peer to peer issues

Hi,

I am creating a peer to peer application that uses a central server to
setup initial connection. After that the communication is completely
peer to peer (i.e. the data is sent directly from one machine to
another without being routed by the central server). If I am correct
sort of like how file transfer works in Yahoo! messenger.

Some of the issues which I can currently identify are-
- How to deal with machines which are behind a firewall/proxy which are
not directly visible on the internet? A related example is the
inability of gnutella to work on my office machines which are behind a
firewall.

- I am not very experienced in this field so would like to know of any
software that works in a similar way and preferebly open source so that
its source can be examined.

Any help will be appreciated.:-)

Ramashish