| avtcore | S. Zhao |
| Internet-Draft | S. Wenger |
| Intended status: Standards Track | Tencent |
| Expires: January 12, 2021 | Y. Sanchez |
| Fraunhofer HHI | |
| July 11, 2020 |
RTP Payload Format for Versatile Video Coding (VVC)
draft-ietf-avtcore-rtp-vvc-02
This memo describes an RTP payload format for the video coding standard ITU-T Recommendation [H.266] and ISO/IEC International Standard [ISO23090-3], both also known as Versatile Video Coding (VVC) and developed by the Joint Video Experts Team (JVET). The RTP payload format allows for packetization of one or more Network Abstraction Layer (NAL) units in each RTP packet payload as well as fragmentation of a NAL unit into multiple RTP packets. The payload format has wide applicability in videoconferencing, Internet video streaming, and high-bitrate entertainment-quality video, among other applications.
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The Versatile Video Coding [VVC] specification, formally published as both ITU-T Recommendation H.266 and ISO/IEC International Standard 23090-3 [ISO23090-3], is currently in the ISO/IEC approval process and is planned for ratification in mid 2020. H.266 is reported to provide significant coding efficiency gains over H.265 and earlier video codec formats.
This memo describes an RTP payload format for VVC. It shares its basic design with the NAL (Network Abstraction Layer) unit-based RTP payload formats of, H.264 Video Coding [RFC6184], Scalable Video Coding (SVC) [RFC6190], High Efficiency Video Coding (HEVC) [RFC7798] and their respective predecessors. With respect to design philosophy, security, congestion control, and overall implementation complexity, it has similar properties to those earlier payload format specifications. This is a conscious choice, as at least RFC 6184 is widely deployed and generally known in the relevant implementer communities. Certain mechanisms known from [RFC6190] were incorporated in VVC, as VVC version 1 supports temporal, spatial, and signal-to-noise ratio (SNR) scalability.
[VVC] and [HEVC] share a similar hybrid video codec design. In this memo, we provide a very brief overview of those features of VVC that are, in some form, addressed by the payload format specified herein. Implementers have to read, understand, and apply the ITU- T/ISO/IEC specifications pertaining to [VVC] to arrive at interoperable, well-performing implementations.
Conceptually, both [VVC] and [HEVC] include a Video Coding Layer (VCL), which is often used to refer to the coding-tool features, and a NAL, which is often used to refer to the systems and transport interface aspects of the codecs.
Coding tool features are described below with occasional reference to the coding tool set of [HEVC], which is well known in the community.
Similar to earlier hybrid-video-coding-based standards, including HEVC, the following basic video coding design is employed by VVC. A prediction signal is first formed by either intra- or motion- compensated prediction, and the residual (the difference between the original and the prediction) is then coded. The gains in coding efficiency are achieved by redesigning and improving almost all parts of the codec over earlier designs. In addition, [VVC] includes several tools to make the implementation on parallel architectures easier.
Finally, [VVC] includes temporal, spatial, and SNR scalability as well as multiview coding support.
Coding blocks and transform structure
Among major coding-tool differences between HEVC and VVC, one of the important improvements is the more flexible coding tree structure in VVC, i.e., multi-type tree. In addition to quadtree, binary and ternary trees are also supported, which contributes significant improvement in coding efficiency. Moreover, the maximum size of Coding Tree Unit (CTU) is increased from 64x64 to 128x128. To improve the coding efficiency of chroma signal, luma chroma separated trees at CTU level may be employed for intra-slices. The square transforms in HEVC are extended to non-square transforms for rectangular blocks resulting from binary and ternary tree splits. Besides, [VVC] supports multiple transform sets (MTS), including DCT-2, DST-7, and DCT-8 as well as the non-separable secondary transform. The transforms used in [VVC] can have different sizes with support for larger transform sizes. For DCT-2, the transform sizes range from 2x2 to 64x64, and for DST-7 and DCT-8, the transform sizes range from 4x4 to 32x32. In addition, [VVC] also support sub-block transform for both intra and inter coded blocks. For intra coded blocks, intra sub-partitioning (ISP) may be used to allow sub-block based intra prediction and transform. For inter blocks, sub-block transform may be used assuming that only a part of an inter-block has non-zero transform coefficients.
Entropy coding
Similar to HEVC , [VVC] uses a single entropy-coding engine, which is based on Context Adaptive Binary Arithmetic Coding (CABAC) [CABAC], but with the support of multi-window sizes. The window sizes can be initialized differently for different context models. Due to such a design, it has more efficient adaptation speed and better coding efficiency. A joint chroma residual coding scheme is applied to further exploit the correlation between the residuals of two color components. In VVC, different residual coding schemes are applied for regular transform coefficients and residual samples generated using transform-skip mode.
