Layer 2 payload compression squeezes Layer 2 payloads (the entire Layer 3 packet). Layer 2 payload compression increases the throughput and decreases the latency in transmission, because smaller packets (with compressed payloads) take less time to transmit than the larger, uncompressed packets. Layer 2 payload compression is performed on a link-by-link basis.
Header compression methods work by not transmitting repeated information in packet headers throughout a session. The two peers on a PPP Layer 2 connection (a dial-up link) agree on session indices which index a dictionary of packet headers. The dictionary is built at the start of every session and is used for all subsequent (non-initial) packets. Only changing, or non-constant, parameters in the headers are actually sent along with the session index.
It is important to note that header compression is performed on a link-by-link basis. Header compression cannot be performed across multiple routers because routers need full Layer 3 header information to be able to route packets to the next hop.
Compressed RTP (cRTP) maintains the state for session contexts. A session context is defined by the combination of the IP source and destination addresses, the User Datagram Protocol (UDP) source and destination ports, and the RTP synchronization source (SSRC) field. A compressor implementation might use a hash function on these fields to index a table of stored session contexts. The compressed packet carries a small integer, called the session context identifier, to indicate in which session context that packet should be interpreted. The decompressor can use the context identifier to index its table of stored session contexts directly.
Note Refer to RFC 2507, 2508, and 1144 for more information on header compression operations.
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