Praise for Fundamentals of WiMAX

256 Chapter 7 • Networking and Services Aspects of Broadband WirelessCorrespondentNode (CN)HomeAgent (HA)ForeignAgent (FA)Home NetworkInternet/ IP NetworkVisited NetworkMobile Node (MN)Figure 7.14 Mobile IP componentsMobile Node (MN)7.4.3.2 Limitations and Work-aroundsMobile IP has a number of limitations, most stemming from the use of triangular routing, thatmake it a suboptimal mobility-management solution. These limitations are as follows:• Inefficient routing. Triangular routing can be extremely wasteful if the mobile roams to alocation that is topologically far away from the home network. For example, if a personwith a U.S. home network were to access a Web site in Korea while in Korea, packets fromthe Web site in Korea would have to go the U.S. home agent before being tunneled backand delivered to the user in Korea. One proposed solution to this problem is the optionalextension to mobile IP, called route optimization, which allows the CN to send packetsdirectly to the MN, in response to a binding update from the HA, informing the CN of theMN’s new CoA. Implementing route optimization, however, requires changes to the CN’sprotocol stack, which is not practical in many scenarios.• Ingress filtering issues. Many firewalls do not allow packets coming from a topologicallyincorrect source address. Since the MN uses its home address as its source address evenwhen in a visited network, firewalls in the visited network may discard packets from theMN. A solution to this problem is a technique called reverse tunneling, which is anotheroptional extension to mobile IP [40]. This solution requires that the MN establish a tunnelfrom the CoA to the HA, where it can be decapsulated and forwarded to the correct CN.(Figure 7.16).• Private address issues. Mobile IP does not allow for the use of private addressing usingnetwork address translation (NAT), wherein one public IP address is shared by manynodes using different port numbers. Since packets are tunneled from the HA (and CN, inthe case of route optimization) using IP-in-IP encapsulation to the MN’s publicly routable