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16.6. Resource Lists

In some scenarios, a subscriber is interested in the presence information related to a list of resources. Instead of sending a SUBSCRIBE request for each resource, which would generate substantial message traffic, [RFC 4662] specifies a mechanism for requesting and conveying notifications for list of resources. The list of resources is identified by a URI (PRES or SIP URI), and it represents a list of zero or more URIs. The notifier for the list is called a Resource List Server (RLS), and the document that describes the list members and their state is called an RLMI (Resource List Meta-Information). The RLS may have direct information about some of the resources in the list. If it does not, the RLS could subscribe to any nonlocal resources specified by the list resource. This is referred to as back-end subscription, and it might be a SIP subscription or any other type of subscription.

Figure 16.7 shows an example of how this works. In this example, John has a list of buddies comprising Alice, Robert, and Peter; this buddy list is maintained in RLS1. The Resource List Server RLS1 is authoritative for the presence information of Alice and Robert, but does not have information about Peter. The authoritative server for Peter’s presence information is PA1.


  

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