[Twisted-Python] UDP File Copy

Antoine Pitrou solipsis at pitrou.net
Wed Apr 6 05:18:53 EDT 2005

Le mardi 05 avril 2005 à 15:31 -0400, Tom Carmichael a écrit :
> UDP has it's place, but typically vanilla file transfer isn't it. 
> UDP traffic typically is harder to NAT rather than easier.  Since it
> is a stateless protocol, what is meant by a 'reply'?  How do you keep
> a forward and reverse translation table for the communications?

It is only harder for people who design NAT devices. ;)

But the real difference here between UDP and TCP, is that when you open
an outbound TCP connection, incoming TCP traffic will only be allowed
*for that connection*. Whereas if you send an outbound UDP message, *all
incoming UDP traffic* towards the originating address/port will be
enabled (at least on some NAT boxes).

This makes it easy for P2P systems to exchange messages between two
NATted clients without any proxying/tunnelling mechanism: you just have
to "punch a hole" by sending sporadic outgoing UDP traffic from your UDP
socket and then you can receive all UDP traffic coming to that socket.
You can't do that with TCP.



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