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Mobile IP Efficiency Issues
(Page 1 of 2)
Having the home
agent forward all datagrams to the mobile node wherever it may be
is a convenient solution to mobility, but is also a rather inefficient
one. Since every datagram must be sent first to the home network and
then be forwarded to the mobile node, datagrams are going to travel
over some part of the internetwork twice. The degree of inefficiency
represented by forwarding can be significant, and may lead to problems
with certain applications.
To see what the problem is, let's
consider a traveling mobile node M and a regular device that
wants to send to it, device A. The degree of the inefficiency
of Mobile IP is a function of the internetwork distance between device
A and M's home network, compared to the internetwork distance
between device A and M's current network. By distance
here, I mean the term as it is used in determining routes on an internetwork;
two devices are closer when it takes less time and/or fewer
hops to communicate between them, and farther when it takes
more. (I use geography in my examples below to represent this notion
of distance, but remember that geographical distance is only one factor
in internetwork distance.)
The Impact on Efficiency of Sending Device Location
Let's consider the case where mobile
node M is on a foreign network quite far from home, and a sending
device, device A, wants to send a datagram using node Ms
home IP address. Suppose the home network is in London and the device
is again in Tokyo, Japan. The following examples are arranged in order
of increasing inefficiency of Mobile IP, compared to the alternative
of having the mobile node just get a new temporary IP address on the
foreign network and not use Mobile IP:
- Sending Device On Home Network: In this
situation, device A will send a datagram that is immediately
intercepted by the home agent on the home network and forwarded to the
mobile node. There is really no inefficiency here at all (except for
overhead for encapsulation and such) because even if A sent directly
to the mobile node with a new foreign address, it would probably be
routed through the home agent router anyway.
- Sending Device On Network Close To Home Network:
Here, let's say a device in Paris, France wants to send to the mobile
node. The datagram goes from Paris to London and then to Tokyo. That's
not too bad.
- Sending Device On Network Close To Foreign
Network: Now, suppose the sending device is in Taipei, Taiwan. In
this situation, Mobile IP becomes quite inefficient.
The datagram must be sent from Taipei all the way to London, and then
all the way back to Tokyo.
- Sending Device On Foreign Network: The
greatest inefficiency results when the sending device is actually on
the foreign network that the mobile node is visiting. If device A
is on the mobile node's current network in Tokyo, it must send all the
way to London and then have the result forwarded all the way back again
to Tokyo. Without Mobile IP, all we would need to do is use ARP and
then deliver directly at layer two without routing needed at all! This
scenario is illustrated in Figure 136.
Figure 136: A Mobile IP Inefficiency Worst Case Scenario
This diagram show the worst possible case of Mobile IP inefficiency: when a device on the foreign network where the mobile is located tries to send to it. The sender here, 188.8.131.52, uses the mobile nodes home address, so the transmission must be routed all the way back to London and then forwarded back to Tokyo, even though the two devices might be sitting on the same desk!
Unfortunately, the worst case
scenario outlined in the last bullet point is one that occurs
quite often. It's common for a mobile device to connect with a foreign
network specifically to communicate with the hosts on that network
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Version 3.0 - Version Date: September 20, 2005
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