What is the NAT/firewall traversal problem?
While NATs and firewall serve important purposes by allowing multiple computers to share few public IP address and protecting local computers from unwanted traffic and access, they also pose major challenges for IP communications through the Internet. The above figure illustrates the NAT /firewall traversal problem for IP communications using SIP (Session Initiation Protocol) – a widely adopted standard for VoIP. In SIP, two user-agents needs to exchange IP addresses and media transport port in the SIP INVITE message to establish media exchange between them.
