SIP Signaling From the Wire Up
A 12-part series that takes you from understanding what SIP is to reading packet captures and diagnosing real call problems. Covers message anatomy, call flows, registration, NAT traversal, security, trunking, and troubleshooting.
What SIP Actually Is (and What It Isn't)
SIP handles call signaling, not voice audio. Understanding that distinction is the foundation for diagnosing every VoIP problem you will ever encounter.
Anatomy of a SIP Message: Reading the Headers
A line-by-line walkthrough of a real SIP INVITE message, explaining what every major header does and how requests and responses relate to each other.
Your First Call Flow: INVITE to BYE
Walk through every SIP message in a basic call from INVITE to BYE. Learn what normal looks like so you can spot problems fast.
SIP Registration: How Your Phone Tells the World Where It Is
How SIP REGISTER works at the protocol level: authentication sequence, registration lifetime, and common failure modes.
SDP and Media Negotiation: How Two Phones Agree on How to Talk
How SDP works inside SIP messages to negotiate codecs, IP addresses, ports, and media direction between two VoIP endpoints.
NAT Traversal: Why SIP and NAT Don't Get Along
Why NAT breaks SIP and RTP, causing one-way audio and registration failures, and how STUN, TURN, ICE, and SBCs work around it.
SIP Response Codes: A Practical Reference
A field reference to the SIP response codes you will actually encounter when troubleshooting VoIP calls, registrations, and transfers.
Authentication and Security in SIP
How SIP digest authentication works, the role of TLS and SRTP in protecting VoIP traffic, and practical defenses against toll fraud.
Call Features in SIP: Hold, Transfer, and Forwarding
How SIP handles hold, transfer, forwarding, and conferencing at the protocol level, and why these features cause interop problems.
SIP Trunking: Connecting to the Outside World
How SIP trunks connect your PBX to the PSTN: authentication, codec negotiation, capacity planning, and SIP trunking vs. hosted PBX.
Reading a SIP Trace: Practical Troubleshooting
Capture and analyze SIP traffic with Wireshark and sngrep. Includes example traces for normal calls, NAT issues, and failed registrations.
Common SIP Problems and What They Look Like
Symptom-based troubleshooting for common SIP problems: one-way audio, dropped calls, registration failures, and caller ID issues.