Internet Engineering Task Force (IETF) S. Zhao
Request for Comments: 9584 Intel
Category: Standards Track S. Wenger
ISSN: 2070-1721 Tencent
Y. Lim
Samsung Electronics
May
June 2024
RTP Payload Format for Essential Video Coding (EVC)
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 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9584.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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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
1.1. Overview of the EVC Codec
1.1.1. Coding-Tool Features (Informative)
1.1.2. Systems and Transport Interfaces
1.1.3. Parallel Processing Support (Informative)
1.1.4. NAL Unit Header
1.2. Overview of the Payload Format
2. Conventions
3. Definitions and Abbreviations
3.1. Definitions
3.1.1. Definitions from the EVC Standard
3.1.2. Definitions Specific to This Document
3.2. Abbreviations
4. RTP Payload Format
4.1. RTP Header Usage
4.2. Payload Header Usage
4.3. Payload Structures
4.3.1. Single NAL Unit Packets
4.3.2. Aggregation Packets (APs)
4.3.3. Fragmentation Units (FUs)
4.4. Decoding Order Number
5. Packetization Rules
6. De-packetization Process
7. Payload Format Parameters
7.1. Media Type Registration
7.2. Optional Parameters Definition
7.3. SDP Parameters
7.3.1. Mapping of Payload Type Parameters to SDP
7.3.2. Usage with SDP Offer/Answer Model
7.3.3. Multicast
7.3.4. Usage in Declarative Session Descriptions
7.3.5. Considerations for Parameter Sets
8. Use with Feedback Messages
8.1. Picture Loss Indication (PLI)
8.2. Full Intra Request (FIR)
9. Security Considerations
10. Congestion Control
11. IANA Considerations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The Essential Video Coding [EVC] standard, which is formally
designated as ISO/IEC International Standard 23094-1 [EVC], was
published in 2020. One of MPEG's goals 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 [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 the RTP Payload Format for
H.264 video as described in [RFC6184] is widely deployed and
generally known in the relevant implementer communities. Certain
mechanisms described in [RFC6190] were incorporated, as EVC supports
temporal scalability. EVC currently does not offer higher forms of
scalability.
1.1. Overview of the EVC Codec
The codings described in [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
[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 to what of the many independent
coding tools are exercised in the bitstream, in a spirit similar to
the general_constraint_info 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 HEVC-like quad-tree-blocks partitioning that
allows a block to be divided horizontally and vertically into 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 or
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 RLE-
based approach to encode the quantized coefficients.
Entropy coding
EVC uses a similar binary arithmetic coding mechanism as HEVC
CABAC (context adaptive binary arithmetic coding) 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 [VIDEO-CODING]. 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 signals of up to
25 different filters to be sent for the luma components; 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
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 Enhanced Intra
Prediction Directions (EIPDs). 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 sequence parameter set (SPS).
For the Main profile, DPB management can be handled much more
flexibly using explicitly signaled Reference Picture Lists (RPLs)
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 (i.e.,
SPS), two picture-level parameter sets (i.e., 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 NAL design of EVC as
well as this document are described below:
Sequence parameter set
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 starts 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 Multipoint Control Units (MCUs)
involved), a CVS can be as long in duration as the whole session.
Picture and adaptation parameter set
The Picture Parameter Set (PPS) and the Adaptation Parameter Set
(APS) 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 be confused with the
"toolset" discussed below) that a decoder compliant with this
profile has to support. In EVC, profiles are defined in Annex A
of [EVC]. 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 constrained 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 elements. These syntax elements,
toolset_idc_h and toolset_idc_l (located in the SPS), are bitmasks
that allow 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
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 clean
random access (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, similar to HEVC.
In the Main profile, substantially all reference picture list
manipulations available in HEVC are specified, including explicit
transmissions or 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 [VSEI] are also mapped.
