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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols
           9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)

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SLIP and PPP Overview and Role In TCP/IP
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Serial Line Internet Protocol (SLIP)
(Page 1 of 3)

The need for a data link layer protocol to let IP operate over serial links was identified very early on in the development of TCP/IP. Engineers working on the Internet Protocol needed a way to send IP datagrams over serial connections linking computers together. To solve the problem they created a very simple protocol that would frame IP datagrams for transmission across the serial line. This protocol is called the Serial Line Internet Protocol, or SLIP for short.

SLIP Overview and History

SLIP is different from most TCP/IP protocols in that it has never been defined as a formalized standard. It was created informally in the early 1980s and its use spread as a de facto standard before it was ever described in an RFC document. Even when it was eventually published, in 1988, the decision was specifically made that SLIP would not be designated an official Internet standard. The authors of the paper that describes it, RFC 1055, made sure nobody would miss this point, by naming it A Nonstandard For Transmission Of IP Datagrams Over Serial Lines: SLIP.

On The Web: This is but one of several places where the IETF engineers let their sense of humor shine through. For another interesting example, see RFC 1313. J


Why was SLIP designated as a “nonstandard” instead of a standard? Well, it was developed as a very rudimentary “stopgap” measure to provide layer two framing when needed. It's so simple that there really isn't much to standardize. Also, the protocol has so many deficiencies that the IETF apparently didn't want it given the status of a formalized standard. RFC 1055 makes specific mention of the problems with SLIP (which we'll see below) and the fact that work was already underway at that time to define a more capable successor to SLIP (PPP).

How simple is SLIP? So simple that it is one of the very few technologies in this Guide that I can describe almost completely without complaining that it's complicated, or resorting to telling you to “see the defining document for details”. SLIP performs only one function: framing of data for transmission. It does nothing else.

Key Concept: SLIP provides a layer two framing service for IP datagrams—and no other features or capabilities.



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