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IP routing is based on the IP address, which is uniquely identities a node's point of attachment to the internet . When a device moves its home network and enters a new one, it has to change its IP address and reestablish a new TCP connection. If a communication with this moving device happened at that time, the communication has to be disconnected until it gets the new IP address of the moving device. To solve this mobility issue, a working group within Internet Engineering Task Force (IETF) proposed a solution, which is the Mobile IP Protocol.
The idea of mobile IP is similar to postal service delivery: once you move to a new location, you ask your home post office to send your mail to your new location's address by the local post office there. Thus, a mobile device first leaves its home network and connects to a foreign network. The agent then sends packets locally to the mobile device visiting that network.
Mobile IP provides transparent Routing of IP datagram over Internet. Each mobile node is identified with his home address regardless of where its current location is. When a node is moved outside its home network as the node associated with a Care-of Address (CoA), which provides information on its current position.
Mobile IP specifies how a mobile's device registered with their home agent and how home agent routers connects to the mobile device through a tunnel. Mobile IP provides an efficient and scalable mechanism for Roaming over the Internet. When using Mobile IP, the devices can change their connection to the Internet without changing its IP address. This means that the device can maintain a connection to Transport layer or higher layer when the device moves and changes its location.
A mobile node may have two addresses, a permanent (home) and a temporary address (care-of address), that changes at each new point of attachment. By using those both address a mobile computing device can be changed its location and moved to a new network without changing its home IP address and without loosing existing connections. And the traffic redirect automatically between Home address and care-of address. There are two versions of mobile IP, Mobile IPv4 and Mobile IPv6. Mobile IPv4 will be described in more detail in this chapter.
When IP packets are exchanged between a host and mobile device the following steps occurs that shown in the figure 4.1:
Server x try to connect to mobile device by sending IP packet with A's home address in the IP header. The IP address is routed to the home network.
The home agent intercept the incoming packet and encapsulate the entire datagram inside a new IP care-of address and transmit the datagram as tunneling to the foreign agent.
The outer IP header is removed by the foreign agent and sends the original IP datagram to A through the foreign network.
A mobile device received the message and send IP packet to X using X's IP address to the foreign agent across the foreign network.
The foreign network routed the IP packet to the X server directly across the internet using X's IP address.
Figure 4.1 Mobile IP Operations 
4.2 Mobile IP Terminology
Mobile IP has the following elements and entities that required for optimum functionality:
Mobile Node (MN): is a moving internet connected device which location and point of attachment to the internet can be changed while keeping ongoing communication without interruption via using its home fixed address. This kind of device is usually IP phone, laptop computer or router.
Home Address: An IP address assigned to Mobile device within the network for extended period of time. It remains the same regardless of where the device is attached to the internet.
Home Agent (HA): is a router on the mobile device's home network. It tracks the mobile device location (care of address), intercept and tunnels packets to the mobile device when it is away from home, and maintain s current location information for the mobile device.
Home Network: is the network within which device its identifying home IP address. The IP routing mechanism will deliver packets destined to mobile device's Home Address to the mobile device's Home Network.
Foreign Agent (FA): is a router on mobile device's visited network. It provides the care-of-address to the mobile device and routing service to the mobile device while registered, act as a default router for datagram generated by the mobile device. The foreign agent decapsulate and delivers datagram to the mobile device that were encapsulate by the mobile device's home agent
Foreign Network: Any network other than the mobile device's Home Network, which Mobile device can be operating successfully when away from its home network.
Case-of-address: is a temporary IP address assigned to mobile device while it is away from home network.
Correspondent Node (CN): A device that sends or receives packets to or from mobile device; the correspondent device may be another mobile device or a non mobile internet device.
A mobile device may have two addresses, permanent home address and a Care-of Address (CoA).A care-of address is a temporary IP address that identifies a mobile device's current point of attachment to the internet and allows it to connect from different location by keeping its home address. When a mobile device is leaving its home network and connecting to any foreign network, it is assigned a care-of address. This may be a "foreign agent care-of address" which is a static address of a foreign agent with which the mobile device is registered, and a "co-located care-of address" which is a temporary IP address assigned to the mobile device. A collocated care-of address is assigned by Dynamic Host Configuration Protocol (DHCP), Point -to Point IP control protocol (PPP), or manual configuration.
