SIP ALG stands for Session Initiation Protocol Application Layer Gateway, a feature built into most commercial routers that is designed to help VoIP traffic traverse NAT (Network Address Translation) boundaries. It's a NAT tool that inspects SIP packets and rewrites private IP addresses and ports into public IP addresses and ports to theoretically ensure VoIP calls work correctly behind firewalls.
How it Works
SIP ALG operates by actively monitoring and modifying SIP packets as they pass through your router. When a VoIP packet reaches the router, the ALG inspects the packet contents, rewrites the IP addresses and port numbers in the SIP message headers and SDP (Session Description Protocol) body, recalculates checksums, and forwards the modified packet. This process happens for both incoming and outgoing SIP traffic with the intention of preventing firewall-related issues.
Why it Causes Problems
Despite its intended purpose, SIP ALG typically causes more problems than it solves for modern VoIP systems. The packet rewriting process often disrupts call quality because many routers don't fully understand SIP routing protocols, and their scripting can eliminate critical parts of SIP messages. Modern VoIP and Unified Communications systems already include built-in NAT traversal solutions (like STUN, TURN, and ICE), making SIP ALG unnecessary and problematic.
Common Issues Caused by SIP ALG
When SIP ALG is enabled, you may experience one-way or no audio, dropped calls, calls that fail to connect, persistent ringing after answering, inability to reach other extensions on the same network, and problems with BLF (Busy Lamp Field) or MWI (Message Waiting Indicator) features. These issues occur because the ALG's modifications create mismatches between what the router thinks the packet should contain and what the VoIP system actually needs.