Mobile IP

Mobile IP
Consider the use of Mobile IP if users need to roam to parts of the network associated with a
different IP address than what’s loaded in the appliance. IETF’s RFC 2002 defines Mobile IP,
an enhancement to the standard IP protocol. The main goal of Mobile IP is to enable mobile
stations to roam transparently throughout networks, automatically maintaining proper IP-based connections to their home networks. This avoids the impracticality of changing the IP address
in the appliance when operating in a different area of the network.
The need for Mobile IP arises most often in wireless WAN systems. For example, a user might
need to roam temporarily with a wireless appliance in a foreign network that has a completely
different IP network address than the home network. This situation can also occur in a LAN
when users roam from an access point located on one subnet (that is, router port) of a network
to an access point on another subnet. In these cases, be sure to consider the use of Mobile IP.
Mobile IP uses an address-forwarding mechanism to continue the delivery of packets to a
mobile station as it moves from network to network. This operation is similar to the postal mail
delivery service. Imagine that you’re moving temporarily from Dayton, Ohio, to Washington,
D.C., for a six-month consulting assignment. After you arrive in D.C. and obtain a new mailing
address, you drop off a change of address card at your new D.C. post office, which notifies the
Dayton post office of your new address. Now when the Dayton post office receives mail for
you, it knows to forward the mail to you at your new address in D.C.
The operation of Mobile IP is very similar to this analogy. For example, imagine that you’re a
doctor working in a hospital and you need to wander from your office to the emergency room
to assist a new patient:
1. You carry your mobile station (a wireless pen-based computer), which has an IP address
associated with the part of the network where your office is located, toward the emergency
room. (The wireless access points in the emergency room reside on a different
subnet of the hospital’s network.)
2. As you walk closer to your destination, your pen-based computer associates with the
emergency room’s network.
3. The Mobile IP protocol notifies your home network of a care-of address (IP address
located within the emergency room subnet) that your home network should send packets
relevant to your mobile station.
4. Your home network forwards all packets destined to you to the emergency room’s network
(via the care-of address), which will deliver them to your mobile station.
A positive attribute of Mobile IP is that its implementation does not require changes to routers
or the Domain Name Service (DNS). To implement Mobile IP, you have to include only a few
software elements as the following describes (refer to Figure 6.15):
• Mobile node—The mobile node is an entity contained within a particular wireless
mobile station (such as a handheld PC or data collector) that communicates with other
Mobile IP components. A mobile node is built into a TCP/IP protocol stack or can exist
as a “shim” under a TCP/IP stack.