with the
fields F and TID set to equal the values of the fields F and TID in
the payload header of the FUs, respectively, and with the NAL unit
type set equal to the value of the field FuType in the FU header of
the FUs, as described in Section 4.3.3.
When sprop-max-don-diff is equal to 0, the de-packetization buffer
size is zero bytes, and the NAL units carried in the single RTP
stream are directly passed to the decoder in their transmission
order, which is identical to their decoding order.
When sprop-max-don-diff is greater than 0, the process described in
the remainder of this section applies.
The receiver has two buffering states: initial buffering and
buffering while playing. Initial buffering starts when the reception
is initialized. After initial buffering, decoding and playback are
started, and the buffering-while-playing mode is used.
Regardless of the buffering state, the receiver stores incoming NAL
units in reception order into the de-packetization buffer. NAL units
carried in RTP packets are stored in the de-packetization buffer
individually, and the value of AbsDon is calculated and stored for
each NAL unit.
Initial buffering lasts until the difference between the greatest and
smallest AbsDon values of the NAL units in the de-packetization
buffer is greater than or equal to the value of sprop-max-don-diff.
After initial buffering, whenever the difference between the greatest
and smallest AbsDon values of the NAL units in the de-packetization
buffer is greater than or equal to the value of sprop-max-don-diff,
the following operation is repeatedly applied until this difference
is smaller than sprop-max-don-diff:
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* The NAL unit in the de-packetization buffer with the smallest
value of AbsDon is removed from the de-packetization buffer and
passed to the decoder.
When no more NAL units are flowing into the de-packetization buffer,
all NAL units remaining in the de-packetization buffer are removed
from the buffer and passed to the decoder in the order of increasing
AbsDon values.
7. Payload Format Parameters
This section specifies the optional parameters. A mapping of the
parameters with Session Description Protocol (SDP) [RFC8866] is also
provided for applications that use SDP.
Parameters starting with the string "sprop" for stream properties can
be used by a sender to provide a receiver with the properties of the
stream that is or will be sent. The media sender (and not the
receiver) selects whether, and with what values, "sprop" parameters
are being sent. This uncommon characteristic of the "sprop"
parameters may not be intuitive in the context of some signaling
protocol concepts, especially with offer/answer. Please see
Section 7.3.2 for guidance specific to the use of sprop parameters in
the Offer/Answer case.
7.1. Media Type Registration
The receiver MUST ignore any parameter unspecified in this document.
Type name: video
Subtype name: evc
Required parameters: N/A
Optional parameters: profile-id, level-id, toolset-id, max-recv-
level-id, sprop-sps, sprop-pps, sprop-sei, sprop-max-don-diff, sprop-
depack-buf-bytes, depack-buf-cap (refer to Section 7.2 for
definitions)
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Security considerations:
See Section 9 of RFC XXXX.
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Interoperability considerations: N/A
Published specification:
Please refer to RFC XXXX and EVC standard [EVC].
Applications that use this media type:
Any application that relies on EVC-based video services over RTP
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information:
Stephan Wenger (stewe@stewe.org)
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section of RFC XXXX.
Change controller:
IETF <avtcore@ietf.org>
7.2. Optional Parameters Definition
profile-id, level-id, toolset-id:
These parameters indicate the profile, the level, and constraints
of the bitstream carried by the RTP stream, or a specific set of
the profile, the level, and constraints the receiver supports.
More specifications of these parameters, including how they relate
to syntax elements specified in [EVC] are provided below.
profile-id:
When profile-id is not present, a value of 0 (i.e., the Baseline
profile) MUST be inferred.
When used to indicate properties of a bitstream, profile-id MUST
be derived from the profile_idc in the SPS.
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EVC bitstreams transported over RTP using the technologies of this
document SHOULD refer only to SPSs that have the same value in
profile_idc, unless the sender has a priori knowledge that a
receiver can correctly decode the EVC bitstream with different
profile_idc values (for example in walled garden scenarios). As
exceptions to this rule, if the receiver is known to support
Baseline profile, a bitstream could safely end with CVS referring
to an SPS wherein profile_idc indicates the Baseline Still Picture
profile. A similar exception can be made for Main profile and
Main Still picture profile.
level-id:
When level-id is not present, a value of 90 (corresponding to
level 3, which allows for approximately SD TV resolution and frame
rates; for details please see Annex A of EVC) MUST be inferred.
