[p2p-hackers] UDP file transfer link speed identification

[David Barrett]

How do you decide how fast to send UDP file-transfer packets?

Lots of people on this list make file sharing applications.  Many of 
those applications use UDP.  A big drawback of using UDP for file 
transfers is you can't rely upon the link-speed detection capabilities 
of TCP.  As a result, if you send UDP too slow, you're not maximizing 
your connection.  But if you send it too fast, you'll congest your 
upstream and/or the remote peer's downstream connection, causing packet 
loss at the very least, as well as possibly denial of service.

How have you resolved this problem?  I'm using an approach where the the 
client (receiving peer) provides continuous feedback to the server 
(sending peer) as to how fast it should send data.  Thus the client can 
control the rate at which it receives data from the server.

But the question is: how does the client know how fast to request data? 
  The overall approach I'm using is to request data at a certain rate, 
measure how fast it is actually received, calculate what new rate should 
be tried, repeat.

However, I'm finding this surprisingly tricky.  Either I ramp up too 
slow, or I ramp up and down erratically.  I've considered some FSM 
approaches, and some formulaic approaches, and I still haven't come up 
with something I like.  I've read the TCP RFC for guidance, but it 
leaves the specifics left as an exercise to the reader.  Other than 
that, I'm not sure where else to read for inspiration.

So I'm curious, how have you solved this problem?  What can you advise?

-david