SLIP: Serial Line IP

Serial Line IP (SLIP) is used for point-to-point serial connections running TCP/IP. SLIP is commonly used on dedicated serial links and sometimes for dialup purposes, and is usually used with line speeds between 1200bps and 19.2Kbps. SLIP is useful for allowing mixes of hosts and routers to communicate with one another (host-host, host-router and router-router are all common SLIP network configurations).

SLIP is merely a packet framing protocol: SLIP defines a sequence of characters that frame IP packets on a serial line. It does not provide addressing, packet type identification, error detection/correction or compression mechanisms.

The SLIP protocol defines two special characters: END and ESC. END is octal 300 (decimal 192) and ESC is octal 333 (decimal 219). To send a packet, a SLIP host simply starts sending the data in the packet. If a data byte is the same code as END character, a two byte sequence of ESC and octal 334 (decimal 220) is sent instead. If it the same as an ESC character, an two byte sequence of ESC and octal 335 (decimal 221) is sent instead. When the last byte in the packet has been sent, an END character is then transmitted.  

Because there is no 'standard' SLIP specification, there is no real defined maximum packet size for SLIP. It is probably best to accept the maximum packet size used by the Berkeley UNIX SLIP drivers: 1006 bytes including the IP and transport protocol headers (not including the framing characters).

Compressed Serial Line IP (CSLIP) performs the Van Jacobson header compression on outgoing IP packets. This compression improves throughput for interactive sessions noticeably.