The IMS: IP Multimedia Concepts And Services, Second Edition

8.3. Resource list

The Session Initiation Protocol (SIP) specific event notification mechanism (Section 12.13.1) allows a user (the subscriber) to request notification of changes to a resource state.

In many cases, a subscriber's list of resources for a particular event about which she needs state change information is a lengthy one. Without some aggregating mechanism, this will require the subscriber to generate a SIP SUBSCRIBE request for each resource. This will also result in SIP NOTIFY requests arriving at the subscriber's terminal more rapidly. For congestion control and bandwidth limitations, it is uneconomical to have the user's terminal send a multiplicity of SUBSCRIBE requests, one for each resource. Figure 8.1 demonstrates the problem.

To resolve this problem, [Draft-ietf-simple-event-list] describes an event notification extension that allows users to subscribe to a list of resources with a single SUBSCRIBE request. The list is identified by a URI and contains zero or more URIs pointing to atomic resources or to other lists. In a presence system these resources are presentities. The entity that processes the list's SUBSCRIBE request is referred to as the Resource List Server (RLS). The RLS can generate individual subscriptions for each resource in the list. These subscriptions may or may not be SIP SUBSCRIBE requests. Figure 8.2 demonstrates the solution.

A client sending SUBSCRIBE requests to a list includes a Supported header with an option tag of value "eventlist". ...