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Unformatted text preview: be sent). In either case the client thought everything was OK when its active
open completed, and may have already sent a request to the server. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (34 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination 18.12 Summary
Before two processes can exchange data using TCP, they must establish a connection between
themselves. When they're done they terminate the connection. This chapter has provided a
detailed look at how connections are established using a three-way handshake, and terminated
using four segments.
We used tcpdump to show all the fields in the TCP header. We've also seen how a connection
establishment can time out, how resets are sent, what happens with a half-open connection, and
how TCP provides a half-close, simultaneous opens, and simultaneous closes.
Fundamental to understanding the operation of TCP is its state transition diagram. We've
followed through the steps involved in connection establishment and termination, and the state
transitions that take place. We also looked at the implications of TCP's connection
establishment on the design of concurrent TCP servers.
A TCP connection is uniquely defined by a 4-tuple: the local IP address, local port number,
foreign IP address, and foreign port number. Whenever a connection is terminated, one end
must maintain knowledge of the connection, and we saw that the TIME_WAIT state handles
this. The rule is that the end that does the active open enters this state for twice the
18.1 In Section 18.2 we said that the initial sequence number (ISN) normally starts at 1 and is
incremented by 64,000 every half-second and every time an active open is performed. This
would imply that the low-order three digits of the ISN would always be 001. But in Figure 18.3
these low-order three digits are 521 in each direction. What's going on?
18.2 In Figure 18.15 we typed 12 characters and saw 13 bytes s...
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