SIP Problems

Problem

SIP phone sessions don't work through a Netscreen firewall. Individual symptoms can include

• the phone not connecting
• one-way voice on phone calls
• phone calls terminating after a regular period of time, eg 30 seconds

Solution

Netscreen Firewalls have an Application Layer Gateway defined, where they try to “help along” certain protocols which may not play nice with NAT’ing firewalls. SIP is one of those protocols. And by default, it is turned on.

If you go into Configuration -> Advanced -> ALG, you can select the protocol in question; clear the check box to turn it off.

This should improve VoIP performance through the firewalls.