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Versions: (draft-schierl-payload-rtp-h265) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 7798

Network Working Group                                      Y.-K. Wang
Internet Draft                                               Qualcomm
Intended status: Standards track                           Y. Sanchez
Expires: February 2015                                     T. Schierl
                                                       Fraunhofer HHI
                                                          S. Wenger
                                                              Vidyo
                                                     M. M. Hannuksela
                                                                Nokia
                                                      August 13, 2014




          RTP Payload Format for High Efficiency Video Coding
                   draft-ietf-payload-rtp-h265-06.txt




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 as well as multiple RTP streams.
   The payload format has wide applicability in videoconferencing,
   Internet video streaming, and high bit-rate entertainment-quality
   video, among others.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that




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   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as "work
   in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 13, 2015.

Copyright and License Notice

   Copyright (c) 2014 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.














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Table of Contents

   Abstract.........................................................1
   Status of this Memo..............................................1
   Table of Contents................................................3
   1 Introduction...................................................5
      1.1 Overview of the HEVC Codec................................5
         1.1.1 Coding-Tool Features.................................5
         1.1.2 Systems and Transport Interfaces.....................7
         1.1.3 Parallel Processing Support.........................14
         1.1.4 NAL Unit Header.....................................16
      1.2 Overview of the Payload Format...........................18
   2 Conventions...................................................18
   3 Definitions and Abbreviations.................................19
      3.1 Definitions..............................................19
         3.1.1 Definitions from the HEVC Specification.............19
         3.1.2 Definitions Specific to This Memo...................21
      3.2 Abbreviations............................................22
   4 RTP Payload Format............................................24
      4.1 RTP Header Usage.........................................24
      4.2 Payload Header Usage.....................................26
      4.3 Payload Structures.......................................27
      4.4 Transmission Modes.......................................27
      4.5 Decoding Order Number....................................28
      4.6 Single NAL Unit Packets..................................30
      4.7 Aggregation Packets (APs)................................31
      4.8 Fragmentation Units (FUs)................................36
      4.9 PACI packets.............................................39
         4.9.1 Reasons for the PACI rules (informative)............42
         4.9.2 PACI extensions (Informative).......................43
      4.10 Temporal Scalability Control Information................44
   5 Packetization Rules...........................................46
   6 De-packetization Process......................................47
   7 Payload Format Parameters.....................................49
      7.1 Media Type Registration..................................50
      7.2 SDP Parameters...........................................75
         7.2.1 Mapping of Payload Type Parameters to SDP...........75
         7.2.2 Usage with SDP Offer/Answer Model...................77
         7.2.3 Usage in Declarative Session Descriptions...........86


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         7.2.4 Parameter Sets Considerations.......................87
         7.2.5 Dependency Signaling in Multi-Stream Mode...........87
   8 Use with Feedback Messages....................................88
      8.1 Picture Loss Indication (PLI)............................89
      8.2 Slice Loss Indication....................................89
      8.3 Use of HEVC with the RPSI Feedback Message...............90
      8.4 Full Intra Request (FIR).................................91
   9 Security Considerations.......................................92
   10 Congestion Control...........................................93
   11 IANA Consideration...........................................94
   12 Acknowledgements.............................................94
   13 References...................................................95
      13.1 Normative References....................................95
      13.2 Informative References..................................96
   14 Authors' Addresses...........................................98































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1 Introduction

1.1 Overview of the HEVC Codec

   High Efficiency Video Coding [HEVC], formally known as ITU-T
   Recommendation H.265 and ISO/IEC International Standard 23008-2
   was ratified by ITU-T in April 2013 and reportedly provides
   significant coding efficiency gains over H.264 [H.264].

   As both H.264 [H.264] and its RTP payload format [RFC6184] are
   widely deployed and generally known in the relevant implementer
   communities, frequently only the differences between those two
   specifications 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 H.264 or its RTP payload format.

   H.264 and HEVC share a similar hybrid video codec design.
   Conceptually, both technologies 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

   Similarly 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 either by
   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 contribute significantly to the
   coding efficiency of HEVC is the usage of flexible coding blocks



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   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
   contribute to the major gain of HEVC, especially at high
   resolutions.

   Entropy coding

   HEVC uses a single entropy coding engine, which is based on
   Context Adaptive Binary Arithmetic Coding (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


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   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 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, such as the NAL-unit-based syntax structure, the
   hierarchical syntax and data unit structure from sequence-level
   parameter sets, multi-picture-level or picture-level parameter


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   sets, slice-level header parameters, lower-level parameters, the
   supplemental enhancement information (SEI) message mechanism, the
   hypothetical reference decoder (HRD) based video buffering model,
   and so on.  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
   video parameter set NAL unit is required to be available prior to
   its activation, while the information contained in the video
   parameter set is not necessary for operation of the decoding
   process.  For future HEVC extensions, such as the 3D or scalable
   extensions, the video parameter set 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 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
   level information pertaining to individual temporally scalable
   layers.  The profile indicator indicates 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) [ISOBMFF] and file formats derived based on ISOBMFF,
   such as the 3GPP file format [3GPPFF].  The profile, tier and
   level syntax structure also includes the indications of whether
   the bitstream is free of frame-packed content, whether the
   bitstream is free of interlaced source content and free of field
   pictures, i.e. contains only frame pictures of progressive


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   source, such that clients/players with no support of post-
   processing functionalities for handling of frame-packed or
   interlaced source content or field pictures can reject those
   bitstreams.

   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 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, wherein
   IDR pictures are conventionally referred to as closed group-of-
   pictures (closed-GOP) random access points, and CRA and BLA
   pictures are those 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 (SAP) types as defined in
   the ISOBMFF [ISOBMFF], 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 as
   leading pictures associated with the IRAP picture.  There are two
   types of leading pictures, namely random access decodable leading


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   (RADL) pictures and random access skipped leading (RASL)
   pictures.  RADL pictures are decodable when the decoding started
   at the associated IRAP picture, and RASL pictures are not
   decodable when the decoding started at the associated IRAP
   picture and are usually discarded.  HEVC provides mechanisms to
   enable the specification of conformance of bitstreams with RASL
   pictures being discarded, thus to provide a standard-compliant
   way to enable systems components to discard RASL pictures when
   needed.

   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
   stepwise 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



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   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 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 VPS, SPS, 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, as well as specification of the
   use of the bitstream extraction process in description of
   bitstream conformance tests as part of the hypothetical reference
   decoder (HRD) specification.





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   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 sliding window plus adaptive
   memory management control operation (MMCO) based reference
   picture marking mechanism in H.264, HEVC specifies a reference
   picture set (RPS) based reference picture management and marking
   mechanism, and the RPLC is consequently based on the RPS
   mechanism.  A reference picture set 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
   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 one
   picture interval.  Sub-picture-level coded picture buffer (CPB)
   and DPB parameters may be signaled, and utilization of these
   information for the derivation of CPB timing (wherein the CPB


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   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.

   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
   coded video sequence 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 coded video sequence that
   updates another parameter set of the same type preceding in
   decoding order.  An update of a parameter set refers to the use


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   of the same parameter set ID but with some other parameters
   changed.  If this property is true for all coded video sequences
   in the bitstream, then all parameter sets can be sent out-of-band
   before session start.

   The decoding unit information SEI message provides coded picture
   buffer removal delay information 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 (GDR).
   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 multi-core/multi-
   processor architectures.  Specifically, for parallelization, four
   picture partition strategies are available.

   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


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   form, in H.264.  Slices based parallelization 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 mechanism.  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.


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   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
   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.










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                   +---------------+---------------+
                   |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 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].  HEVC
      declares a value of 1 as a syntax violation.  Note that the
      inclusion of this bit in the NAL unit header is to enable
      transport of HEVC video over MPEG-2 transport systems
      (avoidance of start code emulations) [MPEG2S].

   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 7.4.1 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
      coded video sequence, 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



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      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 [RFC3550]:

   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, namely 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 their decoding order)

   o Media type parameters to be used with the Session Description
     Protocol (SDP) [RFC4566]

   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 [RFC2119].




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   In this document, these key words will appear with that
   interpretation only when in ALL CAPS.  Lower case uses of these
   words are not to be interpreted as carrying the RFC 2119
   significance.

   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 copied 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, are
   consecutive in decoding order, and 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: 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.





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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 inputs.

   random access: The act of starting the decoding process for a
   bitstream at a point other than the beginning of the bitstream.






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   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.

   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 an MSM 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 an SSM 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 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



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      those packets whose elimination produces the least adverse
      effect on the user experience.  After dropping packets, MANEs
      must rewrite RTCP packets to match the changes to the RTP
      stream as specified in Section 7 of [RFC3550].

   multi-stream mode(MSM): Transmission of an HEVC bitstream using
   more than one RTP stream.

   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-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
   NAL-unit-like structures defined in this memo are packet payloads
   of AP, PACI, and 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 [I-D.ietf-avtext-rtp-grouping-taxonomy].  Within
   the scope of this memo, one RTP stream is utilized to transport
   one or more temporal sub-layers.

   single-stream mode (SSM): Transmission of an HEVC bitstream using
   only one RTP stream.

   transmission order: The order of packets in ascending RTP
   sequence number order (in modulo arithmetic).  Within an
   aggregation packet, the NAL unit transmission order is the same
   as the order of appearance of NAL units in the packet.

3.2 Abbreviations

   AP       Aggregation Packet

   BLA      Broken Link Access

   CRA      Clean Random Access



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   CTB      Coding Tree Block

   CTU      Coding Tree Unit

   CVS      Coded Video Sequence

   DPH      Decoded Picture Hash

   FU       Fragmentation Unit

   GDR      Gradual Decoding Refresh

   HRD      Hypothetical Reference Decoder

   IDR      Instantaneous Decoding Refresh

   IRAP     Intra Random Access Point

   MANE     Media Aware Network Element

   MSM      Multi-Stream Mode

   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

   SEI      Supplemental Enhancement Information

   SPS      Sequence Parameter Set


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   SSM      Single-Stream Mode

   STSA     Step-wise Temporal Sub-layer Access

   TSA      Temporal Sub-layer Access

   TCSI     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 [RFC3550] and
   reprinted in 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 4.7 and 4.8, 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 [RFC3550]

   The RTP header information to be set according to this RTP
   payload format is set as follows:


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   Marker bit (M): 1 bit

      Set for the last packet, carried in the current RTP stream, of
      the access unit, in line with the normal use of the M bit in
      video formats, to allow an efficient playout buffer handling.
      When MSM 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 naluX is the last
         NAL unit of an access unit if it is the last NAL unit of
         the bitstream or 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 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 MSM.

