Host-based IP Mobility Management Protocols

2.2.1 Overview

The IP host-based mobility management protocols enable mobile nodes to remain reachable, while roaming in the Internet, with the mobile nodes being involved in managing their mobility. The mobile nodes are, therefore, aware of their mobility; and they need to perform mobility management operations, in order to maintain their reachability. Because of their participation in mobility management, these protocols require a modification of the mobile nodes‟ IP stack, so that they can handle mobility-related signalling. Examples of the host-based IP mobility management protocols include Mobile IPv6 and its extensions, such as Fast MIPv6 [38], Hierarchical MIPv6 [39], Dual stack MIPv6 [18].

The basic principles of operations of the host-based mobility management protocols in IPv6 networks are derived from MIPv6. In the following subsection, therefore, these principles and the mobility challenges of these protocols, in regard to deploying a centralized mobility anchor to handle mobility for all mobile nodes, are discussed, in relation to MIPv6. The goal is to review these mobility management protocols, and to show their limitations in dealing with the current ever increasing number of mobile nodes and the increased data traffic volume. The efforts toward enhancing MIPv6 to overcome such limitations are presented, and the extension of

MIPv6 with distributed mobility management to solve these limitations is discussed in Section 2.4.

2.2.2 Protocol Descriptions

Mobile IPv6 provides mobility support to mobile nodes, which is transparent to the higher layers (i.e. transport and application layers), while the mobile node is roaming across the IPv6 networks. The protocol uses the principle of distinguishing the two roles of IP address (identifier and locator) to provide mobility support for the mobile node. As discussed previously, the mobile node is, therefore, supplied with two IP addresses: the HoA and CoA. The HoA serves as a stable end point identifier to identify a mobile node; and it is obtained from a mobility anchor in the mobile node‟s home network.

As an identifier, the HoA remains unchanged, irrespective of where the mobile node roams in the Internet. The CoA is the temporary address the MN obtains from the network that is not its home network; and this is known as the visited network. This address serves as the locator to indicate the actual point of attachment of the MN to the Internet. So, it changes each time the MN moves to a different network. Moreover, the mobility agents use the CoA to route packets to the MN‟s current location.

The protocol introduces a special router in the MN‟s home network; this is called a Home Agent (HA). The HA allocates the HoA to MNs, and maintains the binding information between the HoA and the CoA of the MNs, allowing for the reachability of the MNs while away from their home networks. To achieve this, the binding information needs to be up-to-date. So, each time an MN changes a visited network, the MN needs to exchange binding update (BU) and binding acknowledgement (BA) messages, with its HA to update its binding information.

The binding update messages enable the HA to know the current location of the MN. The HA updates/creates the binding entry for the MN in a special table known as the binding cache; and it establishes an IP bidirectional tunnel between itself and the MN. Thereafter, the packets from CN(s) to the MN are intercepted by this HA, which then uses its binding cache entry to locate the MN (i.e., MN‟s CoA) and then encapsulates the packets in an IPv6-in-IPv6 tunnel with the destination address of the MN‟s CoA. Figure 2-1 summarises the basic principles of operation of MIPv6.

HoA1 -> CoA1 HoA1->CoA2 MN MN HoA1 CoA1 HoA1 CoA2 movement AR1 AR2 location bindi ng update signaling

HA’s binding cache HA

CN

IPv6-in-IPv6 tunnel

Internet Visited network1 Visited network2 Home network Signaling messages Data packets AR1 AR2

Figure 2-1 Mobility management in MIPv6 without route optimization

As shown in Figure 2-1, the HA maintains the binding information about the MN‟s current location in the binding cache; it intercepts all packets sent from/to the CN, and encapsulates/decapsulates them in IPv6-in-IPv6 tunnel to/from the MN, while the MN is moving around in the visited networks. The MN configures a new CoA, when it changes the visited network; and it informs these changes to its HA through binding updates signalling. The HA then updates the MN‟s entry in the binding cache. The CN communicates with the MN, using the HoA, regardless of the visited network that the MN is currently visiting. This hides the MN movement from the CN. All the MNs‟ data use a tunnel between the MN and the HA.

