SIP ALG Explained - Technical

Many of today's commercial routers implement SIP ALG (Application-level gateway), this feature is almost always enabled by default. Many of the brand such as Cisco, Netgear, D-Link, Mikrotik... While ALG could help in solving NAT-related problems, many routers' ALG implementations are brand specific, simply wrong and break SIP.

There are various solutions for SIP clients behind NAT, some of them in client-side (STUN, TURN, ICE), others in the server-side (Proxy RTP as RtpProxy, MediaProxy). ALG works typically in the client LAN router or gateway. In some scenarios, some client-side solutions are not valid, for example, STUN with a symmetrical NAT router. If the SIP proxy doesn't provide a server-side NAT solution, then an ALG solution could have a place.

An ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol packet inspection of traffic through it. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signaling and audio traffic between the client behind NAT and the SIP endpoint possible.

You can verify if your router is SIP ALG ENABLED by downloading and running our SIP ALG TESTER Program

SIP ALG Tester Download

How to disable SIP ALGs on NetCloud CradlePoint E300 Series or AER1600 series.

Login into the web interface by navigating to the gateway IP. Usually 172.30.0.1

Then:

SIP ALG can be enabled or disabled in your router by navigating to Security > Zone Firewall > Options