how to transfer file between two computers behind different boardband router?

Hi,

I am new to internet programming. I would now like to create a
platform which allow two users behind different boardband router to
exchange files. There are only two users in the system and I have no
right to modify the boardband router. I have check up some p2p
applications like jxta and winmx. I have made some guess how the
system works:

Their is a relay server or a service outside the Internet. Computer A
will connect to this server and establish a connection. Then computer
B will connect to this server and get the routing information to A. So
computer B can send IP packet to computer A by preset a static route
or partly static route to establish a connection.

Am I right? In this case, I need to operate in the network layer. Is
this some kind of socket programming? Is there any reference site
which can clearly explain how's winmx work behind firewall?

Thanks,
Eric