[standards-jig] Re: VoIP, JEP-0065 and UDP?

Jesper Krogh jesper at krogh.cc
Sun Jan 25 17:07:30 UTC 2004

I gmane.network.jabber.standards-jig, skrev Matthias Wimmer:
>  Jesper Krogh schrieb am 2004-01-25 16:17:15:
> > I haven't seen any describtions on how that should be possible.. could
> > you describe it?=20
>  Just by sending each other a UDP packet on the right ports, the
>  association gets stored by the NAT router. The first packet won't get
>  delivered, as the destination NAT does not yet know where to deliver it,
>  but as soon as the destination host has sent the first UDP packet as
>  well, both NAT routers know where to deliver incoming UDP packets.

I don't think so.. lets play the scenario. 

We assume no default ports, if there were that would kill multiple
setups behind same NAT. 

A sends a UDP datagram to B

a:1234 -> b
(A is sending from port 1234 to B at port 1234 ). 
This package gets killed in B's nat unit but sets up a record in its
own nat to forward packages to the client coming on port 1234 to the
B:2345 -> a 
(B is sending from port 2345 to A at port 2345). 
This too gets killed by A's nat unit but sets up a record in it own nat
to forward packages to the client coming on port 2345 to the client. 

Well, the first package should be sent successfully now. 

A can send to B's port 2345 and
B can send to A's port 1234

But if you dont have a proxy, how should the ever know that it is 1234
and 2345 that's been allocated?


./Jesper Krogh, jesper at krogh.cc
Jabber ID: jesper at jabbernet.dk

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