[Serusers] NAT traversal solution by reinvite?

Richard Z rzheng at gmail.com
Wed Aug 31 23:00:16 CEST 2005

I think that the natted rtp port might be changed after the reinvite because 
the traffic is not going to the original ip:port of the rtpproxy. So it is 
probably not going to work.

On 8/31/05, Federico Giannici <giannici at neomedia.it> wrote:
> I'd like to ask to somebody with more knowledge of me if a possible
> solution to NAT traversal is really feasible.
> For various reasons, we DON'T want to use an RTP proxy.
> We'd like to avoid the use of STUN because: 1) creates hairpin problems;
> 2) many UAC have a bad STUN client code implementation; 3) it requires
> additional configuration by the final user.
> It seems to me that with the nathelper's message rewriting functions it
> is possible to solve every problem for the SIP protocol.
> Moreover, as we have the REAL IP of the UA (in the original SIP
> messages) we could also avoid haipin problems: it is sufficient to use
> the original IP/Port of the two UAs if both have the same natted IP (ie
> they are behind the same NAT). This doesn't work when the UAs are behind
> multiple NATs, but this is a relatively uncommon case.
> So, the unresolved problem is with the RTP data, because we don't know
> what will be the NATted port so we cannot correctly mangle the SDP data
> in the INVITE message.
> Am I correct up to this point?
> Now, I'm asking myself if it is feasible to use a "MINI RTP Proxy" that
> receives the initial INVITEs, discovering the NATted RTP ports, and then
> IMMEDIATLY RE-INVITE the two UAs to connect directly each other. So only
> the first RTP packet is actually proxed, all subsequent traffic will be
> directly between the two UAs.
> I think that something similar is done by Asterisk.
> Is this feasible?
> If it is, then we could have a good solution to NAT Traversal:
> 1) No Hairpin problems (for one NAT cases)
> 2) No problems of the normal RTP proxy (waste of bandwidth, longer
> delays, bad scalability).
> 3) Will work with all type of NATs except for symmetric ones (the same
> that work with STUN).
> 4) Simpler UAC configuraton: only username, password and sip server.
> Thanks.
> --
> ___________________________________________________
> __
> |- giannici at neomedia.it
> |ederico Giannici http://www.neomedia.it
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