IM send file almost never works if both parties are behind firewalls. Skype works, but it tends to be very slow since it almost always ends up relaying (sometim

IM send file almost never works if both parties are behind firewalls. Skype works, but it tends to be very slow since it almost always ends up relaying (sometimes several times) to get around the NAT problem. This uses some sophisticated NAT traversal magic. It usually is as fast as a direct TCP connection or nearly so.