File/Data transfer between two arbitrary sources

I'm looking for a simple way to implement this scenario:

Say I have two machines I'd like to share data between. The location/addresses of these machines can change at any time. I'd like both machines to check in to a central server to announce their availability. One of the two systems wants to pull a file from the other. I know that I can have the sink system make a request to the server, who then requests the file from the source, pulls it, then feeds it to the requester. However, this seems inefficient from a bandwidth perspective. The file will be transfered twice. Is there a system in place where the source c开发者_C百科an broadcast it directly to the sink?

Without being able to guarantee things like port forwarding if a system is behind a firewall etc. I don't know of a way.

Thanks.

When machine A wants to send data to machine B, A sends a request to the central server C. C asks B for permission. If accepted, C gives B's IP and port to A. A attempts to connect to B directly. If unsuccessful (ie, if B is behind a router/firewall), then A notifies C of the failure. C then gives A's IP and port to B. B attempts to connect directly to A (which should be able to pass through B's firewall/router). If either connection is successful, then A has a direct connection to send data to B. If both connections are unsuccessful (ie, if A is also behind a firewall/router), then C has to act as a proxy for all transfers between A and B.