change directly between MNs and CNs. Furthermore, SIP is an application- layer protocol. Any SIP-compatible node can possess the mobile capabil- ity without any lower-layer modification. The advanced comparison between Mobile IP and SIP is presented in Section 4.3. 4.2 SIP-Based Network Mobility Network mobility occurs when a group of nodes moves as a unit over the Internet. For example, users take some transportation carriage, e.g., a bus, a train or an aircraft. The transportation carriage can be regarded as a movable network that is made up of a set of nodes and called "mobile network." In con- trast to host mobility in which each node manages its movement individually, there is at least one central point inside a mobile network for managing the movement of the entire mobile network. In order to resolve this problem, the IETF addressed network mobility by NEMO (Lach, Janneteau, and Petrescu 2003; Devarapalli et al. 2005; Ernst 2005), which is an extension of Mobile IP. According to the NEMO basic support protocol (BSP), the central point of each mobile network is called "mobile router" (MR) and operates the Mobile IP mechanism. In other words, when an MR leaves its home network and gets a new CoA, it would re-register its new CoA for keeping the correct mapping between the HoA and the CoA. Once the re-registration process is finished, a bi-directional tunnel would be established between the MR and its home net- work. All traffic to or from the mobile network must pass through the tunnel. Hence, the tunnel of the NEMO BSP would reinforce the sub-optimal route problem of Mobile IP (Ng et al. 2006a; Ng et al. 2006b). A recent work (Huang and Lee 2006; Huang, Lee, and Zheng 2006) has extended SIP to support network mobility. Since the proposed SIP-based net- work mobility (SIP-NEMO) inherits from the SIP, it can avoid the drawbacks © 2009 by Taylor & Francis Group, LLC SIP Mobility Management due to the bi-directional tunnel. In the following statements, the SIP-NEMO would be presented in terms of: (1) system comp