Editing Serial Line Internet Protocol

{{Short description|Internet Protocol encapsulation for serial ports and router connections}}
{{IPstack}}

The '''Serial Line Internet Protocol''' ('''SLIP''')<ref>{{cite news |newspaper=[[The New York Times]]
   |url=http://www.nytimes.com/library/cyber/qa/0531freed.html
   |title=Internet Q&A |date=May 31, 1996}}</ref><ref>{{cite news |newspaper=[[The New York Times]]
|url=https://www.nytimes.com/1994/05/08/business/the-executive-computer-for-befuddled-newcomers-easier-access-to-the-internet.html
|title=The Executive Computer; For Befuddled Newcomers, Easier Access
|date=May 8, 1994}}</ref> is an [[encapsulation (networking)|encapsulation]] of the [[Internet Protocol]]{{efn|SLIP does not actually care about the internal structures of IP: any [[network layer]] protocols can be sent over the wire.}} designed to work over [[serial port]]s and [[router (computing)|router]] connections. It is documented in {{IETF RFC|1055}}. On personal computers, SLIP has largely been replaced by the [[Point-to-Point Protocol]] (PPP), which is better engineered, has more features, and does not require its IP address configuration to be set before it is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 [[Internet Protocol|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">{{IETF RFC|1547}}: "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 {{IETF RFC|914}} appendix D [[Serial Line Interface Protocol]].<ref name="rfc1547" />

==Description==

{| class="wikitable"
|-
! 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 [[computer configuration|configuration]] of 8 [[data]] [[bit]]s, no [[parity bit|parity]], and either [[Electronic Industries Alliance|EIA]] hardware [[flow control (data)|flow control]], or CLOCAL mode (3-wire [[null-modem]]) [[UART]] operation settings.

SLIP does not provide [[error detection]], being reliant on [[upper layer protocol]]s 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 (computing)|header]] [[data compression|compression]] is called '''Compressed SLIP''' ('''CSLIP''').<ref>{{cite book
   |url=https://www.oreilly.com/library/view/understanding-tcpip/9781904811718/ch04s02.html
   |title=Understanding TCP/IP (Chapter 4.2 Compressed SLIP)}}</ref> The compression algorithm used in CSLIP is known as [[Van Jacobson TCP/IP Header Compression]].<ref>{{cite web |first=V. |last=Jacobson |title=Compressing TCP/IP Headers for Low-Speed Serial Links |date=February 1990 |url=http://tools.ietf.org/html/rfc1144}} — 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 [[byte]]s to seven bytes. CSLIP has no effect on [[User Datagram Protocol]] (UDP) datagrams.

== History ==
{{See also|History of the Internet}}
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>{{cite web |title=RFC 1055: Nonstandard for transmission of IP datagrams over serial lines: SLIP |url=https://datatracker.ietf.org/doc/html/rfc1055 |website=IETF Datatracker |language=en |date=1 June 1988}}</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>{{cite web |title=slattach(8) |url=https://man.freebsd.org/cgi/man.cgi?query=slip&apropos=0&sektion=8&manpath=FreeBSD+7.4-RELEASE&arch=default&format=html |website=man.freebsd.org}}</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>{{cite web |last1=Torvalds |first1=Linus |title=History for mkiss.c |website=[[GitHub]] |url=https://github.com/torvalds/linux/blob/9a48d604672220545d209e9996c2a1edbb5637f6/drivers/net/hamradio/mkiss.c |access-date=13 May 2023 |date=13 May 2023}}</ref> The SLIP driver offers a special "6-bit" escaped mode to accommodate modems incapable of handling non-ASCII characters.<ref>{{cite web |last1=Torvalds |first1=Linus |title=drivers/net/slip/Kconfig |website=[[GitHub]] |url=https://github.com/torvalds/linux/blob/master/drivers/net/slip/Kconfig |date=13 May 2023}}</ref> The Linux slattach command (written independently) also has the ability to auto-detect CSLIP support.<ref>{{man|8|slattach|Linux}} "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]]

==References==
{{notelist}}
{{Reflist}}

{{Authority control}}

[[Category:Internet protocols]]
[[Category:Link protocols]]
[[Category:Logical link control]]