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Internet Engineering Task Force (IETF) S. Zhao
Request for Comments: 9328 Intel
Category: Standards Track S. Wenger
ISSN: 2070-1721 Tencent
Y. Sanchez
Fraunhofer HHI
Y.-K. Wang
Bytedance Inc.
M. M Hannuksela
Nokia Technologies
December 2022
RTP Payload Format for Versatile Video Coding (VVC)
Abstract
This memo describes an RTP payload format for the Versatile Video
Coding (VVC) specification, which was published as both ITU-T
Recommendation H.266 and ISO/IEC International Standard 23090-3. VVC
was developed by the Joint Video Experts Team (JVET). The RTP
payload format allows for packetization of one or more Network
Abstraction Layer (NAL) units in each RTP packet payload, as well as
fragmentation of a NAL unit into multiple RTP packets. The payload
format has wide 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/rfc9328.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include 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 VVC Codec
1.1.1. Coding-Tool Features (Informative)
1.1.2. Systems and Transport Interfaces (Informative)
1.1.3. High-Level Picture Partitioning (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 VVC Specification
3.1.2. Definitions Specific to This Memo
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
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 Versatile Video Coding specification was formally published as
both ITU-T Recommendation H.266 [VVC] and ISO/IEC International
Standard 23090-3 [ISO23090-3]. VVC is reported to provide
significant coding efficiency gains over High Efficiency Video Coding
[HEVC], also known as H.265, and other earlier video codecs.
This memo specifies an RTP payload format for VVC. It shares its
basic design with the NAL-unit-based RTP payload formats of Advanced
Video Coding (AVC) [RFC6184], Scalable Video Coding (SVC) [RFC6190],
and High Efficiency Video Coding (HEVC) [RFC7798], as well as their
respective predecessors. With respect to design philosophy,
security, congestion control, and overall implementation complexity,
it has similar properties to those earlier payload format
specifications. This is a conscious choice, as at least [RFC6184] is
widely deployed and generally known in the relevant implementer
communities. Certain scalability-related mechanisms known from
[RFC6190] were incorporated into this document, as VVC version 1
supports temporal, spatial, and signal-to-noise ratio (SNR)
scalability.
1.1. Overview of the VVC Codec
VVC and HEVC share a similar hybrid video codec design. In this
memo, we provide a very brief overview of those features of VVC that
are, in some form, addressed by the payload format specified herein.
Implementers have to read, understand, and apply the ITU-T/ISO/IEC
specifications pertaining to VVC to arrive at interoperable, well-
performing implementations.
Conceptually, both VVC and HEVC include a Video Coding Layer (VCL),
which is often used to refer to the coding-tool features, and a NAL,
which is often used to refer to the systems and transport interface
aspects of the codecs.
1.1.1. Coding-Tool Features (Informative)
Coding-tool features are described below with occasional reference to
the coding-tool set of HEVC, which is well known in the community.
Similar to earlier hybrid-video-coding-based standards, including
HEVC, the following basic video coding design is employed by VVC. A
prediction signal is first formed by either intra- or motion-
compensated prediction, and the residual (the difference between the
original and the prediction) is then coded. The gains in coding
efficiency are achieved by redesigning and improving almost all parts
of the codec over earlier designs. In addition, VVC includes several
tools to make the implementation on parallel architectures easier.
Finally, VVC includes temporal, spatial, and SNR scalability, as well
as multiview coding support.
Coding blocks and transform structure
Among major coding-tool differences between HEVC and VVC, one of
the important improvements is the more flexible coding tree
structure in VVC, i.e., multi-type tree. In addition to quadtree,
binary and ternary trees are also supported, which contributes
significant improvement in coding efficiency. Moreover, the
maximum size of a coding tree unit (CTU) is increased from 64x64
to 128x128. To improve the coding efficiency of chroma signal,
luma-chroma-separated trees at CTU level may be employed for intra
slices. The square transforms in HEVC are extended to non-square
transforms for rectangular blocks resulting from binary and
ternary tree splits. Besides, VVC supports multiple transform
sets (MTSs), including DCT-2, DST-7, and DCT-8, as well as the
non-separable secondary transform. The transforms used in VVC can
have different sizes with support for larger transform sizes. For
DCT-2, the transform sizes range from 2x2 to 64x64, and for DST-7
and DCT-8, the transform sizes range from 4x4 to 32x32. In
addition, VVC also support sub-block transform for both intra- and
inter-coded blocks. For intra-coded blocks, intra sub-
partitioning (ISP) may be used to allow sub-block-based intra
prediction and transform. For inter blocks, sub-block transform
may be used assuming that only a part of an inter block has non-
zero transform coefficients.
Entropy coding
Similar to HEVC, VVC uses a single entropy-coding engine, which is
based on context adaptive binary arithmetic coding [CABAC] but
with the support of multi-window sizes. The window sizes can be
initialized differently for different context models. Due to such
a design, it has more efficient adaptation speed and better coding
efficiency. A joint chroma residual coding scheme is applied to
further exploit the correlation between the residuals of two color
components. In VVC, different residual coding schemes are applied
for regular transform coefficients and residual samples generated
using transform-skip mode.
In-loop filtering
VVC has more feature support in loop filters than HEVC. The
deblocking filter in VVC is similar to HEVC but operates at a
smaller grid. After deblocking and sample adaptive offset (SAO),
an adaptive loop filter (ALF) may be used. As a Wiener filter,
ALF reduces distortion of decoded pictures. Besides, VVC
introduces a new module called luma mapping with chroma scaling to
fully utilize the dynamic range of signal so that rate-distortion
performance of both Standard Dynamic Range (SDR) and High Dynamic
Range (HDR) content is improved.
Motion prediction and coding
Compared to HEVC, VVC introduces several improvements in this
area. First, there is the adaptive motion vector resolution
(AMVR), which can save bit cost for motion vectors by adaptively
signaling motion vector resolution. Then, the affine motion
compensation is included to capture complicated motion-like
zooming and rotation. Meanwhile, prediction refinement with the
optical flow (PROF) with affine mode is further deployed to mimic
affine motion at the pixel level. Thirdly, the decoder-side
motion vector refinement (DMVR) is a method to derive the motion
vector at the decoder side based on block matching so that fewer
bits may be spent on motion vectors. Bidirectional optical flow
(BDOF) is a similar method to PROF. BDOF adds a sample-wise
offset at the 4x4 sub-block level that is derived with equations
based on gradients of the prediction samples and a motion
difference relative to coding-unit (CU) motion vectors.
Furthermore, merge with motion vector difference (MMVD) is a
special mode that further signals a limited set of motion vector
differences on top of merge mode. In addition to MMVD, there are
another three types of special merge modes, i.e., sub-block merge,
triangle, and combined intra/inter prediction (CIIP). The sub-
block merge list includes one candidate of sub-block temporal
motion vector prediction (SbTMVP) and up to four candidates of
affine motion vectors. Triangle is based on triangular block
motion compensation. CIIP combines intra and inter predictions
with weighting. Adaptive weighting may be employed with a block-
level tool called bi-prediction with CU-based weighting (BCW),
which provides more flexibility than in HEVC.
Intra prediction and intra coding
To capture the diversified local image texture directions with
finer granularity, VVC supports 65 angular directions instead of
33 directions in HEVC. The intra mode coding is based on a 6-
most-probable-modes scheme, and the 6 most probable modes are
derived using the neighboring intra prediction directions. In
addition, to deal with the different distributions of intra
prediction angles for different block aspect ratios, a wide-angle-
intra-prediction (WAIP) scheme is applied in VVC by including
intra prediction angles beyond those present in HEVC. Unlike
HEVC, which only allows using the most adjacent line of reference
samples for intra prediction, VVC also allows using two further
reference lines, known as multi-reference-line (MRL) intra
prediction. The additional reference lines can be only used for
the 6 most probable intra prediction modes. To capture the strong
correlation between different color components, in VVC, a cross-
component linear mode (CCLM) is utilized, which assumes a linear
relationship between the luma sample values and their associated
chroma samples. For intra prediction, VVC also applies a
position-dependent prediction combination (PDPC) for refining the
prediction samples closer to the intra prediction block boundary.
Matrix-based intra prediction (MIP) modes are also used in VVC,
which generates an up to 8x8 intra prediction block using a
weighted sum of downsampled neighboring reference samples, and the
weights are hard-coded constants.
Other coding-tool features
VVC introduces dependent quantization (DQ) to reduce quantization
error by state-based switching between two quantizers.
1.1.2. Systems and Transport Interfaces (Informative)
VVC inherits the basic systems and transport interface designs from
HEVC and AVC. These include the NAL-unit-based syntax structure, the
hierarchical syntax and data unit structure, the supplemental
enhancement information (SEI) message mechanism, and the video
buffering model based on the hypothetical reference decoder (HRD).
The scalability features of VVC are conceptually similar to the
scalable extension of HEVC, known as SHVC. The hierarchical syntax
and data unit structure consists of parameter sets at various levels
(i.e., decoder, sequence (pertaining to all), sequence (pertaining to
a single), and picture), picture-level header parameters, slice-level
header parameters, and lower-level parameters.
A number of key components that influenced the network abstraction
layer design of VVC, as well as this memo, are described below
Decoding capability information
The decoding capability information (DCI) includes parameters that
stay constant for the lifetime of a VVC bitstream in the duration
of a video conference, continuous video stream, and similar, i.e.,
any video that is processed by a decoder between setup and
teardown. For streaming, the requirement of constant parameters
pertains through splicing. Such information includes profile,
level, and sub-profile information to determine a maximum
capability interop point that is guaranteed to never be exceeded,
even if splicing of video sequences occurs within a session. It
further includes constraint fields (most of which are flags),
which can optionally be set to indicate that the video bitstream
will be constrained in the use of certain features, as indicated
by the values of those fields. With this, a bitstream can be
labeled as not using certain tools, which allows, among other
things, for resource allocation in a decoder implementation.
