5.2. Payload Structures
5.2. Payload Structures
The payload format defines three different basic payload structures. A receiver can identify the payload structure by the first byte of the RTP packet payload, which co-serves as the RTP payload header and, in some cases, as the first byte of the payload. This byte is always structured as a NAL unit header. The NAL unit type field indicates which structure is present. The possible structures are as follows.
Single NAL Unit Packet: Contains only a single NAL unit in the payload. The NAL header type field is equal to the original NAL unit type, i.e., in the range of 1 to 23, inclusive. Specified in Section 5.6.
Aggregation Packet: Packet type used to aggregate multiple NAL units into a single RTP payload. This packet exists in four versions, the Single-Time Aggregation Packet type A (STAP-A), the Single-Time Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet (MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet (MTAP) with 24-bit offset (MTAP24). The NAL unit type numbers assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and 27, respectively. Specified in Section 5.7.
Fragmentation Unit: Used to fragment a single NAL unit over multiple RTP packets. Exists with two versions, FU-A and FU-B, identified with the NAL unit type numbers 28 and 29, respectively. Specified in Section 5.8.
Informative note: This specification does not limit the size of NAL units encapsulated in single NAL unit packets and fragmentation units. The maximum size of a NAL unit encapsulated in any aggregation packet is 65535 bytes.
Table 1 summarizes NAL unit types and the corresponding RTP packet types when each of these NAL units is directly used as a packet payload, and where the types are described in this memo.
Table 1. Summary of NAL unit types and the corresponding packet types
| NAL Unit Type | Packet Type | Packet Type Name | Section |
|---|---|---|---|
| 0 | reserved | - | |
| 1-23 | NAL unit | Single NAL unit packet | 5.6 |
| 24 | STAP-A | Single-time aggregation packet | 5.7.1 |
| 25 | STAP-B | Single-time aggregation packet | 5.7.1 |
| 26 | MTAP16 | Multi-time aggregation packet | 5.7.2 |
| 27 | MTAP24 | Multi-time aggregation packet | 5.7.2 |
| 28 | FU-A | Fragmentation unit | 5.8 |
| 29 | FU-B | Fragmentation unit | 5.8 |
| 30-31 | reserved | - |