In-loop filtering
[VVC] has more feature support in loop filters than HEVC. The deblocking filter in [VVC] is similar to HEVC but operates at a smaller grid. After deblocking and sample adaptive offset (SAO), an adaptive loop filter (ALF) may be used. As a Wiener filter, ALF reduces distortion of decoded pictures. Besides, [VVC] introduces a new module before deblocking called luma mapping with chroma scaling to fully utilize the dynamic range of signal so that rate-distortion performance of both SDR and HDR content is improved.
Motion prediction and coding
Compared to HEVC, [VVC] introduces several improvements in this area. First, there is the Adaptive motion vector resolution (AMVR), which can save bit cost for motion vectors by adaptively signaling motion vector resolution. Then the Affine motion compensation is included to capture complicated motion like zooming and rotation. Meanwhile, prediction refinement with the optical flow with affine mode (PROF) is further deployed to mimic affine motion at the pixel level. Thirdly the decoder side motion vector refinement (DMVR) is a method to derive MV vector at decoder side based on block matching so that fewer bits may be spent on motion vectors. Bi-directional optical flow (BDOF) is a similar method to PROF. BDOF adds a sample wise offset at 4x4 sub-block level that is derived with equations based on gradients of the prediction samples and a motion difference relative to CU motion vectors. Furthermore, merge with motion vector difference (MMVD) is a special mode, which further signals a limited set of motion vector differences on top of merge mode. In addition to MMVD, there are another three types of special merge modes, i.e., sub-block merge, triangle, and combined intra-/inter- prediction (CIIP). Sub- block merge list includes one candidate of sub-block temporal motion vector prediction (SbTMVP) and up to four candidates of affine motion vectors. Triangle is based on triangular block motion compensation. CIIP combines intra- and inter- predictions with weighting. Adaptive weighting may be employed with a block-level tool called bi-prediction with CU based weighting (BCW) which provides more flexibility than in HEVC.
Intra prediction and intra-coding
To capture the diversified local image texture directions with finer granularity, [VVC] supports 65 angular directions instead of 33 directions in HEVC. The intra mode coding is based on a 6 most probable mode scheme, and the 6 most probable modes are derived using the neighboring intra prediction directions. In addition, to deal with the different distributions of intra prediction angles for different block aspect ratios, a wide-angle intra prediction (WAIP) scheme is applied in [VVC] by including intra prediction angles beyond those present in HEVC. Unlike HEVC which only allows using the most adjacent line of reference samples for intra prediction, [VVC] also allows using two further reference lines, as known as multi-reference-line (MRL) intra prediction. The additional reference lines can be only used for 6 most probable intra prediction modes. To capture the strong correlation between different colour components, in VVC, a cross-component linear mode (CCLM) is utilized which assumes a linear relationship between the luma sample values and their associated chroma samples. For intra prediction, [VVC] also applies a position-dependent prediction combination (PDPC) for refining the prediction samples closer to the intra prediction block boundary. Matrix-based intra prediction (MIP) modes are also used in [VVC] which generates an up to 8x8 intra prediction block using a weighted sum of downsampled neighboring reference samples, and the weightings are hardcoded constants.
Other coding-tool feature
[VVC] introduces dependent quantization (DQ) to reduce quantization error by state-based switching between two quantizers.
[VVC] inherits the basic systems and transport interfaces designs from HEVC and H.264. These include the NAL-unit-based syntax structure, the hierarchical syntax and data unit structure, the Supplemental Enhancement Information (SEI) message mechanism, and the video buffering model based on the Hypothetical Reference Decoder (HRD). The scalability features of [VVC] are conceptually similar to the scalable variant of HEVC known as SHVC. The hierarchical syntax and data unit structure consists of parameter sets at various levels (decoder, sequence (pertaining to all), sequence (pertaining to a single), picture), picture-level header parameters, slice-level header parameters, and lower-level parameters.
A number of key components that influenced the Network Abstraction Layer design of [VVC] as well as this memo are described below
Decoding Capability Information
The Decoding capability information includes parameters that stay constant for the lifetime of a Video Bitstream, which in IETF terms can translate to the lifetime of a session. Decoding capability informations can include profile, level, and sub-profile information to determine a maximum complexity interop point that is guaranteed to be never exceeded, even if splicing of video sequences occurs within a session. It further includes constraint flags, which can optionally be set to indicate that the video bitstream will be constraint in the use of certain features as indicated by the values of those flags. With this, a bitstream can be labelled 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 a Coded Video Sequences (CVS) of multiple layers covering the same range of picture units, and includes, among other information decoding dependency expressed as information for reference picture set 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 and the random access point picture. In MPGEG-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 MCUs involved), a CVS can be as long in duration as the whole session.
