http://www.rfc-editor.org/rfc/rfc6228.txt
RFC 6228: Session Initiation Protocol (SIP) Response Code for Indication of Terminated Dialog, C. Holmberg2011-05-01T23:00:00-00:00This specification defines a new Session Initiation Protocol (SIP) response code, 199 Early Dialog Terminated, that a SIP forking proxy and a User Agent Server (UAS) can use to indicate to upstream SIP entities (including the User Agent Client (UAC)) that an early dialog has been terminated, before a final response is sent towards the SIP entities. [STANDARDS-TRACK]