This document normatively updates RFC 3261@ the Session Initiation Protocol (SIP)@ to address an error in the specified handling of success (2xx class) responses to INVITE requests. Elements following RFC 3261 exactly will misidentify retransmissions of the request as a new@ unassociated request. The correction involves modifying the INVITE transaction state machines. The correction also changes the way responses that cannot be matched to an existing transaction are handled to address a security risk.