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 close enters this state for twice the implementation's MSL.