Internet DRAFT - draft-zhao-avtcore-rtp-vvc
draft-zhao-avtcore-rtp-vvc
Network Working Group S. Zhao
Internet-Draft S. Wenger
Intended status: Standards Track Tencent
Expires: March 26, 2020 September 23, 2019
RTP Payload Format for Versatile Video Coding (VVC)
draft-zhao-avtcore-rtp-vvc-00
Abstract
This memo describes an RTP payload format for the video coding
standard ITU-T Recommendation H.266 and ISO/IEC International
Standard 23090-3, both also known as Versatile Video Coding (VVC) and
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 others.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview of the VVC Codec . . . . . . . . . . . . . . . . 3
1.1.1. Coding-Tool Features (informative) . . . . . . . . . 3
1.1.2. Systems and Transport Interfaces . . . . . . . . . . 6
1.1.3. Parallel Processing Support (informative) . . . . . . 10
1.1.4. NAL Unit Header . . . . . . . . . . . . . . . . . . . 10
1.2. Overview of the Payload Format . . . . . . . . . . . . . 11
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 12
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.1. Definitions from the VVC Specification . . . . . . . 12
3.1.2. Definitions Specific to This Memo . . . . . . . . . . 12
3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 12
4. RTP Payload Format . . . . . . . . . . . . . . . . . . . . . 12
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 12
4.2. Payload Header Usage . . . . . . . . . . . . . . . . . . 14
4.3. Payload Structures . . . . . . . . . . . . . . . . . . . 15
4.3.1. Single NAL Unit Packets . . . . . . . . . . . . . . . 15
4.3.2. Aggregation Packets (APs) . . . . . . . . . . . . . . 16
4.3.3. Fragmentation Units . . . . . . . . . . . . . . . . . 21
4.4. Decoding Order Number . . . . . . . . . . . . . . . . . . 24
5. Packetization Rulesumber . . . . . . . . . . . . . . . . . . 25
6. De-packetization Process . . . . . . . . . . . . . . . . . . 26
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 28
8. Use with Feedback Messages . . . . . . . . . . . . . . . . . 28
8.1. Picture Loss Indication (PLI) . . . . . . . . . . . . . . 28
8.2. Slice Loss Indication (SLI) . . . . . . . . . . . . . . . 29
8.3. Reference Picture Selection Indication (RPSI) . . . . . . 29
8.4. Full Intra Request (FIR) . . . . . . . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 31
11. IANA Considertaions . . . . . . . . . . . . . . . . . . . . . 32
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.1. Normative References . . . . . . . . . . . . . . . . . . 32
13.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
The VVC specification, formally published as both ITU-T
Recommendation H.266 and ISO/IEC International Standard 23090-23
[ISO23090-3], is planned for ratification in mid 2020. A draft
that's currently in the approval process of ISO/IEC can be found as
[VVC]. H.266 is reported to provide significant coding efficiency
gains over H.265 [H.265] and earlier video codec formats.
This memo describes an RTP payload format for [VVC]. It shares its
basic design with the NAL unit-based RTP payload formats of
[RFC7798], [RFC6184] and [RFC6190] . 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 RFC 6184 is
widely deployed and generally known in the relevant implementer
communities. Certain mechanisms known from RFC 6190 were
incorporated as [VVC] version 1 supports all temporal, spatial, and
SNR scalability.
1.1. Overview of the VVC Codec
[VVC] and H.265 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
Network Abstraction Layer (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 believed to be 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.
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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
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. As to
transform, the square transforms in HEVC are extended to non-square
transforms for rectangular blocks resulted from binary and ternary
tree splits. Besides, [VVC] supports multiple transform sets (MTS),
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) [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 colour
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 supports 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 before deblocking called luma mapping with chroma scaling
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to fully utilize the dynamic range of signal so that rate-distortion
performance of both SDR and 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 with affine mode (PROF)
is further deployed to mimic affine motion at the pixel level.
