1.1.1.Coding-Tool Features (Informative)

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

Similar to earlier hybrid-video-coding-based standards, includingHEVC, 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 theoriginal and the prediction) is then coded. The gains in codingefficiency are achieved by redesigning and improving almost all partsof the codec over earlier designs. In addition, VVC includes severaltools 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 ofthe important improvements is the more flexible coding tree structurein VVC, i.e., multi-type tree. In addition to quadtree, binary andternary trees are also supported, which contributes significantimprovement in coding efficiency. Moreover, the maximum size of a coding tree unit (CTU) is increased from 64x64 to 128x128. Toimprove the coding efficiency of chroma signal, luma-chroma-separatedtrees 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, thetransform sizes range from 4x4 to 32x32. In addition, VVC alsosupport sub-block transform for both intra- and inter-coded blocks.For intra-coded blocks, intra sub-partitioning (ISP) may be used toallow sub-block-based intra prediction and transform. For inter blocks, sub-block transform may be used assuming that only a part ofan inter block has non-zero transform coefficients.¶

Entropy coding

Similar to HEVC, VVC uses a single entropy-coding engine, which isbased on context adaptive binary arithmetic coding[CABAC]but with the support of multi-window sizes. The window sizes can beinitialized differently for different context models. Due to such adesign, it has more efficient adaptation speed and better codingefficiency. A joint chroma residual coding scheme is applied tofurther exploit the correlation between the residuals of two colorcomponents. In VVC, different residual coding schemes are appliedfor regular transform coefficients and residual samples generatedusing transform-skip mode.¶

In-loop filtering

VVC has more feature support in loop filters than HEVC. Thedeblocking filter in VVC is similar to HEVC but operates at asmaller grid. After deblocking and sample adaptive offset (SAO), anadaptive loop filter (ALF) may be used. As a Wiener filter, ALFreduces distortion of decoded pictures. Besides, VVC introduces anew module called luma mapping with chroma scaling to fully utilize the dynamic range of signal so that rate-distortionperformance 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), whichcan save bit cost for motion vectors by adaptively signaling motionvector resolution. Then, the affine motion compensation is includedto capture complicated motion-like zooming and rotation. Meanwhile,prediction refinement with the optical flow (PROF) with affine modeis further deployed to mimic affine motion at the pixel level.Thirdly, the decoder-side motion vector refinement (DMVR) is a methodto derive the motion vector at the decoder side based on block matching so that fewer bits may be spenton motion vectors. Bidirectional optical flow (BDOF) is a similarmethod 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 motionvector differences on top of merge mode. In addition to MMVD, thereare another three types of special merge modes, i.e., sub-blockmerge, triangle, and combined intra/inter prediction (CIIP). The sub-block merge list includes one candidate of sub-block temporal motionvector prediction (SbTMVP) and up to four candidates of affine motionvectors. 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 finergranularity, VVC supports 65 angular directions instead of 33directions in HEVC. The intra mode coding is based on a 6-most-probable-modes scheme, and the 6 most probable modes are derived usingthe neighboring intra prediction directions. In addition, to dealwith the different distributions of intra prediction angles fordifferent block aspect ratios, a wide-angle-intra-prediction (WAIP)scheme is applied in VVC by including intra prediction anglesbeyond those present in HEVC. Unlike HEVC, which only allows usingthe most adjacent line of reference samples for intra prediction,VVC also allows using two further reference lines, known asmulti-reference-line (MRL) intra prediction. The additionalreference lines can be only used for the 6 most probable intra predictionmodes. To capture the strong correlation between different colorcomponents, in VVC, a cross-component linear mode (CCLM) isutilized, 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 predictionblock boundary. Matrix-based intra prediction (MIP) modes are alsoused in VVC, which generates an up to 8x8 intra prediction blockusing 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 quantizationerror by state-based switching between two quantizers.¶

1.1.2.Systems and Transport Interfaces (Informative)

VVC inherits the basic systems and transport interface designsfrom HEVC and AVC. These include the NAL-unit-based syntaxstructure, the hierarchical syntax and data unit structure, thesupplemental enhancement information (SEI) message mechanism, and thevideo buffering model based on the hypothetical reference decoder(HRD). The scalability features of VVC are conceptually similar tothe scalable extension of HEVC, known as SHVC. The hierarchical syntaxand data unit structure consists of parameter sets at various levels(i.e., decoder, sequence (pertaining to all), sequence (pertaining to a single), andpicture), 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 referto the DCI, for layers associated with one or more output layer sets specified by the VPS, and for any layerthat 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. VVCuses a two-byte NAL unit header, as shown inFigure 1. The payloadof a NAL unit refers to the NAL unit excluding the NAL unit header.¶

