s service for the first time, her handset initializes the SIP registration routine and connects to the first available register SIP server. During this registration phase Alice is prompted for nickname and password. She is clever enough to hide her real identity by giving false data. She is recognized by the nickname "Mother" to all the friends that would like to call her in the future. From a Mist point of view, the register SIP server considers to be her Lighthouse. The Lighthouse will be the point of contact for the other SIP users in order to get in touch with her. 20.5.2.2 Mobility Issues Alice is visiting a friend in the other part of town and she wants to be reachable but not traceable. She connects to the first available SIP server. From the Mist point of view, this is considered to be her portal. The next step is to set up a Mist circuit between Alice portal and Lighthouse. Note that the portal does not have to send to Alice a list with the available lighthouses since Alice knows that her lighthouse will be the SIP server where she was registered to (i.e., the registrar server of the SIP protocol). Thus the portal, instead of informing Alice about the available lighthouses, retrieves from the Mist LDAP directory information about the Mist network SO Alice will be able to establish a Mist circuit with her Lighthouse. Accordingly, Alice's handset locates Alice Lighthouse and determines the available Mist circuit through which the Lighthouse is reachable. An open issue is the size of the data volume that the handset receives. It is impossible to receive all Mist links and try to determine the route to its Lighthouse. C 2009 by Taylor & Francis Group, LLC Anonymity in SIP 20.5.2.3 Mist Circuit Establishment Her handset encrypts a predefined message with the public key of the Light- house and forwards it to the portal. The latter follows the aforementioned Mist procedure and routes the packet to her Lighthouse. The Lighthouse upon receiving the packet stores the Mist circuit information to the user binding ta