How does SIP get through a firewall?
There are several possible approaches to SIP-capable firewalls. One of the difficulties is that, unlike for, say, HTTP, connections are originated both by hosts inside and outside the firewall. A likely arrangement is that a SIP proxy sits “on” the firewall and relays SIP requests between the Internet and the intranet. Thix proxy would also open up the necessary ports in the firewall to let audio and video flow through, for example using Socks V5. As an alternative, if a firewall or NAT allows outgoing TCP connections, the inside client can open up a TCP connection to an outside proxy. All outgoing and incoming calls would then be handled by that TCP connection.
