What function does IP header compression serve, for both the client and the ISP?
Header compression, developed by Van Jacobson and documented in RFC 1144, is a popular means for improving throughput over low-speed links by reducing the size of TCP/IP packet headers, though no attempt is made to compress the data itself. Think of it as writing a short note on a postcard, instead of mailing a regular letter – the content is the same, there’s just less overhead. Jacobson’s algorithm operates at the link layer, so it isn’t noticeable to any systems except the two that are participating, and any link layer protocol can be used, so you will also hear about CSLIP (compressed SLIP) and CPPP (compressed PPP). TCP/IP header compression (UDP packets don’t get compressed) reduce the packet header from a nominal 40 bytes (20 bytes IP and 20 bytes TCP) to perhaps half a dozen, or maybe just two or three – exact performance varies. For interactive terminal traffic, where a user is typing on a keyboard and every character gets sent as the keys are struck, this can be a big win. No
