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seconds, but not exact, while the second period is almost exactly 24 seconds. Ten more of these
tests were run and the first timeout period took on various values between 5.59 seconds and
5.93 seconds. The second timeout period, however, was always 24.00 (to two decimal places).
What's happening here is that BSD implementations of TCP run a timer that goes off every 500 file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (7 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination ms. This 500-ms timer is used for various TCP timeouts, all of which we cover in later
chapters. When we type in the telnet command, an initial 6-second timer is established (12
clock ticks), but it may expire anywhere between 5.5 and 6 seconds in the future. Figure 18.7
shows what's happening. Figure 18.7 TCP 500-ms timer.
Although the timer is initialized to 12 ticks, the first decrement of the timer can occur between
0 and 500 ms after it is set. From that point on the timer is decremented about every 500 ms,
but the first period can be variable. (We use the qualifier "about" because the time when TCP
gets control every 500 ms can be preempted by other interrupts being handled by the kernel.)
When that 6-second timer expires at the tick labeled 0 in Figure 18.7, the timer is reset for 24
seconds (48 ticks) in the future. This next timer will be close to 24 seconds, since it was set at a
time when the TCP's 500-ms timer handler was called by the kernel.
In Figure 18.6, the notation [tos 0x10] appears. This is the type-of-service(TOS) field in
the IP datagram (Figure 3.2). The BSD/386 Telnet client sets the field for minimum delay. 18.4 Maximum Segment Size
The maximum segment size (MSS) is the largest "chunk" of data that TCP will send to the
other end. When a connection is established, each end can announce its MSS. The values we've
seen have all been 1024. The resulting IP datagram is normally 40 bytes larger: 20 b...
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