stablishment. 20.5.2 Applying MIST in SIP By taking a closer look into the SIP protocol, someone can note that the home server (registrar, redirect or proxy server) has knowledge both for the user's ID and her current location. Our goal would be to distribute this knowledge to more than one entity. By doing so, it will be difficult for eavesdroppers or the home server itself to inference the user IDs. Since on SIP, the user registers to the home server (with her ID) and the latter is the one that all SIP entities refer to in order to locate the former, which we will consider being the user's Lighthouse. Furthermore, we will con- sider as portals all the remote SIP servers that the user is connected to in order to establish communication through the SIP. Based on the above, we could say in general that each SIP server acts as Lighthouse for the users that have registered to it whilst as portal for the remote users that at some point can connect to the former. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security In order to apply Mist to SIP, there should be some small modifications on the SIP. The SIP location service is a LDAP directory that keeps the current position of each registered user. However on the RTTMist, the SIP location service is not required. The Mist user's binding table will be used instead. The latter keeps routing information about the Mist communication circuit with each user. Furthermore, we consider that: Mist hierarchy has been deployed to the SIP network. As we mentioned earlier a Mist hierarchy considers that all Mist servers are ordered in a hierarchical structure. Public key infrastructure (PKI) - A PKI has been established and has created and distributed to Mist servers a pair of keys. Furthermore the keys are accessible from every Mist server. - Each user holds a pair of keys issued to his nickname 20.5.2.1 Registration Phase Let assume that Alice has a 3G handset that allows her to place calls using an SIP-based phone service. When she triggers thi