Thirdly the decoder side motion vector refinement (DMVR) is a method
to derive MV vector at decoder side so that fewer bits may be spent
on motion vectors. Bi-directional optical flow (BDOF) is a similar
method to DMVR but at 4x4 sub-block level. Another difference is
that DMVR is based on block matching while BDOF derives MVs with
equations. Furthermore, merge with motion vector difference (MMVD)
is a special mode, which 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). 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.
Moreover, weighting in bi-prediction has more flexibility then HEVC.
Adaptive weighting may be employed with a block-level tool called bi-
prediction with CU based weighting (BCW).
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 mode 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, as known as
multi-reference-line (MRL) intra prediction. The additional
reference lines can be only used for 6 most probable intra prediction
modes. To capture the strong correlation between different colour
components, in [VVC], a cross-component linear mode (CCLM) is
utilized which assumes a linear relationship between the luma sample
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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 weightings are hardcoded constants.
Other coding-tool feature
[VVC] introduces dependent quantization (DQ) to reduce quantization
error by state-based switching between two quantizers.
1.1.2. Systems and Transport Interfaces
[VVC] inherits the basic systems and transport interfaces designs
from HEVC and H.264. 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 variant of HEVC known as SHVC. The hierarchical syntax
and data unit structure consists of parameter sets at various levels
(decoder, sequence (including layers), sequence (per layer),
picture), slice-level header parameters, and lower-level parameters.
Below described are a number of key components that influenced the
Network Abstraction Layer design of VVC as well as this memo.
Decoder parameter set
The Decoder parameter set includes parameters that stay constant for
the lifetime of a Video Bitstream, which in IETF terms can translate
to the lifetime of a session. Decoder parameter sets can include
profile, level, and sub-profile information to determine a maximum
complexity interop point that is guaranteed to be never exceeded,
even if splicing of video sequences occurs within a session. It
further optionally includes constraint flags, which indicate that the
video bitstream will be constraint of the use of certain features as
indicated by the values of those flags. With this, a bitstream can
be labelled as not using certain tools, which allows among other
things for resource allocation in a decoder implementation. As all
parameter sets, also the decoder parameter set is required to be
present when first referenced, and it is necessarily referenced by
the very first picture in a video sequence, implying that it has to
be sent among the first NAL units in the bitstream (see section xxx
below). While multiple DPSs can be in the bitstream, the value of
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the syntax elements therein cannot be inconsistent when being
referenced.
Video parameter set
The Video Parameter Set (VPS) includes decoding dependency or
information for reference picture set 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. (see Section xxx).
Sequence parameter set
The Sequence Parameter Set (SPS) contains syntax elements pertaining
to a coded video sequence (CVS), which is a group of pictures,
starting with a random access point, and followed by pictures that
may depend on each other and the random access point picture. In
MPGEG-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. In many TV-like applications, a CVS
contains a few hundred milliseconds to a few seconds of video. In
video conferencing (without switching MCUs involved), a CVS can be as
long in duration as the whole session.
Picture and Adaptation parameter set
The Picture Parameter Set and the Adaptation Parameter Set (PPS and
APS, respectively) carry information pertaining to a single picture.
The PPS contains information that is likely to stay constant from
picture to picture-at least for pictures for a certain type-whereas
the APS contains information, such as adaptive loop filter
coefficients, that are likely to change from picture to picture.
Profile, tier, and level
The profile, tier, and level syntax structure can be included in all
DPS, VPS, and SPS. Somewhat oversimplified, they can be viewed to
provide information about maximum bitstream complexity in the
dimensions of tools used (profile), sample count (level), and maximum
bitrate (tier). Level and tier are onion shaped, in that a decoder
that can decode a certain level or tier can also decode lower levels
or tiers. Profiles are not necessarily onion shaped and do not
necessarily form a hierarchy. Therefore, the profile_tier_level
structure in the video bitstream contains a bitmask which allows an
encoder to mark a bitstream to be compatible with multiple profiles.
