Testing NAT types (was: Re: [Serusers] Re: RESOLVED - nathelper / calling UAs behind symmetric NATs)

[salmon at netzquadrat.de]

Quoting Jan Janak <jan at iptel.org>:
> That's too complex and I couldn't see what is this good for. There is no
> way of notifying the registrar. Registrar can only receive REGISTER
> requests, process them and send a reply.
> 
> If I understand it correctly you try to mimic STUN with SIP, but that
> will result in unwanted overhead.

That is exactly what I am trying to do and here is why: In a network with only
STUN-enabled UACs I want to proxy rtp soley for clients behind symmetric NATs as
all other NATs can be traversed without external help. Trouble is, the UAS has
no way of knowing which type of NAT a STUN enabled UAC is located behind, unless
the UAC submits this information (e.g. Grandstream UAs append a 'warning' header
field). To the best of my knowledge there is no defined standard to transmit
this information and Grandstream is the only UA to carry it in a header field.

Hence, the idea of setting up a STUN like mechanism to test for the type of NAT
a UAC is behind and store it in the registrar's location database.

Thilo