TCP header compression as defined in RFC 1144 compresses only the protocol headers, not the
packet data. TCP header compression lowers the overhead generated by the disproportionately
large TCP/IP headers as they are transmitted across the WAN.
It is important to realize that the layer 2 header is not touched, and only the headers at
layers 3 and 4 are compressed. This enables the layer 2 header to direct that packet across a
WAN link.
You would use the header compression on a network with small packets and a few bytes
of data such as Telnet. Cisco’s header compression supports X.25, Frame Relay, and dial-ondemand
WAN link protocols. Because of processing overhead, header compression is generally
used at lower speeds such as 64Kbps links.
TCP header compression is achieved by using the ip tcp header-compression command:
Router(config)#interface serial0
Router(config-if)#ip tcp ?
compression-connections Maximum number of compressed connections
header-compression Enable TCP header compression
Router(config-if)#ip tcp header-compression ?
passive Compress only for destinations which send compressed headers
The passive parameter is optional and is used to instruct the router to compress the headers
of outbound TCP traffic if the other side is also sending compressed TCP headers. If you don’t
include the passive argument, all TCP traffic will use compressed TCP headers.