When used to indicate properties of a bitstream, level-id MUST be
derived from the level_idc in the SPS.
If the level-id parameter is used for capability exchange, the
following applies. If max-recv-level-id is not present, the
default level defined by level-id indicates the highest level the
codec wishes to support. Otherwise, max-recv-level-id indicates
the highest level the codec supports for receiving. For either
receiving or sending, all levels that are lower than the highest
level supported MUST also be supported.
toolset-id:
This parameter is a base64 encoding (Section 4 of [RFC4648])
representation of a 64 bit unsigned integer bit mask derived from
the concatenation, in network byte order, of the syntax elements
toolset_idc_h and toolset_idc_l. When used to indicate properties
of a bitstream, its value MUST be derived from toolset_idh_h and
toolset_idc_l in the sequence parameter set.
max-recv-level-id:
This parameter MAY be used to indicate the highest level a
receiver supports.
The value of max-recv-level-id MUST be in the range of 0 to 255,
inclusive.P.
When max-recv-level-id is not present, the value is inferred to be
equal to level-id.
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max-recv-level-id MUST NOT be present when the highest level the
receiver supports is not higher than the default level.
sprop-sps:
This parameter MAY be used to convey sequence parameter set NAL
units of the bitstream for out-of-band transmission of sequence
parameter sets. The value of the parameter is a comma-separated
(',') list of base64 encoding (Section 4 of [RFC4648])
representations of the sequence parameter set NAL units as
specified in Section 7.3.2.1 of [EVC].
sprop-pps:
This parameter MAY be used to convey picture parameter set NAL
units of the bitstream for out-of-band transmission of picture
parameter sets. The value of the parameter is a comma-separated
(',') list of base64 encoding (Section 4 of [RFC4648])
representations of the picture parameter set NAL units as
specified in Section 7.3.2.2 of [EVC].
sprop-sei:
This parameter MAY be used to convey one or more SEI messages that
describe bitstream characteristics. When present, a decoder can
rely on the bitstream characteristics that are described in the
SEI messages for the entire duration of the session, independently
from the persistence scopes of the SEI messages as specified in
[VSEI].
The value of the parameter is a comma-separated (',') list of
base64 encoding (Section 4 of [RFC4648]) representations of SEI
NAL units as specified in [VSEI].
Informative note: Intentionally, no list of applicable or
inapplicable SEI messages is specified here. Conveying certain
SEI messages in sprop-sei may be sensible in some application
scenarios and meaningless in others. However, a few examples
are described below:
1) In an environment where the bitstream was created from film-
based source material, and no splicing is going to occur during
the lifetime of the session, the film grain characteristics SEI
message is likely meaningful, and sending it in sprop-sei
rather than in the bitstream at each entry point may help with
saving bits and allows one to configure the renderer only once,
avoiding unwanted artifacts.
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2) Examples for SEI messages that would be meaningless to be
conveyed in sprop-sei include the decoded picture hash SEI
message (it is close to impossible that all decoded pictures
have the same hashtag) or the filler payload SEI message (as
there is no point in just having more bits in SDP).
sprop-max-don-diff:
If there is no NAL unit naluA that is followed in transmission
order by any NAL unit preceding naluA in decoding order (i.e., the
transmission order of the NAL units is the same as the decoding
order), the value of this parameter MUST be equal to 0.
Otherwise, this parameter specifies the maximum absolute
difference between the decoding order number (i.e., AbsDon) values
of any two NAL units naluA and naluB, where naluA follows naluB in
decoding order and precedes naluB in transmission order.
The value of sprop-max-don-diff MUST be an integer in the range of
0 to 32767, inclusive.
When not present, the value of sprop-max-don-diff is inferred to
be equal to 0.
sprop-depack-buf-bytes:
This parameter signals the required size of the de-packetization
buffer in units of bytes. The value of the parameter MUST be
greater than or equal to the maximum buffer occupancy (in units of
bytes) of the de-packetization buffer as specified in Section 6.
The value of sprop-depack-buf-bytes MUST be an integer in the
range of 0 to 4294967295, inclusive.