Video parameter set
The video parameter set (VPS) pertains to one or more coded video
sequences (CVSs) of multiple layers covering the same range of
access units and includes, among other information, decoding
dependency expressed as information for reference-picture-list
construction of enhancement layers. The VPS provides a "big
picture" of a scalable sequence, including what types of operation
points are provided; the profile, tier, and level of the operation
points; and some other high-level properties of the bitstream that
can be used as the basis for session negotiation and content
selection, etc. One VPS may be referenced by one or more sequence
parameter sets.
Sequence parameter set
The sequence parameter set (SPS) contains syntax elements
pertaining to a coded layer video sequence (CLVS), which is a
group of pictures belonging to the same layer, starting with a
random access point, and followed by pictures that may depend on
each other until the next random access point picture. In MPEG-2,
the equivalent of a CVS was a group of pictures (GOP), which
normally started with an I frame and was followed by P and B
frames. While more complex in its options of random access
points, VVC retains this basic concept. One remarkable difference
of VVC is that a CLVS may start with a Gradual Decoding Refresh
(GDR) picture without requiring presence of traditional random
access points in the bitstream, such as instantaneous decoding
refresh (IDR) or clean random access (CRA) pictures. 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 zero or more pictures and
zero or more slices, respectively. The PPS contains information
that is likely to stay constant from picture to picture, at least
for pictures for a certain type, whereas the APS contains
information, such as adaptive loop filter coefficients, that are
likely to change from picture to picture or even within a picture.
A single APS is referenced by all slices of the same picture if
that APS contains information about luma mapping with chroma
scaling (LMCS) or a scaling list. Different APSs containing ALF
parameters can be referenced by slices of the same picture.
Picture header
A picture header (PH) contains information that is common to all
slices that belong to the same picture. Being able to send that
information as a separate NAL unit when pictures are split into
several slices allows for saving bitrate, compared to repeating
the same information in all slices. However, there might be
scenarios where low-bitrate video is transmitted using a single
slice per picture. Having a separate NAL unit to convey that
information incurs in an overhead for such scenarios. For such
scenarios, the picture header syntax structure is directly
included in the slice header, instead of its own NAL unit. The
mode of the picture header syntax structure being included in its
own NAL unit or not can only be switched on/off for an entire CLVS
and can only be switched off when, in the entire CLVS, each
picture contains only one slice.
Profile, tier, and level
The profile, tier, and level syntax structures in DCI, VPS, and
SPS contain profile, tier, and level information for all layers
that refer to the DCI, for layers associated with one or more
output layer sets specified by the VPS, and for any layer that
refers to the SPS, respectively.
Sub-profiles
Within the VVC specification, a sub-profile is a 32-bit number,
coded according to ITU-T Recommendation T.35, that does not carry
semantics. It is carried in the profile_tier_level structure and
hence is (potentially) present in the DCI, VPS, and SPS. External
registration bodies can register a T.35 codepoint with ITU-T
registration authorities and associate with their registration a
description of bitstream restrictions beyond the profiles defined
by ITU-T and ISO/IEC. This would allow encoder manufacturers to
label the bitstreams generated by their encoder as complying with
such sub-profile. It is expected that upstream standardization
organizations (such as Digital Video Broadcasting (DVB) and
Advanced Television Systems Committee (ATSC)), as well as walled-
garden video services, will take advantage of this labeled system.
In contrast to "normal" profiles, it is expected that sub-profiles
may indicate encoder choices traditionally left open in the
(decoder-centric) video coding specifications, such as GOP
structures, minimum/maximum Quantizer Parameter (QP) values, and
the mandatory use of certain tools or SEI messages.
General constraint fields
The profile_tier_level structure carries a considerable number of
constraint fields (most of which are flags), which an encoder can
use to indicate to a decoder that it will not use a certain tool
or technology. They were included in reaction to a perceived
market need to label a bitstream as not exercising a certain tool
that has become commercially unviable.
Temporal scalability support
VVC includes support of temporal scalability, by the inclusion of
the signaling of TemporalId in the NAL unit header, the
restriction that pictures of a particular temporal sublayer cannot
be used for inter prediction reference by pictures of a lower
temporal sublayer, the sub-bitstream extraction process, and the
requirement that each sub-bitstream extraction output be a
conforming bitstream. Media-Aware Network Elements (MANEs) can
utilize the TemporalId in the NAL unit header for stream
adaptation purposes based on temporal scalability.
Reference picture resampling (RPR)
In AVC and HEVC, the spatial resolution of pictures cannot change
unless a new sequence using a new SPS starts, with an intra random
access point (IRAP) picture. VVC enables picture resolution
change within a sequence at a position without encoding an IRAP
picture, which is always intra coded. This feature is sometimes
referred to as reference picture resampling (RPR), as the feature
needs resampling of a reference picture used for inter prediction
when that reference picture has a different resolution than the
current picture being decoded. RPR allows resolution change
without the need of coding an IRAP picture and hence avoids a
momentary bit rate spike caused by an IRAP picture in streaming or
video conferencing scenarios, e.g., to cope with network condition
changes. RPR can also be used in application scenarios wherein
zooming of the entire video region or some region of interest is
needed.
Spatial, SNR, and multiview scalability
VVC includes support for spatial, SNR, and multiview scalability.
Scalable video coding is widely considered to have technical
benefits and enrich services for various video applications.
Until recently, however, the functionality has not been included
in the first version of specifications of the video codecs. In
VVC, however, all those forms of scalability are supported in the
first version of VVC natively through the signaling of the
nuh_layer_id in the NAL unit header, the VPS that associates
layers with the given nuh_layer_id to each other, reference
picture selection, reference picture resampling for spatial
scalability, and a number of other mechanisms not relevant for
this memo.
Spatial scalability
With the existence of reference picture resampling (RPR), the
additional burden for scalability support is just a
modification of the high-level syntax (HLS). The inter-layer
prediction is employed in a scalable system to improve the
coding efficiency of the enhancement layers. In addition to
the spatial and temporal motion-compensated predictions that
are available in a single-layer codec, the inter-layer
prediction in VVC uses the possibly resampled video data of the
reconstructed reference picture from a reference layer to
predict the current enhancement layer. The resampling process
for inter-layer prediction, when used, is performed at the
block level, reusing the existing interpolation process for
motion compensation in single-layer coding. It means that no
additional resampling process is needed to support spatial
scalability.
SNR scalability
SNR scalability is similar to spatial scalability except that
the resampling factors are 1:1. In other words, there is no
change in resolution, but there is inter-layer prediction.
Multiview scalability
The first version of VVC also supports multiview scalability,
wherein a multi-layer bitstream carries layers representing
multiple views, and one or more of the represented views can be
output at the same time.
SEI messages
Supplemental enhancement information (SEI) messages are
information in the bitstream that do not influence the decoding
process as specified in the VVC specification but address issues
of representation/rendering of the decoded bitstream, label the
bitstream for certain applications, and other, similar tasks. The
overall concept of SEI messages and many of the messages
themselves has been inherited from the AVC and HEVC
specifications. Except for the SEI messages that affect the
specification of the hypothetical reference decoder (HRD), other
SEI messages for use in the VVC environment, which are generally
useful also in other video coding technologies, are not included
in the main VVC specification but in a companion specification
[VSEI].
1.1.3. High-Level Picture Partitioning (Informative)
VVC inherited the concept of tiles and wavefront parallel processing
(WPP) from HEVC, with some minor to moderate differences. The basic
concept of slices was kept in VVC but designed in an essentially
different form. VVC is the first video coding standard that includes
subpictures as a feature, which provides the same functionality as
HEVC motion-constrained tile sets (MCTSs) but designed differently to
have better coding efficiency and to be friendlier for usage in
application systems. More details of these differences are described
below.
Tiles and WPP
Same as in HEVC, a picture can be split into tile rows and tile
columns in VVC, in-picture prediction across tile boundaries is
disallowed, etc. However, the syntax for signaling of tile
partitioning has been simplified by using a unified syntax design
for both the uniform and the non-uniform mode. In addition,
signaling of entry point offsets for tiles in the slice header is
optional in VVC, while it is mandatory in HEVC. The WPP design in
VVC has two differences compared to HEVC: i) the CTU row delay is
reduced from two CTUs to one CTU, and ii) signaling of entry point
offsets for WPP in the slice header is optional in VVC while it is
mandatory in HEVC.
Slices
In VVC, the conventional slices based on CTUs (as in HEVC) or
macroblocks (as in AVC) have been removed. The main reasoning
behind this architectural change is as follows. The advances in
video coding since 2003 (the publication year of AVC v1) have been
such that slice-based error concealment has become practically
impossible due to the ever-increasing number and efficiency of in-
picture and inter-picture prediction mechanisms. An error-
concealed picture is the decoding result of a transmitted coded
picture for which there is some data loss (e.g., loss of some
slices) of the coded picture or a reference picture, as at least
some part of the coded picture is not error-free (e.g., that
reference picture was an error-concealed picture). For example,
when one of the multiple slices of a picture is lost, it may be
error-concealed using an interpolation of the neighboring slices.
