This REGISTER is modified by the ALG feature (if not the user wouldn't be reachable by the proxy since it indicated a private IP in REGISTER 'Contact' header). Lack of incoming calls: When a UA is switched on it sends a REGISTER request to the proxy in order to be localisable and receive any incoming calls.Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled. This can give you unexpected behaviour, such as phones not registering and incoming calls failing. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. How can it affect VoIP?Д®ven though SIP ALG is intended to assist users who have phones on private IP addresses (Class C .X), in many cases it is implemented poorly and actually causes more problems than it solves. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signalling and audio traffic between the client behind NAT and the SIP endpoint possible. If the SIP proxy doesn't provide a server-side NAT solution, then an ALG solution could have a place.ДЄn 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. In some scenarios, some client-side solutions are not valid, for example, STUN with symmetrical NAT router. Generally speaking, ALG works typically in the client side LAN router or gateway. There are various solutions for SIP clients behind NAT, some of them in the client side (STUN, TURN, ICE), others are in the server side (Proxy RTP as RtpProxy, MediaProxy). Many routers have SIP ALG turned on by default. Its purpose is to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it.

SIP ALG stands for Application Layer Gateway and is common in all many commercial routers.

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