What is SLIP in the Computer Network?

SLIP stands for Serial Line Internet Protocol. SLIP is an Internet protocol that allows users to gain Internet access using a computer modem. Today, SLIP is not used as frequently as its successor, PPP (Point-to-Point Protocol), which provides enhanced error detection and automatic configuration.

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).

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.

Problems of SLIP

The problems associated with SLIP are explained below −

Standardized Datagram Size Specification

SLIP’s maximum datagram size supported is not standardized and depends on the implementation. The usual default is 1006 bytes, which becomes the maximum transmission unit (MTU) for the link. If a different size is used, this must be programmed into the IP layer.

Error Detection/Correction Mechanism

SLIP doesn't provide any way of detecting or correcting errors in transmissions. While such protection is provided at higher layers through IP header checksums and other mechanisms, it is a job “traditionally” also done at layer two.

The reason is that relying on those higher layers means that errors are only detected after an entire datagram has been sent and passed back up the stack at the recipient. Error correction can only come in the form of re-sending any datagrams that were corrupted.

This is inefficient, especially considering that serial links are generally much slower than normal LAN links.

Control Messaging

SLIP provides no way for the two devices to communicate control information between them to manage the link.

Type Identification

Since SLIP includes no headers of its own, it is not possible to identify the protocol it is sending. While developed for IP, you can see that there is no reason another layer three protocols could not be sent using SLIP. However, without type identification, there is no way to mix datagrams from two or more layer three protocols on the same link.

Address Discovery Method

Addressing isn't needed at layer two due to the point-to-point nature of the connection. There are only two devices, so the intended recipient of each message is obvious. However, devices do need some way of learning each other's IP addresses for routing at layer three. SLIP provides no method for this.

Support for Compression

Compression would improve performance over serial lines that are, again, slow compared to other technologies. SLIP provides no compression features. Note that the modems usually do support the compression at layer one for serial connections that use them.

There was also a variant on SLIP called Compressed SLIP or CSLIP that was created in the late 1980s, but it was not as widely deployed as regular SLIP.

Security Features

SLIP provides no methods for authentication of connections or encryption of data, which means even the basics of security are not provided.

Updated on: 04-May-2021

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