path: root/doc/rfc/rfc9328.txt

Internet Engineering Task Force (IETF)                           S. Zhao
Request for Comments: 9328                                         Intel
Category: Standards Track                                      S. Wenger
ISSN: 2070-1721                                                  Tencent
                                                              Y. Sanchez
                                                          Fraunhofer HHI
                                                              Y.-K. Wang
                                                          Bytedance Inc.
                                                         M. M Hannuksela
                                                      Nokia Technologies
                                                           December 2022


          RTP Payload Format for Versatile Video Coding (VVC)

Abstract

   This memo describes an RTP payload format for the Versatile Video
   Coding (VVC) specification, which was published as both ITU-T
   Recommendation H.266 and ISO/IEC International Standard 23090-3.  VVC
   was developed by the Joint Video Experts Team (JVET).  The RTP
   payload format allows for packetization of one or more Network
   Abstraction Layer (NAL) units in each RTP packet payload, as well as
   fragmentation of a NAL unit into multiple RTP packets.  The payload
   format has wide applicability in videoconferencing, Internet video
   streaming, and high-bitrate entertainment-quality video, among other
   applications.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9328.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Overview of the VVC Codec
       1.1.1.  Coding-Tool Features (Informative)
       1.1.2.  Systems and Transport Interfaces (Informative)
       1.1.3.  High-Level Picture Partitioning (Informative)
       1.1.4.  NAL Unit Header
     1.2.  Overview of the Payload Format
   2.  Conventions
   3.  Definitions and Abbreviations
     3.1.  Definitions
       3.1.1.  Definitions from the VVC Specification
       3.1.2.  Definitions Specific to This Memo
     3.2.  Abbreviations
   4.  RTP Payload Format
     4.1.  RTP Header Usage
     4.2.  Payload Header Usage
     4.3.  Payload Structures
       4.3.1.  Single NAL Unit Packets
       4.3.2.  Aggregation Packets (APs)
       4.3.3.  Fragmentation Units
     4.4.  Decoding Order Number
   5.  Packetization Rules
   6.  De-packetization Process
   7.  Payload Format Parameters
     7.1.  Media Type Registration
     7.2.  Optional Parameters Definition
     7.3.  SDP Parameters
       7.3.1.  Mapping of Payload Type Parameters to SDP
       7.3.2.  Usage with SDP Offer/Answer Model
       7.3.3.  Multicast
       7.3.4.  Usage in Declarative Session Descriptions
       7.3.5.  Considerations for Parameter Sets
   8.  Use with Feedback Messages
     8.1.  Picture Loss Indication (PLI)
     8.2.  Full Intra Request (FIR)
   9.  Security Considerations
   10. Congestion Control
   11. IANA Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Versatile Video Coding specification was formally published as
   both ITU-T Recommendation H.266 [VVC] and ISO/IEC International
   Standard 23090-3 [ISO23090-3].  VVC is reported to provide
   significant coding efficiency gains over High Efficiency Video Coding
   [HEVC], also known as H.265, and other earlier video codecs.

   This memo specifies an RTP payload format for VVC.  It shares its
   basic design with the NAL-unit-based RTP payload formats of Advanced
   Video Coding (AVC) [RFC6184], Scalable Video Coding (SVC) [RFC6190],
   and High Efficiency Video Coding (HEVC) [RFC7798], as well as their
   respective predecessors.  With respect to design philosophy,
   security, congestion control, and overall implementation complexity,
   it has similar properties to those earlier payload format
   specifications.  This is a conscious choice, as at least [RFC6184] is
   widely deployed and generally known in the relevant implementer
   communities.  Certain scalability-related mechanisms known from
   [RFC6190] were incorporated into this document, as VVC version 1
   supports temporal, spatial, and signal-to-noise ratio (SNR)
   scalability.

1.1.  Overview of the VVC Codec

   VVC and HEVC share a similar hybrid video codec design.  In this
   memo, we provide a very brief overview of those features of VVC that
   are, in some form, addressed by the payload format specified herein.
   Implementers have to read, understand, and apply the ITU-T/ISO/IEC
   specifications pertaining to VVC to arrive at interoperable, well-
   performing implementations.

   Conceptually, both VVC and HEVC include a Video Coding Layer (VCL),
   which is often used to refer to the coding-tool features, and a NAL,
   which is often used to refer to the systems and transport interface
   aspects of the codecs.

1.1.1.  Coding-Tool Features (Informative)

   Coding-tool features are described below with occasional reference to
   the coding-tool set of HEVC, which is well known in the community.

   Similar to earlier hybrid-video-coding-based standards, including
   HEVC, the following basic video coding design is employed by VVC.  A
   prediction signal is first formed by either intra- or motion-
   compensated prediction, and the residual (the difference between the
   original and the prediction) is then coded.  The gains in coding
   efficiency are achieved by redesigning and improving almost all parts
   of the codec over earlier designs.  In addition, VVC includes several
   tools to make the implementation on parallel architectures easier.

   Finally, VVC includes temporal, spatial, and SNR scalability, as well
   as multiview coding support.

   Coding blocks and transform structure
      Among major coding-tool differences between HEVC and VVC, one of
      the important improvements is the more flexible coding tree
      structure in VVC, i.e., multi-type tree.  In addition to quadtree,
      binary and ternary trees are also supported, which contributes
      significant improvement in coding efficiency.  Moreover, the
      maximum size of a coding tree unit (CTU) is increased from 64x64
      to 128x128.  To improve the coding efficiency of chroma signal,
      luma-chroma-separated trees at CTU level may be employed for intra
      slices.  The square transforms in HEVC are extended to non-square
      transforms for rectangular blocks resulting from binary and
      ternary tree splits.  Besides, VVC supports multiple transform
      sets (MTSs), including DCT-2, DST-7, and DCT-8, as well as the
      non-separable secondary transform.  The transforms used in VVC can
      have different sizes with support for larger transform sizes.  For
      DCT-2, the transform sizes range from 2x2 to 64x64, and for DST-7
      and DCT-8, the transform sizes range from 4x4 to 32x32.  In
      addition, VVC also support sub-block transform for both intra- and
      inter-coded blocks.  For intra-coded blocks, intra sub-
      partitioning (ISP) may be used to allow sub-block-based intra
      prediction and transform.  For inter blocks, sub-block transform
      may be used assuming that only a part of an inter block has non-
      zero transform coefficients.

   Entropy coding
      Similar to HEVC, VVC uses a single entropy-coding engine, which is
      based on context adaptive binary arithmetic coding [CABAC] but
      with the support of multi-window sizes.  The window sizes can be
      initialized differently for different context models.  Due to such
      a design, it has more efficient adaptation speed and better coding
      efficiency.  A joint chroma residual coding scheme is applied to
      further exploit the correlation between the residuals of two color
      components.  In VVC, different residual coding schemes are applied
      for regular transform coefficients and residual samples generated
      using transform-skip mode.

   In-loop filtering
      VVC has more feature support in loop filters than HEVC.  The
      deblocking filter in VVC is similar to HEVC but operates at a
      smaller grid.  After deblocking and sample adaptive offset (SAO),
      an adaptive loop filter (ALF) may be used.  As a Wiener filter,
      ALF reduces distortion of decoded pictures.  Besides, VVC
      introduces a new module called luma mapping with chroma scaling to
      fully utilize the dynamic range of signal so that rate-distortion
      performance of both Standard Dynamic Range (SDR) and High Dynamic
      Range (HDR) content is improved.

   Motion prediction and coding
      Compared to HEVC, VVC introduces several improvements in this
      area.  First, there is the adaptive motion vector resolution
      (AMVR), which can save bit cost for motion vectors by adaptively
      signaling motion vector resolution.  Then, the affine motion
      compensation is included to capture complicated motion-like
      zooming and rotation.  Meanwhile, prediction refinement with the
      optical flow (PROF) with affine mode is further deployed to mimic
      affine motion at the pixel level.  Thirdly, the decoder-side
      motion vector refinement (DMVR) is a method to derive the motion
      vector at the decoder side based on block matching so that fewer
      bits may be spent on motion vectors.  Bidirectional optical flow
      (BDOF) is a similar method to PROF.  BDOF adds a sample-wise
      offset at the 4x4 sub-block level that is derived with equations
      based on gradients of the prediction samples and a motion
      difference relative to coding-unit (CU) motion vectors.
      Furthermore, merge with motion vector difference (MMVD) is a
      special mode that further signals a limited set of motion vector
      differences on top of merge mode.  In addition to MMVD, there are
      another three types of special merge modes, i.e., sub-block merge,
      triangle, and combined intra/inter prediction (CIIP).  The sub-
      block merge list includes one candidate of sub-block temporal
      motion vector prediction (SbTMVP) and up to four candidates of
      affine motion vectors.  Triangle is based on triangular block
      motion compensation.  CIIP combines intra and inter predictions
      with weighting.  Adaptive weighting may be employed with a block-
      level tool called bi-prediction with CU-based weighting (BCW),
      which provides more flexibility than in HEVC.

   Intra prediction and intra coding
      To capture the diversified local image texture directions with
      finer granularity, VVC supports 65 angular directions instead of
      33 directions in HEVC.  The intra mode coding is based on a 6-
      most-probable-modes scheme, and the 6 most probable modes are
      derived using the neighboring intra prediction directions.  In
      addition, to deal with the different distributions of intra
      prediction angles for different block aspect ratios, a wide-angle-
      intra-prediction (WAIP) scheme is applied in VVC by including
      intra prediction angles beyond those present in HEVC.  Unlike
      HEVC, which only allows using the most adjacent line of reference
      samples for intra prediction, VVC also allows using two further
      reference lines, known as multi-reference-line (MRL) intra
      prediction.  The additional reference lines can be only used for
      the 6 most probable intra prediction modes.  To capture the strong
      correlation between different color components, in VVC, a cross-
      component linear mode (CCLM) is utilized, which assumes a linear
      relationship between the luma sample values and their associated
      chroma samples.  For intra prediction, VVC also applies a
      position-dependent prediction combination (PDPC) for refining the
      prediction samples closer to the intra prediction block boundary.
      Matrix-based intra prediction (MIP) modes are also used in VVC,
      which generates an up to 8x8 intra prediction block using a
      weighted sum of downsampled neighboring reference samples, and the
      weights are hard-coded constants.

   Other coding-tool features
      VVC introduces dependent quantization (DQ) to reduce quantization
      error by state-based switching between two quantizers.

1.1.2.  Systems and Transport Interfaces (Informative)

   VVC inherits the basic systems and transport interface designs from
   HEVC and AVC.  These include the NAL-unit-based syntax structure, the
   hierarchical syntax and data unit structure, the supplemental
   enhancement information (SEI) message mechanism, and the video
   buffering model based on the hypothetical reference decoder (HRD).
   The scalability features of VVC are conceptually similar to the
   scalable extension of HEVC, known as SHVC.  The hierarchical syntax
   and data unit structure consists of parameter sets at various levels
   (i.e., decoder, sequence (pertaining to all), sequence (pertaining to
   a single), and picture), picture-level header parameters, slice-level
   header parameters, and lower-level parameters.

   A number of key components that influenced the network abstraction
   layer design of VVC, as well as this memo, are described below

   Decoding capability information
      The decoding capability information (DCI) includes parameters that
      stay constant for the lifetime of a VVC bitstream in the duration
      of a video conference, continuous video stream, and similar, i.e.,
      any video that is processed by a decoder between setup and
      teardown.  For streaming, the requirement of constant parameters
      pertains through splicing.  Such information includes profile,
      level, and sub-profile information to determine a maximum
      capability interop point that is guaranteed to never be exceeded,
      even if splicing of video sequences occurs within a session.  It
      further includes constraint fields (most of which are flags),
      which can optionally be set to indicate that the video bitstream
      will be constrained in the use of certain features, as indicated
      by the values of those fields.  With this, a bitstream can be
      labeled as not using certain tools, which allows, among other
      things, for resource allocation in a decoder implementation.

   Video parameter set
      The video parameter set (VPS) pertains to one or more coded video
      sequences (CVSs) of multiple layers covering the same range of
      access units and includes, among other information, decoding
      dependency expressed as information for reference-picture-list
      construction of enhancement layers.  The VPS provides a "big
      picture" of a scalable sequence, including what types of operation
      points are provided; the profile, tier, and level of the operation
      points; and some other high-level properties of the bitstream that
      can be used as the basis for session negotiation and content
      selection, etc.  One VPS may be referenced by one or more sequence
      parameter sets.

   Sequence parameter set
      The sequence parameter set (SPS) contains syntax elements
      pertaining to a coded layer video sequence (CLVS), which is a
      group of pictures belonging to the same layer, starting with a
      random access point, and followed by pictures that may depend on
      each other until the next random access point picture.  In MPEG-2,
      the equivalent of a CVS was a group of pictures (GOP), which
      normally started with an I frame and was followed by P and B
      frames.  While more complex in its options of random access
      points, VVC retains this basic concept.  One remarkable difference
      of VVC is that a CLVS may start with a Gradual Decoding Refresh
      (GDR) picture without requiring presence of traditional random
      access points in the bitstream, such as instantaneous decoding
      refresh (IDR) or clean random access (CRA) pictures.  In many TV-
      like applications, a CVS contains a few hundred milliseconds to a
      few seconds of video.  In video conferencing (without switching
      Multipoint Control Units (MCUs) involved), a CVS can be as long in
      duration as the whole session.

   Picture and adaptation parameter set
      The picture parameter set (PPS) and the adaptation parameter set
      (APS) carry information pertaining to zero or more pictures and
      zero or more slices, respectively.  The PPS contains information
      that is likely to stay constant from picture to picture, at least
      for pictures for a certain type, whereas the APS contains
      information, such as adaptive loop filter coefficients, that are
      likely to change from picture to picture or even within a picture.
      A single APS is referenced by all slices of the same picture if
      that APS contains information about luma mapping with chroma
      scaling (LMCS) or a scaling list.  Different APSs containing ALF
      parameters can be referenced by slices of the same picture.

   Picture header
      A picture header (PH) contains information that is common to all
      slices that belong to the same picture.  Being able to send that
      information as a separate NAL unit when pictures are split into
      several slices allows for saving bitrate, compared to repeating
      the same information in all slices.  However, there might be
      scenarios where low-bitrate video is transmitted using a single
      slice per picture.  Having a separate NAL unit to convey that
      information incurs in an overhead for such scenarios.  For such
      scenarios, the picture header syntax structure is directly
      included in the slice header, instead of its own NAL unit.  The
      mode of the picture header syntax structure being included in its
      own NAL unit or not can only be switched on/off for an entire CLVS
      and can only be switched off when, in the entire CLVS, each
      picture contains only one slice.

   Profile, tier, and level
      The profile, tier, and level syntax structures in DCI, VPS, and
      SPS contain profile, tier, and level information for all layers
      that refer to the DCI, for layers associated with one or more
      output layer sets specified by the VPS, and for any layer that
      refers to the SPS, respectively.

   Sub-profiles
      Within the VVC specification, a sub-profile is a 32-bit number,
      coded according to ITU-T Recommendation T.35, that does not carry
      semantics.  It is carried in the profile_tier_level structure and
      hence is (potentially) present in the DCI, VPS, and SPS.  External
      registration bodies can register a T.35 codepoint with ITU-T
      registration authorities and associate with their registration a
      description of bitstream restrictions beyond the profiles defined
      by ITU-T and ISO/IEC.  This would allow encoder manufacturers to
      label the bitstreams generated by their encoder as complying with
      such sub-profile.  It is expected that upstream standardization
      organizations (such as Digital Video Broadcasting (DVB) and
      Advanced Television Systems Committee (ATSC)), as well as walled-
      garden video services, will take advantage of this labeled system.
      In contrast to "normal" profiles, it is expected that sub-profiles
      may indicate encoder choices traditionally left open in the
      (decoder-centric) video coding specifications, such as GOP
      structures, minimum/maximum Quantizer Parameter (QP) values, and
      the mandatory use of certain tools or SEI messages.

   General constraint fields
      The profile_tier_level structure carries a considerable number of
      constraint fields (most of which are flags), which an encoder can
      use to indicate to a decoder that it will not use a certain tool
      or technology.  They were included in reaction to a perceived
      market need to label a bitstream as not exercising a certain tool
      that has become commercially unviable.

   Temporal scalability support
      VVC includes support of temporal scalability, by the inclusion of
      the signaling of TemporalId in the NAL unit header, the
      restriction that pictures of a particular temporal sublayer cannot
      be used for inter prediction reference by pictures of a lower
      temporal sublayer, the sub-bitstream extraction process, and the
      requirement that each sub-bitstream extraction output be a
      conforming bitstream.  Media-Aware Network Elements (MANEs) can
      utilize the TemporalId in the NAL unit header for stream
      adaptation purposes based on temporal scalability.

   Reference picture resampling (RPR)
      In AVC and HEVC, the spatial resolution of pictures cannot change
      unless a new sequence using a new SPS starts, with an intra random
      access point (IRAP) picture.  VVC enables picture resolution
      change within a sequence at a position without encoding an IRAP
      picture, which is always intra coded.  This feature is sometimes
      referred to as reference picture resampling (RPR), as the feature
      needs resampling of a reference picture used for inter prediction
      when that reference picture has a different resolution than the
      current picture being decoded.  RPR allows resolution change
      without the need of coding an IRAP picture and hence avoids a
      momentary bit rate spike caused by an IRAP picture in streaming or
      video conferencing scenarios, e.g., to cope with network condition
      changes.  RPR can also be used in application scenarios wherein
      zooming of the entire video region or some region of interest is
      needed.

   Spatial, SNR, and multiview scalability
      VVC includes support for spatial, SNR, and multiview scalability.
      Scalable video coding is widely considered to have technical
      benefits and enrich services for various video applications.
      Until recently, however, the functionality has not been included
      in the first version of specifications of the video codecs.  In
      VVC, however, all those forms of scalability are supported in the
      first version of VVC natively through the signaling of the
      nuh_layer_id in the NAL unit header, the VPS that associates
      layers with the given nuh_layer_id to each other, reference
      picture selection, reference picture resampling for spatial
      scalability, and a number of other mechanisms not relevant for
      this memo.

      Spatial scalability
         With the existence of reference picture resampling (RPR), the
         additional burden for scalability support is just a
         modification of the high-level syntax (HLS).  The inter-layer
         prediction is employed in a scalable system to improve the
         coding efficiency of the enhancement layers.  In addition to
         the spatial and temporal motion-compensated predictions that
         are available in a single-layer codec, the inter-layer
         prediction in VVC uses the possibly resampled video data of the
         reconstructed reference picture from a reference layer to
         predict the current enhancement layer.  The resampling process
         for inter-layer prediction, when used, is performed at the
         block level, reusing the existing interpolation process for
         motion compensation in single-layer coding.  It means that no
         additional resampling process is needed to support spatial
         scalability.

      SNR scalability
         SNR scalability is similar to spatial scalability except that
         the resampling factors are 1:1.  In other words, there is no
         change in resolution, but there is inter-layer prediction.

      Multiview scalability
         The first version of VVC also supports multiview scalability,
         wherein a multi-layer bitstream carries layers representing
         multiple views, and one or more of the represented views can be
         output at the same time.

   SEI messages
      Supplemental enhancement information (SEI) messages are
      information in the bitstream that do not influence the decoding
      process as specified in the VVC specification but address issues
      of representation/rendering of the decoded bitstream, label the
      bitstream for certain applications, and other, similar tasks.  The
      overall concept of SEI messages and many of the messages
      themselves has been inherited from the AVC and HEVC
      specifications.  Except for the SEI messages that affect the
      specification of the hypothetical reference decoder (HRD), other
      SEI messages for use in the VVC environment, which are generally
      useful also in other video coding technologies, are not included
      in the main VVC specification but in a companion specification
      [VSEI].

1.1.3.  High-Level Picture Partitioning (Informative)

   VVC inherited the concept of tiles and wavefront parallel processing
   (WPP) from HEVC, with some minor to moderate differences.  The basic
   concept of slices was kept in VVC but designed in an essentially
   different form.  VVC is the first video coding standard that includes
   subpictures as a feature, which provides the same functionality as
   HEVC motion-constrained tile sets (MCTSs) but designed differently to
   have better coding efficiency and to be friendlier for usage in
   application systems.  More details of these differences are described
   below.

   Tiles and WPP
      Same as in HEVC, a picture can be split into tile rows and tile
      columns in VVC, in-picture prediction across tile boundaries is
      disallowed, etc.  However, the syntax for signaling of tile
      partitioning has been simplified by using a unified syntax design
      for both the uniform and the non-uniform mode.  In addition,
      signaling of entry point offsets for tiles in the slice header is
      optional in VVC, while it is mandatory in HEVC.  The WPP design in
      VVC has two differences compared to HEVC: i) the CTU row delay is
      reduced from two CTUs to one CTU, and ii) signaling of entry point
      offsets for WPP in the slice header is optional in VVC while it is
      mandatory in HEVC.

   Slices
      In VVC, the conventional slices based on CTUs (as in HEVC) or
      macroblocks (as in AVC) have been removed.  The main reasoning
      behind this architectural change is as follows.  The advances in
      video coding since 2003 (the publication year of AVC v1) have been
      such that slice-based error concealment has become practically
      impossible due to the ever-increasing number and efficiency of in-
      picture and inter-picture prediction mechanisms.  An error-
      concealed picture is the decoding result of a transmitted coded
      picture for which there is some data loss (e.g., loss of some
      slices) of the coded picture or a reference picture, as at least
      some part of the coded picture is not error-free (e.g., that
      reference picture was an error-concealed picture).  For example,
      when one of the multiple slices of a picture is lost, it may be
      error-concealed using an interpolation of the neighboring slices.
      While advanced video coding prediction mechanisms provide
      significantly higher coding efficiency, they also make it harder
      for machines to estimate the quality of an error-concealed
      picture, which was already a hard problem with the use of simpler
      prediction mechanisms.  Advanced in-picture prediction mechanisms
      also cause the coding efficiency loss due to splitting a picture
      into multiple slices to be more significant.  Furthermore, network
      conditions become significantly better while, at the same time,
      techniques for dealing with packet losses have become
      significantly improved.  As a result, very few implementations
      have recently used slices for maximum-transmission-unit-size
      matching.  Instead, substantially all applications where low-delay
      error resilience is required (e.g., video telephony and video
      conferencing) rely on system/transport-level error resilience
      (e.g., retransmission or forward error correction) and/or picture-
      based error resilience tools (e.g., feedback-based error
      resilience, insertion of IRAPs, scalability with a higher
      protection level of the base layer, and so on).  Considering all
      the above, nowadays, it is very rare that a picture that cannot be
      correctly decoded is passed to the decoder, and when such a rare
      case occurs, the system can afford to wait for an error-free
      picture to be decoded and available for display without resulting
      in frequent and long periods of picture freezing seen by end
      users.

      Slices in VVC have two modes: rectangular slices and raster-scan
      slices.  The rectangular slice, as indicated by its name, covers a
      rectangular region of the picture.  Typically, a rectangular slice
      consists of several complete tiles.  However, it is also possible
      that a rectangular slice is a subset of a tile and consists of one
      or more consecutive, complete CTU rows within a tile.  A raster-
      scan slice consists of one or more complete tiles in a tile
      raster-scan order; hence, the region covered by raster-scan slices
      need not but could have a non-rectangular shape, but it may also
      happen to have the shape of a rectangle.  The concept of slices in
      VVC is therefore strongly linked to or based on tiles instead of
      CTUs (as in HEVC) or macroblocks (as in AVC).

   Subpictures
      VVC is the first video coding standard that includes the support
      of subpictures as a feature.  Each subpicture consists of one or
      more complete rectangular slices that collectively cover a
      rectangular region of the picture.  A subpicture may be either
      specified to be extractable (i.e., coded independently of other
      subpictures of the same picture and of earlier pictures in
      decoding order) or not extractable.  Regardless of whether a
      subpicture is extractable or not, the encoder can control whether
      in-loop filtering (including deblocking, SAO, and ALF) is applied
      across the subpicture boundaries individually for each subpicture.

      Functionally, subpictures are similar to the motion-constrained
      tile sets (MCTSs) in HEVC.  They both allow independent coding and
      extraction of a rectangular subset of a sequence of coded pictures
      for use cases like viewport-dependent 360-degree video streaming
      optimization and region of interest (ROI) applications.

      There are several important design differences between subpictures
      and MCTSs.  First, the subpictures featured in VVC allow motion
      vectors of a coding block to point outside of the subpicture, even
      when the subpicture is extractable by applying sample padding at
      the subpicture boundaries, in this case, similarly as at picture
      boundaries.  Second, additional changes were introduced for the
      selection and derivation of motion vectors in the merge mode and
      in the decoder-side motion vector refinement process of VVC.  This
      allows higher coding efficiency compared to the non-normative
      motion constraints applied at the encoder-side for MCTSs.  Third,
      rewriting of slice headers (SHs) (and PH NAL units, when present)
      is not needed when extracting one or more extractable subpictures
      from a sequence of pictures to create a sub-bitstream that is a
      conforming bitstream.  In sub-bitstream extractions based on HEVC
      MCTSs, rewriting of SHs is needed.  Note that, in both HEVC MCTSs
      extraction and VVC subpictures extraction, rewriting of SPSs and
      PPSs is needed.  However, typically, there are only a few
      parameter sets in a bitstream, whereas each picture has at least
      one slice; therefore, rewriting of SHs can be a significant burden
      for application systems.  Fourth, slices of different subpictures
      within a picture are allowed to have different NAL unit types.
      Fifth, VVC specifies HRD and level definitions for subpicture
      sequences, thus the conformance of the sub-bitstream of each
      extractable subpicture sequence can be ensured by encoders.

1.1.4.  NAL Unit Header

   VVC maintains the NAL unit concept of HEVC with modifications.  VVC
   uses a two-byte NAL unit header, as shown in Figure 1.  The payload
   of a NAL unit refers to the NAL unit excluding the NAL unit header.

                     +---------------+---------------+
                     |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |F|Z| LayerID   |  Type   | TID |
                     +---------------+---------------+

             Figure 1: The Structure of the VVC NAL Unit Header

   The semantics of the fields in the NAL unit header are as specified
   in VVC and described briefly below for convenience.  In addition to
   the name and size of each field, the corresponding syntax element
   name in VVC is also provided.

   F: 1 bit
      forbidden_zero_bit.  This field is required to be zero in VVC.
      Note that the inclusion of this bit in the NAL unit header was to
      enable transport of VVC video over MPEG-2 transport systems
      (avoidance of start code emulations) [MPEG2S].  In the context of
      this payload format, the value 1 may be used to indicate a syntax
      violation, e.g., for a NAL unit resulted from aggregating a number
      of fragmented units of a NAL unit but missing the last fragment,
      as described in the last sentence of Section 4.3.3.

   Z: 1 bit
      nuh_reserved_zero_bit.  This field is required to be zero in VVC,
      and reserved for future extensions by ITU-T and ISO/IEC.
      This memo does not overload the "Z" bit for local extensions a)
      because overloading the "F" bit is sufficient and b) in order to
      preserve the usefulness of this memo to possible future versions
      of [VVC].

   LayerId: 6 bits
      nuh_layer_id.  This field identifies the layer a NAL unit belongs
      to, wherein a layer may be, e.g., a spatial scalable layer, a
      quality scalable layer, a layer containing a different view, etc.

   Type: 5 bits
      nal_unit_type.  This field specifies the NAL unit type, as defined
      in Table 5 of [VVC].  For a reference of all currently defined NAL
      unit types and their semantics, please refer to Section 7.4.2.2 in
      [VVC].

   TID: 3 bits
      nuh_temporal_id_plus1.  This field specifies the temporal
      identifier of the NAL unit plus 1.  The value of TemporalId is
      equal to TID minus 1.  A TID value of 0 is illegal to ensure that
      there is at least one bit in the NAL unit header equal to 1 in
      order to enable the consideration of start code emulations in the
      NAL unit payload data independent of the NAL unit header.

