| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
TheSerial Line Internet Protocol (SLIP)[1][2] is anencapsulation of theInternet Protocol[a] designed to work overserial ports androuter connections. It is documented inRFC 1055. On personal computers, SLIP has largely been replaced by thePoint-to-Point Protocol (PPP), which is better engineered, has more features, and does not require its IP address configuration to be set before it is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. Onmicrocontrollers, however, SLIP is still the preferred way of encapsulatingIP 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",[3] to avoid confusion with other proposed protocols named "SLIP". Those other protocols include the much more complicatedRFC 914 appendix DSerial Line Interface Protocol.[3]
| 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 standardTCP/IPdatagram by:
SLIP requires a serial portconfiguration of 8databits, noparity, and eitherEIA hardwareflow control, or CLOCAL mode (3-wirenull-modem)UART operation settings.
SLIP does not provideerror detection, being reliant onupper layer protocols for this. Therefore, SLIP on its own is not satisfactory over a link which is error-prone, such as a poor qualitydial-up connection.
SLIP escape characters were also required on some modem connections to escapeHayes command set, allowing therefore to pass binary data through those modems that would recognize some characters as commands.
A version of SLIP withheadercompression is calledCompressed SLIP (CSLIP).[4] The compression algorithm used in CSLIP is known asVan Jacobson TCP/IP Header Compression.[5] 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 theTransmission Control Protocol (TCP) header from twentybytes to seven bytes. CSLIP has no effect onUser Datagram Protocol (UDP) datagrams.
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 popular4.2BSD 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".[6]
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.[7]
Linux formerly used the same code base for SLIP andKISS (TNC). The split occurred before the start of kernel git history (Linux-2.6.12-rc2, 2005).[8] The SLIP driver offers a special "6-bit" escaped mode to accommodate modems incapable of handling non-ASCII characters.[9] The Linux slattach command (written independently) also has the ability to auto-detect CSLIP support.[10]
slattach(8) – Linux Programmer'sManual – Administration and Privileged Commands from Manned.org "Other possible values are slip (normal SLIP), adaptive (adaptive CSLIP/SLIP)...