n: </status> <contact priority="0.8">im:somebody@homepage.net</contact> <note tml:lang="en">Don't block Please!</note> timestamp>2007-10-27T17:10:10z</timestamp> </tuple> <tuple id="dsfds2e"> <status> <basic>open</basic> </status> <contact priority="1.0">mailto:somebody@handbook.com</contact> </tuple> <note>I am going to the supermarket now</note> </presence> Event List-Notification Extension 5.5.1 Introduction The SIP-specific event notification mechanism [6] allows a user (the sub- scriber) to request to be notified of changes in the state of a particular resource. This is accomplished by having the subscriber generate a SUB- SCRIBE request for the resource, which is processed by a notifier that repre- sents the resource. In many cases, a subscriber has a list of resources they are interested in. Without some aggregating mechanism, this will require the subscriber to C 2009 by Taylor & Francis Group, LLC SIP Event Notification and Presence Information generate a SUBSCRIBE request for each resource about which they want information. For environments in which bandwidth is limited, such as wireless networks, subscribing to each resource individually is expensive. The primary benefit of the resource list server is to reduce the overall messaging volume to a subscriber. To solve these problems, this event list mechanism defines an extension to notification mechanism, which allows for requesting and conveying notifica- tions for lists of resources. The notifier for the list is called a "resource list server," or RLS. 5.5.2 Definition Definition 6 (Back-end Subscription) Any subscription (SIP or other- wise) that an RLS creates to learn of the state of a resource. An RLS will create back-end subscriptions to learn of the state of a resource about which the RLS is not an authority. For back-end subscriptions, RLSes act as a sub- scriber. [7] Definition 7 (List Subscription) A subscription to a resource list. In list subscriptions, RLSes act as the notifier. Definition 8 (Resource List) A list of zero or more resources 