http://www.rfc-editor.org/rfc/rfc3861.txt
RFC 3861: Address Resolution for Instant Messaging and Presence, J. Peterson2004-08-01T23:00:00-00:00Presence and instant messaging are defined in RFC 2778. The Common Profiles for Presence and Instant Messaging define two Universal Resource Identifier (URI) schemes: 'im' for INSTANT INBOXes and 'pres' for PRESENTITIES. This document provides guidance for locating the resources associated with URIs that employ these schemes. [STANDARDS-TRACK]