so Dan, don't forget, that is one of the subjects we are waiting for
good explanations from someone that can well explain (and some of us -
I think - even already paid for it?) ;-)
Till
Am 16.03.2005 um 12:13 schrieb Alex Tweedly:
Dan Shafer wrote:
Alex.....
I got it. IOW, opening a socket doesn't work unilaterally. The
"server" has to have a listener on that port first. Right?
Right. When you open a TCP socket (i.e. open socket to "host:port"),
a TCP connection is formed. A packet is sent from your machine's TCP
stack to his, and he sends a reply (assuming some application has done
"accept connection" on the correct port). If there is no-one listening
on that port, the remote machine will either ignore the incoming
packet, or send back a "reject" packet.
When (if) your host receives a positive acknowledgment, then the
connection is successful (from your point of view); your TCP stack
then sends him back a third packet - and only when he receives it does
he consider the connection complete. (He can't consider it complete
until then, because there could be a problem getting his packets to
you - the only way he knows that has succeeded is when you reply).
You may hear this referred to as "TCP's three-way handshake".
Note that when you open a UDP socket (i.e. open datagram socket to
"host:port"), things are completely different. UDP (datagram) is
"connectionless", so the open socket does not cause any network
activity, and will generally succeed provided the "host:port" have
valid values.
I'm going to figure out this server stuff one of these days.
And once you do, you'll realize how simple it all was to begin with,
and wonder why no-one ever wrote a decent explanation of it in the
first place. It's because most of us can't write decent explanations
of anything :-)
--
Alex Tweedly http://www.tweedly.net
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