   Sequence number (SN): 16 bits

      Set and used in accordance with RFC 3550 [RFC3550].



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   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.  In SSM, by
      definition a single SSRC is used for all parts of a single
      bitstream.  In MSM, each SSRC is used for an RTP stream
      containing a subset of the sub-layers for a single (temporally
      scalable) bitstream.  A receiver is required to correctly
      associate the set of SSRCs that are included parts of the same
      bitstream.

         Informative note: The term "bitstream" in this document is
         equivalent to the term "encoded stream" in [I-D.ietf-
         avtext-rtp-grouping-taxonomy].

4.2 Payload Header Usage

   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




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   better protected against transmission losses than less important
   NAL units.

4.3 Payload Structures

   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.

   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 4.6.

   o  Aggregation packet (AP): Contains more than one NAL unit
      within one access unit.  This payload structure is specified
      in section 4.7.

   o  Fragmentation unit (FU): Contains a subset of a single NAL
      unit.  This payload structure is specified in section 4.8.

   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 4.9.

4.4 Transmission Modes

   This memo enables transmission of an HEVC bitstream over a single
   RTP stream or multiple RTP streams.  The concept and working
   principle is inherited from the design of what was called single
   and multiple session transmission in [RFC6190] and follows a
   similar design.  If only one RTP stream is used for transmission



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   of the HEVC bitstream, the transmission mode is referred to as
   single-stream mode (SSM); otherwise (more than one RTP stream is
   used for transmission of the HEVC bitstream), the transmission
   mode is referred to as multi-stream mode (MSM).

   Dependency of one RTP stream on another RTP stream is typically
   indicated as specified in [RFC5583].  When an RTP stream A
   depends on another RTP stream B, the RTP stream B is referred to
   as a dependee RTP stream of the RTP stream A.

      Informative note: An MSM may involve one or more RTP sessions.
      Each RTP stream in an MSM may be in its own RTP session or a
      set of multiple RTP streams in an MSM may belong to the same
      RTP session, e.g. as indicated by the mechanism specified in
      the Internet-Draft [I-D.ietf-avtcore-rtp-multi-stream] or in
      [I-D.ietf-mmusic-sdp-bundle-negotiation].

   SSM SHOULD be used for point-to-point unicast scenarios, while
   MSM 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 MSM 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 "SSM", SSM MUST be
   used.  Otherwise (tx-mode is equal to "MSM"), MSM MUST be used.

   Receivers MUST support both SSM and MSM.

4.5 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.



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   Let NAL unit n be the n-th NAL unit in transmission order within
   an RTP stream.

   If tx-mode is equal to "SSM" and sprop-max-don-diff is equal to
   0, AbsDon[n], the value of AbsDon for NAL unit n, is derived as
   equal to n.

   Otherwise (tx-mode is equal to "MSM" or sprop-max-don-diff is
   greater than 0), AbsDon[n] is derived as follows, where DON[n] is
   the value of the variable DON for NAL unit n:

   o  If n is equal to 0 (i.e. NAL unit n is the very first NAL unit
      in transmission order), AbsDon[0] is set equal to DON[0].

   o  Otherwise (n is greater than 0), the following applies for
      derivation of AbsDon[n]:

            If DON[n] == DON[n-1],
                AbsDon[n] = AbsDon[n-1]

            If (DON[n] > DON[n-1] and DON[n] - DON[n-1] < 32768),
                AbsDon[n] = AbsDon[n-1] + DON[n] - DON[n-1]

            If (DON[n] < DON[n-1] and DON[n-1] - DON[n] >= 32768),
                AbsDon[n] = AbsDon[n-1] + 65536 - DON[n-1] + DON[n]

            If (DON[n] > DON[n-1] and DON[n] - DON[n-1] >= 32768),
                AbsDon[n] = AbsDon[n-1] - (DON[n-1] + 65536 -
            DON[n])

            If (DON[n] < DON[n-1] and DON[n-1] - DON[n] < 32768),
                AbsDon[n] = AbsDon[n-1] - (DON[n-1] - DON[n])

   For any two NAL units m and n, the following applies:

   o  AbsDon[n] greater than AbsDon[m] indicates that NAL unit n
      follows NAL unit m in NAL unit decoding order.

   o  When AbsDon[n] is equal to AbsDon[m], the NAL unit decoding
      order of the two NAL units can be in either order.




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   o  AbsDon[n] less than AbsDon[m] indicates that NAL unit n
      precedes NAL unit m in decoding order.

   When two consecutive NAL units in the NAL unit decoding order
   have different values of AbsDon, the value of AbsDon for the
   second NAL unit in decoding order MUST be greater than the value
   of AbsDon for the first NAL unit, and the absolute difference
   between the two AbsDon values MAY be greater than or equal to 1.

