The protocols related to IP networks, such as the Internet, are developed by the Internet Engineering Task Force (IETF), which has been developing support for mobile IP nodes for both versions of IP (i.e. for IPv4 and IPv6). The main results of this work are the two Mobile IP protocols, Mobile IPv4 (RFC 2002) and Mobile IPv6 (work in progress, assumed to reach RFC status soon).
In both versions of Mobile IP, the packets sent to the so-called home address, which is the permanent address of the mobile node, are forwarded to the mobile node when it is not located at its home address. An element called a home agent is located on the link within which the home address of the mobile node is located, i.e. on the so-called home link. The home agent captures all IP packets sent to the home address of the mobile node while the mobile node is not located on the home link, and forwards them to the current IP address of the mobile node, called the care-of address. In Mobile IPv4, the forwarding process utilizes so-called IP-in-IP tunneling, in which encapsulated packets are forwarded to the mobile node. Thus the destination address in the outer IP header of this IP-in-IP tunneling is the care-of address, while the destination address in the inner IP header is the home address of the mobile node.
In Mobile IPv6, the mobile node signals a change in its care-of address to the home agent by sending to the home agent the new care-of address in a message called a Binding Update. The home agent acknowledges this message by returning to the mobile node a message called a Binding Acknowledgement. In Mobile IPv4, the corresponding messages are the Registration Request sent from the mobile node to the home agent and the Registration Reply sent in the opposite direction.
When a mobile node moves from one sub-network to another, a hand-off (also termed a handover) procedure takes place so that the mobile node can maintain connectivity during the movement. In order to detect the movement, mobile nodes typically rely on so-called agent advertisements broadcast by routers residing in the network. Each router multicasts agent advertisements periodically, whereby the mobile nodes may discover their neighboring routers, and thus also the care-of addresses available, simply by listening for the agent advertisements. However, if a mobile node wishes to obtain a care-of address immediately without waiting for a periodic agent advertisement, it may also broadcast or multicast a so-called agent solicitation message. The agent advertisements and agent solicitations are also commonly termed router advertisements and router solicitations, respectively.
The occurrence of a Mobile IP hand-off procedure is a consequence of a link layer hand-off procedure between two sub-networks. The overall hand-off process may be divided into three phases: the hand-off at the link layer, the detection of movement, and the above-mentioned registration of the new care-of address. The detection of movement here refers to the mechanism by which the mobile node detects that it has moved away from the coverage area of the access router it is still registered with. Although the Mobile IP hand-off is needed to maintain connectivity as a mobile node moves within the network, the impact of latency associated with the above phases is significant and may disrupt an on-going real-time service. Efficient solutions are thus needed for future wireless IP networks to reduce the overall hand-off latency.
As to the MIP protocols, the crucial components of the hand-off latency are thus the delay associated with the detection of movement and the delay associated with the registration of a new care-of address. The latter is mainly due to the end-to-end delay introduced by the registration messages and it may be reduced by so-called micro-mobility architectures in which local mobility gateways are utilized to manage mobility. In this way, the hand-offs may be divided into two categories: global hand-offs, in which a mobile node registers its care-of address with the home agent and local hand-offs, in which local mobility gateways handle mobility in a local network. When a mobile node associates itself for the first time with a local network, a global hand-off is performed. However, when the same mobile node associates with a different access router within the same local network, a local hand-off is performed. In a local hand-off, the local mobility gateway caches the new care-of address, also known as secondary care-of address, and maps it to the primary care-of address obtained in the above-mentioned global hand-off, i.e. when the mobile node first entered the local network. Thus, in a micro-mobility architecture, the migration of the mobile node is hidden from the rest of the network when it occurs in the local network.
The second crucial delay component is the delay associated with movement detection at the IP layer, i.e. the time period it takes the mobile node to detect that it is no more in the coverage area of the router that is still the serving router for the node (i.e. the router providing connectivity for the node). Different algorithms have been developed for this purpose. In the following, the different movement detection methods are discussed briefly.
In general, movement detection methods may be classified into advertisement based methods and hint based methods. Advertisement based movement detection methods require the above-mentioned agent advertisements to determine the position of the mobile node, while hint based methods rely on so-called hand-off hints, i.e. information communicated from the link layer to the Mobile IP sub-layer when a link layer hand-off is performed.
The advertisement based movement detection methods include methods that are commonly called as Lazy Cell Switching (LCS), Early Cell Switching (EyCS) and Eager Cell Switching (ErCS). In the LCS method, a hand-off is avoided until it is absolutely necessary. Any indication of movement is ignored until contact with the serving router (i.e. agent) is lost for the duration of three advertisement periods. If this happens, the mobile node may attempt to associate itself with a new router. In the EyCS method, the mobile node attempts to associate itself with a new router immediately upon discovering an advertisement from a new router. The ErCS method is similar to the EyCS method, except that hand-offs may not be performed more frequently than once in a second.
In a hint-based movement detection algorithm, also known as Hinted Cell Switching (HCS), the link layer triggers the mobile node to broadcast a router solicitation that in turn forces all adjacent routers to respond with a unicast router advertisement. A drawback of this method is that in environments where a large number of roaming nodes broadcast solicitations the throughput of the respective radio link may drop significantly. To overcome this drawback, an enhanced algorithm known as Fast Hinted Cell Switching (FHCS) determines the identity of the potential router through link layer hints. In this method, the link layer thus provides the MIP layer with information like the IP and MAC addresses of the new router. This mechanism does not need any MIP mechanisms for movement detection or router selection, i.e. the mobile node can bypass the broadcast of the solicitation and proceed directly to the registration phase.
As is obvious from the above discussion, a drawback related to the LCS method is the rather high movement detection delay. However, an advantage of the LCS method is its stable and predictable behavior in overlapping regions where the coverage areas of different routers overlap and where the mobile node may thus receive advertisements from two or more routers.
By means of the EyCS and the ErCS methods the movement detection delay may be considerably reduced. However, these methods suffer from so-called registration oscillations, i.e. in an overlapping region the registration of the mobile node tends to oscillate among the routers involved. Due to the oscillation, data packets are periodically sent through different paths (routers), which may cause severe problems if the end-to-end delays of the different paths are clearly different.
The hint based methods do not suffer from the above drawbacks, since they are independent of router advertisements. Experimental results suggest that the hint based methods significantly improve the hand-off performance as compared to the traditional advertisement based methods. However, the hint based methods incapacitate one uniqueness of the IP, i.e. its transparency to bearer technologies. Furthermore, optimum performance of the hint based methods is limited to homogeneous environments only, where a single access technology plays a dominant role. Unfortunately, future networks do not seem to be developing to that direction. On the contrary, with an increasing number of technologies used for networking, future access networks will most likely be more heterogeneous.
The present invention seeks to eliminate or alleviate the above drawbacks related to MIP hand-offs.