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Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation
(Page 2 of 2)
Additional Home Agent Tasks To Deal With ARP
Solving this problem requires the
intervention of, you guessed it, the home agent. It must perform two
tasks to enable local hosts to send to the mobile node:
- ARP Proxying: The home agent must listen
for any ARP Requests sent by nodes on the same network as any
of the mobile nodes that are currently registered to it. When it hears
one, it replies in the mobile node's stead, and specifies its
own data link layer address as the binding for the mobile node's
IP address. This will cause hosts on the home network to send any datagrams
intended for the mobile node to the home agent where they can be forwarded.
This process is illustrated in Figure 135.
- Gratuitous ARP: Proxying helps
with ARP Requests but what about devices that already have cache
entries for the mobile node? As soon as the mobile node leaves the network,
these become automatically stale. To correct them, the home agent sends
what is called a gratuitous ARP message that tells devices on
the local network to associate the mobile node's IP address with the
home agent's data link layer address. The term gratuitous
refers to the fact that the message isn't being sent in order to perform
an actual address resolution but merely to cause caches to be updated.
It may be sent more than once to ensure that every device gets
Figure 135: ARP Proxying By Mobile IP Home Agent
The home agent must take special steps to deal with transmissions from devices on the local network to the mobile node. In this example (using short hardware addresses for simplicity) the hardware address of the mobile node is #48 and that of the home agent #63. A local client on the home network with hardware address #97 sends an ARP Request to find out the hardware address of the mobile node. The home agent responds on the mobiles behalf, specifying not hardware address #48 but rather its own, #63. The client will thus send to the home agent, which can then forward the data to the mobile node on the foreign network.
Once this is done, ARP should function
normally on the home link. Of course, when the mobile device returns
back to the home network, the process must be reversed. Upon deregistration
with the home agent, it will stop proxying for the mobile node. Both
the mobile node and the home agent will also send gratuitous ARP broadcasts
that update local device caches to again associate the mobile node's
IP address with its own layer two address, instead of that of the home
Key Concept: To avoid problems with hosts on the mobile nodes home network trying to send datagrams to it at layer two, the home agent is required to use proxy ARP to direct such devices to send to the home agent so they can be forwarded. It must also use gratuitous ARP to update any existing ARP caches to that effect.
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Version 3.0 - Version Date: September 20, 2005
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