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<TITLE>ACE Tutorial 008</TITLE>
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<CENTER><B><FONT SIZE=+2>ACE Tutorial 008</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Sending and receiving datagrams</FONT></B></CENTER>
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<P>In a lot of IPC programming, the clients know where the servers
are. A mail client, for instance, has a configuration file that says
where the mail host is. Your web browser has a "location" field that
you type into to give it a destination.
<P>What if you have written a server application and then you execute it
on several systems in your network? All of the instances are probably
more or less equal to the client's point of view, so you don't want to
"configure" the clients to a single server each. Likewise, you
want the ability to add and remove servers at any time so you can't just
give the clients a list to choose from.
<P>So... how do the clients know where the servers are?
<P>Let 'em ask!
<P>Datagrams are great for this. You can toss a datagram out onto
the network and any servers listening at the correct port will* hear it.
Like ACE_SOCK_Stream that we've seen before, you can get the peer address
from a datagram. With that, the server can send a response
back to the client. The client, in turn, can pull the peer address
out and know exactly where the server lives.
<P>In this tutorial we'll develop three applications: a server listening
for datagrams, a client that can send to a known host and a client that
can send to the entire (sub)network. In the next tutorial, we'll
expand on this to make the server a bit more prudish.
<BR>
<P><FONT SIZE=-1>* Actually, the servers <I>might</I> hear the datagram.
Datagrams are rather unreliable. (Sort of like some operating systems
I know.) Still, if the network traffic isn't too bad, they generally
get through. Your clients can always send out more queries if there
aren't any responses in a timely fashion.</FONT>
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