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5.7. Aggregation Packets

5.7. Aggregation Packets​

Aggregation packets are the NAL unit aggregation scheme of this payload specification. The scheme is introduced to reflect the dramatically different MTU sizes of two key target networks: wireline IP networks (with an MTU size that is often limited by the Ethernet MTU size, roughly 1500 bytes) and IP-based or non-IP-based (e.g., ITU-T H.324/M) wireless communication systems with preferred transmission unit sizes of 254 bytes or less. To prevent media transcoding between the two worlds, and to avoid undesirable packetization overhead, a NAL unit aggregation scheme is introduced.

Two types of aggregation packets are defined by this specification:

  • Single-time aggregation packet (STAP): aggregates NAL units with identical NALU-times. Two types of STAPs are defined, one without DON (STAP-A) and another including DON (STAP-B).
  • Multi-time aggregation packet (MTAP): aggregates NAL units with potentially differing NALU-times. Two different MTAPs are defined, differing in the length of the NAL unit timestamp offset.

Each NAL unit to be carried in an aggregation packet is encapsulated in an aggregation unit. Please see below for the four different aggregation units and their characteristics.

The structure of the RTP payload format for aggregation packets is presented 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI|  Type   |                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |
|                                                               |
|             one or more aggregation units                     |
|                                                               |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               :...OPTIONAL RTP padding        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3. RTP payload format for aggregation packets

MTAPs and STAPs share the following packetization rules:

  • The RTP timestamp MUST be set to the earliest of the NALU-times of all the NAL units to be aggregated.
  • The type field of the NAL unit type octet MUST be set to the appropriate value, as indicated in Table 4.
  • The F bit MUST be cleared if all F bits of the aggregated NAL units are zero; otherwise, it MUST be set.
  • The value of NRI MUST be the maximum of all the NAL units carried in the aggregation packet.

Table 4. Type field for STAPs and MTAPs

TypePacketTimestamp offset field length (in bits)DON-related fields (DON, DONB, DOND) present
24STAP-A0no
25STAP-B0yes
26MTAP1616yes
27MTAP2424yes

The marker bit in the RTP header is set to the value that the marker bit of the last NAL unit of the aggregated packet would have if it were transported in its own RTP packet.

The payload of an aggregation packet consists of one or more aggregation units. See Sections 5.7.1 and 5.7.2 for the four different types of aggregation units. An aggregation packet can carry as many aggregation units as necessary; however, the total amount of data in an aggregation packet 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. An aggregation packet MUST NOT contain fragmentation units, as specified in Section 5.8. Aggregation packets MUST NOT be nested; that is, an aggregation packet MUST NOT contain another aggregation packet.

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