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

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

F: 1 bit

forbidden_zero_bit. This field is required to be zero in VVC. Note that theinclusion of this bit in the NAL unit header was to enabletransport of VVC video over MPEG-2 transport systems (avoidanceof start code emulations)[MPEG2S]. In the context of this payload format, the value 1 may be used to indicate a syntax violation, e.g., fora NAL unit resulted from aggregating a number of fragmented unitsof a NAL unit but missing the last fragment, as described in the last sentence ofSection 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, whereina layer may be, e.g., a spatial scalable layer, a quality scalablelayer, a layer containing a different view, etc.¶

Type: 5 bits

nal_unit_type. This field specifies the NAL unit type, as definedin Table 5 of[VVC]. For a reference of all currently definedNAL unit types and their semantics, please refer toSection 7.4.2.2 in[VVC].¶

TID: 3 bits

nuh_temporal_id_plus1. This field specifies the temporalidentifier of the NAL unit plus 1. The value of TemporalId isequal to TID minus 1. A TID value of 0 is illegal to ensure thatthere 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.¶

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 componentinto 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 bitstreamconsisting of VCL NAL units with a particular value of the TemporalIdvariable, 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 NALunits 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 amiddlebox, selective forwarding unit, or application-layer gatewaythat is capable of parsing certain aspects of the RTP payload headersor the RTP payload and reacting to their contents.¶

Informative note: The concept of a MANE goes beyond normal routersor 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 packetsto be dropped according to the needs of the media coding. Forexample, if a MANE has to drop packets due to congestion on acertain link, it can identify and remove those packets whoseelimination produces the least adverse effect on the userexperience. After dropping packets, MANEs must rewrite RTCPpackets to match the changes to the RTP stream, as specified inSection 7 of [RFC3550].¶

NAL unit decoding order:

A NAL unit order that conforms to theconstraints 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 sequencenumber 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.¶

4.3.1.Single NAL Unit Packets

A single NAL unit packet contains exactly one NAL unit and consistsof 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 NALunit, as shown inFigure 3.¶

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

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

4.3.2.Aggregation Packets (APs)

Aggregation packets (APs) can reducepacketization 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 itMUST 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 inFigure 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        | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields in the payload header of an AP are set as follows. The F bitMUSTbe equal to 0 if the F bit of each aggregated NAL unit is equal tozero; otherwise, itMUST be equal to 1. The Type fieldMUST be equalto 28.¶

The value of LayerIdMUST be equal to the lowest value of LayerId ofall the aggregated NAL units. The value of TIDMUST be the lowestvalue of TID of all the aggregated NAL units.¶

Informative note: All VCL NAL units in an AP have the same TIDvalue since they belong to the same access unit. However, an APmay contain non-VCL NAL units for which the TID value in the NALunit header may be different than the TID value of the VCL NALunits 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 APMUST carry at least two aggregation units and can carry as manyaggregation units as necessary; however, the total amount of data inan AP obviouslyMUST fit into an IP packet, and the sizeSHOULD bechosen so that the resulting IP packet is smaller than the MTU sizein order to avoid IP layer fragmentation. An APMUST NOT contain the FUsspecified inSection 4.3.3. APsMUST NOT be nested, i.e., an AP cannot contain another AP.¶

The first aggregation unit in an AP consists of a conditional 16-bitDONL field (in network byte order), followed by 16 bits of unsigned sizeinformation (in network byte order) that indicate the size of theNAL unit in bytes (excluding these two octets but including the NALunit header), followed by the NAL unit itself, including its NAL unitheader, as shown inFigure 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                              | |                                                               | |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |                               : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Informative note: The first octet ofFigure 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 NALunit.¶

If sprop-max-don-diff is greater than 0, the DONL fieldMUST 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 fieldMUST 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 APwill be followed immediately by 16 bits of unsigned size information(in network byte order) that indicate thesize of the NAL unit in bytes (excluding these two octets butincluding the NAL unit header), followed by the NAL unit itself,including its NAL unit header, as shown inFigure 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    | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               | |                                                               | |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |                               : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Informative note: The first octet ofFigure 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 aggregationunits, labeled as 1 and 2 in the figure, without the DONL fieldbeing 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 8 presents an example of an AP that contains two aggregationunits, 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        | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.3.Fragmentation Units

