Explain in detail the problem facing the deployment of MIPv6 in the future wireless Internet and how HMIPv6 can tackle these problems by providing fast handover support.
Background
The next generation of mobile/wireless networks are expected to be all IP based, relying exclusively on IP, from mobile station to the gateway towards the Internet. IP was not designed with mobility in mind. However, in the future all-IP based wireless networks are expected to provide service continuity while moving through different heterogeneous wireless systems (e.g 3G and WLANs). To address the issue of mobility at IP layer, IETF has proposed Mobile IPv4 [1] and Mobile IPv6 [2]. The focus is on Mobile IPv6 as it will be the basis of all future IP networks.
Problem Description
With Mobile IPv6 (MIPv6), each time a mobile node (MN) moves from one subnet to another, it gets a new address called care-of-address (CoA). The association between the permanent home address (HoA) and the CoA is known as a binding and the MN registers it with the home agent (HA) using binding update (BU) messages (association of addresses plus the association lifetime). The HA records this binding in its binding cache and intercepts any packets addressed to the MNs home address and tunnels them to MNs CoA. In MIPv6, HAs no longer exclusively deal with the address mapping, but each correspondent node (CN) can have its own binding cache where a binding update is stored. This enables route optimisation compared to triangular routing via the HA and the CN can directly send packets to the MN [2]. These BU’s are sent every time a MN changes its point of attachment to the Internet. Even if the MN doesn’t change its point of attachment, BUs are sent periodically as they have an associated lifetime.
It is a well-known observation that MNs moving frequently as well as far away from their respective home domain or CNs produce significant BU signaling traffic. As per one of the studies [3], 69 % of user mobility is within its home site (within its building or campus). The problem with standard MIPv6 is that it handles local as well as global mobility identically irrespective of the user mobility pattern [4]. This approach is not scalable since the generated signaling load can become quite overwhelming as the number of MN increase in the Internet. Secondly, when a MN changes its point of attachment, it cannot receive IP packets at its new point of attachment until the handover ends. This time includes the new prefix discovery on the new subnet, the new CoA establishment and the time needed to send BU messages. This time is called handover/handoff latency [5] and can be too long for real time applications (e.g VoIP). So we have two problems; one of high signaling traffic and the other of handover latency and consequently packet losses.
IP Micromobility Protocols
Micromobility protocols aim to handle local movement of mobile hosts within a domain without interaction with the Mobile IP enabled Internet. This has the benefit of reduced signaling load experienced by the network by eliminating BUs between MNs and possibly distant HAs and CNs when the MN remain inside their local domain. To minimise poor performance during handover, Micromobility protocols support fast, seamless, local mobility [6]. However to manage the movement of MNs between different domains and across the Internet (macromobility), standard Mobile IP is used. Over the last few years, different micromobility protocols such as Cellular IP, Hawaii and Hierarchical Mobile IP have been proposed. A comparison of different micromobility protocols has been carried out in [6,7] based on different metrics. It should be noted that micromobility protocols are complementary rather than competitive to Mobile IP.
Hierarchical Mobile IPv6
Hierarchical Mobile IPv6 (HMIPv6) is an extension to MIPv6 to support the micromobility approach. It aims to reduce the signaling load due to user mobility inside a local domain. These domains are independent from subnets and are generally managed by a unique administrative authority (e.g a campus). Each domain is connected to the rest of Internet by a Mobility Anchor Point (MAP). The MAP is an Access Router (AR) with a publicly routable IP address at the top of several ARs (figure 2). The whole concept of HMIPv6 is based on bringing the functionality of HA to this MAP. The MAP basically acts as a local HA.
When a MN enters into a new MAP domain, it gets two CoAs; a Regional care-of-address (RCoA) which is the address on MAP’s subnet and an on-link care-of-address (LCoA) which is the address of the link it is attached [8]. The RCoA is the address that MN will use to inform it’s HA and CNs about its current location. The MAP will receive all the packets on behalf of the MN and tunnels them directly to the MNs LC
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