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Sub-Profiles
Within the [VVC] specification, a sub-profile is simply a 32 bit
number coded according to ITU-T Rec. T.35, that does not carry a
semantic. It is carried in the profile_tier_level structure and
hence (potentially) present in the DPS, 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 complexity 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: DVB and ATSC), as well as large walled-garden
video services will take advantage of this labelling 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 specs, such as GOP structures, minimum/maximum
QP values, and the mandatory use of certain tools or SEI messages.
Constraint Flags
The profile_tier_level structure optionally carries a considerable
number of constraint 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 for labelling a
bitstream as not exercising a certain tool that has become
commercially unviable.
Temporal scalability support
Edt. note: this section may need adjustment as JVET work on bitstream
extraction is in progress.
[VVC] includes support of temporal scalability, by inclusion of the
signaling of TemporalId in the NAL unit header, the restriction that
pictures of a particular temporal sub-layer cannot be used for inter
prediction reference by pictures of a lower temporal sub-layer, 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.
Spatial, SNR, View Scalability
[VVC] includes support for spatial, SNR, and View 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 main profiles
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of video codecs and not wide deployed due to additional costs. In
VVC, however, all those forms of scalability are supported natively
through the signaling of the layer_id in the NAL unit header, the VPS
which associates layers with given layer_ids to each other, reference
picture selection, reference picture resampling for spatial
scalability, and a number of other mechanisms not relevant for this
memo. Scalability support can be implemented in a single decoding
"loop" and is widely considered a comparatively lightweight
operation.
Spatial Scalability
With the existence of Reference Picture Resampling, likely in the
"main" profile of VVC, the additional burden for scalability
support is just a minor modification of the high-level syntax
(HLS). In technical aspects, 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
resampled video data of the reconstructed reference picture from a
reference layer to predict the current enhancement layer. Then,
the resampling process for inter-layer prediction is performed at
the block-level, by modifying the existing interpolation process
for motion compensation. It means that no additional resampling
process is needed to support scalability.
SNR Scalability>
SNR scalability is similar to Spatial Scalability except that the
resampling factors are 1:1--in other words, tehre is no change in
resolution, but there is inter-layer prediction.
View Scalability>
Placeholder
SEI Messages
Supplementary Enhancement Information (SEI) messages are codepoints
in the bitstream that do not influence the decoding process as
specified in the [VVC] spec, but address issues of representation/
rendering of the decoded bitstream, label the bitstream for certain
applications, among other, similar tasks. The overall concept of SEI
messages and many of the messages themselves has been inherited from
the H.264 and HEVC specs. In the [VVC] environment, some of the SEI
messages considered to be generally useful also in other video coding
technologies have been moved out of the main specification info a
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companion document (TO DO: add reference once ITU designation is
known).
1.1.3. Parallel Processing Support (informative)
Compared to HEVC [RFC7798], the [VVC] design to support
parallelization offers numerous improvements. Some of those
improvements are still undergoing changes in JVET. Information, to
the extent relevant for this memo, will be added in future versions
of this memo as the standardization in JVET progresses and the
technology stabilizes.
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 |
+---------------+---------------+
The Structure of the [VVC] NAL Unit Header.
Figure 1
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. 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 memo
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
Section TBD.
Z: 1 bit
nuh_reserved_zero_bit. Required to be zero in [VVC], and reserved
for future extensions by ITU-T and ISO/IEC. This memo does not
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overload the "Z" bit for local extensions, as a) overloading the
"F" bit is sufficient and b) to preserve the usefulness of this
memo to possible future versions of [VVC].
LayerId: 6 bits
nuh_layer_id. Identifies the layer a NAL unit belongs to, wherein
a layer may be, e.g., a spatial scalable layer, a quality scalable
layer .
Type: 6 bits
nal_unit_type. This field specifies the NAL unit type as defined
in Table 7-1 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, so to
enable independent considerations of start code emulations in the
NAL unit header and in the NAL unit payload data.