Fragmentation Units (FUs) are introduced to enable fragmenting asingle NAL unit into multiple RTP packets, possibly withoutcooperation or knowledge of the[VVC] encoder. A fragmentof a NAL unit consists of an integer number of consecutive octets ofthat NAL unit. Fragments of the same NAL unitMUST be sent inconsecutive order with ascending RTP sequence numbers (with no otherRTP packets within the same RTP stream being sent between the firstand last fragment).¶

When a NAL unit is fragmented and conveyed within FUs, it is referredto as a fragmented NAL unit. APsMUST NOT be fragmented. FUsMUST 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 inFigure 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        | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields in the payload header are set as follows. The Type fieldMUST be equal to 29. The fields F, LayerId, and TIDMUST be equal tothe fields F, LayerId, and TID, respectively, of the fragmented NALunit.¶

The FU header consists of an S bit, an E bit, an R bit, and a 5-bit FuTypefield, as shown inFigure 10.¶

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

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 NALunit, i.e., the first byte of the FU payload is also the firstbyte of the payload of the fragmented NAL unit. When the FUpayload is not the start of the fragmented NAL unit payload, the SbitMUST be set to 0.¶

E: 1 bit

When set to 1, the E bit indicates the end of a fragmented NALunit, i.e., the last byte of the payload is also the last byte ofthe fragmented NAL unit. When the FU payload is not the lastfragment of a fragmented NAL unit, the E bitMUST 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 bitMUST be set to 0.¶

FuType: 5 bits

The field FuTypeMUST be equal to the field Type of the fragmentedNAL unit.¶

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

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

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

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

If an FU is lost, the receiverSHOULD discard all followingfragmentation units in transmission order, corresponding to the samefragmented NAL unit, unless the decoder in the receiver is known tobe prepared to gracefully handle incomplete NAL units.¶

A receiver in an endpoint or in a MANEMAY aggregate the first n-1fragments of a NAL unit to an (incomplete) NAL unit, even if fragmentn of that NAL unit is not received. In this case, theforbidden_zero_bit of the NAL unitMUST be set to 1 to indicate asyntax violation.¶

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 SDPMUST be video.¶
• The encoding name in the "a=rtpmap" line of SDPMUST be H266 (the media subtype).¶
• The clock rate in the "a=rtpmap" lineMUST be 90000.¶
• TheOPTIONAL 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.¶

• TheOPTIONAL 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 inSection 6.3 of [RFC5576]. For a particular media format (i.e., RTP payload type), sprop-vps, sprop-sps, sprop-pps, sprop-sei, or sprop-dciMUST 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-dciMUST 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.¶

                                    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  Ptier-flag                          C  D  C  D  Plevel-id                           D  D  D  D  Psub-profile-id                     C  D  C  D  Pinterop-constraints                C  D  C  D  Pmax-recv-level-id                  R  R  R  R  -sprop-max-don-diff                 P  P  -  -  Psprop-depack-buf-bytes             P  P  -  -  Pdepack-buf-cap                     R  R  R  R  -max-lsr                            R  R  R  R  -max-fps                            R  R  R  R  -sprop-dci                          P  P  -  -  Psprop-sei                          P  P  -  -  Psprop-vps                          P  P  -  -  Psprop-sps                          P  P  -  -  Psprop-pps                          P  P  -  -  Psprop-sublayer-id                  P  P  -  -  Precv-sublayer-id                   O  O  O  O  -sprop-ols-id                       P  P  -  -  Precv-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

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 answererMUST 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 offerSHOULD also be used in the answer, as specified in[RFC3264]. An answerMUST NOT contain a payload type number used in the offer unless the configuration is the same as in the offer.¶
• Parameter sets receivedMUST be associated with the originating source andMUST 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 parametersMUST be used:¶

  • Declaring actual configuration or bitstream properties:¶

    • profile-id¶
    • tier-flag¶
    • level-id¶
    • interop-constraints¶
    • sub-profile-id¶
    • sprop-dci¶
    • sprop-vps¶
    • sprop-sps¶
    • sprop-pps¶
    • sprop-max-don-diff¶
    • sprop-depack-buf-bytes¶
    • sprop-sublayer-id¶
    • sprop-ols-id¶
    • sprop-sei¶

  • Not usable (when present, theyMUST be ignored):¶

    • max-lsr¶
    • max-fps¶
    • max-recv-level-id¶
    • depack-buf-cap¶
    • recv-sublayer-id¶
    • recv-ols-id¶
  • A receiver of the SDP is required to support all parameters and values of the parameters provided; otherwise, the receiverMUST 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 setsMAY 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.¶