Module 1: Foundations

SIP Headers — The Essential Ones and What They Tell You

Learn about sip headers — the essential ones and what they tell you

Duration: 10 minutes
Level: Advanced
Lesson: 41

Being able to read SIP headers fluently is the single most valuable skill for a telecom NOC engineer. Every SIP message is a story — the headers tell you where the message came from, where it's going, who the call is for, and what path it took. This lesson is your header-by-header field guide.

The Via Header — Tracing the Request Path

Via is arguably the most important header for troubleshooting. Each SIP entity that forwards a request adds a Via header at the top of the stack. Responses follow the Via headers in reverse to get back to the sender.

Via: SIP/2.0/UDP proxy2.telnyx.com:5060;branch=z9hG4bK-2;received=10.10.5.20
Via: SIP/2.0/UDP proxy1.telnyx.com:5060;branch=z9hG4bK-1;received=10.10.1.10
Via: SIP/2.0/UDP 192.168.1.50:5060;branch=z9hG4bKnashds7;rport=5060;received=203.0.113.50

Reading this: The request originated from 192.168.1.50 (a private IP), was received by proxy1 from public IP 203.0.113.50 (NAT detected — received parameter shows the actual source IP), then forwarded to proxy2.

Key parameters:

  • branch: Transaction identifier. Must start with z9hG4bK (magic cookie indicating RFC 3261 compliance). Each transaction gets a unique branch.
  • received: Added by the receiving proxy when the source IP differs from the address in the Via. Critical for NAT detection.
  • rport: Requested by the client (RFC 3581) — tells the proxy to record the actual source port. Essential for NAT traversal.
  • transport: UDP, TCP, TLS, or WSS (WebSocket Secure).

💡 NOC Tip: en debugging connectivity issues, the Via received parameter tells you the actual public IP of the sender. If this differs from the Contact or From addresses, you're dealing with NAT. Compare received values across hops to trace the real network path.

From and To — Dialog Identification (Not Routing!)

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