1.2. Overview of the Payload Format
This payload format defines the following processes required for
transport of [VVC] coded data over RTP [RFC3550]:
o Usage of RTP header with this payload format
o 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
o Transmission of HEVC NAL units of the same bitstream within a
single RTP stream.
o Media type parameters to be used with the Session Description
Protocol (SDP) [RFC4566]
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119]. In
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this document, the above key words will convey that interpretation
only when in ALL CAPS. Lowercase uses of these words are not to be
interpreted as carrying the significance described in RFC 2119. This
specification uses the notion of setting and clearing a bit when bit
fields are handled. Setting a bit is the same as assigning that bit
the value of 1 (On). Clearing a bit is the same as assigning that
bit the value of 0 (Off).
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.
3.1.1. Definitions from the VVC Specification
Placeholder
3.1.2. Definitions Specific to This Memo
Placeholder
3.2. Abbreviations
Placeholder
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
4.4.2 and 4.4.3, respectively.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP Header According to [RFC3550]
Figure 2
The RTP header information to be set according to this RTP payload
format is set as follows:
Marker bit (M): 1 bit
Set for the last packet of the access unit, carried in the current
RTP stream. This is in line with the normal use of the M bit in
video formats to allow an efficient playout buffer handling.
The informative note below needs updating once the NAL unit
type table is stable in the [VVC] spec
Informative note: The content of a NAL unit does not tell
whether or not the NAL unit is the last NAL unit, in decoding
order, of an access unit. An RTP sender implementation may
obtain this information from the video encoder. If, however,
the implementation cannot obtain this information directly from
the encoder, e.g., when the bitstream was pre-encoded, and also
there is no timestamp allocated for each NAL unit, then the
sender implementation can inspect subsequent NAL units in
decoding order to determine whether or not the NAL unit is the
last NAL unit of an access unit as follows. A NAL unit is
determined to be the last NAL unit of an access unit if it is
the last NAL unit of the bitstream. A NAL unit naluX is also
determined to be the last NAL unit of an access unit if both
the following conditions are true: 1) the next VCL NAL unit
naluY in decoding order has the high-order bit of the first
byte after its NAL unit header equal to 1, and 2) all NAL units
between naluX and naluY, when present, have nal_unit_type in
the range of 32 to 35, inclusive, equal to 39, or in the ranges
of 41 to 44, inclusive, or 48 to 55, inclusive.
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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
picture of the access unit in which the NAL unit (according to
Section xxx 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]. However, this does not mean
that picture timing SEI messages in the bitstream should be
discarded, as picture timing SEI messages may contain frame-field
information that is important in appropriately rendering
interlaced video.
Synchronization source (SSRC): 32 bits
Used to identify the source of the RTP packets. When using SRST,
by definition 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 as shown
in Section 1.1.4, irrespective of the type of the payload structure.
The TID value indicates (among other things) the relative importance
of an RTP packet, for example, because NAL units belonging to higher
temporal sub-layers are not used for the decoding of lower temporal
sub-layers. 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.
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For Discussion: quite possibly something similar can be said for the
Layer_id in layered coding, but perhaps not in multiview coding.
(The relevant part of the spec is relatively new, therefore the soft
language). However, for serious layer pruning, interpretation of the
VPS is required. We can add language about the need for starteful
interpretation of LayerID vis-a-vis stateless interpretation of TID
later.
4.3. Payload Structures
Four 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 four different payload structures are as follows:
o 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 4.4.1.
o Aggregation Packet (AP): Contains more than one NAL unit within
one access unit. This payload structure is specified in
Section 4.4.2.
o Fragmentation Unit (FU): Contains a subset of a single NAL unit.
This payload structure is specified in Section 4.4.3.
4.3.1. Single NAL Unit Packets
A single NAL unit packet contains exactly one NAL unit, and consists
of a payload header (denoted as PayloadHdr), 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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr | DONL (conditional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NAL unit payload data |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of a Single NAL Unit Packet
Figure 3
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the contained NAL
unit. If sprop-max-don-diff is greater than 0 for any of the RTP
streams, 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 for all the RTP
streams), the DONL field MUST NOT be present.