Picture and Adaptation parameter set
The Picture Parameter Set and the Adaptation Parameter Set (PPS and APS, respectively) 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 can be referenced by slices of the same picture if that APS contains information about luma mapping with chroma scaling (LMCS) but different APS can be referenced by slices of the same picture if those APS contain information about ALF.
Picture Header
A Picture Header 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. Therefore, VVC specifies signaling that indicates whether Picture Headers are present in the CLVS or not.
Profile, tier, and level
The profile, tier and level syntax structures in DCI, VPS and SPS contain profile, tier, level information for all layers that refer to the DCI, for layers associated with one or more output layer sets specified by the VPS, and for any layer that refers to the SPS, respectively.
Sub-Profiles
Within the [VVC] specification, a sub-profile is a 32-bit number coded according to ITU-T Rec. T.35, that does not carry a semantic. It is carried in the profile_tier_level structure and hence (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 complexity 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: DVB and ATSC), as well as walled-garden video services will take advantage of this labelling 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 specs, such as GOP structures, minimum/maximum QP values, and the mandatory use of certain tools or SEI messages.
Constraint Flags
The profile_tier_level structure carries a considerable number of constraint 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 for labelling a bitstream as not exercising a certain tool that has become commercially unviable.
Temporal scalability support
[VVC] includes support of temporal scalability, by inclusion of the signaling of TemporalId in the NAL unit header, the restriction that pictures of a particular temporal sub-layer cannot be used for inter prediction reference by pictures of a lower temporal sub-layer, the sub-bitstream extraction process, and the requirement that each sub- bitstream extraction output be a conforming bitstream. Media-Aware Network Elements (MANEs) can utilize the TemporalId in the NAL unit header for stream adaptation purposes based on temporal scalability.
Spatial, SNR, View Scalability
[VVC] includes support for spatial, SNR, and View 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 main profiles of video codecs and not wide deployed due to additional costs. In VVC, however, all those forms of scalability are supported natively through the signaling of the layer_id in the NAL unit header, the VPS which associates layers with given layer_ids to each other, reference picture selection, reference picture resampling for spatial scalability, and a number of other mechanisms not relevant for this memo. Scalability support can be implemented in a single decoding "loop" and is widely considered a comparatively lightweight operation.
SEI Messages
Supplementary Enhancement Information (SEI) messages are codepoints in the bitstream that do not influence the decoding process as specified in the [VVC] spec, but address issues of representation/ rendering of the decoded bitstream, label the bitstream for certain applications, among other, similar tasks. The overall concept of SEI messages and many of the messages themselves has been inherited from the H.264 and HEVC specs. In the [VVC] environment, some of the SEI messages considered to be generally useful also in other video coding technologies have been moved out of the main specification into a companion document (TO DO: add reference once ITU designation is known).
Compared to HEVC, the [VVC] design to support parallelization offers numerous improvements. Some of those improvements are still undergoing changes in JVET. Information, to the extent relevant for this memo, will be added in future versions of this memo as the standardization in JVET progresses and the technology stabilizes.
[VVC] maintains the NAL unit concept of HEVC with modifications. VVC uses a two-byte NAL unit header, as shown in Figure 1. The payload of a NAL unit refers to the NAL unit excluding the NAL unit header.
+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|Z| LayerID | Type | TID |
+---------------+---------------+
The Structure of the VVC NAL Unit Header.
Figure 1
The semantics of the fields in the NAL unit header are as specified in [VVC] and described briefly below for convenience. In addition to the name and size of each field, the corresponding syntax element name in [VVC] is also provided.
F: 1 bit
Z: 1 bit
LayerId: 6 bits
Type: 5 bits
TID: 3 bits
This payload format defines the following processes required for transport of [VVC] coded data over RTP [RFC3550]:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown above.
This document uses the terms and definitions of VVC. Section 3.1.1 lists relevant definitions from [VVC] for convenience. Section 3.1.2 provides definitions specific to this memo.
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 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.
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.
Operation point (OP): A temporal subset of an OLS, identified by an OLS index and a highest value of TemporalId.
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 stream.
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.
Sub-layer: A temporal scalable layer of a temporal scalable bitstream consisting of VCL NAL units with a particular value of the TemporalId variable, and the associated non-VCL NAL units.
Subpicture: An rectangular region of one or more slices within a picture.
Sub-layer representation: A subset of the bitstream consisting of NAL units of a particular sub-layer and the lower sub-layers.
Tile: A rectangular region of 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 the constraints on NAL unit order given in Section 7.4.2.4 in [VVC], follow the Order of NAL units in the bitstream.
NAL unit output order: A NAL unit order in which NAL units of different access units are in the output order of the decoded pictures corresponding to the access units, as specified in [VVC], and in which NAL units within an access unit are in their decoding order.
RTP stream: See [RFC7656]. Within the scope of this memo, one RTP stream is utilized to transport one or more temporal sub-layers.
