When the RTP Payload is not encrypted, intermediate network functions may obtain additional information from the RTP payload. The PDU Set information identification based on the RTP payload format is presented in this clause, including information on the RTP payload formats for H.264/AVC [5] and H.265/HEVC [6] codecs. The information about the used RTP Payload format for a service data flow is provided in advance to 5GC (e.g., UPF). It is generally recommended that the network function considers non-VCL NAL units as part of the PDU Set of the associated VCL NALUs, e.g. identified by the same timestamp.

For a video content with H.264/AVC RTP payload, the PDU Set Information can be obtained by the following approach. According to RFC 6184, the first octet in the RTP payload indicates the content of the RTP payload, e.g. coded slice of an IDR frame, coded slice of a P frame, and also the possible structures of the RTP payload, e.g. single NAL unit packet, AP and fragmentation unit (FU). Depending on the indication of the first octet of the RTP payload, a second octet (the FU header) should also be processed.

• For single NAL unit packets and APs, it can be easily detected that each RTP packet can be treated as a complete PDU Set when the first Type field of Figure A.2.2-1 is less than 28.
• In case of APs, the network function may need to process all embedded NAL units.
• When the first Type field in Figure A.2.2-1 is 28 or 29, one NAL unit is carried over multiple RTP packets. In this case, the first byte of RTP payload is also named the FU indicator and the following byte is the FU header. The NAL unit type is contained in the Type field of the FU header (Figure A.2.2-1). In the FU header, the "S" bit and "E" bit separately represents the start and end of the NAL unit. Therefore, based on the NAL unit type (also known as FU indicator for FU) and the FU header, the start/end of the PDU Set can be identified.

With the RTP payload (i.e. NALU header and optionally FU header) and the sequence number in the RTP header, the indication of the End PDU of the PDU Set and the PDU SN within a PDU Set can be derived. When using FUs (Type equals 28 or 29), the size of the NALU can only be determined after reception of the last packet of this FU. Thus, a network function can only determine the PDU Set Size with the reception of the last PDU of this FU. As described in clause 4.2.6.2.2, the Type and NRI value in the NAL unit header indicates the relative transport priority and can be used to set the PSI. Besides, different NRI values can also indicate different requirements, which can be used to provide different protections against transmission losses, e.g. reliabilities (tolerable frame/slice error rate), and priorities.

For a video content with H.265/HEVC RTP payload, the identification of the PDU Set can be realized by following approach. According to RFC 7798, within the RTP packet, the first two octets of the RTP payload indicate the content of RTP packet. Besides, it also indicates the possible structures of the RTP payload, e.g. single NAL unit packet, APs, FUs, and Payload Content Information (PACI) carrying RTP packet.

• For single NAL unit packets and APs, it can be easily detected that each RTP packet can be treated as a single PDU Set when the NAL unit type is less than 49.
• When NAL unit type is 49, one NAL unit is carried over multiple RTP packets. In this case, the first two-byte of RTP payload is also named the payload header (denoted as NAL unit, NALU, header) and the following byte is the FU header. In the FU header, the "S" bit and "E" bit separately represents the start and end of the NAL unit. The FuType field contains the actual NAL unit type. Therefore, based on the Type field of the first two octets (also known as FU indicator for FU) and the FU header, the start/end of the PDU Set can be identified.
• When NAL unit type is 50, this is a PACI packet which may carry a single NAL unit packet or FU. In this case, the first two-byte of RTP payload is also named as the PACI header (denoted as NAL unit header). In the following two bytes, the "A" bit is the copy of "F" bit and cType field is the copy of Type field in the PACI payload NAL unit. Then the following is the PHES field, whose length is determined by the PHSize. Finally, the following is the PACI payload NAL unit, during which the first byte is FU header when cType (within the PACI payload header) is 49. Therefore, based on the PACI header and PACI payload NAL unit, the start/end of the PDU Set can be identified.

With the RTP payload (i.e. NAL unit header and optionally FU header) and the sequence number in the RTP header, the indication of the End PDU of the PDU Set and the PDU SN within a PDU Set can be derived. As described in clause 4.2.6.2.3, the Type field and the TID field in the NAL unit header indicates the relative transport priority and can be used to be mapped to the PSI. They can also indicate different requirements, which can be used to provide different protections against transmission losses, e.g. reliabilities (tolerable frame/slice error rate), and priorities. When using FUs (Type equals 49, or 50 where cType is 49), the size of the NAL unit can only be determined after reception of the last packet of this FU. Thus, a network function can only determine the PDU Set Size with the reception of the last PDU of this FU.