SIP ALG, or Session Initiation Protocol Application Layer Gateway, is a feature found in many routers designed to assist VoIP traffic. However, in modern VoIP deployments, it often becomes a significant source of problems. Instead of improving call quality, SIP ALG can introduce a host of issues that disrupt communication and frustrate users. Understanding why SIP ALG breaks VoIP is crucial for maintaining a stable and efficient phone system.
One of the primary reasons SIP ALG causes problems is its modification of SIP packets. SIP ALG alters the SIP and SDP data within these packets, leading to malformed traffic. This interference disrupts call signaling, causing issues such as one-way audio, dropped calls, and failed call features. Furthermore, SIP ALG often conflicts with encrypted VoIP protocols like TLS and SRTP, compromising the security of your communications.
Modern VoIP providers generally do not require SIP ALG, and in most cases, it should be disabled to ensure reliable communication. Symptoms of SIP ALG interference include intermittent “ghost” issues that are difficult to troubleshoot, phones unregistering, and problems with call transfers or conferencing. By disabling SIP ALG, you can often resolve these issues and improve the overall performance of your VoIP system.
