Subject: RE: on NAT-PT

From: "Christian Huitema" <[email-masked]>
Date: Sun, 26 Jan 2003 23:32:07 -0800

> Of course, folks could build a single box with this functionality
>  - NAT(-PT)
>  - SIP proxy
>  - SIP proxy controlled IP/port mapper
> but you can't just have an independent SIP proxy and NAT(-PT) box do this;
> significant interfaces would be needed between the SIP and NAT functionality.

You could build such a box, but there are some well known limitations. Normally, SIP does not constraint the media flow to follow the same path as the signalling; forcing this single path can result in spectacularly poor performances in some scenarios, e.g. when calls are redirected, or when the proxy is "remote" -- as is the case today with most large instant messaging systems. Also, the rewriting that you describe imposes to rewrite the SDP payload of the SIP messages; if the SIP UA secure the payload with MIME security , this rewriting is impossible.
 
-- Christian Huitema