http://www.rfc-editor.org/rfc/rfc6665.txt
RFC 6665: SIP-Specific Event Notification, A.B. Roach2012-07-01T23:00:00-00:00This document represents a backwards-compatible improvement on the original mechanism described by RFC 3265, taking into account several years of implementation experience. Accordingly, this document obsoletes RFC 3265. This document also updates RFC 4660 slightly to accommodate some small changes to the mechanism that were discussed in that document. [STANDARDS-TRACK]