16.3 Nonblocking connect
When a TCP socket is set to nonblocking and then
connect is called, connect returns immediately
with an error of EINPROGRESS but the TCP three-way
handshake continues. We then check for either a successful or
unsuccessful completion of the connection's establishment using
select. There are three uses for a nonblocking
connect:
-
We can overlap
other processing with the three-way handshake. A connect
takes one RTT to complete (Section 2.6) and
this can be anywhere from a few milliseconds on a LAN to hundreds
of milliseconds or a few seconds on a WAN. There might be other
processing we wish to perform during this time.
-
We can
establish multiple connections at the same time using this
technique. This has become popular with Web browsers, and we will
show an example of this in Section
16.5.
-
Since we wait
for the connection to be established using select, we can
specify a time limit for select, allowing us to shorten
the timeout for the connect. Many implementations have a
timeout for connect that is between 75 seconds and several
minutes. There are times when an application wants a shorter
timeout, and using a nonblocking connect is one way to
accomplish this. Section 14.2 talks
about other ways to place timeouts on socket operations.
As simple as the nonblocking connect
sounds, there are other details we must handle:
-
Even though the socket is nonblocking, if the
server to which we are connecting is on the same host, the
connection is normally established immediately when we call
connect. We must handle this scenario.
-
Berkeley-derived implementations (and POSIX)
have the following two rules regarding select and
nonblocking connects:
-
When the
connection completes successfully, the descriptor becomes writable
(p. 531 of TCPv2).
-
When the
connection establishment encounters an error, the descriptor
becomes both readable and writable (p. 530 of TCPv2).
These two rules regarding select fall
out from our rules in Section 6.3 about
the conditions that make a descriptor ready. A TCP socket is
writable if there is available space in the send buffer (which will
always be the case for a connecting socket since we have not yet
written anything to the socket) and the socket is connected (which occurs only
when the three-way handshake completes). A pending error causes a
socket to be both readable and writable.
There are many portability problems with
nonblocking connects that we mention in the examples that
follow.
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