[Serusers] Theory behind a media proxy and NATed UA - sr-users

Andreas Granig

3 Oct 3 Oct

8:34 p.m.

Felipe Louback wrote:

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?

After the first UDP packet received on the mediaproxy socket it knows where to send the packets to. So both parties have to send at least one packet to the mediaproxy before the streams can be bridged together. This is why some protocols like T.38, where one party waits for data from the other party before replying, don't work with NAT. Andy

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Felipe Louback

5:17 p.m.

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(a)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(a)gmail.com> To: <serusers(a)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(a)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

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Felipe Louback

5:50 p.m.

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(a)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(a)gmail.com> To: "Greger V. Teigre" <greger(a)teigre.com> Cc: <serusers(a)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(a)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(a)gmail.com> To: <serusers(a)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(a)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

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Felipe Louback

5 Oct 5 Oct

1:36 a.m.

...

after the UA, which send the INVITE, received the 200 OK for the INIVTE it has to start sending RTP.

uhm..... I didn't know that. That is a very good explanation. If it has to start sending RTP, then the media proxy now knows what port it is using in tha NAT box. I thought that after the 200 OK, the nated UA would wait for audio to come, since it sent all the parameters in the SDP body. Thanks for the explanation. 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

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Nils Ohlmeier

2:51 p.m.

If one side of the connection is on the public internet and the other one behind NAT you do not need a RTP proxy in all cases. But if both parties are behind NAT then you have a classical chicken-egg problem. For that scenarios you defiantly need a relay in the public internet. Nils On Thursday 06 October 2005 13:17, Felipe Louback wrote:

...

well, if after the NATed UA receives the 200 OK it has to start sending RTP and it is symetric(sends and receive data throught the same port), then there is no need of a media proxy, since the called part knows from which port in the NAT box the NATed UA is using... I am getting confused about why I need to use a media proxy.... Felipe On 10/4/05, Felipe Louback <louback(a)gmail.com> wrote:

after the UA, which send the INVITE, received the 200 OK for the INIVTE it has to start sending RTP.

uhm..... I didn't know that. That is a very good explanation. If it has to start sending RTP, then the media proxy now knows what port it is using in tha NAT box. I thought that after the 200 OK, the nated UA would wait for audio to come, since it sent all the parameters in the SDP body. Thanks for the explanation. 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

-- 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

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