RTP Payload Format for Visual Volumetric Video-based Coding (V3C)

Introduction Volumetric video, similar to traditional 2D video, when uncompressed, is represented by a large amount of data. The Visual Volumetric Video-based Coding (V3C) specification leverages the compression efficiency of existing 2D video codecs to reduce the amount of data needed for storage and transmission of volumetric video. V3C is a generic mechanism for volumetric video coding, and it can be used by applications targeting volumetric content, such as point clouds, immersive video with depth, mesh representations of visual volumetric frames, etc. Examples of such applications are Video-based Point Cloud Compression (V-PCC) , and MPEG Immersive Video (MIV) . V3C encoder converts volumetric frames, i.e., 3D volumetric information, into a collection of 2D images and associated data, known as atlas data. The converted 2D images are subsequently coded using any video or image codec, e.g., ISO/IEC International Standard 14496-10 (Advanced Video Coding, AVC/H.264) , ISO/IEC International Standard 23008-2 (High Efficiency Video Coding, HEVC/H.265) or ISO/IEC International Standard 23090-3 (Versatile Video Coding, VVC/H.266) . The atlas data is coded with mechanisms specified in . V3C utilizes high level syntax (HLS) design, familiar from traditional 2D video codecs, to represent the associated coded data, i.e., atlas data. The coded atlas data is represented by Network Abstraction Layer (NAL) units. Consequently, RTP payload format for V3C atlas data described in this memo shares design philosophy, security, congestion control, and overall implementation complexity with the other NAL unit-based RTP payload formats such as the ones defined in , , and .

Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . All fields defined in this specification related to RTP payload structures SHALL be considered in network order.

Definitions, and abbreviations

Definitions

General This document uses the definitions of . below lists relevant definitions from for convenience.

Definitions from the V3C specification atlas: collection of 2D bounding boxes and their associated information placed onto a rectangular frame and corresponding to a volume in 3D space on which volumetric data is rendered. atlas bitstream: sequence of bits that forms the representation of atlas frames and associated data forming one or more CASs. atlas coding layer NAL unit: collective term for coded atlas tile layer NAL units and the subset of NAL units that have reserved values of nal_unit_type that are classified as being of type class equal to ACL in this document. atlas frame: 2D rectangular array of atlas samples onto which patches are projected and additional information related to the patches, corresponding to a volumetric frame. atlas frame parameter set: syntax structure containing syntax elements that apply to zero or more entire coded atlas frames as determined by the content of a syntax element found in each tile header. atlas sequence parameter set: syntax structure containing syntax elements that apply to zero or more entire coded atlas sequences as determined by the content of a syntax element found in the AFPS referred to by a syntax element found in each tile header. attribute: scalar or vector property optionally associated with each point in a volumetric frame such as colour, reflectance, surface normal, timestamps, material ID, etc. coded atlas sequence: sequence of coded atlas access units, in decoding order, of an IRAP coded atlas access unit, followed by zero or more coded atlas access units that are not IRAP coded atlas access units, including all subsequent access units up to but not including any subsequent coded atlas access unit that is an IRAP coded atlas access unit. coded atlas access unit: set of atlas NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain all atlas NAL units pertaining to one particular output time. intra random access point coded atlas: coded atlas for which each ACL NAL unit has nal_unit_type in the range of NAL_BLA_W_LP to NAL_RSV_IRAP_ACL_29, inclusive. intra random access point coded atlas access unit: access unit in which the coded atlas with nal_layer_id equal to 0 is a IRAP coded atlas. network abstraction layer unit: syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP. patch: rectangular region within an atlas associated with volumetric information. raw byte sequence payload: syntax structure containing an integer number of bytes that is encapsulated in a NAL unit and that 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. tile: independently decodable rectangular region of an atlas frame. visual volumetric video-based coding atlas sub-bitstream: extracted sub-bitstream from the V3C bitstream containing whole or portion of an atlas bitstream. visual volumetric video-based coding video sub-bitstream: extracted sub-bitstream from the V3C bitstream containing whole or portion of a video bitstream. visual volumetric video-based coding component: atlas, occupancy, geometry, or attribute of a particular type that is associated with a V3C volumetric content representation. visual volumetric video-based coding parameter set: syntax structure containing syntax elements that apply to zero or more entire CVSs and may be referred to by syntax elements found in the V3C unit header. volumetric frame: set of 3D points specified by their cartesian coordinates and zero or more corresponding sets of attributes at a particular time instance.

