2.4 SLIP: Serial Line IP

SLIP stands for Serial Line IP. It is a simple form of encapsulation for IP datagrams on serial lines, and is specified in RFC 1055 [Romkey 1988]. SLIP has become popular for connecting home systems to the Internet, through the ubiquitous RS-232 serial port found on almost every computer and high-speed modems.

The following rules specify the framing used by SLIP.

The IP datagram is terminated by the special character called END (0xc0). Also, to prevent any line noise before this datagram from being interpreted as part of this datagram, most implementations transmit an END character at the beginning of the datagram too. (If there was some line noise, this END terminates that erroneous datagram, allowing the current datagram to be transmitted. That erroneous datagram will be thrown away by a higher layer when its contents are detected to be garbage.)
If a byte of the IP datagram equals the END character, the 2-byte sequence 0xdb, 0xdc is transmitted instead. This special character, 0xdb, is called the SLIP ESC character, but its value is different from the ASCII ESC character (0x1b).
If a byte of the IP datagram equals the SLIP ESC character, the 2-byte sequence 0xdb, 0xdd is transmitted instead.

Figure 2.2 shows an example of this framing, assuming that one END character and one ESC character appear in the original IP datagram. In this example the number of bytes transmitted across the serial line is the length of the IP datagram plus 4.

Figure 2.2. SLIP encapsulation.

SLIP is a simple framing method. It has some deficiencies that are worth noting.

Each end must know the other's IP address. There is no method for one end to inform the other of its IP address.
There is no type field (similar to the frame type field in Ethernet frames ). If a serial line is used for SLIP, it can't be used for some other protocol at the same time.
There is no checksum added by SLIP (similar to the CRC field in Ethernet frames). If a noisy phone line corrupts a datagram being transferred by SLIP, it's up to the higher layers to detect this. (Alternately, newer modems can detect and correct corrupted frames.) This makes it essential that the upper layers provide some form of CRC. In Chapters 3 and 17 we'll see that there is always a checksum for the IP header, and for the TCP header and the TCP data. But in Chapter 11 we'll see that the checksum that covers the UDP header and UDP data is optional.

Despite these shortcomings, SLIP is a popular protocol that is widely used.

The history of SLIP dates back to 1984 when Rick Adams implemented it in 4.2BSD. Despite its self-description as a nonstandard, it is becoming more popular as the speed and reliability of modems increase. Publicly available implementations abound, and many vendors support it today.