Serial Line Internet Protocol

Serial Line Internet Protocol ( SLIP ) denotes a network protocol of the data link layer.

It is used to produce an IP network connection between two computers which are connected via a serial interface. Meanwhile, the Point-to -Point Protocol (PPP ) is used mostly.

SLIP is the simplest possible way of such a connection, simply by the octets of an IP packet is sent over the serial cable. The end of the packet is marked with the special END character # 192 ( octal code 300). If this character occurs within the packet to be transmitted, the sequence 219/220 ( octal 333 334 ) and for the character sequence # 219 even the 219/221 ( octal 333 335 ) is transmitted instead. The character # 219 is called the escape character.

A modification of the protocol sends the END character at the beginning of each packet. This makes the transmission more robust against low line noise between packets.

SLIP has no address and no error detection. This must be provided by the higher level protocols are available:

  • SLIP: no error detection and addressing
  • IP: checksum for the IP header data, 32 -bit addresses
  • UDP checksum of the UDP header and data, 16 - bit ports
  • TCP: checksum of the TCP header and data, 16 -bit ports

Differences with PPP

The PPP protocol is much more complex than SLIP. First, SLIP requires that all octets can be passed unaltered through the serial cable. This is often not possible because some terminal drivers bytes specifically interpret, for example, CTRL -Q and CTRL-S as the start / stop characters interpret or before a carriage return or insert a line feed. About PPP certain characters can be banned. These are then replaced by sequences of characters.

PPP also allows a transfer of the network parameters ( IP address, gateway address) and authentication, as it is now required by many Internet Service Providers.

Other parameters such as compression can be negotiated between the parties when establishing a connection over PPP, which is not possible with SLIP.

SLIP can not and is therefore used only in very simple cases; In the past, often in callback modems purpose external Einwählens on workstations in a secure Intranet ( → Extranet).

Van Jacobson compression and CSLIP

Since serial links are often quite slow, the transfer of the IP header with each packet resulting in reduced throughput. Each TCP / IP header has at least 40 characters. On a 19200 - baud line of each character requires at least about 1/2 ms, the TCP / IP headers so only about 20 ms. This limits the transmission rate to less than 50 packets / second, which is often not even enough for Quick Tipper when each keystroke a packet is transmitted.

Van Jacobson has therefore proposed a compression, which transfers only the differences of TCP header between two packets. Thereby, the throughput is substantially increased, as a rule, only 7 instead of 40 characters is required. The Van Jacobson compression is often used in SLIP connections. The protocol with active Van Jacobson compression is also called CSLIP protocol ( ' compressed SLIP = compressed SLIP).

History

The draft (RFC ) to SLIP dates from 1988, the to CSLIP of 1990. Nowadays SLIP is replaced in practice largely or entirely of PPP.

Point-to-Point Protocol Callback (telecommunications) Integrated Authority File
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