Abbreviations ACL atlas coding layer AFPS atlas frame parameter set AP aggregation packet ASPS atlas sequence parameter set AU aggregation unit CAS coded atlas sequence DON decoding order number IRAP intra random access point MRMT Multiple RTP streams on Multiple media Transports MRST Multiple RTP streams over a Single media Transport MTU maximum transmission unit NAL network abstraction layer NALU NAL unit RBSP raw byte sequence payload SRST Single RTP stream on a Single media Transport V3C visual volumetric video-based coding VPS V3C parameter set

Media format description

Overview of the V3C codec (informative) V3C encoding of a volumetric frame is achieved through a conversion of the volumetric frame from its 3D representation into multiple 2D representations and a generation of associated data documenting such conversions and transformations. The associated data, also known as the atlas data, provides information on how to reproject the 2D representations back into the 3D volumetric frame. 2D representations, known as V3C video components, of volumetric frame are encoded using traditional 2D video codecs. V3C video component may, for example, include occupancy, geometry, or attribute data. The occupancy data informs a V3C decoder which pixels in other V3C video components contribute to reconstructed 3D representation. The geometry data describes information on the position of the reconstructed voxels, while attribute data provides additional properties for the voxels, e.g., colour or material information. Atlas data, known as V3C atlas component, provides information to interpret V3C video components and enables the reconstruction from a 2D representation back into a 3D representation of volumetric frame. Atlas data is composed of a collection of patches. Each patch identifies a region in the V3C video components and provides information necessary to perform the appropriate inverse projection of the indicated region back into 3D space. The shape of the patch region is determined by a 2D bounding box associated with each patch as well as their coding order. The shape of these patches is also further refined based on occupancy data. To enable parallelization, random access, as well as a variety of other functionalities, an atlas frame can be divided into one or more rectangular partitions referred to as tiles. Tiles are not allowed to overlap and should be independently decodable. An atlas frame may contain regions that are not associated with any tile or patch. The binary form of V3C video components, i.e., video bitstream, and V3C atlas components, i.e., V3C atlas bitstream, can be grouped and represented by a single V3C bitstream. The V3C bitstream is composed of a set of V3C units. Each V3C unit has a V3C unit header and a V3C unit payload. The V3C unit header describes the V3C unit type for the payload. V3C unit payload contains V3C video components, V3C atlas components or a V3C parameter set. V3C video components, i.e., occupancy, geometry, or attribute components, correspond to video data units (e.g., NAL units defined in ) that could be decoded by an appropriate video decoder. An example of V3C bitstream consisting of a V3C parameter set, V3C atlas bitstream and three video component bitsreams (geometry, occupancy, attribute) is provided in .

Example of V3C bitstream

V3C parameter set (informative) While this memo intends to describe encapsulation of V3C atlas data, aspects related to signalling of V3C parameter set need to be considered. V3C parameter set is encapsulated in its own V3C unit, which allows decoupling the transmission of V3C parameter set from the V3C video and atlas components. V3C parameter set may be transmitted by external means (e.g., as a result of the capability exchange) or through a (reliable or unreliable) control protocol. This memo provides information on how a V3C parameter set may be signalled as part of session description protocol, see . Generally, it is useful to signal V3C parameter set out-of-band, because it describes what overall resources are needed to decode and reconstruct the associated V3C bitstream. Signalling it dynamically as part of an RTP stream might result in undefined behaviour when receiver does not have the required capabilities to decode the received V3C video component sub-bitstreams or when reconstruction process relies on information that the receiver does not support.

V3C atlas and video components (informative)

General In V3C bitstream the atlas component is identified by vuh_unit_type equal to V3C_AD, or V3C_CAD in case of common atlas data, in the V3C unit header. The V3C atlas component consists of atlas NAL units that define header and payload pairs, see . V3C video components are identified by vuh_unit_type equal to V3C_OVD, V3C_GVD, V3C_AVD, and V3C_PVD. V3C video components can be further differentiated by other values in the V3C unit header such as vuh_attribute_index, vuh_attribute_partition_index, vuh_map_index and vuh_auxiliary_video_flag. By mapping V3C parameter set information to vuh_attribute_index, a V3C decoder identifies which attribute a given V3C video component contains, e.g., colour. The information supplied by V3C unit header should be provided in one form or another to a V3C decoder, e.g., as part of SDP as described in this memo in . The four-byte V3C unit header syntax and semantics are copied below as defined in . vuh_unit_type indicates the V3C unit type for the V3C component as specified in . As a convenience, the mapping table from vuh_unit_type values to semantics is copied below in . V3C unit type semantics

vuh_unit_type	Identifier	V3C unit type	Description
0	V3C_VPS	V3C parameter set	V3C level parameters
1	V3C_AD	Atlas data	Atlas information
2	V3C_OVD	Occupancy video data	Occupancy information
3	V3C_GVD	Geometry video data	Geometry information
4	V3C_AVD	Attribute video data	Attribute information
5	V3C_PVD	Packed video data	Packing information
6	V3C_CAD	Common atlas data	Information that is common for atlases in a CVS. Specified in ISO/IEC 23090-12
7...31	V3C_RSVD	Reserved	-

vuh_v3c_parameter_set_id specifies the value of vps_v3c_parameter_set_id for the active V3C VPS. vuh_atlas_id specifies the ID of the atlas that corresponds to the current V3C unit. vuh_attribute_index indicates the index of the attribute data carried in the Attribute Video Data unit. vuh_attribute_partition_index indicates the index of the attribute dimension group carried in the attribute video data unit. vuh_map_index when present indicates the map index of the current geometry or attribute stream. When not present, the map index of the current geometry or attribute sub-bitstream is derived based on the type of the sub-bitstream. vuh_auxiliary_video_flag equal indicates if the associated geometry or attribute video data unit is a RAW and/or EOM coded points video only sub-bitstream.

