SIP ALG, Session Border Controllers, and Media Anchoring

Session Border Controllers: The Proper Fix

SBCs solve NAT by being the "man in the middle." They terminate SIP on both sides, rewrite headers appropriately, and relay media when needed.

How SBCs Handle NAT

Phone (NAT'd)          SBC (Public)           Carrier
192.168.1.100  ──────→  203.0.113.10  ──────→  198.51.100.5
SIP + SDP              SIP rewritten          Clean SIP
                         + SDP relay
             ←────────                ←──────
           Media relayed           Media

The SBC's job:

Receives SIP from private endpoint
Rewrites Contact, Via, SDP to use SBC's public address
Forwards clean SIP to destination
Receives RTP on SBC's port
Forwards RTP through NAT hole to private endpoint

SBC Functions Beyond NAT

Security: Topology hiding, DoS protection, call admission control
Interoperability: Normalize quirky SIP implementations
Media handling: Transcoding, recording, A/B leg separation
Regulatory: Lawful intercept, emergency call handling

💡 NOC Tip: Telnyx, the B2BUA acts as the SBC. When you see calls from a customer behind NAT, verify the call flow shows the B2BUA handling both legs. Call-site-finder can tell you which B2BUA handled a specific call.

Media Anchoring: Forcing RTP Through Known Paths

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