Under the Hood - TCP/IP Sockets in Java Practical Guide for Programmers - page 105

Call close()/shutdownOutput()

Returns immediately

Established Closing Half closed Time-Wait

Start
close
handshake

Close
handshake
complete s

Close
handshake
initiated by
remote
completes

Local port P

A.B.C.D

Local port P

A.B.C.D

Local port P

A.B.C.D

Local port P

A.B.C.D Local IP Local IP Local IP Local IP

Remote port
Remote IP

Q
W.X.Y.Z

Remote port
Remote IP

Q
W.X.Y.Z

Remote port
Remote IP

Q
W.X.Y.Z

Remote port
Remote IP

Q
W.X.Y.Z

Figure 5.10: Closing a TCP connection first.

compression example of Section 4.5. It works like this: the application indicates that it is
finished sending data on a connected socket by calling close() or by calling shutdownOutput() .
At that point, the underlying TCP implementation first transmits any data remaining in SendQ
(subject to available space in RecvQ at the other end), and then sends a closing TCP handshake
message to the other end. This closing handshake message can be thought of as an end-of-
transmission marker: it tells the receiving TCP that no more bytes will be placed in RecvQ .
(Note that the closing handshake message itself is not passed to the receiving application,
but that its position in the byte stream is indicated by read() returning 1.) The closing TCP

waits for an acknowledgment of its closing handshake message, which indicates that all data
sent on the connection made it safely to RecvQ . Once that acknowledgment is received, the
connection is “Half closed.” It is not completely closed until a symmetric handshake happens
in the other direction—that is, until both ends have indicated that they have no more data to
send.

−

The closing event sequence in TCP can happen in two ways: either one application calls
close() (or shutdownOutput() ) and completes its closing handshake before the other calls
close() , or both call close() simultaneously, so that their closing handshake messages cross
in the network. Figure 5.10 shows the sequence of events in the implementation when the
application invokes close() before the other end closes. The closing handshake message is
sent, the state of the socket structure is set to “Closing”, and the call returns. After this
point, further reads and writes on the Socket are disallowed (they throw an exception). When
the acknowledgment for the close handshake is received, the state changes to “Half closed”,
where it remains until the other end's close handshake message is received. Note that if the
remote endpoint goes away while the connection is in this state, the local underlying structure
will stay around indefinitely. When the other end's close handshake message arrives, an
acknowledgment is sent and the state is changed to “Time-Wait”. Although the corresponding
Socket instance in the application program may have long since vanished, the associated
underlying structure continues to exist in the implementation for a minute or more; the reasons
for this are discussed on page 107.