Atlas NAL units Atlas NAL unit (nal_unit(NumBytesInNalUnit)) is a byte-aligned syntax structure defined by to carry atlas data. Atlas NAL unit always contains a 16-bit NAL unit header (nal_unit_header()), which indicates among other things the type of the NAL unit (nal_unit_type). The payload of a NAL unit refers to the NAL unit excluding the NAL unit header. The Atlas NAL unit syntax and semantics are copied here as defined in . nal_forbidden_zero_bit MUST be equal to 0. (F) nal_unit_type indicates the type of the RBSP data structure contained in the NAL unit (NUT) nal_layer_id indicates the identifier of the layer to which an ACL NAL unit belongs or the identifier of a layer to which a non-ACL NAL unit applies. (NLI) nal_temporal_id_plus1 minus 1 indicates a temporal identifier for the NAL unit. The value of nal_temporal_id_plus1 MUST NOT be equal to 0. (TID)

Systems and transport interfaces (informative) In addition to releasing specifications on V3C applications and , MPEG conducted further systems level work on file formats to encapsulate compressed V3C content. The seventh edition of the ISOBMFF specification introduces a new media handler 'volv', intended to support volumetric visual media. It also specifies other structures to enable development of derived specifications detailing how various volumetric visual media may be stored in ISOBMFF. One of such derived specifications is , which defines how V3C content can be stored in a file and streamed over DASH. To a large extent ISO/IEC 23090-10 focuses on describing how ISOBMFF boxes and syntax elements may be used to store volumetric media, but in some cases new boxes and syntax elements are introduced to accommodate the fundamentally different type of new media. While the specification is not directly relevant for defining RTP payload format for V3C atlas data, it is a useful resource that may be considered especially when designing ingestion of encoded V3C content into RTP streaming pipelines.

V3C atlas RTP payload format

General This section describes details related to V3C atlas RTP payload format definitions. Aspects related to RTP header, RTP payload header and general payload structure are considered along with different packetization modes.

RTP header The format of the RTP header is specified in and replicated below in for convenience. This payload format uses the fields of the header in a manner consistent with that specification.

RTP Header The RTP header information to be set according to this RTP payload format is set as follows: Marker bit (M): 1 bit Set for the last packet of the access unit, carried in the current RTP stream. This is in line with the normal use of the M bit in video formats to allow an efficient playout buffer handling. When MRST or MRMT is in use, if an access unit appears in multiple RTP streams, the marker bit is set on each RTP stream's last packet of the access unit. Payload Type (PT): 7 bits The assignment of an RTP payload type for this new packet format is outside the scope of this document and will not be specified here. The assignment of a payload type MUST be performed either through the profile used or in a dynamic way. NOTE: (informative) It is not required to use different payload type values for different RTP streams in MRST or MRMT. Sequence Number (SN): 16 bits Set and used in accordance with 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 atlas of the access unit in which the NAL unit (according to Section 8.4.5.3 of ) is included. Receivers MUST use the RTP timestamp for the display process, even when the bitstream contains atlas frame timing SEI messages as specified in . Synchronization source (SSRC): 32 bits Used to identify the source of the RTP packets. When using SRST, by definition a single SSRC is used for all parts of a single bitstream. In MRST or MRMT, different SSRCs are used for each RTP stream containing a subset of the sub-layers of the single (temporally scalable) bitstream. A receiver is required to correctly associate the set of SSRCs that are included parts of the same bitstream. The remaining RTP header fields are used as specified in .

RTP payload header 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, NUT, NLI, and TID) as the NAL unit header as shown in , irrespective of the type of the payload structure. For convenience the structure of RTP payload header is described below in .

RTP Payload Header F: nal_forbidden_zero_bit as specified in MUST be equal to 0. NUT: nal_unit_type as specified in defines the type of the RBSP data structure contained in the NAL unit payload. NUT value could carry other meaning depending on the RTP packet type. NLI: nal_layer_id as specified in defines the identifier of the layer to which an ACL NAL unit belongs or the identifier of a layer to which a non-ACL NAL unit applies. TID: nal_temporal_id_plus1 minus 1 as specified in defines a temporal identifier for the NAL unit. The value of nal_temporal_id_plus1 MUST NOT be equal to 0.

