onents, (2) re-registration, (3) re-invitation and (4) route optimization. 4.2.1 System Components Figure 4.3 depicts the architecture of the SIP-NEMO. In order to apply SIP into the network mobility environment, three types of the SIP Back-To-Back User Agent (B2BUA) which are: (1) SIP Home Server (SIP-HS), (2) SIP Foreign Server (SIP-FS) and (3) SIP Network Mobility Server (SIP-NMS) are defined and configured over the Internet. Correspondence Node (CN) server server Visited network Home network SIP Home Server (SIP-HS) SIP Foreign SIP Foreign Server (SIP-FS) Server (SIP-FS) SIP Network Mobility Server (SIP-NMS) Mobile Node (MN) Mobile network FIGURE 4.3 The SIP-NEMO architecture. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security A SIP Network Mobility Server (SIP-NMS) is the application-layer gateway (ALG) of a mobile network and has the capability of routing between its attached network and its carried mobile network. In other words, the SIP- NMS is the central point of a mobile network and responsible for managing all traffic to and from the mobile network. A mobile network can consist of nested subnets, i.e., a SIP-NMS can be attached to another SIP-NMS. For convenience, the upstream SIP-NMS is called "parent-SIPNMS" and the downstream SIP-NMS is called "sub-SIP-NMS." The SIP-NMS at the top level of nesting is called "root-SIP-NMS." Each SIP-NMS has one corresponding SIP Home Server (SIP-HS) in its home network. A SIP-HS takes charge of recording the current location of the registered SIP-NMSs. When each SIP-NMS gets a new point of attachment, it should re-register its new contact address to the corresponding SIP-HS like the pre-call operation of the SIP-based host mobility. In addition, since mobile network may form a complex levels of nesting, the SIP-HS can help to determine routing paths inside a nested mobile network. A lot of SIP Foreign Servers (SIP-FSs) distributed in the Internet play the role of providing a URI-list service. When a SIP-NMS roams over the Intern