An INVITE Is Only the Start of the Call Contract

Call setup includes dialog state, SDP negotiation and route continuity after the first request.

I reached An INVITE Is Only the Start of the Call Contract through a repeatable lab problem rather than a design slogan. LOUP call testing had to preserve transaction and dialog state across provisional responses, final responses, ACK and later BYE.

A proxy or endpoint can appear to handle the initial INVITE while losing Contact, Route, tags or Call-ID relationships needed for in-dialog requests. Capturing the complete dialog is therefore more useful than looking at one request.

The useful debugging step was separating signal path, state and timing instead of modifying all three and then guessing which change mattered. Evidence marker: invite-dialog.

SIP debugging should follow the dialog lifecycle, not stop at the first 200 OK. That distinction also makes regressions cheaper to isolate when hardware, firmware and infrastructure are changing in parallel.

Project evidence

LOUP engineering marker: invite-dialog. This note records the design or debugging lesson without publishing device credentials, private keys, customer data or manufacturing secrets.