 assumed that the MN acquires a local address at the visited domain, known as the Care-of Address (CoA). A binding between the MN's home address and CoA is then established, SO that the MN's identity continues to be specified by its home address, but its current location is specified by its CoA for routing purposes. To support the registration of this binding and the routing of packets to the MN using this binding, MIP requires the deployment of two types of mobility support agents in IP domains associated with a MN: a home agent (HA) at the MN's home domain, and a foreign agent (FA) in each foreign network domain that the MN is visiting. In the visited domain, the FA relays the binding registration from the MN to the HA at the MN's home domain. The HA authenticates the MN, acknowledges the positive authentication to the FA, and stores the binding in a cache SO that incoming packets addressed to the MN can be forwarded to the MN's current CoA by encapsulating them with the CoA as the destination address and tunnelling the packets to the FA. The FA in turn caches the binding SO that packets arriving from the HA via the tunnel can be forwarded to the MN. Thus, packets sent by a CN to the MN undergo triangular routing via the MN's HA, but outgoing packets from the MN to a CN are sent directly to the CN. Usually, the address of the FA also serves as the CoA of the visiting MN, although the MN is allowed to serve as its own FA if it is able to independently obtain a CoA via other © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security means, such as using Dynamic Host Configuration Protocol (DHCP) or IPv6 address auto-configuration. To avoid stale information, binding registrations and binding caches are soft states that need to be renewed before the timers for the respective information expire and the information is deleted. To reduce the number of registration messages, MIP has been extended with paging functionality to reduce the signalling cost for the address update [9]. Furthermo