st routers is extremely difficult since the routers are unaware of each other's identity. It should be noted also that the architecture is applicable to a variety of network environments since it uses a general purpose protocol that can enable privacy on IP networks despite the underlying technology (i.e., Ethernet-based fixed networks, WLANs, 2/3G Mobile systems). In short, the Mist architecture consists of a number of routers, called "Mist routers" ordered in a hierarchical structure. A typical structure is shown in Figure 20.1. The leaf nodes of hierarchy are called Portals. Portals are the gateways that interface the virtual world with the physical one. In other words, they are connection points where users can connect to the Mist system. Let us assume that user A requires a network connection that ensures privacy and data confidentiality. User A has to register himself to the Mist system. His mobile device locates and interfaces directly with one of the available portals in the surrounding physical space. The portal, upon receiving a registration request, replies with a list of its ancestral Mist routers that exist at a higher level within the Mist hierarchy and are willing to act as a Lighthouse for the user. A Lighthouse is a Mist router that acts as a point of contact for © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security user A. Users that intend to communicate with user A have to contact his Lighthouse. Following Lighthouse selection, a virtual circuit (Mist circuit) must be established between user A and the corresponding Lighthouse. This process, called "Mist Circuit Establishment," aims to entitle user A Lighthouse to authenticate user A without revealing his/her physical location while hiding, at the same time, from the portal, user's identity and the designated Light- house. Furthermore, the Mist circuit applies a hop-to-hop handle-based rout- ing technique for packet transmission between source and destination nodes and in combination with data encryption mana