      Informative note: There are multiple reasons to allow for the
      absolute difference of the values of AbsDon for two
      consecutive NAL units in the NAL unit decoding order to be
      greater than one.  An increment by one is not required, as at
      the time of associating values of AbsDon to NAL units, it may
      not be known whether all NAL units are to be delivered to the
      receiver.  For example, a gateway may not forward VCL NAL
      units of higher sub-layers or some SEI NAL units when there is
      congestion in the network.  In another example, the first
      intra-coded picture of a pre-encoded clip is transmitted in
      advance to ensure that it is readily available in the
      receiver, and when transmitting the first intra-coded picture,
      the originator does not exactly know how many NAL units will
      be encoded before the first intra-coded picture of the pre-
      encoded clip follows in decoding order.  Thus, the values of
      AbsDon for the NAL units of the first intra-coded picture of
      the pre-encoded clip have to be estimated when they are
      transmitted, and gaps in values of AbsDon may occur.  Another
      example is MSM where the AbsDon values must indicate cross-
      layer decoding order for NAL units conveyed in all the RTP
      streams.

4.6 Single NAL Unit Packets

   A single NAL unit packet contains exactly one NAL unit, and
   consists of a payload header (denoted as PayloadHdr), a
   conditional 16-bit DONL field (in network byte order), and the
   NAL unit payload data (the NAL unit excluding its NAL unit
   header) of the contained NAL unit, as shown in Figure 3.






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   0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           PayloadHdr          |      DONL (conditional)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                  NAL unit payload data                        |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3 The structure a single NAL unit packet

   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 tx-mode is equal to "MSM" or sprop-max-don-diff is
   greater than 0, the DONL field MUST be present, and the variable
   DON for the contained NAL unit is derived as equal to the value
   of the DONL field.  Otherwise (tx-mode is equal to "SSM" and
   sprop-max-don-diff is equal to 0), the DONL field MUST NOT be
   present.

4.7 Aggregation Packets (APs)

   Aggregation packets (APs) are introduced to enable the reduction
   of packetization overhead for small NAL units, such as most of
   the non-VCL NAL units, which are often only a few octets in size.

   An AP aggregates NAL units within one access unit.  Each NAL unit
   to be carried in an AP is encapsulated in an aggregation unit.
   NAL units aggregated in one AP are in NAL unit decoding order.

   An AP consists of a payload header (denoted as PayloadHdr)
   followed by two or more aggregation units, as shown in Figure 4.




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   0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PayloadHdr (Type=48)       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |             two or more aggregation units                     |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            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.

      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 Fragmentation Units (FUs) specified in section 4.8.  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,



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   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 tx-mode is equal to "MSM" or sprop-max-don-diff is greater
   than 0, the DONL field MUST be present in an aggregation unit
   that is the first aggregation unit in an AP, and the variable DON
   for the aggregated NAL unit is derived as equal to the value of
   the DONL field.  Otherwise (tx-mode is equal to "SSM" and sprop-
   max-don-diff is equal to 0), the DONL field MUST NOT be present
   in an aggregation unit that is the first aggregation unit in an
   AP.

   An aggregation unit that is not the first aggregation unit in an
   AP consists of a conditional 8-bit DOND field followed by a 16-
   bit unsigned size information (in network byte order) that
   indicates the size of the NAL unit in bytes (excluding these two
   octets, but including the NAL unit header), followed by the NAL
   unit itself, including its NAL unit header, as shown in Figure 6.








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   0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   : DOND (cond)   |          NALU size            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                       NAL unit                                |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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 tx-mode is equal to "MSM" or sprop-max-don-diff is greater
   than 0, 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 (tx-mode is
   equal to "SSM" and sprop-max-don-diff is equal to 0), the DOND
   field MUST NOT be present in an aggregation unit that is not the
   first aggregation unit in an AP, and in this case the
   transmission order and decoding order of NAL units carried in the
   AP are the same as the order the NAL units appear in the AP.

   Figure 7 presents an example of an AP that contains two
   aggregation units, labeled as 1 and 2 in the figure, without the
   DONL and DOND fields being present.










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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   PayloadHdr (Type=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

   Figure 8 presents an example of an AP that contains two
   aggregation units, labeled as 1 and 2 in the figure, with the
   DONL and DOND fields being present.


















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   PayloadHdr (Type=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

4.8 Fragmentation Units (FUs)

   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.



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   The RTP timestamp of an RTP packet carrying an FU is set to the
   NALU-time of the fragmented NAL unit.

   An FU consists of a payload header (denoted as PayloadHdr), an FU
   header of one octet, a conditional 16-bit DONL field (in network
   byte order), and an FU payload, as shown in Figure 9.

    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







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   The semantics of the FU header fields are as follows:
   S: 1 bit
      When set to one, 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 zero.

   E: 1 bit
      When set to one, 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 zero.

   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 tx-mode is equal to "MSM" or sprop-max-don-diff is greater
   than 0, and the S bit is equal to 1, the DONL field MUST be
   present in the FU, and the variable DON for the fragmented NAL
   unit is derived as equal to the value of the DONL field.
   Otherwise (tx-mode is equal to "SSM" and sprop-max-don-diff is
   equal to 0, or the S bit is equal to 0), the DONL field MUST NOT
   be present in the FU.

   A non-fragmented NAL unit MUST NOT be transmitted in one FU; i.e.
   the Start bit and End bit MUST NOT both be set to one 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



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   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 one to indicate
   a syntax violation.