1.2.  Overview of the Payload Format

   This payload format defines the following processes required for
   transport of VVC coded data over RTP [RFC3550]:

   *  usage of the RTP header with this payload format

   *  packetization of VVC coded NAL units into RTP packets using three
      types of payload structures: a single NAL unit packet, aggregation
      packet, and fragment unit

   *  transmission of VVC NAL units of the same bitstream within a
      single RTP stream

   *  media type parameters to be used with the Session Description
      Protocol (SDP) [RFC8866]

   *  usage of RTCP feedback messages

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Definitions and Abbreviations

3.1.  Definitions

   This document uses the terms and definitions of VVC.  Section 3.1.1
   lists relevant definitions from [VVC] for convenience.  Section 3.1.2
   provides definitions specific to this memo.  All the used terms and
   definitions in this memo are verbatim copies from the [VVC]
   specification.

3.1.1.  Definitions from the VVC Specification

   Access unit (AU):
      A set of PUs that belong to different layers and contain coded
      pictures associated with the same time for output from the DPB.

   Adaptation parameter set (APS):
      A syntax structure containing syntax elements that apply to zero
      or more slices as determined by zero or more syntax elements found
      in slice headers.

   Bitstream:
      A sequence of bits, in the form of a NAL unit stream or a byte
      stream, that forms the representation of a sequence of AUs forming
      one or more coded video sequences (CVSs).

   Coded picture:
      A coded representation of a picture comprising VCL NAL units with
      a particular value of nuh_layer_id within an AU and containing all
      CTUs of the picture.

   Clean random access (CRA) PU:
      A PU in which the coded picture is a CRA picture.

   Clean random access (CRA) picture:
      An IRAP picture for which each VCL NAL unit has nal_unit_type
      equal to CRA_NUT.

   Coded video sequence (CVS):
      A sequence of AUs that consists, in decoding order, of a CVSS AU,
      followed by zero or more AUs that are not CVSS AUs, including all
      subsequent AUs up to but not including any subsequent AU that is a
      CVSS AU.

   Coded video sequence start (CVSS) AU:
      An AU in which there is a PU for each layer in the CVS and the
      coded picture in each PU is a CLVSS picture.

   Coded layer video sequence (CLVS):
      A sequence of PUs with the same value of nuh_layer_id that
      consists, in decoding order, of a CLVSS PU, followed by zero or
      more PUs that are not CLVSS PUs, including all subsequent PUs up
      to but not including any subsequent PU that is a CLVSS PU.

   Coded layer video sequence start (CLVSS) PU:
      A PU in which the coded picture is a CLVSS picture.

   Coded layer video sequence start (CLVSS) picture:
      A coded picture that is an IRAP picture with
      NoOutputBeforeRecoveryFlag equal to 1 or a GDR picture with
      NoOutputBeforeRecoveryFlag equal to 1.

   Coding Tree Block (CTB):
      An NxN block of samples for some value of N such that the division
      of a component into CTBs is a partitioning.

   Coding tree unit (CTU):
      A CTB of luma samples, two corresponding CTBs of chroma samples of
      a picture that has three sample arrays, or a CTB of samples of a
      monochrome picture or a picture that is coded using three separate
      colour planes and syntax structures used to code the samples.

   Coding Unit (CU):
      A coding block of luma samples, two corresponding coding blocks of
      chroma samples of a picture that has three sample arrays in the
      single tree mode, or a coding block of luma samples of a picture
      that has three sample arrays in the dual tree mode, or two coding
      blocks of chroma samples of a picture that has three sample arrays
      in the dual tree mode, or a coding block of samples of a
      monochrome picture, and syntax structures used to code the
      samples.

   Decoding Capability Information (DCI):
      A syntax structure containing syntax elements that apply to the
      entire bitstream.

   Decoded picture buffer (DPB):
      A buffer holding decoded pictures for reference, output
      reordering, or output delay specified for the hypothetical
      reference decoder.

   Gradual decoding refresh (GDR) picture:
      A picture for which each VCL NAL unit has nal_unit_type equal to
      GDR_NUT.

   Instantaneous decoding refresh (IDR) PU:
      A PU in which the coded picture is an IDR picture.

   Instantaneous decoding refresh (IDR) picture:
      An IRAP picture for which each VCL NAL unit has nal_unit_type
      equal to IDR_W_RADL or IDR_N_LP.

   Intra random access point (IRAP) AU:
      An AU in which there is a PU for each layer in the CVS and the
      coded picture in each PU is an IRAP picture.

   Intra random access point (IRAP) PU:
      A PU in which the coded picture is an IRAP picture.

   Intra random access point (IRAP) picture:
      A coded picture for which all VCL NAL units have the same value of
      nal_unit_type in the range of IDR_W_RADL to CRA_NUT, inclusive.

   Layer:
      A set of VCL NAL units that all have a particular value of
      nuh_layer_id and the associated non-VCL NAL units.

   Network abstraction layer (NAL) unit:
      A syntax structure containing an indication of the type of data to
      follow and bytes containing that data in the form of an RBSP
      interspersed as necessary with emulation prevention bytes.

   Network abstraction layer (NAL) unit stream:
      A sequence of NAL units.

   Output Layer Set (OLS):
      A set of layers for which one or more layers are specified as the
      output layers.

   Operation point (OP):
      A temporal subset of an OLS, identified by an OLS index and a
      highest value of TemporalId.

   Picture Header (PH):
      A syntax structure containing syntax elements that apply to all
      slices of a coded picture.

   Picture parameter set (PPS):
      A syntax structure containing syntax elements that apply to zero
      or more entire coded pictures as determined by a syntax element
      found in each slice header.

   Picture unit (PU):
      A set of NAL units that are associated with each other according
      to a specified classification rule, are consecutive in decoding
      order, and contain exactly one coded picture.

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

   Raw Byte Sequence Payload (RBSP):
      A syntax structure containing an integer number of bytes that is
      encapsulated in a NAL unit and is either empty or has the form of
      a string of data bits containing syntax elements followed by an
      RBSP stop bit and zero or more subsequent bits equal to 0.

   Sequence parameter set (SPS):
      A syntax structure containing syntax elements that apply to zero
      or more entire CLVSs as determined by the content of a syntax
      element found in the PPS referred to by a syntax element found in
      each picture header.

   Slice:
      An integer number of complete tiles or an integer number of
      consecutive complete CTU rows within a tile of a picture that are
      exclusively contained in a single NAL unit.

   Slice header (SH):
      A part of a coded slice containing the data elements pertaining to
      all tiles or CTU rows within a tile represented in the slice.

   Sublayer:
      A temporal scalable layer of a temporal scalable bitstream
      consisting of VCL NAL units with a particular value of the
      TemporalId variable, and the associated non-VCL NAL units.

   Subpicture:
      A rectangular region of one or more slices within a picture.

   Sublayer representation:
      A subset of the bitstream consisting of NAL units of a particular
      sublayer and the lower sublayers.

   Tile:
      A rectangular region of CTUs within a particular tile column and a
      particular tile row in a picture.

   Tile column:
      A rectangular region of CTUs having a height equal to the height
      of the picture and a width specified by syntax elements in the
      picture parameter set.

   Tile row:
      A rectangular region of CTUs having a height specified by syntax
      elements in the picture parameter set and a width equal to the
      width of the picture.

   Video coding layer (VCL) NAL unit:
      A collective term for coded slice NAL units and the subset of NAL
      units that have reserved values of nal_unit_type that are
      classified as VCL NAL units in this Specification.

3.1.2.  Definitions Specific to This Memo

   Media-Aware Network Element (MANE):
      A network element, such as a middlebox, selective forwarding unit,
      or application-layer gateway that is capable of parsing certain
      aspects of the RTP payload headers or the RTP payload and reacting
      to their contents.

         |  Informative note: The concept of a MANE goes beyond normal
         |  routers or gateways in that a MANE has to be aware of the
         |  signaling (e.g., to learn about the payload type mappings of
         |  the media streams), and in that it has to be trusted when
         |  working with Secure RTP (SRTP).  The advantage of using
         |  MANEs is that they allow packets to be dropped according to
         |  the needs of the media coding.  For example, if a MANE has
         |  to drop packets due to congestion on a certain link, it can
         |  identify and remove those packets whose elimination produces
         |  the least adverse effect on the user experience.  After
         |  dropping packets, MANEs must rewrite RTCP packets to match
         |  the changes to the RTP stream, as specified in Section 7 of
         |  [RFC3550].

   NAL unit decoding order:
      A NAL unit order that conforms to the constraints on NAL unit
      order given in Section 7.4.2.4 in [VVC], follow the order of NAL
      units in the bitstream.

   RTP stream (see [RFC7656]):
      Within the scope of this memo, one RTP stream is utilized to
      transport a VVC bitstream, which may contain one or more layers,
      and each layer may contain one or more temporal sublayers.

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

3.2.  Abbreviations

   AU      Access Unit

   AP      Aggregation Packet

   APS     Adaptation Parameter Set

   CTU     Coding Tree Unit

   CVS     Coded Video Sequence

   DPB     Decoded Picture Buffer

   DCI     Decoding Capability Information

   DON     Decoding Order Number

   FIR     Full Intra Request

   FU      Fragmentation Unit

   GDR     Gradual Decoding Refresh

   HRD     Hypothetical Reference Decoder

   IDR     Instantaneous Decoding Refresh

   IRAP    Intra Random Access Point

   MANE    Media-Aware Network Element

   MTU     Maximum Transfer Unit

   NAL     Network Abstraction Layer

   NALU    Network Abstraction Layer Unit

   OLS     Output Layer Set

   PLI     Picture Loss Indication

   PPS     Picture Parameter Set

   RPSI    Reference Picture Selection Indication

   SEI     Supplemental Enhancement Information

   SLI     Slice Loss Indication

   SPS     Sequence Parameter Set

   VCL     Video Coding Layer

   VPS     Video Parameter Set

4.  RTP Payload Format

4.1.  RTP Header Usage

   The format of the RTP header is specified in [RFC3550] (reprinted as
   Figure 2 for convenience).  This payload format uses the fields of
   the header in a manner consistent with that specification.

   The RTP payload (and the settings for some RTP header bits) for
   aggregation packets and fragmentation units are specified in Sections
   4.3.2 and 4.3.3, respectively.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                             ....                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: RTP Header According to RFC 3550

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

   Marker bit (M): 1 bit
      Set for the last packet, in transmission order, among each set of
      packets that contain NAL units of one access unit.  This is in
      line with the normal use of the M bit in video formats to allow an
      efficient playout buffer handling.

   Payload Type (PT): 7 bits
      The assignment of an RTP payload type for this new packet format
      is outside the scope of this document and will not be specified
      here.  The assignment of a payload type has to be performed either
      through the profile used or in a dynamic way.

   Sequence Number (SN): 16 bits
      Set and used in accordance with [RFC3550].

   Timestamp: 32 bits
      The RTP timestamp is set to the sampling timestamp of the content.
      A 90 kHz clock rate MUST be used.  If the NAL unit has no timing
      properties of its own (e.g., parameter set and SEI NAL units), the
      RTP timestamp MUST be set to the RTP timestamp of the coded
      pictures of the access unit in which the NAL unit (according to
      Section 7.4.2.4 of [VVC]) is included.  Receivers MUST use the RTP
      timestamp for the display process, even when the bitstream
      contains picture timing SEI messages or decoding unit information
      SEI messages, as specified in [VVC].

         |  Informative note: When picture timing SEI messages are
         |  present, the RTP sender is responsible to ensure that the
         |  RTP timestamps are consistent with the timing information
         |  carried in the picture timing SEI messages.

   Synchronization source (SSRC): 32 bits
      Used to identify the source of the RTP packets.  A single SSRC is
      used for all parts of a single bitstream.

4.2.  Payload Header Usage

   The first two bytes of the payload of an RTP packet are referred to
   as the payload header.  The payload header consists of the same
   fields (F, Z, LayerId, Type, and TID) as the NAL unit header shown in
   Section 1.1.4, irrespective of the type of the payload structure.

   The TID value indicates (among other things) the relative importance
   of an RTP packet, for example, because NAL units belonging to higher
   temporal sublayers are not used for the decoding of lower temporal
   sublayers.  A lower value of TID indicates a higher importance.  More
   important NAL units MAY be better protected against transmission
   losses than less-important NAL units.

4.3.  Payload Structures

   Three different types of RTP packet payload structures are specified.
   A receiver can identify the type of an RTP packet payload through the
   Type field in the payload header.

   The three different payload structures are as follows:

   *  Single NAL unit packet: Contains a single NAL unit in the payload,
      and the NAL unit header of the NAL unit also serves as the payload
      header.  This payload structure is specified in Section 4.3.1.

   *  Aggregation Packet (AP): Contains more than one NAL unit within
      one access unit.  This payload structure is specified in
      Section 4.3.2.

   *  Fragmentation Unit (FU): Contains a subset of a single NAL unit.
      This payload structure is specified in Section 4.3.3.

4.3.1.  Single NAL Unit Packets

   A single NAL unit packet contains exactly one NAL unit and consists
   of a payload header, as defined in Table 5 of [VVC] (denoted here as
   PayloadHdr), following with a conditional 16-bit DONL field (in
   network byte order), and the NAL unit payload data (the NAL unit
   excluding its NAL unit header) of the contained NAL unit, as shown in
   Figure 3.

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

            Figure 3: The Structure of a Single NAL Unit Packet

   The DONL field, when present, specifies the value of the 16 least
   significant bits of the decoding order number of the contained NAL
   unit.  If sprop-max-don-diff (defined in Section 7.2) is greater than
   0, the DONL field MUST be present, and the variable DON for the
   contained NAL unit is derived as equal to the value of the DONL
   field.  Otherwise (sprop-max-don-diff is equal to 0), the DONL field
   MUST NOT be present.

4.3.2.  Aggregation Packets (APs)

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

   An AP aggregates NAL units of one access unit, and it MUST NOT
   contain NAL units from more than one AU.  Each NAL unit to be carried
   in an AP is encapsulated in an aggregation unit.  NAL units
   aggregated in one AP are included in NAL-unit-decoding order.

   An AP consists of a payload header, as defined in Table 5 of [VVC]
   (denoted here as PayloadHdr with Type=28), followed by two or more
   aggregation units, as shown in Figure 4.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    PayloadHdr (Type=28)       |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                                                               |
    |             two or more aggregation units                     |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: The Structure of an Aggregation Packet

   The fields in the payload header of an AP are set as follows.  The F
   bit MUST be equal to 0 if the F bit of each aggregated NAL unit is
   equal to zero; otherwise, it MUST be equal to 1.  The Type field MUST
   be equal to 28.

