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Mobile IP Data Encapsulation and Tunneling
(Page 2 of 3)
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:
- The mobile node sends a request from
the foreign network to some third party device somewhere on the internetwork.
- 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.
- 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.
Figure 134: Mobile IP Encapsulation and Tunneling
This example illustrates how a typical request/reply message exchange in Mobile IP results in a triangle of communication. In step #1, the mobile node sends a request to a remote server somewhere on the Internet. It uses its own home address as the source for this request, so in step #2 the reply goes back to the home agent. Step #3 consists of the home agent tunneling the reply back to the mobile node.
Key Concept: Once Mobile IP is set up and operational, it works by having the home agent encapsulate and tunnel received datagrams to the mobile node. The mobile device normally sends datagrams directly to Internet hosts, which respond back to the mobiles home agent which forwards those datagrams to the mobile node. This means a request/reply communication takes three transmissions.
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