Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation
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Mobile IP is a protocol that does a good job of implementing a difficult function: transparently allowing an IP device to travel to a different network. Unfortunately, a problem with any protocol that tries to change how IP works is dealing with special cases. Having a home agent intercept datagrams and tunnel them to the mobile node works well in general terms, but there are some places where extra work is required. One of these is the use of the TCP/IP Address Resolution Protocol (ARP), which under Mobile IP breaks unless we take special steps.
To understand the problem with ARP, consider a mobile node that is on a foreign network and has successfully registered with its home agent. The home agent will intercept all datagrams that come onto the home network intended for the mobile node, and then encapsulate and forward them. For this to happen, though, the home agent (home router) must see the datagram. This normally only occurs when a datagram comes onto the home network from the outside and is processed by the router.
What happens when a local device on the home network itself wants to transmit to a mobile node that has traveled elsewhere? Remember that this device may not be mobile itself and probably knows nothing about Mobile IP. It will follow the standard process for deciding what to do with a datagram it needs to send as explained in the topic discussing direct and indirect delivery of IP datagrams. It will compare its network ID to that of the mobile node and realize it doesn't need to route its datagram; it can send it directly to the mobile node.
The local host will attempt to use ARP to find the data link layer address of the mobile node so it can send the datagram to it directly. It will start by looking in its ARP cache and if it finds the node's data link layer address there, will use it to send at layer two. Of course the mobile node is no longer on the local network segment so the message will never be received. If there is no ARP cache entry, the host on the home network will attempt to send an ARP Request to the mobile node to determine its layer two address. Again, the mobile node has traveled away, so this request will go unanswered.
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