   The value of LayerId MUST be equal to the lowest value of LayerId of
   all the aggregated NAL units.  The value of TID MUST be the lowest
   value of TID of all the aggregated NAL units.

      |  Informative note: All VCL NAL units in an AP have the same TID
      |  value since they belong to the same access unit.  However, an
      |  AP may contain non-VCL NAL units for which the TID value in the
      |  NAL unit header may be different than the TID value of the VCL
      |  NAL units in the same AP.

      |  Informative note: If a system envisions subpicture-level or
      |  picture-level modifications, for example, by removing
      |  subpictures or pictures of a particular layer, a good design
      |  choice on the sender's side would be to aggregate NAL units
      |  belonging to only the same subpicture or picture of a
      |  particular layer.

   An AP MUST carry at least two aggregation units and can carry as many
   aggregation units as necessary; however, the total amount of data in
   an AP obviously MUST fit into an IP packet, and the size SHOULD be
   chosen so that the resulting IP packet is smaller than the MTU size
   in order to avoid IP layer fragmentation.  An AP MUST NOT contain the
   FUs specified in Section 4.3.3.  APs MUST NOT be nested, i.e., an AP
   cannot contain another AP.

   The first aggregation unit in an AP consists of a conditional 16-bit
   DONL field (in network byte order), followed by 16 bits of unsigned
   size information (in network byte order) that indicate the size of
   the NAL unit in bytes (excluding these two octets but including the
   NAL unit header), followed by the NAL unit itself, including its NAL
   unit header, as shown in Figure 5.

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

       Figure 5: The Structure of the First Aggregation Unit in an AP

      |  Informative note: The first octet of Figure 5 (indicated by the
      |  first colon) belongs to a previous aggregation unit.  It is
      |  depicted to emphasize that aggregation units are octet aligned
      |  only.  Similarly, the NAL unit carried in the aggregation unit
      |  can terminate at the octet boundary.

   The DONL field, when present, specifies the value of the 16 least
   significant bits of the decoding order number of the aggregated NAL
   unit.

   If sprop-max-don-diff is greater than 0, the DONL field MUST be
   present in an aggregation unit that is the first aggregation unit in
   an AP, and the variable DON for the aggregated NAL unit is derived as
   equal to the value of the DONL field, and the variable DON for an
   aggregation unit that is not the first aggregation unit in an AP-
   aggregated NAL unit is derived as equal to the DON of the preceding
   aggregated NAL unit in the same AP plus 1 modulo 65536.  Otherwise
   (sprop-max-don-diff is equal to 0), the DONL field MUST NOT be
   present in an aggregation unit that is the first aggregation unit in
   an AP.

   An aggregation unit that is not the first aggregation unit in an AP
   will be followed immediately by 16 bits of unsigned size information
   (in network byte order) that indicate the size of the NAL unit in
   bytes (excluding these two octets but including the NAL unit header),
   followed by the NAL unit itself, including its NAL unit header, as
   shown in Figure 6.

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

    Figure 6: The Structure of an Aggregation Unit That Is Not the First
                         Aggregation Unit in an AP

      |  Informative note: The first octet of Figure 6 (indicated by the
      |  first colon) belongs to a previous aggregation unit.  It is
      |  depicted to emphasize that aggregation units are octet aligned
      |  only.  Similarly, the NAL unit carried in the aggregation unit
      |  can terminate at the octet boundary.

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

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   PayloadHdr (Type=28)        |         NALU 1 Size           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          NALU 1 HDR           |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         NALU 1 Data           |
    |                   . . .                                       |
    |                                                               |
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  . . .        | NALU 2 Size                   | NALU 2 HDR    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | NALU 2 HDR    |                                               |
    +-+-+-+-+-+-+-+-+              NALU 2 Data                      |
    |                   . . .                                       |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 7: An Example of an AP Packet Containing Two Aggregation
                        Units without the DONL Field

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

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   PayloadHdr (Type=28)        |        NALU 1 DONL            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          NALU 1 Size          |            NALU 1 HDR         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                 NALU 1 Data   . . .                           |
    |                                                               |
    +        . . .                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :          NALU 2 Size          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          NALU 2 HDR           |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+          NALU 2 Data          |
    |                                                               |
    |        . . .                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 8: An Example of an AP Containing Two Aggregation Units
                            with the DONL Field

4.3.3.  Fragmentation Units

   Fragmentation Units (FUs) are introduced to enable fragmenting a
   single NAL unit into multiple RTP packets, possibly without
   cooperation or knowledge of the [VVC] encoder.  A fragment of a NAL
   unit consists of an integer number of consecutive octets of that NAL
   unit.  Fragments of the same NAL unit MUST be sent in consecutive
   order with ascending RTP sequence numbers (with no other RTP packets
   within the same RTP stream being sent between the first and last
   fragment).

   When a NAL unit is fragmented and conveyed within FUs, it is referred
   to as a fragmented NAL unit.  APs MUST NOT be fragmented.  FUs MUST
   NOT be nested, i.e., an FU cannot contain a subset of another FU.

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

   An FU consists of a payload header as defined in Table 5 of [VVC]
   (denoted here as PayloadHdr with Type=29), an FU header of one octet,
   a conditional 16-bit DONL field (in network byte order), and an FU
   payload (as shown in Figure 9).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   PayloadHdr (Type=29)        |   FU header   | DONL (cond)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
    |   DONL (cond) |                                               |
    |-+-+-+-+-+-+-+-+                                               |
    |                         FU payload                            |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9: The Structure of an FU

   The fields in the payload header are set as follows.  The Type field
   MUST be equal to 29.  The fields F, LayerId, and TID MUST be equal to
   the fields F, LayerId, and TID, respectively, of the fragmented NAL
   unit.

   The FU header consists of an S bit, an E bit, an R bit, and a 5-bit
   FuType field, as shown in Figure 10.

                             +---------------+
                             |0|1|2|3|4|5|6|7|
                             +-+-+-+-+-+-+-+-+
                             |S|E|P|  FuType |
                             +---------------+

                 Figure 10: The Structure of the FU Header

   The semantics of the FU header fields are as follows:

   S: 1 bit
      When set to 1, the S bit indicates the start of a fragmented NAL
      unit, i.e., the first byte of the FU payload is also the first
      byte of the payload of the fragmented NAL unit.  When the FU
      payload is not the start of the fragmented NAL unit payload, the S
      bit MUST be set to 0.

   E: 1 bit
      When set to 1, the E bit indicates the end of a fragmented NAL
      unit, i.e., the last byte of the payload is also the last byte of
      the fragmented NAL unit.  When the FU payload is not the last
      fragment of a fragmented NAL unit, the E bit MUST be set to 0.

   P: 1 bit
      When set to 1, the P bit indicates the last FU of the last VCL NAL
      unit of a coded picture, i.e., the last byte of the FU payload is
      also the last byte of the last VCL NAL unit of the coded picture.
      When the FU payload is not the last fragment of the last VCL NAL
      unit of a coded picture, the P bit MUST be set to 0.

   FuType: 5 bits
      The field FuType MUST be equal to the field Type of the fragmented
      NAL unit.

   The DONL field, when present, specifies the value of the 16 least
   significant bits of the decoding order number of the fragmented NAL
   unit.

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

   A non-fragmented NAL unit MUST NOT be transmitted in one FU, i.e.,
   the Start bit and End bit must not both be set to 1 in the same FU
   header.

   The FU payload consists of fragments of the payload of the fragmented
   NAL unit so that, if the FU payloads of consecutive FUs, starting
   with an FU with the S bit equal to 1 and ending with an FU with the E
   bit equal to 1, are sequentially concatenated, the payload of the
   fragmented NAL unit can be reconstructed.  The NAL unit header of the
   fragmented NAL unit is not included as such in the FU payload, but
   rather the information of the NAL unit header of the fragmented NAL
   unit is conveyed in the F, LayerId, and TID fields of the FU payload
   headers of the FUs and the FuType field of the FU header of the FUs.
   An FU payload MUST NOT be empty.

   If an FU is lost, the receiver SHOULD discard all following
   fragmentation units in transmission order, corresponding to the same
   fragmented NAL unit, unless the decoder in the receiver is known to
   be prepared to gracefully handle incomplete NAL units.

   A receiver in an endpoint or in a MANE MAY aggregate the first n-1
   fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
   n of that NAL unit is not received.  In this case, the
   forbidden_zero_bit of the NAL unit MUST be set to 1 to indicate a
   syntax violation.

4.4.  Decoding Order Number

   For each NAL unit, the variable AbsDon is derived, representing the
   decoding order number that is indicative of the NAL unit decoding
   order.

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

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

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

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

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

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

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

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

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

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

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

   *  When AbsDon[n] is greater than AbsDon[m], this indicates that NAL
      unit n follows NAL unit m in NAL unit decoding order.

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

   *  When AbsDon[n] is less than AbsDon[m], this indicates that NAL
      unit n precedes NAL unit m in decoding order.

      |  Informative note: When two consecutive NAL units in the NAL
      |  unit decoding order have different values of AbsDon, the
      |  absolute difference between the two AbsDon values may be
      |  greater than or equal to 1.

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

5.  Packetization Rules

   The following packetization rules apply:

   *  If sprop-max-don-diff is greater than 0, the transmission order of
      NAL units carried in the RTP stream MAY be different than the NAL
      unit decoding order.  Otherwise (sprop-max-don-diff is equal to
      0), the transmission order of NAL units carried in the RTP stream
      MUST be the same as the NAL unit decoding order.

   *  A NAL unit of a small size SHOULD be encapsulated in an
      aggregation packet together with one or more other NAL units in
      order to avoid the unnecessary packetization overhead for small
      NAL units.  For example, non-VCL NAL units, such as access unit
      delimiters, parameter sets, or SEI NAL units, are typically small
      and can often be aggregated with VCL NAL units without violating
      MTU size constraints.

   *  Each non-VCL NAL unit SHOULD, when possible from an MTU size match
      viewpoint, be encapsulated in an aggregation packet together with
      its associated VCL NAL unit, as typically a non-VCL NAL unit would
      be meaningless without the associated VCL NAL unit being
      available.

   *  For carrying exactly one NAL unit in an RTP packet, a single NAL
      unit packet MUST be used.

6.  De-packetization Process

   The general concept behind de-packetization is to get the NAL units
   out of the RTP packets in an RTP stream and pass them to the decoder
   in the NAL unit decoding order.

   The de-packetization process is implementation dependent.  Therefore,
   the following description should be seen as an example of a suitable
   implementation.  Other schemes may be used as well, as long as the
   output for the same input is the same as the process described below.
   The output is the same when the set of output NAL units and their
   order are both identical.  Optimizations relative to the described
   algorithms are possible.

   All normal RTP mechanisms related to buffer management apply.  In
   particular, duplicated or outdated RTP packets (as indicated by the
   RTP sequence number and the RTP timestamp) are removed.  To determine
   the exact time for decoding, factors, such as a possible intentional
   delay to allow for proper inter-stream synchronization, MUST be
   factored in.

   NAL units with NAL unit type values in the range of 0 to 27,
   inclusive, may be passed to the decoder.  NAL-unit-like structures
   with NAL unit type values in the range of 28 to 31, inclusive, MUST
   NOT be passed to the decoder.

   The receiver includes a receiver buffer, which is used to compensate
   for transmission delay jitter within individual RTP streams and to
   reorder NAL units from transmission order to the NAL unit decoding
   order.  In this section, the receiver operation is described under
   the assumption that there is no transmission delay jitter within an
   RTP stream.  To make a difference from a practical receiver buffer
   that is also used for compensation of transmission delay jitter, the
   receiver buffer is hereafter called the de-packetization buffer in
   this section.  Receivers should also prepare for transmission delay
   jitter, that is, either reserve separate buffers for transmission
   delay jitter buffering and de-packetization buffering or use a
   receiver buffer for both transmission delay jitter and de-
   packetization.  Moreover, receivers should take transmission delay
   jitter into account in the buffering operation, e.g., by additional
   initial buffering before starting of decoding and playback.

   The de-packetization process extracts the NAL units from the RTP
   packets in an RTP stream as follows.  When an RTP packet carries a
   single NAL unit packet, the payload of the RTP packet is extracted as
   a single NAL unit, excluding the DONL field, i.e., third and fourth
   bytes, when sprop-max-don-diff is greater than 0.  When an RTP packet
   carries an aggregation packet, several NAL units are extracted from
   the payload of the RTP packet.  In this case, each NAL unit
   corresponds to the part of the payload of each aggregation unit that
   follows the NALU size field, as described in Section 4.3.2.  When an
   RTP packet carries a Fragmentation Unit (FU), all RTP packets from
   the first FU (with the S field equal to 1) of the fragmented NAL unit
   up to the last FU (with the E field equal to 1) of the fragmented NAL
   unit are collected.  The NAL unit is extracted from these RTP packets
   by concatenating all FU payloads in the same order as the
   corresponding RTP packets and appending the NAL unit header with the
   fields F, LayerId, and TID set to equal the values of the fields F,
   LayerId, and TID in the payload header of the FUs, respectively, and
   with the NAL unit type set equal to the value of the field FuType in
   the FU header of the FUs, as described in Section 4.3.3.

   When sprop-max-don-diff is equal to 0, the de-packetization buffer
   size is zero bytes, and the NAL units carried in the single RTP
   stream are directly passed to the decoder in their transmission
   order, which is identical to their decoding order.

   When sprop-max-don-diff is greater than 0, the process described in
   the remainder of this section applies.

   There are two buffering states in the receiver: initial buffering and
   buffering while playing.  Initial buffering starts when the reception
   is initialized.  After initial buffering, decoding and playback are
   started, and the buffering-while-playing mode is used.

   Regardless of the buffering state, the receiver stores incoming NAL
   units in reception order into the de-packetization buffer.  NAL units
   carried in RTP packets are stored in the de-packetization buffer
   individually, and the value of AbsDon is calculated and stored for
   each NAL unit.

   Initial buffering lasts until the difference between the greatest and
   smallest AbsDon values of the NAL units in the de-packetization
   buffer is greater than or equal to the value of sprop-max-don-diff.

   After initial buffering, whenever the difference between the greatest
   and smallest AbsDon values of the NAL units in the de-packetization
   buffer is greater than or equal to the value of sprop-max-don-diff,
   the following operation is repeatedly applied until this difference
   is smaller than sprop-max-don-diff:

      The NAL unit in the de-packetization buffer with the smallest
      value of AbsDon is removed from the de-packetization buffer and
      passed to the decoder.