When sprop-max-don-diff is present and greater than 0, this
parameter MUST be present and the value MUST be greater than 0.
When not present, the value of sprop-depack-buf-bytes is inferred
to be equal to 0.
Informative note: The value of sprop-depack-buf-bytes indicates
the required size of the de-packetization buffer only. When
network jitter can occur, an appropriately sized jitter buffer
has to be available as well.
depack-buf-cap:
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This parameter signals the capabilities of a receiver
implementation and indicates the amount of de-packetization buffer
space in units of bytes that the receiver has available for
reconstructing the NAL unit decoding order from NAL units carried
in the RTP stream. A receiver is able to handle any RTP stream
for which the value of the sprop-depack-buf-bytes parameter is
smaller than or equal to this parameter.
When not present, the value of depack-buf-cap is inferred to be
equal to 4294967295. The value of depack-buf-cap MUST be an
integer in the range of 1 to 4294967295, inclusive.
Informative note: depack-buf-cap indicates the maximum possible
size of the de-packetization buffer of the receiver only,
without allowing for network jitter.
7.3. SDP Parameters
The receiver MUST ignore any parameter unspecified in this document.
7.3.1. Mapping of Payload Type Parameters to SDP
The media type video/evc string is mapped to fields in the Session
Description Protocol (SDP) [RFC8866] as follows:
* The media name in the "m=" line of SDP MUST be video.
* The encoding name in the "a=rtpmap" line of SDP MUST be evc (the
media subtype).
* The clock rate in the "a=rtpmap" line MUST be 90000.
* The OPTIONAL parameters profile-id, level-id, toolset-id, max-
recv-level-id, sprop-max-don-diff, sprop-depack-buf-bytes, and
depack-buf-cap, when present, MUST be included in the "a=fmtp"
line of SDP. The fmtp line is expressed as a media type string,
in the form of a semicolon-separated list of parameter=value
pairs.
* The OPTIONAL parameters sprop-sps, sprop-pps, and sprop-sei, when
present, MUST be included in the "a=fmtp" line of SDP or conveyed
using the "fmtp" source attribute as specified in Section 6.3 of
[RFC5576]. For a particular media format (i.e., RTP payload
type), sprop-sps, sprop-pps, or sprop-sei MUST NOT be both
included in the "a=fmtp" line of SDP and conveyed using the "fmtp"
source attribute. When included in the "a=fmtp" line of SDP,
those parameters are expressed as a media type string, in the form
of a semicolon-separated list of parameter=value pairs. When
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conveyed in the "a=fmtp" line of SDP for a particular payload
type, the parameters sprop-sps, sprop-pps, and sprop-sei MUST be
applied to each SSRC with the payload type. When conveyed using
the "fmtp" source attribute, these parameters are only associated
with the given source and payload type as parts of the "fmtp"
source attribute.
Informative note: Conveyance of sprop-sps and sprop-pps using the
"fmtp" source attribute allows for out-of-band transport of
parameter sets in topologies like Topo-Video-switch-MCU, as
specified in [RFC7667].
A general usage of media representation in SDP is as follows:
m=video 49170 RTP/AVP 98
a=rtpmap:98 evc/90000
a=fmtp:98 profile-id=1;
sprop-sps=<sequence parameter set data>;
sprop-pps=<picture parameter set data>;
A SIP offer/answer exchange wherein both parties are expected to both
send and receive could look like the following. Only the media
codec-specific parts of the SDP are shown.
Offerer->Answerer:
m=video 49170 RTP/AVP 98
a=rtpmap:98 evc/90000
a=fmtp:98 profile-id=1; level_id=90;
The above represents an offer for symmetric video communication
using [EVC] and its payload specification at the main profile and
level 3.0. Informally speaking, this offer tells the receiver of
the offer that the sender is willing to receive up to xKpxx
resolution at the maximum bitrates specified in [EVC]. At the
same time, if this offer were accepted "as is", the offer can
expect that the answerer would be able to receive and properly
decode EVC media up to and including level 3.0.
Answerer->Offerer:
m=video 49170 RTP/AVP 98
a=rtpmap:98 evc/90000
a=fmtp:98 profile-id=1; level_id=60
Informative note: level_id shall be set equal to a value of 30
times the level number specified in Table A.1 of EVC.
