Serial Line Internet Protocol (SLIP)

Serial Line Internet Protocol (SLIP) is a simple protocol that works with TCP/IP for communication over serial ports and routers. They provide communications between machines that were previously configured for direct communication with each other.

For example, a client may be connected to the Internet service provider (ISP) with a slower SLIP line. When a service is required, the client places a request to the ISP. The ISP responds to the request and passes it over to the Internet via high speed multiplexed lines. The ISP then sends the results back to the client via the SLIP lines.

SLIP was developed by Rick Adams in 1984. It is documented in RFC 1055.

How SLIP Works

SLIP operates by encapsulating IP packets within a simple frame structure for transmission over serial links. It provides a method to send IP datagrams over serial connections such as telephone lines, which were commonly used for dial-up internet access.

SLIP Frame Structure

SLIP frame has a very simple format, comprising of payload and a flag that acts as an end delimiter. The flag is generally a special character equivalent to decimal 192 (0xC0). If this flag is present in the data, then an escape sequence precedes it, so that the receiver does not consider it as the end of the frame.

Escape Sequences

SLIP uses two special characters for framing and escaping:

• END (0xC0) − Marks the end of a frame

• ESC (0xDB) − Used to escape special characters within data

Advantages of SLIP

• Minimal overhead − Very small frame overhead makes it suitable for usage in microcontrollers and low-bandwidth connections.

• Reuses existing infrastructure − Can utilize existing dial-up connections and telephone lines.

• IP support − Supports the widely used Internet Protocol (IP), ensuring ease of deployment.

• Simplicity − Easy to implement due to its straightforward framing mechanism.

Disadvantages of SLIP

• No error detection − SLIP does not provide any error detection or correction mechanisms.

• No addressing − Cannot handle multiple protocols or provide addressing information.

• No compression − Does not support data compression, limiting efficiency over slow links.

• Limited functionality − Largely superseded by PPP (Point-to-Point Protocol) which addresses these limitations.

Conclusion

SLIP provided a simple method for transmitting IP packets over serial connections, particularly useful in early dial-up internet access. While it has been largely replaced by more advanced protocols like PPP, its simplicity made it valuable for basic serial communication applications.