4.3.2. Aggregation Packets (APs)
Aggregation Packets (APs) are introduced to enable the reduction of
packetization overhead for small NAL units, such as most of the non-
VCL NAL units, which are often only a few octets in size.
An AP aggregates NAL units within one access unit. Each NAL unit to
be carried in an AP is encapsulated in an aggregation unit. NAL
units aggregated in one AP are in NAL unit decoding order.
An AP consists of a payload header (denoted as PayloadHdr) followed
by two or more aggregation units, as shown in Figure 4.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=48) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| two or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Packet
Figure 4
The fields in the payload header 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 48.
NOTE: double check #48 against post-geneva [VVC] spec
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.
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
so to avoid IP layer fragmentation. An AP MUST NOT contain FUs
specified in Section 4.4.3. APs MUST NOT be nested; i.e., an AP must
not contain another AP.
The first aggregation unit in an AP consists of a conditional 16-bit
DONL field (in network byte order) followed by a 16-bit unsigned size
information (in network byte order) that indicates the size of the
NAL unit in bytes (excluding these two octets, but including the NAL
unit header), followed by the NAL unit itself, including its NAL unit
header, as shown in Figure 5.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : DONL (conditional) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU size | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of the First Aggregation Unit in an AP
Figure 5
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 for any of the RTP streams,
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. Otherwise (sprop-max-don-diff is equal to 0 for all the RTP
streams), 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
consists of a conditional 8-bit DOND field followed by a 16-bit
unsigned size information (in network byte order) that indicates the
size of the NAL unit in bytes (excluding these two octets, but
including the NAL unit header), followed by the NAL unit itself,
including its NAL unit header, as shown in Figure 6.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : DOND (cond) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NAL unit |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Unit That Is Not the First
Aggregation Unit in an AP
Figure 6
When present, the DOND field plus 1 specifies the difference between
the decoding order number values of the current aggregated NAL unit
and the preceding aggregated NAL unit in the same AP.
If sprop-max-don-diff is greater than 0 for any of the RTP streams,
the DOND field MUST be present in an aggregation unit that is not the
first aggregation unit in an AP, and the variable DON for the
aggregated NAL unit is derived as equal to the DON of the preceding
aggregated NAL unit in the same AP plus the value of the DOND field
plus 1 modulo 65536. Otherwise (sprop-max-don-diff is equal to 0 for
all the RTP streams), the DOND field MUST NOT be present in an
aggregation unit that is not the first aggregation unit in an AP, and
in this case the transmission order and decoding order of NAL units
carried in the AP are the same as the order the NAL units appear in
the AP.
Figure 7 presents an example of an AP that contains two aggregation
units, labeled as 1 and 2 in the figure, without the DONL and DOND
fields being present.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=XX) | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 1 Data |
| . . . |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+ NALU 2 Data |
| . . . |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Packet Containing Two Aggregation Units without
the DONL and DOND Fields
Figure 7
Figure 8 presents an example of an AP that contains two aggregation
units, labeled as 1 and 2 in the figure, with the DONL and DOND
fields being present.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=XX) | NALU 1 DONL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NALU 1 Data . . . |
| |
+ . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 DOND | NALU 2 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 2 Data |
| |
| . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Containing Two Aggregation Units with the DONL
and DOND Fields
Figure 8
4.3.3. Fragmentation Units
Fragmentation Units (FUs) are introduced to enable fragmenting a
single NAL unit into multiple RTP packets, possibly without
cooperation or knowledge of the HEVC [RFC7798] encoder. A fragment
of a NAL unit consists of an integer number of consecutive octets of
that NAL unit. Fragments of the same NAL unit MUST be sent in
consecutive order with ascending RTP sequence numbers (with no other
RTP packets within the same RTP stream being sent between the first
and last fragment).