While advanced video coding prediction mechanisms provide
significantly higher coding efficiency, they also make it harder
for machines to estimate the quality of an error-concealed
picture, which was already a hard problem with the use of simpler
prediction mechanisms. Advanced in-picture prediction mechanisms
also cause the coding efficiency loss due to splitting a picture
into multiple slices to be more significant. Furthermore, network
conditions become significantly better while, at the same time,
techniques for dealing with packet losses have become
significantly improved. As a result, very few implementations
have recently used slices for maximum-transmission-unit-size
matching. Instead, substantially all applications where low-delay
error resilience is required (e.g., video telephony and video
conferencing) rely on system/transport-level error resilience
(e.g., retransmission or forward error correction) and/or picture-
based error resilience tools (e.g., feedback-based error
resilience, insertion of IRAPs, scalability with a higher
protection level of the base layer, and so on). Considering all
the above, nowadays, it is very rare that a picture that cannot be
correctly decoded is passed to the decoder, and when such a rare
case occurs, the system can afford to wait for an error-free
picture to be decoded and available for display without resulting
in frequent and long periods of picture freezing seen by end
users.
Slices in VVC have two modes: rectangular slices and raster-scan
slices. The rectangular slice, as indicated by its name, covers a
rectangular region of the picture. Typically, a rectangular slice
consists of several complete tiles. However, it is also possible
that a rectangular slice is a subset of a tile and consists of one
or more consecutive, complete CTU rows within a tile. A raster-
scan slice consists of one or more complete tiles in a tile
raster-scan order; hence, the region covered by raster-scan slices
need not but could have a non-rectangular shape, but it may also
happen to have the shape of a rectangle. The concept of slices in
VVC is therefore strongly linked to or based on tiles instead of
CTUs (as in HEVC) or macroblocks (as in AVC).
Subpictures
VVC is the first video coding standard that includes the support
of subpictures as a feature. Each subpicture consists of one or
more complete rectangular slices that collectively cover a
rectangular region of the picture. A subpicture may be either
specified to be extractable (i.e., coded independently of other
subpictures of the same picture and of earlier pictures in
decoding order) or not extractable. Regardless of whether a
subpicture is extractable or not, the encoder can control whether
in-loop filtering (including deblocking, SAO, and ALF) is applied
across the subpicture boundaries individually for each subpicture.
Functionally, subpictures are similar to the motion-constrained
tile sets (MCTSs) in HEVC. They both allow independent coding and
extraction of a rectangular subset of a sequence of coded pictures
for use cases like viewport-dependent 360-degree video streaming
optimization and region of interest (ROI) applications.
There are several important design differences between subpictures
and MCTSs. First, the subpictures featured in VVC allow motion
vectors of a coding block to point outside of the subpicture, even
when the subpicture is extractable by applying sample padding at
the subpicture boundaries, in this case, similarly as at picture
boundaries. Second, additional changes were introduced for the
selection and derivation of motion vectors in the merge mode and
in the decoder-side motion vector refinement process of VVC. This
allows higher coding efficiency compared to the non-normative
motion constraints applied at the encoder-side for MCTSs. Third,
rewriting of slice headers (SHs) (and PH NAL units, when present)
is not needed when extracting one or more extractable subpictures
from a sequence of pictures to create a sub-bitstream that is a
conforming bitstream. In sub-bitstream extractions based on HEVC
MCTSs, rewriting of SHs is needed. Note that, in both HEVC MCTSs
extraction and VVC subpictures extraction, rewriting of SPSs and
PPSs is needed. However, typically, there are only a few
parameter sets in a bitstream, whereas each picture has at least
one slice; therefore, rewriting of SHs can be a significant burden
for application systems. Fourth, slices of different subpictures
within a picture are allowed to have different NAL unit types.
Fifth, VVC specifies HRD and level definitions for subpicture
sequences, thus the conformance of the sub-bitstream of each
extractable subpicture sequence can be ensured by encoders.
1.1.4. NAL Unit Header
VVC maintains the NAL unit concept of HEVC with modifications. VVC
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|Z| LayerID | Type | TID |
+---------------+---------------+
Figure 1: The Structure of the VVC NAL Unit Header
The semantics of the fields in the NAL unit header are as specified
in VVC and described briefly below for convenience. In addition to
the name and size of each field, the corresponding syntax element
name in VVC is also provided.
F: 1 bit
forbidden_zero_bit. This field is required to be zero in VVC.
Note that the inclusion of this bit in the NAL unit header was to
enable transport of VVC video over MPEG-2 transport systems
(avoidance of start code emulations) [MPEG2S]. In the context of
this payload format, the value 1 may be used to indicate a syntax
violation, e.g., for a NAL unit resulted from aggregating a number
of fragmented units of a NAL unit but missing the last fragment,
as described in the last sentence of Section 4.3.3.
Z: 1 bit
nuh_reserved_zero_bit. This field is required to be zero in VVC,
and reserved for future extensions by ITU-T and ISO/IEC.
This memo does not overload the "Z" bit for local extensions a)
because overloading the "F" bit is sufficient and b) in order to
preserve the usefulness of this memo to possible future versions
of [VVC].
LayerId: 6 bits
nuh_layer_id. This field identifies the layer a NAL unit belongs
to, wherein a layer may be, e.g., a spatial scalable layer, a
quality scalable layer, a layer containing a different view, etc.
Type: 5 bits
nal_unit_type. This field specifies the NAL unit type, as defined
in Table 5 of [VVC]. For a reference of all currently defined NAL
unit types and their semantics, please refer to Section 7.4.2.2 in
[VVC].
TID: 3 bits
nuh_temporal_id_plus1. This field specifies the temporal
identifier of the NAL unit plus 1. The value of TemporalId is
equal to TID minus 1. A TID value of 0 is illegal to ensure that
there is at least one bit in the NAL unit header equal to 1 in
order to enable the consideration of start code emulations in the
NAL unit payload data independent of the NAL unit header.
1.2. Overview of the Payload Format
This payload format defines the following processes required for
transport of VVC coded data over RTP [RFC3550]:
* usage of the RTP header with this payload format
* packetization of VVC coded NAL units into RTP packets using three
types of payload structures: a single NAL unit packet, aggregation
packet, and fragment unit
* transmission of VVC NAL units of the same bitstream within a
single RTP stream
* 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 VVC. Section 3.1.1
lists relevant definitions from [VVC] for convenience. Section 3.1.2
provides definitions specific to this memo. All the used terms and
definitions in this memo are verbatim copies from the [VVC]
specification.
3.1.1. Definitions from the VVC Specification
Access unit (AU):
A set of PUs that belong to different layers and contain coded
pictures associated with the same time for output from the DPB.
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 a sequence of AUs forming
one or more coded video sequences (CVSs).
Coded picture:
A coded representation of a picture comprising VCL NAL units with
a particular value of nuh_layer_id within an AU and containing all
CTUs of the picture.
Clean random access (CRA) PU:
A PU in which the coded picture is a CRA picture.
Clean random access (CRA) picture:
An IRAP picture for which each VCL NAL unit has nal_unit_type
equal to CRA_NUT.
Coded video sequence (CVS):
A sequence of AUs that consists, in decoding order, of a CVSS AU,
followed by zero or more AUs that are not CVSS AUs, including all
subsequent AUs up to but not including any subsequent AU that is a
CVSS AU.
Coded video sequence start (CVSS) AU:
An AU in which there is a PU for each layer in the CVS and the
coded picture in each PU is a CLVSS picture.
Coded layer video sequence (CLVS):
A sequence of PUs with the same value of nuh_layer_id that
consists, in decoding order, of a CLVSS PU, followed by zero or
more PUs that are not CLVSS PUs, including all subsequent PUs up
to but not including any subsequent PU that is a CLVSS PU.
Coded layer video sequence start (CLVSS) PU:
A PU in which the coded picture is a CLVSS picture.
Coded layer video sequence start (CLVSS) picture:
A coded picture that is an IRAP picture with
NoOutputBeforeRecoveryFlag equal to 1 or a GDR picture with
NoOutputBeforeRecoveryFlag equal to 1.
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
colour planes and syntax structures used to code the samples.
Coding Unit (CU):
A coding block of luma samples, two corresponding coding blocks of
chroma samples of a picture that has three sample arrays in the
single tree mode, or a coding block of luma samples of a picture
that has three sample arrays in the dual tree mode, or two coding
blocks of chroma samples of a picture that has three sample arrays
in the dual tree mode, or a coding block of samples of a
monochrome picture, and syntax structures used to code the
samples.
Decoding Capability Information (DCI):
A syntax structure containing syntax elements that apply to the
entire bitstream.
Decoded picture buffer (DPB):
A buffer holding decoded pictures for reference, output
reordering, or output delay specified for the hypothetical
reference decoder.
Gradual decoding refresh (GDR) picture:
A picture for which each VCL NAL unit has nal_unit_type equal to
GDR_NUT.
Instantaneous decoding refresh (IDR) PU:
A PU in which the coded picture is an IDR picture.
Instantaneous decoding refresh (IDR) picture:
An IRAP picture for which each VCL NAL unit has nal_unit_type
equal to IDR_W_RADL or IDR_N_LP.
Intra random access point (IRAP) AU:
An AU in which there is a PU for each layer in the CVS and the
coded picture in each PU is an IRAP picture.
Intra random access point (IRAP) PU:
A PU in which the coded picture is an IRAP picture.
Intra random access point (IRAP) picture:
A coded picture for which all VCL NAL units have the same value of
nal_unit_type in the range of IDR_W_RADL to CRA_NUT, inclusive.
Layer:
A set of VCL NAL units that all have a particular value of
nuh_layer_id and the associated non-VCL NAL units.
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 with emulation prevention bytes.
Network abstraction layer (NAL) unit stream:
A sequence of NAL units.
Output Layer Set (OLS):
A set of layers for which one or more layers are specified as the
output layers.
Operation point (OP):
A temporal subset of an OLS, identified by an OLS index and a
highest value of TemporalId.
Picture Header (PH):
A syntax structure containing syntax elements that apply to all
slices of a coded picture.
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 unit (PU):
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.
Random access:
The act of starting the decoding process for a bitstream at a
point other than the beginning of the bitstream.
