Main ROHC Compression Principles
The ROHC protocol takes advantage of the information redundancy in the different headers of:
- one single network packet (eg. the payload lengths in IP and UDP headers),
- several network packets that belongs to one single stream (eg. the IP addresses).
Redundant information is transmitted in first packets only. Next packets contain variable information only, eg. identifiers or sequence numbers. Theses fields are furthermore transmitted in a adequate compressed form to save some more bits.
For better performances, the packets must be classified into streams before being compressed. This is a requirement to take advantage of inter-packet redundancy. The classification algorithm is not defined by the ROHC protocol itself but left to the implementations. Once a stream of packets was identified, it is compressed according to the compression profile that fits best. A compression profile defines the way to compress the different fields in the network headers. Several compression profiles are available: Uncompressed, IP-only, IP/UDP, IP/UDP-Lite, IP/ESP, IP/UDP/RTP, IP/UDP-Lite/RTP and IP/TCP.
Read more about this topic: Robust Header Compression
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