2.5 Compressed SLIP

Since SLIP lines are often slow (19200 bits/sec or below) and frequently used for interactive traffic (such as Telnet and Rlogin, both of which use TCP), there tend to be many small TCP packets exchanged across a SLIP line. To carry 1 byte of data requires a 20-byte IP header and a 20-byte TCP header, an overhead of 40 bytes. (Section 19.2 shows the flow of these small packets when a simple command is typed during an Rlogin session.)

Recognizing this performance drawback, a newer version of SLIP, called CSLIP (for compressed SLIP), is specified in RFC 1144 [Jacobson 1990a]. CSLIP normally reduces the 40-byte header to 3 or 5 bytes. It maintains the state of up to 16 TCP connections on each end of the CSLIP link and knows that some of the fields in the two headers for a given connection normally don't change. Of the fields that do change, most change by a small positive amount. These smaller headers greatly improve the interactive response time.

Most SLIP implementations today support CSLIP. Both SLIP links on the author's subnet (see inside front cover) are CSLIP links.