Raw Byte Sequence Payload (RBSP):
A syntax structure containing an integer number of bytes that is
encapsulated in a NAL unit and 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 CLVSs as determined by the content of a syntax
element found in the PPS referred to by a syntax element found in
each picture header.
Slice:
An integer number of complete tiles or an integer number of
consecutive complete CTU rows within a tile of a picture that are
exclusively contained in a single NAL unit.
Slice header (SH):
A part of a coded slice containing the data elements pertaining to
all tiles or CTU rows within a tile represented in the slice.
Sublayer:
A temporal scalable layer of a temporal scalable bitstream
consisting of VCL NAL units with a particular value of the
TemporalId variable, and the associated non-VCL NAL units.
Subpicture:
A rectangular region of one or more slices within a picture.
Sublayer representation:
A subset of the bitstream consisting of NAL units of a particular
sublayer and the lower sublayers.
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 a width specified by syntax elements in the
picture parameter set.
Tile row:
A rectangular region of CTUs having a height specified by syntax
elements in the picture parameter set and a width equal to the
width 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 nal_unit_type that are
classified as VCL NAL units in this Specification.
3.1.2. Definitions Specific to This Memo
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 7.4.2.4 in [VVC], follow the order of NAL
units in the bitstream.
RTP stream (see [RFC7656]):
Within the scope of this memo, one RTP stream is utilized to
transport a VVC bitstream, which may contain one or more layers,
and each layer may contain one or more temporal sublayers.
Transmission order:
The order of packets in ascending RTP sequence number order (in
modulo arithmetic). Within an aggregation packet, the NAL unit
transmission order is the same as the order of appearance of NAL
units in the packet.
3.2. Abbreviations
AU Access Unit
AP Aggregation Packet
APS Adaptation Parameter Set
CTU Coding Tree Unit
CVS Coded Video Sequence
DPB Decoded Picture Buffer
DCI Decoding Capability Information
DON Decoding Order Number
FIR Full Intra Request
FU Fragmentation Unit
GDR Gradual Decoding Refresh
HRD Hypothetical Reference Decoder
IDR Instantaneous Decoding Refresh
IRAP Intra Random Access Point
MANE Media-Aware Network Element
MTU Maximum Transfer Unit
NAL Network Abstraction Layer
NALU Network Abstraction Layer Unit
OLS Output Layer Set
PLI Picture Loss Indication
PPS Picture Parameter Set
RPSI Reference Picture Selection Indication
SEI Supplemental Enhancement Information
SLI Slice Loss Indication
SPS Sequence Parameter Set
VCL Video Coding Layer
VPS Video Parameter Set
4. RTP Payload Format
4.1. RTP Header Usage
The format of the RTP header is specified in [RFC3550] (reprinted as
Figure 2 for convenience). This payload format uses the fields of
the header in a manner consistent with that specification.
The RTP payload (and the settings for some RTP header bits) for
aggregation packets and fragmentation units are specified in 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, in transmission order, among each set of
packets that contain NAL units of one access unit. 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 packet 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 set and SEI NAL units), the
RTP timestamp MUST be set to the RTP timestamp of the coded
pictures of the access unit in which the NAL unit (according to
Section 7.4.2.4 of [VVC]) is included. 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 [VVC].
| Informative note: When picture timing SEI messages are
| present, the RTP sender is responsible to ensure that the
| RTP timestamps are consistent with the timing information
| carried in the picture timing SEI messages.
Synchronization source (SSRC): 32 bits
Used to identify the source of the RTP packets. 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, Z, LayerId, Type, and TID) as the NAL unit header 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 belonging to higher
temporal sublayers are not used for the decoding of lower temporal
sublayers. 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 5 of [VVC] (denoted here as
PayloadHdr), following with 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 (sprop-max-don-diff is equal to 0), the DONL field
MUST NOT be present.
4.3.2. Aggregation Packets (APs)
Aggregation packets (APs) can reduce 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 5 of [VVC]
(denoted here as PayloadHdr with Type=28), 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=28) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| 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 28.
The value of LayerId MUST be equal to the lowest value of LayerId of
all the aggregated NAL units. The value of TID MUST be the lowest
value of TID of all the aggregated NAL units.
| 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 than the TID value of the VCL
| NAL units in the same AP.
| Informative note: If a system envisions subpicture-level or
| picture-level modifications, for example, by removing
| subpictures or pictures of a particular layer, a good design
| choice on the sender's side would be to aggregate NAL units
| belonging to only the same subpicture or picture of a
| particular layer.
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 MTU size
in order to avoid IP layer fragmentation. An AP MUST NOT contain the
FUs specified in Section 4.3.3. APs MUST NOT be nested, i.e., an AP
cannot contain another AP.
The first aggregation unit in an AP consists of a conditional 16-bit
DONL field (in network byte order), followed by 16 bits of unsigned
size information (in network byte order) that indicate 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, and the variable DON for the aggregated NAL unit is derived as
equal to the value of the DONL field, and the variable DON for an
aggregation unit that is not the first aggregation unit in an AP-
aggregated NAL unit is derived as equal to the DON of the preceding
aggregated NAL unit in the same AP plus 1 modulo 65536. Otherwise
(sprop-max-don-diff is equal to 0), the DONL field MUST NOT be
present in an aggregation unit that is the first aggregation unit in
an AP.
An aggregation unit that is not the first aggregation unit in an AP
will be followed immediately by 16 bits of unsigned size information
(in network byte order) that indicate 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 as 1 and 2 in the figure, 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=28) | 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 as 1 and 2 in the figure, 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=28) | 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
Fragmentation Units (FUs) are introduced to enable fragmenting a
single NAL unit into multiple RTP packets, possibly without
cooperation or knowledge of the [VVC] 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 cannot 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 5 of [VVC]
(denoted here as PayloadHdr with Type=29), 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=29) | 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 29. The fields F, LayerId, and TID MUST be equal to
the fields F, LayerId, and TID, respectively, of the fragmented NAL
unit.
The FU header consists of an S bit, an E bit, an R bit, and a 5-bit
FuType field, as shown in Figure 10.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|P| FuType |
+---------------+
Figure 10: The Structure of the 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.
P: 1 bit
When set to 1, the P bit indicates the last FU of the last VCL NAL
unit of a coded picture, i.e., the last byte of the FU payload is
also the last byte of the last VCL NAL unit of the coded picture.
When the FU payload is not the last fragment of the last VCL NAL
unit of a coded picture, the P bit MUST be set to 0.
FuType: 5 bits
The field FuType MUST be equal to the field Type of the fragmented
NAL unit.
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the fragmented NAL
unit.
If sprop-max-don-diff is greater than 0, and the S bit is equal to 1,
the DONL field MUST be present in the FU, and the variable DON for
the fragmented NAL unit is derived as equal to the value of the DONL
field. Otherwise (sprop-max-don-diff is equal to 0, or the S bit is
equal to 0), the DONL field MUST NOT be present in the FU.
A non-fragmented NAL unit MUST NOT be transmitted in one FU, i.e.,
the Start bit and End bit must not both be set to 1 in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that, if the FU payloads of consecutive FUs, starting
with an FU with the S bit equal to 1 and ending with an FU with the E
bit equal to 1, are sequentially concatenated, the payload of the
fragmented NAL unit can be reconstructed. The NAL unit header of the
fragmented NAL unit is not included as such in the FU payload, but
rather the information of the NAL unit header of the fragmented NAL
unit is conveyed in the F, LayerId, and TID 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
be prepared to gracefully handle incomplete NAL units.
A receiver in an endpoint or in 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, representing the
decoding order number that is indicative of the NAL unit decoding
order.
Let NAL unit n be the n-th NAL unit in transmission order within an
RTP stream.
If sprop-max-don-diff is equal to 0, AbsDon[n], the value of AbsDon
for NAL unit n, is derived as equal to n.
Otherwise (sprop-max-don-diff is greater than 0), AbsDon[n] is
derived as follows, where DON[n] is the value of the variable DON for
NAL unit n:
* If n is equal to 0 (i.e., NAL unit n is the very first NAL unit in
transmission order), AbsDon[0] is set equal to DON[0].
* Otherwise (n is greater than 0), the following applies for
derivation of AbsDon[n]:
If DON[n] == DON[n-1],
AbsDon[n] = AbsDon[n-1]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] < 32768),
AbsDon[n] = AbsDon[n-1] + DON[n] - DON[n-1]
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] >= 32768),
AbsDon[n] = AbsDon[n-1] + 65536 - DON[n-1] + DON[n]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] >= 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] + 65536 - DON[n])
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] < 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] - DON[n])
For any two NAL units (m and n), the following applies:
* When AbsDon[n] is greater than AbsDon[m], this indicates that NAL
unit n follows NAL unit m in NAL unit decoding order.
* When AbsDon[n] is equal to AbsDon[m], the NAL unit decoding order
of the two NAL units can be in either order.
* When AbsDon[n] is less than AbsDon[m], this indicates that NAL
unit n precedes NAL unit m in decoding order.
| Informative note: When two consecutive NAL units in the NAL
| unit decoding order have 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 for the
| absolute difference of the values of AbsDon for two consecutive
| NAL units in the NAL unit decoding order to be greater than
| one. An increment by one is not required, as at the time of
| associating values of AbsDon to NAL units, it may not be known
| whether all NAL units are to be delivered to the receiver. For
| example, a gateway might not forward VCL NAL units of higher
| sublayers 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, and when transmitting the
| first intra-coded picture, the originator does not exactly know
| 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 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 than the NAL
unit decoding order. Otherwise (sprop-max-don-diff is equal to
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 a small size SHOULD be encapsulated in an
aggregation packet together with one or more other NAL units in
order to avoid the unnecessary packetization overhead for small
NAL units. For example, non-VCL NAL units, such as access unit
delimiters, parameter sets, or SEI NAL units, are typically small
and can often be aggregated with VCL NAL units without violating
MTU size constraints.
