by Asela Bandara

Mobile Computing is becoming increasingly important due to the rise in the number of portable computers and the desire to have continuous network connectivity to the Internet irrespective of their physical location. The introduction of new mobile communication devices with improved networking and processing performance has convinced that requirement further-up to accommodate their communication needs. The current IT trend stated that the solution would be easily interoperable to the existing Internet topology, to incorporate the mobile devices with stationary device to have a single Internet which allows everyone to be connected from everywhere at any time.

Figure 1

The Internet infrastructure is built on top of a collection of protocols, called the TCP/IP protocol suite. IP requires the location of any host connected to the Internet to be uniquely identified by an assigned IP address. This raises one of the most important issues in mobility, because when a host moves to another physical location, it has to change its IP address dynamically. The Mobile Internet Protocol (Mobile IP) is an extension to the Internet Protocol that addresses this issue.

Mobile IP design Concerns

The IP address of a host consists of two parts:

1) The higher order bits of the address determine the network on which the host resides.

2) The remaining low-order bits determine the host number.

IP decides the next-hop by determining the network information from the destination IP address of the packet.

Figure 2

Thus, while trying to support mobility on the Internet under the existing protocol suite, it focused on two mutually conflicting requirements:

(1) A mobile node has to change its IP address whenever it changes its point of attachment (so that packets destined to the node are routed correctly)

(2) To maintain existing TCP connections (the mobile node has to keep its IP address the same)

Protocol Remarks

Mobile IP is designed to solve the aforementioned problem by allowing each mobile node to have two IP addresses and by transparently maintaining the binding between the two addresses.

1. Permanent home address: assigned at the home network and is used to identify communication endpoints.

2. Temporary care-of address: represents the current location of the host.

Here the IP packet header contains the agents’ source, destination pair while its’ payload filled with another IP packet with the real (original) information (This is called IP Tunnelling).

The mobility binding is maintained by some specialized routers known as mobility agents. Mobility agents are of two types:

1. Home agents:  A designated router in the home network of the mobile node maintains the mobility binding in a mobility binding table where each entry is identified by the tuple:

<Permanent home address, temporary care-of address, association lifetime>

Figure 3: Mobility Binding Table

2. Foreign agents: Specialized routers on the foreign network where the mobile node is currently visiting. They maintain a visitor list which contains information about the mobile nodes currently visiting that network. Each entry in the visitor list is identified by the tuple:

< permanent home address, home agent address, media address of the mobile node, association lifetime>.

Figure 4: Visitor List

The basic Mobile IP protocol Stages:

             1. Agent Discovery:

Uses the ‘Agent Advertisement message’ to discovery the available agent which is nearest to the mobile node

2. Registration:

Follow the registration process to get registered and served with an agent.

Figure 5: Registration process in Mobile IP

3. In Service:

Use ‘tunneling’ to exchange the IP packets.

Figure 6: Tunneling Operation in Mobile IP

4. Deregistration

The mobile node achieves this by sending a Registration Request with the lifetime set to zero. This is more often handled by the Home agent when the mobile node gets attached to a different ‘Foreign agent’ than the last previous entry.

Security Concerns

Security is very important in Mobile IP as mobile nodes are often connected to the Internet via wireless links which are very vulnerable to security attacks. Mobile IP solves this problem by specifying a security association between the home agent and the mobile node. The default algorithm for the Mobile IPs’ authentication is keyed MD5 with a key size of 128 bits. Also each registration contains unique data to avoid valid registration recording by malicious nodes.

Two methods are used to generate the unique data:

• timestamps - The node generating the message inserts the time-of-day, and the node receiving the message checks whether it is sufficiently close to its time-of-day.
• nonces - Node A generates a new random number in every message to node B, and checks whether node B returns the same number in its next message to node A. Both messages use an authentication code to protect against alteration by an outsider. Node B can also generate random numbers and use them in its messages.
  

Consideration:

Mobility support in IPv6 solves many of the problems of basic Mobile IP. A comparison on those two will be discussed in a later version of this series of articles.

References:

1. Figure 1: http://www.panoulu.net/img/MobileIP.png

2. Figure 2: http://www.wichorus.com/UserFiles/mobile5_outline_large.jpg

3.http://en.wikipedia.org/wiki/Mobile_IP

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