Transmission modes This memo enables transmission of an V3C atlas bitstream over:

a Single RTP stream on a Single media Transport (SRST),
Multiple RTP streams over a Single media Transport (MRST), or
Multiple RTP streams on Multiple media Transports (MRMT).

When in MRST or MRMT, multiple RTP streams MAY be grouped together as specified in and . SRST or MRST SHOULD be used for point-to-point unicast scenarios, whereas MRMT SHOULD be used for point-to-multipoint multicast scenarios where different receivers require different operation points of the same V3C atlas bitstream, to improve bandwidth utilizing efficiency. NOTE: A multicast may degrade to a unicast at some point when only one receiver is left. This is a justification of the first "SHOULD" instead of "MUST". There might be scenarios where MRMT is desirable but not possible, e.g., when IP multicast is not deployed in a certain network. This is a justification of the second "SHOULD" instead of "MUST". The transmission mode is indicated by the tx-mode media parameter. If tx-mode is equal to "SRST", SRST MUST be used. Otherwise, if tx-mode is equal to "MRST", MRST MUST be used. Otherwise (tx-mode is equal to "MRMT"), MRMT MUST be used. NOTE: (informative) When an RTP stream does not depend on other RTP streams, any of SRST, MRST, or MRMT may be in use for the RTP stream. Receivers MUST support all of SRST, MRST, and MRMT. The required support of MRMT by receivers does not imply that multicast must be supported by receivers.

Payload structures

General Three different types of RTP packet payload structures are specified. A receiver can identify the payload structure by the first two bytes of the RTP packet payload, which co-serves as the RTP payload header. These two bytes are always structured as a NAL unit header. The NAL unit type field indicates which structure is present in the payload. The three different payload structures are as follows:

Single NAL Unit Packet: Contains a single NAL unit in the payload. This payload structure is specified in .
Aggregation Packet: Contains multiple NAL units in a single RTP payload. This payload structure is specified in .
Fragmentation Unit: Contains a subset of a single NAL unit. This payload structure is specified in .

NOTE: (informative) This memo does not limit the size of NAL units encapsulated in NAL unit packets and fragmentation units. does not restrict the maximum size of a NAL unit directly either. Instead, a NAL unit sample stream format may be used, which provides flexibility to signal NAL unit size up to UINT64_MAX bytes.

Single NAL unit packet Single NAL unit packet contains exactly one NAL unit, and consists of an RTP payload header and following conditional fields: 16-bit DONL and 16-bit v3c-tile-id. The rest of the payload data contain the NAL unit payload data (excluding the NAL unit header). Single NAL unit packet MUST only contain atlas NAL units of the types defined in Table 4 of . The structure of the single NAL unit packet is shown below in .

Single NAL unit packet RTP payload header MUST be an exact copy of the NAL unit header of the contained NAL unit. A NAL unit stream composed by de-packetizing single NAL unit packets in RTP sequence number order MUST conform to the NAL unit decoding order, when DONL is not present. The DONL field, when present, specifies the value of the 16-bit decoding order number of the contained NAL unit. If sprop-max-don-diff is greater than 0 for any of the RTP streams, the DONL field MUST be present, and the variable DONL 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 for all the RTP streams), the DONL field MUST NOT be present. The v3c-tile-id field, when present, specifies the 16-bit tile identifier for the NAL unit, as signalled in V3C atlas tile header defined in . If v3c-tile-id-pres is equal to 1 and RTP payload header NUT is in range 0-35, inclusive, the v3c-tile-id field MUST be present. Otherwise, the v3c-tile-id field MUST NOT be present. NOTE: (informative) Only values for NAL unit type (NUT) in range 0-35, inclusive, are allocated for atlas tile layer data in .

Aggregation packet Aggregation Packets (APs) enable the reduction of packetization overhead for small NAL units, such as most of the non-ACL NAL units, which are often only a few octets in size. Aggregation packets MAY be used wrap multiple NAL units belonging to the same access unit in a single RTP payload. The first two bytes of the AP MUST contain RTP payload header. The NAL unit type (NUT) for the NAL unit header contained in the RTP payload header MUST be equal to 56, which falls in the unspecified range of the NAL unit types defined in . AP MAY contain a conditional v3c-tile-id field. AP MUST contain two or more aggregation units. The structure of AP is shown in .

