Praise for Fundamentals of WiMAX

7.4 Mobility Management 255moved to a new IP subnet. Mobile IP is also transparent to the network in the sense that the routingprotocols or routers need not be changed.7.4.3.1 ComponentsFigure 7.14 shows the basic components of mobile IP. The MIP client is implemented in the terminalthat is moving (MS in WiMAX) and is referred to as the mobile node (MN). The IP hostwith which the MN is communicating is called the correspondent node (CN). Mobile IP definestwo addresses for each MN. The first address, the address issued to the MN by its home network,is called the home address (HoA). This IP address can be thought of as identifying the mobile tothe IP network. The second address, the care-of address (CoA), is a temporary IP address that isassigned to the MN by the visited network. This IP address can be thought of as providing informationabout the current logical location of the MN.In order to manage mobility, dynamic mapping is needed between the fixed identifier IPaddress and the CoA. This need is met through the use a mobility agent, the home agent (HA),located in the home network, working with another mobility agent, the foreign agent (FA), locatedin the visited network. Both of these mobility agents can be thought of as specialized routers.There is also the option of colocating the FA with the MN itself; this scenario is referred toas a colocated foreign agent. The CoA of the MN is the address of the FA. Whenever the MNmoves away from the home network to a visited network, this movement is detected through theuse of location-discovery protocols that are based on extensions to ICMP (Internet control messageprotocol) router discovery protocol [18]. Mobility agents advertise their presence to enablediscovery by the MN. Once in a visited network, the MN obtains a new address and sends anupdate message to the HA, informing it of the address of the new FA. This update registrationcan be done directly by the MN for a colocated CoA or is relayed by the FA if the visited subnetFA address is used as the CoA.Once the HA is updated with the new CoA, all packets destined to the MN that arrive at thehome network are forwarded to the appropriate FA CoA by encapsulating them in a tunnelingprotocol. IP-in-IP encapsulation as defined in RFC 2003 [45] is used for this tunneling. Minimalencapsulation (RFC 2004) [47] or GRE (generic routing encapsulation) tunneling (RFC 1701)[56] may optionally be supported as well. The FA decapsulates the packets and delivers them tothe MN. By having the HA act as the anchor point for all packets destined to the MN, mobile IPis able to deliver all packets to the MN regardless of its location.Mobile IP is required only for delivering packets destined to the MN. Packets from the MNcan be carried directly without the need for mobile IP, except, of course, if the CN is also mobile,in which case it will have to go through the HA of the CN.Clearly, packets destined for the MN take a different path from those originating from theMN. This triangular routing is illustrated in Figure 7.15. Triangular routing causes some problemsand is one of the key limitations of mobile IP.