Imagine this scenario. You normally work in you lovely air-conditioned office in Dallas. One
day your boss informs you that you have to go to Ericsson Stockholm to give a presentation. You spend a few hours preparing the presentation, which remains on a server on your home network. You then take your lap top computer across the Atlantic to your meeting in Stockholm. There you plug in to the LAN. Without having to change any configuration parameters or addresses, you can immediately access your home LAN in Dallas and can easily give your presentation. Also, external users can still contact you with your original IP address, as that has moved with you. This transparent mobility is the job of Mobile IP.
So, how does it work? Well, conceptually it's quite simple. Here you can see your home network in Dallas and your foreign network in Stockholm. When you are on your home network you are addressed using the IP address 220.127.116.11. Users on your own network or on other networks connected to the Internet can talk to your computer using that IP address, 18.104.22.168. The router on your home network ensures that packets bearing your IP address are delivered to you.
Now, what happens when you pick up your laptop, spend 10 hours on a plane and arrive at
Ericsson in Stockholm? Well, when you plug your laptop into this new foreign network your laptop talks to the router on the foreign network, which is called the Foreign Agent.
Your laptop basically sends a message to the Foreign Agent saying, "Hi, I'm laptop
22.214.171.124 and I'd like to use your network." The Foreign Agent then sends a message
to the router on the home network, the Home Agent, and says "The mobile with IP address 126.96.36.199 isn't on your network anymore, but it can be reached by sending packets to me at IP address 188.8.131.52." The Home Agent accepts this request and then we're ready to roll.
So, what happens when some host out on the Internet starts to send packets to your laptop? Well, the routers on the Internet have no knowledge of your boss's decision to send you to Stockholm, so they forward the packets to your home network as usual. The router on your home network, the Home Agent, sees these packets and remembers the care-ofaddress it received from the Foreign Agent. So, it takes your IP packets and sends them through an IP tunnel over the Internet to the Foreign Agent. The Foreign Agent looks at the packets and thinks, "This IP address isn't on my network, but I remember receiving information about it, so I can forward it on directly." In this way packets reach the mobile node.
What happens when the mobile node wishes to send packets out to the Internet? Well, this
causes no problem whatsoever, the laptop builds IP packets with the desired destination address and the usual source address (that is, 184.108.40.206) and sends them directly out to the Internet as usual.
Of course Mobile IP isn't perfect. One of the biggest drawbacks is the routing inefficiency it
introduces. Packets destined for our laptop pass through the Internet to our home network and then back out onto the Internet to our foreign network. This doglegged routing means that received packets suffer from higher latency than normal. It also means that the Internet is loaded more as the same packets must pass through it twice. There are also some important security implications to consider with Mobile IP. The mobile node must be carefully verified to ensure that packets are not forwarded to impostors intent on stealing others information.
Mobile IP is only one of a host of macro mobility techniques. Others, such as Dynamic DNS, L2TP and IP security are also capable of solving the problem of nodes moving between interconnected networks.