* Each non-VCL NAL unit SHOULD, when possible from an MTU size match
viewpoint, be encapsulated in an aggregation packet together with
its associated VCL NAL unit, as typically a non-VCL NAL unit would
be meaningless without the associated VCL NAL unit being
available.
* For carrying exactly one NAL unit in an RTP packet, a single NAL
unit packet MUST be used.
6. De-packetization Process
The general concept behind de-packetization is to get the NAL units
out of the RTP packets in an RTP stream and pass them to the decoder
in the NAL unit decoding order.
The de-packetization process is implementation dependent. Therefore,
the following description should be seen as an example of a suitable
implementation. Other schemes may be used as well, as long as the
output for the same input is the same as the process described below.
The output is the same when the set of output NAL units and their
order are both identical. Optimizations relative to the described
algorithms are possible.
All normal RTP mechanisms related to buffer management apply. In
particular, duplicated or outdated RTP packets (as indicated by the
RTP sequence number and the RTP timestamp) are removed. To determine
the exact time for decoding, factors, such as a possible intentional
delay to allow for proper inter-stream synchronization, MUST be
factored in.
NAL units with NAL unit type values in the range of 0 to 27,
inclusive, may be passed to the decoder. NAL-unit-like structures
with NAL unit type values in the range of 28 to 31, 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 make a difference from a practical receiver buffer
that is also used for compensation of transmission delay jitter, the
receiver buffer is hereafter called the de-packetization buffer in
this section. Receivers should also prepare for transmission delay
jitter, that is, either reserve separate buffers for transmission
delay jitter buffering and de-packetization buffering or use a
receiver buffer for both transmission delay jitter and de-
packetization. Moreover, receivers should take transmission delay
jitter into account in the buffering operation, e.g., by additional
initial buffering before starting of decoding and playback.
The de-packetization process extracts the NAL units from the RTP
packets in an RTP stream as follows. When an RTP packet carries a
single NAL unit packet, the payload of the RTP packet is extracted as
a single NAL unit, excluding the DONL field, i.e., third and fourth
bytes, when sprop-max-don-diff is greater than 0. When an RTP packet
carries an aggregation packet, several NAL units are extracted from
the payload of the RTP packet. In this case, each NAL unit
corresponds to the part of the payload of each aggregation unit that
follows the NALU size field, as described in Section 4.3.2. When an
RTP packet carries a Fragmentation Unit (FU), all RTP packets from
the first FU (with the S field equal to 1) of the fragmented NAL unit
up to the last FU (with the E field equal to 1) of the fragmented NAL
unit are collected. The NAL unit is extracted from these RTP packets
by concatenating all FU payloads in the same order as the
corresponding RTP packets and appending the NAL unit header with the
fields F, LayerId, and TID set to equal the values of the fields F,
LayerId, 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.
There are two buffering states in the receiver: 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 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 memo.
Type name: video
Subtype name: H266
Required parameters: N/A
Optional parameters: profile-id, tier-flag, sub-profile-id, interop-
constraints, level-id, sprop-sublayer-id, sprop-ols-id, recv-
sublayer-id, recv-ols-id, max-recv-level-id, sprop-dci, sprop-vps,
sprop-sps, sprop-pps, sprop-sei, max-lsr, max-fps, 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 9328.
Interoperability considerations: N/A
Published specification: Please refer to RFC 9328 and VVC coding
specification [VVC].
Applications that use this media type: Any application that relies
on VVC-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 9328.
Change controller: IETF <avtcore@ietf.org>
7.2. Optional Parameters Definition
profile-id, tier-flag, sub-profile-id, interop-constraints, and
level-id:
These parameters indicate the profile, the tier, the default
level, the sub-profile, and some constraints of the bitstream
carried by the RTP stream, or a specific set of the profile, the
tier, the default level, the sub-profile, and some constraints the
receiver supports.
The subset of coding tools that may have been used to generate the
bitstream or that the receiver supports, as well as some
additional constraints, are indicated collectively by profile-id,
sub-profile-id, and interop-constraints.
| Informative note: There are 128 values of profile-id. The
| subset of coding tools identified by profile-id can be
| further constrained with up to 255 instances of sub-profile-
| id. In addition, 68 bits included in interop-constraints,
| which can be extended up to 324 bits, provide means to
| further restrict tools from existing profiles. To be able
| to support this fine-granular signaling of coding-tool
| subsets with profile-id, sub-profile-id, and interop-
| constraints, it would be safe to require symmetric use of
| these parameters in SDP offer/answer unless recv-ols-id is
| included in the SDP answer for choosing one of the layers
| offered.
The tier is indicated by tier-flag. The default level is
indicated by level-id. The tier and the default level specify the
limits on values of syntax elements or arithmetic combinations of
values of syntax elements that are followed when generating the
bitstream or that the receiver supports.
In SDP offer/answer, when the SDP answer does not include the
recv-ols-id parameter that is less than the sprop-ols-id parameter
in the SDP offer, the following applies:
* The tier-flag, profile-id, sub-profile-id, and interop-
constraints parameters MUST be used symmetrically, i.e., the
value of each of these parameters in the offer MUST be the same
as that in the answer, either explicitly signaled or implicitly
inferred.
* The level-id parameter is changeable as long as the highest
level indicated by the answer is either equal to or lower than
that in the offer. Note that the highest level higher than
level-id in the offer for receiving can be included as max-
recv-level-id.
In SDP offer/answer, when the SDP answer does include the recv-
ols-id parameter that is less than the sprop-ols-id parameter in
the SDP offer, the set of tier-flag, profile-id, sub-profile-id,
interop-constraints, and level-id parameters included in the
answer MUST be consistent with that for the chosen output layer
set as indicated in the SDP offer, with the exception that the
level-id parameter in the SDP answer is changeable as long as the
highest level indicated by the answer is either lower than or
equal to that in the offer.
More specifications of these parameters, including how they relate
to syntax elements specified in [VVC], are provided below.
profile-id:
When profile-id is not present, a value of 1 (i.e., the Main 10
profile) MUST be inferred.
When used to indicate properties of a bitstream, profile-id is
derived from the general_profile_idc syntax element that applies
to the bitstream in an instance of the profile_tier_level( )
syntax structure.
VVC bitstreams transported over RTP using the technologies of this
memo SHOULD contain only a single profile_tier_level( ) structure
in the DCI, unless the sender can assure that a receiver can
correctly decode the VVC bitstream, regardless of which
profile_tier_level( ) structure contained in the DCI was used for
deriving profile-id and other parameters for the SDP offer/answer
exchange.
As specified in [VVC], a profile_tier_level( ) syntax structure
may be contained in an SPS NAL unit, and one or more
profile_tier_level( ) syntax structures may be contained in a VPS
NAL unit and in a DCI NAL unit. One of the following three cases
applies to the container NAL unit of the profile_tier_level( )
syntax structure containing syntax elements used to derive the
values of profile-id, tier-flag, level-id, sub-profile-id, or
interop-constraints:
1. The container NAL unit is an SPS, the bitstream is a single-
layer bitstream, and the profile_tier_level( ) syntax
structures in all SPSs referenced by the CVSs in the bitstream
have the same values respectively for those
profile_tier_level( ) syntax elements.
2. The container NAL unit is a VPS, the profile_tier_level( )
syntax structure is the one in the VPS that applies to the OLS
corresponding to the bitstream, and the profile_tier_level( )
syntax structures applicable to the OLS corresponding to the
bitstream in all VPSs referenced by the CVSs in the bitstream
have the same values respectively for those
profile_tier_level( ) syntax elements.
3. The container NAL unit is a DCI NAL unit, and the
profile_tier_level( ) syntax structures in all DCI NAL units
in the bitstream have the same values respectively for those
profile_tier_level( ) syntax elements.
[VVC] allows for multiple profile_tier_level( ) structures in a
DCI NAL unit, which may contain different values for the syntax
elements used to derive the values of profile-id, tier-flag,
level-id, sub-profile-id, or interop-constraints in the different
entries. However, herein defined is only a single profile-id,
tier-flag, level-id, sub-profile-id, or interop-constraints. When
signaling these parameters and a DCI NAL unit is present with
multiple profile_tier_level( ) structures, these values SHOULD be
the same as the first profile_tier_level structure in the DCI,
unless the sender has ensured that the receiver can decode the
bitstream when a different value is chosen.
tier-flag, level-id:
The value of tier-flag MUST be in the range of 0 to 1, inclusive.
The value of level-id MUST be in the range of 0 to 255, inclusive.
If the tier-flag and level-id parameters are used to indicate
properties of a bitstream, they indicate the tier and the highest
level the bitstream complies with.
If the tier-flag and level-id parameters are 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.
If no tier-flag is present, a value of 0 MUST be inferred; if no
level-id is present, a value of 51 (i.e., level 3.1) MUST be
inferred.
| Informative note: The level values currently defined in the
| VVC specification are in the form of "majorNum.minorNum",
| and the value of the level-id for each of the levels is
| equal to majorNum * 16 + minorNum * 3. It is expected that,
| if any levels are defined in the future, the same convention
| will be used, but this cannot be guaranteed.
When used to indicate properties of a bitstream, the tier-flag and
level-id parameters are derived respectively from the syntax
element general_tier_flag, and the syntax element
general_level_idc or sub_layer_level_idc[j], that apply to the
bitstream in an instance of the profile_tier_level( ) syntax
structure.
