Mobile Inter-networking with IPv6 : Concepts, Principles and Practices

11

Movement Detection

Never confuse movement with action.    –Ernest Hemingway

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Movement detection is one of the crucial events that affects the performance of handover. Yet, it is not easy to do it quickly and reliably for several reasons. First, movement detection at the IP layer is dependent on reliably detecting the movement at the lower layer, namely the link layer. In wireless technologies, a link may be lost and regained, which does not mean movement. However, the IP operations are typically performed anyway when a link "comes up." Movement may actually happen from one link to another, for instance from one WLAN access point to another. However, that may not constitute movement to a new subnet. The challenge is to determine when a subnet change has actually taken place; this requires performing at least some operations, and these operations inevitably introduce delay. Second, multiple prefixes may be advertised by the same router on a single link, for instance to balance the traffic load on the network. So, hearing a new prefix does not mean a new subnet. Third, a router may use the same link-local address on multiple interfaces but advertise different prefixes. In this case, the mobile node needs to perform an IP handover, but it may not recognize that the router is still reachable at the previous address. Finally, there may be multiple routers on the same link advertising ...