VoIP glossary · Application Layer Gateway

What is SIP ALG?

SIP ALG is a router feature that tries to rewrite SIP messages as they pass through NAT, and it breaks VoIP calls so often that the standard advice is to switch it off.

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SIP ALG explained

In theory an ALG helps devices behind NAT by replacing private addresses in SIP headers with the public one. In practice many implementations rewrite the wrong fields, corrupt messages, or interfere with a PBX that is already handling NAT correctly. Symptoms include registrations that drop, calls that end after 30 seconds and one-way audio.

If your PBX is configured with its public address, disable SIP ALG on the router. The setting may be called "SIP ALG", "SIP Helper", "SIP Fixup" or "SIP Transformations" depending on the brand.

Example

Calls through a home-office router drop exactly 32 seconds after answer. Turning off "SIP Helper" in the router settings stops the drops.

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