Aggregation Packet (AP) The fields in the payload header are set as follows. The F bit MUST be equal to 0 if the F bit of each aggregated NAL unit is equal to zero; otherwise, it MUST be equal to 1. The NUT field MUST be equal to 56. The value of NLI MUST be equal to the lowest value of NLI of all the aggregated NAL units. The value of TID MUST be the lowest value of TID of all the aggregated NAL units. All ACL NAL units in an aggregation packet have the same TID value since they belong to the same access unit. However, the packet MAY contain non-ACL NAL units for which the TID value in the NAL unit header MAY be different than the TID value of the ACL NAL units in the same AP. The v3c-tile-id field, when present, specifies the 16-bit tile identifier for all ACL NAL units in the AP. If v3c-tile-id-pres is equal to 1, the v3c-tile-id field MUST be present. Otherwise, the v3c-tile-id field MUST NOT be present. AP MUST carry at least two aggregation units (AU) and can carry as many aggregation units as necessary. However, the total amount of data in an AP 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. The structure of the AU depends both on the presence of the decoding order number, the sequence order of the AU in the AP and the presence of v3c-tile-id field. The structure of an AU is shown in .

Aggregation Unit (AU) If sprop-max-don-diff is greater than 0 for any of the RTP streams, an AU begins with the DOND / DONL field. The first AU in the AP contains DONL field, which specifies the 16-bit value of the decoding order number of the aggregated NAL unit. The variable DON for the aggregated NAL unit is derived as equal to the value of the DONL field. All subsequent AUs in the AP MUST contain an (8-bit) DOND field, which specifies the difference between the decoding order number values of the current aggregated NAL unit and the preceding aggregated NAL unit in the same AP. The variable DON for the aggregated NAL unit is derived as equal to the DON of the preceding aggregated NAL unit in the same AP plus the value of the DOND field plus 1 modulo 65536. When sprop-max-don-diff is equal to 0 for all the RTP streams, DOND / DONL fields MUST NOT be present in an aggregation unit. The aggregation units MUST be stored in the aggregation packet so that the decoding order of the containing NAL units is preserved. This means that the first aggregation unit in the aggregation packet SHOULD contain the NAL unit that SHOULD be decoded first. If v3c-tile-id-pres is equal to 2 and the AU NAL unit header type is in range 0-35, inclusive, the 16-bit v3c-tile-id field MUST be present in the aggregation unit after the conditional DOND/DONL field. Otherwise v3c-tile-id field MUST NOT be present in the aggregation unit. The conditional fields of the aggregation unit are followed by a 16-bit NALU size field, which provides the size of the NAL unit (in bytes) in the aggregation unit. The remainder of the data in the aggregation unit SHOULD contain the NAL unit (including the unmodified NAL unit header).

Fragmentation unit Fragmentation Units (FUs) are introduced to enable fragmenting a single NAL unit into multiple RTP packets, possibly without co-operation or knowledge of the 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. Aggregation packets MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU MUST NOT contain a subset of another FU. The RTP header timestamp of an RTP packet carrying an FU is set to the NALU-time of the fragmented NAL unit. A FU consists of an RTP payload header with NUT equal to 57, an 8-bit FU header, a conditional 16-bit DONL field, a conditional 16-bit v3c-tile-id field and an FU payload. The structure of an FU is illustrated below in .

Fragmentation Unit The fields in the RTP payload header are set as follows. The NUT field MUST be equal to 57. The rest of the fields MUST be equal to the fragmented NAL unit. The FU header consists of an S bit, an E bit, and a 6-bit FUT field. The structure of FU header is illustrated below in .

Fragmentation unit header When set to 1, the S bit indicates the start of a fragmented NAL unit, i.e., the first byte of the FU payload is also the first byte of the payload of the fragmented NAL unit. When the FU payload is not the start of the fragmented NAL unit payload, the S bit MUST be set to 0. When set to 1, the E bit indicates the end of a fragmented NAL unit, i.e., the last byte of the payload is also the last byte of the fragmented NAL unit. When the FU payload is not the last fragment of a fragmented NAL unit, the E bit MUST be set to 0. The field FUT MUST be equal to the nal_unit_type field of the fragmented NAL unit. 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 DONL field, when present, specifies the value of the 16-bit decoding order number of the fragmented NAL unit. If sprop-max-don-diff is greater than 0 for any of the RTP streams, 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 for all the RTP streams, or the S bit is equal to 0), the DONL field MUST NOT be present in the FU. The v3c-tile-id field, when present, specifies the 16-bit tile identifier for the fragmented NAL unit. If v3c-tile-id-pres is equal to 1, FUT is in range 0-35, and the S bit is equal to 1, the v3c-tile-id field MUST be present after the conditional DONL field. Otherwise, the v3c-tile-id field MUST NOT be present. 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, NLI, and TID fields of the RTP payload headers of the FUs and the FUT field of the FU header. 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.

Example of fragmentation unit (informative) This example illustrates how fragmentation unit may be used to divide one NAL unit into two RTP packets. The depicts the structure of the first packet with the first part of the fragmented NAL unit.

First packet of fragmented NAL unit The visualizes the structure of the second packet with the rest of the fragmented NAL unit.