Transmission order: The order of packets in ascending RTP sequence number order (in modulo arithmetic). Within an aggregation packet, the NAL unit transmission order is the same as the order of appearance of NAL units in the packet.
AU Access Unit
AP Aggregation Packet
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
HRD Hypothetical Reference Decoder
IDR Instantaneous Decoding Refresh
MANE Media-Aware Network Element
MTU Maximum Transfer Unit
NAL Network Abstraction Layer
NALU Network Abstraction Layer Unit
PLI Picture Loss Indication
PPS Picture Parameter Set
RPS Reference Picture 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
The format of the RTP header is specified in [RFC3550] (reprinted as Figure 2 for convenience). This payload format uses the fields of the header in a manner consistent with that specification.
The RTP payload (and the settings for some RTP header bits) for aggregation packets and fragmentation units are specified in Section 4.3.2 and Section 4.3.3, respectively.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP Header According to {{RFC3550}}
Figure 2
The RTP header information to be set according to this RTP payload format is set as follows:
Marker bit (M): 1 bit
Payload Type (PT): 7 bits
Sequence Number (SN): 16 bits
Timestamp: 32 bits
Synchronization source (SSRC): 32 bits
The first two bytes of the payload of an RTP packet are referred to as the payload header. The payload header consists of the same fields (F, Z, LayerId, Type, and TID) as the NAL unit header as shown in Section 1.1.4, irrespective of the type of the payload structure.
The TID value indicates (among other things) the relative importance of an RTP packet, for example, because NAL units belonging to higher temporal sub-layers are not used for the decoding of lower temporal sub-layers. A lower value of TID indicates a higher importance. More-important NAL units MAY be better protected against transmission losses than less-important NAL units.
Three different types of RTP packet payload structures are specified. A receiver can identify the type of an RTP packet payload through the Type field in the payload header.
The three different payload structures are as follows:
A single NAL unit packet contains exactly one NAL unit, and consists of a payload header (denoted as PayloadHdr), a conditional 16-bit DONL field (in network byte order), and the NAL unit payload data (the NAL unit excluding its NAL unit header) of the contained NAL unit, as shown in Figure 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr | DONL (conditional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NAL unit payload data |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of a Single NAL Unit Packet
Figure 3
The DONL field, when present, specifies the value of the 16 least significant bits of the decoding order number of the contained NAL unit. If sprop-max-don-diff is greater than 0, the DONL field MUST be present, and the variable DON for the contained NAL unit is derived as equal to the value of the DONL field. Otherwise (sprop-max-don-diff is equal to 0), the DONL field MUST NOT be present.
Aggregation Packets (APs) can reduce of packetization overhead for small NAL units, such as most of the non- VCL NAL units, which are often only a few octets in size.
An AP aggregates NAL units of one access unit. Each NAL unit to be carried in an AP is encapsulated in an aggregation unit. NAL units aggregated in one AP are included in NAL unit decoding order.
An AP consists of a payload header (denoted as PayloadHdr) followed by two or more aggregation units, as shown in Figure 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=28) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| two or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Packet
Figure 4
The fields in the payload header of an AP are set as follows. The F bit MUST be equal to 0 if the F bit of each aggregated NAL unit is equal to zero; otherwise, it MUST be equal to 1. The Type field MUST be equal to 28.
The value of LayerId MUST be equal to the lowest value of LayerId of all the aggregated NAL units. The value of TID MUST be the lowest value of TID of all the aggregated NAL units.
An AP MUST carry at least two aggregation units and can carry as many aggregation units as necessary; however, the total amount of data in an AP obviously MUST fit into an IP packet, and the size SHOULD be chosen so that the resulting IP packet is smaller than the MTU size so to avoid IP layer fragmentation. An AP MUST NOT contain FUs specified in Section 4.3.3. APs MUST NOT be nested; i.e., an AP can not contain another AP.
The first aggregation unit in an AP consists of a conditional 16-bit DONL field (in network byte order) followed by a 16-bit unsigned size information (in network byte order) that indicates the size of the NAL unit in bytes (excluding these two octets, but including the NAL unit header), followed by the NAL unit itself, including its NAL unit header, as shown in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : DONL (conditional) | NALU size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU size | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of the First Aggregation Unit in an AP
Figure 5
The DONL field, when present, specifies the value of the 16 least significant bits of the decoding order number of the aggregated NAL unit.
If sprop-max-don-diff is greater than 0, the DONL field MUST be present in an aggregation unit that is the first aggregation unit in an AP, and the variable DON for the aggregated NAL unit is derived as equal to the value of the DONL field. Otherwise (sprop-max-don-diff is equal to 0), the DONL field MUST NOT be present in an aggregation unit that is the first aggregation unit in an AP.