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With this answer to the offer above, the system receiving the offer
advises the offerer that it is incapable of handing evc at level 3.0
but is capable of decoding level 2. As EVC video codecs must support
decoding at all levels below the maximum level they implement, the
resulting user experience would likely be that both systems send
video at level 2. However, nothing prevents an encoder from further
downgrading its sending to, for example, level 1 if it were short of
cycles or bandwidth or for other reasons.
7.3.2. Usage with SDP Offer/Answer Model
This section describes the negotiation of unicast messages using the
offer/answer model described in [RFC3264] and its updates.
This section applies to all profiles defined in [EVC], specifically
to Baseline, Main, and the associated still image profiles.
The following limitations and rules pertaining to the media
configuration apply:
The parameters identifying a media format configuration for EVC are
profile-id and level-id. Profile_id MUST be used symmetrically.
The answerer MUST structure its answer according to one of the
following three options:
- maintain all configuration parameters with the values remaining
the same as in the offer for the media format (payload type),
with the exception that the value of level-id is changeable as
long as the highest level indicated by the answer is not higher
than that indicated by the offer; or
- remove the media format (payload type) completely (when one or
more of the parameter values are not supported).
Informative note: The above requirement for symmetric use does not
apply for level-id and does not apply for the other bitstream or RTP
stream properties and capability parameters, as described in
Section 7.3.2.1 (Payload format config) below.
To simplify handling and matching of these configurations, the same
RTP payload type number used in the offer SHOULD also be used in the
answer, as specified in [RFC3264].
The answer MUST NOT contain a payload type number used in the offer
for the media subtype unless the configuration is the same as in the
offer or the configuration in the answer only differs from that in
the offer with a different value of level-id.
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7.3.2.1. Payload Format Configuration
The following limitations and rules pertain to the configuration of
the payload format buffer management.
The parameters sprop-max-don-diff and sprop-depack-buf-bytes describe
the properties of an RTP stream that the offerer or the answerer is
sending for the media format configuration. This differs from the
normal usage of the offer/answer parameters; normally, such
parameters declare the properties of the bitstream or RTP stream that
the offerer or the answerer is able to receive. When dealing with
EVC, the offerer assumes that the answerer will be able to receive
media encoded using the configuration being offered.
Informative note: The above parameters apply for any RTP stream, when
present, sent by a declaring entity with the same configuration. In
other words, the applicability of the above parameters to RTP streams
depends on the source endpoint. Rather than being bound to the
payload type, the values may have to be applied to another payload
type when being sent, as they apply for the configuration.
When an offerer offers an interleaved stream, indicated by the
presence of sprop-max-don-diff with a value larger than zero, the
offerer MUST include the size of the de-packetization buffer sprop-
depack-buf-bytes.
To enable the offerer and answerer to inform each other about their
capabilities for de-packetization buffering in receiving RTP streams,
both parties are RECOMMENDED to include depack-buf-cap.
The parameters sprop-sps, or sprop-pps, when present (included in the
"a=fmtp" line of SDP or conveyed using the "fmtp" source attribute,
as specified in Section 6.3 of [RFC5576]), are used for out-of-band
transport of the parameter sets (SPS or PPS, respectively). The
answerer MAY use either out-of-band or in-band transport of parameter
sets for the bitstream it is sending, regardless of whether out-of-
band parameter sets transport has been used in the offerer-to-
answerer direction. Parameter sets included in an answer are
independent of those parameter sets included in the offer, as they
are used for decoding two different bitstreams; one from the answerer
to the offerer and the other in the opposite direction. In case some
RTP packets are sent before the SDP offer/answer settles down, in-
band parameter sets MUST be used for those RTP stream parts sent
before the SDP offer/answer.
The following rules apply to transport of parameter sets in the
offerer-to-answerer direction.
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An offer MAY include sprop-sps, and/or sprop-pps. If none of these
parameters are present in the offer, then only in-band transport of
parameter sets is used.