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. They can be encoded to have
coding dependencies with other slices from the previous picture but
not with other slices in the same 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|
+-------------+-----------------+
Figure 1: The Structure of the EVC NAL Unit Header
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:
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 of [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 Annex A of [EVC] 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 Annex A of [EVC] 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
* usage of 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 here.
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 (AU):
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 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.
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
color 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 the [EVC] standard.
Dynamic Range Adjustment (DRA):
A mapping process that is applied to the decoded picture prior to
cropping and output as part of the decoding process; it 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:
An access unit in which the coded picture is an IDR picture.
IDR Picture:
The 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.
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 DPB)
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 DPB.
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:
An integer number of tiles of a picture in the tile scan of the
picture, exclusively contained in a single NAL unit.
Tile:
A rectangular region of CTUs within a particular tile column and a
particular tile row in a picture.
Tile Column:
A 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.
| 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 of [EVC] and follows 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 an 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 (AP), 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
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 Red, Green, and Blue
SAR Sample Aspect Ratio
SEI Supplemental Enhancement Information
SODB String Of Data Bits
SPS Sequence Parameter Set
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] (included 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 APs
and Fragmentation Units (FUs) are specified in Sections 4.3.2 and
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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RTP Header According to RFC 3550
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 and 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
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
values are not used to decode the ones with smaller TID values. 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: The Structure of a Single NAL Unit Packet
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 (where sprop-max-don-diff is equal to 0), the DONL
field MUST NOT be present.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The Structure of an Aggregation Packet
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.
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
cannot contain another AP.
| Informative note: If a receiver encounters nested APs, 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; and 3)
| implement support for nested APs and extract the 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 |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The Structure of the First Aggregation Unit in an AP
| 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.
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 Decoding Order
Number (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 (where 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The Structure of an Aggregation Unit That Is Not the First
Aggregation Unit in an AP
| 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 "NALU 1" and "NALU 2", without the DONL field being
present.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: An Example of an AP Packet Containing Two Aggregation
Units without the DONL Field
Figure 8 presents an example of an AP that contains two aggregation
units, labeled "NALU 1" and "NALU 2", with the DONL field being
present.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: An Example of an AP Containing Two Aggregation Units
with the DONL Field
4.3.3. Fragmentation Units (FUs)
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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: The Structure of an FU
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 |
+---------------+
Figure 10: The Structure of FU Header
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.
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 (where sprop-max-don-diff is equal to 0, or where
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 S-bit and E-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.
Instead, 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.
4.4. Decoding Order Number
For each NAL unit, the variable AbsDon is derived; it represents 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, then AbsDon[n] (the value of
AbsDon for NAL unit n) is derived as equal to n.
Otherwise (where 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 (where 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:
* When AbsDon[n] is greater than AbsDon[m], the 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.
* When AbsDon[n] is less than AbsDon[m], the NAL unit n precedes NAL
unit m in decoding order.
| Informative note: When two consecutive NAL units in the NAL
| unit decoding order has different values of AbsDon, the
| absolute difference between the two AbsDon values may be
| greater than or equal to 1.
| Informative note: There are multiple reasons to allow 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 (where 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 AP 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 AP with its associated VCL NAL
unit as, typically, a non-VCL NAL unit would be meaningless
without the associated VCL NAL unit being available.
* A single NAL unit packet MUST be used for carrying precisely one
NAL unit in an RTP packet.
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
considered.
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 decoding and
playback.
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 AP, 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:
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 the 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 [RFC3550].
Security considerations: See Section 9 of RFC 9584.
Interoperability considerations: N/A
Published specification: Please refer to RFC 9584 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 9584.
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.
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 a
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 standard-definition
television (SD TV) resolution and frame rates; 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 representation (Section 4 of
[RFC4648]) 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.
When max-recv-level-id is not present, the value is inferred to be
equal to level-id.
