TCP IP Illustrated

TCP IP Illustrated

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Unformatted text preview: cable from svr4. Line 6 shows "and hi" being sent. Lines 7-18 are 12 retransmissions of that segment, and line 19 is when the sending TCP finally gives up and sends a reset. Examine the time difference between successive retransmissions: with rounding they occur 1, 3, 6, 12, 24, 48, and then 64 seconds apart. We'll see later in this chapter that the first timeout is actually set for 1.5 seconds after the first transmission. (The reason it occurs 1.0136 seconds after the first transmission, and not exactly 1.5 seconds, was explained in Figure 18.7.) After this the timeout value is doubled for each retransmission, with an upper limit of 64 seconds. This doubling is called an exponential backoff. Compare this to the TFTP example in Section 6.5, where every retransmission occurred 5 seconds after the previous. The time difference between the first transmission of the packet (line 6 at time 24.480) and the reset (line 19 at time 566.488) is about 9 minutes. Modem TCP's are persistent when trying to send data! On most implementations this total timeout value is not tunable. Solaris 2.2 allows the administrator to change this (the tcp_ip_abort_interval variable in Section E.4) and its default is only 2 minutes, not the more common 9 minutes. 21.3 Round-Trip Time Measurement Fundamental to TCP's timeout and retransmission is the measurement of the round-trip time file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (3 of 29) [12/09/2001 14.47.26] Chapter 21. TCP Timeout and Retransmission (RTT) experienced on a given connection. We expect this can change over time, as routes might change and as network traffic changes, and TCP should track these changes and modify its timeout accordingly. First TCP must measure the RTT between sending a byte with a particular sequence number and receiving an acknowledgment that covers that sequence number. Recall from the previous chapter that normally there is not a one-to-one correspondence between data segments and ACKs. In Figure 20.1 this means that one R...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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