Re: [Ekiga-list] NAT issues with VoIP clients

[ael <law_ence dev ntlworld com>]

On Thu, Mar 18, 2010 at 01:28:34PM +0000, My Name wrote:
> Indeed, this requires explanation.  This is surely the job of the SIP server.  I certainly do not need a separate STUN server to use a Web browser.  The Web site has no trouble determining my external IP address.

That is because there are only two parties involved: you contact the
web server which thus has your "call back" IP/port which has been
opened on the NAT as part of setting up the connection.

But in sip voip, the sip server only "introduces" the two ends of a call:
the actual call packets go directly rather than via the server. Thus
in effect the server introduces a 3rd party. There has to be a mechanism
to get that 3rd party traffic through the NAT: hence STUN. Otherwise
any "3rd party" spam etc. would get through.

Above is a very simplistic explanation, but I hope captures the essence.

ael