   When no more NAL units are flowing into the de-packetization buffer,
   all NAL units remaining in the de-packetization buffer are removed
   from the buffer and passed to the decoder in the order of increasing
   AbsDon values.

7.  Payload Format Parameters

   This section specifies the optional parameters.  A mapping of the
   parameters with Session Description Protocol (SDP) [RFC8866] is also
   provided for applications that use SDP.

   Parameters starting with the string "sprop" for stream properties can
   be used by a sender to provide a receiver with the properties of the
   stream that is or will be sent.  The media sender (and not the
   receiver) selects whether, and with what values, "sprop" parameters
   are being sent.  This uncommon characteristic of the "sprop"
   parameters may not be intuitive in the context of some signaling
   protocol concepts, especially with offer/answer.  Please see
   Section 7.3.2 for guidance specific to the use of sprop parameters in
   the offer/answer case.

7.1.  Media Type Registration

   The receiver MUST ignore any parameter unspecified in this memo.

   Type name:  video

   Subtype name:  H266

   Required parameters:  N/A

   Optional parameters:  profile-id, tier-flag, sub-profile-id, interop-
      constraints, level-id, sprop-sublayer-id, sprop-ols-id, recv-
      sublayer-id, recv-ols-id, max-recv-level-id, sprop-dci, sprop-vps,
      sprop-sps, sprop-pps, sprop-sei, max-lsr, max-fps, sprop-max-don-
      diff, sprop-depack-buf-bytes, depack-buf-cap (refer to Section 7.2
      for definitions).

   Encoding considerations:  This type is only defined for transfer via
      RTP [RFC3550].

   Security considerations:  See Section 9 of RFC 9328.

   Interoperability considerations:  N/A

   Published specification:  Please refer to RFC 9328 and VVC coding
      specification [VVC].

   Applications that use this media type:  Any application that relies
      on VVC-based video services over RTP

   Fragment identifier considerations:  N/A

   Additional information:  N/A

   Person & email address to contact for further information:
      Stephan Wenger (stewe@stewe.org)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section of RFC 9328.

   Change controller:  IETF <avtcore@ietf.org>

7.2.  Optional Parameters Definition

   profile-id, tier-flag, sub-profile-id, interop-constraints, and
   level-id:
      These parameters indicate the profile, the tier, the default
      level, the sub-profile, and some constraints of the bitstream
      carried by the RTP stream, or a specific set of the profile, the
      tier, the default level, the sub-profile, and some constraints the
      receiver supports.

      The subset of coding tools that may have been used to generate the
      bitstream or that the receiver supports, as well as some
      additional constraints, are indicated collectively by profile-id,
      sub-profile-id, and interop-constraints.

         |  Informative note: There are 128 values of profile-id.  The
         |  subset of coding tools identified by profile-id can be
         |  further constrained with up to 255 instances of sub-profile-
         |  id.  In addition, 68 bits included in interop-constraints,
         |  which can be extended up to 324 bits, provide means to
         |  further restrict tools from existing profiles.  To be able
         |  to support this fine-granular signaling of coding-tool
         |  subsets with profile-id, sub-profile-id, and interop-
         |  constraints, it would be safe to require symmetric use of
         |  these parameters in SDP offer/answer unless recv-ols-id is
         |  included in the SDP answer for choosing one of the layers
         |  offered.

      The tier is indicated by tier-flag.  The default level is
      indicated by level-id.  The tier and the default level specify the
      limits on values of syntax elements or arithmetic combinations of
      values of syntax elements that are followed when generating the
      bitstream or that the receiver supports.

      In SDP offer/answer, when the SDP answer does not include the
      recv-ols-id parameter that is less than the sprop-ols-id parameter
      in the SDP offer, the following applies:

      *  The tier-flag, profile-id, sub-profile-id, and interop-
         constraints parameters MUST be used symmetrically, i.e., the
         value of each of these parameters in the offer MUST be the same
         as that in the answer, either explicitly signaled or implicitly
         inferred.

      *  The level-id parameter is changeable as long as the highest
         level indicated by the answer is either equal to or lower than
         that in the offer.  Note that the highest level higher than
         level-id in the offer for receiving can be included as max-
         recv-level-id.

      In SDP offer/answer, when the SDP answer does include the recv-
      ols-id parameter that is less than the sprop-ols-id parameter in
      the SDP offer, the set of tier-flag, profile-id, sub-profile-id,
      interop-constraints, and level-id parameters included in the
      answer MUST be consistent with that for the chosen output layer
      set as indicated in the SDP offer, with the exception that the
      level-id parameter in the SDP answer is changeable as long as the
      highest level indicated by the answer is either lower than or
      equal to that in the offer.

      More specifications of these parameters, including how they relate
      to syntax elements specified in [VVC], are provided below.

   profile-id:
      When profile-id is not present, a value of 1 (i.e., the Main 10
      profile) MUST be inferred.

      When used to indicate properties of a bitstream, profile-id is
      derived from the general_profile_idc syntax element that applies
      to the bitstream in an instance of the profile_tier_level( )
      syntax structure.

      VVC bitstreams transported over RTP using the technologies of this
      memo SHOULD contain only a single profile_tier_level( ) structure
      in the DCI, unless the sender can assure that a receiver can
      correctly decode the VVC bitstream, regardless of which
      profile_tier_level( ) structure contained in the DCI was used for
      deriving profile-id and other parameters for the SDP offer/answer
      exchange.

      As specified in [VVC], a profile_tier_level( ) syntax structure
      may be contained in an SPS NAL unit, and one or more
      profile_tier_level( ) syntax structures may be contained in a VPS
      NAL unit and in a DCI NAL unit.  One of the following three cases
      applies to the container NAL unit of the profile_tier_level( )
      syntax structure containing syntax elements used to derive the
      values of profile-id, tier-flag, level-id, sub-profile-id, or
      interop-constraints:

      1.  The container NAL unit is an SPS, the bitstream is a single-
          layer bitstream, and the profile_tier_level( ) syntax
          structures in all SPSs referenced by the CVSs in the bitstream
          have the same values respectively for those
          profile_tier_level( ) syntax elements.

      2.  The container NAL unit is a VPS, the profile_tier_level( )
          syntax structure is the one in the VPS that applies to the OLS
          corresponding to the bitstream, and the profile_tier_level( )
          syntax structures applicable to the OLS corresponding to the
          bitstream in all VPSs referenced by the CVSs in the bitstream
          have the same values respectively for those
          profile_tier_level( ) syntax elements.

      3.  The container NAL unit is a DCI NAL unit, and the
          profile_tier_level( ) syntax structures in all DCI NAL units
          in the bitstream have the same values respectively for those
          profile_tier_level( ) syntax elements.

      [VVC] allows for multiple profile_tier_level( ) structures in a
      DCI NAL unit, which may contain different values for the syntax
      elements used to derive the values of profile-id, tier-flag,
      level-id, sub-profile-id, or interop-constraints in the different
      entries.  However, herein defined is only a single profile-id,
      tier-flag, level-id, sub-profile-id, or interop-constraints.  When
      signaling these parameters and a DCI NAL unit is present with
      multiple profile_tier_level( ) structures, these values SHOULD be
      the same as the first profile_tier_level structure in the DCI,
      unless the sender has ensured that the receiver can decode the
      bitstream when a different value is chosen.

   tier-flag, level-id:
      The value of tier-flag MUST be in the range of 0 to 1, inclusive.
      The value of level-id MUST be in the range of 0 to 255, inclusive.

      If the tier-flag and level-id parameters are used to indicate
      properties of a bitstream, they indicate the tier and the highest
      level the bitstream complies with.

      If the tier-flag and level-id parameters are used for capability
      exchange, the following applies.  If max-recv-level-id is not
      present, the default level defined by level-id indicates the
      highest level the codec wishes to support.  Otherwise, max-recv-
      level-id indicates the highest level the codec supports for
      receiving.  For either receiving or sending, all levels that are
      lower than the highest level supported MUST also be supported.

      If no tier-flag is present, a value of 0 MUST be inferred; if no
      level-id is present, a value of 51 (i.e., level 3.1) MUST be
      inferred.

         |  Informative note: The level values currently defined in the
         |  VVC specification are in the form of "majorNum.minorNum",
         |  and the value of the level-id for each of the levels is
         |  equal to majorNum * 16 + minorNum * 3.  It is expected that,
         |  if any levels are defined in the future, the same convention
         |  will be used, but this cannot be guaranteed.

      When used to indicate properties of a bitstream, the tier-flag and
      level-id parameters are derived respectively from the syntax
      element general_tier_flag, and the syntax element
      general_level_idc or sub_layer_level_idc[j], that apply to the
      bitstream in an instance of the profile_tier_level( ) syntax
      structure.

      If the tier-flag and level-id are derived from the
      profile_tier_level( ) syntax structure in a DCI NAL unit, the
      following applies:

      *  tier-flag = general_tier_flag

      *  level-id = general_level_idc

      Otherwise, if the tier-flag and level-id are derived from the
      profile_tier_level( ) syntax structure in an SPS or VPS NAL unit,
      and the bitstream contains the highest sublayer representation in
      the OLS corresponding to the bitstream, the following applies:

      *  tier-flag = general_tier_flag

      *  level-id = general_level_idc

      Otherwise, if the tier-flag and level-id are derived from the
      profile_tier_level( ) syntax structure in an SPS or VPS NAL unit,
      and the bitstream does not contain the highest sublayer
      representation in the OLS corresponding to the bitstream, the
      following applies, with j being the value of the sprop-sublayer-id
      parameter:

      *  tier-flag = general_tier_flag

      *  level-id = sub_layer_level_idc[j]

   sub-profile-id:
      The value of the parameter is a comma-separated (',') list of data
      using base64 encoding (Section 4 of [RFC4648]) representation
      without "==" padding.

      When used to indicate properties of a bitstream, sub-profile-id is
      derived from each of the ptl_num_sub_profiles
      general_sub_profile_idc[i] syntax elements that apply to the
      bitstream in a profile_tier_level( ) syntax structure.

   interop-constraints:
      A base64 encoding (Section 4 of [RFC4648]) representation of the
      data that includes the ptl_frame_only_constraint_flag syntax
      element, the ptl_multilayer_enabled_flag syntax element, and the
      general_constraints_info( ) syntax structure that apply to the
      bitstream in an instance of the profile_tier_level( ) syntax
      structure.

      If the interop-constraints parameter is not present, the following
      MUST be inferred:

      *  ptl_frame_only_constraint_flag = 1

      *  ptl_multilayer_enabled_flag = 0

      *  gci_present_flag in the general_constraints_info( ) syntax
         structure = 0

      Using interop-constraints for capability exchange results in a
      requirement on any bitstream to be compliant with the interop-
      constraints.

   sprop-sublayer-id:
      This parameter MAY be used to indicate the highest allowed value
      of TID in the bitstream.  When not present, the value of sprop-
      sublayer-id is inferred to be equal to 6.

      The value of sprop-sublayer-id MUST be in the range of 0 to 6,
      inclusive.

   sprop-ols-id:
      This parameter MAY be used to indicate the OLS that the bitstream
      applies to.  When not present, the value of sprop-ols-id is
      inferred to be equal to TargetOlsIdx, as specified in
      Section 8.1.1 of [VVC].  If this optional parameter is present,
      sprop-vps MUST also be present or its content MUST be known a
      priori at the receiver.

      The value of sprop-ols-id MUST be in the range of 0 to 256,
      inclusive.

         |  Informative note: VVC allows having up to 257 output layer
         |  sets indicated in the VPS, as the number of output layer
         |  sets minus 2 is indicated with a field of 8 bits.

   recv-sublayer-id:
      This parameter MAY be used to signal a receiver's choice of the
      offered or declared sublayer representations in sprop-vps and
      sprop-sps.  The value of recv-sublayer-id indicates the TID of the
      highest sublayer that a receiver supports.  When not present, the
      value of recv-sublayer-id is inferred to be equal to the value of
      the sprop-sublayer-id parameter in the SDP offer.

      The value of recv-sublayer-id MUST be in the range of 0 to 6,
      inclusive.

   recv-ols-id:
      This parameter MAY be used to signal a receiver's choice of the
      offered or declared output layer sets in sprop-vps.  The value of
      recv-ols-id indicates the OLS index of the bitstream that a
      receiver supports.  When not present, the value of recv-ols-id is
      inferred to be equal to the value of the sprop-ols-id parameter
      inferred from or indicated in the SDP offer.  When present, the
      value of recv-ols-id must be included only when sprop-ols-id was
      received and must refer to an output layer set in the VPS that
      includes no layers other than all or a subset of the layers of the
      OLS referred to by sprop-ols-id.  If this optional parameter is
      present, sprop-vps must have been received or its content must be
      known a priori at the receiver.

      The value of recv-ols-id MUST be in the range of 0 to 256,
      inclusive.

   max-recv-level-id:
      This parameter MAY be used to indicate the highest level a
      receiver supports.

      The value of max-recv-level-id MUST be in the range of 0 to 255,
      inclusive.

      When max-recv-level-id is not present, the value is inferred to be
      equal to level-id.

      max-recv-level-id MUST NOT be present when the highest level the
      receiver supports is not higher than the default level.

   sprop-dci:
      This parameter MAY be used to convey a decoding capability
      information NAL unit of the bitstream for out-of-band
      transmission.  The parameter MAY also be used for capability
      exchange.  The value of the parameter is a base64 encoding
      (Section 4 of [RFC4648]) representation of the decoding capability
      information NAL unit, as specified in Section 7.3.2.1 of [VVC].

   sprop-vps:
      This parameter MAY be used to convey any video parameter set to
      the NAL unit of the bitstream for out-of-band transmission of
      video parameter sets.  The parameter MAY also be used for
      capability exchange and to indicate substream characteristics
      (i.e., properties of output layer sets and sublayer
      representations, as defined in [VVC]).  The value of the parameter
      is a comma-separated (',') list of base64 encoding (Section 4 of
      [RFC4648]) representations of the video parameter set NAL units,
      as specified in Section 7.3.2.3 of [VVC].

