Members of the IETF SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE) Working Group have released an initial Internet Draft for "Optimizing Federated Presence with View Sharing." Presence federation refers to the exchange of presence information between systems. One of the primary challenges in presence federation is scale. With a large number of watchers in one domain obtaining presence for many presentities in another, the amount of notification traffic is large.
This document describes an extension to the Session Initiation Protocol (SIP) event framework, called view sharing. View sharing can substantially reduce the amount of traffic, but requires a certain level of trust between domains. View sharing allows the amount of presence traffic between domains to achieve the theoretical lower bound on information exchange in any presence system. The key idea with view sharing is that when there are many watchers in a domain to a single presentity in another domain, each of which is actually going to get the exact same presence document, the domain of the watchers extends a single subscription to the domain of the presentity, and the domain of the presentity sends a single copy of the presence document back to the domain of the watcher. In the case of a pair of large providers that are peering with each other, this mechanism can result in a significant savings. The ACL document informs a watching domain of the set of views that can be received by that domain, and associates specific watchers with specific views. An ACL document is an XML document that must be well-formed and must be valid according to schemas, including extension schemas, available to the validater and applicable to the XML document (see XML Schemas in Section 5.5). ACL documents must be based on XML 1.0 and must be encoded using UTF-8.
Read the IETF Internet Draft by Jonathan Rosenberg, Steve Donovan, Kathleen McMurry as cited in Cover Pages.