[SR-Users] Cannot hear voice with symmetric NAT and STUN

[Mitan Lopez]

So RTP proxy servers are used most of the time in Internet to communicate between 2 UA's? (in this case why bother to check all those lines in kamailio cfg?)


Mitan Lopez
mitan23 at aol.com




-----Original Message-----
From: Alex Balashov <abalashov at evaristesys.com>
To: sr-users <sr-users at lists.sip-router.org>
Sent: Mon, Mar 4, 2013 11:30 am
Subject: Re: [SR-Users] Cannot hear voice with symmetric NAT and STUN


On 03/04/2013 02:55 AM, Khoa Pham wrote:
> @Alex: thanks for reply
>
> In this IPv4 world, most devices are in NAT, and it only matters if they
> are symmetric NAT or not.
>
> So if 1 client is behind symmetric NAT, I want to use rtpproxy,
> otherwise, I do not.
> According to what I read from kamailio cfg, Kamailio only knows if
> client is NATed or not, it doesnot know the NAT type

That's correct.

The only way you can infer whether a client is symmetric is through its 
insertion of the 'rport' parameter into the Via header.  However, 
symmetric clients are not required to insert it;  it is only so that a 
client can request symmetric behaviour from a server.  AS RFC 3581 says:

     A client, compliant to this specification (clients include UACs and
     proxies), MAY include an "rport" parameter in the top Via header
     field value of requests it generates.

Certainly, there's nothing saying it must do so.

However, almost all UAs out there are symmetric these days.  It's got to 
be over 95%+.  Have you run into one that isn't?

-- Alex

-- 
Alex Balashov - Principal
Evariste Systems LLC
235 E Ponce de Leon Ave
Suite 106
Decatur, GA 30030
United States
Tel: +1-678-954-0670
Web: http://www.evaristesys.com/, http://www.alexbalashov.com/

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