Second packet of fragmented NAL unit

Decoding order number For each atlas 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 for all the RTP streams carrying the V3C atlas bitstream, 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 for any of the RTP streams), AbsDon[n] is derived as follows, where DON[n] is the value of the variable DON for NAL unit 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:

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

Packetization and de-packetization rules The following packetization rules apply:

If sprop-max-don-diff is greater than 0 for any of the RTP streams, the transmission order of NAL units carried in the RTP stream MAY be different than the NAL unit decoding order and the NAL unit output order. Otherwise (sprop-max-don-diff is equal to 0 for all the RTP streams), the transmission order of NAL units carried in the RTP stream MUST be the same as the NAL unit decoding order and, when tx-mode is equal to "MRST" or "MRMT", MUST also be the same as the NAL unit output 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-ACL NAL units such as access unit delimiters, parameter sets, or SEI NAL units are typically small and can often be aggregated with ACL NAL units without violating MTU size constraints.
Each non-ACL NAL unit SHOULD, when possible, from an MTU size perspective, be encapsulated in an aggregation packet together with its associated ACL NAL unit, as typically a non-ACL NAL unit would be meaningless without the associated ACL NAL unit being available.
For carrying exactly one NAL unit in an RTP packet, a single NAL unit packet MUST be used

The general concept behind de-packetization is to get the NAL units out of the RTP packets in an RTP stream and all RTP streams the RTP stream depends on, if any, and pass them to the decoder in the NAL unit decoding order. The de-packetization process is implementation dependent. Therefore, the following de-packetization rules SHOULD be taken as an example.

All normal RTP mechanisms related to buffer management apply. In particular, duplicated or outdated RTP packets (as indicated by the RTP sequence number and the RTP timestamp) are removed. To determine the exact time for decoding, factors such as a possible intentional delay to allow for proper inter-stream synchronization must be factored in.
NAL units with NAL unit type values in the range of 0 to 55, inclusive, MAY be passed to the decoder. NAL-unit-like structures with NAL unit type values in the range of 56 to 63, inclusive, MUST NOT be passed to the decoder.
When sprop-max-don-diff is equal to 0 for the received RTP stream, the NAL units carried in the RTP stream MAY be directly passed to the decoder in their transmission order, which is identical to their decoding order.
When sprop-max-don-diff is greater than 0 for any of the received RTP streams, the received NAL units need to be arranged into decoding order before handing them over to the decoder.
For further de-packetization examples, the reader is referred to Section 6 of .

Payload format parameters This section specifies the optional parameters. A mapping of the parameters into the Session Description Protocol (SDP) is also provided for applications that use SDP. Equivalent parameters could be defined elsewhere for use with control protocols that do not use SDP.

Media type registration The receiver MUST ignore any parameter unspecified in this memo. Type name: application Subtype name: v3c Required parameters: N/A Optional parameters: v3c-unit-header, v3c-unit-type, v3c-vps-id, v3c-atlas-id, v3c-attr-idx, v3c-attr-part-idx, v3c-map-idx, v3c-aux-video-flag, v3c-parameter-set, v3c-tile-id, v3c-tile-id-pres, v3c-atlas-data, v3c-common-atlas-data, v3c-sei, v3c-ptl-level-idc, v3c-ptl-tier-flag, v3c-ptl-codec-idc, v3c-ptl-toolset-idc, v3c-ptl-rec-idc, tx-mode and sprop-max-don-diff. Encoding considerations: This type is only defined for transfer via RTP . Security considerations: Please see . Interoperability considerations: N/A Published specification: Please refer to Applications that use this media type: Any application that relies on V3C-based media services over RTP Additional information: N/A Person & email address to contact for further information: Intended usage: COMMON Restrictions on usage: N/A Author: See Authors' Addresses section of this memo. Change controller: IETF avtcore@ietf.org Provisional registration? (standards tree only): No

