Q15Mobile Computing

In standard TCP/IP, if a laptop moves to a new subnet, its IP address must physically change, violently terminating all active TCP sockets (downloads crash). Mobile IP mathematically solves this by assigning two IP addresses: a permanent Home Address (HoA) and a temporary Care-of-Address (CoA). The delivery mechanism relies on catastrophic tunneling.

Assume a Correspondent Node (CN) wants to send a packet to the Mobile Node (MN).

• The CN is completely ignorant of the MN's physical location. It mathematically addresses the packet to the MN's permanent Home Address.
• The standard internet routers blindly blast the packet toward the MN's Home Network.
• Because the MN is gone, the Home Agent (HA) (a router on the home network) violently intercepts the packet using Proxy ARP.
• The HA mathematically executes IP-in-IP Encapsulation. It takes the original packet, wraps it in a brand new IP header addressed to the MN's current Care-of-Address (CoA), and blasts it through a secure Tunnel.
• The Foreign Agent (FA) (or the MN itself) receives the tunneled packet, violently strips off the outer header, and delivers the original packet to the MN.

This is mathematically much simpler.

• When the MN replies to the CN, it does NOT need to use the tunnel.
• It constructs a standard IP packet with its Home Address as the Source, and the CN's address as the Destination.
• It blasts the packet to the standard local router, which routes it directly to the CN, completely bypassing the Home Agent (unless Reverse Tunneling is strictly forced by a firewall).