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

Krzysztof Szyntar criss at poczta.neostrada.pl
Mon Jan 26 14:28:35 UTC 2004

>>> So if we take advantage of this behaviour, we can definitely construct
>>> a UDP proxy.
>> Why a proxy if it is possible with the same technique without it?

JH> Take a look at RFC 3489, STUN.  It describes this approach pretty 
JH> thoroughly.

OK, let me get back to the matter again. I think most NATs nowadays
work as symmetrical NATS, i.e. they map different destinations to
different source ports. Moreover, once they create the mapping,
they don't allow incoming traffic from a different address and port
than the destination.

In that case it is impossible to transfer datagrams without proxy.
Using proxy just to help both sides exchange their addresses doesn't
help either.

Let's consider an example:
- A sends a UDP to PROXY
- B sends a UDP to PROXY
- proxy replies, giving B the address and port of A
- proxy replies, giving A the address and port of B

In ideal world, A and B would now be able to exchange datagrams,
however, a typical NAT will check the source address of
incoming datagram against the mapping (it mapped the port to PROXY
address), and it just won't let it through.

So we *do* have to send datagrams through proxy.
The question remains - is there a need in the Jabber community
of decribing the negotiation with UDP proxy and putting it into a JEP?

Best regards,
 Krzysztof                            mailto:criss at poczta.neostrada.pl

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