OK, well to rephrase the requirement:

I have a 20KB/min (!) connection and the OS is currently buffering 3-4 mins worth of data, so I need to find a way to crank down the amount of buffering between the two apps, ie all I can do is affect the advertised receive window of the server socket which the remove app connects to (over localhost)

Given a set of machines running XP and Vista can anyone think of a way to control advertised receive window?

Curiously under Vista it seems that setting SO_RCVBUF controls the advertised window nicely - however, it looks like something wierd like 40KB ish + SO+RCVBUF on XP... (Need to repeat that test to remind myself of the magic number, however, I think it *wasn't* 64KB which would have been the obvious answer...)

I can post some test code if it's useful?

20KB/min? hmmm... for very slow connections, sometimes UDP would be a better option.

Your point is fair, but in reality I can't see that it buys me much here. I'm proxying a TCP stream, so I need it reliable, so all I end up doing is re-implementing TCP on top of UDP... It would be possible to avoid the TCP backoff problems due to packet errors (not the same as congestion if you see the point), however, some simple benchmarks suggest that there isn't much to gain here...

TCCP looks interesting mind! However, I need this thing cross platform and stable...

Right now we use lots of compression and multiplexing of connection and even carefully limiting packet size to squeeze absolutely everything out of the link. My remaining issue is to control the OS buffering on the source side. Remember that I don't have control over the sender that I am buffering, so I need to indirectly affect their send speed - I really need to try and drop down the size of the OS buffers here - right now 64KB is a buffer of 3 mins or so!

Any suggestions please...?!!