chat program behind a router

[Dar Scott]

On Sunday, April 7, 2002, at 12:21 AM, Mark Talluto wrote:

> By TCP, do you mean:
>
> accept connections on port 80 with message "connectiong"
> vs
> accept datagram connections on port 80 with message "connecting"

The first uses TCP.  The second uses UDP.

UDP would make a great chat method on a local network.  It can 
broadcast, too.  It can be done simply.  I think mc UDP will have 
trouble with a firewall, though.  You might be able to send packets 
from behind the firewall to outside the firewall, but will have 
trouble receiving them behind the firewall.

TCP creates a virtual wire between a port on one computer and a 
port on another computer.  A port is simply a 16-bit number.  Bytes 
can be sent down the "wire" both directions.  The connection is not 
symmetrical.  The connection is initiated by one end, popularly 
called the client.  The other end accepts the connection and is 
often called the server.  In the sockets model of TCP, the server 
can have lots of "wires" connected to the same port.  They are 
differentiated by the port and computer at the other end.  Other 
than that, ports are not shared.

One computer (client) uses "open" to initiate the connection.  The 
other (server) uses "accept" to accept one or, perhaps, more 
connections.  You provide the connection string with "open" and you 
are provided one with the message from "accept".

You are most likely to succeed with the client using "open" behind 
the firewall and the other computer (outside the firewall) being 
the server using "accept".

A typical low-security firewall will allow you to make a TCP 
connection from behind the firewall to outside the firewall, but 
not the other way.

A typical firewall will also use NAT (network address 
translation).  The most common NAT method (and a firewall might use 
several) makes all connections look--from the outside--as though 
they are from the firewall itself.  This means that the IP address 
of the client as seen from the server might not be that of the 
actual computer making the link.

Though using port 80 can solve some problems, it can also create 
problems.  Try a high port number, first.

In your initial tests, use the same computer for both ends.

Depending on your situation, you might want to take the person 
responsible for the firewall settings to lunch and explain what you 
are doing.

Dar Scott