      The sprop-vps parameter MAY contain one or more than one video
      parameter set NAL units.  However, all other video parameter sets
      contained in the sprop-vps parameter MUST be consistent with the
      first video parameter set in the sprop-vps parameter.  A video
      parameter set vpsB is said to be consistent with another video
      parameter set vpsA if the number of OLSs in vpsA and vpsB are the
      same and any decoder that conforms to the profile, tier, level,
      and constraints indicated by the data starting from the syntax
      element general_profile_idc to the syntax structure
      general_constraints_info(), inclusive, in the profile_tier_level(
      ) syntax structure corresponding to any OLS with index olsIdx in
      vpsA can decode any CVS(s) referencing vpsB when TargetOlsIdx is
      equal to olsIdx that conforms to the profile, tier, level, and
      constraints indicated by the data starting from the syntax element
      general_profile_idc to the syntax structure
      general_constraints_info(), inclusive, in the profile_tier_level(
      ) syntax structure corresponding to the OLS with index
      TargetOlsIdx in vpsB.

   sprop-sps:
      This parameter MAY be used to convey sequence parameter set NAL
      units of the bitstream for out-of-band transmission of sequence
      parameter sets.  The value of the parameter is a comma-separated
      (',') list of base64 encoding (Section 4 of [RFC4648])
      representations of the sequence parameter set NAL units, as
      specified in Section 7.3.2.4 of [VVC].

      A sequence parameter set spsB is said to be consistent with
      another sequence parameter set spsA if any decoder that conforms
      to the profile, tier, level, and constraints indicated by the data
      starting from the syntax element general_profile_idc to the syntax
      structure general_constraints_info(), inclusive, in the
      profile_tier_level( ) syntax structure in spsA can decode any
      CLVS(s) referencing spsB that conforms to the profile, tier,
      level, and constraints indicated by the data starting from the
      syntax element general_profile_idc to the syntax structure
      general_constraints_info(), inclusive, in the profile_tier_level(
      ) syntax structure in spsB.

   sprop-pps:
      This parameter MAY be used to convey picture parameter set NAL
      units of the bitstream for out-of-band transmission of picture
      parameter sets.  The value of the parameter is a comma-separated
      (',') list of base64 encoding (Section 4 of [RFC4648])
      representations of the picture parameter set NAL units, as
      specified in Section 7.3.2.5 of [VVC].

   sprop-sei:
      This parameter MAY be used to convey one or more SEI messages that
      describe bitstream characteristics.  When present, a decoder can
      rely on the bitstream characteristics that are described in the
      SEI messages for the entire duration of the session, independently
      from the persistence scopes of the SEI messages, as specified in
      [VSEI].

      The value of the parameter is a comma-separated (',') list of
      base64 encoding (Section 4 of [RFC4648]) representations of SEI
      NAL units, as specified in [VSEI].

         |  Informative note: Intentionally, no list of applicable or
         |  inapplicable SEI messages is specified here.  Conveying
         |  certain SEI messages in sprop-sei may be sensible in some
         |  application scenarios and meaningless in others.  However, a
         |  few examples are described below:
         |  
         |  In an environment where the bitstream was created from film-
         |  based source material, and no splicing is going to occur
         |  during the lifetime of the session, the film grain
         |  characteristics SEI message is likely meaningful, and
         |  sending it in sprop-sei, rather than in the bitstream at
         |  each entry point, may help with saving bits and allows one
         |  to configure the renderer only once, avoiding unwanted
         |  artifacts.
         |  
         |  Examples for SEI messages that would be meaningless to be
         |  conveyed in sprop-sei include the decoded picture hash SEI
         |  message (it is close to impossible that all decoded pictures
         |  have the same hashtag) or the filler payload SEI message (as
         |  there is no point in just having more bits in SDP).

   max-lsr:
      The max-lsr MAY be used to signal the capabilities of a receiver
      implementation and MUST NOT be used for any other purpose.  The
      value of max-lsr is an integer indicating the maximum processing
      rate in units of luma samples per second.  The max-lsr parameter
      signals that the receiver is capable of decoding video at a higher
      rate than is required by the highest level.

         |  Informative note: When the OPTIONAL media type parameters
         |  are used to signal the properties of a bitstream, and max-
         |  lsr is not present, the values of tier-flag, profile-id,
         |  sub-profile-id, interop-constraints, and level-id must
         |  always be such that the bitstream complies fully with the
         |  specified profile, sub-profile, tier, level, and interop-
         |  constraints.

      When max-lsr is signaled, the receiver MUST be able to decode
      bitstreams that conform to the highest level, with the exception
      that the MaxLumaSr value in Table A.3 of [VVC] for the highest
      level is replaced with the value of max-lsr.  Senders MAY use this
      knowledge to send pictures of a given size at a higher picture
      rate than is indicated in the highest level.

      When not present, the value of max-lsr is inferred to be equal to
      the value of MaxLumaSr given in Table A.3 of [VVC] for the highest
      level.

      The value of max-lsr MUST be in the range of MaxLumaSr to 16 *
      MaxLumaSr, inclusive, where MaxLumaSr is given in Table A.3 of
      [VVC] for the highest level.

   max-fps:
      The value of max-fps is an integer indicating the maximum picture
      rate in units of pictures per 100 seconds that can be effectively
      processed by the receiver.  The max-fps parameter MAY be used to
      signal that the receiver has a constraint in that it is not
      capable of processing video effectively at the full picture rate
      that is implied by the highest level and, when present, max-lsr.

      The value of max-fps is not necessarily the picture rate at which
      the maximum picture size can be sent; it constitutes a constraint
      on maximum picture rate for all resolutions.

         |  Informative note: The max-fps parameter is semantically
         |  different from max-lsr in that max-fps is used to signal a
         |  constraint, lowering the maximum picture rate from what is
         |  implied by other parameters.

      The encoder MUST use a picture rate equal to or less than this
      value.  In cases where the max-fps parameter is absent, the
      encoder is free to choose any picture rate according to the
      highest level and any signaled optional parameters.

      The value of max-fps MUST be smaller than or equal to the full
      picture rate that is implied by the highest level and, when
      present, max-lsr.

   sprop-max-don-diff:
      If there is no NAL unit naluA that is followed in transmission
      order by any NAL unit preceding naluA in decoding order (i.e., the
      transmission order of the NAL units is the same as the decoding
      order), the value of this parameter MUST be equal to 0.

      Otherwise, this parameter specifies the maximum absolute
      difference between the decoding order number (i.e., AbsDon) values
      of any two NAL units naluA and naluB, where naluA follows naluB in
      decoding order and precedes naluB in transmission order.

      The value of sprop-max-don-diff MUST be an integer in the range of
      0 to 32767, inclusive.

      When not present, the value of sprop-max-don-diff is inferred to
      be equal to 0.

   sprop-depack-buf-bytes:
      This parameter signals the required size of the de-packetization
      buffer in units of bytes.  The value of the parameter MUST be
      greater than or equal to the maximum buffer occupancy (in units of
      bytes) of the de-packetization buffer, as specified in Section 6.

      The value of sprop-depack-buf-bytes MUST be an integer in the
      range of 0 to 4294967295, inclusive.

      When sprop-max-don-diff is present and greater than 0, this
      parameter MUST be present and the value MUST be greater than 0.
      When not present, the value of sprop-depack-buf-bytes is inferred
      to be equal to 0.

         |  Informative note: The value of sprop-depack-buf-bytes
         |  indicates the required size of the de-packetization buffer
         |  only.  When network jitter can occur, an appropriately sized
         |  jitter buffer has to be available as well.

   depack-buf-cap:
      This parameter signals the capabilities of a receiver
      implementation and indicates the amount of de-packetization buffer
      space in units of bytes that the receiver has available for
      reconstructing the NAL unit decoding order from NAL units carried
      in the RTP stream.  A receiver is able to handle any RTP stream
      for which the value of the sprop-depack-buf-bytes parameter is
      smaller than or equal to this parameter.

      When not present, the value of depack-buf-cap is inferred to be
      equal to 4294967295.  The value of depack-buf-cap MUST be an
      integer in the range of 1 to 4294967295, inclusive.

         |  Informative note: depack-buf-cap indicates the maximum
         |  possible size of the de-packetization buffer of the receiver
         |  only, without allowing for network jitter.

7.3.  SDP Parameters

   The receiver MUST ignore any parameter unspecified in this memo.

7.3.1.  Mapping of Payload Type Parameters to SDP

   The media type video/H266 string is mapped to fields in the Session
   Description Protocol (SDP) [RFC8866] as follows:

   *  The media name in the "m=" line of SDP MUST be video.

   *  The encoding name in the "a=rtpmap" line of SDP MUST be H266 (the
      media subtype).

   *  The clock rate in the "a=rtpmap" line MUST be 90000.

   *  The OPTIONAL parameters profile-id, tier-flag, sub-profile-id,
      interop-constraints, level-id, sprop-sublayer-id, sprop-ols-id,
      recv-sublayer-id, recv-ols-id, max-recv-level-id, max-lsr, max-
      fps, sprop-max-don-diff, sprop-depack-buf-bytes, and depack-buf-
      cap, when present, MUST be included in the "a=fmtp" line of SDP.
      The fmtp line is expressed as a media type string, in the form of
      a semicolon-separated list of parameter=value pairs.

   *  The OPTIONAL parameters sprop-vps, sprop-sps, sprop-pps, sprop-
      sei, and sprop-dci, when present, MUST be included in the "a=fmtp"
      line of SDP or conveyed using the "fmtp" source attribute as
      specified in Section 6.3 of [RFC5576].  For a particular media
      format (i.e., RTP payload type), sprop-vps, sprop-sps, sprop-pps,
      sprop-sei, or sprop-dci MUST NOT be both included in the "a=fmtp"
      line of SDP and conveyed using the "fmtp" source attribute.  When
      included in the "a=fmtp" line of SDP, those parameters are
      expressed as a media type string, in the form of a semicolon-
      separated list of parameter=value pairs.  When conveyed in the
      "a=fmtp" line of SDP for a particular payload type, the parameters
      sprop-vps, sprop-sps, sprop-pps, sprop-sei, and sprop-dci MUST be
      applied to each SSRC with the payload type.  When conveyed using
      the "fmtp" source attribute, these parameters are only associated
      with the given source and payload type as parts of the "fmtp"
      source attribute.

      |  Informative note: Conveyance of sprop-vps, sprop-sps, and
      |  sprop-pps using the "fmtp" source attribute allows for out-of-
      |  band transport of parameter sets in topologies like Topo-Video-
      |  switch-MCU, as specified in [RFC7667].

   A general usage of media representation in SDP is as follows:

           m=video 49170 RTP/AVP 98
           a=rtpmap:98 H266/90000
           a=fmtp:98 profile-id=1;
             sprop-vps=<video parameter sets data>;
             sprop-sps=<sequence parameter set data>;
             sprop-pps=<picture parameter set data>;

   A SIP offer/answer exchange wherein both parties are expected to both
   send and receive could look like the following.  Only the media
   codec-specific parts of the SDP are shown.  Some lines are wrapped
   due to text constraints.

     Offerer->Answerer:
           m=video 49170 RTP/AVP 98
           a=rtpmap:98 H266/90000
           a=fmtp:98 profile-id=1; level_id=83;

   The above represents an offer for symmetric video communication using
   [VVC] and its payload specification at the main profile and level 5.1
   (and as the levels are downgradable, all lower levels).  Informally
   speaking, this offer tells the receiver of the offer that the sender
   is willing to receive up to 4Kp60 resolution at the maximum bitrates
   specified in [VVC].  At the same time, if this offer were accepted
   "as is", the offer can expect that the answerer would be able to
   receive and properly decode H.266 media up to and including level
   5.1.

     Answerer->Offerer:
           m=video 49170 RTP/AVP 98
           a=rtpmap:98 H266/90000
           a=fmtp:98 profile-id=1; level_id=67

   With this answer to the offer above, the system receiving the offer
   advises the offerer that it is incapable of handing H.266 at level
   5.1 but is capable of decoding 1080p60.  As H.266 video codecs must
   support decoding at all levels below the maximum level they
   implement, the resulting user experience would likely be that both
   systems send video at 1080p60.  However, nothing prevents an encoder
   from further downgrading its sending to, for example, 720p30 if it
   were short of cycles or bandwidth or for other reasons.

7.3.2.  Usage with SDP Offer/Answer Model

   This section describes the negotiation of unicast messages using the
   offer/answer model as described in [RFC3264] and its updates.  The
   section is split into subsections, covering a) media format
   configurations not involving non-temporal scalability; b) scalable
   media format configurations; c) the description of the use of those
   parameters not involving the media configuration itself but rather
   the parameters of the payload format design; and d) multicast.

7.3.2.1.  Non-scalable Media Format Configuration

   A non-scalable VVC media configuration is such a configuration where
   no non-temporal scalability mechanisms are allowed.  In [VVC] version
   1, it is implied that general_profile_idc indicates one of the
   following profiles: Main 10, Main 10 Still Picture, Main 10 4:4:4, or
   Main 10 4:4:4 Still Picture, with general_profile_idc values of 1,
   65, 33, and 97, respectively.  Note that non-scalable media
   configurations include temporal scalability inline with VVC's design
   philosophy and profile structure.

   The following limitations and rules pertaining to the media
   configuration apply:

   *  The parameters identifying a media format configuration for VVC
      are profile-id, tier-flag, sub-profile-id, level-id, and interop-
      constraints.  These media configuration parameters, except level-
      id, MUST be used symmetrically.

      The answerer MUST structure its answer according to one of the
      following three options:

      1.  maintain all configuration parameters with the values
          remaining the same as in the offer for the media format
          (payload type), with the exception that the value of level-id
          is changeable as long as the highest level indicated by the
          answer is not higher than that indicated by the offer;

      2.  include in the answer the recv-sublayer-id parameter, with a
          value less than the sprop-sublayer-id parameter in the offer,
          for the media format (payload type), and maintain all
          configuration parameters with the values remaining the same as
          in the offer for the media format (payload type), with the
          exception that the value of level-id is changeable as long as
          the highest level indicated by the answer is not higher than
          the level indicated by sprop-sps or sprop-vps in offer for the
          chosen sublayer representation; or

      3.  remove the media format (payload type) completely (when one or
          more of the parameter values are not supported).

      |  Informative note: The above requirement for symmetric use does
      |  not apply for level-id and does not apply for the other
      |  bitstream or RTP stream properties and capability parameters,
      |  as described in Section 7.3.2.3 below.

