Reliable Provisional Response (100rel) is now supported in Linphone, as Liblinphone library implements the reliability mechanisms described in RFC 3262.

The Session Initiation Protocol (SIP - RFC 3261) is a request-response protocol for initiating and managing communications sessions. SIP defines two types of responses, provisional and final. Final responses convey the result of the request processing, and are sent reliably, whereas Provisional responses provide information on the progress of the request processing, but are not sent reliably.

However, reliability proved to be important in several cases, including interoperability scenarios with the PSTN (standard telephony). Therefore, an optional capability was needed to support reliable transmission of provisional responses.

Reliable Provisional Response (100rel) is now supported in Linphone, as Liblinphone library implements the reliability mechanisms described in RFC 3262.