Thanks Anonymous for the heads up on this article over at the Microsoft XP Expertzone. Will Windows Messenger voice and video real time communications work behind a residential home gateway that includes Network Address Translation (NAT) support? Until recently, I didn"t think it would ever happen. I"ve been using developmental firmware and I"ve got really good news: Voice and video now work behind a NAT box in my home office with Windows XP Windows Messenger!
In the traditional world of residential/broadband gateways and NAT boxes, applications such as NetMeeting and many games depend on a fixed port and/or range of ports being available and usually can be accessed only after a manual configuration of ports, triggers, and DMZ zones. Windows Messenger is one of the first applications to take advantage of session initiation protocol (SIP), an emerging Internet Engineering Task Force (IETF) standard. When it comes to media streams for voice and video, SIP has the same problems behind a NAT box since voice and video are transmitted using the UDP protocol, not TCP. The port number of the end point PC will be wrong in the SIP headers due to the NAT. To fix this problem, Microsoft has implemented the Universal Plug and Play (UPnP) protocols in Windows XP, but that will only work if the NAT device also implements the UPnP protocol. (Without this support, voice and video using Windows Messenger will not work reliably behind a NAT device).
Here is also a link sent in by Vla that covers NAT and Messenger in more depth: