I work with NENA, the North American Number Association. We're engaged in a standards effort for "Next Generation 9-1-1", moving the emergency call system to an IP network. Our work is based on IETF work, and uses SIP Event Notification (RFC3265). We need to define event packages for use within private IP networks connecting public safety agencies, and we do not expect use of these packages to escape into the wild. As a practical matter, defining an event package means defining one or more XML structures that are passed in the event subscription and/or notification. These structures are passed in SIP bodies, and thus need MIME types, typically within the application tree. We will request registration using a prefix of vnd.nena. My question is whether to register each of these MIME types individually with IANA, or register a subtree (vnd.nena.ng911) and create a registry in NENA for the subtree (vnd.nena.ng911.queuestate). I would appreciate advise on that subject. We would expect a dozen or so event packages. We will also need some other MIME types for other SIP related use, for which similar issues apply. NENA is creating a registry management service not unlike a very small IANA, that could manage the subname space if that was deemed appropriate. Brian