If the tier-flag and level-id are derived from the
profile_tier_level( ) syntax structure in a DCI NAL unit, the
following applies:
* tier-flag = general_tier_flag
* level-id = general_level_idc
Otherwise, if the tier-flag and level-id are derived from the
profile_tier_level( ) syntax structure in an SPS or VPS NAL unit,
and the bitstream contains the highest sublayer representation in
the OLS corresponding to the bitstream, the following applies:
* tier-flag = general_tier_flag
* level-id = general_level_idc
Otherwise, if the tier-flag and level-id are derived from the
profile_tier_level( ) syntax structure in an SPS or VPS NAL unit,
and the bitstream does not contain the highest sublayer
representation in the OLS corresponding to the bitstream, the
following applies, with j being the value of the sprop-sublayer-id
parameter:
* tier-flag = general_tier_flag
* level-id = sub_layer_level_idc[j]
sub-profile-id:
The value of the parameter is a comma-separated (',') list of data
using base64 encoding (Section 4 of [RFC4648]) representation
without "==" padding.
When used to indicate properties of a bitstream, sub-profile-id is
derived from each of the ptl_num_sub_profiles
general_sub_profile_idc[i] syntax elements that apply to the
bitstream in a profile_tier_level( ) syntax structure.
interop-constraints:
A base64 encoding (Section 4 of [RFC4648]) representation of the
data that includes the ptl_frame_only_constraint_flag syntax
element, the ptl_multilayer_enabled_flag syntax element, and the
general_constraints_info( ) syntax structure that apply to the
bitstream in an instance of the profile_tier_level( ) syntax
structure.
If the interop-constraints parameter is not present, the following
MUST be inferred:
* ptl_frame_only_constraint_flag = 1
* ptl_multilayer_enabled_flag = 0
* gci_present_flag in the general_constraints_info( ) syntax
structure = 0
Using interop-constraints for capability exchange results in a
requirement on any bitstream to be compliant with the interop-
constraints.
sprop-sublayer-id:
This parameter MAY be used to indicate the highest allowed value
of TID in the bitstream. When not present, the value of sprop-
sublayer-id is inferred to be equal to 6.
The value of sprop-sublayer-id MUST be in the range of 0 to 6,
inclusive.
sprop-ols-id:
This parameter MAY be used to indicate the OLS that the bitstream
applies to. When not present, the value of sprop-ols-id is
inferred to be equal to TargetOlsIdx, as specified in
Section 8.1.1 of [VVC]. If this optional parameter is present,
sprop-vps MUST also be present or its content MUST be known a
priori at the receiver.
The value of sprop-ols-id MUST be in the range of 0 to 256,
inclusive.
| Informative note: VVC allows having up to 257 output layer
| sets indicated in the VPS, as the number of output layer
| sets minus 2 is indicated with a field of 8 bits.
recv-sublayer-id:
This parameter MAY be used to signal a receiver's choice of the
offered or declared sublayer representations in sprop-vps and
sprop-sps. The value of recv-sublayer-id indicates the TID of the
highest sublayer that a receiver supports. When not present, the
value of recv-sublayer-id is inferred to be equal to the value of
the sprop-sublayer-id parameter in the SDP offer.
The value of recv-sublayer-id MUST be in the range of 0 to 6,
inclusive.
recv-ols-id:
This parameter MAY be used to signal a receiver's choice of the
offered or declared output layer sets in sprop-vps. The value of
recv-ols-id indicates the OLS index of the bitstream that a
receiver supports. When not present, the value of recv-ols-id is
inferred to be equal to the value of the sprop-ols-id parameter
inferred from or indicated in the SDP offer. When present, the
value of recv-ols-id must be included only when sprop-ols-id was
received and must refer to an output layer set in the VPS that
includes no layers other than all or a subset of the layers of the
OLS referred to by sprop-ols-id. If this optional parameter is
present, sprop-vps must have been received or its content must be
known a priori at the receiver.
The value of recv-ols-id MUST be in the range of 0 to 256,
inclusive.
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-dci:
This parameter MAY be used to convey a decoding capability
information NAL unit of the bitstream for out-of-band
transmission. The parameter MAY also be used for capability
exchange. The value of the parameter is a base64 encoding
(Section 4 of [RFC4648]) representation of the decoding capability
information NAL unit, as specified in Section 7.3.2.1 of [VVC].
sprop-vps:
This parameter MAY be used to convey any video parameter set to
the NAL unit of the bitstream for out-of-band transmission of
video parameter sets. The parameter MAY also be used for
capability exchange and to indicate substream characteristics
(i.e., properties of output layer sets and sublayer
representations, as defined in [VVC]). The value of the parameter
is a comma-separated (',') list of base64 encoding (Section 4 of
[RFC4648]) representations of the video parameter set NAL units,
as specified in Section 7.3.2.3 of [VVC].
The sprop-vps parameter MAY contain one or more than one video
parameter set NAL units. However, all other video parameter sets
contained in the sprop-vps parameter MUST be consistent with the
first video parameter set in the sprop-vps parameter. A video
parameter set vpsB is said to be consistent with another video
parameter set vpsA if the number of OLSs in vpsA and vpsB are the
same and any decoder that conforms to the profile, tier, level,
and constraints indicated by the data starting from the syntax
element general_profile_idc to the syntax structure
general_constraints_info(), inclusive, in the profile_tier_level(
) syntax structure corresponding to any OLS with index olsIdx in
vpsA can decode any CVS(s) referencing vpsB when TargetOlsIdx is
equal to olsIdx that conforms to the profile, tier, level, and
constraints indicated by the data starting from the syntax element
general_profile_idc to the syntax structure
general_constraints_info(), inclusive, in the profile_tier_level(
) syntax structure corresponding to the OLS with index
TargetOlsIdx in vpsB.
sprop-sps:
This parameter MAY be used to convey sequence parameter set NAL
units of the bitstream for out-of-band transmission of sequence
parameter sets. The value of the parameter is a comma-separated
(',') list of base64 encoding (Section 4 of [RFC4648])
representations of the sequence parameter set NAL units, as
specified in Section 7.3.2.4 of [VVC].
A sequence parameter set spsB is said to be consistent with
another sequence parameter set spsA if any decoder that conforms
to the profile, tier, level, and constraints indicated by the data
starting from the syntax element general_profile_idc to the syntax
structure general_constraints_info(), inclusive, in the
profile_tier_level( ) syntax structure in spsA can decode any
CLVS(s) referencing spsB that conforms to the profile, tier,
level, and constraints indicated by the data starting from the
syntax element general_profile_idc to the syntax structure
general_constraints_info(), inclusive, in the profile_tier_level(
) syntax structure in spsB.
sprop-pps:
This parameter MAY be used to convey picture parameter set NAL
units of the bitstream for out-of-band transmission of picture
parameter sets. The value of the parameter is a comma-separated
(',') list of base64 encoding (Section 4 of [RFC4648])
representations of the picture parameter set NAL units, as
specified in Section 7.3.2.5 of [VVC].
sprop-sei:
This parameter MAY be used to convey one or more SEI messages that
describe bitstream characteristics. When present, a decoder can
rely on the bitstream characteristics that are described in the
SEI messages for the entire duration of the session, independently
from the persistence scopes of the SEI messages, as specified in
[VSEI].
The value of the parameter is a comma-separated (',') list of
base64 encoding (Section 4 of [RFC4648]) representations of SEI
NAL units, as specified in [VSEI].
| Informative note: Intentionally, no list of applicable or
| inapplicable SEI messages is specified here. Conveying
| certain SEI messages in sprop-sei may be sensible in some
| application scenarios and meaningless in others. However, a
| few examples are described below:
|
| In an environment where the bitstream was created from film-
| based source material, and no splicing is going to occur
| during the lifetime of the session, the film grain
| characteristics SEI message is likely meaningful, and
| sending it in sprop-sei, rather than in the bitstream at
| each entry point, may help with saving bits and allows one
| to configure the renderer only once, avoiding unwanted
| artifacts.
|
| 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).
max-lsr:
The max-lsr MAY be used to signal the capabilities of a receiver
implementation and MUST NOT be used for any other purpose. The
value of max-lsr is an integer indicating the maximum processing
rate in units of luma samples per second. The max-lsr parameter
signals that the receiver is capable of decoding video at a higher
rate than is required by the highest level.
| Informative note: When the OPTIONAL media type parameters
| are used to signal the properties of a bitstream, and max-
| lsr is not present, the values of tier-flag, profile-id,
| sub-profile-id, interop-constraints, and level-id must
| always be such that the bitstream complies fully with the
| specified profile, sub-profile, tier, level, and interop-
| constraints.
When max-lsr is signaled, the receiver MUST be able to decode
bitstreams that conform to the highest level, with the exception
that the MaxLumaSr value in Table A.3 of [VVC] for the highest
level is replaced with the value of max-lsr. Senders MAY use this
knowledge to send pictures of a given size at a higher picture
rate than is indicated in the highest level.
When not present, the value of max-lsr is inferred to be equal to
the value of MaxLumaSr given in Table A.3 of [VVC] for the highest
level.
The value of max-lsr MUST be in the range of MaxLumaSr to 16 *
MaxLumaSr, inclusive, where MaxLumaSr is given in Table A.3 of
[VVC] for the highest level.
max-fps:
The value of max-fps is an integer indicating the maximum picture
rate in units of pictures per 100 seconds that can be effectively
processed by the receiver. The max-fps parameter MAY be used to
signal that the receiver has a constraint in that it is not
capable of processing video effectively at the full picture rate
that is implied by the highest level and, when present, max-lsr.
The value of max-fps is not necessarily the picture rate at which
the maximum picture size can be sent; it constitutes a constraint
on maximum picture rate for all resolutions.
| Informative note: The max-fps parameter is semantically
| different from max-lsr in that max-fps is used to signal a
| constraint, lowering the maximum picture rate from what is
| implied by other parameters.
The encoder MUST use a picture rate equal to or less than this
value. In cases where the max-fps parameter is absent, the
encoder is free to choose any picture rate according to the
highest level and any signaled optional parameters.
