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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

Once a mobile node on a foreign network has completed a successful registration with its home agent, the Mobile IP datagram forwarding process described in the general operation topic will be fully “activated”. The home agent will intercept datagrams intended for the mobile node as they are routed to its home network, and forward them to the mobile node. This is done by encapsulating the datagrams and then sending them to the node's care-of address.

Encapsulation is required because each datagram we intercept and forward needs to be resent over the network to the device's care-of address. In theory, the designers might conceivably have done this by just having the home agent change the destination address and stick it back out on the network, but there are various complications that make this unwise. It makes more sense to take the entire datagram and wrap it in a new set of headers before retransmitting. In our mail analogy, this is comparable to taking a letter received for our traveling consultant and putting it into a fresh envelope for forwarding, as opposed to just crossing off the original address and putting a new one on.

The default encapsulation process used in Mobile IP is called IP Encapsulation Within IP, defined in RFC 2003 and commonly abbreviated IP-in-IP. It is a relatively simple method that describes how to take an IP datagram and make it the payload of another IP datagram. In Mobile IP, the new headers specify how to send the encapsulated datagram to the mobile node's care-of address.

In addition to IP-in-IP, two other encapsulation methods may be optionally used: Minimal Encapsulation Within IP, defined in RFC 2004, and Generic Routing Encapsulation (GRE), defined in RFC 1701. To use either of these, the mobile node must request the appropriate method in its Registration Request and the home agent must agree to use it. If foreign agent care-of addressing is used, the foreign agent also must support the method desired.

The encapsulation process creates a logical construct called a tunnel between the device that encapsulates and the one that decapsulates. This is the same idea of a tunnel used in discussions of virtual private networks (VPNs), IPSec tunnel mode, or the various other tunneling protocols used for security. The tunnel represents a conduit over which datagrams are forwarded across an arbitrary internetwork, with the details of the encapsulated datagram (meaning the original IP headers) temporarily hidden.

In Mobile IP, the start of the tunnel is the home agent, which does the encapsulation. The end of the tunnel depends on what sort of care-of address is being used:

• Foreign Agent Care-Of Address: The foreign agent is the end of the tunnel. It receives encapsulated messages from the home agent, strips off the outer IP header and then delivers the datagram to the mobile node. This is generally done using layer two, because the mobile node and foreign agent are on the same local network, and of course, the mobile node does not have its own IP address on that network (it is using that of the foreign agent.)
  
  
• Co-Located Care-Of Address: The mobile node itself is the end of the tunnel and strips off the outer header.

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