   *  To simplify handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [RFC3264].

   *  The same RTP payload type number used in the offer for the media
      subtype H266 MUST be used in the answer when the answer includes
      recv-sublayer-id.  When the answer does not include recv-sublayer-
      id, the answer MUST NOT contain a payload type number used in the
      offer for the media subtype H266 unless the configuration is
      exactly the same as in the offer or the configuration in the
      answer only differs from that in the offer with a different value
      of level-id.  The answer MAY contain the recv-sublayer-id
      parameter if a VVC bitstream contains multiple operation points
      (using temporal scalability and sublayers) and sprop-sps or sprop-
      vps is included in the offer where information of sublayers are
      present in the first sequence parameter set or video parameter set
      contained in sprop-sps or sprop-vps, respectively.  If sprop-sps
      or sprop-vps is provided in an offer, an answerer MAY select a
      particular operation point indicated in the first sequence
      parameter set or video parameter set contained in sprop-sps or
      sprop-vps, respectively.  When the answer includes a recv-
      sublayer-id that is less than a sprop-sublayer-id in the offer,
      the following applies:

      1.  When the sprop-sps parameter is present, all sequence
          parameter sets contained in the sprop-sps parameter in the SDP
          answer and all sequence parameter sets sent in-band for either
          the offerer-to-answerer direction or the answerer-to-offerer
          direction MUST be consistent with the first sequence parameter
          set in the sprop-sps parameter of the offer (see the semantics
          of sprop-sps in Section 7.1 of this document on one sequence
          parameter set being consistent with another sequence parameter
          set).

      2.  When the sprop-vps parameter is present, all video parameter
          sets contained in the sprop-vps parameter in the SDP answer
          and all video parameter sets sent in-band for either the
          offerer-to-answerer direction or the answerer-to-offerer
          direction MUST be consistent with the first video parameter
          set in the sprop-vps parameter of the offer (see the semantics
          of sprop-vps in Section 7.1 of this document on one video
          parameter set being consistent with another video parameter
          set).

      3.  The bitstream sent in either direction MUST conform to the
          profile, tier, level, and constraints of the chosen sublayer
          representation, as indicated by the profile_tier_level( )
          syntax structure in the first sequence parameter set in the
          sprop-sps parameter or by the first profile_tier_level( )
          syntax structure in the first video parameter set in the
          sprop-vps parameter of the offer.

      |  Informative note: When an offerer receives an answer that does
      |  not include recv-sublayer-id, it has to compare payload types
      |  not declared in the offer based on the media type (i.e., video/
      |  H266) and the above media configuration parameters with any
      |  payload types it has already declared.  This will enable it to
      |  determine whether the configuration in question is new or if it
      |  is equivalent to configuration already offered, since a
      |  different payload type number may be used in the answer.  The
      |  ability to perform operation point selection enables a receiver
      |  to utilize the temporal scalable nature of a VVC bitstream.

7.3.2.2.  Scalable Media Format Configuration

   A scalable VVC media configuration is such a configuration where non-
   temporal scalability mechanisms are allowed.  In [VVC] version 1, it
   is implied that general_profile_idc indicates one of the following
   profiles: Multilayer Main 10 and Multilayer Main 10 4:4:4, with
   general_profile_idc values of 17 and 49, respectively.

   The following limitations and rules pertaining to the media
   configuration apply.  They are listed in an order that would be
   logical for an implementation to follow:

   *  The parameters identifying a media format configuration for
      scalable VVC are profile-id, tier-flag, sub-profile-id, level-id,
      interop-constraints, and sprop-vps.  These media configuration
      parameters, except level-id, MUST be used symmetrically, except as
      noted below.

   *  The answerer MAY include a level-id that MUST be lower than or
      equal to the level-id indicated in the offer (either expressed by
      level-id in the offer or implied by the default level, as
      specified in Section 7.1).

   *  When sprop-ols-id is present in an offer, sprop-vps MUST also be
      present in the same offer and include at least one valid VPS so to
      allow the answerer to meaningfully interpret sprop-ols-id and
      select recv-ols-id (see below).

   *  The answerer MUST NOT include recv-ols-id unless the offer
      includes sprop-ols-id.  When present, recv-ols-id MUST indicate a
      supported output layer set in the VPS that includes no layers
      other than all or a subset of the layers of the OLS referred to by
      sprop-ols-id.  If unable, the answerer MUST remove the media
      format.

      |  Informative note: If an offerer wants to offer more than one
      |  output layer set, it can do so by offering multiple VVC media
      |  with different payload types.

   *  The offerer MAY include sprop-sublayer-id, which indicates the
      highest allowed value of TID in the bitstream.  The answerer MAY
      include recv-sublayer-id, which can be used to reduce the number
      of sublayers from the value of sprop-sublayer-id.

   *  When the answerer includes recv-ols-id and configuration
      parameters profile-id, tier-flag, sub-profile-id, level-id, and
      interop-constraints, it MUST use the configuration parameter
      values as signaled in the sprop-vps for the operating point with
      the largest number of sublayers for the chosen output layer set,
      with the exception that the value of level-id is changeable as
      long as the highest level indicated by the answer is not higher
      than the level indicated by sprop-vps in offer for the operating
      point with the largest number of sublayers for the chosen output
      layer set.

7.3.2.3.  Payload Format Configuration

   The following limitations and rules pertain to the configuration of
   the payload format buffer management mostly and apply to both
   scalable and non-scalable VVC.

   *  The parameters sprop-max-don-diff and sprop-depack-buf-bytes
      describe the properties of an RTP stream that the offerer or the
      answerer is sending for the media format configuration.  This
      differs from the normal usage of the offer/answer parameters;
      normally, such parameters declare the properties of the bitstream
      or RTP stream that the offerer or the answerer is able to receive.
      When dealing with VVC, the offerer assumes that the answerer will
      be able to receive media encoded using the configuration being
      offered.

      |  Informative note: The above parameters apply for any RTP
      |  stream, when present, sent by a declaring entity with the same
      |  configuration.  In other words, the applicability of the above
      |  parameters to RTP streams depends on the source endpoint.
      |  Rather than being bound to the payload type, the values may
      |  have to be applied to another payload type when being sent, as
      |  they apply for the configuration.

   *  The capability parameter max-lsr MAY be used to declare further
      capabilities of the offerer or answerer for receiving.  It MUST
      NOT be present when the direction attribute is sendonly.

   *  The capability parameter max-fps MAY be used to declare lower
      capabilities of the offerer or answerer for receiving.  It MUST
      NOT be present when the direction attribute is sendonly.

   *  When an offerer offers an interleaved stream, indicated by the
      presence of sprop-max-don-diff with a value larger than zero, the
      offerer MUST include the size of the de-packetization buffer
      sprop-depack-buf-bytes.

   *  To enable the offerer and answerer to inform each other about
      their capabilities for de-packetization buffering in receiving RTP
      streams, both parties are RECOMMENDED to include depack-buf-cap.

   *  The parameters sprop-dci, sprop-vps, sprop-sps, or sprop-pps, when
      present (included in the "a=fmtp" line of SDP or conveyed using
      the "fmtp" source attribute, as specified in Section 6.3 of
      [RFC5576]), are used for out-of-band transport of the parameter
      sets (DCI, VPS, SPS, or PPS, respectively).

   *  The answerer MAY use either out-of-band or in-band transport of
      parameter sets for the bitstream it is sending, regardless of
      whether out-of-band parameter sets transport has been used in the
      offerer-to-answerer direction.  Parameter sets included in an
      answer are independent of those parameter sets included in the
      offer, as they are used for decoding two different bitstreams; one
      from the answerer to the offerer and the other in the opposite
      direction.  In case some RTP packets are sent before the SDP
      offer/answer settles down, in-band parameter sets MUST be used for
      those RTP stream parts sent before the SDP offer/answer.

   *  The following rules apply to transport of parameter sets in the
      offerer-to-answerer direction.

      -  An offer MAY include sprop-dci, sprop-vps, sprop-sps, and/or
         sprop-pps.  If none of these parameters are present in the
         offer, then only in-band transport of parameter sets is used.

      -  If the level to use in the offerer-to-answerer direction is
         equal to the default level in the offer, the answerer MUST be
         prepared to use the parameter sets included in sprop-vps,
         sprop-sps, and sprop-pps (either included in the "a=fmtp" line
         of SDP or conveyed using the "fmtp" source attribute) for
         decoding the incoming bitstream, e.g., by passing these
         parameter set NAL units to the video decoder before passing any
         NAL units carried in the RTP streams.  Otherwise, the answerer
         MUST ignore sprop-vps, sprop-sps, and sprop-pps (either
         included in the "a=fmtp" line of SDP or conveyed using the
         "fmtp" source attribute) and the offerer MUST transmit
         parameter sets in-band.

   *  The following rules apply to transport of parameter sets in the
      answerer-to-offerer direction.

      -  An answer MAY include sprop-dci, sprop-vps, sprop-sps, and/or
         sprop-pps.  If none of these parameters are present in the
         answer, then only in-band transport of parameter sets is used.

      -  The offerer MUST be prepared to use the parameter sets included
         in sprop-vps, sprop-sps, and sprop-pps (either included in the
         "a=fmtp" line of SDP or conveyed using the "fmtp" source
         attribute) for decoding the incoming bitstream, e.g., by
         passing these parameter set NAL units to the video decoder
         before passing any NAL units carried in the RTP streams.

   *  When sprop-dci, sprop-vps, sprop-sps, and/or sprop-pps are
      conveyed using the "fmtp" source attribute, as specified in
      Section 6.3 of [RFC5576], the receiver of the parameters MUST
      store the parameter sets included in sprop-dci, sprop-vps, sprop-
      sps, and/or sprop-pps and associate them with the source given as
      part of the "fmtp" source attribute.  Parameter sets associated
      with one source (given as part of the "fmtp" source attribute)
      MUST only be used to decode NAL units conveyed in RTP packets from
      the same source (given as part of the "fmtp" source attribute).
      When this mechanism is in use, SSRC collision detection and
      resolution MUST be performed as specified in [RFC5576].

   Figure 11 lists the interpretation of all the parameters that MAY be
   used for the various combinations of offer, answer, and direction
   attributes.

                                       sendonly --+
               answer: recvonly, recv-ols-id --+  |
                 recvonly w/o recv-ols-id --+  |  |
         answer: sendrecv, recv-ols-id --+  |  |  |
           sendrecv w/o recv-ols-id --+  |  |  |  |
                                      |  |  |  |  |
   profile-id                         C  D  C  D  P
   tier-flag                          C  D  C  D  P
   level-id                           D  D  D  D  P
   sub-profile-id                     C  D  C  D  P
   interop-constraints                C  D  C  D  P
   max-recv-level-id                  R  R  R  R  -
   sprop-max-don-diff                 P  P  -  -  P
   sprop-depack-buf-bytes             P  P  -  -  P
   depack-buf-cap                     R  R  R  R  -
   max-lsr                            R  R  R  R  -
   max-fps                            R  R  R  R  -
   sprop-dci                          P  P  -  -  P
   sprop-sei                          P  P  -  -  P
   sprop-vps                          P  P  -  -  P
   sprop-sps                          P  P  -  -  P
   sprop-pps                          P  P  -  -  P
   sprop-sublayer-id                  P  P  -  -  P
   recv-sublayer-id                   O  O  O  O  -
   sprop-ols-id                       P  P  -  -  P
   recv-ols-id                        X  O  X  O  -

   Legend:

    C: configuration for sending and receiving bitstreams
    D: changeable configuration, same as C, except possible
       to answer with a different but consistent value (see the
       semantics of the six parameters related to profile, tier,
       and level on these parameters being consistent)
    P: properties of the bitstream to be sent
    R: receiver capabilities
    O: operation point selection
    X: MUST NOT be present
    -: not usable, when present MUST be ignored

      Figure 11: Interpretation of Parameters for Various Combinations
       of Offers, Answers, and Direction Attributes, with and without
                                recv-ols-id.

   Parameters used for declaring receiver capabilities are, in general,
   downgradable, i.e., they express the upper limit for a sender's
   possible behavior.  Thus, a sender MAY select to set its encoder
   using only lower/lesser or equal values of these parameters.

   When the answer does not include a recv-ols-id that is less than the
   sprop-ols-id in the offer, parameters declaring a configuration point
   are not changeable, with the exception of the level-id parameter for
   unicast usage, and these parameters express values a receiver expects
   to be used and MUST be used verbatim in the answer as in the offer.

   When a sender's capabilities are declared with the configuration
   parameters, these parameters express a configuration that is
   acceptable for the sender to receive bitstreams.  In order to achieve
   high interoperability levels, it is often advisable to offer multiple
   alternative configurations.  It is impossible to offer multiple
   configurations in a single payload type.  Thus, when multiple
   configuration offers are made, each offer requires its own RTP
   payload type associated with the offer.  However, it is possible to
   offer multiple operation points using one configuration in a single
   payload type by including sprop-vps in the offer and recv-ols-id in
   the answer.

   An implementation SHOULD be able to understand all media type
   parameters (including all optional media type parameters), even if it
   doesn't support the functionality related to the parameter.  This, in
   conjunction with proper application logic in the implementation,
   allows the implementation, after having received an offer, to create
   an answer by potentially downgrading one or more of the optional
   parameters to the point where the implementation can cope, leading to
   higher chances of interoperability beyond the most basic interop
   points (for which, as described above, no optional parameters are
   necessary).

      |  Informative note: In implementations of previous H.26x payload
      |  formats, it was occasionally observed that implementations were
      |  incapable of parsing most (or all) of the optional parameters.
      |  As a result, the offer/answer exchange resulted in a baseline
      |  performance (using the default values for the optional
      |  parameters) with the resulting suboptimal user experience.
      |  However, there are valid reasons to forego the implementation
      |  complexity of implementing the parsing of some or all of the
      |  optional parameters, for example, when there is predetermined
      |  knowledge, not negotiated by an SDP-based offer/answer process,
      |  of the capabilities of the involved systems (walled gardens,
      |  baseline requirements defined in application standards higher
      |  up in the stack, and similar).

