12. Hooks for additional header compression

The following hook is supplied to allow additional header compression schemes for headers on top of UDP. The initial chain of subheaders is then compressed as described here, and the other header compression scheme is applied to the header above the UDP header. An example of such additional header compression is Compressed RTP by Casner and Jacobson [CRTP]. To allow some error detection, such schemes typically need a sequence number that may need to be passed in full headers as well as compressed UDP headers.

The D-bit and Data octet (see section 6) provides the necessary mechanism. When a sequence number, say, needs to be passed in a FULL_HEADER or COMPRESSED_NON_TCP header, the D-bit is set and the sequence number is placed in the Data field. The decompressor must then extract and make the Data field available to the additional header compression scheme.

Use of additional header compression schemes like CRTP must be negotiated. The D-bit and Data octet mechanism must automatically be enabled whenever use of additional header compression schemes has been negotiated.