avtcore S. Zhao
Internet-Draft S. Wenger
Intended status: Standards Track Tencent
Expires: 11 June 2021 8 December 2020
RTP Payload Format for Essential Video Coding (EVC)
draft-ietf-avtcore-rtp-evc-00
Abstract
This memo describes an RTP payload format for the video coding
standard ISO/IEC International Standard 23094-1 [ISO23094-1], also
known as Essential Video Coding [EVC] and developed by ISO/IEC
JTC1/SC29/WG11. 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
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This Internet-Draft will expire on 11 June 2021.
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Please review these documents carefully, as they describe your rights
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extracted from this document must include Simplified BSD License text
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview of the EVC Codec . . . . . . . . . . . . . . . . 3
1.1.1. Coding-Tool Features (informative) . . . . . . . . . 4
1.1.2. Systems and Transport Interfaces . . . . . . . . . . 6
1.1.3. Parallel Processing Support (informative) . . . . . . 8
1.1.4. NAL Unit Header . . . . . . . . . . . . . . . . . . . 8
1.2. Overview of the Payload Format . . . . . . . . . . . . . 9
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 10
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1. Definitions from the EVC Specification . . . . . . . 10
3.1.2. Definitions Specific to This Memo . . . . . . . . . . 12
3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 13
4. RTP Payload Format . . . . . . . . . . . . . . . . . . . . . 14
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 15
4.2. Payload Header Usage . . . . . . . . . . . . . . . . . . 16
4.3. Payload Structures . . . . . . . . . . . . . . . . . . . 17
4.3.1. Single NAL Unit Packets . . . . . . . . . . . . . . . 17
4.3.2. Aggregation Packets (APs) . . . . . . . . . . . . . . 18
4.3.3. Fragmentation Units . . . . . . . . . . . . . . . . . 22
4.4. Decoding Order Number . . . . . . . . . . . . . . . . . . 25
5. Packetization Rules . . . . . . . . . . . . . . . . . . . . . 26
6. De-packetization Process . . . . . . . . . . . . . . . . . . 27
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 29
7.1. Media Type Registration . . . . . . . . . . . . . . . . . 29
7.2. SDP Parameters . . . . . . . . . . . . . . . . . . . . . 29
7.2.1. Mapping of Payload Type Parameters to SDP . . . . . . 29
7.2.2. Usage with SDP Offer/Answer Model . . . . . . . . . . 30
7.2.3. SDP Example . . . . . . . . . . . . . . . . . . . . . 30
8. Use with Feedback Messages . . . . . . . . . . . . . . . . . 30
8.1. Picture Loss Indication (PLI) . . . . . . . . . . . . . . 30
8.2. Full Intra Request (FIR) . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 31
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.1. Normative References . . . . . . . . . . . . . . . . . . 32
13.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
The [EVC] specification, which will be formally designated (once
approved) as ISO/IEC International Standard 23094-1 [ISO23094-1], is
planned for ratification in early 2020. A draft that's currently in
the approval process of ISO/IEC can be found as [EVC]. One goal of
MPEG is to keep [EVC]'s baseline essentially royalty free by
agreement among the key contributors, whereas more advanced profiles
follow a reasonable and non-discriminatory licensing terms policy.
Both baseline and higher profiles of [EVC] are reported to provide
coding efficiency gains over [HEVC] and [AVC] under certain
configurations.
editor-note 1: Is it necessary to add comparison with [VVC]?
This memo describes an RTP payload format for [EVC]. It shares its
basic design with the NAL unit-based RTP payload formats of H.264
Video Coding [RFC6184], Scalable Video Coding (SVC) [RFC6190], High
Efficiency Video Coding (HEVC) [RFC7798], and Versatile Video Coding
(VVC)[I-D.ietf-avtcore-rtp-vvc]. 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 [RFC6190] were
incorporated as EVC supports temporal scalability. [EVC] does not
offer higher forms of scalability.
1.1. Overview of the EVC Codec
[EVC], [AVC], [HEVC] and [VVC] share a similar hybrid video codec
design. In this memo, we provide a very brief overview of those
features of EVC that are, in some form, addressed by the payload
format specified herein. Implementers have to read, understand, and
apply the ISO/IEC specifications pertaining to EVC to arrive at
interoperable, well-performing implementations. The EVC standard has
a baseline profile and on top of that, a main profile, the latter
including more advanced features. A "toolset" syntax element allows
encoders to mark a bitstream as to what of the many independent
coding tools are exercised in the bitstream, in a spirit similar to
the general_constraint_flags of [VVC].
Conceptually, all [EVC], [AVC], [HEVC] and [VVC] 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.
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1.1.1. Coding-Tool Features (informative)
Coding blocks and transform structure
[EVC] uses a traditional quad-tree coding structure, which divides
the encoded image into blocks of up to 128x128 luma samples, which
can be recursively divided into smaller blocks. The main profile
adds two advanced coding structure tools: Binary Ternary Tree (BTT)
that allows non-square coding units and segmentation that changes the
processing order of the segmentation unit from traditional left-
scanning order processing to right-scanning order processing Unit
Coding Order (SUCO). In the main profile, the picture can be divided
into rectangular tiles, and these tiles can be independently encoded
and/or decoded in parallel.
When predicting a data block using intra prediction or inter
prediction, the remaining data is usually added to the prediction
block. The residual data is added to the prediction block. The
residual data is obtained by applying an inverse quantization process
and an inverse transform. [EVC] includes integer discrete cosine
transform (DCT2) and scalar quantization. For the main profile,
Improved Quantization and Transform (IQT) uses a different mapping/
clipping function for quantization. An inverse zig-zag scanning
order is used for coefficient coding. Advanced Coefficient Coding
(ADCC) in the main profile can code coefficient values more
efficiently, for example, indicated by the last non-zero coefficient.
In main profile, Adaptive Transformation Selection (ATS) is also
available and can be applied to integer versions of DST7 or DCT8, and
not just DCT2.
Entropy coding
[EVC] uses a similar binary arithmetic coding mechanism as [AVC].
The mechanism includes a binarization step and a probability update
defined by a lookup table. In the main profile, the derivation
process of syntax elements based on adjacent blocks makes the context
modeling and initialization process more efficient.
In-loop filtering
The baseline profile of [EVC] uses the deblocking filter defined in
H.263 Annex J. In the main profile, compared to the deblocking
filter in the baseline profile, an Advanced Deblocking Filter (ADDB)
can be used, which can further reduce artifacts. The main profile
also defines two additional in-loop filters that can be used to
improve the quality of decoded pictures before output and/or for
inter prediction. A Walsh-Hadamard Transform Domain Filter (HTDF) is
applied to the luma samples before deblocking, and the scanning
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process is used to determine 4 adjacent samples for filtering. An
adaptive Loop Filter (ALF) allows to send signals of up to 25
different filters for the luma components, and the best filter can be
selected through the classification process for each 4x4 block. The
filter parameters of the ALF filter are signaled in the Adaptation
Parameter Set (APS).
Inter-prediction
The basis of [EVC] inter prediction is motion compensation using
interpolation filters with a quarter sample resolution. In baseline
profile, a motion vector signal is transmitted using one of three
spatially neighboring motion vectors and a temporally collocated
motion vector as a predictor. The motion vector difference may be
signaled relative to the selected predictor, but for the case where
no motion vector difference is signaled and there is no remaining
data in the block, there is a specific mode called a skip mode. The
main profile includes six additional tools to provide improved inter
prediction. With advanced Motion Interpolation and Signaling (AMIS),
adjacent blocks can be conceptually merged to indicate that they use
the same motion, but more advanced schemes can also be used to create
predictions from the basic model list of candidate predictors. The
Merge with Motion Vector Difference (MMVD) tool uses a process
similar to the concept of merging neighboring blocks, but also allows
the use of expressions that include a starting point, motion
amplitude, and direction of motion to send a motion vector signal.
Using Advanced Motion Vector Prediction (AMVP), candidate motion
vector predictions for the block can be derived from its neighboring
blocks in the same picture and collocated blocks in the reference
picture. The Adaptive Motion Vector Resolution (AMVR) tool provides
a way to reduce the accuracy of a motion vector from a quarter sample
to half sample, full sample, double sample, or quad sample, which
provides the efficiency advantage, such as when sending large motion
vector differences. The main profile also includes the Decoder-side
Motion Vector Refinement (DMVR), which uses a bilateral template
matching process to refine the motion vectors in a bidirectional
fashion.
Intra prediction and intra-coding
Intra prediction in [EVC] is performed on adjacent samples of coding
units in a partitioned structure. For the baseline profile, all
coding units are square, and there are five different prediction
modes: DC (mean value of the neighborhood), horizontal, vertical, and
two different diagonal directions. In the main profile, intra
prediction can be applied to any rectangular coding unit, and there
are 28 additional direction modes available in the so-called Enhanced
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Intra Prediction Directions (EIPD). In the main profile, an encoder
can also use Intra Block Copy (IBC), where a previously decoded
sample blocks of the same picture is used as a predictor. A
displacement vector in integer sample precision is signaled to
indicate where the prediction block in the current picture is used
for this mode.
Decoded picture buffer management
In the previous technology, decoded pictures can be stored in a
decoded picture buffer (Decoded Picture Buffer, DPB) for predicting
pictures that follow them in decoding order. In the baseline
profile, the management of the DPB (i.e. the process of adding and
deleting reference pictures) is controlled by the information in the
SPS. For the main profile, if a Reference Picture List (RPL) scheme
is used, DPB management can be controlled by information that is
signaled at the picture level.
1.1.2. Systems and Transport Interfaces
[EVC] inherited the basic systems and transport interfaces designs
from [AVC] and [HEVC]. These include the NAL-unit-based syntax
structure, the hierarchical syntax and data unit structure and the
Supplemental Enhancement Information (SEI) message mechanism. The
hierarchical syntax and data unit structure consists of a sequence-
level parameter set (SPS), two picture-level parameter sets (PPS and
APS, each of which can apply to one or more pictures), slice-level
header parameters, and lower-level parameters.
A number of key components that influenced the Network Abstraction
Layer design of [EVC] as well as this memo are described below
Sequence parameter set
The Sequence Parameter Set (SPS) contains syntax elements pertaining
to a coded video sequence (CVS), which is a group of pictures,
starting with a random access point, 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, EVC
retains this basic concept. In many TV-like applications, a CVS
contains a few hundred milliseconds to a few seconds of video. In
video conferencing (without switching MCUs involved), a CVS can be as
long in duration as the whole session.
Picture and Adaptation parameter set
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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, level and toolsets
Profiles and levels follow the same design considerations ask known
form [AVC], [HEVC], and in fact video codecs as old as MPEG-1 visual.
A profile defines a set of tools (not to confuse with the "toolset"
discussed below) that a decoder compliant with this profile has to
support. In [EVC], profiles are defined in Annex A. Formally, they
are defined as a set of constraints that a bitstream needs to conform
to. In [EVC], the baseline profile is much more severely constraint
than main profile, reducing implementation complexity. Levels relate
to bitstream complexity in dimensions such as maximum sample decoding
rate, maximum picture size, and similar parameters that are directly
related to computational complexity.
Profiles and levels are signaled in the highest parameter set
available, the SPS.
[EVC] contains another mechanism related to the use of coding tools,
known as the toolset syntax element. This syntax element, also
located in the SPS, is a bitmask that allows encoders to indicate
which coding tools they are using, within the menu of profiles
offered by the profile that is also signaled. No decoder conformance
point is associated with the toolset, but a bitstream that were using
a coding tool that is indicated as not used in the toolset syntax
element would obviously be non-compliant. While MPEG specifically
rules out the use of the toolset syntax element as a conformance
point, walled garden implementations could do so without incurring
the interoperability problems MPEG fears, and create bitstreams and
decoders that do not support one or more given tools. That, in turn,
may be useful to mitigate certain patent related risks.
Bitstream and elementary stream
Above the Coded Video Sequence (CVS), [EVC] defines a video bitstream
that can be used in the MPEG systems context as an elementary stream.
For the purpose of this memo, this is not relevant.
Random access support
editor-note 2: At this point, the authors believe [EVC] supports
only clean random access. WG input is solicited.
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Temporal scalability support
[EVC] includes support for temporal scalability through the
generalized reference picture selection approach known since
[AVC]/SVC. Up to six temporal layers are supported. The temporal
layer is signaled in the NAL unit header (which co-serves as the
payload header in this memo), in the nuh_temporal_id field.
Reference picture management
placeholder
SEI Message
[EVC] inherits many of [HEVC]'s SEI Messages, occasionally with
changes in syntax and/or semantics making them applicable to EVC.
1.1.3. Parallel Processing Support (informative)
Placeholder
1.1.4. NAL Unit Header
[EVC] maintains the NAL unit concept of [HEVC] with different
parameter options. EVC also uses a two-byte NAL unit header, as
shown in Figure 1. The payload of a NAL unit refers to the NAL unit
excluding the NAL unit header.
+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| Type | TID | Reserve |E|
+-------------+-----------------+
The Structure of the EVC NAL Unit Header
Figure 1
The semantics of the fields in the NAL unit header are as specified
in [EVC] and described briefly below for convenience. In addition to
the name and size of each field, the corresponding syntax element
name in [EVC] is also provided.
F: 1 bit
forbidden_zero_bit. Required to be zero in [EVC]. Note that the
inclusion of this bit in the NAL unit header was included to
enable transport of EVC video over MPEG-2 transport systems
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(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 xxx. (section # placeholder)
Type: 6 bits
nal_unit_type_plus1. This field specifies the NAL unit type as
defined in Table 4 of [EVC]. If the value of this field is less
than and equal to 23, the NAL unit is a VCL NAL unit. Otherwise,
the NAL unit is a non-VCL NAL unit. For a reference of all
currently defined NAL unit types and their semantics, please refer
to Section 7.4.2.2 in [EVC].
TID: 3 bits
nuh_temporal_id. This field specifies the temporal identifier of
the NAL unit. The value of TemporalId is equal to TID.
TemporalId shall be equal to 0 if it is a IDR NAL unit type (NAL
unit type 1).
Reserve: 5 bits
nuh_reserved_zero_5bits. This field shall be equal to the version
of the [EVC] specification. Values of nuh_reserved_zero_5bits
greater than 0 are reserved for future use by ISO/IEC. Decoders
conforming to a profile specified in [EVC] Annex A shall ignore
(i.e., remove from the bitstream and discard) all NAL units with
values of nuh_reserved_zero_5bits greater than 0.
E: 1 bit
nuh_extension_flag. This field shall be equal the version of the
[EVC] specification. Value of nuh_extesion_flag equal to 1 is
reserved for future use by ISO/IEC. Decoders conforming to a
profile specified in Annex A shall ignore (i.e., remove from the
bitstream and discard) all NAL units with values of
nuh_extension_flag equal to 1.
1.2. Overview of the Payload Format
This payload format defines the following processes required for
transport of [EVC] coded data over RTP [RFC3550]:
* Usage of RTP header with this payload format
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* Packetization of [EVC] coded NAL units into RTP packets using
three types of payload structures: a single NAL unit, aggregation,
and fragment unit packet
* Transmission of [EVC] NAL units of the same bitstream within a
single RTP stream.
* Media type parameters to be used with the Session Description
Protocol (SDP) [RFC4566]
* Frame-marking mapping [FrameMarking]
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown above.
3. Definitions and Abbreviations
3.1. Definitions
This document uses the terms and definitions of EVC. Section 3.1.1
lists relevant definitions from [EVC] for convenience. Section 3.1.2
provides definitions specific to this memo.
3.1.1. Definitions from the EVC Specification
Access Unit: A set of NAL units that are associated with each other
according to a specified classification rule, are consecutive in
decoding order, and contain exactly one coded picture.
Bitstream: A sequence of bits, in the form of a NAL unit stream or a
byte stream, that forms the representation of coded pictures and
associated data forming one or more coded video sequences (CVSs).
Coded Picture: A coded representation of a picture containing all
CTUs of the picture.
Coded Video Sequence (CVS): A sequence of access units that consists,
in decoding order, of an IDR access unit, followed by zero or more
access units that are not IDR access units, including all subsequent
access units up to but not including any subsequent access unit that
is an IDR access unit.
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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.
Decoded Picture: A decoded picture is derived by decoding a coded
picture.
Decoded Picture Buffer (DPB): A buffer holding decoded pictures for
reference, output reordering, or output delay specified for the
hypothetical reference decoder in Annex C of [EVC] specification.
Dynamic Range Adjustment (DRA): A mapping process that is applied to
decoded picture prior to cropping and output as part of the decoding
process and is controlled by parameters conveyed in an Adaptation
Parameter Set (APS).
Hypothetical Reference Decoder (HRD): A hypothetical decoder model
that specifies constraints on the variability of conforming NAL unit
streams or conforming byte streams that an encoding process may
produce.
Instantaneous Decoding Refresh (IDR) access unit: An access unit in
which the coded picture is an IDR picture.
Instantaneous Decoding Refresh (IDR) picture: A coded picture for
which each VCL NAL unit has NalUnitType equal to IDR_NUT.
Level: A defined set of constraints on the values that may be taken
by the syntax elements and variables of this document, or the value
of a transform coefficient prior to scaling.
Network Abstraction Layer (NAL) unit: A syntax structure containing
an indication of the type of data to follow and bytes containing that
data in the form of an RBSP interspersed as necessary.
Network Abstraction Layer (NAL) Unit Stream: A sequence of NAL units.
Non-IDR Picture: A coded picture that is not an IDR picture.
Non-VCL NAL Unit: A NAL unit that is not a VCL NAL unit.
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Picture Parameter Set (PPS): A syntax structure containing syntax
elements that apply to zero or more entire coded pictures as
determined by a syntax element found in each slice header.
Picture Order Count (POC): A variable that is associated with each
picture, uniquely identifies the associated picture among all
pictures in the CVS, and, when the associated picture is to be output
from the decoded picture buffer, indicates the position of the
associated picture in output order relative to the output order
positions of the other pictures in the same CVS that are to be output
from the decoded picture buffer.
Raw Byte Sequence Payload (RBSP): A syntax structure containing an
integer number of bytes that is encapsulated in a NAL unit and that
is either empty or has the form of a string of data bits containing
syntax elements followed by an RBSP stop bit and zero or more
subsequent bits equal to 0.
Sequence Parameter Set (SPS): A syntax structure containing syntax
elements that apply to zero or more entire CVSs as determined by the
content of a syntax element found in the PPS referred to by a syntax
element found in each slice header.
Tile row: A rectangular region of CTUs having a height specified by
syntax elements in the PPS and a width equal to the width of the
picture.
Tile scan: A specific sequential ordering of CTUs partitioning a
picture in which the CTUs are ordered consecutively in CTU raster
scan in a tile whereas tiles in a picture are ordered consecutively
in a raster scan of the tiles of the picture.
Video coding layer (VCL) NAL unit: A collective term for coded slice
NAL units and the subset of NAL units that have reserved values of
NalUnitType that are classified as VCL NAL units in this document.
3.1.2. Definitions Specific to This 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.
editor-note 3: the following informative needs to be updated along
with frame marking update
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Informative note: The concept of a MANE goes beyond normal routers
or gateways in that a MANE has to be aware of the signaling (e.g.,
to learn about the payload type mappings of the media streams),
and in that it has to be trusted when working with Secure RTP
(SRTP). The advantage of using MANEs is that they allow packets
to be dropped according to the needs of the media coding. For
example, if a MANE has to drop packets due to congestion on a
certain link, it can identify and remove those packets whose
elimination produces the least adverse effect on the user
experience. After dropping packets, MANEs must rewrite RTCP
packets to match the changes to the RTP stream, as specified in
Section 7 of [RFC3550].
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in Section 8.2 and 8.3 in [EVC],
follow the Order of NAL units in the bitstream.
NAL unit output order: A NAL unit order in which NAL units of
different access units are in the output order of the decoded
pictures corresponding to the access units, as specified in [EVC],
and in which NAL units within an access unit are in their decoding
order.
RTP stream: See [RFC7656]. Within the scope of this memo, one RTP
stream is utilized to transport one or more temporal sub-layers.
Transmission order: The order of packets in ascending RTP sequence
number order (in modulo arithmetic). Within an aggregation packet,
the NAL unit transmission order is the same as the order of
appearance of NAL units in the packet.
3.2. Abbreviations
APS Adaptation Parameter Set
ATS Adaptive Transform Selection
B Bi-predictive
CBR Constant Bit Rate
CPB Coded Picture Buffer
CTB Coding Tree Block
CTU Coding Tree Unit
CVS Coded Video Sequence
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DPB Decoded Picture Buffer
HRD Hypothetical Reference Decoder
HSS Hypothetical Stream Scheduler
I Intra
IDR Instantaneous Decoding Refresh
LSB Least Significant Bit
LTRP Long-Term Reference Picture
MMVD Merge with Motion Vector Difference
MSB Most Significant Bit
NAL Network Abstraction Layer
P Predictive
POC Picture Order Count
PPS Picture Parameter Set
QP Quantization Parameter
RBSP Raw Byte Sequence Payload
RGB Same as GBR
SAR Sample Aspect Ratio
SEI Supplemental Enhancement Information
SODB String Of Data Bits
SPS Sequence Parameter Set
STRP Short-Term Reference Picture
VBR Variable Bit Rate
VCL Video Coding Layer
4. RTP Payload Format
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4.1. RTP Header Usage
The format of the RTP header is specified in [RFC3550] (reprinted as
Figure 2 for convenience). This payload format uses the fields of
the header in a manner consistent with that specification.
The RTP payload (and the settings for some RTP header bits) for
aggregation packets and fragmentation units are specified in
Section 4.3.2 and Section 4.3.3, respectively.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP Header According to {{RFC3550}}
Figure 2
The RTP header information to be set according to this RTP payload
format is set as follows:
Marker bit (M): 1 bit
Set for the last packet of the access unit, carried in the current
RTP stream. This is in line with the normal use of the M bit in
video formats to allow an efficient playout buffer handling.
editor-note 4: The informative note below needs updating once
the NAL unit type table is stable in the [EVC] 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
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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 or nal_unit_type
equal to 27, and 2) all NAL units between naluX and naluY, when
present, have nal_unit_type in the range of 24 to 26,
inclusive, equal to 28 or 29.
Payload Type (PT): 7 bits
The assignment of an RTP payload type for this new payload format
is outside the scope of this document and will not be specified
here. The assignment of a payload type has to be performed either
through the profile used or in a dynamic way.
Sequence Number (SN): 16 bits
Set and used in accordance with [RFC3550].
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the content.
A 90 kHz clock rate MUST be used. If the NAL unit has no timing
properties of its own (e.g., parameter sets or certain SEI NAL
units), the RTP timestamp MUST be set to the RTP timestamp of the
coded picture of the access unit in which the NAL unit (according
to Annex D of [EVC]) 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 [EVC].
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, TID, Reserve and E) as the NAL unit header as shown in
Section 1.1.4, irrespective of the type of the payload structure.
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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.
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 (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.
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
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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) 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| two or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Packet
Figure 4
The fields in the payload header are set as follows. The F bit MUST
be equal to 0 if the F bit of each aggregated NAL unit is equal to
zero; otherwise, it MUST be equal to 1. The Type field MUST be equal
to 56.
The value of TID MUST be the lowest value of TID of all the
aggregated NAL units. The value of Reserve and E Must match the
version of [EVC] specification.
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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 path MTU
size so to avoid IP layer fragmentation. An AP MUST NOT contain FUs
specified in Section 4.3.3. APs MUST NOT be nested; i.e., an AP can
not contain another AP.
The first aggregation unit in an AP consists of a conditional 16-bit
DONL field (in network byte order) followed by a 16-bit unsigned size
information (in network byte order) that indicates the size of the
NAL unit in bytes (excluding these two octets, but including the NAL
unit header), followed by the NAL unit itself, including its NAL unit
header, as shown in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : DONL (conditional) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU size | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of the First Aggregation Unit in an AP
Figure 5
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.
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An aggregation unit that is not the first aggregation unit in an AP
will be followed immediately by a 16-bit unsigned size information
(in network byte order) that indicates the size of the NAL unit in
bytes (excluding these two octets, but including the NAL unit
header), followed by the NAL unit itself, including its NAL unit
header, as shown in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU size | NAL unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Unit That Is Not the First
Aggregation Unit in an AP
Figure 6
Figure 7 presents an example of an AP that contains two aggregation
units, labeled as NALU 1 and NALU 2 in the figure, without the DONL
field being present.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 1 Data |
| . . . |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+ NALU 2 Data |
| . . . |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Packet Containing
Two Aggregation Units without the DONL Field
Figure 7
Figure 8 presents an example of an AP that contains two aggregation
units, labeled as NALU 1 and NALU 2 in the figure, with the DONL
field being present.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=56) | NALU 1 DONL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NALU 1 Data . . . |
| |
+ . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU 2 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 2 Data |
| |
| . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Containing
Two Aggregation Units with the DONL Field
Figure 8
4.3.3. Fragmentation Units
Fragmentation Units (FUs) are introduced to enable fragmenting a
single NAL unit into multiple RTP packets, possibly without
cooperation or knowledge of the EVC [EVC] encoder. A fragment of a
NAL unit consists of an integer number of consecutive octets of that
NAL unit. Fragments of the same NAL unit MUST be sent in consecutive
order with ascending RTP sequence numbers (with no other RTP packets
within the same RTP stream being sent between the first and last
fragment).
When a NAL unit is fragmented and conveyed within FUs, it is referred
to as a fragmented NAL unit. APs MUST NOT be fragmented. FUs MUST
NOT be nested; i.e., an FU must not contain a subset of another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU-
time of the fragmented NAL unit.
An FU consists of a payload header (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=57) | FU header | DONL (cond) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| DONL (cond) | |
|-+-+-+-+-+-+-+-+ |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an FU
Figure 9
The fields in the payload header are set as follows. The Type field
MUST be equal to 57. The fields F, TID, Reserve and E MUST be equal
to the fields F, TID, Reserve and E, respectively, of the fragmented
NAL unit.
The FU header consists of an S bit, an E bit, and a 6-bit FuType
field, as shown in Figure 10.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E| FuType |
+---------------+
The Structure of FU Header
Figure 10
The semantics of the FU header fields are as follows:
S: 1 bit
When set to 1, the S bit indicates the start of a fragmented NAL
unit, i.e., the first byte of the FU payload is also the first
byte of the payload of the fragmented NAL unit. When the FU
payload is not the start of the fragmented NAL unit payload, the S
bit MUST be set to 0.
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, TID, Reserve and E fields of the FU payload
headers of the FUs and the FuType field of the FU header of the FUs.
An FU payload MUST NOT be empty.
If an FU is lost, the receiver SHOULD discard all following
fragmentation units in transmission order corresponding to the same
fragmented NAL unit, unless the decoder in the receiver is known to
gracefully handle incomplete NAL units.
A receiver in an endpoint or 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:
* If n is equal to 0 (i.e., NAL unit n is the very first NAL unit in
transmission order), AbsDon[0] is set equal to DON[0].
* Otherwise (n is greater than 0), the following applies for
derivation of AbsDon[n]:
If DON[n] == DON[n-1],
AbsDon[n] = AbsDon[n-1]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] < 32768),
AbsDon[n] = AbsDon[n-1] + DON[n] - DON[n-1]
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] >= 32768),
AbsDon[n] = AbsDon[n-1] + 65536 - DON[n-1] + DON[n]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] >= 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] + 65536 -
DON[n])
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] < 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] - DON[n])
For any two NAL units m and n, the following applies:
* AbsDon[n] greater than AbsDon[m] indicates that NAL unit n follows
NAL unit m in NAL unit decoding order.
* When AbsDon[n] is equal to AbsDon[m], the NAL unit decoding order
of the two NAL units can be in either order.
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* 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 might not forward VCL NAL units of higher
sub-layers or some SEI NAL units when there is congestion in
the network. In another example, the first intra-coded picture
of a pre-encoded clip is transmitted in advance to ensure that
it is readily available in the receiver, 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 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.
* 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 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.
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* 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 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 55,
inclusive, may be passed to the decoder. NAL-unit-like structures
with NAL unit type values in the range of 56 to 63, 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. 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
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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 the received RTP stream,
the de-packetization buffer size is zero bytes, and the process
described in the remainder of this paragraph applies. The NAL units
carried in the RTP stream are directly passed to the decoder in their
transmission order, which is identical to their decoding order. 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 the received RTP stream
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 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) 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:
* 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.
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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) [RFC4556] is also
provided for applications that use SDP.
7.1. Media Type Registration
The receiver MUST ignore any parameter unspecified in this memo.
Type name: video
Subtype name: evc
Required parameters: none
Optional parameters:
editor-note 5: To be updated
7.2. SDP Parameters
The receiver MUST ignore any parameter unspecified in this memo.
7.2.1. Mapping of Payload Type Parameters to SDP
The media type video/evc string is mapped to fields in the Session
Description Protocol (SDP) [RFC4566] as follows:
* The media name in the "m=" line of SDP MUST be video.
* The encoding name in the "a=rtpmap" line of SDP MUST be evc (the
media subtype).
* The clock rate in the "a=rtpmap" line MUST be 90000.
* OPTIONAL PARAMETERS:
editor-note 6: To be updated
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7.2.2. Usage with SDP Offer/Answer Model
When [EVC] is offered over RTP using SDP in an offer/answer model
[RFC3264] for negotiation for unicast usage, the following
limitations and rules apply:
editor-note 7: to be updated
7.2.3. SDP Example
editor-note 8: to be updated
8. Use with Feedback Messages
Placeholder
8.1. Picture Loss Indication (PLI)
Placeholder
8.2. Full Intra Request (FIR)
Placeholder
9. Security Considerations
The scope of this Security Considerations section is limited to the
payload format itself and to one feature of [EVC] that may pose a
particularly serious security risk if implemented naively. The
payload format, in isolation, does not form a complete system.
Implementers are advised to read and understand relevant security-
related documents, especially those pertaining to RTP (see the
Security Considerations section in [RFC3550] ), and the security of
the call-control stack chosen (that may make use of the media type
registration of this memo). Implementers should also consider known
security vulnerabilities of video coding and decoding implementations
in general and avoid those.
Within this RTP payload format, 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]
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discusses, it is not an RTP payload format's responsibility to
discuss or mandate what solutions are used to meet the basic security
goals like confidentiality, integrity and source authenticity for RTP
in general. This responsibility lays on anyone using RTP in an
application. They can find guidance on available security mechanisms
and important considerations in "Options for Securing RTP Sessions"
[RFC7201]. Applications SHOULD use one or more appropriate strong
security mechanisms. The rest of this section discusses the security
impacting properties of the payload format itself.
Because the data compression used with this payload format is applied
end-to-end, any encryption needs to be performed after compression.
A potential denial-of-service threat exists for data encodings using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the bitstream that are complex to decode and that cause the
receiver to be overloaded. EVC is particularly vulnerable to such
attacks, as it is extremely simple to generate datagrams containing
NAL units that affect the decoding process of many future NAL units.
Therefore, the usage of data origin authentication and data integrity
protection of at least the RTP packet is RECOMMENDED, for example,
with SRTP [RFC3711].
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.
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The bitrate adaptation necessary for obeying the congestion control
principle is easily achievable when real-time encoding is used, for
example, by adequately tuning the quantization parameter. However,
when pre-encoded content is being transmitted, bandwidth adaptation
requires the pre-coded bitstream to be tailored for such adaptivity.
The key mechanism available in [EVC] is temporal scalability. A
media sender can remove NAL units belonging to higher temporal sub-
layers (i.e., those NAL. units with a high value of TID) until the
sending bitrate drops to an acceptable range.
The mechanisms mentioned above 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 Considerations
Placeholder
12. Acknowledgements
Large 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.
13. References
13.1. Normative References
[EVC] "ISO/IEC FDIS 23094-1 Essential Video Coding", 2020,
<https://www.iso.org/standard/57797.html>.
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[ISO23094-1]
"ISO/IEC DIS Information technology --- General video
coding --- Part 1 Essential video coding", n.d.,
<https://www.iso.org/standard/57797.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/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>.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556,
DOI 10.17487/RFC4556, June 2006,
<https://www.rfc-editor.org/info/rfc4556>.
[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>.
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[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>.
[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>.
[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>.
13.2. Informative References
[AVC] "ITU-T Recommendation H.264 - Advanced video coding for
generic audiovisual services", 2014,
<https://www.iso.org/standard/66069.html>.
[FrameMarking]
Berger, E, ., Nandakumar, S, ., and . Zanaty M, "Frame
Marking RTP Header Extension", Work in Progress draft-
berger-avtext-framemarking , 2015.
[HEVC] "High efficiency video coding, ITU-T Recommendation
H.265", 2017, <https://www.iso.org/standard/69668.html>.
[I-D.ietf-avtcore-rtp-vvc]
Zhao, S., Wenger, S., Sanchez, Y., and Y. Wang, "RTP
Payload Format for Versatile Video Coding (VVC)", Work in
Progress, Internet-Draft, draft-ietf-avtcore-rtp-vvc-05, 2
November 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-avtcore-rtp-vvc-05.txt>.
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[MPEG2S] IS0/IEC, ., "Information technology - Generic coding
ofmoving 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.-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>.
[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>.
[VVC] "ISO/IEC FDIS 23090-3 Information technology --- Coded
representation of immersive media --- Part 3 - Versatile
video coding", 2020,
<https://www.iso.org/standard/73022.html>.
Authors' Addresses
Shuai Zhao
Tencent
2747 Park Blvd
Palo Alto, 94588
United States of America
Email: shuai.zhao@ieee.org
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Stephan Wenger
Tencent
2747 Park Blvd
Palo Alto, 94588
United States of America
Email: stewe@stewe.org
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