An aggregation unit that is not the first aggregation unit in an AP will be followed immediately by a 16-bit unsigned size information (in network byte order) that indicates the size of the NAL unit in bytes (excluding these two octets, but including the NAL unit header), followed by the NAL unit itself, including its NAL unit header, as shown in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU size | NAL unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an Aggregation Unit That Is Not the First
Aggregation Unit in an AP
Figure 6
Figure 7 presents an example of an AP that contains two aggregation units, labeled as 1 and 2 in the figure, without the DONL field being present.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=28) | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 1 Data |
| . . . |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+ NALU 2 Data |
| . . . |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Packet Containing
Two Aggregation Units without the DONL Field
Figure 7
Figure 8 presents an example of an AP that contains two aggregation units, labeled as 1 and 2 in the figure, with the DONL field being present.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=28) | NALU 1 DONL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NALU 1 Data . . . |
| |
+ . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : NALU 2 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NALU 2 Data |
| |
| . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Example of an AP Containing
Two Aggregation Units with the DONL Field
Figure 8
Fragmentation Units (FUs) are introduced to enable fragmenting a single NAL unit into multiple RTP packets, possibly without cooperation or knowledge of the [VVC] encoder. A fragment of a NAL unit consists of an integer number of consecutive octets of that NAL unit. Fragments of the same NAL unit MUST be sent in consecutive order with ascending RTP sequence numbers (with no other RTP packets within the same RTP stream being sent between the first and last fragment).
When a NAL unit is fragmented and conveyed within FUs, it is referred to as a fragmented NAL unit. APs MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU can not contain a subset of another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU- time of the fragmented NAL unit.
An FU consists of a payload header (denoted as PayloadHdr), an FU header of one octet, a conditional 16-bit DONL field (in network byte order), and an FU payload, as shown in Figure 9}.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadHdr (Type=29) | FU header | DONL (cond) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| DONL (cond) | |
|-+-+-+-+-+-+-+-+ |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Structure of an FU
Figure 9
The fields in the payload header are set as follows. The Type field MUST be equal to 29. The fields F, LayerId, and TID MUST be equal to the fields F, LayerId, and TID, respectively, of the fragmented NAL unit.
The FU header consists of an S bit, an E bit, an R bit and a 5-bit FuType field, as shown in Figure 10.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|R| FuType |
+---------------+
The Structure of FU Header
Figure 10
The semantics of the FU header fields are as follows:
S: 1 bit
E: 1 bit
Reserved: 1 bit
FuType: 5 bits
The DONL field, when present, specifies the value of the 16 least significant bits of the decoding order number of the fragmented NAL unit.
If sprop-max-don-diff is greater than 0, and the S bit is equal to 1, the DONL field MUST be present in the FU, and the variable DON for the fragmented NAL unit is derived as equal to the value of the DONL field. Otherwise (sprop-max-don-diff is equal to 0, or the S bit is equal to 0), the DONL field MUST NOT be present in the FU.
A non-fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the Start bit and End bit must not both be set to 1 in the same FU header.
The FU payload consists of fragments of the payload of the fragmented NAL unit so that if the FU payloads of consecutive FUs, starting with an FU with the S bit equal to 1 and ending with an FU with the E bit equal to 1, are sequentially concatenated, the payload of the fragmented NAL unit can be reconstructed. The NAL unit header of the fragmented NAL unit is not included as such in the FU payload, but rather the information of the NAL unit header of the fragmented NAL unit is conveyed in F, LayerId, and TID fields of the FU payload headers of the FUs and the FuType field of the FU header of the FUs. An FU payload MUST NOT be empty.
If an FU is lost, the receiver SHOULD discard all following fragmentation units in transmission order corresponding to the same fragmented NAL unit, unless the decoder in the receiver is known to be prepared to gracefully handle incomplete NAL units.
A receiver in an endpoint or in a MANE MAY aggregate the first n-1 fragments of a NAL unit to an (incomplete) NAL unit, even if fragment n of that NAL unit is not received. In this case, the forbidden_zero_bit of the NAL unit MUST be set to 1 to indicate a syntax violation.
For each NAL unit, the variable AbsDon is derived, representing the decoding order number that is indicative of the NAL unit decoding order.
Let NAL unit n be the n-th NAL unit in transmission order within an RTP stream.
If sprop-max-don-diff is equal to 0, AbsDon[n], the value of AbsDon for NAL unit n, is derived as equal to n.