2.2.3 MIPv6 Limitations

In MIPv6, the MN communicates with its CNs by using its HoA, which is topologically anchored to its HA. As a result, all the packets sent to MN are firstly sent to HA, and then tunnelled to the MN. The packets, therefore, encounter a sub-optimal path, called triangular routing [40]. The triangular routing causes a substantial communication delay, especially when the MN moves topologically far away from its home network; and this results in the inefficient use of the resources in the backhaul of the communication network [40][22][14].

In order to overcome the triangular routing problems, the basic specification of MIPv6 protocol specifies the route optimization; and this is called the Return Routability Procedure (RRP) [20]. The RRP allows the MN to inform its CN(s) of its current location; a direct routing

path between the MN and the CN(s) is then established; and the data packets are then exchanged through this path, thereby bypassing the HA. This reduces the communication delay and the data traffic load on the HA. However, the MN reveals its temporary identifier, CoA, to the CN(s), which infringes the location privacy of the MN [41]. Furthermore, to make the route optimization feasible, the CN(s) must support the RRP; but it is impossible to have all the IPv6 nodes employing RRP. Consequently, legacy IPv6 nodes cannot utilize the benefit of route optimization; and all the data traffic for these nodes will traverse the HA, irrespective of the route optimization.

Consequently, the specified route optimization is limited only to scenarios, where the CN(s) supports the RRP. The RRP, however, still requires the involvement of HA for security reasons; and that further makes the HA a single point of failure. Moreover, it causes a significant signalling load, especially when a huge number of MNs are communicating with various CN(s), and opt to perform route optimization.

The MNs binding information in MIPv6 are stored on the HA, but considering the anticipated growth in the number of mobile nodes and the traffic volume [4], the HA cannot scale well with such increase; as it needs to maintain mobility context, and it manages the tunnel for large numbers of MNs. Moreover, the HA is a single point of failure. This is because if the HA is unavailable, the communication for all MNs is interrupted. Furthermore, if the route optimization is favoured, the RRP cannot be accomplished.

Besides these limitations, the MIPv6 requires a modification of the MN‟s IP stack, in order for the MN to support its own mobility, which increases complexity of the MN. Moreover, the signalling exchanges and the data tunnelling over the wireless link do not promise well in terms of spectrum usage, given the current rapid growth in the number of MNs. This is because the wireless spectrum is limited, and cannot be increased [3]; and hence, it needs to be used with great care.

2.2.4 MIPv6 Extension

The host-based mobility management protocol defined in IETF has been Mobile IPv6. Numerous extensions have been standardized in IETF, in order to enhance the performance of MIPv6 in solving some of the mentioned limitations. To enhance the performance of MIPv6 in

terms of signalling overhead to the HA, Hierarchical MIPv6 (HMIPv6) [39] was developed. HMIPv6 introduces a Mobility Anchor Point (MAP), a local HA, which limits the signalling exchange between the MAP and the MN, while the MN moves within the MAP domain. This approach brings the anchor (MAP) close to the MN, which reduces binding update delay, signalling overhead to the HA, and the handover latency. Nevertheless, HMIPv6 still requires modification of the MN IP stack, and the packets for legacy IPv6 nodes still encounter triangular routing. Moreover, HMIPv6 still inherits the HA centralization problems that include a single point of failure.

Another improvement to MIPv6 is the Mobile IPv6 Fast handover (FMIPv6) [38] designed to mitigate the handover delay and packet loss due to MIPv6 handover procedures. FMIPv6 uses predictive and reactive handover mechanisms. The predictive mechanism enables the MN to set up IP connectivity prior to actual handover and establishment of the tunnel between the previous and new access routers of the MN. This approach reduces/removes the time needed for IP CoA configuration; hence, it reduces the handover delay, and the tunnel created further reduces the packet loss. Yet, the MN IP stack modification is still needed. Moreover, the HA remains a traffic anchoring point and a manager for all MNs‟ binding information.

Both approaches, HMIPv6 and FMIPv6, require support from the MNs in mobility management operation. As a result, these extensions still make the MNs complex and difficult to deploy, because all the existing IPv6 nodes need change in their IP stack. Therefore, these protocols still inherit most of the limitations of MIPv6.