The value of max-fps MUST be smaller than or equal to the full
picture rate that is implied by the highest level and, when
present, max-lsr.
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: depack-buf-cap indicates the maximum
| possible size of the de-packetization buffer of the receiver
| only, without allowing for network jitter.
7.3. SDP Parameters
The receiver MUST ignore any parameter unspecified in this memo.
7.3.1. Mapping of Payload Type Parameters to SDP
The media type video/H266 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 H266 (the
media subtype).
* The clock rate in the "a=rtpmap" line MUST be 90000.
* The OPTIONAL parameters profile-id, tier-flag, sub-profile-id,
interop-constraints, level-id, sprop-sublayer-id, sprop-ols-id,
recv-sublayer-id, recv-ols-id, max-recv-level-id, max-lsr, max-
fps, sprop-max-don-diff, sprop-depack-buf-bytes, and depack-buf-
cap, when present, MUST be included in the "a=fmtp" line of SDP.
The fmtp line is expressed as a media type string, in the form of
a semicolon-separated list of parameter=value pairs.
* The OPTIONAL parameters sprop-vps, sprop-sps, sprop-pps, sprop-
sei, and sprop-dci, 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-vps, sprop-sps, sprop-pps,
sprop-sei, or sprop-dci 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-vps, sprop-sps, sprop-pps, sprop-sei, and sprop-dci 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-vps, 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 H266/90000
a=fmtp:98 profile-id=1;
sprop-vps=<video parameter sets data>;
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. Some lines are wrapped
due to text constraints.
Offerer->Answerer:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H266/90000
a=fmtp:98 profile-id=1; level_id=83;
The above represents an offer for symmetric video communication using
[VVC] and its payload specification at the main profile and level 5.1
(and as the levels are downgradable, all lower levels). Informally
speaking, this offer tells the receiver of the offer that the sender
is willing to receive up to 4Kp60 resolution at the maximum bitrates
specified in [VVC]. 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 H.266 media up to and including level
5.1.
Answerer->Offerer:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H266/90000
a=fmtp:98 profile-id=1; level_id=67
With this answer to the offer above, the system receiving the offer
advises the offerer that it is incapable of handing H.266 at level
5.1 but is capable of decoding 1080p60. As H.266 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 1080p60. However, nothing prevents an encoder
from further downgrading its sending to, for example, 720p30 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 as described in [RFC3264] and its updates. The
section is split into subsections, covering a) media format
configurations not involving non-temporal scalability; b) scalable
media format configurations; c) the description of the use of those
parameters not involving the media configuration itself but rather
the parameters of the payload format design; and d) multicast.
7.3.2.1. Non-scalable Media Format Configuration
A non-scalable VVC media configuration is such a configuration where
no non-temporal scalability mechanisms are allowed. In [VVC] version
1, it is implied that general_profile_idc indicates one of the
following profiles: Main 10, Main 10 Still Picture, Main 10 4:4:4, or
Main 10 4:4:4 Still Picture, with general_profile_idc values of 1,
65, 33, and 97, respectively. Note that non-scalable media
configurations include temporal scalability inline with VVC's design
philosophy and profile structure.
The following limitations and rules pertaining to the media
configuration apply:
* The parameters identifying a media format configuration for VVC
are profile-id, tier-flag, sub-profile-id, level-id, and interop-
constraints. These media configuration parameters, except level-
id, MUST be used symmetrically.
The answerer MUST structure its answer according to one of the
following three options:
1. 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;
2. include in the answer the recv-sublayer-id parameter, with a
value less than the sprop-sublayer-id parameter in the offer,
for the media format (payload type), and 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
the level indicated by sprop-sps or sprop-vps in offer for the
chosen sublayer representation; or
3. 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 7.3.2.3 below.
* To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be used
in the answer, as specified in [RFC3264].
* The same RTP payload type number used in the offer for the media
subtype H266 MUST be used in the answer when the answer includes
recv-sublayer-id. When the answer does not include recv-sublayer-
id, the answer MUST NOT contain a payload type number used in the
offer for the media subtype H266 unless the configuration is
exactly 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. The answer MAY contain the recv-sublayer-id
parameter if a VVC bitstream contains multiple operation points
(using temporal scalability and sublayers) and sprop-sps or sprop-
vps is included in the offer where information of sublayers are
present in the first sequence parameter set or video parameter set
contained in sprop-sps or sprop-vps, respectively. If sprop-sps
or sprop-vps is provided in an offer, an answerer MAY select a
particular operation point indicated in the first sequence
parameter set or video parameter set contained in sprop-sps or
sprop-vps, respectively. When the answer includes a recv-
sublayer-id that is less than a sprop-sublayer-id in the offer,
the following applies:
1. When the sprop-sps parameter is present, all sequence
parameter sets contained in the sprop-sps parameter in the SDP
answer and all sequence parameter sets sent in-band for either
the offerer-to-answerer direction or the answerer-to-offerer
direction MUST be consistent with the first sequence parameter
set in the sprop-sps parameter of the offer (see the semantics
of sprop-sps in Section 7.1 of this document on one sequence
parameter set being consistent with another sequence parameter
set).
2. When the sprop-vps parameter is present, all video parameter
sets contained in the sprop-vps parameter in the SDP answer
and all video parameter sets sent in-band for either the
offerer-to-answerer direction or the answerer-to-offerer
direction MUST be consistent with the first video parameter
set in the sprop-vps parameter of the offer (see the semantics
of sprop-vps in Section 7.1 of this document on one video
parameter set being consistent with another video parameter
set).
3. The bitstream sent in either direction MUST conform to the
profile, tier, level, and constraints of the chosen sublayer
representation, as indicated by the profile_tier_level( )
syntax structure in the first sequence parameter set in the
sprop-sps parameter or by the first profile_tier_level( )
syntax structure in the first video parameter set in the
sprop-vps parameter of the offer.
| Informative note: When an offerer receives an answer that does
| not include recv-sublayer-id, it has to compare payload types
| not declared in the offer based on the media type (i.e., video/
| H266) and the above media configuration parameters with any
| payload types it has already declared. This will enable it to
| determine whether the configuration in question is new or if it
| is equivalent to configuration already offered, since a
| different payload type number may be used in the answer. The
| ability to perform operation point selection enables a receiver
| to utilize the temporal scalable nature of a VVC bitstream.
7.3.2.2. Scalable Media Format Configuration
A scalable VVC media configuration is such a configuration where non-
temporal scalability mechanisms are allowed. In [VVC] version 1, it
is implied that general_profile_idc indicates one of the following
profiles: Multilayer Main 10 and Multilayer Main 10 4:4:4, with
general_profile_idc values of 17 and 49, respectively.
The following limitations and rules pertaining to the media
configuration apply. They are listed in an order that would be
logical for an implementation to follow:
* The parameters identifying a media format configuration for
scalable VVC are profile-id, tier-flag, sub-profile-id, level-id,
interop-constraints, and sprop-vps. These media configuration
parameters, except level-id, MUST be used symmetrically, except as
noted below.
* The answerer MAY include a level-id that MUST be lower than or
equal to the level-id indicated in the offer (either expressed by
level-id in the offer or implied by the default level, as
specified in Section 7.1).
* When sprop-ols-id is present in an offer, sprop-vps MUST also be
present in the same offer and include at least one valid VPS so to
allow the answerer to meaningfully interpret sprop-ols-id and
select recv-ols-id (see below).
* The answerer MUST NOT include recv-ols-id unless the offer
includes sprop-ols-id. When present, recv-ols-id MUST indicate a
supported output layer set in the VPS that includes no layers
other than all or a subset of the layers of the OLS referred to by
sprop-ols-id. If unable, the answerer MUST remove the media
format.
| Informative note: If an offerer wants to offer more than one
| output layer set, it can do so by offering multiple VVC media
| with different payload types.
* The offerer MAY include sprop-sublayer-id, which indicates the
highest allowed value of TID in the bitstream. The answerer MAY
include recv-sublayer-id, which can be used to reduce the number
of sublayers from the value of sprop-sublayer-id.
* When the answerer includes recv-ols-id and configuration
parameters profile-id, tier-flag, sub-profile-id, level-id, and
interop-constraints, it MUST use the configuration parameter
values as signaled in the sprop-vps for the operating point with
the largest number of sublayers for the chosen output layer set,
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 the level indicated by sprop-vps in offer for the operating
point with the largest number of sublayers for the chosen output
layer set.
7.3.2.3. Payload Format Configuration
The following limitations and rules pertain to the configuration of
the payload format buffer management mostly and apply to both
scalable and non-scalable VVC.
* 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 VVC, 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.
* The capability parameter max-lsr MAY be used to declare further
capabilities of the offerer or answerer for receiving. It MUST
NOT be present when the direction attribute is sendonly.
* The capability parameter max-fps MAY be used to declare lower
capabilities of the offerer or answerer for receiving. It MUST
NOT be present when the direction attribute is sendonly.
* 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-dci, sprop-vps, 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 (DCI, VPS, 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-dci, sprop-vps, 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-vps,
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-dci, sprop-vps, 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-vps, 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-dci, sprop-vps, 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-dci, sprop-vps, 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 --+
answer: recvonly, recv-ols-id --+ |
recvonly w/o recv-ols-id --+ | |
answer: sendrecv, recv-ols-id --+ | | |
sendrecv w/o recv-ols-id --+ | | | |
| | | | |
profile-id C D C D P
tier-flag C D C D P
level-id D D D D P
sub-profile-id C D C D P
interop-constraints C D C D P
max-recv-level-id R R R R -
sprop-max-don-diff P P - - P
sprop-depack-buf-bytes P P - - P
depack-buf-cap R R R R -
max-lsr R R R R -
max-fps R R R R -
sprop-dci P P - - P
sprop-sei P P - - P
sprop-vps P P - - P
sprop-sps P P - - P
sprop-pps P P - - P
sprop-sublayer-id P P - - P
recv-sublayer-id O O O O -
sprop-ols-id P P - - P
recv-ols-id X O X O -
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 six parameters related to profile, tier,
and level on these parameters being consistent)
P: properties of the bitstream to be sent
R: receiver capabilities
O: operation point selection
X: MUST NOT be present
-: not usable, when present MUST be ignored
Figure 11: Interpretation of Parameters for Various Combinations
of Offers, Answers, and Direction Attributes, with and without
recv-ols-id.
