Nonstandard for transmission of IP datagrams over serial lines: SLIP
RFC 1055

Document Type RFC - Internet Standard (June 1988; Errata)
Also known as STD 47
Last updated 2016-02-17
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Network Working Group                                          J. Romkey
Request for Comments: 1055                                     June l988

 A NONSTANDARD FOR TRANSMISSION OF IP DATAGRAMS OVER SERIAL LINES: SLIP

INTRODUCTION

   The TCP/IP protocol family runs over a variety of network media:
   IEEE 802.3 (ethernet) and 802.5 (token ring) LAN's, X.25 lines,
   satellite links, and serial lines.  There are standard encapsulations
   for IP packets defined for many of these networks, but there is no
   standard for serial lines.  SLIP, Serial Line IP, is a currently a de
   facto standard, commonly used for point-to-point serial connections
   running TCP/IP.  It is not an Internet standard.  Distribution of
   this memo is unlimited.

HISTORY

   SLIP has its origins in the 3COM UNET TCP/IP implementation from the
   early 1980's.  It is merely a packet framing protocol: SLIP defines a
   sequence of characters that frame IP packets on a serial line, and
   nothing more. It provides no addressing, packet type identification,
   error detection/correction or compression mechanisms.  Because the
   protocol does so little, though, it is usually very easy to
   implement.

   Around 1984, Rick Adams implemented SLIP for 4.2 Berkeley Unix and
   Sun Microsystems workstations and released it to the world.  It
   quickly caught on as an easy reliable way to connect TCP/IP hosts and
   routers with serial lines.

   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.  It 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).

AVAILABILITY

   SLIP is available for most Berkeley UNIX-based systems.  It is
   included in the standard 4.3BSD release from Berkeley.  SLIP is
   available for Ultrix, Sun UNIX and most other Berkeley-derived UNIX
   systems.  Some terminal concentrators and IBM PC implementations also
   support it.

   SLIP for Berkeley UNIX is available via anonymous FTP from
   uunet.uu.net in pub/sl.shar.Z.  Be sure to transfer the file in
   binary mode and then run it through the UNIX uncompress program. Take

Romkey                                                          [Page 1]
RFC 1055                     Serial Line IP                    June 1988

   the resulting file and use it as a shell script for the UNIX /bin/sh
   (for instance, /bin/sh sl.shar).

PROTOCOL

   The SLIP protocol defines two special characters: END and ESC. END is
   octal 300 (decimal 192) and ESC is octal 333 (decimal 219) not to be
   confused with the ASCII ESCape character; for the purposes of this
   discussion, ESC will indicate the SLIP ESC character.  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.

   Phil Karn suggests a simple change to the algorithm, which is to
   begin as well as end packets with an END character.  This will flush
   any erroneous bytes which have been caused by line noise.  In the
   normal case, the receiver will simply see two back-to-back END
   characters, which will generate a bad IP packet.  If the SLIP
   implementation does not throw away the zero-length IP packet, the IP
   implementation certainly will.  If there was line noise, the data
   received due to it will be discarded without affecting the following
   packet.

   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).  Therefore any new SLIP implementations
   should be prepared to accept 1006 byte datagrams and should not send
   more than 1006 bytes in a datagram.

DEFICIENCIES

   There are several features that many users would like SLIP to provide
   which it doesn't.  In all fairness, SLIP is just a very simple
   protocol designed quite a long time ago when these problems were not
   really important issues.  The following are commonly perceived
   shortcomings in the existing SLIP protocol:

      - addressing:

         both computers in a SLIP link need to know each other's IP
         addresses for routing purposes.  Also, when using SLIP for
         hosts to dial-up a router, the addressing scheme may be quite
         dynamic and the router may need to inform the dialing host of

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RFC 1055                     Serial Line IP                    June 1988
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