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Unformatted text preview: ) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm 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 [Rornkey 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.
1. 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.)
2. 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 (0xib).
3. 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.
1. Each end must know the other's IP address. There is no method for one end to inform the
other of its IP address.
2. 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.
3. There is no checksum added by SLIP (similar to the CRC field...
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