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 the answer does not include a recv-ols-id that is less than the
sprop-ols-id in the offer, parameters declaring a configuration point
are not changeable, with the exception of the level-id parameter for
unicast usage, and these parameters express values a receiver expects
to be used and MUST be used verbatim in the answer as in the offer.
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. However, it is possible to
offer multiple operation points using one configuration in a single
payload type by including sprop-vps in the offer and recv-ols-id in
the answer.
An implementation SHOULD be able to understand all media type
parameters (including all optional media type parameters), even if it
doesn't support the functionality related to the parameter. This, in
conjunction with proper application logic in the implementation,
allows the implementation, after having received an offer, to create
an answer by potentially downgrading one or more of the optional
parameters to the point where the implementation can cope, leading to
higher chances of interoperability beyond the most basic interop
points (for which, as described above, no optional parameters are
necessary).
| Informative note: In implementations of previous H.26x payload
| formats, it was occasionally observed that implementations were
| incapable of parsing most (or all) of the optional parameters.
| As a result, the offer/answer exchange resulted in a baseline
| performance (using the default values for the optional
| parameters) with the resulting suboptimal user experience.
| However, there are valid reasons to forego the implementation
| complexity of implementing the parsing of some or all of the
| optional parameters, for example, when there is predetermined
| knowledge, not negotiated by an SDP-based offer/answer process,
| of the capabilities of the involved systems (walled gardens,
| baseline requirements defined in application standards higher
| up in the stack, and similar).
An answerer MAY extend the offer with additional media format
configurations. However, to enable their usage, in most cases, a
second offer is required from the offerer to provide the bitstream
property parameters that the media sender will use. This also has
the effect that the offerer has to be able to receive this media
format configuration, not only to send it.
7.3.3. Multicast
For bitstreams being delivered over multicast, the following rules
apply:
* The media format configuration is identified by profile-id, tier-
flag, sub-profile-id, level-id, and interop-constraints. 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 VVC over RTP is offered with SDP in a declarative style, as in
Real Time Streaming Protocol (RTSP) [RFC7826] or Session Announcement
Protocol (SAP) [RFC2974], the following considerations are necessary.
* 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,
tier-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 tier-flag
o level-id
o interop-constraints
o sub-profile-id
o sprop-dci
o sprop-vps
o sprop-sps
o sprop-pps
o sprop-max-don-diff
o sprop-depack-buf-bytes
o sprop-sublayer-id
o sprop-ols-id
o sprop-sei
- Not usable (when present, they MUST be ignored):
o max-lsr
o max-fps
o max-recv-level-id
o depack-buf-cap
o recv-sublayer-id
o recv-ols-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) and Full Intra Request (FIR) feedback messages with
[VVC]. The PLI is defined in [RFC4585], and the FIR message is
defined in [RFC5104]. In accordance with this memo, unlike [HEVC], a
sender MUST NOT send Slice Loss Indication (SLI) or Reference Picture
Selection Indication (RPSI), and a receiver SHOULD ignore RPSI and
treat a received SLI as a 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, non-IRAP decoder refresh points,
picture structures, and so forth), a reaction to the reception of a
PLI by a VVC sender SHOULD be to send an IRAP picture and relevant
parameter sets, potentially with sufficient redundancy so to ensure
correct reception. However, sometimes information about the
bitstream structure is known. For example, such information can be
parameter sets that have been conveyed out of band through mechanisms
not defined in this document and that are known to stay static for
the duration of the session. In that case, it is obviously
unnecessary to send them in-band as a result of the reception of a
PLI. Other examples could be devised based on a priori knowledge of
different aspects of the bitstream structure. In all cases, the
timing and congestion control mechanisms of [RFC4585] MUST be
observed.
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 [VVC] that may pose a particularly serious security
risk if implemented naively. The payload format, in isolation, does
not form a complete system. Implementers are advised to read and
understand relevant security-related documents, especially those
pertaining to RTP (see the Security Considerations section in
[RFC3550]) and the security of the call-control stack chosen (that
may make use of the media type registration of this memo).
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 memo 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 lays on anyone using RTP in an
application. They can find guidance on available security mechanisms
and important considerations in "Options for Securing RTP Sessions"
[RFC7201]. 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. [VVC] 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 but NOT REQUIRED
based on the thoughts of [RFC7202].
Like HEVC [RFC7798], [VVC] 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. [VVC] 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, it would be a bad and insecure implementation
practice to forward any JavaScript a decoder implementation detects
to a web browser. 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. This on-path
inclusion of the MANE forgoes end-to-end security guarantees for the
end points.
10. Congestion Control
Congestion control for RTP SHALL be used in accordance with RTP
[RFC3550] and with any applicable RTP profile, e.g., AVP [RFC3551] or
AVPF [RFC4585]. If best-effort service is being used, an additional
requirement is that users of this payload format MUST monitor packet
loss to ensure that the packet loss rate is within an acceptable
range. Packet loss is considered acceptable if a TCP flow across the
same network path and experiencing the same network conditions would
achieve an average throughput, measured on a reasonable timescale,
that is not less than all RTP streams combined are achieved. This
condition can be satisfied by implementing congestion-control
mechanisms to adapt the transmission rate, by implementing the number
of layers subscribed for a layered multicast session, or by arranging
for a receiver to leave the session if the loss rate is unacceptably
high.
The bitrate adaptation necessary for obeying the congestion control
principle is easily achievable when real-time encoding is used, for
example, by adequately tuning the quantization parameter. However,
when pre-encoded content is being transmitted, bandwidth adaptation
requires the pre-coded bitstream to be tailored for such adaptivity.
The key mechanisms available in [VVC] are temporal scalability and
spatial/SNR scalability. A media sender can remove NAL units
belonging to higher temporal sublayers (i.e., those NAL units with a
high value of TID) or higher spatio-SNR layers until the sending
bitrate drops to an acceptable range.
The mechanisms mentioned above generally work within a defined
profile and level; therefore no renegotiation of the channel is
required. Only when non-downgradable parameters (such as profile)
are required to be changed does it become necessary to terminate and
restart the RTP stream(s). This may be accomplished by using
different RTP payload types.
MANEs MAY remove certain unusable packets from the RTP stream when
that RTP stream was damaged due to previous packet losses. This can
help reduce the network load in certain special cases. For example,
MANEs can remove those FUs where the leading FUs belonging to the
same NAL unit have been lost or those dependent slice segments when
the leading slice segments belonging to the same slice have been
lost, because the trailing FUs or dependent slice segments 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
A new media type has been registered with IANA; see Section 7.1.
12. References
12.1. Normative References
[ISO23090-3]
International Organization for Standardization,
"Information technology - Coded representation of
immersive media - Part 3: Versatile video coding", ISO/
IEC 23090-3:2022, September 2022,
<https://www.iso.org/standard/73022.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>.
[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>.
[VSEI] ITU-T, "Versatile supplemental enhancement information
messages for coded video bitstreams", ITU-T
Recommendation H.274, May 2022,
<https://www.itu.int/rec/T-REC-H.274>.
[VVC] ITU-T, "Versatile Video Coding", ITU-T
Recommendation H.266, April 2022,
<http://www.itu.int/rec/T-REC-H.266>.
12.2. Informative References
[CABAC] Sole, J., et al., "Transform coefficient coding in HEVC",
IEEE Transactions on Circuits and Systems for Video
Technology, DOI 10.1109/TCSVT.2012.2223055, December 2012,
<https://doi.org/10.1109/TCSVT.2012.2223055>.
[HEVC] ITU-T, "High efficiency video coding", ITU-T
Recommendation H.265, August 2021,
<https://www.itu.int/rec/T-REC-H.265>.
[MPEG2S] International Organization for Standardization,
"Information technology - Generic coding of moving
pictures and associated audio information - Part 1:
Systems", ISO/IEC 13818-1:2022, September 2022.
[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>.
[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>.
Acknowledgements
Dr. Byeongdoo Choi is thanked for the video-codec-related technical
discussion and other aspects in this memo. Xin Zhao and Dr. Xiang Li
are thanked for their contributions on [VVC] specification
descriptive content. Spencer Dawkins is thanked for his valuable
review comments that led to great improvements of this memo. Some
parts of this specification share text with the RTP payload format
for HEVC [RFC7798]. We thank the authors of that specification for
their excellent work.
Authors' Addresses
Shuai Zhao
Intel
2200 Mission College Blvd
Santa Clara, 95054
United States of America
Email: shuai.zhao@ieee.org
Stephan Wenger
Tencent
2747 Park Blvd
Palo Alto, 94588
United States of America
Email: stewe@stewe.org
Yago Sanchez
Fraunhofer HHI
Einsteinufer 37
10587 Berlin
Germany
Email: yago.sanchez@hhi.fraunhofer.de
Ye-Kui Wang
Bytedance Inc.
8910 University Center Lane
San Diego, 92122
United States of America
Email: yekui.wang@bytedance.com
Miska M. Hannuksela
Nokia Technologies
Hatanpään valtatie 30
FI-33100 Tampere
Finland
Email: miska.hannuksela@nokia.com