If the level to use in the offerer-to-answerer direction is equal to
the default level in the offer, the answerer MUST be prepared to use
the parameter sets included in sprop-sps, and sprop-pps (either
included in the "a=fmtp" line of SDP or conveyed using the "fmtp"
source attribute) for decoding the incoming bitstream, e.g., by
passing these parameter set NAL units to the video decoder before
passing any NAL units carried in the RTP streams. Otherwise, the
answerer MUST ignore sprop-vps, sprop-sps, and sprop-pps (either
included in the "a=fmtp" line of SDP or conveyed using the "fmtp"
source attribute) and the offerer MUST transmit parameter sets in-
band.
The following rules apply to transport of parameter sets in the
answerer-to-offerer direction.
An answer MAY include sprop-sps, and/or sprop-pps. If none of these
parameters are present in the answer, then only in-band transport of
parameter sets is used.
The offerer MUST be prepared to use the parameter sets included in
sprop-sps and sprop-pps (either included in the "a=fmtp" line of SDP
or conveyed using the "fmtp" source attribute) for decoding the
incoming bitstream, e.g., by passing these parameter set NAL units to
the video decoder before passing any NAL units carried in the RTP
streams.
When sprop-sps and/or sprop-pps are conveyed using the "fmtp" source
attribute, as specified in Section 6.3 of [RFC5576], the receiver of
the parameters MUST store the parameter sets included in sprop-sps
and/or sprop-pps and associate them with the source given as part of
the "fmtp" source attribute. Parameter sets associated with one
source (given as part of the "fmtp" source attribute) MUST only be
used to decode NAL units conveyed in RTP packets from the same source
(given as part of the "fmtp" source attribute). When this mechanism
is in use, SSRC collision detection and resolution MUST be performed
as specified in [RFC5576].
Figure 11 lists the interpretation of all the parameters that MAY be
used for the various combinations of offer, answer, and direction
attributes.
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sendonly --+
recvonly --+ |
sendrecv --+ | |
| | |
profile-id C C P
level-id D D P
toolset-id C C P
max-recv-level-id R R -
sprop-max-don-diff P - P
sprop-depack-buf-bytes P - P
depack-buf-cap R R -
sprop-sei P - P
sprop-sps P - P
sprop-pps P - P
Legend:
C: configuration for sending and receiving bitstreams
D: changeable configuration, same as C, except possible to
answer with a different but consistent value (see the semantics
of the level-id parameter on these parameters being
consistent-basically, level down-grading is allowed)
P: properties of the bitstream to be sent
R: receiver capabilities
-: not usable, when present MUST be ignored
Interpretation of Parameters for Various Combinations of
Offers, Answers, and Direction Attributes.
Figure 11
Parameters used for declaring receiver capabilities are, in general,
downgradable, i.e., they express the upper limit for a sender's
possible behavior. Thus, a sender MAY select to set its encoder
using only lower/lesser or equal values of these parameters.
When a sender's capabilities are declared with the configuration
parameters, these parameters express a configuration that is
acceptable for the sender to receive bitstreams. In order to achieve
high interoperability levels, it is often advisable to offer multiple
alternative configurations. It is impossible to offer multiple
configurations in a single payload type. Thus, when multiple
configuration offers are made, each offer requires its own RTP
payload type associated with the offer.
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An implementation SHOULD be able to understand all media type
parameters (including all optional media type parameters), even if it
doesn't support the functionality related to the parameter. This, in
conjunction with proper application logic in the implementation,
allows the implementation, after having received an offer, to create
an answer by potentially downgrading one or more of the optional
parameters to the point where the implementation can cope, leading to
higher chances of interoperability beyond the most basic interop
points (for which, as described above, no optional parameters are
necessary).
Informative note: In implementations of various H.26x video coding
payload Formats including those for [AVC] and [HEVC], it was
occasionally observed that implementations were incapable of parsing
most (or all) of the optional parameters and hence rejected offers
other than the most basic offers. As a result, the offer/answer
exchange resulted in a baseline performance (using the default values
for the optional parameters) with the resulting suboptimal user
experience. However, there are valid reasons to forego the
implementation complexity of implementing the parsing of some or all
of the optional parameters, for example, when there is predetermined
knowledge, not negotiated by an SDP-based offer/answer process, of
the capabilities of the involved systems (walled gardens, baseline
requirements defined in application standards higher up in the stack,
and similar).
