[Serusers] Theory behind a media proxy and NATed UA

[Felipe Louback]

My Nated UA does work fine. I use X-lite. Now I am trying to
understand why it works. It has everything not to work.... I mean,
after the Nated client send an INVITE, the proxy replaces the unvalid
IP of the SDP field and puts its valid IP, so it can be in the middle
of the conversation. But through what I studied in SIP, after a client
send an INVITE, it expects audio to get audio(that is why it sends the
port and ip in the SDP body). Since it is expecting to get audio, how
does the media proxy talks to it, since there isn't any port opened in
the NAT device?

The problem with NAT is that the called part cannot open an audio
channel with the Nated callee because in the SDP body, there is an
invalid IP. In theory, it seems that a media proxy has the same
problem the called has to open an audio channel..... I just couldn't
figure out this...

Thanks,

Felipe


On 10/4/05, Greger V. Teigre <greger at teigre.com> wrote:
> Mediaproxy cannot force the nated ua to send rtp (unless it supports active
> media, which most don't, AFAIK).
> Some UAs (ex. Grandstream old firmware) has an issue where it takes a couple
> of seconds before you get sound.  However, not getting anything means that
> the UA does not have anything to send and that silence suppression is
> probably turned on. If you speak using the nated UA, does it start sending
> rtp?
> g-)
>
> ----- Original Message -----
> From: "Felipe Louback" <louback at gmail.com>
> To: "Greger V. Teigre" <greger at teigre.com>
> Cc: <serusers at lists.iptel.org>
> Sent: Tuesday, October 04, 2005 04:17 PM
> Subject: Re: [Serusers] Theory behind a media proxy and NATed UA
>
>
> > Andy told that after the first UDP packet is received on the
> > mediaproxy socket it knows
> > where to send the packets to. That is the problem.... after the
> > invite, the NAT-ed UA will wait for traffic, and not send traffic.
> > Greger, I read Section 3.5 of the document you said, but they dont
> > explain my doubt. There you can find how the register process is done
> > and how the NAT problem can be solved. In the document it is said the
> > proxy put his address in the SDP payload, but how does the media proxy
> > force the NATed UA to send a UDP packet? By default, the UA is waiting
> > for audio after it sends an INVITE....
> >
> > Felipe
> >
> >
> > On 10/4/05, Greger V. Teigre <greger at teigre.com> wrote:
> >> Section 3.5 of the ONsip.org Getting Started document provides an intro
> >> from
> >> a SIP/SER point of view.  Tell me if you miss something :-)
> >> g-)
> >>
> >> ----- Original Message -----
> >> From: "Felipe Louback" <louback at gmail.com>
> >> To: <serusers at lists.iptel.org>
> >> Sent: Monday, October 03, 2005 07:27 PM
> >> Subject: [Serusers] Theory behind a media proxy and NATed UA
> >>
> >>
> >> > The problem with NAT is because the IP address of the SDP body is a
> >> > non routeable IP. So how does a media proxy works?
> >> >
> >> > I read that a media proxy put his address at the sdp body and both UA
> >> > talks with the media proxy instead of each other. At the time a NAT
> >> > user sends an INVITE, there is no corresponding port on the NAT. So
> >> > How does a media proxy contact the NATed UA?
> >> > Isn't this the same problem the called part will have to contact the
> >> > NATed party?
> >> >
> >> > I read a couple of papers about it, but in all of them it is said that
> >> > both ends talk to the media proxy and that is all. No explanation
> >> > about how things work. I know it works because I am using mediaproxy.
> >> >
> >> > If anyone could point me how this works or where I can find a document
> >> > explaining....
> >> >
> >> > Thanks,
> >> >
> >> > Felipe
> >> >
> >> > --
> >> > Master Student - Electrical Engineering Department
> >> > Computer Engineering and Telecommunications Research Group
> >> > Universidade Federal de Minas Gerais - Brazil
> >> >
> >> > "For God so loved the world that he gave his one and only Son, that
> >> > whoever believes in him shall not perish but have eternal life."
> >> > John 3:16
> >> >
> >> > _______________________________________________
> >> > Serusers mailing list
> >> > serusers at lists.iptel.org
> >> > http://lists.iptel.org/mailman/listinfo/serusers
> >> >
> >>
> >>
> >
> >
> > --
> > Master Student - Electrical Engineering Department
> > Computer Engineering and Telecommunications Research Group
> > Universidade Federal de Minas Gerais - Brazil
> >
> > "For God so loved the world that he gave his one and only Son, that
> > whoever believes in him shall not perish but have eternal life."
> > John 3:16
> >
> >
>
>


--
Master Student - Electrical Engineering Department
Computer Engineering and Telecommunications Research Group
Universidade Federal de Minas Gerais - Brazil

"For God so loved the world that he gave his one and only Son, that
whoever believes in him shall not perish but have eternal life."
John 3:16