Siemens Enterprise Communications This document surveys existing mechanisms in the Session Initiation Protocol (SIP) for identifying and authenticating the source of a SIP request (or caller identification). It describes how identification and authentication are not always end-to-end and the problems that this can lead to, particularly since media security based on techniques such as DTLS-SRTP is dependent on end-to-end authenticated identification of parties. This work is being discussed on the sip@ietf.org mailing list.