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SLIP is a protocol for sending IP datagrams over a serial link, using unique byte markers. It lacks security and error handling, and PPP is now more commonly used. CSLIP compresses TCP headers, but PPP is a more robust solution.
Serial Line Internet Protocol, also known by the acronym SLIP, is a means of sending Internet Protocol datagrams over a serial link. Transmits each byte of a packet, adding one or more unique byte markers to the stream as needed. SLIP can be used by two systems to communicate over a direct cable connection or a modem link. Most personal computers use Point-to-Point Protocol (PPP) instead, but SLIP is often used when very low overhead is required.
When transmitting a datagram, the serial line Internet protocol may first send an “end” byte marker over the serial link. Then send every byte in the packet, unless it’s one of two decimal values: 219 or 192. If it’s 192, the value of the end marker, SLIP sends two special bytes in its place; 219 followed by 220. If 219, SLIP sends a different two-byte code: 219 followed by 221. When all bytes of the Internet Protocol packet have been sent, SLIP sends the 192 byte end marker.
When a packet is received by SLIP, it removes the end markers and replaces any special byte pairs with their original values. Some implementations of SLIP can also drop zero-length packets. The resulting datagram, if any, is then passed on to the next network layer without further analysis. Error detection and correction is left to the next level, which must request a packet retransmission if an error is discovered.
The SLIP specification was released by the Internet Engineering Task Force (IETF) in 1988 as Request For Comments (RFC) 1055. It identifies several SLIP deficiencies, including error handling and lack of control messages. Another common problem with Serial Line Internet Protocol is the absence of packet protocol identification. Since SLIP was only designed to handle Internet Protocol datagrams, this may not be a frequent problem. SLIP is quite capable of handling multiple Internet protocols, however, given a means of identifying them.
Lack of security is another consideration for using Serial Line Internet Protocol. There is no mechanism for authenticating the connection or any kind of data encryption at this level with SLIP. Also, it is not possible to discover Internet Protocol addresses for routing to the next network layer. Regarding the maximum packet size, the SLIP specification is vague, suggesting a limit used by another implementation.
Serial Line Internet Protocol does not compress any data packets, although the modem on the serial link will, if a modem is in use. Compressed SLIP (CSLIP), defined in 1990 by RFC 1144, makes an effort in this direction. It reduces each packet’s Transmission Control Protocol (TCP) header to seven bytes from the original 20. However, the rest of the package remains unchanged. In 1994, PPP, defined in RFC 1661, was created as a new robust protocol that solved many SLIP problems.
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