http://www.rfc-editor.org/rfc/rfc4662.txt
RFC 4662: A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists, A. B. Roach, B. Campbell, J. Rosenberg2006-08-01T23:00:00-00:00This document presents an extension to the Session Initiation Protocol (SIP)-Specific Event Notification mechanism for subscribing to a homogeneous list of resources. Instead of sending a SUBSCRIBE for each resource individually, the subscriber can subscribe to an entire list and then receive notifications when the state of any of the resources in the list changes. [STANDARDS-TRACK]