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.

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].

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.

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

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 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, 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.

• 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) [RFC 8866]
• usage of RTCP feedback messages

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.

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.

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.

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.

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

    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             |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 invideo formats to allow an efficient playout buffer handling.

Payload Type (PT): 7 bits

The assignment of an RTP payload type for this new packet formatis outside the scope of this document and will not be specifiedhere. The assignment of a payload type has to be performed eitherthrough the profile used or in a dynamic way.

Sequence Number (SN): 16 bits

Set and used in accordance with [RFC 3550].

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].

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.

• 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.

  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        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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 Sbit MUST 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 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 fragmentedNAL unit.

• 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])

• 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.

• 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.

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 [RFC 3550].

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>

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.

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.

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 RFC 4648) 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 RFC 4648) 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.

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 RFC 4648) 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 RFC 4648) 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 RFC 4648) 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 RFC 4648) 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 RFC 4648) representations of SEI NAL units, as specified in [VSEI].

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.

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.

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.

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.

• 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 RFC 5576. 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.

        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>;

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

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

• 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).

• To simplify handling and matching of these configurations, the same RTP payload type number used in the offer SHOULD also be used in the answer, as specified in [RFC 3264].
• 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.

• 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.

• 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.

• 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.

• 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 RFC 5576), 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 RFC 5576, 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 [RFC 5576].

                                    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

• 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 [RFC 3264]. An answer MUST NOT contain a payload type number used in the offer unless the configuration is the same as in the offer.
• 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.

• 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:
    • 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, they MUST 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 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.

[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]

S. Bradner, "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]

J. Rosenberg, 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]

H. Schulzrinne, S. Casner, R. Frederick, 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]

H. Schulzrinne, 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]

M. Baugher, D. McGrew, M. Naslund, E. Carrara, 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]

J. Ott, S. Wenger, N. Sato, C. Burmeister, 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]

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

[RFC5104]

S. Wenger, U. Chandra, M. Westerlund, 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]

J. Ott, 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]

J. Lennox, J. Ott, 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]

S. Wenger, J. Lennox, B. Burman, 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]

B. Leiba, "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]

A. Begen, P. Kyzivat, C. Perkins, 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>.

[CABAC]

Sole, J., et al., "Transform coefficient coding in HEVC", 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]

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

[RFC6184]

Y.-K. Wang, R. Even, T. Kristensen, 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]

S. Wenger, Y.-K. Wang, T. Schierl, 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]

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

[RFC7202]

C. Perkins, 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]

J. Lennox, K. Gross, S. Nandakumar, G. Salgueiro, and B. Burman, "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]

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

[RFC7798]

Y.-K. Wang, Y. Sanchez, T. Schierl, S. Wenger, 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]

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

Shuai Zhao

Stephan Wenger

Yago Sanchez

Ye-Kui Wang

Miska M. Hannuksela