[Serusers] B2BUA and NAT.

Amos Nungu iw03_amn at it.kth.se
Fri Apr 15 14:22:19 CEST 2005


Thanks Alex.

Just wondering...
I know that STUN won't solve cascading NATs (e.g. 2 nats in front of a UA), 
How about RTP Proxying (rtp/media proxy)? How?


----- Original Message ----- 
From: "Alex Mack" <amack at fhm.edu>
To: "Amos Nungu" <iw03_amn at it.kth.se>
Cc: <serusers at lists.iptel.org>
Sent: Friday, April 15, 2005 12:51 PM
Subject: Re: [Serusers] B2BUA and NAT.


> Hi Amos!
>
>> 1.Does it solve NAT because it stays in the middle and proxy everything 
>> (+ media) or
>> 2.can it be involved in the call only when NAT is detected like rtp proxy 
>> ?
>
> You can always put a B2BUA in the media path, not just for NAT traversal. 
> Other applications like billing or privacy service are also common. 
> Downsides are media quality, scalability and representing a single point 
> of failure.
>
>> 3. iF 2, THEN how does it work in corporation with SER or any SIP Server?
>
> B2BUA means a system standing in the signalling AND the media path.
> So a proxy in the normal SIP message path could "catch" the session, i.e. 
> terminating the incoming call ("picking up the receiver") and build up new 
> outgoing call to the target. Then it relays the voice from one call the 
> other. It can change codec and other information as well on the way.
>
> In this respect SER isn't a B2BUA and will never be.
> SER itself just can handle and mangle SIP messages very good but can have 
> only influence on the media *routing* with e.g. nathelper/rtpproxy or 
> mediaproxy. It doesn't terminate SIP transactions or media sessions.
> If you're looking for a real B2BUA you might want to trake a look at 
> Asterisk.
>
>> 4. Which situation will rtp proxy (rtpproxy/mediaproxy) fail to solve NAT 
>> issues
>
> RTP relay seems to be a cure-all regarding NAT problems - desipte the 
> disadvantages. The only clients not working with RTP relay where those who 
> didn't support symmetric SIP and symmetric RTP (i.e. sending and receiving 
> data on the same port).
> Another set of clients not working are more than two STUN clients behind 
> the same NAT and the NAT not supporting hairpin of media. In this 
> situation SER assumes both UAs to have public adresses (i.e. not being 
> behind a NAT) and to be able to communicate with each other, which isn't 
> the case with the lack of hairpin of media. Signalling would work but you 
> wonÄt hear voice.
>
> Alex Mack 




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