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Unformatted text preview: ve probes are sent, 75 seconds apart, before the
sending host gives up. The error returned to the application generates a different message
this time: "No route to host." We saw in Figure 6.12 that this corresponds to the ICMP
network unreachable error. 23.4 Summary
As we said earlier, the keepalive feature is controversial. Protocol experts continue to
debate whether it belongs in the transport layer, or should be handled entirely by the
It operates by sending a probe packet across a connection after the connection has been
idle for 2 hours. Four different scenarios can occur: the other end is still there, the other
end has crashed, the other end has crashed and rebooted, or the other end is currently
unreachable. We saw each of these scenarios with an example, and saw different errors
returned for the last three conditions.
In the first two examples that we looked at, had this feature not been provided, and
without any application-level timer, our client would never have known that the other end
had crashed, or crashed and rebooted. In the final example, however, nothing was wrong
with the other end, the connection between them was temporarily down. We must be
aware of this limitation when using keepalives.
Exercises file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/tcp_keep.htm (8 of 9) [12/09/2001 14.47.30] Chapter 23. TCP Keepalive Timer 23.1 List some advantages of the keepalive feature.
23.2 List some disadvantages of the keepalive feature. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/tcp_keep.htm (9 of 9) [12/09/2001 14.47.30] Chapter 24. TCP Futures and Performance TCP Futures and Performance
TCP has operated for many years over data links ranging from 1200 bits/sec dialup SLIP
links to Ethernets. Ethernets were the predominant form of data link for TCP/IP in the
1980s and early 1990s. Although TCP operates correctly at speeds higher than an
Ethernet (T3 phone lines, FDDI, and gigabit networks, for example), certain TCP limits
start to be encountered at these higher speeds...
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- Spring '12