http://www.rfc-editor.org/rfc/rfc4320.txt
RFC 4320: Actions Addressing Identified Issues with the Session Initiation Protocol's (SIP) Non-INVITE Transaction, R. Sparks2006-01-01T23:00:00-00:00This document describes modifications to the Session Initiation Protocol (SIP) to address problems that have been identified with the SIP non-INVITE transaction. These modifications reduce the probability of messages losing the race condition inherent in the non-INVITE transaction and reduce useless network traffic. They also improve the robustness of SIP networks when elements stop responding. These changes update behavior defined in RFC 3261. [STANDARDS-TRACK]