Optional parameters definition provides a V3C unit header bytes defined in . The value contains base64 encoded representation of the 4 bytes of V3C unit header. v3c-unit-type provides a V3C unit type value corresponding to vuh_unit_type defined in , i.e., defines V3C sub-bitstream type. v3c-vps-id provides a value corresponding to vuh_v3c_parameter_set_id defined in . v3c-atlas-id provides a value corresponding to vuh_atlas_id defined in . v3c-attr-idx provides a value corresponding to vuh_attribute_index defined in . v3c-attr-part-idx provides a value corresponding to vuh_attribute_partition_index defined in . v3c-map-idx provides a value corresponding to vuh_map_index defined in . v3c-aux-video-flag provides a value corresponding to vuh_auxiliary_video_flag defined in . v3c-parameter-set provides V3C parameter set bytes as defined in . The value contains base64 encoded representation of the V3C parameter set bytes. v3c-tile-id indicates that the RTP stream contains only portion of the tiles in the atlas. v3c-tile-id is a comma-separated (',') list of integer values, which indicate the v3c-tile-ids that are present in the RTP stream. v3c-tile-id-pres indicates that the RTP packets contain v3c-tile-id field. v3c-atlas-data MAY be used to convey any atlas data NAL units of the V3C atlas sub bitstream for out-of-band transmission. The value is a comma-separated (',') list of encoded representations of the atlas NAL units as specified in . The NAL units SHOULD be encoded as base64 representations. v3c-common-atlas-data MAY be used to convey common atlas data NAL units of the V3C common atlas sub bitstream for out-of-band transmission. The value is a comma-separated (',') list of encoded representations of the common atlas NAL units (i.e., NAL_CASPS and NAL_CAF_IDR) as specified in . The NAL units SHOULD be encoded as base64 representations. v3c-sei MAY be used to convey SEI NAL units of V3C atlas and common atlas sub bitstreams for out-of-band transmission. The value is a comma-separated (',') list of encoded representations of SEI NAL units (i.e., NAL_PREFIX_NSEI and NAL_SUFFIX_NSEI, NAL_PREFIX_ESEI, NAL_SUFFIX_ESEI) as specified in . The SEI NAL units SHOULD be encoded as base64 representations. v3c-ptl-level-idc provides a value corresponding to ptl_level_idc defined in . v3c-ptl-tier-flag provides a value corresponding to ptl_tier_flag defined in . v3c-ptl-codec-idc provides a value corresponding to ptl_profile_codec_group_idc defined in . v3c-ptl-toolset-idc provides a value corresponding to ptl_profile_toolset_idc defined in . v3c-ptl-rec-idc provides a value corresponding to ptl_profile_reconstruction_idc defined in . This parameter indicates whether the transmission mode is SRST, MRST, or MRMT. The value of tx-mode MUST be equal to "SRST", "MRST" or "MRMT". When not present, the value of tx-mode is inferred to be equal to "SRST". If the value is equal to "MRST", MRST MUST be in use. Otherwise, if the value is equal to "MRMT", MRMT MUST be in use. Otherwise (the value is equal to "SRST"), SRST MUST be in use. The value of tx-mode MUST be equal to "MRST" for all RTP streams in an MRST. The value of tx-mode MUST be equal to "MRMT" for all RTP streams in an MRMT. If the transmission order of NAL units in the RTP stream(s) is the same as the decoding and NAL unit output order, this parameter must be equal to 0. Otherwise, if the decoding order of the NAL units of the RTP stream(s) is the same as the NAL unit transmission order but not the same as NAL unit output order, the value of this parameter MUST be equal to 1. 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.

Congestion control considerations Congestion control for RTP SHALL be used in accordance with , and with any applicable RTP profile: e.g., . An additional requirement if best-effort service is being used is users of this payload format MUST monitor packet loss to ensure that the packet loss rate is within acceptable parameters. Simple bitrate adaptation for congestion control can be achieved when real-time coding is used for V3C video components, where quality parameter can be adaptively tuned. Video coding specifications MAY define further adaptation techniques. Circuit Breakers is an update to RTP that defines criteria for when one is required to stop sending RTP Packet Streams. The circuit breakers is to be implemented and followed.

Session description protocol The mapping of above defined payload format media type to the corresponding fields in the Session Description Protocol (SDP) is done according to .

Mapping of payload type parameters to SDP

For V3C atlas components

The media name in the "m=" line of SDP MUST be application.
The encoding name in the "a=rtpmap" line of SDP MUST be v3c
The clock rate in the "a=rtpmap" line MUST be 90000.
The OPTIONAL parameters v3c-unit-header, v3c-unit-type, v3c-vps-id, v3c-atlas-id, v3c-attr-idx, v3c-attr-part-idx, v3c-map-idx, v3c-aux-video-flag, sprop-max-don-diff, v3c-parameter-set, v3c-atlas-data, v3c-common-atlas-data, v3c-sei, v3c-tile-id, v3c-tile-id-pres, v3c-ptl-level-idc, v3c-ptl-tier-flag, v3c-ptl-codec-idc, v3c-ptl-toolset-idc, v3c-ptl-rec-idc, when present, MUST be included in the "a=fmtp" line of SDP. This parameter is expressed as a media type string, in the form of a semicolon-separated list of parameter=value pairs.

An example of media representation corresponding to atlas data component (V3C_AD) in SDP is as follows:

For V3C video components

The media name in the "m=" line of SDP MUST be video.
The encoding name in the "a=rtpmap" line of SDP can be any video subtype, e.g., H.264, H.265, H.266 etc.
The clock rate in the "a=rtpmap" line MUST be 90000.
The OPTIONAL parameters v3c-unit-header, v3c-unit-type, v3c-vps-id, v3c-atlas-id, v3c-attr-idx, v3c-attr-part-idx, v3c-map-idx, v3c-aux-video-flag, sprop-max-don-diff, v3c-parameter-set, v3c-atlas-data, v3c-common-atlas-data, v3c-sei, v3c-tile-id, v3c-tile-id-pres, v3c-ptl-level-idc, v3c-ptl-tier-flag, v3c-ptl-codec-idc, v3c-ptl-toolset-idc, v3c-ptl-rec-idc, when present, MUST be included in the "a=fmtp" line of SDP. This parameter is expressed as a media type string, in the form of a semicolon-separated list of parameter=value pairs.
The OPTIONAL parameters MAY include any optional parameters from the respective video payload specifications.

