Serial Line Internet Protocol

The Serial Line Internet Protocol (SLIP)<ref>Template:Cite news</ref><ref>Template:Cite news</ref> is an encapsulation of the Internet ProtocolTemplate:Efn designed to work over serial ports and router connections. It is documented in Template:IETF RFC. On personal computers, SLIP has largely been replaced by the Point-to-Point Protocol (PPP), which is better engineered, has more features, and does not require its IP address configuration to be set before it is established. On microcontrollers, however, SLIP is still the preferred way of encapsulating IP packets, due to its very small overhead.

Some people refer to the successful and widely used RFC 1055 Serial Line Internet Protocol as "Rick Adams' SLIP",<ref name="rfc1547">Template:IETF RFC: "Requirements for an Internet Standard Point-to-Point Protocol" </ref> to avoid confusion with other proposed protocols named "SLIP". Those other protocols include the much more complicated Template:IETF RFC appendix D Serial Line Interface Protocol.<ref name="rfc1547" />

Description

Hex value	Dec Value	Oct Value	Abbreviation	Description
0xC0	192	300	END	Frame End
0xDB	219	333	ESC	Frame Escape
0xDC	220	334	ESC_END	Transposed Frame End
0xDD	221	335	ESC_ESC	Transposed Frame Escape

SLIP modifies a standard TCP/IP datagram by:

• appending a special "END" byte to it, which distinguishes datagram boundaries in the byte stream,
• if the END byte occurs in the data to be sent, the two byte sequence ESC, ESC_END is sent instead,
• if the ESC byte occurs in the data, the two byte sequence ESC, ESC_ESC is sent.
• variants of the protocol may begin, as well as end, packets with END.

SLIP requires a serial port configuration of 8 data bits, no parity, and either EIA hardware flow control, or CLOCAL mode (3-wire null-modem) UART operation settings.

SLIP does not provide error detection, being reliant on upper layer protocols for this. Therefore, SLIP on its own is not satisfactory over a link which is error-prone, such as a poor quality dial-up connection.

SLIP escape characters were also required on some modem connections to escape Hayes command set, allowing therefore to pass binary data through those modems that would recognize some characters as commands.

CSLIP

A version of SLIP with header compression is called Compressed SLIP (CSLIP).<ref>Template:Cite book</ref> The compression algorithm used in CSLIP is known as Van Jacobson TCP/IP Header Compression.<ref>Template:Cite web — introduced the Van Jacobson TCP/IP Header Compression used by CSLIP</ref> CSLIP has no effect on the data payload of a packet and is independent of any compression by the serial line modem used for transmission. It reduces the Transmission Control Protocol (TCP) header from twenty bytes to seven bytes. CSLIP has no effect on User Datagram Protocol (UDP) datagrams.

History

Template:See also RFC 1055, a "non-standard" for SLIP, traces its origins to the 3COM UNET TCP/IP implementation from the 1980s. Rick Adams added SLIP to the popular 4.2BSD in 1984 and it "quickly caught on". By the time of the RFC (1988), it is described as "commonly used on dedicated serial links and sometimes for dialup purposes".<ref name=rfc1055>Template:Cite web</ref>

The last version of FreeBSD to include "slattach" (a command for connecting to slip) in the manual database is FreeBSD 7.4, released 2011. The manual claims that auto-negotiation exists for CSLIP. The FreeBSD version is inherited from 4.3BSD.<ref>Template:Cite web</ref>

Linux formerly used the same code base for SLIP and KISS (TNC). The split occurred before the start of kernel git history (Linux-2.6.12-rc2, 2005).<ref>Template:Cite web</ref> The SLIP driver offers a special "6-bit" escaped mode to accommodate modems incapable of handling non-ASCII characters.<ref>Template:Cite web</ref> The Linux slattach command (written independently) also has the ability to auto-detect CSLIP support.<ref>Template:Man "Other possible values are slip (normal SLIP), adaptive (adaptive CSLIP/SLIP)...</ref>

See also

• Parallel Line Internet Protocol
• Slirp
• KA9Q
• Direct cable connection
• In-band signaling
• KISS (amateur radio protocol)
• Consistent Overhead Byte Stuffing