An answerer MAY extend the offer with additional media format
configurations. However, to enable their usage, in most cases, a
second offer is required from the offerer to provide the bitstream
property parameters that the media sender will use. This also has
the effect that the offerer has to be able to receive this media
format configuration, not only to send it.
7.3.3. Multicast
For bitstreams being delivered over multicast, the following rules
apply:
The media format configuration is identified by profile-id and level-
id. These media format configuration parameters, including level-id,
MUST be used symmetrically; that is, the answerer MUST either
maintain all configuration parameters or remove the media format
(payload type) completely. Note that this implies that the level-id
for offer/answer in multicast is not changeable.
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To simplify the handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be used in
the answer, as specified in [RFC3264]. An answer MUST NOT contain a
payload type number used in the offer unless the configuration is the
same as in the offer.
Parameter sets received MUST be associated with the originating
source and MUST only be used in decoding the incoming bitstream from
the same source.
The rules for other parameters are the same as above for unicast as
long as the three above rules are obeyed.
7.3.4. Usage in Declarative Session Descriptions
When EVC over RTP is offered with SDP in a declarative style, as in
Real Time Streaming Protocol (RTSP) [RFC7826] or Session Announcement
Protocol (SAP) [RFC2974], the following considerations apply.
All parameters capable of indicating both bitstream properties and
receiver capabilities are used to indicate only bitstream properties.
For example, in this case, the parameters profile-id and level-id
declare the values used by the bitstream, not the capabilities for
receiving bitstreams. As a result, the following interpretation of
the parameters MUST be used:
Declaring actual configuration or bitstream properties:
profile-id level-id sprop-sps sprop-pps sprop-max-don-diff sprop-
depack-buf-bytes sprop-sei
Not usable (when present, they MUST be ignored):
depack-buf-cap recv-sublayer-id
A receiver of the SDP is required to support all parameters and
values of the parameters provided; otherwise, the receiver MUST
reject (RTSP) or not participate in (SAP) the session. It falls on
the creator of the session to use values that are expected to be
supported by the receiving application.
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7.3.5. Considerations for Parameter Sets
When out-of-band transport of parameter sets is used, parameter sets
MAY still be additionally transported in-band unless explicitly
disallowed by an application, and some of these additional parameter
sets may update some of the out-of-band transported parameter sets.
An update of a parameter set refers to the sending of a parameter set
of the same type using the same parameter set ID but with different
values for at least one other parameter of the parameter set.
8. Use with Feedback Messages
The following subsections define the use of the Picture Loss
Indication (PLI) and Full Intra Request (FIR) feedback messages with
[EVC]. The PLI is defined in [RFC4585], and the FIR message is
defined in [RFC5104].
In accordance with this document, a sender MUST NOT send Slice Loss
Indication (SLI) or Reference Picture Selection Indication (RPSI),
and a receiver MUST ignore RPSI and MUST treat a received SLI as a
received PLI, ignoring the "First", "Number", and "PictureID" fields
of the PLI.
8.1. Picture Loss Indication (PLI)
As specified in Section 6.3.1 of [RFC4585], the reception of a PLI by
a media sender indicates "the loss of an undefined amount of coded
video data belonging to one or more pictures". Without having any
specific knowledge of the setup of the bitstream (such as use and
location of in-band parameter sets, IDR picture locations, picture
structures, and so forth), a reaction to the reception of a PLI by a
EVC sender SHOULD be to send an IDR picture and relevant parameter
sets, potentially with sufficient redundancy so to ensure correct
reception. However, sometimes information about the bitstream
structure is known. For example, such information can be parameter
sets that have been conveyed out of band through mechanisms not
defined in this document and that are known to stay static for the
duration of the session. In that case, it is obviously unnecessary
to send them in-band as a result of the reception of a PLI. Other
examples could be devised based on a priori knowledge of different
aspects of the bitstream structure. In all cases, the timing and
congestion control mechanisms of [RFC4585] MUST be observed.
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8.2. Full Intra Request (FIR)
The purpose of the FIR message is to force an encoder to send an
independent decoder refresh point as soon as possible while observing
applicable congestion-control-related constraints, such as those set
out in [RFC8082].
Upon reception of a FIR, a sender MUST send an IDR picture.