An example of media representation corresponding to occupancy video component (V3C_OVD) in SDP is as follows: When v3c-unit-header or v3c-unit-type indicate V3C unit type V3C_PVD, v3c-parameter-set, v3c-atlas-data or v3c-common-atlas-data MAY be signalled along the video stream. When v3c-parameter-set, v3c-atlas-data or v3c-common-atlas-data are present it indicates that the provided data is static for the whole duration of the stream. When v3c-parameter-set, v3c-atlas-data or v3c-common-atlas-data are signalled along the video stream it is expected the respective v3c-parameter-set, v3c-atlas-data or v3c-common-atlas-data remain static for the duration of the stream. Below is an example of media representation corresponding to packed video component (V3C_PVD), where V3C parameter set, atlas data and common atlas data are carried out-of-band in SDP.

Grouping framework Different V3C components MAY be represented by their own respective RTP streams. A grouping tool, as defined in , is extended to support V3C grouping. Group attribute with V3C type is provided to allow application to identify "m" lines that belong to the same V3C bitstream. Grouping type V3C MUST be used with the group attribute. The tokens that follow are mapped to 'mid'-values of individual media lines in the SDP. ]]> The V3C grouping type attribute related v3c-specific session level parameters MAY include the following optional information: v3c-atlas-data= v3c-common-atlas-data= v3c-sei= ]]> When signalled as a session level parameter, the data is considered to be static for the duration of the stream. The following example shows an SDP including four media lines, three describing V3C video components (PT:96=occupancy, PT:97=geometry, PT:98=attribute) and one V3C atlas component (PT:100). All the media lines are grouped under one V3C group which provides the V3C parameter set. V3C group attribute type can be used as follows to indicate different V3C components (PT:96=occupancy, PT:97=geometry, PT:98=attribute) and associate static atlas data with them. The following example describes how every V3C video component is packed into a single stream (V3C_PVD) and associated with static atlas data. The example below describes how content with two atlases can be signalled as separate streams. V3C parameter set and common atlas data are carried as group attribute parameters. PT equal to 96, 97, 98 and 100 correspond to occupancy, geometry and attribute video component as well as atlas data component for atlas zero. PT equal to 101, 102, 103 and 104 correspond to respective components for atlas one.

Offer and answer considerations An example of offer which only sends V3C content. The following example contains video components as three different versions (H.264, H.265, H.266). Further differences between the alternatives would be signaled as part of the media attribute parameters, as is the practice with regular video streams. An example of answer which only receives V3C data with the selected versions. An example offer, which allows bundling different V3C components on one stream, based on . An example answer, which accepts bundling of different V3C components.

Declarative SDP considerations When V3C content over RTP is offered with SDP in a declarative style, the parameters capable of indicating both bitstream properties as well as receiver capabilities are used to indicate only bitstream properties. For example, in this case, the parameters v3c-ptl-level-idc, v3c-ptl-tier-flag, v3c-ptl-codec-idc, v3c-ptl-toolset-idc and v3c-ptl-rec-idc declare the values used by the bitstream, not the capabilities for receiving bitstreams. A receiver of the SDP is required to support all parameters and values of the parameters provided; otherwise, the receiver MUST reject or not participate in the session. It falls on the creator of the session to use values that are expected to be supported by the receiving application.

IANA considerations A new media type will be registered with IANA; see Section . Furthermore new group type (V3C) for the group attribute will be registered as defined in . This document registers the semantics in with IANA in the "Semantics for the 'group' SDP Attribute" subregistry (under the "Session Description Protocol (SDP) Parameters" registry): Additional semantics for V3C SDP group type

Semantics	Token	Mux Category	Reference
V3C grouping	V3C	NORMAL	"this memo"

NOTE: (informative) "this memo" to be replaced wit the RFC number, once it becomes available.

Security considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification , and in any applicable RTP profile such as RTP/AVP , RTP/AVPF , RTP/SAVP , or RTP/SAVPF . However, as "Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single Media Security Solution" 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" . Applications SHOULD use one or more appropriate strong security mechanisms. The rest of this Security Considerations section discusses the security impacting properties of the payload format itself. This RTP payload format and its media decoder do not exhibit any significant non-uniformity in the receiver-side computational complexity for packet processing, and thus are unlikely to pose a denial-of-service threat due to the receipt of pathological data. Nor does the RTP payload format contain any active content.