SIP Fundamentals

SIP REGISTER

REGISTER is the SIP method by which a user agent or PBX tells a SIP registrar 'I am at this IP address right now — route traffic for sip:user@domain to me.' It is the standard authentication + presence handshake.

Flow

1. Endpoint sends REGISTER with Contact: <sip:user@1.2.3.4> and Expires: 3600.
2. Registrar replies 401 Unauthorized with a digest challenge.
3. Endpoint re-sends REGISTER with Authorization header containing the digest response.
4. Registrar replies 200 OK; the binding is now active.
5. Endpoint refreshes before expiry (typically every 60-300 seconds).

Sample exchange

REGISTER sip:sip.didhub.io SIP/2.0
From: <sip:trunk_x@sip.didhub.io>;tag=ab
To:   <sip:trunk_x@sip.didhub.io>
Contact: <sip:trunk_x@198.51.100.10:5060>
Expires: 300
CSeq: 1 REGISTER

--> 401 Unauthorized + WWW-Authenticate

REGISTER sip:sip.didhub.io SIP/2.0
Authorization: Digest username="trunk_x", realm="sip.didhub.io",
  nonce="abc...", uri="sip:sip.didhub.io", response="def..."

--> 200 OK + Contact + Expires

Registration vs IP-ACL

Many carriers (DIDHub included) support both:

• Registration mode: the customer's PBX registers periodically. Works behind any NAT. Required for dynamic IPs.
• IP-ACL mode: the customer pins their static public IP on the trunk. No registration needed. Lower overhead, more deterministic, recommended for cloud / static deployments.

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