Mobile IP

Mobile IP is a communication protocol that enables a host to move from one network to another while still being connected to its home network. Mobile IP allows a mobile host to communicate with the remote host being in a foreign network. It is the improvised version of Internetworking Protocol (IP) that boosts mobile communication.

In this section, we will discuss mobile IP, it’s working, several entities of mobile IP, advantages and disadvantages of mobile IP.

Content: Mobile IP

1. Why is Mobile IP needed
2. What is Mobile IP?
3. Entities of Mobile IP
4. Working of Mobile IP
5. Advantages & Disadvantages
6. Key Takeaways

Why do we need Mobile IP?

Mobile Internetworking Protocol (Mobile IP) is an advanced version of Internetworking Protocol (IP). IP addresses were designed for the stationary host that always remains attached to one specific network.

The prefix of the IP address identifies the network to which the host belongs. The suffix of the IP address identifies the particular host in the corresponding network.

Thus, we need to modify the structure of the IP address. The two proposed solutions to modify IP address are:

Changing the IP address each time the host enters a new network. This attempt was not as successful as it has several drawbacks. Every time the computer enters a new network it has to reboot. We have to keep revising the DNS table to let every host on the internet have knowledge about changes. Each time we have to change the configuration files. If the host moves to another network in-between the transmission of data, the data exchange would be interrupted.

This solution is more feasible here, a host will have two addresses:

1. An original address which would identify the host in the home network.
2. A temporary address (care-of address) that would identify the host in the foreign network. The care-of address will keep on changing whenever the host will move to a new network.

The second solution contributes to the design of Mobile IP. It allows a host to move to another network by maintaining its original IP address and still is able to communicate with the remote hosts over the internet.

What is Mobile IP?

Mobile IP is an improvised version of IP that allows a mobile host to move from one network to another. The Mobile IP address includes two addresses:

1. Home Address – The home address is permanent and it doesn’t change even if the host moves to another network.
2. Care-of Address – The care-of address changes as soon as the host moves to another network. This change of address requires a home agent and foreign agent that we have described in the section below.

Entities of Mobile IP

The functional entities in the Mobile IP protocol are:

• Home Network is a network to which the mobile host actually belongs. It is a permanent network of the mobile host.
• Foreign Network is a new network into which the mobile host has moved.
• Remote Network is a network which is neither the home network nor the foreign network.
• Mobile Host is a host of the home network which has moved to the foreign network.
• Remote Host is a host in a remote network.
• Home Agent is a router attached to the home network that allows the mobile host to send and receive data from the remote host over the internet.
• Foreign Agent is a router attached to the foreign network that allows a mobile host to send and receive data from a remote host over the internet.
• Care-of address is a temporary IP address provided by the foreign agent to the mobile host till it is in the foreign network.
• Home Address is the address of the mobile host in its home network.

Working

Mobile IP communication protocol allows the mobile host to communicate with the remote host even if it is in a network other than its home network.
If a mobile host wants to communicate with the remote host being itself in the foreign network, then it has to go through three phases:

Agent Discovery (Phase I)

Agent discovery is the first phase and requires the involvement of a mobile host, a home agent and a foreign agent. This phase also has two sub-phases as described below:

1. The mobile host has to discover the home agent’s address before it moves away from the home network.
2. As the mobile host moves to a new network (foreign network), it has to discover the foreign agent’s address and also the care-of address.

The agent discovery phase includes two types of messages: agent advertisement and agent solicitation.

Agent Advertisement

A simple router advertises its existence on the network with the ‘ICMP router advertisement’ packet. If the router is playing the role of an agent, it ‘appends’ the agent advertisement message to the ICMP advertisement packet.

If the advertisement is done by a foreign agent, then it sends a ‘list of addresses’ available as care-of addresses. From this list of addresses, the mobile host has to choose one. The announcement of the care-of address chosen by the mobile host is done in the registration request.

Agent Solicitation

In case, if a host in the network doesn’t receive the ‘router ICMP advertisement’ packet. The host can initiate itself by sending the ‘router ICMP solicitation packet’. If a mobile host has not received the ‘agent advertisement’, it can use the ‘router ICMP solicitation packet’ to send the ‘agent solicitation’ message.

Registration (Phase II)

This is the second phase and it also requires the involvement of a mobile host, a home agent and a foreign agent. After discovering the home agent and foreign agent address the mobile host has to register itself to the foreign agent and the home agent.

The mobile host has to renew its registration if it has expired. While returning back to the home network the mobile host has to cancel or deregister its registration. This registration process involves two messages: registration request and registration reply. These registration messages are first encapsulated in a UDP user datagram.

Registration Request

The mobile host sends the registration request to the foreign agent declaring its chosen ‘care-of address’. Along with this it also declares its ‘home address’ and ‘home agent’s address’. When a foreign agent receives the registration request message, it registers that mobile host and further forwards the message to the ‘home agent’ in the IP packet.

So, now the home agent knows the address of the foreign agent where its mobile host is present now because the IP address would have the foreign agent’s address in its source address field of the IP packet.

Registration Reply

The reply packet is sent from the ‘home agent’ to the ‘mobile host’ via a ‘foreign agent’. This registration reply has the confirmation of whether the request is accepted or denied.

Data Transfer (Phase III)

Now finally, after the agent discovery and registration process, the mobile host is in the foreign network and can communicate with the remote host. Let us see how?

The remote host would definitely send the packet with its own address in the source address field and the mobile host’s home address in the destination address field of the packet.

As the mobile host is not it’s the home network, the packet sent by the remote host is received by the home agent on behalf of the mobile host.

The ‘home agent’ encapsulates the received IP datagram into another IP datagram (with the home agent’s address in the source address field and the foreign agent address in the destination address field) and relays it to the foreign agent.

The foreign agent receives the packet, removes the encapsulation and sees the home address of the mobile host in the destination address field of the original packet sent by the remote host. The foreign agent reviews its registry table, observes which care-of address has been registered to the corresponding home address, and then forwards the corresponding mobile host packet.

Next, if a mobile host being in the foreign network wants to reply or communicate with the remote host, it simply prepares a packet and puts its home address in the source address field of the packet and the remote host’s address in the destination address field of the packet. The mobile host then directly sends the packet from the foreign network to the remote host.

Advantages and Disadvantages of Mobile IP

Advantages

• A user with its network devices can move to any other network without losing its connection with its home address.
• Mobile IP provides transparency while the data transfer process. It hides the fact that the mobile host is not in its home network and is communicating from a foreign network.

Disadvantages

• When the ‘remote host’ and ‘mobile host’ both are in a foreign network and still the data transfer is occurring through the ‘home agent’ then the data packet has to travel more distance though both the host are in the same network.
• As we have seen above, if the mobile host in the foreign network wants to send the data packet to the remote host it sends it directly from the foreign network with its home address as the source and the remote host address in the destination.
  But, if a remote host wants to send a packet to a mobile host in a foreign network, the data packet has to travel to the mobile host via its home agent. So, here it has to travel the extra distance.

So, this is all about mobile IP that allows mobile communication i.e. a mobile host can communicate with a remote host without losing the connection with the home network.