pre-call operation and (2) the mid-call operation. The pre-call operation is per- formed when a session is not established yet; the mid-call operation is performed when a session has been established. On the other hand, the handoff operation of Mobile IP is a "binding update," i.e., the process of re-registering a new CoA with the corresponding home agent (HA). 4.3.4 Data Delivery According to the SIP proposal, data delivery between MNs and CNs is estab- lished by the INVITE method. No matter where MNs or CNs are located, they can determine each other's current locations by the CONTACT field of an INVITE request. Once the session is established successfully, data are exchanged directly between MNs and CNs. On the contrary, MNs using Mobile IPv4 must re-register their new CoAs by sending Binding Updates (BU) back to their home agents (HA). Then, the Mobile IPv4 is able to tunnel data from HA to foreign agents (FA). In order to improve performance, Mobile IPv6 not only sends the BU message to the corresponding HA but also sends BU to all communicating CNs. Therefore, data delivery using Mobile IPv6 can become direct after a short time of tunneling. 4.3.5 Mobility Support The last difference between two protocols is that Mobile IP is capable of handling IP mobility only, including host mobility and network mobility. On C 2009 by Taylor & Francis Group, LLC SIP Mobility Management TABLE 4.2 SIP-NEMO VS. MIPv6-NEMO SIP-NEMO MIPv6-NEMO Signaling Mechanism Off-plane In-plane Signaling Overhead Improved Light Header overhead Heavy Involved Entities SIP-HS, SIP-NMS, SIP-FS HA and MR Transmission Route Optimal Sub-optimal Handoff Latency Determined by CNs Determined by HA Nesting Impact Light Heavy the contrary, SIP possesses a flexible addressing mechanism and provides a dynamic name mapping and redirect service. In addition to IP mobility intro- duced in this chapter, SIP is also able to support several other types of mobil- ity. For example, SIP can support session mobility, which is the ability of a user to keep ongoin