Figure 4.2 Mobile IP Components 
4.3 Mobile IP Functionality
Mobile IP function can be dividing by using three mechanisms :
Discovering the Care-of Address;
Registering the Care-of Address;
Tunneling to the Care-of Address;
4.3.1 Discovering the Case-of Address
The discovery process in Mobile IP is based on the ICMP (Internet Control Message Protocol). The mobile device is responsible for discovery process by determine if it is attached to its home network or foreign network. Because handoff from home network to the foreign network occurs at the physical layer, a transition between those two different networks can be happened at any time without notification to the upper layer (network layer, i.e. the IP layer). The discovery process for mobile device is a continuous. A special message called Agent Advertisement is periodically broadcasting by the home agent or foreign agent to advertise their availability to any attached links. A mobile device listens to these Agent Advertisement message and compare the network portion of the router's IP address with the network portion of its own home address. If the network portions match, then the mobile device is on a home network; but if it doesn't match, then the mobile device is on the foreign network.
If the mobile device stays in its home network, then it will work without Mobile IP functionality.
If the mobile device gets a foreign agent care of address, then the foreign agent will be the end of the tunnel and will perform the de-capsulation and will deliver the message to the mobile device. In the other case if the mobile device gets a collocated care-of address, then the mobile device it self will perform the decapsulation for the tunneling.
4.3.2 Registering the Care-of Address
Once the mobile device gets care of address and recognized that it is on a foreign network, it is need to tell the home agent where it is. And request home agent to forward its IP traffic. This done according to the following process that is shown in the (Figure 4.3):
Mobile device requests forwarding process by sending a registration request to the foreign agent that mobile device moved to. This request contains home address of mobile device, care-of address of the mobile device and the registration lifetime.
The foreign agent passes this request to the home agent of the mobile device. In some cases, the mobile device may register directly with the home agent.
Home agent receives the registration request, it either accepted or denied. If the request accepted, the home agent updates its route table associating the home IP address of the mobile device with its care-of address. The home agent keeps the association until the registration lifetime expired.
The foreign agent forward this reply to the mobile device
Registration in Mobile IP must be secured so that malicious registration can be detected and rejected .Otherwise, attacker in the internet could disrupt communication between the home agent and the mobile device .Though Mobile IP provide some authentication methods like identification field and timestamp.
Figure 4.3- Registration process
4.3.3 Tunneling to the Care-of Address
Once the mobile device registers to its home agent, the home agent will be able to intercept the IP packets that send to the mobile device's home address so that these packets can de transmitted using tunneling. Encapsulation mechanism is used to forward the IP datagram to the care-of-address by inserting the entire IP datagram into an outer IP datagram by the home agent. Three different type of encapsulation are used in Mobile IP: IP-within IP Encapsulation, Minimal Encapsulation and Generic Routing Encapsulation (GRE).
IP-within-IP Encapsulation, in this scenario the entire original IP packet becomes the payload in the new IP packet (figure 4.4). In the old IP header, the source address refers to the corresponded device that is sending the original message and the destination address is the home address of the mobile device. In the new IP header the source address refer to the IP address of the home agent and the destination address refer to the care-of address for the mobile device.
Minimal Encapsulation, in this scenario the new header is inserted between the original IP header and the original IP payload (figure 4.5) this can be used if both home and foreign agent accepted to do so. Minimal encapsulation results in less overhead.
Home agent encapsulates the datagram as it shown in the (figure 4.5) through the tunneling and send across the internet to the case-of address. This minimal header is de-capsulated to the original header by either foreign agent or the mobile device itself.
Generic Routing Encapsulation (GRE) is an optional tunneling method used by mobile IP. GRE tunnel built a virtual point-to-point link between two routers at remote point over an IP internetwork. GRE is good in certain application because it is support multiprotocol and provide prevention of recursive encapsulation.
Figure 4.4 IP within IP encapsulation 
Figure 4.5 Minimal Encapsulation 
 H. Hansen, "IPSec and Mobile IP in Mobile Ad Hoc Networking " , Helsinki University of Technology, April 2000.
 C. Perkins, "Mobile Networking through Mobile IP", online tutorial
http://www.computer.org/internet/v2n1/perkins.htm. October 2002