 from its home network to other networks, it needs a certain mechanism to let it use the same IP address for communication. In order to solve this problem, the IETF addressed host mobility by Mobile IP (Perkins 2000, 2002; Johnson, Perkins, and Arkko 2004), which is a network-layer protocol. Based on the Mobile IP proposal, each node has its own and unique IP address called home address (HoA) in its home network. Once it leaves its home network, it acquires a new IP address © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Correspondence Node (CN) 4. Invite 5. Invite server server 6. Moved temporarily 7. Invite 1. Register 3. Re-register Mobile Mobile Node (MN) 2. Move Node (MN) Visited network Home network FIGURE 4.1 The pre-call operation. called care-of-address (CoA). The node should register its new CoA to its home network. Therefore, the home network is able to tunnel those packets destined to this node to the current CoA. Although Mobile IP can handle host mobility and hide the host mobility issue from upper layers, it would induce an indirect routing path and require protocol modification of each node. Past works (Banerjee, Basu, and Das 2003) have discussed and improved the fact that SIP has the built-in characteristics for solving the host mobil- ity problem. According to the handoff situation, SIP supports two kinds of handoff operations which are (1) the pre-call operation and (2) the mid-call operation. 4.1.1 Pre-Call Operation Pre-call means that a mobile node (MN) moves to a new visited network before a session is established. The pre-call operation utilizes the SIP REGIS- TER method to resolve the MNs' reachability. Figure 4.1 depicts the pre-call handoff operation. An MN is supposed to be located in its home network. The detailed steps are described as follows: 1. The MN must register itself with its SIP server using the SIP REGIS- TER method. © 2009 by Taylor & Francis Group, LLC SIP Mobility Management 2. The MN roams to another network and acquires a new c