![]() Breaking SIP signalling: Many of the actual common routers with inbuilt SIP ALG modify SIP headers and the SDP body incorrectly,īreaking SIP and making communication just impossible.Mantain the keepalive (so incoming calls will be not possible). The REGISTER request so the proxy doesn't detect the NAT and doesn't Or NOTIFY messages to the UA, but they just do it when the UA has beenĭetected as natted during the registration. Many SIP proxies mantain the UDP keepalive by sending OPTIONS Port forwarding is ended and incoming packets are discarded by the ![]() "conntection" open for a while (30-60 seconds) so after that time the Wouldn't be reachable by the proxy since it indicated a private IP in This REGISTER is modified by the ALG feature (if not the user Lack of incoming calls: When a UA is switched on it sends a REGISTER to the proxy in order to be localizable and receive incomingĬalls.Useful for outgoing calls, but not for incoming calls: Most commercial routers and the fact that this technology is just The main problem is the poor implementation at SIP protocol level of Source What is SIP ALG and why does Gradwell recommend that I turn it off? This can give you unexpected behaviour, such as phones not registering and incoming calls failing. Gradwell and other providers often find that the SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. If you have one of these problem routers it is recommended that the SIP-ALG is disabled and other methods used to ensure VoIP works correctly. Unfortunately the SIP-ALG in many routers is poorly implemented and causes more problems than it solves. Without SIP-ALG you may have one-way audio on VoIP conversations (see below). ![]() This make signaling and audio traffic between the client behind NAT (a VoIP phone for example) and the SIP endpoint possible. The intention is to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it. SIP-ALG allows a NAT router to re-write information within the SIP messages (SIP headers and SDP body). If SIP-ALG is such an issue with VoIP phones, why is it always enabled by default or yet alone even used as a protocol?
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