   An answerer MAY extend the offer with additional media format
   configurations.  However, to enable their usage, in most cases, a
   second offer is required from the offerer to provide the bitstream
   property parameters that the media sender will use.  This also has
   the effect that the offerer has to be able to receive this media
   format configuration, not only to send it.

7.3.3.  Multicast

   For bitstreams being delivered over multicast, the following rules
   apply:

   *  The media format configuration is identified by profile-id, tier-
      flag, sub-profile-id, level-id, and interop-constraints.  These
      media format configuration parameters, including level-id, MUST be
      used symmetrically; that is, the answerer MUST either maintain all
      configuration parameters or remove the media format (payload type)
      completely.  Note that this implies that the level-id for offer/
      answer in multicast is not changeable.

   *  To simplify the handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [RFC3264].  An answer MUST NOT
      contain a payload type number used in the offer unless the
      configuration is the same as in the offer.

   *  Parameter sets received MUST be associated with the originating
      source and MUST only be used in decoding the incoming bitstream
      from the same source.

   *  The rules for other parameters are the same as above for unicast
      as long as the three above rules are obeyed.

7.3.4.  Usage in Declarative Session Descriptions

   When VVC over RTP is offered with SDP in a declarative style, as in
   Real Time Streaming Protocol (RTSP) [RFC7826] or Session Announcement
   Protocol (SAP) [RFC2974], the following considerations are necessary.

   *  All parameters capable of indicating both bitstream properties and
      receiver capabilities are used to indicate only bitstream
      properties.  For example, in this case, the parameters profile-id,
      tier-id, and level-id declare the values used by the bitstream,
      not the capabilities for receiving bitstreams.  As a result, the
      following interpretation of the parameters MUST be used:

      -  Declaring actual configuration or bitstream properties:

         o  profile-id

         o  tier-flag

         o  level-id

         o  interop-constraints

         o  sub-profile-id

         o  sprop-dci

         o  sprop-vps

         o  sprop-sps

         o  sprop-pps

         o  sprop-max-don-diff

         o  sprop-depack-buf-bytes

         o  sprop-sublayer-id

         o  sprop-ols-id

         o  sprop-sei

      -  Not usable (when present, they MUST be ignored):

         o  max-lsr

         o  max-fps

         o  max-recv-level-id

         o  depack-buf-cap

         o  recv-sublayer-id

         o  recv-ols-id

      -  A receiver of the SDP is required to support all parameters and
         values of the parameters provided; otherwise, the receiver MUST
         reject (RTSP) or not participate in (SAP) the session.  It
         falls on the creator of the session to use values that are
         expected to be supported by the receiving application.

7.3.5.  Considerations for Parameter Sets

   When out-of-band transport of parameter sets is used, parameter sets
   MAY still be additionally transported in-band unless explicitly
   disallowed by an application, and some of these additional parameter
   sets may update some of the out-of-band transported parameter sets.
   An update of a parameter set refers to the sending of a parameter set
   of the same type using the same parameter set ID but with different
   values for at least one other parameter of the parameter set.

8.  Use with Feedback Messages

   The following subsections define the use of the Picture Loss
   Indication (PLI) and Full Intra Request (FIR) feedback messages with
   [VVC].  The PLI is defined in [RFC4585], and the FIR message is
   defined in [RFC5104].  In accordance with this memo, unlike [HEVC], a
   sender MUST NOT send Slice Loss Indication (SLI) or Reference Picture
   Selection Indication (RPSI), and a receiver SHOULD ignore RPSI and
   treat a received SLI as a PLI.

8.1.  Picture Loss Indication (PLI)

   As specified in Section 6.3.1 of [RFC4585], the reception of a PLI by
   a media sender indicates "the loss of an undefined amount of coded
   video data belonging to one or more pictures".  Without having any
   specific knowledge of the setup of the bitstream (such as use and
   location of in-band parameter sets, non-IRAP decoder refresh points,
   picture structures, and so forth), a reaction to the reception of a
   PLI by a VVC sender SHOULD be to send an IRAP picture and relevant
   parameter sets, potentially with sufficient redundancy so to ensure
   correct reception.  However, sometimes information about the
   bitstream structure is known.  For example, such information can be
   parameter sets that have been conveyed out of band through mechanisms
   not defined in this document and that are known to stay static for
   the duration of the session.  In that case, it is obviously
   unnecessary to send them in-band as a result of the reception of a
   PLI.  Other examples could be devised based on a priori knowledge of
   different aspects of the bitstream structure.  In all cases, the
   timing and congestion control mechanisms of [RFC4585] MUST be
   observed.

8.2.  Full Intra Request (FIR)

   The purpose of the FIR message is to force an encoder to send an
   independent decoder refresh point as soon as possible while observing
   applicable congestion-control-related constraints, such as those set
   out in [RFC8082].

   Upon reception of a FIR, a sender MUST send an IDR picture.
   Parameter sets MUST also be sent, except when there is a priori
   knowledge that the parameter sets have been correctly established.  A
   typical example for that is an understanding between the sender and
   receiver, established by means outside this document, that parameter
   sets are exclusively sent out of band.

9.  Security Considerations

   The scope of this section is limited to the payload format itself and
   to one feature of [VVC] that may pose a particularly serious security
   risk if implemented naively.  The payload format, in isolation, does
   not form a complete system.  Implementers are advised to read and
   understand relevant security-related documents, especially those
   pertaining to RTP (see the Security Considerations section in
   [RFC3550]) and the security of the call-control stack chosen (that
   may make use of the media type registration of this memo).
   Implementers should also consider known security vulnerabilities of
   video coding and decoding implementations in general and avoid those.

   Within this RTP payload format, and with the exception of the user
   data SEI message as described below, no security threats other than
   those common to RTP payload formats are known.  In other words,
   neither the various media-plane-based mechanisms nor the signaling
   part of this memo seem to pose a security risk beyond those common to
   all RTP-based systems.

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] and in any applicable RTP profile, such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
   SAVPF [RFC5124].  However, as "Securing the RTP Framework: Why RTP
   Does Not Mandate a Single Media Security Solution" [RFC7202]
   discusses, it is not an RTP payload format's responsibility to
   discuss or mandate what solutions are used to meet the basic security
   goals, like confidentiality, integrity, and source authenticity for
   RTP in general.  This responsibility lays on anyone using RTP in an
   application.  They can find guidance on available security mechanisms
   and important considerations in "Options for Securing RTP Sessions"
   [RFC7201].  The rest of this section discusses the security impacting
   properties of the payload format itself.

   Because the data compression used with this payload format is applied
   end to end, any encryption needs to be performed after compression.
   A potential denial-of-service threat exists for data encodings using
   compression techniques that have non-uniform receiver-end
   computational load.  The attacker can inject pathological datagrams
   into the bitstream that are complex to decode and that cause the
   receiver to be overloaded.  [VVC] is particularly vulnerable to such
   attacks, as it is extremely simple to generate datagrams containing
   NAL units that affect the decoding process of many future NAL units.
   Therefore, the usage of data origin authentication and data integrity
   protection of at least the RTP packet is RECOMMENDED but NOT REQUIRED
   based on the thoughts of [RFC7202].

   Like HEVC [RFC7798], [VVC] includes a user data Supplemental
   Enhancement Information (SEI) message.  This SEI message allows
   inclusion of an arbitrary bitstring into the video bitstream.  Such a
   bitstring could include JavaScript, machine code, and other active
   content.  [VVC] leaves the handling of this SEI message to the
   receiving system.  In order to avoid harmful side effects of the user
   data SEI message, decoder implementations cannot naively trust its
   content.  For example, it would be a bad and insecure implementation
   practice to forward any JavaScript a decoder implementation detects
   to a web browser.  The safest way to deal with user data SEI messages
   is to simply discard them, but that can have negative side effects on
   the quality of experience by the user.

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

10.  Congestion Control

   Congestion control for RTP SHALL be used in accordance with RTP
   [RFC3550] and with any applicable RTP profile, e.g., AVP [RFC3551] or
   AVPF [RFC4585].  If best-effort service is being used, an additional
   requirement is that users of this payload format MUST monitor packet
   loss to ensure that the packet loss rate is within an acceptable
   range.  Packet loss is considered acceptable if a TCP flow across the
   same network path and experiencing the same network conditions would
   achieve an average throughput, measured on a reasonable timescale,
   that is not less than all RTP streams combined are achieved.  This
   condition can be satisfied by implementing congestion-control
   mechanisms to adapt the transmission rate, by implementing the number
   of layers subscribed for a layered multicast session, or by arranging
   for a receiver to leave the session if the loss rate is unacceptably
   high.

   The bitrate adaptation necessary for obeying the congestion control
   principle is easily achievable when real-time encoding is used, for
   example, by adequately tuning the quantization parameter.  However,
   when pre-encoded content is being transmitted, bandwidth adaptation
   requires the pre-coded bitstream to be tailored for such adaptivity.
   The key mechanisms available in [VVC] are temporal scalability and
   spatial/SNR scalability.  A media sender can remove NAL units
   belonging to higher temporal sublayers (i.e., those NAL units with a
   high value of TID) or higher spatio-SNR layers until the sending
   bitrate drops to an acceptable range.

   The mechanisms mentioned above generally work within a defined
   profile and level; therefore no renegotiation of the channel is
   required.  Only when non-downgradable parameters (such as profile)
   are required to be changed does it become necessary to terminate and
   restart the RTP stream(s).  This may be accomplished by using
   different RTP payload types.

   MANEs MAY remove certain unusable packets from the RTP stream when
   that RTP stream was damaged due to previous packet losses.  This can
   help reduce the network load in certain special cases.  For example,
   MANEs can remove those FUs where the leading FUs belonging to the
   same NAL unit have been lost or those dependent slice segments when
   the leading slice segments belonging to the same slice have been
   lost, because the trailing FUs or dependent slice segments are
   meaningless to most decoders.  MANE can also remove higher temporal
   scalable layers if the outbound transmission (from the MANE's
   viewpoint) experiences congestion.

11.  IANA Considerations

   A new media type has been registered with IANA; see Section 7.1.

12.  References

12.1.  Normative References

   [ISO23090-3]
              International Organization for Standardization,
              "Information technology - Coded representation of
              immersive media - Part 3: Versatile video coding", ISO/
              IEC 23090-3:2022, September 2022,
              <https://www.iso.org/standard/73022.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <https://www.rfc-editor.org/info/rfc5104>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <https://www.rfc-editor.org/info/rfc5576>.

   [RFC8082]  Wenger, S., Lennox, J., Burman, B., and M. Westerlund,
              "Using Codec Control Messages in the RTP Audio-Visual
              Profile with Feedback with Layered Codecs", RFC 8082,
              DOI 10.17487/RFC8082, March 2017,
              <https://www.rfc-editor.org/info/rfc8082>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,
              <https://www.rfc-editor.org/info/rfc8866>.

   [VSEI]     ITU-T, "Versatile supplemental enhancement information
              messages for coded video bitstreams", ITU-T
              Recommendation H.274, May 2022,
              <https://www.itu.int/rec/T-REC-H.274>.

   [VVC]      ITU-T, "Versatile Video Coding", ITU-T
              Recommendation H.266, April 2022,
              <http://www.itu.int/rec/T-REC-H.266>.

12.2.  Informative References

   [CABAC]    Sole, J., et al., "Transform coefficient coding in HEVC",
              IEEE Transactions on Circuits and Systems for Video
              Technology, DOI 10.1109/TCSVT.2012.2223055, December 2012,
              <https://doi.org/10.1109/TCSVT.2012.2223055>.

   [HEVC]     ITU-T, "High efficiency video coding", ITU-T
              Recommendation H.265, August 2021,
              <https://www.itu.int/rec/T-REC-H.265>.

   [MPEG2S]   International Organization for Standardization,
              "Information technology - Generic coding of moving
              pictures and associated audio information - Part 1:
              Systems", ISO/IEC 13818-1:2022, September 2022.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
              October 2000, <https://www.rfc-editor.org/info/rfc2974>.

   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184,
              DOI 10.17487/RFC6184, May 2011,
              <https://www.rfc-editor.org/info/rfc6184>.

   [RFC6190]  Wenger, S., Wang, Y.-K., Schierl, T., and A.
              Eleftheriadis, "RTP Payload Format for Scalable Video
              Coding", RFC 6190, DOI 10.17487/RFC6190, May 2011,
              <https://www.rfc-editor.org/info/rfc6190>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <https://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <https://www.rfc-editor.org/info/rfc7202>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <https://www.rfc-editor.org/info/rfc7656>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <https://www.rfc-editor.org/info/rfc7667>.

   [RFC7798]  Wang, Y.-K., Sanchez, Y., Schierl, T., Wenger, S., and M.
              M. Hannuksela, "RTP Payload Format for High Efficiency
              Video Coding (HEVC)", RFC 7798, DOI 10.17487/RFC7798,
              March 2016, <https://www.rfc-editor.org/info/rfc7798>.

   [RFC7826]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, Ed., "Real-Time Streaming Protocol
              Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
              2016, <https://www.rfc-editor.org/info/rfc7826>.

Acknowledgements

   Dr. Byeongdoo Choi is thanked for the video-codec-related technical
   discussion and other aspects in this memo.  Xin Zhao and Dr. Xiang Li
   are thanked for their contributions on [VVC] specification
   descriptive content.  Spencer Dawkins is thanked for his valuable
   review comments that led to great improvements of this memo.  Some
   parts of this specification share text with the RTP payload format
   for HEVC [RFC7798].  We thank the authors of that specification for
   their excellent work.

Authors' Addresses

   Shuai Zhao
   Intel
   2200 Mission College Blvd
   Santa Clara,  95054
   United States of America
   Email: shuai.zhao@ieee.org


   Stephan Wenger
   Tencent
   2747 Park Blvd
   Palo Alto,  94588
   United States of America
   Email: stewe@stewe.org


   Yago Sanchez
   Fraunhofer HHI
   Einsteinufer 37
   10587 Berlin
   Germany
   Email: yago.sanchez@hhi.fraunhofer.de


   Ye-Kui Wang
   Bytedance Inc.
   8910 University Center Lane
   San Diego,  92122
   United States of America
   Email: yekui.wang@bytedance.com


   Miska M. Hannuksela
   Nokia Technologies
   Hatanpään valtatie 30
   FI-33100 Tampere
   Finland
   Email: miska.hannuksela@nokia.com