Otherwise (sprop-max-don-diff is greater than 0), AbsDon[n] is derived as follows, where DON[n] is the value of the variable DON for NAL unit n:
If DON[n] == DON[n-1],
AbsDon[n] = AbsDon[n-1]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] < 32768),
AbsDon[n] = AbsDon[n-1] + DON[n] - DON[n-1]
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] >= 32768),
AbsDon[n] = AbsDon[n-1] + 65536 - DON[n-1] + DON[n]
If (DON[n] > DON[n-1] and DON[n] - DON[n-1] >= 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] + 65536 -
DON[n])
If (DON[n] < DON[n-1] and DON[n-1] - DON[n] < 32768),
AbsDon[n] = AbsDon[n-1] - (DON[n-1] - DON[n])
For any two NAL units m and n, the following applies:
The following packetization rules apply:
The general concept behind de-packetization is to get the NAL units out of the RTP packets in an RTP stream and pass them to the decoder in the NAL unit decoding order.
The de-packetization process is implementation dependent. Therefore, the following description should be seen as an example of a suitable implementation. Other schemes may be used as well, as long as the output for the same input is the same as the process described below. The output is the same when the set of output NAL units and their order are both identical. Optimizations relative to the described algorithms are possible.
All normal RTP mechanisms related to buffer management apply. In particular, duplicated or outdated RTP packets (as indicated by the RTP sequences number and the RTP timestamp) are removed. To determine the exact time for decoding, factors such as a possible intentional delay to allow for proper inter-stream synchronization MUST be factored in.
NAL units with NAL unit type values in the range of 0 to 27, inclusive, may be passed to the decoder. NAL-unit-like structures with NAL unit type values in the range of 28 to 31, inclusive, MUST NOT be passed to the decoder.
The receiver includes a receiver buffer, which is used to compensate for transmission delay jitter within individual RTP streams and across RTP streams, to reorder NAL units from transmission order to the NAL unit decoding order. In this section, the receiver operation is described under the assumption that there is no transmission delay jitter within an RTP stream and across RTP streams. To make a difference from a practical receiver buffer that is also used for compensation of transmission delay jitter, the receiver buffer is hereafter called the de-packetization buffer in this section. Receivers should also prepare for transmission delay jitter; that is, either reserve separate buffers for transmission delay jitter buffering and de-packetization buffering or use a receiver buffer for both transmission delay jitter and de- packetization. Moreover, receivers should take transmission delay jitter into account in the buffering operation, e.g., by additional initial buffering before starting of decoding and playback.
When sprop-max-don-diff is equal to 0, the de-packetization buffer size is zero bytes, and the process described in the remainder of this paragraph applies.
The NAL units carried in the single RTP stream are directly passed to the decoder in their transmission order, which is identical to their decoding order. When there are several NAL units of the same RTP stream with the same NTP timestamp, the order to pass them to the decoder is their transmission order.
When sprop-max-don-diff is greater than 0, the process described in the remainder of this section applies.
There are two buffering states in the receiver: initial buffering and buffering while playing. Initial buffering starts when the reception is initialized. After initial buffering, decoding and playback are started, and the buffering-while-playing mode is used.
Regardless of the buffering state, the receiver stores incoming NAL units, in reception order, into the de-packetization buffer. NAL units carried in RTP packets are stored in the de-packetization buffer individually, and the value of AbsDon is calculated and stored for each NAL unit.
Initial buffering lasts until condition A (the difference between the greatest and smallest AbsDon values of the NAL units in the de- packetization buffer is greater than or equal to the value of sprop- max-don-diff) or condition B (the number of NAL units in the de-packetization buffer is greater than the value of sprop-depack-buf-nalus) is true.
After initial buffering, whenever condition A or condition B is true, the following operation is repeatedly applied until both condition A and condition B become false:
When no more NAL units are flowing into the de-packetization buffer, all NAL units remaining in the de-packetization buffer are removed from the buffer and passed to the decoder in the order of increasing AbsDon values.
This section specifies the optional parameters. A mapping of the parameters with Session Description Protocol (SDP) [RFC4556] is also provided for applications that use SDP.
The receiver MUST ignore any parameter unspecified in this memo.
Type name: Video
Subtype name: H266
Required parameters: none
Optional parameters:
The receiver MUST ignore any parameter unspecified in this memo.
The media type video/H266 string is mapped to fields in the Session Description Protocol (SDP) [RFC4566] as follows:
An example 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>
When [VVC] is offered over RTP using SDP in an offer/answer model [RFC3264] for negotiation for unicast usage, the following limitations and rules apply:
Placeholder: To add limitations and considerations.
The following subsections define the use of the Picture Loss Indication (PLI), Slice Lost Indication (SLI), Reference Picture Selection Indication (RPSI), and Full Intra Request (FIR) feedback messages with HEVC. The PLI, SLI, and RPSI messages are defined in [RFC4585], and the FIR message is defined in [RFC5104].