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 representations (Section 4 of
[RFC4648]) 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 representations (Section 4 of
[RFC4648]) 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 representations (Section 4 of [RFC4648]) 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
| couple of 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 allow one
| to configure the renderer only once, avoiding unwanted
| artifacts.
|
| 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:
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: The value of depack-buf-cap indicates the
| maximum possible size of the de-packetization buffer of the
| receiver only, without allowing for network jitter. When
| network jitter occurs, an appropriately sized jitter buffer
| has to be available as well.
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 "a=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
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.
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.
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].
With this answer to the offer above, the system receiving the offer
advises the Offerer that it is incapable of handling evc at level 3
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 two 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
| Configuration").
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.
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.
- 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.
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
Figure 11: Interpretation of Parameters for Various Combinations
of Offers, Answers, and Direction Attributes
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.
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. This
leads 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, and 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.
* 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
the 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:
o profile-id
o level-id
o sprop-sps
o sprop-pps
o sprop-max-don-diff
o sprop-depack-buf-bytes
o sprop-sei
- Not usable (when present, they MUST be ignored):
o depack-buf-cap
o 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.
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) [RFC4585] and Full Intra Request (FIR) [RFC5104]
feedback messages with [EVC].
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 an
EVC sender SHOULD be to send an IDR picture and relevant parameter
sets, potentially with sufficient redundancy so as 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.
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 in Section 14 of
[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.
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 lies 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 [RFC7202].
Like HEVC [RFC7798] and VVC [VVC], EVC [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 [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
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. 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 the
profile) are required to be changed does it become necessary to
terminate and restart the RTP streams. 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.
11. IANA Considerations
The media type specified in Section 7.1 has been registered with
IANA.
12. References
12.1. Normative References
[EVC] "Information technology -- General video coding -- Part 1:
Essential video coding", ISO/IEC 23094-1:2020, October
2020, <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/info/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/info/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/info/rfc3550>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/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/info/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/info/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/info/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/info/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/info/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/info/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/info/rfc8866>.
[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/info/rfc9328>.
[VSEI] ITU-T, "Versatile supplemental enhancement information
messages for coded video bitstreams", ITU-T
Recommendation H.274, March 2024,
<https://www.itu.int/rec/T-REC-H.274>.
12.2. Informative References
[AVC] ITU-T, "Part 10: Advanced video coding", ITU-T
Recommendation H.264, October 2014,
<https://www.iso.org/standard/66069.html>.
[HEVC] ITU-T, "High efficiency video coding", ITU-T
Recommendation H.265, November 2019,
<https://www.itu.int/rec/T-REC-H.265>.
[MPEG2S] IS0/IEC, "Information technology - Generic coding of
moving pictures and associated audio information - Part 1:
Systems", ISO/IEC 13818-1:2013, June 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/info/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/info/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/info/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/info/rfc7201>.
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
2014, <https://www.rfc-editor.org/info/rfc7202>.
[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/info/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/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/info/rfc7798>.
[VIDEO-CODING]
ITU-T, "Video coding for low bit rate communication",
ITU-T Recommendation H.263, January 2005,
<https://www.itu.int/rec/T-REC-H.263>.
[VVC] ITU-T, "Versatile video coding", ITU-T
Recommendation H.266, August 2020,
<http://www.itu.int/rec/T-REC-H.266>.
Acknowledgements
Large parts of this specification share text with the RTP payload
format for VVC [RFC9328]. Roman Chernyak is thanked for his valuable
review comments. We thank the authors of that specification for
their excellent work.
Authors' Addresses
Shuai Zhao
Intel
2200 Mission College Blvd
Santa Clara, California 95054
United States of America
Email: shuai.zhao@ieee.org
Stephan Wenger
Tencent
2747 Park Blvd
Palo Alto, California 94588
United States of America
Email: stewe@stewe.org
Youngkwon Lim
Samsung Electronics
6625 Excellence Way
Plano, Texas 75013
United States of America
Email: yklwhite@gmail.com