Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 284

Connection Termination
A connection can terminate in two ways. The first is graceful termination , and the second is an
aborted connection .

For graceful connection termination, both sides must send a finish (FIN) signal and expect an
acknowledgment from the other side. It is similar to connection establishment, but in this case,
it is a connection termination. It takes four, not three, segments to terminate a connection.
Because the connection is full-duplex, each side must shut down independently. Either side can
send a FIN to terminate the connection. As shown in Figure 6-8, Host A sends the FIN flag,
which is acknowledged by Host B. Host B also sends a FIN flag in a separate segment, which
is acknowledged by Host A. At this point, the connection is closed.

Connection Termination Figure 6-8

Host A Host B

FIN(X)

ACK(X)

Connection

termination

FIN(Y)

ACK(Y)

Host A Host B

RST

Aborted
termination

For an aborted connection, a RST signal is sent instead of a FIN, as shown in Figure 6-8.
The receiving side of the RST aborts the connection, dumps any data, and does not send an
acknowledgment. The sending side of the RST does not have to wait for an ACK to end the
connection. An example of an aborted connection is when in a Telnet connection a CTRL-D
disconnects. An aborted connection is also known as a ha r d close .