Module 1: Foundations

SIP Registration — How Endpoints Make Themselves Reachable

Learn about sip registration — how endpoints make themselves reachable

Duration: 7 minutes
Level: Advanced
Lesson: 44

The Problem Registration Solves

In the PSTN, your phone number is physically wired to a specific copper pair that terminates at a specific switch port. The network always knows where you are. In SIP, endpoints move — a softphone on a laptop might be at a coffee shop today and the office tomorrow. Its IP address changes. How does the network find it?

Registration is SIP's answer. An endpoint tells a registrar: "I'm user@example.com, and right now you can reach me at this IP:port." The registrar stores this binding and uses it to route incoming calls. Without registration, inbound calls have nowhere to go.

The REGISTER Transaction

Address of Record vs. Contact

Two concepts are central to registration:

  • Address of Record (AOR): Your public identity — like sip:alice@telnyx.com. This is what callers dial. It's stable and permanent.
  • Contact: Your current physical location — like sip:alice@192.168.1.50:5060. This changes whenever your network changes.

Registration creates a binding between the AOR and one or more Contacts. When a call arrives for the AOR, the registrar looks up the current Contact(s) and forwards the INVITE there.

The REGISTER Message

REGISTER sip:telnyx.com SIP/2.0
Via: SIP/2.0/UDP 192.168.1.50:5060;branch=z9hG4bK776
From: <sip:alice@telnyx.com>;tag=abc123
To: <sip:alice@telnyx.com>
Contact: <sip:alice@192.168.1.50:5060>;expires=3600
Call-ID: reg-unique-id@192.168.1.50
CSeq: 1 REGISTER
Expires: 3600

Key observations:

  • The Request-URI is the domain, not a specific user — the registrar is a domain-level service
  • From and To are the same — you're registering yourself
  • Contact is where you want to receive calls
  • Expires is how long the binding should last (in seconds)

The Authentication Dance

Most registrars require authentication. The flow is:

  1. Client sends REGISTER (no credentials)
  2. Registrar responds 401 Unauthorized with a WWW-Authenticate header containing a nonce
  3. Client recalculates REGISTER with an Authorization header containing the digest response (hash of username, password, nonce, realm, URI)
  4. Registrar verifies the hash and responds 200 OK

This two-step dance happens every time. The 401 is not an error — it's step one of authentication. (See Lesson 43 for deeper coverage.)

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