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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Mobility Support (Mobile IP)

Internet Protocol Mobility Support (Mobile IP) 1 2 3 4 Mobile IP Concepts and General Operation

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.

Internet Protocol Mobility Support (Mobile IP) 1 2 3 4 Mobile IP Concepts and General Operation