Parameter sets MUST also be sent, except when there is a priori
knowledge that the parameter sets have been correctly established. A
typical example for that is an understanding between the sender and
receiver, established by means outside this document, that parameter
sets are exclusively sent out of band.
9. Security Considerations
The scope of this section is limited to the payload format itself and
to one feature of [EVC] that may pose a particularly serious security
risk if implemented naively. The payload format, in isolation, does
not form a complete system. Implementers are advised to read and
understand relevant security-related documents, especially those
pertaining to RTP (see the Security Considerations section in
[RFC3550]) and the security of the call-control stack chosen (that
may make use of the media type registration of this document).
Implementers should also consider known security vulnerabilities of
video coding and decoding implementations in general and avoid those.
Within this RTP payload format, and with the exception of the user
data SEI message as described below, no security threats other than
those common to RTP payload formats are known. In other words,
neither the various media-plane-based mechanisms nor the signaling
part of this document seem to pose a security risk beyond those
common to all RTP-based systems.
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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 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 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"
[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.
Because the data compression used with this payload format is applied
end-to-end, any encryption needs to be performed after compression.
A potential denial-of-service threat exists for data encodings using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the bitstream that are complex to decode and that cause the
receiver to be overloaded.
EVC is particularly vulnerable to such attacks, as it is extremely
simple to generate datagrams containing NAL units that affect the
decoding process of many future NAL units. Therefore, the usage of
data origin authentication and data integrity protection of at least
the RTP packet is RECOMMENDED based on the thoughts of [RFC7202].
Like HEVC [RFC7798] and [VVC], [EVC] includes a user data
Supplemental Enhancement Information (SEI) message. This SEI message
allows inclusion of an arbitrary bitstring into the video bitstream.
Such a bitstring could include JavaScript, machine code, and other
active content.
[EVC] leaves the handling of this SEI message to the receiving
system. In order to avoid harmful side effects of the user data SEI
message, decoder implementations cannot naively trust its content.
For example, forwarding all received JavaScript code detected by a
decoder implementation to a web-browser unchecked would be a bad and
insecure implementation practice. The safest way to deal with user
data SEI messages is to simply discard them, but that can have
negative side effects on the quality of experience by the user.
End-to-end security with authentication, integrity, or
confidentiality protection will prevent a MANE from performing media-
aware operations other than discarding complete packets. In the case
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of confidentiality protection, it will even be prevented from
discarding packets in a media-aware way. To be allowed to perform
such operations, a MANE is required to be a trusted entity that is
included in the security context establishment.
10. Congestion Control
Congestion control for RTP SHALL be used in accordance with RTP
[RFC3550] and with any applicable RTP profile, e.g., AVP [RFC3551] or
AVPF [RFC4585]. If best-effort service is being used, an additional
requirement is that users of this payload format MUST monitor packet
loss to ensure that the packet loss rate is within an acceptable
range. Packet loss is considered acceptable if a TCP flow across the
same network path and experiencing the same network conditions would
achieve an average throughput, measured on a reasonable timescale,
that is not less than all RTP streams combined are achieved. This
condition can be satisfied by implementing congestion-control
mechanisms to adapt the transmission rate, by implementing the number
of layers subscribed for a layered multicast session, or by arranging
for a receiver to leave the session if the loss rate is unacceptably
high.
The bitrate adaptation necessary for obeying the congestion control
principle is easily achievable when real-time encoding is used, for
example, by adequately tuning the quantization parameter. However,
when pre-encoded content is being transmitted, bandwidth adaptation
requires the pre-coded bitstream to be tailored for such adaptivity.
The key mechanism available in [EVC] is temporal scalability. A
media sender can remove NAL units belonging to higher temporal sub-
layers (i.e., those NAL units with a large value of TID) until the
sending bitrate drops to an acceptable range.
The mechanisms mentioned above generally work within a defined
profile and level; therefore no renegotiation of the channel is
required. Only when non-downgradable parameters (such as profile)
are required to be changed does it become necessary to terminate and
restart the RTP stream(s). This may be accomplished by using
different RTP payload types.
MANEs MAY remove certain unusable packets from the RTP stream when
that RTP stream was damaged due to previous packet losses. This can
help reduce the network load in certain special cases. For example,
MANEs can remove those FUs where the leading FUs belonging to the
same NAL unit have been lost, because the trailing FUs are
meaningless to most decoders. MANE can also remove higher temporal
scalable layers if the outbound transmission (from the MANE's
viewpoint) experiences congestion.
