Shareaza

I think this would be a good feature to put in. A lot of people are firewalled.

This would require a intermediate server. Using UPnP is enough, and Shareaza already supports it.

"intermediate server" is maybe overstating it. Firewall-to-firewall does require a third party but this not more demanding than the push forwarding that hubs already do. Basically client A would send a SYN packet from its listen port to the listen port of client B and send a F2F request thru the push route, and client B would then open a connection from its own listen port to the listen port of the client A which should now be open in the firewall. (In practice it is of course more complicated since the TCP connection Shareaza currently uses probably does not support doing this.)

I agree UPnP is a better solution, though.

I think it could be implemented. The extra amount of traffic produced wouldn't be too much as once a connection is established, no further intervention/help from outside is required.

However if this feature were to be implemented, there should be red warning signs all over Shareaza telling the user about him being limited by his current situation.

Incidentally, if somebody wants to experiment with this, it would probably be easier to use UDP connection. IIRC uTP is open source and it would be simpler to add F2F (if it doesn't already have it) to that than to a TCP based protocol. And backwards compatibility is not really possible here anyway...

Well, this would be a problem of course - however for file transfers, it were preferable IMHO to use TCP even if it is more difficult to establish such a connection as TCP has much less overhead if you have to transfer a continuous stream of data - which an upload clearly is, don't you think?

The overhead should be pretty much the same, possibly slightly lower as most UDP based transfer protocols do slightly less synchronization than TCP does by default. In any case there is no magic that lets you do the same work with less overhead by switching whether the packets are TCP or UDP or anything else. Only real functional difference between TCP and UDP packets is that routers understand TCP packets for handshakes and closing connections whereas UDP should be just passed on with a redirect established for an unknown number of response packets in near future.

Of course it is much easier to do uploads with TCP, unless you can use a ready code library to handle all the dirty work TCP usually does for you.