next router on the hierarchy. This process is repeated on each of the intermedi- ate Mist routers until the packet reaches its final destination. In the case that decryption of the payload is successful, this indicates that the user selected the current Mist router to act as his Lighthouse. Finally, the Lighthouse answers back to user A and confirms the registration. From this point, a secure cir- cuit is established through which user A can communicate securely with his Lighthouse. Note that even though the Lighthouse of user A can infer that his/her physical location is underneath Mist router "Y", it is very difficult to determine the exact position. Following circuit establishment, the Lighthouse undertakes the role of rep- resenting the end user. An issue that has to be addressed is the detection of the user's Lighthouse. A public directory (i.e., LDAP server) or a Web server can be used for that purpose. Let us assume now that user B intends to communicate with user A (Fig- ure 20.2) and both of them have previously established a Mist circuit with LighthouseB and LighthouseA, respectively. User B transmits to his Light- house a packet indicating that he wants to set up a connection with user A. The LighthouseB verifies that the originator of the message is user B, locates the Lighthouse of user A and performs the initialization procedure for connection establishment with the Lighthouse of user B. As soon as the communication path is established, user A and user B are able to communicate. Note that the intermediate routers are unaware of the two ends of the communication. Moreover, it is impossible for user B to determine the location of user A and vice versa. C 2009 by Taylor & Francis Group, LLC Anonymity in SIP User binding table Downward HandleID User A's User C lighthouse User A User B Mist routing table Upward Downward HandleID HandleID Mist routing table Upward Downward HandleID HandleID Portal's presence table User A's Upward Downward portal HandleID Anon-1 Anon-2 User A FIGURE 20.2 The Mist circuit e