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Unformatted text preview: 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. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (4 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm 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. 2.5 Compressed SLIP
Since SLIP lines are often slow (19200 bits/sec or below) and frequently used for interactive
traffic (such as Telnet and Rlogin, both of which use TCP), there tend to be many small TCP
packets exchanged across a SLIP line. To carry I byte of data requires a 20-byte IP header and a
20-byte TCP header, an overhead of 40 bytes. (Section 19.2 shows the flow of these small packets
when a simple command is typed during an Rlogin session.)
Recognizing this performance drawback, a newer version of SLIP, called CSLIP (for compressed
SLIP), is specified in RFC 1144 [Jacobson 1990a]. CSLIP normally reduces the 40-byte header to
3 or 5 bytes. It maintains the state of up to 16 TCP connections on each end of the CSLIP link and
knows that some of the fields in the two headers for a given connection normally don't change. Of
the fields that do change, most change by a small positive amoun...
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