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Collaboration, Voice & AV · Intermediate

SIP Application Layer Gateway (SIP ALG)

A firewall or router feature that inspects and rewrites SIP messages and opens pinholes for RTP media across NAT.

Definition

A SIP ALG parses SIP headers and SDP bodies, translating embedded private addresses and dynamically opening the RTP ports negotiated in SDP. When implemented poorly or when signaling is encrypted with TLS, the ALG can corrupt messages or fail to open ports. Many UC vendors recommend disabling ALG and using an SBC instead.

Example

A Webex Calling site has registration failures until the SIP ALG is disabled on the internet router.

Exam tip

The ALG cannot inspect TLS-encrypted SIP, so it is useless or harmful with secure signaling.

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