Nokia Siemens Networks Helsinki University of Technology Columbia U. The Boeing Company Ericsson This document investigates the interworking between the Session Initiation Protocol (SIP) and the Host Identity Protocol (HIP) and the benefits that may arise from their combined operation. The aspect of exchanging Host Identities (or Host Identity Tags) in SIP/SDP for later usage with the Host Identity Protocol Protocol (HIP) is described in more detail as an example of this interworking.