Re: network programming: how does s.accept() work?
- From: Thomas Bellman <bellman@xxxxxxxxxxxxxx>
- Date: Mon, 25 Feb 2008 17:56:25 +0000 (UTC)
7stud <bbxx789_05ss@xxxxxxxxx> wrote:
The question I'm really trying to answer is: if a client connects to a
host at a specific port, but the server changes the port when it
creates a new socket with accept(), how does data sent by the client
arrive at the correct port? Won't the client be sending data to the
original port e.g. port 5052 in the client code above?
The answer is that the server *doesn't* change its port. As you
could see in the output of your server, the socket that accept()
returned also had local port 5052. Each *client* will however
get a unique local port at *its* end.
A TCP connection is identified by a four-tuple:
( localaddr, localport, remoteaddr, remoteport )
Note that what is local and what is remote is relative to which
process you are looking from. If the four-tuple for a specific
TCP connection is ( 127.0.0.1, 5052, 127.0.0.1, 50816 ) in your
server, it will be ( 127.0.0.1, 50816, 127.0.0.1, 5052 ) in the
client for the very same TCP connection.
Since your client hasn't bound its socket to a specific port, the
kernel will chose a local port for you when you do a connect().
The chosen port will be more or less random, but it will make
sure that the four-tuple identifying the TCP connection will be
unique.
--
Thomas Bellman, Lysator Computer Club, Linköping University, Sweden
"There are many causes worth dying for, but ! bellman @ lysator.liu.se
none worth killing for." -- Gandhi ! Make Love -- Nicht Wahr!
.
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