troller (RNC, functioning as the base station controllers of second generation (2G) or 2.5G cellular networks) mobility, and the GGSN manages inter-SGSN mobil- ity. When the serving RNC (i.e., the one to which the MH/UE is attached) changes and it is within the scope of the same SGSN, a redirection of the GTP C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security tunnel takes place between the SGSN and the RNC. This changeover takes place in the subnetwork layer, and the solution works fine until the MH/UE leaves the GPRS or UMTS network. This type of mobility management mech- anism has its own location update, registration, and paging scheme for main- taining connection to the MH/UE. However, seamless mobility between the GPRS/UMTS network and other data networks still remains elusive. 17.4.2 Network-Layer Mobility The value of network-layer mobility is open to question with the wide deploy- ment and popularity of Dynamic Host Configuration Protocol (DHCP) [10], Post Office Protocol (POP3) [11], roaming user profile support for Web brow- sers through Lightweight Directory Access Protocol (LDAP) [12], and the general move to client-server applications where the MH initiates communi- cation. However, to-date the network layer remains the most dominant of all the layers in the protocol stack for mobility protocols. As mentioned earlier, network-layer mobility protocols can be broadly classified into macro- and micro-mobility protocols depending on the scope of their operation. 1) Macro-mobility Protocols: Macro-mobility refers to user mobility that is infrequent and also spans considerable space, often between several adminis- trative domains. We have chosen Mobile IP (in short, MIP) as the representa- tive network layer (or macro) mobility solution since it has been standardized by the Internet Engineering Task Force (IETF). MIP was proposed with the objective of supporting mobile users with appli- cation transparency and the possibility of seamless roaming. This was done by addin