http://www.rfc-editor.org/rfc/rfc7852.txt
RFC 7852: Additional Data Related to an Emergency Call, R. Gellens, B. Rosen, H. Tschofenig, R. Marshall, J. Winterbottom2016-07-01T23:00:00-00:00The mechanisms permit the data to be conveyed by reference (as an external resource) or by value (within the body of a SIP message or a location object). This follows the tradition of prior emergency services standardization work where data can be conveyed by value within the call signaling (i.e., in the body of the SIP message) or by reference.