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Collaboration, Voice & AV · Intermediate

VoIP NAT Traversal

Techniques that allow SIP signaling and RTP media to work across NAT devices that rewrite IP addresses and ports.

Definition

SIP and SDP embed IP addresses in message bodies, so NAT that rewrites only packet headers can cause one-way or no audio. Solutions include SIP-aware ALGs, SBCs that rewrite SDP and relay media, STUN/TURN/ICE, symmetric RTP (comedia) and keeping NAT bindings alive with frequent registrations or keepalives. SIP ALGs on consumer routers frequently break calls and are often disabled.

Example

Disabling the SIP ALG on a branch firewall fixes one-way audio caused by the ALG rewriting SDP incorrectly.

Exam tip

One-way audio is the classic symptom of NAT or routing problems with RTP; check SDP addresses and firewall pinholes.

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