When a NAL unit is fragmented and conveyed within FUs, it is referred
to as a fragmented NAL unit. APs MUST NOT be fragmented. FUs MUST
NOT be nested; i.e., an FU must not contain a subset of another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU-
time of the fragmented NAL unit.
An FU consists of a payload header (denoted as PayloadHdr), 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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=XX) | FU header | DONL (cond) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| DONL (cond) | |
|-+-+-+-+-+-+-+-+ |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an FU
Figure 9
The fields in the payload header are set as follows. The Type field
MUST be equal to XX. 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, and a 6-bit FuType
field, as shown in Figure 10.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E| FuType |
+---------------+
The Structure of FU Header
Figure 10
The semantics of the FU header fields are as follows:
S: 1 bit
When set to 1, the S bit indicates the start of a fragmented NAL
unit, i.e., the first byte of the FU payload is also the first
byte of the payload of the fragmented NAL unit. When the FU
payload is not the start of the fragmented NAL unit payload, the S
bit MUST be set to 0.
E: 1 bit
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When set to 1, the E bit indicates the end of a fragmented NAL
unit, i.e., the last byte of the payload is also the last byte of
the fragmented NAL unit. When the FU payload is not the last
fragment of a fragmented NAL unit, the E bit MUST be set to 0.
FuType: 6 bits
The field FuType MUST be equal to the field Type of the fragmented
NAL unit.
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the fragmented NAL
unit.
If sprop-max-don-diff is greater than 0 for any of the RTP streams,
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 for all the RTP streams, or the S bit is equal to 0),
the DONL field MUST NOT be present in the FU.
A non-fragmented NAL unit MUST NOT be transmitted in one FU; i.e.,
the Start bit and End bit must not both be set to 1 in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that if the FU payloads of consecutive FUs, starting with
an FU with the S bit equal to 1 and ending with an FU with the E bit
equal to 1, are sequentially concatenated, the payload of the
fragmented NAL unit can be reconstructed. The NAL unit header of the
fragmented NAL unit is not included as such in the FU payload, but
rather the information of the NAL unit header of the fragmented NAL
unit is conveyed in F, 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.
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4.4. Decoding Order Number
For each NAL unit, the variable AbsDon is derived, representing the
decoding order number that is indicative of the NAL unit decoding
order.
Let NAL unit n be the n-th NAL unit in transmission order within an
RTP stream.
If sprop-max-don-diff is equal to 0 for all the RTP streams carrying
the HEVC bitstream, 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 for any of the RTP
streams), AbsDon[n] is derived as follows, where DON[n] is the value
of the variable DON for NAL unit n:
o 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].
o 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:
o AbsDon[n] greater than AbsDon[m] indicates that NAL unit n follows
NAL unit m in NAL unit decoding order.
o When AbsDon[n] is equal to AbsDon[m], the NAL unit decoding order
of the two NAL units can be in either order.
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o AbsDon[n] less than AbsDon[m] 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 may not forward VCL NAL units of higher sub-
layers or some SEI NAL units when there is congestion in the
network. In another example, the first intra-coded picture of
a pre-encoded clip is transmitted in advance to ensure that it
is readily available in the receiver, 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. Another example is MRST or MRMT with sprop-
max-don-diff greater than 0, where the AbsDon values must
indicate cross-layer decoding order for NAL units conveyed in
all the RTP streams.
5. Packetization Rulesumber
The following packetization rules apply:
o If sprop-max-don-diff is greater than 0 for any of the RTP
streams, the transmission order of NAL units carried in the RTP
stream MAY be different than the NAL unit decoding order and the
NAL unit output order. Otherwise (sprop-max-don-diff is equal to
0 for all the RTP streams), the transmission order of NAL units
carried in the RTP stream MUST be the same as the NAL unit
decoding order and, when tx-mode is equal to "MRST" or "MRMT",
MUST also be the same as the NAL unit output order.
o 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
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and can often be aggregated with VCL NAL units without violating
MTU size constraints.
o 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.
o 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 all RTP streams the RTP
stream depends on, if any, 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 sequences 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 XX,
inclusive, may be passed to the decoder. NAL-unit-like structures
with NAL unit type values in the range of XX to XX, 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
across RTP streams, to reorder NAL units from transmission order to
the NAL unit decoding order, and to recover the NAL unit decoding
order in MRST or MRMT, when applicable. In this section, the
receiver operation is described under the assumption that there is no
transmission delay jitter within an RTP stream and across RTP
streams. To make a difference from a practical receiver buffer that
is also used for compensation of transmission delay jitter, the
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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.
