Session Initiation Protocol Application-Level Gateway or SIP ALG as it is more commonly known refers to a router’s specific ability or function that enables it to navigate through a network’s firewall. It is a default feature or inclusion in most commercial routers and is supposed to perform the function of a troubleshooter by solving issues arising due to router firewalls. The primary aim of SIP ALG is to monitor as well as modify VoIP traffic as and when required in order to tackle any problems or irregularities that may arise.
Why SIP ALG Should Be Disabled on Commercial Routers and How It Can Hamper Proper Functioning of VoIP
Although the aim of SIP ALG is to help people who own phones that operate on private IP addresses, typically belonging to Class C and in the 192.168.X.X format, more often than not, it gives rise to more issues than it resolves and is executed in a manner that is frankly, quite lacking. Instead of troubleshooting, what SIP ALG usually ends up doing is making tweaks and changes in SIP traffic or packets in a way that it corrupts them and making them quite impossible to read. This leads to bigger and more worrisome problems such as incoming calls not being able to come through and sometimes, even rendering the system unable to register phone calls altogether. Thus, it is highly recommended that if you are facing all or any of the above-mentioned issues, you check to see if your router’s SIP ALG has been turned on as a default setting and disable it to take care of your current problems and avoid any more of the same kind in the future.
In a normal set-up, what should ideally happen is that on being turned on, the UA should send a request to the proxy to ‘REGISTER’ to be available to local channels and enable incoming calls to come through. However, what a SIP ALG ends up doing is that it modifies the ‘REGISTER’ request in such a way that the proxy is unable to read the NAT, thus making it impossible for incoming calls to come through the router network.
Another problem that routers face when they have their default SIP ALG feature turned on is the inaccurate modification or tweaking of the SDP body and SIP headers, thus, ultimately destroying the Session Initiation Protocol or SIP and making any kind of communication nothing short of a herculean task, something it shouldn’t be, especially when SIP ALG is purported to aid the process, not completely complicate and disrupt it.
SIP ALG also disrupts and makes it impossible to receive server-side solutions. Your SIP proxy will usually provide you with a server-side NAT solution. But, if the SIP ALG feature in your router has been turned on by default, it will interrupt, break and corrupt the SIP signal, making it impossible for you to establish any kind of communication with your proxy.
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