http://www.rfc-editor.org/rfc/rfc7135.txt
RFC 7135: Registering a SIP Resource Priority Header Field Namespace for Local Emergency Communications, J. Polk2014-05-01T23:00:00-00:00This document creates the new Session Initiation Protocol (SIP) Resource Priority header field namespace 'esnet' and registers this namespace with IANA. The new header field namespace allows for local emergency session establishment to a public safety answering point (PSAP), between PSAPs, and between a PSAP and first responders and their organizations.