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Mobile IP Overview, History and Motivation
(Page 2 of 4)
Difficulties with Older Mobile Node Solutions
The tight binding of network identifier
and host IP address means that there are only two real options under
conventional IP when a mobile device moves from one network to another:
- Change IP Address: We can change the IP
address of the host to a new address that includes the network ID of
the network to which it is moving.
- Decouple IP Routing From Address: We can
change the way routing is done for the device, so that instead of routers
sending datagrams to it based on its network ID, they route based on
its entire address.
These both seem like viable options
at first glance, and if only a few devices tried them they might work.
Unfortunately, they are both inefficient, often impractical, and neither
is scalable, meaning, practical when thousands or millions
of devices try them:
- Changing the IP address each time a device moves
is time-consuming and normally requires manual intervention. In addition,
the entire TCP/IP stack would need to be restarted, breaking any existing
connections.
- If we change the mobile device’s IP address,
how do we communicate the change of address to other devices on the
Internet? These devices will only have the mobile node’s original
home address, which means they won’t be able to find it even if
we give it a new address matching its new location.
- Routing based on the entire address of a host
would mean the entire Internet would be flooded with routing information
for each and every mobile computer. Considering how much trouble has
gone into developing technologies like classless addressing to reduce
routing table entries, it's obvious this is a Pandora's Box nobody wants
to touch.
 Key Concept: The basic problem with supporting mobile devices in IP internetworks is that routing is performed using the IP address, which means the IP address of a device is tied to the network where that device is located. If a device changes networks, data sent to its old address cannot be delivered by conventional means. Traditional workarounds such as routing by the full IP address or changing IP addresses manually often create more problems.
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