4.9 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 Temporal
   Scalability Control Information as specified in section 4.10
   below.  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 4.9.2 below 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




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   section 4.3 above.  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; that
   is, 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

   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.









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   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 4.9.2.

   Y: 1 bit
      MUST be 0, available for future extensions, see section 4.9.2.

   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.

         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


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         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.

4.9.1 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 would get 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 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


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   such small NAL units, AP 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.

4.9.2 PACI extensions (Informative)

   This subsection 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 (Temporal
   Scalability Control Information, described below in section 4.10),
   and, 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 to, recursively, add further F and Y
   bits to extend the mechanism beyond 3 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
   3 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 7
   flags and another Y bit.  The size of this list of flags is
   subject to the limits specified in section 4.9 (32 octets for all
   flag-extensions and the PHES information combined).



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   Each of the F bits can indicate either the presence of
   information in the Payload Header Extension Structure (PHES),
   described below, or a given F bit can indicate a certain
   condition, without including additional information in the 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.
     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
        to be able to either use those bits when present, or at
        least be able to skip over them.

4.10 Temporal Scalability Control Information

   This section describes the single payload header extension
   defined in this specification, known as Temporal Scalability
   Control Information (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:












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     0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
   1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   +-+
      |    PayloadHdr (Type=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
      [H.265] 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 [H.265] for the
      access unit containing the NAL unit in the PACI.

   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.





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      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.

5 Packetization Rules

   The following packetization rules apply:

   o  If tx-mode is equal to "MSM" or sprop-max-don-diff is greater
      than 0 for an RTP stream, the transmission order of NAL units
      carried in the RTP stream MAY be different than the NAL unit
      decoding order.  Otherwise (tx-mode is equal to "SSM" and sprop-
      max-don-diff is equal to 0 for an RTP stream), the transmission
      order of NAL units carried in the RTP stream MUST be the same as
      the NAL unit decoding order.




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   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.

   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



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   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 MSM, 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 here after
   called the de-packetization buffer in this section.  Receivers
   should also prepare for transmission delay jitter; i.e. 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.

   If only one RTP stream is being received and sprop-max-don-diff
   of the only RTP stream being received is equal to 0, the de-
   packetization buffer size is zero bytes, i.e. the NAL units
   carried in the RTP stream are directly passed to the decoder in
   their transmission order, which is identical to the decoding
   order of the NAL units. Otherwise, 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 MSM is in use, NAL


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   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.

   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 A 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) [RFC4566] is also provided for
   applications that use SDP.  Equivalent parameters could be
   defined elsewhere for use with control protocols that do not use
   SDP.





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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.

   Media Type name:     video

   Media subtype name:  H265

   Required parameters: none

   OPTIONAL parameters:

      profile-space, tier-flag, profile-id, profile-compatibility-
      indicator, interop-constraints, and level-id:

         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 [HEVC draft v2], 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-



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            constraints - this (rather unexpected way of profile
            signalling) 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.

         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
              signalled 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



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              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 changable 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]:

            If the RTP stream is the highest RTP stream, the
            following applies:

            o profile_space = general_profile_space
            o profile_id = general_profile_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 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
         and 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:





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            o tier-flag = general_tier_flag
            o level-id = general_level_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 tier-flag = sub_layer_tier_flag[j]
            o level-id = sub_layer_level_idc[j]

      interop-constraints:

         A base16 [RFC4648] (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]:





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            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
            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 [RFC4648] 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



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         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.

            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.







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      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.

      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 SSM
         or MSM.


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         The value of tx-mode MUST be equal to either "MSM" or "SSM".
         When not present, the value of tx-mode is inferred to be
         equal to "SSM".

         If the value is equal to "MSM", MSM MUST be in use.  Otherwise
         (the value is equal to "SSM"), SSM MUST be in use.

         The value of tx-mode MUST be equal to "MSM" for all RTP
         sessions in an MSM.

      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
         [RFC4648] representations of the video parameter set NAL
         units as specified in Section 7.3.2.1 of [HEVC].

         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_id, 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_id, inclusive, in the first
         profile_tier_level( ) syntax structure in vpsB.






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      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 [RFC4648]
         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 [RFC4648]
         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 [RFC4648] representations of SEI NAL units as
         specified in Section 7.3.2.4 of [HEVC].

            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,



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               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 saving
               bits and allows 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 hash-tag),
               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 such 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 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


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         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


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         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 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.


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            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 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
         ) )



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              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 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


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         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.

         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.




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      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.

         Senders MAY use this knowledge to send pictures utilizing a
         larger number of tile columns than the value allowed by the
         highest level.




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         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.

      sprop-max-don-diff:




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         The value of this parameter MUST be equal to 0, if the RTP
         stream does not depend on other RTP streams and there is no
         NAL unit naluA that is followed in transmission order by
         any NAL unit preceding naluA in decoding order.  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.

         When the RTP stream depends on one or more other RTP
         streams (in this case tx-mode MUST be equal to "MSM" and
         MSM is in use), this parameter MUST be present and the
         value MUST be greater than 0.

            Informative note: When the RTP stream does not depend on
            other RTP streams, either MSM or SSM may be in use.

