ain" handoffs by localizing the address updates within the foreign network. According to their modes of operation, these solutions can be classified into tunnel-based management [12,13] and routing-based man- agement [14,15] schemes. In tunnel-based schemes, a number of registration agents are distributed hierarchically in the foreign network and data packets are tunnelled between the agents at different levels. Whenever the MN roams between two subnets in the visited domain, it will register its new CoA with the nearest common agents instead of its HA, which needs to be informed only about the network domain that the MN is visiting. In routing-based schemes, the MN will keep the same CoA address in the visited domain, which modifies the routing tables of its routers to follow the current location of the MN. Generally, network layer solutions can provide universal mobility manage- ment over IP networks. However, this approach has two major drawbacks. First, it requires establishment of mobility support agents in all network C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks domains that support host mobility. Second, for route optimization, it also requires CNs to implement the route optimization scheme and support tun- nelling at the IP layer. The design of MIP assumes that each MN is assigned a static IP address by its home network, which is globally known by all CNs that wish to communicate with it. In IPv4 networks, due to shortage of IP addresses, MNs are most likely to be assigned temporary IP addresses via DHCP. Thus, the design premise of MIP tends not to be met in practice and the usefulness of MIP is therefore questionable. 11.2.2 Transport Layer Solutions Transport layer solutions follow the end-to-end principle [16] that makes the Internet such a useful and flexible network: anything that can be done in the end systems should be done there and the network layer should concentrate on efficient routing of packets between the end systems. Since the transpor