TCP IP Illustrated

We look at how typical implementations measure the

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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 congestion avoidance. 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 Trying 140.252.13.34... Connected to svr4. Escape character is '^]'. Hello, world and hi 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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