that can have their individual states subscribed to with a single subscription. Definition 9 (RLMI) Resource List Meta-Information. RLMI is a docu- ment that describes the state of the virtual subscriptions associated with a list subscription. Definition 10 (RLS) Resource List Server. RLSes accept subscriptions to resource lists and send notifications to update subscribers of the state of the resources in a resource list. Definition 11 (Virtual Subscription) Virtual subscriptions are a logical construct within an RLS that represent subscriptions to the resources in a resource list. For each list subscription it services, an RLS creates at least one virtual subscription for every resource in the resource list being subscribed to. 5.5.3 Operation When users wish to subscribe to the resource of a list of resources, they can use the mechanisms described in this section. The first step is creation of a resource list. This resource list is represented by a SIP URI. The list contains a set of URIs, each of which represents a resource for which the subscriber wants to receive information. To learn the resource state of the set of elements on the list, the user sends a single SUBSCRIBE request targeted to the URI of the list. This will be © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security routed to an RLS for that URI. The RLS acts as a notifier and accepts the subscription. The RLS may have direct information about some or all of the resources specified by the list. If it does not, it could subscribe to any non-local resources specified by the list resource. As the state of resources in the list change, the RLS generates notifications to the list subscribers. The list notifications contain a body of type multipart/related. The root section of the multipart/related content is an XML document that provides meta-information about each resource present in the list. The remaining sec- tions contain the actual state information for each resource. Multipart/related and application/rlmi+xml 