The Defense Information Systems Agency of the United States Department of Defense, with is contractors, has proposed a service architecture for military (NATO and related agencies) telephone systems. This is called the Assured Service, and is defined in two documents: 'Architecture for Assured Service Capabilities in Voice over IP' and 'Requirements for Assured Service Capabilities in Voice over IP'. Responding to these are three documents: 'Reason Header Field for the Session Initiation Protocol', 'Extending the Session Initiation Protocol Reason Header to account for Preemption Events', 'Communications Resource Priority for the Session Initiation Protocol'.