      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 the RTP stream depends on one or more other RTP
         streams (in this case tx-mode MUST be equal to "MSM" and
         MSM is in use), 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 the RTP stream depends on one or more other RTP
         streams (in this case tx-mode MUST be equal to "MSM" and
         MSM is in use) or sprop-max-don-diff is present and greater
         than 0, this parameter MUST be present and the value MUST
         be greater than 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.





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            Informative note: depack-buf-cap indicates the maximum
            possible size of the de-packetization buffer of the
            receiver only.  When network jitter can occur, an
            appropriately sized jitter buffer has to be available as
            well.

      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 [RFC4648] 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



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         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 [RFC5234] is as follows:

            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 (';').


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         The profile of all capability points is determined by
         profile-space and profile-id that 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.

         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


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         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.

         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-



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         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).

      Security considerations:

         See Section 9 of RFC XXXX.




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      Public specification:

         Please refer to Section 13 of RFC XXXX.

      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 (yekuiw@qti.qualcomm.com).

      Intended usage: COMMON

      Author: See Section 14 of RFC XXXX.

      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) [RFC4566] 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-


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      compatibility-indicator", "sprop-sub-layer-id", "recv-sub-
      layer-id", "max-recv-level-id", "tx-mode", "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 [RFC5576].  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 [RFC5117].

   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>




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7.2.2 Usage with SDP Offer/Answer Model

   When HEVC is offered over RTP using SDP in an Offer/Answer model
   [RFC3264] 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 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 signalled 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.




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   o  The profile-compatibility-indicator, when offered as sendonly,
      describe 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 [RFC3264].

   o  The same RTP payload type number used in the offer 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 in the offer
      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 recv-sub-
      layer-id that is less than 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


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      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.

          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; i.e. they are dependent on their 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".


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   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 MSM transmission mode.  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 MSM, it is also
      RECOMMENDED to consider offering multiple payload types with
      different buffering requirements when the capabilities of the
      receiver are unknown.

   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"


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      source attribute as specified in section 6.3 of [RFC5576]),
      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 stream(s) are
      sent before 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.

       o 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.

       o 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.

       o In MSM, 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



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          parameter set NAL units to the video decoder before
          passing any NAL units carried in the RTP streams.

   o  The following rules apply to transport of parameter set in the
      answerer-to-offerer direction.

       o 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.

       o 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.

       o In MSM, 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
      [RFC5576], 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
      [RFC5576].






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   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.

   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 [RFC3264].  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.

                                          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 --+  |  |  |  |



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                                         |  |  |  |  |
      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: changable 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


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      R: receiver capabilities
      O: operation point selection
      X: MUST NOT be present
      -: not usable, when present SHOULD be ignored

   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 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.



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   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) [RFC2326] or Session
   Announcement Protocol (SAP) [RFC2974], the following
   considerations are necessary.

   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.  This results in
      that the following interpretation of the parameters MUST be
      used:

      o 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

      o Not usable (when present, they SHOULD be ignored):
         - max-lps
         - max-lsr
         - max-cpb
         - max-dpb
         - max-br


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         - 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.

7.2.4 Parameter Sets Considerations

   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
   additionally in-band transported parameter sets may update some
   of the out-of-band transported parameter sets.  Update of a
   parameter set refers to 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.

   If MSM is used, the rules on signaling media decoding dependency
   in SDP as defined in [RFC5583] apply.  The rules on "hierarchical
   or layered encoding" with multicast in Section 5.7 of [RFC4566]
   do not apply, i.e. the notation for Connection Data "c=" SHALL
   NOT be used with more than one address.  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.

7.2.5 Dependency Signaling in Multi-Stream Mode

   If MSM is used, the rules on signaling media decoding dependency
   in SDP as defined in [RFC5583] apply.  The rules on "hierarchical
   or layered encoding" with multicast in Section 5.7 of [RFC4566]
   do not apply, i.e. the notation for Connection Data "c=" SHALL


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   NOT be used with more than one address.  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

   As specified in section 6.1 of RFC 4585 [RFC4585], payload
   Specific Feedback messages are identified by the RTCP packet type
   value PSFB (206).  AVPF [RFC4585] defines three payload-specific
   feedback messages and one application layer feedback message, and
   CCM [RFC5104] specifies four payload-specific feedback messages.

   These feedback messages are identified by means of the feedback
   message type (FMT) parameter as follows:

   Assigned in [RFC4585]:

      1:     Picture Loss Indication (PLI)
      2:     Slice Lost Indication (SLI)
      3:     Reference Picture Selection Indication (RPSI)
      15:    Application layer FB message
      31:    reserved for future expansion of the number space

   Assigned in [RFC5104]:

      4:     Full Intra Request (FIR) Command
      5:     Temporal-Spatial Trade-off Request (TSTR)
      6:     Temporal-Spatial Trade-off Notification (TSTN)
      7:     Video Back Channel Message (VBCM)

   Unassigned:

      0:      unassigned
      8-14:   unassigned
      16-30:  unassigned

   The following subsections define the use of the PLI, SLI, RPSI,
   and FIR feedback messages with HEVC.





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8.1 Picture Loss Indication (PLI)

   As specified in RFC 4585 section 6.3.1, the reception of a
   picture loss indication 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 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

   RFC 4585's Slice Loss Indication can be used to indicate, to a
   sender, the loss of a number of Coded Tree Blocks (CTBs) in 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_segement_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" are 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.


