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Twelve cited cards

The JSON files next to this page hold the full cited deck (88) and the idea-pool deck (1384). This page is the one-page pass.

QuestionAnswerCite
What does a SIP REGISTER request ask the registrar to store?A binding from an address-of-record to one or more Contact URIs, for an expiry time. It does not create a call.RFC 3261 section 10
On a REGISTER, what is the difference between the Request-URI, To, and Contact?The Request-URI is the registrar domain, such as sip:example.com. To is the address-of-record being registered. Contact is the device URI to bind to that address.RFC 3261 section 10
Why is the first REGISTER often answered with 401?The registrar is issuing a digest challenge. WWW-Authenticate carries the realm and nonce. The phone sends a new REGISTER with Authorization.RFC 3261 section 22
How is 401 different from 407?401 comes from the registrar and uses WWW-Authenticate. 407 comes from a proxy and uses Proxy-Authenticate. The matching response headers are Authorization and Proxy-Authorization.RFC 3261
What does Expires: 0 do on a REGISTER Contact?It removes that binding. Contact: * with Expires: 0 removes all bindings for the address-of-record.RFC 3261 section 10
Does a successful registration mean media can reach the phone?No. Registration publishes a signaling Contact. NAT, firewalls, and the SDP addresses used later are separate problems.RFC 3261
What identifies a SIP dialog?The Call-ID plus the local tag and the remote tag. From carries the UAC's tag. To carries the UAS's tag once a response creates it.RFC 3261 section 12
When is a dialog early, and when is it confirmed?A provisional response with a To tag creates an early dialog. The dialog is confirmed when the UAC receives a 2xx to the INVITE.RFC 3261
Is 100 Trying forwarded end to end?No. 100 Trying is hop-by-hop. A proxy generates its own 100 toward the caller so the caller stops retransmitting the INVITE.RFC 3261
How does ACK for a 2xx differ from ACK for a failure such as 486?ACK for a 2xx is a new end-to-end transaction and follows the dialog route set. ACK for a non-2xx final response is hop-by-hop and belongs to the INVITE transaction.RFC 3261 section 17
What does Record-Route change about later requests?It builds a route set so that in-dialog requests such as ACK for the 2xx, re-INVITE, and BYE still pass through that proxy. Without it, they go to the remote Contact.RFC 3261
Where is the SDP in a typical early-offer call, and where is it in a delayed offer?Early offer: offer in the INVITE, answer in a reliable 18x or in the 200 OK. Delayed offer: the INVITE has no SDP, the offer is in the 200 OK, and the answer is in the ACK.RFC 3261 and RFC 3264

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