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For time-sensitive SIP applications , this may be unacceptable . At issue is the Enhanced Vehicle Inspection and Maintenance Program , a critical element of the 1994 SIP that was designed to identify highly polluting vehicles and ensure their repair or removal from the road . Note that a failure to complete HTTPS or starttls ( for example , due to certificate or encryption mismatch ) after having accepted the indirect content in the SIP request is not the same as rejecting the SIP request , and it may require additional user-user communication for correction . To ensure that Windows Messenger has mass appeal , Microsoft has made the product compliant with the Session Initiation Protocol ( SIP ) advocated by the Internet Engineering Task Force . A SIP interoperability test event is a meeting of developers of SIP implementations .