As specified in RFC 4585, Section 6.3.1, the reception of a PLI by a media sender indicates "the loss of an undefined amount of coded video data belonging to one or more pictures". Without having any specific knowledge of the setup of the bitstream (such as use and location of in-band parameter sets, non-IRAP decoder refresh points, picture structures, and so forth), a reaction to the reception of an PLI by a [VVC] sender SHOULD be to send an IRAP picture and relevant parameter sets; potentially with sufficient redundancy so to ensure correct reception. However, sometimes information about the bitstream structure is known. For example, state could have been established outside of the mechanisms defined in this document that parameter sets are conveyed out of band only, and stay static for the duration of the session. In that case, it is obviously unnecessary to send them in-band as a result of the reception of a PLI. Other examples could be devised based on a priori knowledge of different aspects of the bitstream structure. In all cases, the timing and congestion control mechanisms of RFC 4585 MUST be observed.
For further study. Maybe remove as there are no known implementations of SDLI in [HEVC] based systems
Feedback-based reference picture selection has been shown as a powerful tool to stop temporal error propagation for improved error resilience [Girod99] [Wang05]. In one approach, the decoder side tracks errors in the decoded pictures and informs the encoder side that a particular picture that has been decoded relatively earlier is correct and still present in the decoded picture buffer; it requests the encoder to use that correct picture-availability information when encoding the next picture, so to stop further temporal error propagation. For this approach, the decoder side should use the RPSI feedback message.
Encoders can encode some long-term reference pictures as specified in [VVC] for purposes described in the previous paragraph without the need of a huge decoded picture buffer. As shown in [Wang05], with a flexible reference picture management scheme, as in VVC, even a decoded picture buffer size of two picture storage buffers would work for the approach described in the previous paragraph.
The text above is copy-paste from RFC 7798. If we keep the RPSI message, it needs adaptation to the [VVC] syntax. Doing so shouldn't be too hard as the [VVC] reference picture mechanism is not too different from the [HEVC] one.
The purpose of the FIR message is to force an encoder to send an independent decoder refresh point as soon as possible, while observing applicable congestion-control-related constraints, such as those set out in [RFC8082]).
Upon reception of a FIR, a sender MUST send an IDR picture. Parameter sets MUST also be sent, except when there is a priori knowledge that the parameter sets have been correctly established. A typical example for that is an understanding between sender and receiver, established by means outside this document, that parameter sets are exclusively sent out-of-band.
[FrameMarking] provides an extension mechanism for RTP. The codec-agnostic meta-data in the [FrameMarking] header provides valuable video frame information. Its usage with [VVC] is defined in this section. Refer [FrameMarking] for any unspecified fields. Two header extensions are RECOMMENDED:
The fields for the short extension, as shown in Figure 11, are used as described in the following.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | L=0 |S|E|I|D|0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Short Frame Marking RTP Extension for [VVC]
Figure 11
The I bit MUST be 1 when the NAL unit type is 7-9 (inclusive), otherwise it MUST be 0.
The D bit MUST be 1 when the syntax element ph_non_ref_pic_flag for a picture is equal to 1, otherwise it MUST be 0.
The S bit MUST be set to 1 if any of the following conditions is true and MUST be set to 0 otherwise:
The E bit MUST be set to 1 if any of the following conditions is true and MUST be set to 0 otherwise:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | L=2 |S|E|I|D|B| TID |0|0| LayerID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Long Frame Marking RTP Extension for [VVC]
Figure 12
The fields for the long extension for scalable streams, as shown in Figure 12, are used as described in the following.
The LayerID (6 bits) and TID (3 bits) from the NAL unit header Section 1.1.4 are mapped to the generic LID and TID fields in [FrameMarking] as shown in Figure 12.
The I bit MUST be 1 when the NAL unit type is 7-9 (inclusive), otherwise it MUST be 0.
The D bit MUST be 1 when the syntax element ph_non_ref_pic_flag for a picture is equal to 1, otherwise it MUST be 0.
The S bit MUST be set to 1 if any of the following conditions is true and MUST be set to 0 otherwise:
The E bit MUST be set to 1 if any of the following conditions is true and MUST be set to 0 otherwise:
The scope of this Security Considerations section is limited to the payload format itself and to one feature of [VVC] that may pose a particularly serious security risk if implemented naively. The payload format, in isolation, does not form a complete system. Implementers are advised to read and understand relevant security- related documents, especially those pertaining to RTP (see the Security Considerations section in [RFC3550] ), and the security of the call-control stack chosen (that may make use of the media type registration of this memo). Implementers should also consider known security vulnerabilities of video coding and decoding implementations in general and avoid those.
Within this RTP payload format, and with the exception of the user data SEI message as described below, no security threats other than those common to RTP payload formats are known. In other words, neither the various media-plane-based mechanisms, nor the signaling part of this memo, seems to pose a security risk beyond those common to all RTP-based systems.
