[Cryptography] Firewall penetration

[James A. Donald]

Suppose a server talks to two clients, which are connected to the 
internet by ordinary consumer type connections.  Is their any reliable, 
practical, generally useful way whereby it can arrange for the two 
clients to talk directly to each other, or is it more practical for all 
data to be stored on the server by one client, and then collected by the 
other client?

When last I looked at this issue, direct communication was getting 
harder, and workarounds were like bugs that were likely to be fixed.