When sprop-max-don-diff is equal to 0 for all the received RTP
streams, the de-packetization buffer size is zero bytes, and the
process described in the remainder of this paragraph applies. When
there is only one RTP stream received, 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
there is more than one RTP stream received, the NAL units carried in
the multiple RTP streams are passed to the decoder in their NTP
timestamp order. When there are several NAL units of different RTP
streams with the same NTP timestamp, the order to pass them to the
decoder is their dependency order, where NAL units of a dependee RTP
stream are passed to the decoder prior to the NAL units of the
dependent RTP stream. When there are several NAL units of the same
RTP stream with the same NTP timestamp, the order to pass them to the
decoder is their transmission order.
Informative note: The mapping between RTP and NTP timestamps is
conveyed in RTCP SR packets. In addition, the mechanisms for
faster media timestamp synchronization discussed in [RFC6051] may
be used to speed up the acquisition of the RTP-to-wall-clock
mapping.
When sprop-max-don-diff is greater than 0 for any the received RTP
streams, 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. When MRST or MRMT is in use, NAL units of all RTP
streams of a bitstream are stored in the same de-packetization
buffer. When NAL units carried in any two RTP streams are available
to be placed into the de-packetization buffer, those NAL units
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carried in the RTP stream that is lower in the dependency tree are
placed into the buffer first. For example, if RTP stream A depends
on RTP stream B, then NAL units carried in RTP stream B are placed
into the buffer first.
Initial buffering lasts until condition A (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 of the highest RTP stream) or condition B (the number of
NAL units in the de-packetization buffer is greater than the value of
sprop-depack-buf-nalus) is true.
After initial buffering, whenever condition A or condition B is true,
the following operation is repeatedly applied until both condition A
and condition B become false:
o 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
Placeholder
8. Use with Feedback Messages
The following subsections define the use of the Picture Loss
Indication (PLI), Slice Lost Indication (SLI), Reference Picture
Selection Indication (RPSI), and Full Intra Request (FIR) feedback
messages with HEVC. The PLI, SLI, and RPSI messages are defined in
[RFC4585] , and the FIR message is defined in [RFC5104] .
8.1. Picture Loss Indication (PLI)
As specified in RFC 4585, Section 6.3.1, 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-IDR decoder refresh points,
picture structures, and so forth), a reaction to the reception of an
PLI by a [VVC] sender SHOULD be to send an IDR picture and relevant
parameter sets; potentially with sufficient redundancy so to ensure
correct reception. However, sometimes information about the
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bitstream structure is known. For example, state could have been
established outside of the mechanisms defined in this document that
parameter sets are conveyed out of band only, and 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 RFC 4585 MUST be observed.
8.2. Slice Loss Indication (SLI)
For further study. Maybe remove as there are no known
implementations of SDLI in H.265 based systems
8.3. Reference Picture Selection Indication (RPSI)
Feedback-based reference picture selection has been shown as a
powerful tool to stop temporal error propagation for improved error
resilience [Girod99] [Wang05]. In one approach, the decoder side
tracks errors in the decoded pictures and informs the encoder side
that a particular picture that has been decoded relatively earlier is
correct and still present in the decoded picture buffer; it requests
the encoder to use that correct picture-availability information when
encoding the next picture, so to stop further temporal error
propagation. For this approach, the decoder side should use the RPSI
feedback message.
