What is maximum waiting time for SIP INVITE method

Question

I have a scheme like this https://prnt.sc/176ry00

What is maximum waiting time for 200 OK response?

Answer 1

There isn't. There is a max waiting time for the first non-100 provisional response to the SIP INVITE request, which is equal to the transaction timeout (Timer B, 64*T1). After the provisional response has been received it is up to the application or user to decide when to stop waiting for the final response.

See Client INVITE transaction in RFC 3261.

Answer 2

SIP INVITE transaction does not have timeout on receiving final response (2xx, 3xx, 4xx, 5xx, 6xx) after provisional response (1xx). But you definitely should be aware about Timer C. This timer is proxy-specific and defines that transaction maybe cancelled if no provisioning response was received during Timer C timeout (by SIP specification should be set to more than 3 minutes).

Therefore if you are implementing UAS then you need periodically send additional 1xx provisional responses to keep INVITE transaction "fresh" on intermediate proxy servers that are involved in processing of your SIP request.