Subject: [Sip] STUN state machine question

From: "Frank W. Miller" <[email-masked]>
Date: Sun, 21 Aug 2005 14:41:17 -0400

Greetings,

I've been working with STUN a bit lately and have run into an
interesting situation.  This may have been discussed elsewhere and I
just haven't seen it.  If so, I apologize and would appreciate a pointer
to that discussion.

In the state diagram, Figure 2 of RFC 3489, there is a path to repeat
Test I.  That is, if the first execution of Test I yields a different IP
address than the current host interface IP address and no response is
received from the STUN server for Test II, the client repeats Test I
sending the Binding Request to the IP address specified in the
CHANGED-ADDRESS attribute from the first execution of Test I.

My question is, what should the client assume if no response is received
for the repeated execution of Test I?

It seems to me that in theory, this situation should never occur.  The
reason is because the first execution of Test I returned a response and
the second execution is doing the same thing, just with a different IP
address and port number.  However, I have encountered situations in
practice where this does occur.

My guess would be that you can certainly assume a NAT and use the
MAPPED-ADDRESS that you get from the response to the first Test I.
Would that be a good assumption?  Also, what type of NAT would I then
call that?

Thoughts?
FM

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