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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)

Mobile IP Home Agent Registration and Registration Messages 1 2 3 Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation

Mobile IP Conventional Tunneling

Normally, the tunnel described above is used only for datagrams that have been sent to the mobile node and captured by the home agent. When the mobile nodes wants to send a datagram, it doesn't tunnel it back to the home agent; this would be needlessly inefficient. Instead it just sends out the datagram directly using whatever router it can find on its current network, which may or may not be a foreign agent. When it does this, it uses its own home address as the source address for any requests it sends. As a result, any response to those requests will go back to the home network. This sets up a “triangle” of sorts for these kinds of transactions:

1. The mobile node sends a request from the foreign network to some third party device somewhere on the internetwork.
   
   
2. The third party device responds back to the mobile node. However, this sends the reply back to the mobile node's home address on its home network.
   
   
3. The home agent intercepts the response on the home network and tunnels it back to the mobile node.

This process is illustrated in Figure 134. The reverse transaction would be pretty much the same, just in the reverse order. In that case the third party (Internet) device would send a request to mobile node, which would be received and forwarded by the home agent. The mobile node would reply back directly to the Internet host.

Mobile IP Home Agent Registration and Registration Messages 1 2 3 Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation