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IETF RFC 7798
Last modified on Wednesday, March 23rd, 2016
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Internet Engineering Task Force (IETF) Y.-K. Wang
Request for Comments: 7798 Qualcomm
Category: Standards Track Y. Sanchez
ISSN: 2070-1721 T. Schierl
Fraunhofer HHI
S. Wenger
Vidyo
M. M. Hannuksela
Nokia
March 2016
RTP Payload Format for High Efficiency Video Coding (HEVC)
 Abstract
This memo describes an RTP payload format for the video coding
standard ITU-T Recommendation H.265 and ISO/IEC International
Standard 23008-2, both also known as High Efficiency Video Coding
(HEVC) and developed by the Joint Collaborative Team on Video Coding
(JCT-VC). 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.
Furthermore, it supports transmission of an HEVC bitstream over a
single stream as well as multiple RTP streams. When multiple RTP
streams are used, a single transport or multiple transports may be
utilized. The payload format has wide applicability in
videoconferencing, Internet video streaming, and high-bitrate
entertainment-quality video, among others.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7798.
Wang, et al. Standards Track PAGE 1 
RFC 7798 RTP Payload Format for HEVC March 2016
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Overview of the HEVC Codec .................................4
1.1.1. Coding-Tool Features ................................4
1.1.2. Systems and Transport Interfaces ....................6
1.1.3. Parallel Processing Support ........................11
1.1.4. NAL Unit Header ....................................13
1.2. Overview of the Payload Format ............................14
2. Conventions ....................................................15
3. Definitions and Abbreviations ..................................15
3.1. Definitions ...............................................15
3.1.1. Definitions from the HEVC Specification ...........15
3.1.2. Definitions Specific to This Memo .................17
3.2. Abbreviations .............................................19
4. RTP Payload Format .............................................20
4.1. RTP Header Usage ..........................................20
4.2. Payload Header Usage ......................................22
4.3. Transmission Modes ........................................23
4.4. Payload Structures ........................................24
4.4.1. Single NAL Unit Packets ............................24
4.4.2. Aggregation Packets (APs) ..........................25
4.4.3. Fragmentation Units ................................29
4.4.4. PACI Packets .......................................32
4.4.4.1. Reasons for the PACI Rules (Informative) ..34
4.4.4.2. PACI Extensions (Informative) .............35
4.5. Temporal Scalability Control Information ..................36
4.6. Decoding Order Number .....................................37
5. Packetization Rules ............................................39
6. De-packetization Process .......................................40
7. Payload Format Parameters ......................................42
7.1. Media Type Registration ...................................42
7.2. SDP Parameters ............................................64
Wang, et al. Standards Track PAGE 2
RFC 7798 RTP Payload Format for HEVC March 2016
7.2.1. Mapping of Payload Type Parameters to SDP ..........64
7.2.2. Usage with SDP Offer/Answer Model ..................65
7.2.3. Usage in Declarative Session Descriptions ..........73
7.2.4. Considerations for Parameter Sets ..................75
7.2.5. Dependency Signaling in Multi-Stream Mode ..........75
8. Use with Feedback Messages .....................................75
8.1. Picture Loss Indication (PLI) .............................75
8.2. Slice Loss Indication (SLI) ...............................76
8.3. Reference Picture Selection Indication (RPSI) .............77
8.4. Full Intra Request (FIR) ..................................77
9. Security Considerations ........................................78
10. Congestion Control ............................................79
11. IANA Considerations ...........................................80
12. References ....................................................80
12.1. Normative References .....................................80
12.2. Informative References ...................................82
Acknowledgments ...................................................85
Authors' Addresses ................................................86
1. Introduction
The High Efficiency Video Coding specification, formally published as
both ITU-T Recommendation H.265 [HEVC] and ISO/IEC International
Standard 23008-2 [ISO23008-2], was ratified by the ITU-T in April
2013; reportedly, it provides significant coding efficiency gains
over H.264 [H.264].
This memo describes an RTP payload format for HEVC. It shares its
basic design with the RTP payload formats of [RFC 6184] and [RFC 6190].
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. Mechanisms from RFC 6190 were
incorporated as HEVC version 1 supports temporal scalability.
In order to help the overlapping implementer community, frequently
only the differences between RFCs 6184 and 6190 and the HEVC payload
format are highlighted in non-normative, explanatory parts of this
memo. Basic familiarity with both specifications is assumed for
those parts. However, the normative parts of this memo do not
require study of RFCs 6184 or 6190.
Wang, et al. Standards Track PAGE 3
RFC 7798 RTP Payload Format for HEVC March 2016
1.1. Overview of the HEVC Codec
H.264 and HEVC share a similar hybrid video codec design. In this
memo, we provide a very brief overview of those features of HEVC 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 HEVC to arrive at interoperable, well-
performing implementations. Implementers should consider testing
their design (including the interworking between the payload format
implementation and the core video codec) using the tools provided by
ITU-T/ISO/IEC, for example, conformance bitstreams as specified in
[H.265.1]. Not doing so has historically led to systems that perform
badly and that are not secure.
Conceptually, both H.264 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
Similar to earlier hybrid-video-coding-based standards, including
H.264, the following basic video coding design is employed by HEVC.
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, HEVC includes
several tools to make the implementation on parallel architectures
easier. Below is a summary of HEVC coding-tool features.
Quad-tree block and transform structure
One of the major tools that contributes significantly to the coding
efficiency of HEVC is the use of flexible coding blocks and
transforms, which are defined in a hierarchical quad-tree manner.
Unlike H.264, where the basic coding block is a macroblock of fixed-
size 16x16, HEVC defines a Coding Tree Unit (CTU) of a maximum size
of 64x64. Each CTU can be divided into smaller units in a
hierarchical quad-tree manner and can represent smaller blocks down
to size 4x4. Similarly, the transforms used in HEVC can have
different sizes, starting from 4x4 and going up to 32x32. Utilizing
large blocks and transforms contributes to the major gain of HEVC,
especially at high resolutions.
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Entropy coding
HEVC uses a single entropy-coding engine, which is based on Context
Adaptive Binary Arithmetic Coding (CABAC) [CABAC], whereas H.264 uses
two distinct entropy coding engines. CABAC in HEVC shares many
similarities with CABAC of H.264, but contains several improvements.
Those include improvements in coding efficiency and lowered
implementation complexity, especially for parallel architectures.
In-loop filtering
H.264 includes an in-loop adaptive deblocking filter, where the
blocking artifacts around the transform edges in the reconstructed
picture are smoothed to improve the picture quality and compression
efficiency. In HEVC, a similar deblocking filter is employed but
with somewhat lower complexity. In addition, pictures undergo a
subsequent filtering operation called Sample Adaptive Offset (SAO),
which is a new design element in HEVC. SAO basically adds a pixel-
level offset in an adaptive manner and usually acts as a de-ringing
filter. It is observed that SAO improves the picture quality,
especially around sharp edges, contributing substantially to visual
quality improvements of HEVC.
Motion prediction and coding
There have been a number of improvements in this area that are
summarized as follows. The first category is motion merge and
Advanced Motion Vector Prediction (AMVP) modes. The motion
information of a prediction block can be inferred from the spatially
or temporally neighboring blocks. This is similar to the DIRECT mode
in H.264 but includes new aspects to incorporate the flexible quad-
tree structure and methods to improve the parallel implementations.
In addition, the motion vector predictor can be signaled for improved
efficiency. The second category is high-precision interpolation.
The interpolation filter length is increased to 8-tap from 6-tap,
which improves the coding efficiency but also comes with increased
complexity. In addition, the interpolation filter is defined with
higher precision without any intermediate rounding operations to
further improve the coding efficiency.
Intra prediction and intra-coding
Compared to 8 intra prediction modes in H.264, HEVC supports angular
intra prediction with 33 directions. This increased flexibility
improves both objective coding efficiency and visual quality as the
edges can be better predicted and ringing artifacts around the edges
can be reduced. In addition, the reference samples are adaptively
smoothed based on the prediction direction. To avoid contouring
Wang, et al. Standards Track PAGE 5
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artifacts a new interpolative prediction generation is included to
improve the visual quality. Furthermore, Discrete Sine Transform
(DST) is utilized instead of traditional Discrete Cosine Transform
(DCT) for 4x4 intra-transform blocks.
Other coding-tool features
HEVC includes some tools for lossless coding and efficient screen-
content coding, such as skipping the transform for certain blocks.
These tools are particularly useful, for example, when streaming the
user interface of a mobile device to a large display.
1.1.2. Systems and Transport Interfaces
HEVC inherited the basic systems and transport interfaces designs
from 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 hierarchical syntax and data unit structure consists of sequence-
level parameter sets, multi-picture-level or picture-level parameter
sets, slice-level header parameters, and lower-level parameters. In
the following, a list of differences in these aspects compared to
H.264 is summarized.
Video parameter set
A new type of parameter set, called Video Parameter Set (VPS), was
introduced. For the first (2013) version of [HEVC], the VPS NAL unit
is required to be available prior to its activation, while the
information contained in the VPS is not necessary for operation of
the decoding process. For future HEVC extensions, such as the 3D or
scalable extensions, the VPS is expected to include information
necessary for operation of the decoding process, e.g., decoding
dependency or information for reference picture set construction of
enhancement layers. The VPS provides a "big picture" of a bitstream,
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 7.1).
Profile, tier, and level
The profile, tier, and level syntax structure that can be included in
both the VPS and Sequence Parameter Set (SPS) includes 12 bytes of
data to describe the entire bitstream (including all temporally
scalable layers, which are referred to as sub-layers in the HEVC
specification), and can optionally include more profile, tier, and
Wang, et al. Standards Track PAGE 6
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level information pertaining to individual temporally scalable
layers. The profile indicator shows the "best viewed as" profile
when the bitstream conforms to multiple profiles, similar to the
major brand concept in the ISO Base Media File Format (ISOBMFF)
[IS014496-12] [IS015444-12] and file formats derived based on
ISOBMFF, such as the 3GPP file format [3GPPFF]. The profile, tier,
and level syntax structure also includes indications such as 1)
whether the bitstream is free of frame-packed content, 2) whether the
bitstream is free of interlaced source content, and 3) whether the
bitstream is free of field pictures. When the answer is yes for both
2) and 3), the bitstream contains only frame pictures of progressive
source. Based on these indications, clients/players without support
of post-processing functionalities for the handling of frame-packed,
interlaced source content or field pictures can reject those
bitstreams that contain such pictures.
Bitstream and elementary stream
HEVC includes a definition of an elementary stream, which is new
compared to H.264. An elementary stream consists of a sequence of
one or more bitstreams. An elementary stream that consists of two or
more bitstreams has typically been formed by splicing together two or
more bitstreams (or parts thereof). When an elementary stream
contains more than one bitstream, the last NAL unit of the last
access unit of a bitstream (except the last bitstream in the
elementary stream) must contain an end of bitstream NAL unit, and the
first access unit of the subsequent bitstream must be an Intra-Random
Access Point (IRAP) access unit. This IRAP access unit may be a
Clean Random Access (CRA), Broken Link Access (BLA), or Instantaneous
Decoding Refresh (IDR) access unit.
Random access support
HEVC includes signaling in the NAL unit header, through NAL unit
types, of IRAP pictures beyond IDR pictures. Three types of IRAP
pictures, namely IDR, CRA, and BLA pictures, are supported: IDR
pictures are conventionally referred to as closed group-of-pictures
(closed-GOP) random access points whereas CRA and BLA pictures are
conventionally referred to as open-GOP random access points. BLA
pictures usually originate from splicing of two bitstreams or part
thereof at a CRA picture, e.g., during stream switching. To enable
better systems usage of IRAP pictures, altogether six different NAL
units are defined to signal the properties of the IRAP pictures,
which can be used to better match the stream access point types as
defined in the ISOBMFF [IS014496-12] [IS015444-12], which are
utilized for random access support in both 3GP-DASH [3GPDASH] and
MPEG DASH [MPEGDASH]. Pictures following an IRAP picture in decoding
order and preceding the IRAP picture in output order are referred to
Wang, et al. Standards Track PAGE 7
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as leading pictures associated with the IRAP picture. There are two
types of leading pictures: Random Access Decodable Leading (RADL)
pictures and Random Access Skipped Leading (RASL) pictures. RADL
pictures are decodable when the decoding started at the associated
IRAP picture; RASL pictures are not decodable when the decoding
started at the associated IRAP picture and are usually discarded.
HEVC provides mechanisms to enable specifying the conformance of a
bitstream wherein the originally present RASL pictures have been
discarded. Consequently, system components can discard RASL
pictures, when needed, without worrying about causing the bitstream
to become non-compliant.
Temporal scalability support
HEVC includes an improved 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.
Temporal sub-layer switching support
HEVC specifies, through NAL unit types present in the NAL unit
header, the signaling of Temporal Sub-layer Access (TSA) and Step-
wise Temporal Sub-layer Access (STSA). A TSA picture and pictures
following the TSA picture in decoding order do not use pictures prior
to the TSA picture in decoding order with TemporalId greater than or
equal to that of the TSA picture for inter prediction reference. A
TSA picture enables up-switching, at the TSA picture, to the sub-
layer containing the TSA picture or any higher sub-layer, from the
immediately lower sub-layer. An STSA picture does not use pictures
with the same TemporalId as the STSA picture for inter prediction
reference. Pictures following an STSA picture in decoding order with
the same TemporalId as the STSA picture do not use pictures prior to
the STSA picture in decoding order with the same TemporalId as the
STSA picture for inter prediction reference. An STSA picture enables
up-switching, at the STSA picture, to the sub-layer containing the
STSA picture, from the immediately lower sub-layer.
Sub-layer reference or non-reference pictures
The concept and signaling of reference/non-reference pictures in HEVC
are different from H.264. In H.264, if a picture may be used by any
other picture for inter prediction reference, it is a reference
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picture; otherwise, it is a non-reference picture, and this is
signaled by two bits in the NAL unit header. In HEVC, a picture is
called a reference picture only when it is marked as "used for
reference". In addition, the concept of sub-layer reference picture
was introduced. If a picture may be used by another other picture
with the same TemporalId for inter prediction reference, it is a sub-
layer reference picture; otherwise, it is a sub-layer non-reference
picture. Whether a picture is a sub-layer reference picture or sub-
layer non-reference picture is signaled through NAL unit type values.
Extensibility
Besides the TemporalId in the NAL unit header, HEVC also includes the
signaling of a six-bit layer ID in the NAL unit header, which must be
equal to 0 for a single-layer bitstream. Extension mechanisms have
been included in the VPS, SPS, Picture Parameter Set (PPS), SEI NAL
unit, slice headers, and so on. All these extension mechanisms
enable future extensions in a backward-compatible manner, such that
bitstreams encoded according to potential future HEVC extensions can
be fed to then-legacy decoders (e.g., HEVC version 1 decoders), and
the then-legacy decoders can decode and output the base-layer
bitstream.
Bitstream extraction
HEVC includes a bitstream-extraction process as an integral part of
the overall decoding process. The bitstream extraction process is
used in the process of bitstream conformance tests, which is part of
the HRD buffering model.
Reference picture management
The reference picture management of HEVC, including reference picture
marking and removal from the Decoded Picture Buffer (DPB) as well as
Reference Picture List Construction (RPLC), differs from that of
H.264. Instead of the reference picture marking mechanism based on a
sliding window plus adaptive Memory Management Control Operation
(MMCO) described in H.264, HEVC specifies a reference picture
management and marking mechanism based on Reference Picture Set
(RPS), and the RPLC is consequently based on the RPS mechanism. An
RPS consists of a set of reference pictures associated with a
picture, consisting of all reference pictures that are prior to the
associated picture in decoding order, that may be used for inter
prediction of the associated picture or any picture following the
associated picture in decoding order. The reference picture set
consists of five lists of reference pictures; RefPicSetStCurrBefore,
RefPicSetStCurrAfter, RefPicSetStFoll, RefPicSetLtCurr, and
RefPicSetLtFoll. RefPicSetStCurrBefore, RefPicSetStCurrAfter, and
Wang, et al. Standards Track PAGE 9
RFC 7798 RTP Payload Format for HEVC March 2016
RefPicSetLtCurr contain all reference pictures that may be used in
inter prediction of the current picture and that may be used in inter
prediction of one or more of the pictures following the current
picture in decoding order. RefPicSetStFoll and RefPicSetLtFoll
consist of all reference pictures that are not used in inter
prediction of the current picture but may be used in inter prediction
of one or more of the pictures following the current picture in
decoding order. RPS provides an "intra-coded" signaling of the DPB
status, instead of an "inter-coded" signaling, mainly for improved
error resilience. The RPLC process in HEVC is based on the RPS, by
signaling an index to an RPS subset for each reference index; this
process is simpler than the RPLC process in H.264.
Ultra-low delay support
HEVC specifies a sub-picture-level HRD operation, for support of the
so-called ultra-low delay. The mechanism specifies a standard-
compliant way to enable delay reduction below a one-picture interval.
Coded Picture Buffer (CPB) and DPB parameters at the sub-picture
level may be signaled, and utilization of this information for the
derivation of CPB timing (wherein the CPB removal time corresponds to
decoding time) and DPB output timing (display time) is specified.
Decoders are allowed to operate the HRD at the conventional access-
unit level, even when the sub-picture-level HRD parameters are
present.
New SEI messages
HEVC inherits many H.264 SEI messages with changes in syntax and/or
semantics making them applicable to HEVC. Additionally, there are a
few new SEI messages reviewed briefly in the following paragraphs.
The display orientation SEI message informs the decoder of a
transformation that is recommended to be applied to the cropped
decoded picture prior to display, such that the pictures can be
properly displayed, e.g., in an upside-up manner.
The structure of pictures SEI message provides information on the NAL
unit types, picture-order count values, and prediction dependencies
of a sequence of pictures. The SEI message can be used, for example,
for concluding what impact a lost picture has on other pictures.
The decoded picture hash SEI message provides a checksum derived from
the sample values of a decoded picture. It can be used for detecting
whether a picture was correctly received and decoded.
Wang, et al. Standards Track PAGE 10
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The active parameter sets SEI message includes the IDs of the active
video parameter set and the active sequence parameter set and can be
used to activate VPSs and SPSs. In addition, the SEI message
includes the following indications: 1) An indication of whether "full
random accessibility" is supported (when supported, all parameter
sets needed for decoding of the remaining of the bitstream when
random accessing from the beginning of the current CVS by completely
discarding all access units earlier in decoding order are present in
the remaining bitstream, and all coded pictures in the remaining
bitstream can be correctly decoded); 2) An indication of whether
there is no parameter set within the current CVS that updates another
parameter set of the same type preceding in decoding order. An
update of a parameter set refers to the use of the same parameter set
ID but with some other parameters changed. If this property is true
for all CVSs in the bitstream, then all parameter sets can be sent
out-of-band before session start.
The decoding unit information SEI message provides information
regarding coded picture buffer removal delay for a decoding unit.
The message can be used in very-low-delay buffering operations.
The region refresh information SEI message can be used together with
the recovery point SEI message (present in both H.264 and HEVC) for
improved support of gradual decoding refresh. This supports random
access from inter-coded pictures, wherein complete pictures can be
correctly decoded or recovered after an indicated number of pictures
in output/display order.
1.1.3. Parallel Processing Support
The reportedly significantly higher encoding computational demand of
HEVC over H.264, in conjunction with the ever-increasing video
resolution (both spatially and temporally) required by the market,
led to the adoption of VCL coding tools specifically targeted to
allow for parallelization on the sub-picture level. That is,
parallelization occurs, at the minimum, at the granularity of an
integer number of CTUs. The targets for this type of high-level
parallelization are multicore CPUs and DSPs as well as multiprocessor
systems. In a system design, to be useful, these tools require
signaling support, which is provided in Section 7 of this memo. This
section provides a brief overview of the tools available in [HEVC].
Many of the tools incorporated in HEVC were designed keeping in mind
the potential parallel implementations in multicore/multiprocessor
architectures. Specifically, for parallelization, four picture
partition strategies, as described below, are available.
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Slices are segments of the bitstream that can be reconstructed
independently from other slices within the same picture (though there
may still be interdependencies through loop filtering operations).
Slices are the only tool that can be used for parallelization that is
also available, in virtually identical form, in H.264.
Parallelization based on slices does not require much inter-processor
or inter-core communication (except for inter-processor or inter-core
data sharing for motion compensation when decoding a predictively
coded picture, which is typically much heavier than inter-processor
or inter-core data sharing due to in-picture prediction), as slices
are designed to be independently decodable. However, for the same
reason, slices can require some coding overhead. Further, slices (in
contrast to some of the other tools mentioned below) also serve as
the key mechanism for bitstream partitioning to match Maximum
Transfer Unit (MTU) size requirements, due to the in-picture
independence of slices and the fact that each regular slice is
encapsulated in its own NAL unit. In many cases, the goal of
parallelization and the goal of MTU size matching can place
contradicting demands to the slice layout in a picture. The
realization of this situation led to the development of the more
advanced tools mentioned below.
Dependent slice segments allow for fragmentation of a coded slice
into fragments at CTU boundaries without breaking any in-picture
prediction mechanisms. They are complementary to the fragmentation
mechanism described in this memo in that they need the cooperation of
the encoder. As a dependent slice segment necessarily contains an
integer number of CTUs, a decoder using multiple cores operating on
CTUs can process a dependent slice segment without communicating
parts of the slice segment's bitstream to other cores.
Fragmentation, as specified in this memo, in contrast, does not
guarantee that a fragment contains an integer number of CTUs.
In Wavefront Parallel Processing (WPP), the picture is partitioned
into rows of CTUs. Entropy decoding and prediction are allowed to
use data from CTUs in other partitions. Parallel processing is
possible through parallel decoding of CTU rows, where the start of
the decoding of a row is delayed by two CTUs, so to ensure that data
related to a CTU above and to the right of the subject CTU is
available before the subject CTU is being decoded. Using this
staggered start (which appears like a wavefront when represented
graphically), parallelization is possible with up to as many
processors/cores as the picture contains CTU rows.
Because in-picture prediction between neighboring CTU rows within a
picture is allowed, the required inter-processor/inter-core
communication to enable in-picture prediction can be substantial.
The WPP partitioning does not result in the creation of more NAL
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units compared to when it is not applied; thus, WPP cannot be used
for MTU size matching, though slices can be used in combination for
that purpose.
Tiles define horizontal and vertical boundaries that partition a
picture into tile columns and rows. The scan order of CTUs is
changed to be local within a tile (in the order of a CTU raster scan
of a tile), before decoding the top-left CTU of the next tile in the
order of tile raster scan of a picture. Similar to slices, tiles
break in-picture prediction dependencies (including entropy decoding
dependencies). However, they do not need to be included into
individual NAL units (same as WPP in this regard); hence, tiles
cannot be used for MTU size matching, though slices can be used in
combination for that purpose. Each tile can be processed by one
processor/core, and the inter-processor/inter-core communication
required for in-picture prediction between processing units decoding
neighboring tiles is limited to conveying the shared slice header in
cases a slice is spanning more than one tile, and loop-filtering-
related sharing of reconstructed samples and metadata. Insofar,
tiles are less demanding in terms of inter-processor communication
bandwidth compared to WPP due to the in-picture independence between
two neighboring partitions.
1.1.4. NAL Unit Header
HEVC maintains the NAL unit concept of H.264 with modifications.
HEVC 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 | LayerId | TID |
+-------------+-----------------+
Figure 1: The Structure of the HEVC NAL Unit Header
The semantics of the fields in the NAL unit header are as specified
in [HEVC] and described briefly below for convenience. In addition
to the name and size of each field, the corresponding syntax element
name in [HEVC] is also provided.
F: 1 bit
forbidden_zero_bit. Required to be zero in [HEVC]. Note that the
inclusion of this bit in the NAL unit header was to enable
transport of HEVC video over MPEG-2 transport systems (avoidance
of start code emulations) [MPEG2S]. In the context of this memo,
Wang, et al. Standards Track PAGE 13
RFC 7798 RTP Payload Format for HEVC March 2016
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 4.4.3.
Type: 6 bits
nal_unit_type. This field specifies the NAL unit type as defined
in Table 7-1 of [HEVC]. If the most significant bit of this field
of a NAL unit is equal to 0 (i.e., the value of this field is less
than 32), 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
in [HEVC].
LayerId: 6 bits
nuh_layer_id. Required to be equal to zero in [HEVC]. It is
anticipated that in future scalable or 3D video coding extensions
of this specification, this syntax element will be used to
identify additional layers that may be present in the CVS, wherein
a layer may be, e.g., a spatial scalable layer, a quality scalable
layer, a texture view, or a depth view.
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 HEVC coded data over RTP [RFC 3550]:
o Usage of RTP header with this payload format
o Packetization of HEVC 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 or multiple RTP streams (within one or more RTP
sessions), where within an RTP stream transmission of NAL units
may be either non-interleaved (i.e., the transmission order of NAL
units is the same as their decoding order) or interleaved (i.e.,
the transmission order of NAL units is different from the decoding
order)
Wang, et al. Standards Track PAGE 14
RFC 7798 RTP Payload Format for HEVC March 2016
o Media type parameters to be used with the Session Description
Protocol (SDP) [RFC 4566]
o A payload header extension mechanism and data structures for
enhanced support of temporal scalability based on that extension
mechanism.
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 [RFC 2119].
In 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 [HEVC]. Section
3.1.1 lists relevant definitions from [HEVC] for convenience.
Section 3.1.2 provides definitions specific to this memo.
3.1.1. Definitions from the HEVC Specification
access unit: A set of NAL units that are associated with each other
according to a specified classification rule, that are consecutive in
decoding order, and that contain exactly one coded picture.
BLA access unit: An access unit in which the coded picture is a BLA
picture.
BLA picture: An IRAP picture for which each VCL NAL unit has
nal_unit_type equal to BLA_W_LP, BLA_W_RADL, or BLA_N_LP.
Coded Video Sequence (CVS): A sequence of access units that consists,
in decoding order, of an IRAP access unit with NoRaslOutputFlag equal
to 1, followed by zero or more access units that are not IRAP access
units with NoRaslOutputFlag equal to 1, including all subsequent
access units up to but not including any subsequent access unit that
is an IRAP access unit with NoRaslOutputFlag equal to 1.
Wang, et al. Standards Track PAGE 15
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Informative note: An IRAP access unit may be an IDR access unit, a
BLA access unit, or a CRA access unit. The value of
NoRaslOutputFlag is equal to 1 for each IDR access unit, each BLA
access unit, and each CRA access unit that is the first access
unit in the bitstream in decoding order, is the first access unit
that follows an end of sequence NAL unit in decoding order, or has
HandleCraAsBlaFlag equal to 1.
CRA access unit: An access unit in which the coded picture is a CRA
picture.
CRA picture: A RAP picture for which each VCL NAL unit has
nal_unit_type equal to CRA_NUT.
IDR access unit: An access unit in which the coded picture is an IDR
picture.
IDR picture: A RAP picture for which each VCL NAL unit has
nal_unit_type equal to IDR_W_RADL or IDR_N_LP.
IRAP access unit: An access unit in which the coded picture is an
IRAP picture.
IRAP picture: A coded picture for which each VCL NAL unit has
nal_unit_type in the range of BLA_W_LP (16) to RSV_IRAP_VCL23 (23),
inclusive.
layer: A set of VCL NAL units that all have a particular value of
nuh_layer_id and the associated non-VCL NAL units, or one of a set of
syntactical structures having a hierarchical relationship.
operation point: bitstream created from another bitstream by
operation of the sub-bitstream extraction process with the another
bitstream, a target highest TemporalId, and a target-layer identifier
list as input.
random access: The act of starting the decoding process for a
bitstream at a point other than the beginning of the bitstream.
sub-layer: A temporal scalable layer of a temporal scalable bitstream
consisting of VCL NAL units with a particular value of the TemporalId
variable, and the associated non-VCL NAL units.
sub-layer representation: A subset of the bitstream consisting of NAL
units of a particular sub-layer and the lower sub-layers.
tile: A rectangular region of coding tree blocks within a particular
tile column and a particular tile row in a picture.
Wang, et al. Standards Track PAGE 16
RFC 7798 RTP Payload Format for HEVC March 2016
tile column: A rectangular region of coding tree blocks having a
height equal to the height of the picture and a width specified by
syntax elements in the picture parameter set.
tile row: A rectangular region of coding tree blocks having a height
specified by syntax elements in the picture parameter set and a width
equal to the width of the picture.
3.1.2. Definitions Specific to This Memo
dependee RTP stream: An RTP stream on which another RTP stream
depends. All RTP streams in a Multiple RTP streams on a Single media
Transport (MRST) or Multiple RTP streams on Multiple media Transports
(MRMT), except for the highest RTP stream, are dependee RTP streams.
highest RTP stream: The RTP stream on which no other RTP stream
depends. The RTP stream in a Single RTP stream on a Single media
Transport (SRST) is the highest RTP stream.
Media-Aware Network Element (MANE): A network element, such as a
middlebox, selective forwarding unit, or application-layer gateway
that is capable of parsing certain aspects of the RTP payload headers
or the RTP payload and reacting to their contents.
Informative note: The concept of a MANE goes beyond normal routers
or gateways in that a MANE has to be aware of the signaling (e.g.,
to learn about the payload type mappings of the media streams),
and in that it has to be trusted when working with Secure RTP
(SRTP). The advantage of using MANEs is that they allow packets
to be dropped according to the needs of the media coding. For
example, if a MANE has to drop packets due to congestion on a
certain link, it can identify and remove those packets whose
elimination produces the least adverse effect on the user
experience. After dropping packets, MANEs must rewrite RTCP
packets to match the changes to the RTP stream, as specified in
Section 7 of [RFC 3550].
Media Transport: As used in the MRST, MRMT, and SRST definitions
below, Media Transport denotes the transport of packets over a
transport association identified by a 5-tuple (source address, source
port, destination address, destination port, transport protocol).
See also Section 2.1.13 of [RFC 7656].
Informative note: The term "bitstream" in this document is
equivalent to the term "encoded stream" in [RFC 7656].
Wang, et al. Standards Track PAGE 17
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Multiple RTP streams on a Single media Transport (MRST): Multiple
RTP streams carrying a single HEVC bitstream on a Single Transport.
See also Section 3.5 of [RFC 7656].
Multiple RTP streams on Multiple media Transports (MRMT): Multiple
RTP streams carrying a single HEVC bitstream on Multiple Transports.
See also Section 3.5 of [RFC 7656].
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in Section 7.4.2.4 in [HEVC].
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 [HEVC],
and in which NAL units within an access unit are in their decoding
order.
NAL-unit-like structure: A data structure that is similar to NAL
units in the sense that it also has a NAL unit header and a payload,
with a difference that the payload does not follow the start code
emulation prevention mechanism required for the NAL unit syntax as
specified in Section 7.3.1.1 of [HEVC]. Examples of NAL-unit-like
structures defined in this memo are packet payloads of Aggregation
Packet (AP), PAyload Content Information (PACI), and Fragmentation
Unit (FU) packets.
NALU-time: The value that the RTP timestamp would have if the NAL
unit would be transported in its own RTP packet.
RTP stream: See [RFC 7656]. Within the scope of this memo, one RTP
stream is utilized to transport one or more temporal sub-layers.
Single RTP stream on a Single media Transport (SRST): Single RTP
stream carrying a single HEVC bitstream on a Single (Media)
Transport. See also Section 3.5 of [RFC 7656].
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.
Wang, et al. Standards Track PAGE 18
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3.2. Abbreviations
AP Aggregation Packet
BLA Broken Link Access
CRA Clean Random Access
CTB Coding Tree Block
CTU Coding Tree Unit
CVS Coded Video Sequence
DPH Decoded Picture Hash
FU Fragmentation Unit
HRD Hypothetical Reference Decoder
IDR Instantaneous Decoding Refresh
IRAP Intra Random Access Point
MANE Media-Aware Network Element
MRMT Multiple RTP streams on Multiple media Transports
MRST Multiple RTP streams on a Single media Transport
MTU Maximum Transfer Unit
NAL Network Abstraction Layer
NALU Network Abstraction Layer Unit
PACI PAyload Content Information
PHES Payload Header Extension Structure
PPS Picture Parameter Set
RADL Random Access Decodable Leading (Picture)
RASL Random Access Skipped Leading (Picture)
RPS Reference Picture Set
Wang, et al. Standards Track PAGE 19
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SEI Supplemental Enhancement Information
SPS Sequence Parameter Set
SRST Single RTP stream on a Single media Transport
STSA Step-wise Temporal Sub-layer Access
TSA Temporal Sub-layer Access
TSCI Temporal Scalability Control Information
VCL Video Coding Layer
VPS Video Parameter Set
4. RTP Payload Format
4.1. RTP Header Usage
The format of the RTP header is specified in [RFC 3550] (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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RTP Header According to [RFC 3550]
Wang, et al. Standards Track PAGE 20
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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. When
MRST or MRMT is in use, if an access unit appears in multiple RTP
streams, the marker bit is set on each RTP stream's last packet of
the access unit.
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.
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.
Informative note: It is not required to use different payload
type values for different RTP streams in MRST or MRMT.
Sequence Number (SN): 16 bits
Set and used in accordance with [RFC 3550].
Wang, et al. Standards Track PAGE 21
RFC 7798 RTP Payload Format for HEVC March 2016
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 7.4.2.4.4 of [HEVC]) 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 [HEVC].
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. In MRST or MRMT, different SSRCs are used for each RTP
stream containing a subset of the sub-layers of the single
(temporally scalable) bitstream. A receiver is required to
correctly associate the set of SSRCs that are included parts of
the same 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, Type, LayerId, 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.
Wang, et al. Standards Track PAGE 22
RFC 7798 RTP Payload Format for HEVC March 2016
4.3. Transmission Modes
This memo enables transmission of an HEVC bitstream over:
o a Single RTP stream on a Single media Transport (SRST),
o Multiple RTP streams over a Single media Transport (MRST), or
o Multiple RTP streams on Multiple media Transports (MRMT).
Informative note: While this specification enables the use of MRST
within the H.265 RTP payload, the signaling of MRST within SDP
offer/answer is not fully specified at the time of this writing.
See [RFC 5576] and [RFC 5583] for what is supported today as well as
[RTP-MULTI-STREAM] and [SDP-NEG] for future directions.
When in MRMT, the dependency of one RTP stream on another RTP stream
is typically indicated as specified in [RFC 5583]. [RFC 5583] can also
be utilized to specify dependencies within MRST, but only if the RTP
streams utilize distinct payload types.
SRST or MRST SHOULD be used for point-to-point unicast scenarios,
whereas MRMT SHOULD be used for point-to-multipoint multicast
scenarios where different receivers require different operation
points of the same HEVC bitstream, to improve bandwidth utilizing
efficiency.
Informative note: A multicast may degrade to a unicast after all
but one receivers have left (this is a justification of the first
"SHOULD" instead of "MUST"), and there might be scenarios where
MRMT is desirable but not possible, e.g., when IP multicast is not
deployed in certain network (this is a justification of the second
"SHOULD" instead of "MUST").
The transmission mode is indicated by the tx-mode media parameter
(see Section 7.1). If tx-mode is equal to "SRST", SRST MUST be used.
Otherwise, if tx-mode is equal to "MRST", MRST MUST be used.
Otherwise (tx-mode is equal to "MRMT"), MRMT MUST be used.
Informative note: When an RTP stream does not depend on other RTP
streams, any of SRST, MRST, or MRMT may be in use for the RTP
stream.
Receivers MUST support all of SRST, MRST, and MRMT.
Informative note: The required support of MRMT by receivers does
not imply that multicast must be supported by receivers.
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RFC 7798 RTP Payload Format for HEVC March 2016
4.4. 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.
o PACI carrying RTP packet: Contains a payload header (that differs
from other payload headers for efficiency), a Payload Header
Extension Structure (PHES), and a PACI payload. This payload
structure is specified in Section 4.4.4.
4.4.1. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: The Structure of a Single NAL Unit Packet
Wang, et al. Standards Track PAGE 24
RFC 7798 RTP Payload Format for HEVC March 2016
The payload header SHOULD be an exact copy of the NAL unit header of
the contained NAL unit. However, the Type (i.e., nal_unit_type)
field MAY be changed, e.g., when it is desirable to handle a CRA
picture to be a BLA picture [JCTVC-J0107].
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.
4.4.2. 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The Structure of an Aggregation Packet
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. 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.
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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 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: DONL (conditional) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU size | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The Structure of the First Aggregation Unit in an AP
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
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.
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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The Structure of an Aggregation Unit That Is Not the
First Aggregation Unit in an AP
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.
Wang, et al. Standards Track PAGE 27
RFC 7798 RTP Payload Format for HEVC March 2016
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=48) | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 1 Data |
| . . . |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+ NALU 2 Data |
| . . . |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: An Example of an AP Packet Containing Two Aggregation
Units without the DONL and DOND Fields
Wang, et al. Standards Track PAGE 28
RFC 7798 RTP Payload Format for HEVC March 2016
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.
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=48) | 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: An Example of an AP Containing Two Aggregation Units
with the DONL and DOND Fields
4.4.3. 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 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.
Wang, et al. Standards Track PAGE 29
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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.
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=49) | FU header | DONL (cond) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| DONL (cond) | |
|-+-+-+-+-+-+-+-+ |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: The Structure of an FU
The fields in the payload header are set as follows. The Type field
MUST be equal to 49. 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 |
+---------------+
Figure 10: The Structure of FU Header
The semantics of the FU header fields are as follows:
S: 1 bit
When set to 1, the S bit indicates the start of a fragmented NAL
unit, i.e., the first byte of the FU payload is also the first
byte of the payload of the fragmented NAL unit. When the FU
payload is not the start of the fragmented NAL unit payload, the S
bit MUST be set to 0.
Wang, et al. Standards Track PAGE 30
RFC 7798 RTP Payload Format for HEVC March 2016
E: 1 bit
When set to 1, the E bit indicates the end of a fragmented NAL
unit, i.e., the last byte of the payload is also the last byte of
the fragmented NAL unit. When the FU payload is not the last
fragment of a fragmented NAL unit, the E bit MUST be set to 0.
FuType: 6 bits
The field FuType MUST be equal to the field Type of the fragmented
NAL unit.
The DONL field, when present, specifies the value of the 16 least
significant bits of the decoding order number of the fragmented NAL
unit.
If sprop-max-don-diff is greater than 0 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.
Wang, et al. Standards Track PAGE 31
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4.4.4. PACI Packets
This section specifies the PACI packet structure. The basic payload
header specified in this memo is intentionally limited to the 16 bits
of the NAL unit header so to keep the packetization overhead to a
minimum. However, cases have been identified where it is advisable
to include control information in an easily accessible position in
the packet header, despite the additional overhead. One such control
information is the TSCI as specified in Section 4.5. PACI packets
carry this and future, similar structures.
The PACI packet structure is based on a payload header extension
mechanism that is generic and extensible to carry payload header
extensions. In this section, the focus lies on the use within this
specification. Section 4.4.4.2 provides guidance for the
specification designers in how to employ the extension mechanism in
future specifications.
A PACI packet consists of a payload header (denoted as PayloadHdr),
for which the structure follows what is described in Section 4.2.
The payload header is followed by the fields A, cType, PHSsize,
F[0..2], and Y.
Figure 11 shows a PACI packet in compliance with this memo, i.e.,
without any extensions.
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=50) |A| cType | PHSsize |F0..2|Y|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Header Extension Structure (PHES) |
|=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=|
| |
| PACI payload: NAL unit |
| . . . |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: The Structure of a PACI
Wang, et al. Standards Track PAGE 32
RFC 7798 RTP Payload Format for HEVC March 2016
The fields in the payload header are set as follows. The F bit MUST
be equal to 0. The Type field MUST be equal to 50. The value of
LayerId MUST be a copy of the LayerId field of the PACI payload NAL
unit or NAL-unit-like structure. The value of TID MUST be a copy of
the TID field of the PACI payload NAL unit or NAL-unit-like
structure.
The semantics of other fields are as follows:
A: 1 bit
Copy of the F bit of the PACI payload NAL unit or NAL-unit-like
structure.
cType: 6 bits
Copy of the Type field of the PACI payload NAL unit or NAL-unit-
like structure.
PHSsize: 5 bits
Indicates the length of the PHES field. The value is limited to
be less than or equal to 32 octets, to simplify encoder design for
MTU size matching.
F0:
This field equal to 1 specifies the presence of a temporal
scalability support extension in the PHES.
F1, F2:
MUST be 0, available for future extensions, see Section 4.4.4.2.
Receivers compliant with this version of the HEVC payload format
MUST ignore F1=1 and/or F2=1, and also ignore any information in
the PHES indicated as present by F1=1 and/or F2=1.
Informative note: The receiver can do that by first decoding
information associated with F0=1, and then skipping over any
remaining bytes of the PHES based on the value of PHSsize.
Y: 1 bit
MUST be 0, available for future extensions, see Section 4.4.4.2.
Receivers compliant with this version of the HEVC payload format
MUST ignore Y=1, and also ignore any information in the PHES
indicated as present by Y.
PHES: variable number of octets
A variable number of octets as indicated by the value of PHSsize.
PACI Payload:
The single NAL unit packet or NAL-unit-like structure (such as: FU
or AP) to be carried, not including the first two octets.
Wang, et al. Standards Track PAGE 33
RFC 7798 RTP Payload Format for HEVC March 2016
Informative note: The first two octets of the NAL unit or NAL-
unit-like structure carried in the PACI payload are not
included in the PACI payload. Rather, the respective values
are copied in locations of the PayloadHdr of the RTP packet.
This design offers two advantages: first, the overall structure
of the payload header is preserved, i.e., there is no special
case of payload header structure that needs to be implemented
for PACI. Second, no additional overhead is introduced.
A PACI payload MAY be a single NAL unit, an FU, or an AP. PACIs
MUST NOT be fragmented or aggregated. The following subsection
documents the reasons for these design choices.
4.4.4.1. Reasons for the PACI Rules (Informative)
A PACI cannot be fragmented. If a PACI could be fragmented, and a
fragment other than the first fragment got lost, access to the
information in the PACI would not be possible. Therefore, a PACI
must not be fragmented. In other words, an FU must not carry
(fragments of) a PACI.
A PACI cannot be aggregated. Aggregation of PACIs is inadvisable
from a compression viewpoint, as, in many cases, several to be
aggregated NAL units would share identical PACI fields and values
which would be carried redundantly for no reason. Most, if not all,
of the practical effects of PACI aggregation can be achieved by
aggregating NAL units and bundling them with a PACI (see below).
Therefore, a PACI must not be aggregated. In other words, an AP must
not contain a PACI.
The payload of a PACI can be a fragment. Both middleboxes and
sending systems with inflexible (often hardware-based) encoders
occasionally find themselves in situations where a PACI and its
headers, combined, are larger than the MTU size. In such a scenario,
the middlebox or sender can fragment the NAL unit and encapsulate the
fragment in a PACI. Doing so preserves the payload header extension
information for all fragments, allowing downstream middleboxes and
the receiver to take advantage of that information. Therefore, a
sender may place a fragment into a PACI, and a receiver must be able
to handle such a PACI.
The payload of a PACI can be an aggregation NAL unit. HEVC
bitstreams can contain unevenly sized and/or small (when compared to
the MTU size) NAL units. In order to efficiently packetize such
small NAL units, APs were introduced. The benefits of APs are
independent from the need for a payload header extension. Therefore,
a sender may place an AP into a PACI, and a receiver must be able to
handle such a PACI.
Wang, et al. Standards Track PAGE 34
RFC 7798 RTP Payload Format for HEVC March 2016
4.4.4.2. PACI Extensions (Informative)
This section includes recommendations for future specification
designers on how to extent the PACI syntax to accommodate future
extensions. Obviously, designers are free to specify whatever
appears to be appropriate to them at the time of their design.
However, a lot of thought has been invested into the extension
mechanism described below, and we suggest that deviations from it
warrant a good explanation.
This memo defines only a single payload header extension (TSCI,
described in Section 4.5); therefore, only the F0 bit carries
semantics. F1 and F2 are already named (and not just marked as
reserved, as a typical video spec designer would do). They are
intended to signal two additional extensions. The Y bit allows one
to, recursively, add further F and Y bits to extend the mechanism
beyond three possible payload header extensions. It is suggested to
define a new packet type (using a different value for Type) when
assigning the F1, F2, or Y bits different semantics than what is
suggested below.
When a Y bit is set, an 8-bit flag-extension is inserted after the Y
bit. A flag-extension consists of 7 flags F[n..n+6], and another Y
bit.
The basic PACI header already includes F0, F1, and F2. Therefore,
the Fx bits in the first flag-extensions are numbered F3, F4, ...,
F9; the F bits in the second flag-extension are numbered F10, F11,
..., F16, and so forth. As a result, at least three Fx bits are
always in the PACI, but the number of Fx bits (and associated types
of extensions) can be increased by setting the next Y bit and adding
an octet of flag-extensions, carrying seven flags and another Y bit.
The size of this list of flags is subject to the limits specified in
Section 4.4.4 (32 octets for all flag-extensions and the PHES
information combined).
Each of the F bits can indicate either the presence or the absence of
certain information in the Payload Header Extension Structure (PHES).
When a spec developer devises a new syntax that takes advantage of
the PACI extension mechanism, he/she must follow the constraints
listed below; otherwise, the extension mechanism may break.
1) The fields added for a particular Fx bit MUST be fixed in
length and not depend on what other Fx bits are set (no parsing
dependency).
2) The Fx bits must be assigned in order.
Wang, et al. Standards Track PAGE 35
RFC 7798 RTP Payload Format for HEVC March 2016
3) An implementation that supports the n-th Fn bit for any value
of n must understand the syntax (though not necessarily the
semantics) of the fields Fk (with k < n), so as to be able to
either use those bits when present, or at least be able to skip
over them.
4.5. Temporal Scalability Control Information
This section describes the single payload header extension defined in
this specification, known as TSCI. If, in the future, additional
payload header extensions become necessary, they could be specified
in this section of an updated version of this document, or in their
own documents.
When F0 is set to 1 in a PACI, this specifies that the PHES field
includes the TSCI fields TL0PICIDX, IrapPicID, S, and E as follows:
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=50) |A| cType | PHSsize |F0..2|Y|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TL0PICIDX | IrapPicID |S|E| RES | |
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| .... |
| PACI payload: NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: The Structure of a PACI with a PHES Containing a TSCI
TL0PICIDX (8 bits)
When present, the TL0PICIDX field MUST be set to equal to
temporal_sub_layer_zero_idx as specified in Section D.3.22 of
[HEVC] for the access unit containing the NAL unit in the PACI.
IrapPicID (8 bits)
When present, the IrapPicID field MUST be set to equal to
irap_pic_id as specified in Section D.3.22 of [HEVC] for the
access unit containing the NAL unit in the PACI.
Wang, et al. Standards Track PAGE 36
RFC 7798 RTP Payload Format for HEVC March 2016
S (1 bit)
The S bit MUST be set to 1 if any of the following conditions is
true and MUST be set to 0 otherwise:
o The NAL unit in the payload of the PACI is the first VCL NAL
unit, in decoding order, of a picture.
o The NAL unit in the payload of the PACI is an AP, and the NAL
unit in the first contained aggregation unit is the first VCL
NAL unit, in decoding order, of a picture.
o The NAL unit in the payload of the PACI is an FU with its S bit
equal to 1 and the FU payload containing a fragment of the
first VCL NAL unit, in decoding order, of a picture.
E (1 bit)
The E bit MUST be set to 1 if any of the following conditions is
true and MUST be set to 0 otherwise:
o The NAL unit in the payload of the PACI is the last VCL NAL
unit, in decoding order, of a picture.
o The NAL unit in the payload of the PACI is an AP and the NAL
unit in the last contained aggregation unit is the last VCL NAL
unit, in decoding order, of a picture.
o The NAL unit in the payload of the PACI is an FU with its E bit
equal to 1 and the FU payload containing a fragment of the last
VCL NAL unit, in decoding order, of a picture.
RES (6 bits)
MUST be equal to 0. Reserved for future extensions.
The value of PHSsize MUST be set to 3. Receivers MUST allow other
values of the fields F0, F1, F2, Y, and PHSsize, and MUST ignore any
additional fields, when present, than specified above in the PHES.
4.6. 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.
Wang, et al. Standards Track PAGE 37
RFC 7798 RTP Payload Format for HEVC March 2016
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.
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.
Wang, et al. Standards Track PAGE 38
RFC 7798 RTP Payload Format for HEVC March 2016
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 Rules
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
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.
Wang, et al. Standards Track PAGE 39
RFC 7798 RTP Payload Format for HEVC March 2016
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 47,
inclusive, may be passed to the decoder. NAL-unit-like structures
with NAL unit type values in the range of 48 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, 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
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.
Wang, et al. Standards Track PAGE 40
RFC 7798 RTP Payload Format for HEVC March 2016
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 [RFC 6051] 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
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.
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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
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features or
properties of the bitstream or the RTP stream. The parameters are
specified here as part of the media type registration for the HEVC
codec. A mapping of the parameters into the Session Description
Protocol (SDP) [RFC 4566] is also provided for applications that use
SDP. Equivalent parameters could be defined elsewhere for use with
control protocols that do not use SDP.
7.1. Media Type Registration
The media subtype for the HEVC codec is allocated from the IETF tree.
The receiver MUST ignore any unrecognized parameter.
Type name: video
Subtype name: H265
Required parameters: none
OPTIONAL parameters:
profile-space, tier-flag, profile-id, profile-compatibility-
indicator, interop-constraints, and level-id:
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These parameters indicate the profile, tier, default level, and
some constraints of the bitstream carried by the RTP stream and
all RTP streams the RTP stream depends on, or a specific set of
the profile, tier, default level, and some constraints the
receiver supports.
The profile and some constraints are indicated collectively by
profile-space, profile-id, profile-compatibility-indicator, and
interop-constraints. The profile specifies the subset of
coding tools that may have been used to generate the bitstream
or that the receiver supports.
Informative note: There are 32 values of profile-id, and
there are 32 flags in profile-compatibility-indicator, each
flag corresponding to one value of profile-id. According to
HEVC version 1 in [HEVC], when more than one of the 32 flags
is set for a bitstream, the bitstream would comply with all
the profiles corresponding to the set flags. However, in a
draft of HEVC version 2 in [HEVCv2], Subclause A.3.5, 19
Format Range Extensions profiles have been specified, all
using the same value of profile-id (4), differentiated by
some of the 48 bits in interop-constraints; this (rather
unexpected way of profile signaling) means that one of the
32 flags may correspond to multiple profiles. To be able to
support whatever HEVC extension profile that might be
specified and indicated using profile-space, profile-id,
profile-compatibility-indicator, and interop-constraints in
the future, it would be safe to require symmetric use of
these parameters in SDP offer/answer unless recv-sub-layer-
id is included in the SDP answer for choosing one of the
sub-layers offered.
The tier is indicated by tier-flag. The default level is
indicated by level-id. The tier and the default level specify
the limits on values of syntax elements or arithmetic
combinations of values of syntax elements that are followed
when generating the bitstream or that the receiver supports.
A set of profile-space, tier-flag, profile-id, profile-
compatibility-indicator, interop-constraints, and level-id
parameters ptlA is said to be consistent with another set of
these parameters ptlB if any decoder that conforms to the
profile, tier, level, and constraints indicated by ptlB can
decode any bitstream that conforms to the profile, tier, level,
and constraints indicated by ptlA.
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In SDP offer/answer, when the SDP answer does not include the
recv-sub-layer-id parameter that is less than the sprop-sub-
layer-id parameter in the SDP offer, the following applies:
o The profile-space, tier-flag, profile-id, profile-
compatibility-indicator, and interop-constraints
parameters MUST be used symmetrically, i.e., the value of
each of these parameters in the offer MUST be the same as
that in the answer, either explicitly signaled or
implicitly inferred.
o The level-id parameter is changeable as long as the
highest level indicated by the answer is either equal to
or lower than that in the offer. Note that the highest
level is indicated by level-id and max-recv-level-id
together.
In SDP offer/answer, when the SDP answer does include the recv-
sub-layer-id parameter that is less than the sprop-sub-layer-id
parameter in the SDP offer, the set of profile-space, tier-
flag, profile-id, profile-compatibility-indicator, interop-
constraints, and level-id parameters included in the answer
MUST be consistent with that for the chosen sub-layer
representation as indicated in the SDP offer, with the
exception that the level-id parameter in the SDP answer is
changeable as long as the highest level indicated by the answer
is either lower than or equal to that in the offer.
More specifications of these parameters, including how they
relate to the values of the profile, tier, and level syntax
elements specified in [HEVC] are provided below.
profile-space, profile-id:
The value of profile-space MUST be in the range of 0 to 3,
inclusive. The value of profile-id MUST be in the range of 0
to 31, inclusive.
When profile-space is not present, a value of 0 MUST be
inferred. When profile-id is not present, a value of 1 (i.e.,
the Main profile) MUST be inferred.
When used to indicate properties of a bitstream, profile-space
and profile-id are derived from the profile, tier, and level
syntax elements in SPS or VPS NAL units as follows, where
general_profile_space, general_profile_idc,
sub_layer_profile_space[j], and sub_layer_profile_idc[j] are
specified in [HEVC]:
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If the RTP stream is the highest RTP stream, the following
applies:
o profile-space = general_profile_space
o profile-id = general_profile_idc
Otherwise (the RTP stream is a dependee RTP stream), the
following applies, with j being the value of the sprop-sub-
layer-id parameter:
o profile-space = sub_layer_profile_space[j]
o profile-id = sub_layer_profile_idc[j]
tier-flag, level-id:
The value of tier-flag MUST be in the range of 0 to 1,
inclusive. The value of level-id MUST be in the range of 0 to
255, inclusive.
If the tier-flag and level-id parameters are used to indicate
properties of a bitstream, they indicate the tier and the
highest level the bitstream complies with.
If the tier-flag and level-id parameters are used for
capability exchange, the following applies. If max-recv-level-
id is not present, the default level defined by level-id
indicates the highest level the codec wishes to support.
Otherwise, max-recv-level-id indicates the highest level the
codec supports for receiving. For either receiving or sending,
all levels that are lower than the highest level supported MUST
also be supported.
If no tier-flag is present, a value of 0 MUST be inferred; if
no level-id is present, a value of 93 (i.e., level 3.1) MUST be
inferred.
When used to indicate properties of a bitstream, the tier-flag
and level-id parameters are derived from the profile, tier, and
level syntax elements in SPS or VPS NAL units as follows, where
general_tier_flag, general_level_idc, sub_layer_tier_flag[j],
and sub_layer_level_idc[j] are specified in [HEVC]:
If the RTP stream is the highest RTP stream, the following
applies:
o tier-flag = general_tier_flag
o level-id = general_level_idc
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Otherwise (the RTP stream is a dependee RTP stream), the
following applies, with j being the value of the sprop-sub-
layer-id parameter:
o tier-flag = sub_layer_tier_flag[j]
o level-id = sub_layer_level_idc[j]
interop-constraints:
A base16 [RFC 4648] (hexadecimal) representation of six bytes of
data, consisting of progressive_source_flag,
interlaced_source_flag, non_packed_constraint_flag,
frame_only_constraint_flag, and reserved_zero_44bits.
If the interop-constraints parameter is not present, the
following MUST be inferred:
o progressive_source_flag = 1
o interlaced_source_flag = 0
o non_packed_constraint_flag = 1
o frame_only_constraint_flag = 1
o reserved_zero_44bits = 0
When the interop-constraints parameter is used to indicate
properties of a bitstream, the following applies, where
general_progressive_source_flag,
general_interlaced_source_flag,
general_non_packed_constraint_flag,
general_non_packed_constraint_flag,
general_frame_only_constraint_flag,
general_reserved_zero_44bits,
sub_layer_progressive_source_flag[j],
sub_layer_interlaced_source_flag[j],
sub_layer_non_packed_constraint_flag[j],
sub_layer_frame_only_constraint_flag[j], and
sub_layer_reserved_zero_44bits[j] are specified in [HEVC]:
If the RTP stream is the highest RTP stream, the following
applies:
o progressive_source_flag = general_progressive_source_flag
o interlaced_source_flag = general_interlaced_source_flag
o non_packed_constraint_flag =
general_non_packed_constraint_flag
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o frame_only_constraint_flag =
general_frame_only_constraint_flag
o reserved_zero_44bits = general_reserved_zero_44bits
Otherwise (the RTP stream is a dependee RTP stream), the
following applies, with j being the value of the sprop-sub-
layer-id parameter:
o progressive_source_flag =
sub_layer_progressive_source_flag[j]
o interlaced_source_flag =
sub_layer_interlaced_source_flag[j]
o non_packed_constraint_flag =
sub_layer_non_packed_constraint_flag[j]
o frame_only_constraint_flag =
sub_layer_frame_only_constraint_flag[j]
o reserved_zero_44bits = sub_layer_reserved_zero_44bits[j]
Using interop-constraints for capability exchange results in
a requirement on any bitstream to be compliant with the
interop-constraints.
profile-compatibility-indicator:
A base16 [RFC 4648] representation of four bytes of data.
When profile-compatibility-indicator is used to indicate
properties of a bitstream, the following applies, where
general_profile_compatibility_flag[j] and
sub_layer_profile_compatibility_flag[i][j] are specified in
[HEVC]:
The profile-compatibility-indicator in this case indicates
additional profiles to the profile defined by profile-space,
profile-id, and interop-constraints the bitstream conforms
to. A decoder that conforms to any of all the profiles the
bitstream conforms to would be capable of decoding the
bitstream. These additional profiles are defined by
profile-space, each set bit of profile-compatibility-
indicator, and interop-constraints.
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If the RTP stream is the highest RTP stream, the following
applies for each value of j in the range of 0 to 31,
inclusive:
o bit j of profile-compatibility-indicator =
general_profile_compatibility_flag[j]
Otherwise (the RTP stream is a dependee RTP stream), the
following applies for i equal to sprop-sub-layer-id and for
each value of j in the range of 0 to 31, inclusive:
o bit j of profile-compatibility-indicator =
sub_layer_profile_compatibility_flag[i][j]
Using profile-compatibility-indicator for capability exchange
results in a requirement on any bitstream to be compliant with
the profile-compatibility-indicator. This is intended to
handle cases where any future HEVC profile is defined as an
intersection of two or more profiles.
If this parameter is not present, this parameter defaults to
the following: bit j, with j equal to profile-id, of profile-
compatibility-indicator is inferred to be equal to 1, and all
other bits are inferred to be equal to 0.
sprop-sub-layer-id:
This parameter MAY be used to indicate the highest allowed
value of TID in the bitstream. When not present, the value of
sprop-sub-layer-id is inferred to be equal to 6.
The value of sprop-sub-layer-id MUST be in the range of 0 to 6,
inclusive.
recv-sub-layer-id:
This parameter MAY be used to signal a receiver's choice of the
offered or declared sub-layer representations in the sprop-vps.
The value of recv-sub-layer-id indicates the TID of the highest
sub-layer of the bitstream that a receiver supports. When not
present, the value of recv-sub-layer-id is inferred to be equal
to the value of the sprop-sub-layer-id parameter in the SDP
offer.
The value of recv-sub-layer-id MUST be in the range of 0 to 6,
inclusive.
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max-recv-level-id:
This parameter MAY be used to indicate the highest level a
receiver supports. The highest level the receiver supports is
equal to the value of max-recv-level-id divided by 30.
The value of max-recv-level-id MUST be in the range of 0 to
255, inclusive.
When max-recv-level-id is not present, the value is inferred to
be equal to level-id.
max-recv-level-id MUST NOT be present when the highest level
the receiver supports is not higher than the default level.
tx-mode:
This parameter indicates whether the transmission mode is SRST,
MRST, or MRMT.
The value of tx-mode MUST be equal to "SRST", "MRST" or "MRMT".
When not present, the value of tx-mode is inferred to be equal
to "SRST".
If the value is equal to "MRST", MRST MUST be in use.
Otherwise, if the value is equal to "MRMT", MRMT MUST be in
use. Otherwise (the value is equal to "SRST"), SRST MUST be in
use.
The value of tx-mode MUST be equal to "MRST" for all RTP
streams in an MRST.
The value of tx-mode MUST be equal to "MRMT" for all RTP
streams in an MRMT.
sprop-vps:
This parameter MAY be used to convey any video parameter set
NAL unit of the bitstream for out-of-band transmission of video
parameter sets. The parameter MAY also be used for capability
exchange and to indicate sub-stream characteristics (i.e.,
properties of sub-layer representations as defined in [HEVC]).
The value of the parameter is a comma-separated (',') list of
base64 [RFC 4648] representations of the video parameter set NAL
units as specified in Section 7.3.2.1 of [HEVC].
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The sprop-vps parameter MAY contain one or more than one video
parameter set NAL unit. However, all other video parameter sets
contained in the sprop-vps parameter MUST be consistent with
the first video parameter set in the sprop-vps parameter. A
video parameter set vpsB is said to be consistent with another
video parameter set vpsA if any decoder that conforms to the
profile, tier, level, and constraints indicated by the 12 bytes
of data starting from the syntax element general_profile_space
to the syntax element general_level_idc, inclusive, in the
first profile_tier_level( ) syntax structure in vpsA can decode
any bitstream that conforms to the profile, tier, level, and
constraints indicated by the 12 bytes of data starting from the
syntax element general_profile_space to the syntax element
general_level_idc, inclusive, in the first profile_tier_level(
) syntax structure in vpsB.
sprop-sps:
This parameter MAY be used to convey sequence parameter set NAL
units of the bitstream for out-of-band transmission of sequence
parameter sets. The value of the parameter is a comma-
separated (',') list of base64 [RFC 4648] representations of the
sequence parameter set NAL units as specified in Section
7.3.2.2 of [HEVC].
sprop-pps:
This parameter MAY be used to convey picture parameter set NAL
units of the bitstream for out-of-band transmission of picture
parameter sets. The value of the parameter is a comma-
separated (',') list of base64 [RFC 4648] representations of the
picture parameter set NAL units as specified in Section 7.3.2.3
of [HEVC].
sprop-sei:
This parameter MAY be used to convey one or more SEI messages
that describe bitstream characteristics. When present, a
decoder can rely on the bitstream characteristics that are
described in the SEI messages for the entire duration of the
session, independently from the persistence scopes of the SEI
messages as specified in [HEVC].
The value of the parameter is a comma-separated (',') list of
base64 [RFC 4648] representations of SEI NAL units as specified
in Section 7.3.2.4 of [HEVC].
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Informative note: Intentionally, no list of applicable or
inapplicable SEI messages is specified here. Conveying
certain SEI messages in sprop-sei may be sensible in some
application scenarios and meaningless in others. However, a
few examples are described below:
1) In an environment where the bitstream was created from
film-based source material, and no splicing is going
to occur during the lifetime of the session, the film
grain characteristics SEI message or the tone mapping
information SEI message are likely meaningful, and
sending them in sprop-sei rather than in the bitstream
at each entry point may help with saving bits and
allows one to configure the renderer only once,
avoiding unwanted artifacts.
2) The structure of pictures information SEI message in
sprop-sei can be used to inform a decoder of
information on the NAL unit types, picture-order count
values, and prediction dependencies of a sequence of
pictures. Having such knowledge can be helpful for
error recovery.
3) Examples for SEI messages that would be meaningless to
be conveyed in sprop-sei include the decoded picture
hash SEI message (it is close to impossible that all
decoded pictures have the same hashtag), the display
orientation SEI message when the device is a handheld
device (as the display orientation may change when the
handheld device is turned around), or the filler
payload SEI message (as there is no point in just
having more bits in SDP).
max-lsr, max-lps, max-cpb, max-dpb, max-br, max-tr, max-tc:
These parameters MAY be used to signal the capabilities of a
receiver implementation. These parameters MUST NOT be used for
any other purpose. The highest level (specified by max-recv-
level-id) MUST be the highest that the receiver is fully
capable of supporting. max-lsr, max-lps, max-cpb, max-dpb,
max-br, max-tr, and max-tc MAY be used to indicate capabilities
of the receiver that extend the required capabilities of the
highest level, as specified below.
When more than one parameter from the set (max-lsr, max-lps,
max-cpb, max-dpb, max-br, max-tr, max-tc) is present, the
receiver MUST support all signaled capabilities simultaneously.
For example, if both max-lsr and max-br are present, the
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highest level with the extension of both the picture rate and
bitrate is supported. That is, the receiver is able to decode
bitstreams in which the luma sample rate is up to max-lsr
(inclusive), the bitrate is up to max-br (inclusive), the coded
picture buffer size is derived as specified in the semantics of
the max-br parameter below, and the other properties comply
with the highest level specified by max-recv-level-id.
Informative note: When the OPTIONAL media type parameters
are used to signal the properties of a bitstream, and max-
lsr, max-lps, max-cpb, max-dpb, max-br, max-tr, and max-tc
are not present, the values of profile-space, tier-flag,
profile-id, profile-compatibility-indicator, interop-
constraints, and level-id must always be such that the
bitstream complies fully with the specified profile, tier,
and level.
max-lsr:
The value of max-lsr is an integer indicating the maximum
processing rate in units of luma samples per second. The max-
lsr parameter signals that the receiver is capable of decoding
video at a higher rate than is required by the highest level.
When max-lsr is signaled, the receiver MUST be able to decode
bitstreams that conform to the highest level, with the
exception that the MaxLumaSR value in Table A-2 of [HEVC] for
the highest level is replaced with the value of max-lsr.
Senders MAY use this knowledge to send pictures of a given size
at a higher picture rate than is indicated in the highest
level.
When not present, the value of max-lsr is inferred to be equal
to the value of MaxLumaSR given in Table A-2 of [HEVC] for the
highest level.
The value of max-lsr MUST be in the range of MaxLumaSR to 16 *
MaxLumaSR, inclusive, where MaxLumaSR is given in Table A-2 of
[HEVC] for the highest level.
max-lps:
The value of max-lps is an integer indicating the maximum
picture size in units of luma samples. The max-lps parameter
signals that the receiver is capable of decoding larger picture
sizes than are required by the highest level. When max-lps is
signaled, the receiver MUST be able to decode bitstreams that
conform to the highest level, with the exception that the
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MaxLumaPS value in Table A-1 of [HEVC] for the highest level is
replaced with the value of max-lps. Senders MAY use this
knowledge to send larger pictures at a proportionally lower
picture rate than is indicated in the highest level.
When not present, the value of max-lps is inferred to be equal
to the value of MaxLumaPS given in Table A-1 of [HEVC] for the
highest level.
The value of max-lps MUST be in the range of MaxLumaPS to 16 *
MaxLumaPS, inclusive, where MaxLumaPS is given in Table A-1 of
[HEVC] for the highest level.
max-cpb:
The value of max-cpb is an integer indicating the maximum coded
picture buffer size in units of CpbBrVclFactor bits for the VCL
HRD parameters and in units of CpbBrNalFactor bits for the NAL
HRD parameters, where CpbBrVclFactor and CpbBrNalFactor are
defined in Section A.4 of [HEVC]. The max-cpb parameter
signals that the receiver has more memory than the minimum
amount of coded picture buffer memory required by the highest
level. When max-cpb is signaled, the receiver MUST be able to
decode bitstreams that conform to the highest level, with the
exception that the MaxCPB value in Table A-1 of [HEVC] for the
highest level is replaced with the value of max-cpb. Senders
MAY use this knowledge to construct coded bitstreams with
greater variation of bitrate than can be achieved with the
MaxCPB value in Table A-1 of [HEVC].
When not present, the value of max-cpb is inferred to be equal
to the value of MaxCPB given in Table A-1 of [HEVC] for the
highest level.
The value of max-cpb MUST be in the range of MaxCPB to 16 *
MaxCPB, inclusive, where MaxLumaCPB is given in Table A-1 of
[HEVC] for the highest level.
Informative note: The coded picture buffer is used in the
hypothetical reference decoder (Annex C of [HEVC]). The use
of the hypothetical reference decoder is recommended in HEVC
encoders to verify that the produced bitstream conforms to
the standard and to control the output bitrate. Thus, the
coded picture buffer is conceptually independent of any
other potential buffers in the receiver, including de-
packetization and de-jitter buffers. The coded picture
buffer need not be implemented in decoders as specified in
Annex C of [HEVC], but rather standard-compliant decoders
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can have any buffering arrangements provided that they can
decode standard-compliant bitstreams. Thus, in practice,
the input buffer for a video decoder can be integrated with
de-packetization and de-jitter buffers of the receiver.
max-dpb:
The value of max-dpb is an integer indicating the maximum
decoded picture buffer size in units decoded pictures at the
MaxLumaPS for the highest level, i.e., the number of decoded
pictures at the maximum picture size defined by the highest
level. The value of max-dpb MUST be in the range of 1 to 16,
respectively. The max-dpb parameter signals that the receiver
has more memory than the minimum amount of decoded picture
buffer memory required by default, which is MaxDpbPicBuf as
defined in [HEVC] (equal to 6). When max-dpb is signaled, the
receiver MUST be able to decode bitstreams that conform to the
highest level, with the exception that the MaxDpbPicBuff value
defined in [HEVC] as 6 is replaced with the value of max-dpb.
Consequently, a receiver that signals max-dpb MUST be capable
of storing the following number of decoded pictures
(MaxDpbSize) in its decoded picture buffer:
if( PicSizeInSamplesY <= ( MaxLumaPS >> 2 ) )
MaxDpbSize = Min( 4 * max-dpb, 16 )
else if ( PicSizeInSamplesY <= ( MaxLumaPS >> 1 ) )
MaxDpbSize = Min( 2 * max-dpb, 16 )
else if ( PicSizeInSamplesY <= ( ( 3 * MaxLumaPS ) >> 2
) )
MaxDpbSize = Min( (4 * max-dpb) / 3, 16 )
else
MaxDpbSize = max-dpb
Wherein MaxLumaPS given in Table A-1 of [HEVC] for the highest
level and PicSizeInSamplesY is the current size of each decoded
picture in units of luma samples as defined in [HEVC].
The value of max-dpb MUST be greater than or equal to the value
of MaxDpbPicBuf (i.e., 6) as defined in [HEVC]. Senders MAY
use this knowledge to construct coded bitstreams with improved
compression.
When not present, the value of max-dpb is inferred to be equal
to the value of MaxDpbPicBuf (i.e., 6) as defined in [HEVC].
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
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decoded picture buffer stores reconstructed samples. There
is no relationship between the size of the decoded picture
buffer and the buffers used in RTP, especially de-
packetization and de-jitter buffers.
max-br:
The value of max-br is an integer indicating the maximum video
bitrate in units of CpbBrVclFactor bits per second for the VCL
HRD parameters and in units of CpbBrNalFactor bits per second
for the NAL HRD parameters, where CpbBrVclFactor and
CpbBrNalFactor are defined in Section A.4 of [HEVC].
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bitrate than
is required by the highest level.
When max-br is signaled, the video codec of the receiver MUST
be able to decode bitstreams that conform to the highest level,
with the following exceptions in the limits specified by the
highest level:
o The value of max-br replaces the MaxBR value in Table A-2
of [HEVC] for the highest level.
o When the max-cpb parameter is not present, the result of
the following formula replaces the value of MaxCPB in
Table A-1 of [HEVC]:
(MaxCPB of the highest level) * max-br / (MaxBR of the
highest level)
For example, if a receiver signals capability for Main profile
Level 2 with max-br equal to 2000, this indicates a maximum
video bitrate of 2000 kbits/sec for VCL HRD parameters, a
maximum video bitrate of 2200 kbits/sec for NAL HRD parameters,
and a CPB size of 2000000 bits (2000000 / 1500000 * 1500000).
Senders MAY use this knowledge to send higher bitrate video as
allowed in the level definition of Annex A of [HEVC] to achieve
improved video quality.
When not present, the value of max-br is inferred to be equal
to the value of MaxBR given in Table A-2 of [HEVC] for the
highest level.
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The value of max-br MUST be in the range of MaxBR to 16 *
MaxBR, inclusive, where MaxBR is given in Table A-2 of [HEVC]
for the highest level.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
assumption that the network is capable of handling such
bitrates at any given time cannot be made from the value of
this parameter. In particular, no conclusion can be drawn
that the signaled bitrate is possible under congestion
control constraints.
max-tr:
The value of max-tr is an integer indication the maximum number
of tile rows. The max-tr parameter signals that the receiver
is capable of decoding video with a larger number of tile rows
than the value allowed by the highest level.
When max-tr is signaled, the receiver MUST be able to decode
bitstreams that conform to the highest level, with the
exception that the MaxTileRows value in Table A-1 of [HEVC] for
the highest level is replaced with the value of max-tr.
Senders MAY use this knowledge to send pictures utilizing a
larger number of tile rows than the value allowed by the
highest level.
When not present, the value of max-tr is inferred to be equal
to the value of MaxTileRows given in Table A-1 of [HEVC] for
the highest level.
The value of max-tr MUST be in the range of MaxTileRows to 16 *
MaxTileRows, inclusive, where MaxTileRows is given in Table A-1
of [HEVC] for the highest level.
max-tc:
The value of max-tc is an integer indication the maximum number
of tile columns. The max-tc parameter signals that the
receiver is capable of decoding video with a larger number of
tile columns than the value allowed by the highest level.
When max-tc is signaled, the receiver MUST be able to decode
bitstreams that conform to the highest level, with the
exception that the MaxTileCols value in Table A-1 of [HEVC] for
the highest level is replaced with the value of max-tc.
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Senders MAY use this knowledge to send pictures utilizing a
larger number of tile columns than the value allowed by the
highest level.
When not present, the value of max-tc is inferred to be equal
to the value of MaxTileCols given in Table A-1 of [HEVC] for
the highest level.
The value of max-tc MUST be in the range of MaxTileCols to 16 *
MaxTileCols, inclusive, where MaxTileCols is given in Table A-1
of [HEVC] for the highest level.
max-fps:
The value of max-fps is an integer indicating the maximum
picture rate in units of pictures per 100 seconds that can be
effectively processed by the receiver. The max-fps parameter
MAY be used to signal that the receiver has a constraint in
that it is not capable of processing video effectively at the
full picture rate that is implied by the highest level and,
when present, one or more of the parameters max-lsr, max-lps,
and max-br.
The value of max-fps is not necessarily the picture rate at
which the maximum picture size can be sent, it constitutes a
constraint on maximum picture rate for all resolutions.
Informative note: The max-fps parameter is semantically
different from max-lsr, max-lps, max-cpb, max-dpb, max-br,
max-tr, and max-tc in that max-fps is used to signal a
constraint, lowering the maximum picture rate from what is
implied by other parameters.
The encoder MUST use a picture rate equal to or less than this
value. In cases where the max-fps parameter is absent, the
encoder is free to choose any picture rate according to the
highest level and any signaled optional parameters.
The value of max-fps MUST be smaller than or equal to the full
picture rate that is implied by the highest level and, when
present, one or more of the parameters max-lsr, max-lps, and
max-br.
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sprop-max-don-diff:
If tx-mode is equal to "SRST" and there is no NAL unit naluA
that is followed in transmission order by any NAL unit
preceding naluA in decoding order (i.e., the transmission order
of the NAL units is the same as the decoding order), the value
of this parameter MUST be equal to 0.
Otherwise, if tx-mode is equal to "MRST" or "MRMT", the
decoding order of the NAL units of all the RTP streams is the
same as the NAL unit transmission order and the NAL unit output
order, the value of this parameter MUST be equal to either 0 or
1.
Otherwise, if tx-mode is equal to "MRST" or "MRMT" and the
decoding order of the NAL units of all the RTP streams is the
same as the NAL unit transmission order but not the same as the
NAL unit output order, the value of this parameter MUST be
equal to 1.
Otherwise, this parameter specifies the maximum absolute
difference between the decoding order number (i.e., AbsDon)
values of any two NAL units naluA and naluB, where naluA
follows naluB in decoding order and precedes naluB in
transmission order.
The value of sprop-max-don-diff MUST be an integer in the range
of 0 to 32767, inclusive.
When not present, the value of sprop-max-don-diff is inferred
to be equal to 0.
sprop-depack-buf-nalus:
This parameter specifies the maximum number of NAL units that
precede a NAL unit in transmission order and follow the NAL
unit in decoding order.
The value of sprop-depack-buf-nalus MUST be an integer in the
range of 0 to 32767, inclusive.
When not present, the value of sprop-depack-buf-nalus is
inferred to be equal to 0.
When sprop-max-don-diff is present and greater than 0, this
parameter MUST be present and the value MUST be greater than 0.
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sprop-depack-buf-bytes:
This parameter signals the required size of the de-
packetization buffer in units of bytes. The value of the
parameter MUST be greater than or equal to the maximum buffer
occupancy (in units of bytes) of the de-packetization buffer as
specified in Section 6.
The value of sprop-depack-buf-bytes MUST be an integer in the
range of 0 to 4294967295, inclusive.
When sprop-max-don-diff is present and greater than 0, this
parameter MUST be present and the value MUST be greater than 0.
When not present, the value of sprop-depack-buf-bytes is
inferred to be equal to 0.
Informative note: The value of sprop-depack-buf-bytes
indicates the required size of the de-packetization buffer
only. When network jitter can occur, an appropriately sized
jitter buffer has to be available as well.
depack-buf-cap:
This parameter signals the capabilities of a receiver
implementation and indicates the amount of de-packetization
buffer space in units of bytes that the receiver has available
for reconstructing the NAL unit decoding order from NAL units
carried in one or more RTP streams. A receiver is able to
handle any RTP stream, and all RTP streams the RTP stream
depends on, when present, for which the value of the sprop-
depack-buf-bytes parameter is smaller than or equal to this
parameter.
When not present, the value of depack-buf-cap is inferred to be
equal to 4294967295. The value of depack-buf-cap MUST be an
integer in the range of 1 to 4294967295, inclusive.
Informative note: depack-buf-cap indicates the maximum
possible size of the de-packetization buffer of the receiver
only, without allowing for network jitter.
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sprop-segmentation-id:
This parameter MAY be used to signal the segmentation tools
present in the bitstream and that can be used for
parallelization. The value of sprop-segmentation-id MUST be an
integer in the range of 0 to 3, inclusive. When not present,
the value of sprop-segmentation-id is inferred to be equal to
0.
When sprop-segmentation-id is equal to 0, no information about
the segmentation tools is provided. When sprop-segmentation-id
is equal to 1, it indicates that slices are present in the
bitstream. When sprop-segmentation-id is equal to 2, it
indicates that tiles are present in the bitstream. When sprop-
segmentation-id is equal to 3, it indicates that WPP is used in
the bitstream.
sprop-spatial-segmentation-idc:
A base16 [RFC 4648] representation of the syntax element
min_spatial_segmentation_idc as specified in [HEVC]. This
parameter MAY be used to describe parallelization capabilities
of the bitstream.
dec-parallel-cap:
This parameter MAY be used to indicate the decoder's additional
decoding capabilities given the presence of tools enabling
parallel decoding, such as slices, tiles, and WPP, in the
bitstream. The decoding capability of the decoder may vary
with the setting of the parallel decoding tools present in the
bitstream, e.g., the size of the tiles that are present in a
bitstream. Therefore, multiple capability points may be
provided, each indicating the minimum required decoding
capability that is associated with a parallelism requirement,
which is a requirement on the bitstream that enables parallel
decoding.
Each capability point is defined as a combination of 1) a
parallelism requirement, 2) a profile (determined by profile-
space and profile-id), 3) a highest level, and 4) a maximum
processing rate, a maximum picture size, and a maximum video
bitrate that may be equal to or greater than that determined by
the highest level. The parameter's syntax in ABNF [RFC 5234] is
as follows:
Wang, et al. Standards Track PAGE 60
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dec-parallel-cap = "dec-parallel-cap={" cap-point *(","
cap-point) "}"
cap-point = ("w" / "t") ":" spatial-seg-idc 1*(";"
cap-parameter)
spatial-seg-idc = 1*4DIGIT ; (1-4095)
cap-parameter = tier-flag / level-id / max-lsr
/ max-lps / max-br
tier-flag = "tier-flag" EQ ("0" / "1")
level-id = "level-id" EQ 1*3DIGIT ; (0-255)
max-lsr = "max-lsr" EQ 1*20DIGIT ; (0-
18,446,744,073,709,551,615)
max-lps = "max-lps" EQ 1*10DIGIT ; (0-4,294,967,295)
max-br = "max-br" EQ 1*20DIGIT ; (0-
18,446,744,073,709,551,615)
EQ = "="
The set of capability points expressed by the dec-parallel-cap
parameter is enclosed in a pair of curly braces ("{}"). Each
set of two consecutive capability points is separated by a
comma (','). Within each capability point, each set of two
consecutive parameters, and, when present, their values, is
separated by a semicolon (';').
The profile of all capability points is determined by profile-
space and profile-id, which are outside the dec-parallel-cap
parameter.
Each capability point starts with an indication of the
parallelism requirement, which consists of a parallel tool
type, which may be equal to 'w' or 't', and a decimal value of
the spatial-seg-idc parameter. When the type is 'w', the
capability point is valid only for H.265 bitstreams with WPP in
use, i.e., entropy_coding_sync_enabled_flag equal to 1. When
the type is 't', the capability point is valid only for H.265
bitstreams with WPP not in use (i.e.,
entropy_coding_sync_enabled_flag equal to 0). The capability-
point is valid only for H.265 bitstreams with
min_spatial_segmentation_idc equal to or greater than spatial-
seg-idc.
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After the parallelism requirement indication, each capability
point continues with one or more pairs of parameter and value
in any order for any of the following parameters:
o tier-flag
o level-id
o max-lsr
o max-lps
o max-br
At most, one occurrence of each of the above five parameters is
allowed within each capability point.
The values of dec-parallel-cap.tier-flag and dec-parallel-
cap.level-id for a capability point indicate the highest level
of the capability point. The values of dec-parallel-cap.max-
lsr, dec-parallel-cap.max-lps, and dec-parallel-cap.max-br for
a capability point indicate the maximum processing rate in
units of luma samples per second, the maximum picture size in
units of luma samples, and the maximum video bitrate (in units
of CpbBrVclFactor bits per second for the VCL HRD parameters
and in units of CpbBrNalFactor bits per second for the NAL HRD
parameters where CpbBrVclFactor and CpbBrNalFactor are defined
in Section A.4 of [HEVC]).
When not present, the value of dec-parallel-cap.tier-flag is
inferred to be equal to the value of tier-flag outside the dec-
parallel-cap parameter. When not present, the value of dec-
parallel-cap.level-id is inferred to be equal to the value of
max-recv-level-id outside the dec-parallel-cap parameter. When
not present, the value of dec-parallel-cap.max-lsr, dec-
parallel-cap.max-lps, or dec-parallel-cap.max-br is inferred to
be equal to the value of max-lsr, max-lps, or max-br,
respectively, outside the dec-parallel-cap parameter.
The general decoding capability, expressed by the set of
parameters outside of dec-parallel-cap, is defined as the
capability point that is determined by the following
combination of parameters: 1) the parallelism requirement
corresponding to the value of sprop-segmentation-id equal to 0
for a bitstream, 2) the profile determined by profile-space,
profile-id, profile-compatibility-indicator, and interop-
constraints, 3) the tier and the highest level determined by
tier-flag and max-recv-level-id, and 4) the maximum processing
rate, the maximum picture size, and the maximum video bitrate
determined by the highest level. The general decoding
capability MUST NOT be included as one of the set of capability
points in the dec-parallel-cap parameter.
Wang, et al. Standards Track PAGE 62
RFC 7798 RTP Payload Format for HEVC March 2016
For example, the following parameters express the general
decoding capability of 720p30 (Level 3.1) plus an additional
decoding capability of 1080p30 (Level 4) given that the
spatially largest tile or slice used in the bitstream is equal
to or less than 1/3 of the picture size:
a=fmtp:98 level-id=93;dec-parallel-cap={t:8;level- id=120}
For another example, the following parameters express an
additional decoding capability of 1080p30, using dec-parallel-
cap.max-lsr and dec-parallel-cap.max-lps, given that WPP is
used in the bitstream:
a=fmtp:98 level-id=93;dec-parallel-cap={w:8;
max-lsr=62668800;max-lps=2088960}
Informative note: When min_spatial_segmentation_idc is
present in a bitstream and WPP is not used, [HEVC] specifies
that there is no slice or no tile in the bitstream
containing more than 4 * PicSizeInSamplesY / (
min_spatial_segmentation_idc + 4 ) luma samples.
include-dph:
This parameter is used to indicate the capability and
preference to utilize or include Decoded Picture Hash (DPH) SEI
messages (see Section D.3.19 of [HEVC]) in the bitstream. DPH
SEI messages can be used to detect picture corruption so the
receiver can request picture repair, see Section 8. The value
is a comma-separated list of hash types that is supported or
requested to be used, each hash type provided as an unsigned
integer value (0-255), with the hash types listed from most
preferred to the least preferred. Example: "include-dph=0,2",
which indicates the capability for MD5 (most preferred) and
Checksum (less preferred). If the parameter is not included or
the value contains no hash types, then no capability to utilize
DPH SEI messages is assumed. Note that DPH SEI messages MAY
still be included in the bitstream even when there is no
declaration of capability to use them, as in general SEI
messages do not affect the normative decoding process and
decoders are allowed to ignore SEI messages.
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Wang, et al. Standards Track PAGE 63
RFC 7798 RTP Payload Format for HEVC March 2016
Security considerations:
See Section 9 of RFC 7798.
Published specification:
Please refer to RFC 7798 and its Section 12.
Additional information: None
File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Ye-Kui Wang (yekui.wang@gmail.com)
Intended usage: COMMON
Author: See Authors' Addresses section of RFC 7798.
Change controller:
IETF Audio/Video Transport Payloads working group delegated from
the IESG.
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/H265 string is mapped to fields in the Session
Description Protocol (SDP) [RFC 4566] as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be H265 (the
media subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The OPTIONAL parameters profile-space, profile-id, tier-flag,
level-id, interop-constraints, profile-compatibility-indicator,
sprop-sub-layer-id, recv-sub-layer-id, max-recv-level-id, tx-mode,
Wang, et al. Standards Track PAGE 64
RFC 7798 RTP Payload Format for HEVC March 2016
max-lsr, max-lps, max-cpb, max-dpb, max-br, max-tr, max-tc, max-
fps, sprop-max-don-diff, sprop-depack-buf-nalus, sprop-depack-buf-
bytes, depack-buf-cap, sprop-segmentation-id, sprop-spatial-
segmentation-idc, dec-parallel-cap, and include-dph, when present,
MUST be included in the "a=fmtp" line of SDP. This parameter is
expressed as a media type string, in the form of a semicolon-
separated list of parameter=value pairs.
o The OPTIONAL parameters sprop-vps, sprop-sps, and sprop-pps, when
present, MUST be included in the "a=fmtp" line of SDP or conveyed
using the "fmtp" source attribute as specified in Section 6.3 of
[RFC 5576]. For a particular media format (i.e., RTP payload
type), sprop-vps sprop-sps, or sprop-pps MUST NOT be both included
in the "a=fmtp" line of SDP and conveyed using the "fmtp" source
attribute. When included in the "a=fmtp" line of SDP, these
parameters are expressed as a media type string, in the form of a
semicolon-separated list of parameter=value pairs. When conveyed
in the "a=fmtp" line of SDP for a particular payload type, the
parameters sprop-vps, sprop-sps, and sprop-pps MUST be applied to
each SSRC with the payload type. When conveyed using the "fmtp"
source attribute, these parameters are only associated with the
given source and payload type as parts of the "fmtp" source
attribute.
Informative note: Conveyance of sprop-vps, sprop-sps, and
sprop-pps using the "fmtp" source attribute allows for out-of-
band transport of parameter sets in topologies like Topo-Video-
switch-MCU as specified in [RFC 7667].
An example of media representation in SDP is as follows:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H265/90000
a=fmtp:98 profile-id=1;
sprop-vps=<video parameter sets data>
7.2.2. Usage with SDP Offer/Answer Model
When HEVC is offered over RTP using SDP in an offer/answer model
[RFC 3264] for negotiation for unicast usage, the following
limitations and rules apply:
o The parameters identifying a media format configuration for HEVC
are profile-space, profile-id, tier-flag, level-id, interop-
constraints, profile-compatibility-indicator, and tx-mode. These
media configuration parameters, except level-id, MUST be used
symmetrically when the answerer does not include recv-sub-layer-id
Wang, et al. Standards Track PAGE 65
RFC 7798 RTP Payload Format for HEVC March 2016
in the answer for the media format (payload type) or the included
recv-sub-layer-id is equal to sprop-sub-layer-id in the offer.
The answerer MUST:
1) maintain all configuration parameters with the values remaining
the same as in the offer for the media format (payload type),
with the exception that the value of level-id is changeable as
long as the highest level indicated by the answer is not higher
than that indicated by the offer;
2) include in the answer the recv-sub-layer-id parameter, with a
value less than the sprop-sub-layer-id parameter in the offer,
for the media format (payload type), and maintain all
configuration parameters with the values being the same as
signaled in the sprop-vps for the chosen sub-layer
representation, with the exception that the value of level-id
is changeable as long as the highest level indicated by the
answer is not higher than the level indicated by the sprop-vps
in offer for the chosen sub-layer representation; or
3) remove the media format (payload type) completely (when one or
more of the parameter values are not supported).
Informative note: The above requirement for symmetric use
does not apply for level-id, and does not apply for the
other bitstream or RTP stream properties and capability
parameters.
o The profile-compatibility-indicator, when offered as sendonly,
describes bitstream properties. The answerer MAY accept an RTP
payload type even if the decoder is not capable of handling the
profile indicated by the profile-space, profile-id, and interop-
constraints parameters, but capable of any of the profiles
indicated by the profile-space, profile-compatibility-indicator,
and interop-constraints. However, when the profile-compatibility-
indicator is used in a recvonly or sendrecv media description, the
bitstream using this RTP payload type is required to conform to
all profiles indicated by profile-space, profile-compatibility-
indicator, and interop-constraints.
o To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be used
in the answer, as specified in [RFC 3264].
o The same RTP payload type number used in the offer for the media
subtype H265 MUST be used in the answer when the answer includes
recv-sub-layer-id. When the answer does not include recv-sub-
layer-id, the answer MUST NOT contain a payload type number used
Wang, et al. Standards Track PAGE 66
RFC 7798 RTP Payload Format for HEVC March 2016
in the offer for the media subtype H265 unless the configuration
is exactly the same as in the offer or the configuration in the
answer only differs from that in the offer with a different value
of level-id. The answer MAY contain the recv-sub-layer-id
parameter if an HEVC bitstream contains multiple operation points
(using temporal scalability and sub-layers) and sprop-vps is
included in the offer where information of sub-layers are present
in the first video parameter set contained in sprop-vps. If the
sprop-vps is provided in an offer, an answerer MAY select a
particular operation point indicated in the first video parameter
set contained in sprop-vps. When the answer includes a recv-sub-
layer-id that is less than a sprop-sub-layer-id in the offer, all
video parameter sets contained in the sprop-vps parameter in the
SDP answer and all video parameter sets sent in-band for either
the offerer-to-answerer direction or the answerer-to-offerer
direction MUST be consistent with the first video parameter set in
the sprop-vps parameter of the offer (see the semantics of sprop-
vps in Section 7.1 of this document on one video parameter set
being consistent with another video parameter set), and the
bitstream sent in either direction MUST conform to the profile,
tier, level, and constraints of the chosen sub-layer
representation as indicated by the first profile_tier_level( )
syntax structure in the first video parameter set in the sprop-vps
parameter of the offer.
Informative note: When an offerer receives an answer that does
not include recv-sub-layer-id, it has to compare payload types
not declared in the offer based on the media type (i.e.,
video/H265) and the above media configuration parameters with
any payload types it has already declared. This will enable it
to determine whether the configuration in question is new or if
it is equivalent to configuration already offered, since a
different payload type number may be used in the answer. The
ability to perform operation point selection enables a receiver
to utilize the temporal scalable nature of an HEVC bitstream.
o The parameters sprop-max-don-diff, sprop-depack-buf-nalus, and
sprop-depack-buf-bytes describe the properties of an RTP stream,
and all RTP streams the RTP stream depends on, when present, that
the offerer or the answerer is sending for the media format
configuration. This differs from the normal usage of the
offer/answer parameters: normally such parameters declare the
properties of the bitstream or RTP stream that the offerer or the
answerer is able to receive. When dealing with HEVC, the offerer
assumes that the answerer will be able to receive media encoded
using the configuration being offered.
Wang, et al. Standards Track PAGE 67
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Informative note: The above parameters apply for any RTP
stream and all RTP streams the RTP stream depends on, when
present, sent by a declaring entity with the same
configuration. In other words, the applicability of the above
parameters to RTP streams depends on the source endpoint.
Rather than being bound to the payload type, the values may
have to be applied to another payload type when being sent, as
they apply for the configuration.
o The capability parameters max-lsr, max-lps, max-cpb, max-dpb, max-
br, max-tr, and max-tc MAY be used to declare further capabilities
of the offerer or answerer for receiving. These parameters MUST
NOT be present when the direction attribute is sendonly.
o The capability parameter max-fps MAY be used to declare lower
capabilities of the offerer or answerer for receiving. The
parameters MUST NOT be present when the direction attribute is
sendonly.
o The capability parameter dec-parallel-cap MAY be used to declare
additional decoding capabilities of the offerer or answerer for
receiving. Upon receiving such a declaration of a receiver, a
sender MAY send a bitstream to the receiver utilizing those
capabilities under the assumption that the bitstream fulfills the
parallelism requirement. A bitstream that is sent based on
choosing a capability point with parallel tool type 'w' from dec-
parallel-cap MUST have entropy_coding_sync_enabled_flag equal to 1
and min_spatial_segmentation_idc equal to or larger than dec-
parallel-cap.spatial-seg-idc of the capability point. A bitstream
that is sent based on choosing a capability point with parallel
tool type 't' from dec-parallel-cap MUST have
entropy_coding_sync_enabled_flag equal to 0 and
min_spatial_segmentation_idc equal to or larger than dec-parallel-
cap.spatial-seg-idc of the capability point.
o An offerer has to include the size of the de-packetization buffer,
sprop-depack-buf-bytes, as well as sprop-max-don-diff and sprop-
depack-buf-nalus, in the offer for an interleaved HEVC bitstream
or for the MRST or MRMT transmission mode when sprop-max-don-diff
is greater than 0 for at least one of the RTP streams. To enable
the offerer and answerer to inform each other about their
capabilities for de-packetization buffering in receiving RTP
streams, both parties are RECOMMENDED to include depack-buf-cap.
For interleaved RTP streams or in MRST or MRMT, it is also
RECOMMENDED to consider offering multiple payload types with
different buffering requirements when the capabilities of the
receiver are unknown.
Wang, et al. Standards Track PAGE 68
RFC 7798 RTP Payload Format for HEVC March 2016
o The capability parameter include-dph MAY be used to declare the
capability to utilize decoded picture hash SEI messages and which
types of hashes in any HEVC RTP streams received by the offerer or
answerer.
o The sprop-vps, sprop-sps, or sprop-pps, when present (included in
the "a=fmtp" line of SDP or conveyed using the "fmtp" source
attribute as specified in Section 6.3 of [RFC 5576]), are used for
out-of-band transport of the parameter sets (VPS, SPS, or PPS,
respectively).
o The answerer MAY use either out-of-band or in-band transport of
parameter sets for the bitstream it is sending, regardless of
whether out-of-band parameter sets transport has been used in the
offerer-to-answerer direction. Parameter sets included in an
answer are independent of those parameter sets included in the
offer, as they are used for decoding two different bitstreams, one
from the answerer to the offerer and the other in the opposite
direction. In case some RTP streams are sent before the SDP
offer/answer settles down, in-band parameter sets MUST be used for
those RTP stream parts sent before the SDP offer/answer.
o The following rules apply to transport of parameter set in the
offerer-to-answerer direction.
+ An offer MAY include sprop-vps, sprop-sps, and/or sprop-pps.
If none of these parameters is present in the offer, then only
in-band transport of parameter sets is used.
+ If the level to use in the offerer-to-answerer direction is
equal to the default level in the offer, the answerer MUST be
prepared to use the parameter sets included in sprop-vps,
sprop-sps, and sprop-pps (either included in the "a=fmtp" line
of SDP or conveyed using the "fmtp" source attribute) for
decoding the incoming bitstream, e.g., by passing these
parameter set NAL units to the video decoder before passing any
NAL units carried in the RTP streams. Otherwise, the answerer
MUST ignore sprop-vps, sprop-sps, and sprop-pps (either
included in the "a=fmtp" line of SDP or conveyed using the
"fmtp" source attribute) and the offerer MUST transmit
parameter sets in-band.
+ In MRST or MRMT, the answerer MUST be prepared to use the
parameter sets out-of-band transmitted for the RTP stream and
all RTP streams the RTP stream depends on, when present, for
decoding the incoming bitstream, e.g., by passing these
parameter set NAL units to the video decoder before passing any
NAL units carried in the RTP streams.
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o The following rules apply to transport of parameter set in the
answerer-to-offerer direction.
+ An answer MAY include sprop-vps, sprop-sps, and/or sprop-pps.
If none of these parameters is present in the answer, then only
in-band transport of parameter sets is used.
+ The offerer MUST be prepared to use the parameter sets included
in sprop-vps, sprop-sps, and sprop-pps (either included in the
"a=fmtp" line of SDP or conveyed using the "fmtp" source
attribute) for decoding the incoming bitstream, e.g., by
passing these parameter set NAL units to the video decoder
before passing any NAL units carried in the RTP streams.
+ In MRST or MRMT, the offerer MUST be prepared to use the
parameter sets out-of-band transmitted for the RTP stream and
all RTP streams the RTP stream depends on, when present, for
decoding the incoming bitstream, e.g., by passing these
parameter set NAL units to the video decoder before passing any
NAL units carried in the RTP streams.
o When sprop-vps, sprop-sps, and/or sprop-pps are conveyed using the
"fmtp" source attribute as specified in Section 6.3 of [RFC 5576],
the receiver of the parameters MUST store the parameter sets
included in sprop-vps, sprop-sps, and/or sprop-pps and associate
them with the source given as part of the "fmtp" source attribute.
Parameter sets associated with one source (given as part of the
"fmtp" source attribute) MUST only be used to decode NAL units
conveyed in RTP packets from the same source (given as part of the
"fmtp" source attribute). When this mechanism is in use, SSRC
collision detection and resolution MUST be performed as specified
in [RFC 5576].
For bitstreams being delivered over multicast, the following rules
apply:
o The media format configuration is identified by profile-space,
profile-id, tier-flag, level-id, interop-constraints, profile-
compatibility-indicator, and tx-mode. These media format
configuration parameters, including level-id, MUST be used
symmetrically; that is, the answerer MUST either maintain all
configuration parameters or remove the media format (payload
type) completely. Note that this implies that the level-id for
offer/answer in multicast is not changeable.
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o To simplify the handling and matching of these configurations,
the same RTP payload type number used in the offer SHOULD also
be used in the answer, as specified in [RFC 3264]. An answer
MUST NOT contain a payload type number used in the offer unless
the configuration is the same as in the offer.
o Parameter sets received MUST be associated with the originating
source and MUST only be used in decoding the incoming bitstream
from the same source.
o The rules for other parameters are the same as above for
unicast as long as the three above rules are obeyed.
Table 1 lists the interpretation of all the parameters that MUST be
used for the various combinations of offer, answer, and direction
attributes. Note that the two columns wherein the recv-sub-layer-id
parameter is used only apply to answers, whereas the other columns
apply to both offers and answers.
Table 1. Interpretation of parameters for various combinations of
offers, answers, direction attributes, with and without recv-sub-
layer-id. Columns that do not indicate offer or answer apply to
both.
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sendonly --+
answer: recvonly, recv-sub-layer-id --+ |
recvonly w/o recv-sub-layer-id --+ | |
answer: sendrecv, recv-sub-layer-id --+ | | |
sendrecv w/o recv-sub-layer-id --+ | | | |
| | | | |
profile-space C D C D P
profile-id C D C D P
tier-flag C D C D P
level-id D D D D P
interop-constraints C D C D P
profile-compatibility-indicator C D C D P
tx-mode C C C C P
max-recv-level-id R R R R -
sprop-max-don-diff P P - - P
sprop-depack-buf-nalus P P - - P
sprop-depack-buf-bytes P P - - P
depack-buf-cap R R R R -
sprop-segmentation-id P P P P P
sprop-spatial-segmentation-idc P P P P P
max-br R R R R -
max-cpb R R R R -
max-dpb R R R R -
max-lsr R R R R -
max-lps R R R R -
max-tr R R R R -
max-tc R R R R -
max-fps R R R R -
sprop-vps P P - - P
sprop-sps P P - - P
sprop-pps P P - - P
sprop-sub-layer-id P P - - P
recv-sub-layer-id X O X O -
dec-parallel-cap R R R R -
include-dph R R R R -
Legend:
C: configuration for sending and receiving bitstreams
D: changeable configuration, same as C except possible
to answer with a different but consistent value (see the
semantics of the six parameters related to profile, tier,
and level on these parameters being consistent)
P: properties of the bitstream to be sent
R: receiver capabilities
O: operation point selection
X: MUST NOT be present
-: not usable, when present MUST be ignored
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Parameters used for declaring receiver capabilities are, in general,
downgradable; i.e., they express the upper limit for a sender's
possible behavior. Thus, a sender MAY select to set its encoder
using only lower/lesser or equal values of these parameters.
When the answer does not include a recv-sub-layer-id that is less
than the sprop-sub-layer-id in the offer, parameters declaring a
configuration point are not changeable, with the exception of the
level-id parameter for unicast usage, and these parameters express
values a receiver expects to be used and MUST be used verbatim in the
answer as in the offer.
When a sender's capabilities are declared with the configuration
parameters, these parameters express a configuration that is
acceptable for the sender to receive bitstreams. In order to achieve
high interoperability levels, it is often advisable to offer multiple
alternative configurations. It is impossible to offer multiple
configurations in a single payload type. Thus, when multiple
configuration offers are made, each offer requires its own RTP
payload type associated with the offer. However, it is possible to
offer multiple operation points using one configuration in a single
payload type by including sprop-vps in the offer and recv-sub-layer-
id in the answer.
A receiver SHOULD understand all media type parameters, even if it
only supports a subset of the payload format's functionality. This
ensures that a receiver is capable of understanding when an offer to
receive media can be downgraded to what is supported by the receiver
of the offer.
An answerer MAY extend the offer with additional media format
configurations. However, to enable their usage, in most cases a
second offer is required from the offerer to provide the bitstream
property parameters that the media sender will use. This also has
the effect that the offerer has to be able to receive this media
format configuration, not only to send it.
7.2.3. Usage in Declarative Session Descriptions
When HEVC over RTP is offered with SDP in a declarative style, as in
Real Time Streaming Protocol (RTSP) [RFC 2326] or Session Announcement
Protocol (SAP) [RFC 2974], the following considerations are necessary.
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o All parameters capable of indicating both bitstream properties
and receiver capabilities are used to indicate only bitstream
properties. For example, in this case, the parameter profile-
tier-level-id declares the values used by the bitstream, not
the capabilities for receiving bitstreams. As a result, the
following interpretation of the parameters MUST be used:
+ Declaring actual configuration or bitstream properties:
- profile-space
- profile-id
- tier-flag
- level-id
- interop-constraints
- profile-compatibility-indicator
- tx-mode
- sprop-vps
- sprop-sps
- sprop-pps
- sprop-max-don-diff
- sprop-depack-buf-nalus
- sprop-depack-buf-bytes
- sprop-segmentation-id
- sprop-spatial-segmentation-idc
+ Not usable (when present, they MUST be ignored):
- max-lps
- max-lsr
- max-cpb
- max-dpb
- max-br
- max-tr
- max-tc
- max-fps
- max-recv-level-id
- depack-buf-cap
- sprop-sub-layer-id
- dec-parallel-cap
- include-dph
o A receiver of the SDP is required to support all parameters and
values of the parameters provided; otherwise, the receiver MUST
reject (RTSP) or not participate in (SAP) the session. It
falls on the creator of the session to use values that are
expected to be supported by the receiving application.
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7.2.4. Considerations for Parameter Sets
When out-of-band transport of parameter sets is used, parameter sets
MAY still be additionally transported in-band unless explicitly
disallowed by an application, and some of these additional parameter
sets may update some of the out-of-band transported parameter sets.
Update of a parameter set refers to the sending of a parameter set of
the same type using the same parameter set ID but with different
values for at least one other parameter of the parameter set.
7.2.5. Dependency Signaling in Multi-Stream Mode
If MRST or MRMT is used, the rules on signaling media decoding
dependency in SDP as defined in [RFC 5583] apply. The rules on
"hierarchical or layered encoding" with multicast in Section 5.7 of
[RFC 4566] do not apply. This means that the notation for Connection
Data "c=" SHALL NOT be used with more than one address, i.e., the
sub-field <number of addresses> in the sub-field <connection-address>
of the "c=" field, described in [RFC 4566], must not be present. The
order of session dependency is given from the RTP stream containing
the lowest temporal sub-layer to the RTP stream containing the
highest temporal sub-layer.
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
[RFC 4585], and the FIR message is defined in [RFC 5104].
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 an HEVC sender SHOULD be to send an IDR picture and relevant
parameter sets; potentially with sufficient redundancy so to ensure
correct reception. However, sometimes information about the
bitstream structure is known. For example, 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
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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)
The SLI described in RFC 4585 can be used to indicate, to a sender,
the loss of a number of Coded Tree Blocks (CTBs) in a CTB raster scan
order of a picture. In the SLI's Feedback Control Indication (FCI)
field, the subfield "First" MUST be set to the CTB address of the
first lost CTB. Note that the CTB address is in CTB-raster-scan
order of a picture. For the first CTB of a slice segment, the CTB
address is the value of slice_segment_address when present, or 0 when
the value of first_slice_segment_in_pic_flag is equal to 1; both
syntax elements are in the slice segment header. The subfield
"Number" MUST be set to the number of consecutive lost CTBs, again in
CTB-raster-scan order of a picture. Note that due to both the
"First" and "Number" being counted in CTBs in CTB-raster-scan order,
of a picture, not in tile-scan order (which is the bitstream order of
CTBs), multiple SLI messages may be needed to report the loss of one
tile covering multiple CTB rows but less wide than the picture.
The subfield "PictureID" MUST be set to the 6 least significant bits
of a binary representation of the value of PicOrderCntVal, as defined
in [HEVC], of the picture for which the lost CTBs are indicated.
Note that for IDR pictures the syntax element slice_pic_order_cnt_lsb
is not present, but then the value is inferred to be equal to 0.
As described in RFC 4585, an encoder in a media sender can use this
information to "clean up" the corrupted picture by sending intra
information, while observing the constraints described in RFC 4585,
for example, with respect to congestion control. In many cases,
error tracking is required to identify the corrupted region in the
receiver's state (reference pictures) because of error import in
uncorrupted regions of the picture through motion compensation.
Reference-picture selection can also be used to "clean up" the
corrupted picture, which is usually more efficient and less likely to
generate congestion than sending intra information.
In contrast to the video codecs contemplated in RFCs 4585 and 5104
[RFC 5104], in HEVC, the "macroblock size" is not fixed to 16x16 luma
samples, but is variable. That, however, does not create a
conceptual difficulty with SLI, because the setting of the CTB size
is a sequence-level functionality, and using a slice loss indication
across CVS boundaries is meaningless as there is no prediction across
sequence boundaries. However, a proper use of SLI messages is not as
straightforward as it was with older, fixed-macroblock-sized video
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codecs, as the state of the sequence parameter set (where the CTB
size is located) has to be taken into account when interpreting the
"First" subfield in the FCI.
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
H.264 or HEVC 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
H.264 and HEVC, even a decoded picture buffer size of two picture
storage buffers would work for the approach described in the previous
paragraph.
The field "Native RPSI bit string defined per codec" is a base16
[RFC 4648] representation of the 8 bits consisting of the 2 most
significant bits equal to 0 and 6 bits of nuh_layer_id, as defined in
[HEVC], followed by the 32 bits representing the value of the
PicOrderCntVal (in network byte order), as defined in [HEVC], for the
picture that is indicated by the RPSI feedback message.
The use of the RPSI feedback message as positive acknowledgement with
HEVC is deprecated. In other words, the RPSI feedback message MUST
only be used as a reference picture selection request, such that it
can also be used in multicast.
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
knowledge that the parameter sets have been correctly established. A
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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 HEVC 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 [RFC 3550]), 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 [RFC 3550], and in any applicable RTP profile such as
RTP/AVP [RFC 3551], RTP/AVPF [RFC 4585], RTP/SAVP [RFC 3711], or
RTP/SAVPF [RFC 5124]. However, as "Securing the RTP Framework: Why
RTP Does Not Mandate a Single Media Security Solution" [RFC 7202]
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"
[RFC 7201]. 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. H.265 is particularly vulnerable to such
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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 [RFC 3711].
Like [H.264], HEVC 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. HEVC
leaves the handling of this SEI message to the receiving system. In
order to avoid harmful side effects of the user data SEI message,
decoder implementations cannot naively trust its content. For
example, it would be a bad and insecure implementation practice to
forward any JavaScript a decoder implementation detects to a web
browser. The safest way to deal with user data SEI messages is to
simply discard them, but that can have negative side effects on the
quality of experience by the user.
End-to-end security with authentication, integrity, or
confidentiality protection will prevent a MANE from performing media-
aware operations other than discarding complete packets. In the case
of confidentiality protection, it will even be prevented from
discarding packets in a media-aware way. To be allowed to perform
such operations, a MANE is required to be a trusted entity that is
included in the security context establishment.
10. Congestion Control
Congestion control for RTP SHALL be used in accordance with RTP
[RFC 3550] and with any applicable RTP profile, e.g., AVP [RFC 3551].
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 mechanism available in HEVC 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. HEVC
contains mechanisms that allow the lightweight identification of
switching points in temporal enhancement layers, as discussed in
Section 1.1.2 of this memo. An HEVC media sender can send packets
belonging to NAL units of temporal enhancement layers starting from
these switching points to probe for available bandwidth and to
utilized bandwidth that has been shown to be available.
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 Considerations
A new media type, as specified in Section 7.1 of this memo, has been
registered with IANA.
12. References
12.1. Normative References
[H.264] ITU-T, "Advanced video coding for generic audiovisual
services", ITU-T Recommendation H.264, April 2013.
[HEVC] ITU-T, "High efficiency video coding", ITU-T Recommendation
H.265, April 2013.
Wang, et al. Standards Track PAGE 80
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[ISO23008-2]
ISO/IEC, "Information technology -- High efficiency coding
and media delivery in heterogeneous environments -- Part 2:
High efficiency video coding", ISO/IEC 23008-2, 2013.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC 3264, June 2002,
<http://www.rfc-editor.org/info/RFC 3264>.
[RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC 3550, July
2003, <http://www.rfc-editor.org/info/RFC 3550>.
[RFC 3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC 3551, July 2003,
<http://www.rfc-editor.org/info/RFC 3551>.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC 3711, March 2004,
<http://www.rfc-editor.org/info/RFC 3711>.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC 4566, July
2006, <http://www.rfc-editor.org/info/RFC 4566>.
[RFC 4585] 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/RFC 4585, July 2006,
<http://www.rfc-editor.org/info/RFC 4585>.
[RFC 4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC 4648, October 2006,
<http://www.rfc-editor.org/info/RFC 4648>.
[RFC 5104] 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/RFC 5104,
February 2008, <http://www.rfc-editor.org/info/RFC 5104>.
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[RFC 5124] 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/RFC 5124, February
2008, <http://www.rfc-editor.org/info/RFC 5124>.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC 5234, January 2008,
<http://www.rfc-editor.org/info/RFC 5234>.
[RFC 5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media
Attributes in the Session Description Protocol (SDP)",
RFC 5576, DOI 10.17487/RFC 5576, June 2009,
<http://www.rfc-editor.org/info/RFC 5576>.
[RFC 5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)",
RFC 5583, DOI 10.17487/RFC 5583, July 2009,
<http://www.rfc-editor.org/info/RFC 5583>.
12.2. Informative References
[3GPDASH] 3GPP, "Transparent end-to-end Packet-switched Streaming
Service (PSS); Progressive Download and Dynamic Adaptive
Streaming over HTTP (3GP-DASH)", 3GPP TS 26.247 12.1.0,
December 2013.
[3GPPFF] 3GPP, "Transparent end-to-end packet switched streaming
service (PSS); 3GPP file format (3GP)", 3GPP TS 26.244
12.20, December 2013.
[CABAC] Sole, J., Joshi, R., Nguyen, N., Ji, T., Karczewicz, M.,
Clare, G., Henry, F., and Duenas, A., "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 Faerber, F., "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.
[H.265.1] ITU-T, "Conformance specification for ITU-T H.265 high
efficiency video coding", ITU-T Recommendation H.265.1,
October 2014.
Wang, et al. Standards Track PAGE 82
RFC 7798 RTP Payload Format for HEVC March 2016
[HEVCv2] Flynn, D., Naccari, M., Rosewarne, C., Sharman, K., Sole,
J., Sullivan, G. J., and T. Suzuki, "High Efficiency Video
Coding (HEVC) Range Extensions text specification: Draft
7", JCT-VC document JCTVC-Q1005, 17th JCT-VC meeting,
Valencia, Spain, March/April 2014.
[IS014496-12]
IS0/IEC, "Information technology - Coding of audio-visual
objects - Part 12: ISO base media file format", IS0/IEC
14496-12, 2015.
[IS015444-12]
IS0/IEC, "Information technology - JPEG 2000 image coding
system - Part 12: ISO base media file format", IS0/IEC
15444-12, 2015.
[JCTVC-J0107]
Wang, Y.-K., Chen, Y., Joshi, R., and Ramasubramonian, K.,
"AHG9: On RAP pictures", JCT-VC document JCTVC-L0107, 10th
JCT-VC meeting, Stockholm, Sweden, July 2012.
[MPEG2S] ISO/IEC, "Information technology - Generic coding of moving
pictures and associated audio information - Part 1:
Systems", ISO International Standard 13818-1, 2013.
[MPEGDASH] ISO/IEC, "Information technology - Dynamic adaptive
streaming over HTTP (DASH) -- Part 1: Media presentation
description and segment formats", ISO International
Standard 23009-1, 2012.
[RFC 2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, DOI 10.17487/RFC 2326,
April 1998, <http://www.rfc-editor.org/info/RFC 2326>.
[RFC 2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, DOI 10.17487/RFC 2974,
October 2000, <http://www.rfc-editor.org/info/RFC 2974>.
[RFC 6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, DOI 10.17487/RFC 6051, November 2010,
<http://www.rfc-editor.org/info/RFC 6051>.
[RFC 6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC 6184, May 2011,
<http://www.rfc-editor.org/info/RFC 6184>.
Wang, et al. Standards Track PAGE 83
RFC 7798 RTP Payload Format for HEVC March 2016
[RFC 6190] Wenger, S., Wang, Y.-K., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
DOI 10.17487/RFC 6190, May 2011,
<http://www.rfc-editor.org/info/RFC 6190>.
[RFC 7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC 7201, April 2014,
<http://www.rfc-editor.org/info/RFC 7201>.
[RFC 7202] Perkins, C. and M. Westerlund, "Securing the RTP Framework:
Why RTP Does Not Mandate a Single Media Security Solution",
RFC 7202, DOI 10.17487/RFC 7202, April 2014,
<http://www.rfc-editor.org/info/RFC 7202>.
[RFC 7656] 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/RFC 7656, November 2015,
<http://www.rfc-editor.org/info/RFC 7656>.
[RFC 7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC 7667, November 2015,
<http://www.rfc-editor.org/info/RFC 7667>.
[RTP-MULTI-STREAM]
Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session",
Work in Progress, draft-ietf-avtcore-rtp-multi-stream-11,
December 2015.
[SDP-NEG] Holmberg, C., Alvestrand, H., and C. Jennings, "Negotiating
Medai Multiplexing Using Session Description Protocol
(SDP)", Work in Progress,
draft-ietf-mmusic-sdp-bundle-negotiation-25, January 2016.
[Wang05] Wang, Y.-K., Zhu, C., and Li, H., "Error resilient video
coding using flexible reference fames", Visual
Communications and Image Processing 2005 (VCIP 2005),
Beijing, China, July 2005.
Wang, et al. Standards Track PAGE 84
RFC 7798 RTP Payload Format for HEVC March 2016
Acknowledgements
Muhammed Coban and Marta Karczewicz are thanked for discussions on
the specification of the use with feedback messages and other aspects
in this memo. Jonathan Lennox and Jill Boyce are thanked for their
contributions to the PACI design included in this memo. Rickard
Sjoberg, Arild Fuldseth, Bo Burman, Magnus Westerlund, and Tom
Kristensen are thanked for their contributions to signaling related
to parallel processing. Magnus Westerlund, Jonathan Lennox, Bernard
Aboba, Jonatan Samuelsson, Roni Even, Rickard Sjoberg, Sachin
Deshpande, Woo Johnman, Mo Zanaty, Ross Finlayson, Danny Hong, Bo
Burman, Ben Campbell, Brian Carpenter, Qin Wu, Stephen Farrell, and
Min Wang made valuable review comments that led to improvements.
Wang, et al. Standards Track PAGE 85
RFC 7798 RTP Payload Format for HEVC March 2016
Authors' Addresses
Ye-Kui Wang
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121
United States
Phone: +1-858-651-8345
Email: yekui.wang@gmail.com
Yago Sanchez
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49 30 31002-663
Email: yago.sanchez@hhi.fraunhofer.de
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
Email: thomas.schierl@hhi.fraunhofer.de
Stephan Wenger
Vidyo, Inc.
433 Hackensack Ave., 7th floor
Hackensack, NJ 07601
United States
Phone: +1-415-713-5473
Email: stewe@stewe.org
Miska M. Hannuksela
Nokia Corporation
P.O. Box 1000
33721 Tampere
Finland
Phone: +358-7180-08000
Email: miska.hannuksela@nokia.com
Wang, et al. Standards Track PAGE 86
RFC TOTAL SIZE: 209680 bytes
PUBLICATION DATE: Wednesday, March 23rd, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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