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11. IANA Considerations
A new media type, as specified in Section 7.1 of this document, has
been registered with IANA.
12. Acknowledgements
Large parts of this specification share text with the RTP payload
format for VVC [RFC9328]. Roman Chernyak is thanksed for his
valueable review comments. We thank the authors of that
specification for their excellent work.
13. References
13.1. Normative References
[EVC] "ISO/IEC 23094-1 Essential Video Coding", 2020,
<https://www.iso.org/standard/57797.html>.
[ISO23094-1]
"ISO/IEC DIS Information technology --- General video
coding --- Part 1 Essential video coding", n.d.,
<https://www.iso.org/standard/57797.html>.
[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/rfc/rfc2119>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/rfc/rfc3264>.
[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/rfc/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/rfc/rfc3551>.
[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/rfc/rfc3711>.
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[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/rfc/rfc4585>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/rfc/rfc5104>.
[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/rfc/rfc5124>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<https://www.rfc-editor.org/rfc/rfc5576>.
[RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, Ed., "Real-Time Streaming Protocol
Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
2016, <https://www.rfc-editor.org/rfc/rfc7826>.
[RFC8082] Wenger, S., Lennox, J., Burman, B., and M. Westerlund,
"Using Codec Control Messages in the RTP Audio-Visual
Profile with Feedback with Layered Codecs", RFC 8082,
DOI 10.17487/RFC8082, March 2017,
<https://www.rfc-editor.org/rfc/rfc8082>.
[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/rfc/rfc8174>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/rfc/rfc8866>.
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[RFC9328] Zhao, S., Wenger, S., Sanchez, Y., Wang, Y.-K., and M. M.
Hannuksela, "RTP Payload Format for Versatile Video Coding
(VVC)", RFC 9328, DOI 10.17487/RFC9328, December 2022,
<https://www.rfc-editor.org/rfc/rfc9328>.
[VSEI] "Versatile supplemental enhancement information messages
for coded video bitstreams", 2020,
<https://www.itu.int/rec/T-REC-H.274>.
13.2. Informative References
[AVC] "ITU-T Recommendation H.264 - Advanced video coding for
generic audiovisual services", 2014,
<https://www.iso.org/standard/66069.html>.
[HEVC] "High efficiency video coding, ITU-T Recommendation
H.265", 2019, <https://www.itu.int/rec/T-REC-H.265>.
[MPEG2S] IS0/IEC, "Information technology - Generic coding ofmoving
pictures and associated audio information - Part
1:Systems, ISO International Standard 13818-1", 2013.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
October 2000, <https://www.rfc-editor.org/rfc/rfc2974>.
[RFC6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<https://www.rfc-editor.org/rfc/rfc6184>.
[RFC6190] Wenger, S., Wang, Y.-K., Schierl, T., and A.
Eleftheriadis, "RTP Payload Format for Scalable Video
Coding", RFC 6190, DOI 10.17487/RFC6190, May 2011,
<https://www.rfc-editor.org/rfc/rfc6190>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/rfc/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/rfc/rfc7202>.
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[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<https://www.rfc-editor.org/rfc/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/rfc/rfc7667>.
[RFC7798] Wang, Y.-K., Sanchez, Y., Schierl, T., Wenger, S., and M.
M. Hannuksela, "RTP Payload Format for High Efficiency
Video Coding (HEVC)", RFC 7798, DOI 10.17487/RFC7798,
March 2016, <https://www.rfc-editor.org/rfc/rfc7798>.
[VVC] "Versatile Video Coding, ITU-T Recommendation H.266",
2020, <http://www.itu.int/rec/T-REC-H.266>.
Authors' Addresses
Shuai Zhao
Intel
2200 Mission College Blvd
Santa Clara, 95054
United States of America
Email: shuai.zhao@ieee.org
Stephan Wenger
Tencent
2747 Park Blvd
Palo Alto, 94588
United States of America
Email: stewe@stewe.org
Youngkwon Lim
Samsung Electronics
6625 Excellence Way
Plano, 75013
United States of America
Email: yklwhite@gmail.com
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