24-07-2012, 03:05 PM
Mobile Routing
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Basic Notions of Mobility
When a host moves, its point of attachment in the
network changes. This is called a handoff.
The point of attachment is a base station (BS) for cellular
network, or an access point (AP) for WLAN.
We will assume WLAN in our examples. Problem essentially
the same for cellular networks.
Handoff can be handled completely in the link layer, if
both the APs involved are in the same network.
Otherwise, a route change in the IP layer may be
needed. This is called network layer handoff.
Mobile IP is a standard protocol for handling network
layer handoff.
Motivation for Mobile IP
How IP routing works?
IP addresses are assigned in a topologically significant fashion. All
hosts in a network shares the same network address (e.g.,
129.13.42).
Routing is generally based on prefixes. For example, a packet for
129.13.42.12 is forwarded to network 129.13.42 unless there is a
different host-specific route.
What if a hosts moves to a different network? Two solutions:
The host carries the old IP address. Maintain host-specific routes
for all mobile hosts on all routers. Not scalable, because of too
many individual routes need to be maintained.
Change IP address of the mobile host to an address in the new
network. Upper layer connections (such as TCP) will break. Also,
not easy to find IP address for a mobile host that moves frequently.
Mobile IP solution:
Preserve the original address, but also use a new care-of-address
(COA).
Core Idea of Mobile IP
Transform the mobility problem into a routing
problem.
Use two IP addresses for the mobile.
Preserve the original IP address. But borrow a new
IP address (COA) in the newly visited network.
Use forwarding: A specific host (typically the router)
in the “home” network forwards IP datagrams to the
new COA.
Mobile IP does not need wireless network. The
handoff between two networks may involve physical
wireline connection/disconnection.
Idea is to simply be able to make the change in
network transparent to upper layer.
Mobile IP does not dictate how the change might
happen.
A Note on IP Routing
Why do we really need forwarding? Why cannot the MH simply
use the original IP address in the foreign network? Why a new
COA?
In the post office analogy, this means that you will continue
using your home address while in Stony Brook.
This is ridiculous in the post office system.
In IP network this is not impossible. But impractical.
IP routing is hierarchical. IP addresses are assigned in a
topologically significant fashion. All hosts in a network share the
same network address (e.g., 132.4.16.).
Routing is generally based on prefixes. For example, a packet for
Foo (132.4.16.Z) is forwarded to network 132.4.16. unless there is
a different host-specific route.
Providing host specific routes to all mobiles is not scalable, given
that such routes must be updated when the move.