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Unformatted text preview: smission timer is used when expecting an acknowledgment from the other
end. This chapter looks at this timer in detail, along with related issues such as
2. A persist timer keeps window size information flowing even if the other end closes its
receive window. Chapter 22 describes this timer.
3. A keepalive timer detects when the other end on an otherwise idle connection crashes
or reboots. Chapter 23 describes this timer.
4. A 2MSL timer measures the time a connection has been in the TIME_WAIT state. We
described this state in Section 18.6.
In this chapter we start with a simple example of TCP's timeout and retransmission and then
move to a larger example that lets us look at all the details involved in TCP's timer
management. We look at how typical implementations measure the round-trip time of TCP
segments and how TCP uses these measurements to estimate the retransmission timeout of
the next segment it transmits. We then look at TCP's congestion avoidance-what TCP does
when packets are lost-and follow through an actual example where packets are lost. We also
look at the newer fast retransmit and fast recovery algorithms, and see how they let TCP
detect lost packets faster than waiting for a timer to expire. 21.2 Simple Timeout and Retransmission Example
Let's first look at the retransmission strategy used by TCP. We'll establish a connection, send
some data to verify that everything is OK, disconnect the cable, send some more data, and
watch what TCP does: file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (1 of 29) [12/09/2001 14.47.26] Chapter 21. TCP Timeout and Retransmission bsdi % telnet svr4 discard
Connected to svr4.
Escape character is '^]'.
Connection closed by foreign
host. send this line normally
disconnect cable before sending this line
output whenTCP gives up after 9 minutes Figure 21.1 shows the tcpdump output. (We have removed all th...
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- Spring '12