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550] , and in any applicable RTP profile such as RTP/AVP [RFC3551] , RTP/AVPF [RFC4585] , RTP/SAVP [RFC3711] , or RTP/SAVPF [RFC5124] . However, as "Securing the RTP Framework: Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202] discusses, it is not an RTP payload format's responsibility to discuss or mandate what solutions are used to meet the basic security goals like confidentiality, integrity and source authenticity for RTP in general. This responsibility lays on anyone using RTP in an application. They can find guidance on available security mechanisms and important considerations in "Options for Securing RTP Sessions" [RFC7201] . The rest of this section discusses the security impacting properties of the payload format itself.
Because the data compression used with this payload format is applied end-to-end, any encryption needs to be performed after compression. A potential denial-of-service threat exists for data encodings using compression techniques that have non-uniform receiver-end computational load. The attacker can inject pathological datagrams into the bitstream that are complex to decode and that cause the receiver to be overloaded. [VVC] is particularly vulnerable to such attacks, as it is extremely simple to generate datagrams containing NAL units that affect the decoding process of many future NAL units. Therefore, the usage of data origin authentication and data integrity protection of at least the RTP packet is RECOMMENDED, for example, with SRTP [RFC3711] .
Like HEVC [RFC7798], [VVC] includes a user data Supplemental Enhancement Information (SEI) message. This SEI message allows inclusion of an arbitrary bitstring into the video bitstream. Such a bitstring could include JavaScript, machine code, and other active content. [VVC] leaves the handling of this SEI message to the receiving system. In order to avoid harmful side effects the user data SEI message, decoder implementations cannot naively trust its content. For example, it would be a bad and insecure implementation practice to forward any JavaScript a decoder implementation detects to a web browser. The safest way to deal with user data SEI messages is to simply discard them, but that can have negative side effects on the quality of experience by the user.
End-to-end security with authentication, integrity, or confidentiality protection will prevent a MANE from performing media- aware operations other than discarding complete packets. In the case of confidentiality protection, it will even be prevented from discarding packets in a media-aware way. To be allowed to perform such operations, a MANE is required to be a trusted entity that is included in the security context establishment.
Congestion control for RTP SHALL be used in accordance with RTP [RFC3550] and with any applicable RTP profile, e.g., AVP [RFC3551]. If best-effort service is being used, an additional requirement is that users of this payload format MUST monitor packet loss to ensure that the packet loss rate is within an acceptable range. Packet loss is considered acceptable if a TCP flow across the same network path, and experiencing the same network conditions, would achieve an average throughput, measured on a reasonable timescale, that is not less than all RTP streams combined are achieving. This condition can be satisfied by implementing congestion-control mechanisms to adapt the transmission rate, the number of layers subscribed for a layered multicast session, or by arranging for a receiver to leave the session if the loss rate is unacceptably high.
The bitrate adaptation necessary for obeying the congestion control principle is easily achievable when real-time encoding is used, for example, by adequately tuning the quantization parameter. However, when pre-encoded content is being transmitted, bandwidth adaptation requires the pre-coded bitstream to be tailored for such adaptivity. The key mechanisms available in [VVC] are temporal scalability, and spatial/SNR scalability. A media sender can remove NAL units belonging to higher temporal sub-layers (i.e., those NAL units with a high value of TID) or higher spatio-SNR layers (as indicated by interpreting the VPS) until the sending bitrate drops to an acceptable range.
The mechanisms mentioned above generally work within a defined profile and level and, therefore, no renegotiation of the channel is required. Only when non-downgradable parameters (such as profile) are required to be changed does it become necessary to terminate and restart the RTP stream(s). This may be accomplished by using different RTP payload types.
MANEs MAY remove certain unusable packets from the RTP stream when that RTP stream was damaged due to previous packet losses. This can help reduce the network load in certain special cases. For example, MANES can remove those FUs where the leading FUs belonging to the same NAL unit have been lost or those dependent slice segments when the leading slice segments belonging to the same slice have been lost, because the trailing FUs or dependent slice segments are meaningless to most decoders. MANES can also remove higher temporal scalable layers if the outbound transmission (from the MANE's viewpoint) experiences congestion.
Placeholder
Dr. Byeongdoo Choi is thanked for the video codec related technical discussion and other aspects in this memo. Xin Zhao and Dr. Xiang Li are thanked for their contributions on [VVC] specification descriptive content. Spencer Dawkins is thanked for his valuable review comments that led to great improvements of this memo. Some parts of this specification share text with the RTP payload format for HEVC [RFC7798]. We thank the authors of that specification for their excellent work.
draft-zhao-payload-rtp-vvc-00 …….. initial version
draft-zhao-payload-rtp-vvc-01 …….. editorial clarifications and corrections
draft-ietf-payload-rtp-vvc-00 …….. initial WG draft
draft-ietf-payload-rtp-vvc-01 …….. VVC specification update