Troubleshooting your experience: SIP-aware Routers/Firewalls

[Michael Glover]

When calling into a SPOT system, you want to be sure your network route to SPOT supports SIP based communication. To understand what this means, let's look at an example that doesn't work.

The caller is at home on his broadband internet connection. He is using a simple network router that procures a single IP address(1.2.3.4) from his ISP. The router/firewall uses DHCP to provide the 192.168.1.2 IP addresses to his host. This IP range is not reachable across the internet and is only used internally on the home network to communication between systems and the router.

The caller opens his soft-phone application and calls SPOT(sip:####@stp.iivip.com). The SIP INVITE message is created and sent across the internet to the SPOT system. The message that was created would have a fields that reference the user on his host(IP address 192.168.1.2).

The user doesn't know it yet, but his router/firewall is not SIP aware. It doesn't examine the SIP request and update the contact information to help identify the responses as needing to come to the 1.2.3.4 IP address the router has, before it can go to the 192.168.1.2 IP address. The message makes it to SPOT and the contact information points to an IP address than cannot be routed back to the user.

What the user will experience is a system that is not responding. If he opens the SIP communication using a TCP/IP connection rather than UDP/IP, he'll get call progression, but it is unlikely that he'll hear any audio as it will also be routed to the wrong destination.

A scenario like this shows why a SIP aware router/firewall can be key to a successful call. Making sure your firewall supports SIP is important piece of troubleshooting your call experience when you are not getting any audio.