Re: Proposal: UP&P

On Sat, Mar 10, 2007 at 01:37:01AM +0000, Michael Rogers wrote:
> Matthew Toseland wrote:
> > That's another interesting point: port rewriting. Right now we don't
> > take any steps to identify our external port number, apart from using it
> > as well as our real port number if we hear it from successful connectees.
> 
> STUNT uses the following trick, based on the fact that most NATs 
> increase the external port number by 0, 1 or 2 for successive mappings 
> with the same internal address and port number:
> 
> * Contact first STUNT server, learn external port p1
> * Contact second STUNT server, learn external port p2
> * Predicted port for next connection = p2 + (p2 - p1)
> * Peers exchange their predicted ports out of band, then try to connect

For a symmetric NAT? Nice.
> 
> This might also work with STUN - I'm not sure whether UDP mappings are 
> allocated in the same way but I don't see why they wouldn't be. However, 
> the predicted port doesn't stay fresh for very long unless p1 = p2 (full 
> cone), so it might not be very useful unless the peers have a 
> low-latency channel to exchange predicted ports (eg a mutual friend's node).

p1 = p2 on anything except symmetric iirc.
> 
> Cheers,
> Michael

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