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   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
   RFC4585, 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 RFC 4585 and RFC
   5104 [RFC5104], in HEVC, the "macroblock size" is not fixed to
   16x16 luma samples, but 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 coded video sequence 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 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 Use of HEVC with the RPSI Feedback Message

   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 to the
   encoder side that a particular picture that has been decoded
   relatively earlier is correct and still present in the decoded
   picture buffer and requests the encoder to use that correct


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   picture for reference 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 would work for the approach described in the previous
   paragraph.

   The field "Native RPSI bit string defined per codec" is a base16
   [RFC4648] representation of the 8 bits consisting of 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 requested to be used for
   reference when encoding the next picture.

   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 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.






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9 Security Considerations

   RTP packets using the payload format defined in this
   specification are subject to the security considerations
   discussed in the RTP specification [RFC3550], and in any
   applicable RTP profile such as RTP/AVP [RFC3551], RTP/AVPF
   [RFC4585], RTP/SAVP [RFC3711], or RTP/SAVPF [RFC5124].  However,
   as RFC 7202 [RFC7202] discusses it is not an RTP payload format's
   responsibility to discuss or mandate what solutions are used to
   meet the basic security goals like confidentiality, integrity,
   and source authenticity for RTP in general.  This responsibility
   lays on anyone using RTP in an application.  They can find
   guidance on available security mechanisms and important
   considerations as discussed in RFC 7201 [RFC7201].

   The rest of this section discusses the security impacting
   properties of the payload format itself.

   Because the data compression used with this payload format is
   applied end-to-end, any encryption needs to be performed after
   compression.  A potential denial-of-service threat exists for
   data encodings using compression techniques that have non-uniform
   receiver-end computational load.  The attacker can inject
   pathological datagrams into the bitstream that are complex to
   decode and that cause the receiver to be overloaded.  H.265 is
   particularly vulnerable to such attacks, as it is extremely
   simple to generate datagrams containing NAL units that affect the
   decoding process of many future NAL units.  Therefore, the usage
   of data origin authentication and data integrity protection of at
   least the RTP packet is RECOMMENDED, for example, with SRTP
   [RFC3711].

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on
   the application and on the transport and signaling protocols
   employed.  Thus, although SRTP is given as an example above,
   other possible choices exist.

   Decoders MUST exercise caution with respect to the handling of
   user data SEI messages, particularly if they contain active



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   elements, and MUST restrict their domain of applicability to the
   presentation containing the bitstream.

   End-to-end security with authentication, integrity, or
   confidentiality protection will prevent a MANE from performing
   media-aware operations other than discarding complete packets.
   In the case of confidentiality protection, it will even be
   prevented from discarding packets in a media-aware way.  To be
   allowed to perform such operations, a MANE is required to be a
   trusted entity that is included in the security context
   establishment.

10 Congestion Control

   Congestion control for RTP SHALL be used in accordance with RTP
   [RFC3550] and with any applicable RTP profile, e.g. AVP
   [RFC3551].  If best-effort service is being used, an additional
   requirement is that users of this payload format MUST monitor
   packet loss to ensure that the packet loss rate is within an
   acceptable range.  Packet loss is considered acceptable if a TCP
   flow across the same network path, and experiencing the same
   network conditions, would achieve an average throughput, measured
   on a reasonable timescale, that is not less than all RTP streams
   combined is achieving.  This condition can be satisfied by
   implementing congestion control mechanisms to adapt the
   transmission rate, the number of layers subscribed for a layered
   multicast session, or by arranging for a receiver to leave the
   session if the loss rate is unacceptably high.

   The bitrate adaptation necessary for obeying the congestion
   control principle is easily achievable when real-time encoding is
   used, for example by adequately tuning the quantization
   parameter.

   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


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   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 Consideration

   A new media type, as specified in Section 7.1 of this memo,
   should be registered with IANA.

12 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
   parallel processing related signalling.  Magnus Westerlund,
   Jonathan Lennox, Bernard Aboba, Jonatan Samuelsson, Roni Even,
   Rickard Sjoberg, Sachin Deshpande, Woo Johnman, Mo Zanaty, Ross


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   Finlayson, and Danny Hong made valuable reviewing comments that
   led to improvements.

   This document was prepared using 2-Word-v2.0.template.dot.

13 References

13.1 Normative References

   [HEVC]    ITU-T Recommendation H.265, "High efficiency video
             coding", April 2013.

   [H.264]   ITU-T Recommendation H.264, "Advanced video coding for
             generic audiovisual services", April 2013.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
             Model with Session Description Protocol (SDP)", RFC
             3264, June 2002.

   [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and
             Jacobson, V., "RTP: A Transport Protocol for Real-Time
             Applications", STD 64, RFC 3550, July 2003.

   [RFC3551] Schulzrinne, H. and Casner, S., "RTP Profile for Audio
             and Video Conferences with Minimal Control", STD 65,
             RFC 3551, July 2003.

   [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
             Norrman, K., "The Secure Real-time Transport Protocol
             (SRTP)", RFC 3711, March 2004.

   [RFC4566] Handley, M., Jacobson, V., and Perkins, C., "SDP:
             Session Description Protocol", RFC 4566, July 2006.

   [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and Rey,
             J., "Extended RTP Profile for Real-time Transport
             Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC
             4585, July 2006.



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   [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
             Encodings", RFC 4648, October 2006.

   [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and Burman,
             B., "Codec Control Messages in the RTP Audio-Visual
             Profile with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC5124] Ott, J. and Carrara, E., "Extended Secure RTP Profile
             for Real-time Transport Control Protocol (RTCP)-Based
             Feedback (RTP/SAVPF)", RFC 5124, February 2008.

   [RFC5234] Crocker, D. and Overell, P., "Augmented BNF for Syntax
             Specifications: ABNF", RFC 5234, January 2008.

   [RFC5576] Lennox, J., Ott, J., and Schierl, T., "Source-Specific
             Media Attributes in the Session Description Protocol",
             RFC 5576, June 2009.

   [RFC5583] Schierl, T. and Wenger, S., "Signaling Media Decoding
             Dependency in the Session Description Protocol (SDP)",
             RFC 5583, July 2009.

   [RFC6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup,
             "RTP Payload Format for H.264 Video", RFC 6184, May
             2011.

   [RFC6190] Wenger, S., Wang, Y.-K., Schierl, T., and A.
             Eleftheriadis, "RTP Payload Format for Scalable Video
             Coding", RFC 6190, May 2011.

13.2 Informative References

   [3GPDASH] 3GPP TS 26.247, "Transparent end-to-end Packet-switched
             Streaming Service (PSS); Progressive Download and
             Dynamic Adaptive Streaming over HTTP (3GP-DASH)",
             v12.1.0, December 2013.

   [3GPPFF]  3GPP TS 26.244, "Transparent end-to-end packet switched
             streaming service (PSS); 3GPP file format (3GP)",
             v12.20, December 2013.




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   [Girod99] Girod, B. and Faerber, F., "Feedback-based error
             control for mobile video transmission", Proceedings
             IEEE, Vol. 87, No. 10, pp. 1707-1723, October 1999.

   [HEVC draft v2]
             Draft version 2 of HEVC, "High Efficiency Video Coding
             (HEVC) Range Extensions text specification: Draft 7",
             JCT-VC document JCTVC-Q1005, 17th JCT-VC meeting, 27
             March - 4 April 2014, Valencia, Spain.

   [I-D.ietf-avtcore-rtp-multi-stream]
             Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
             "Sending Multiple Media Streams in a Single RTP
             Session", draft-ietf-avtcore-rtp-multi-stream-05 (work
             in progress), July 2014.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
             Holmberg, C., Alvestrand, H., and C. Jennings,
             "Multiplexing Negotiation Using Session Description
             Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
             bundle-negotiation-07 (work in progress), April 2014.

   [I-D.ietf-avtext-rtp-grouping-taxonomy]
             Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G.,
             and Burman, B. "A Taxonomy of Grouping Semantics and
             Mechanisms for Real-Time Transport", draft-ietf-avtext-
             rtp-grouping-taxonomy-02 (work in progress), June 2014.

   [ISOBMFF] IS0/IEC 14496-12 | 15444-12: "Information technology -
             Coding of audio-visual objects - Part 12: ISO base
             media file format" | "Information technology - JPEG
             2000 image coding system - Part 12: ISO base media file
             format", 2012.

   [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, July 2012, Stockholm,
             Sweden.





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   [MPEG2S]  ISO/IEC 13818-1, "Information technology - Generic
             coding of moving pictures and associated audio
             information: Systems", 2013.

   [MPEGDASH] ISO/IEC 23009-1, "Information technology - Dynamic
             adaptive streaming over HTTP (DASH) - Part 1: Media
             presentation description and segment formats", 2012.

   [RFC2326] Schulzrinne, H., Rao, A., and Lanphier R., "Real Time
             Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC2974] Handley, M., Perkins C., and Whelan E., "Session
             Announcement Protocol", RFC 2974, October 2000.

   [RFC5117] Westerlund, M. and Wenger, S., "RTP Topologies", RFC
             5117, January 2008.

   [RFC7201] Westerlund, M. and Perkins, C., "Options for Securing
             RTP Sessions", RFC 7201, April 2014.

   [RFC7202] Perkins, C. and Westerlund, M., "Securing the RTP
             Framework: Why RTP Does Not Mandate a Single Media
             Security Solution", RFC 7202, April 2014.

   [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),
             July 2005, Beijing, China.

14 Authors' Addresses

   Ye-Kui Wang
   Qualcomm Incorporated
   5775 Morehouse Drive
   San Diego, CA 92121, USA
   Phone: +1-858-651-8345
   EMail: yekuiw@qti.qualcomm.com

   Yago Sanchez
   Fraunhofer HHI
   Einsteinufer 37



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   D-10587 Berlin, Germany
   Phone: +49-30-31002-227
   Email: yago.sanchez@hhi.fraunhofer.de

   Thomas Schierl
   Fraunhofer HHI
   Einsteinufer 37
   D-10587 Berlin, Germany
   Phone: +49-30-31002-227
   Email: ts@thomas-schierl.de

   Stephan Wenger
   Vidyo, Inc.
   433 Hackensack Ave., 7th floor
   Hackensack, N.J. 07601, USA
   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





















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