Encoders can encode some long-term reference pictures as specified in
[VVC] for purposes described in the previous paragraph without the
need of a huge decoded picture buffer. As shown in [Wang05], with a
flexible reference picture management scheme, as in [VVC], even a
decoded picture buffer size of two picture storage buffers would work
for the approach described in the previous paragraph.
the text below is copy-paste from RFC 7798. If we keep the RPSI
message, it needs adaptation to the [VVC] syntax. Doing so shouldn't
be too hard as the [VVC] reference picture mechanism is not too
different from the H.265 one.
8.4. 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 (observing, for
example, the congestion-control-related constraints set out in RFC
5104).
Upon reception of a FIR, a sender MUST send an IDR picture.
Parameter sets MUST also be sent, except when there is a priori
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knowledge that the parameter sets have been correctly established. A
typical example for that is an understanding between 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 Security Considerations 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, seems 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] . Applications SHOULD use one or more appropriate strong
security mechanisms. The rest of this section discusses the security
impacting properties of the payload format itself.
Because the data compression used with this payload format is applied
end-to-end, any encryption needs to be performed after compression.
A potential denial-of-service threat exists for data encodings using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the bitstream that are complex to decode and that cause the
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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, for example,
with SRTP [RFC3711] .
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 rganization
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.
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] .
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 is achieving. This condition can
be satisfied by implementing congestion-control mechanisms to adapt
the transmission rate, 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.
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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 sub-layers (i.e., those NAL
units with a high value of TID) or higher spatio-SNR layers (as
indicated by interpreting the VPS) until the sending bitrate drops to
an acceptable range.
Above mechanisms generally work within a defined profile and level
and, 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. MANES can also remove higher temporal
scalable layers if the outbound transmission (from the MANE's
viewpoint) experiences congestion.
11. IANA Considertaions
Placeholder
12. Acknowledgements
Large parts of this specification share text with the RTP payload
format for HEVC [RFC7798], RFC 7798. We thank the authors of that
specification for their excellent work. We also thank BD Choi for
his contribution towards the [VVC] descriptive text.
13. References
13.1. Normative References
[ISO23090-3]
ISO and IEC, "Versatile video coding -- not yet
published", August 2020.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>.
[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>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[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>.
[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>.
[VVC] ITU-T, "Versatile video coding - JVET-O2001-vE, available
from http://phenix.it-
sudparis.eu/jvet/doc_end_user/documents/15_Gothenburg/
wg11/JVET-O2001-v14.zip", August 2019.
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13.2. Informative References
[CABAC] Sole, J., Joshi, R., Nguyen, N., Ji, T., Karczewicz, M.,
Clare, G., Henry, F., and A. Duenas, "Transform
coefficient coding in HEVC", IEEE Transactions on Circuts
and Systems for Video Technology Vol. 22 No. 12 pp.
1765-1777, DOI 10.1109/TCSVT.2012.2223055, December 2012.
[Girod99] Girod, B. and F. Faerber, "Feedback-based error control
for mobile video transmission", Proceedings of the
IEEE Vol. 87, No. 10, pp. 1707-1723, DOI 10.1109/5.790632,
October 1999.
[MPEG2S] IS0/IEC, "Information technology - Generic coding of
moving pictures and associated audio information - Part 1:
Systems", ISO International Standard 13818-1, 2013.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, DOI 10.17487/RFC6051, November 2010,
<https://www.rfc-editor.org/info/rfc6051>.
[RFC6184] Wang, Y., 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., 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>.
[RFC7798] Wang, Y., Sanchez, Y., Schierl, T., Wenger, S., and 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>.
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Appendix A. Change History
draft-zhao-payload-rtp-vvc-00 ........ initial version
Authors' Addresses
Shuai Zhao
Tencent
2747 Park Blvd.
Palo Alto, CA 94306
US
Email: shuaiizhao@tencent.com
Stephan Wenger
Tencent
2747 Park Blvd.
Palo Alto, CA 94306
US
Email: stewe@stewe.org
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