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Mobile IP Addressing: Home and "Care-Of" Addresses
(Page 1 of 2)
Just as most of us have only a single
address used for our mail, most IP devices have only a single address.
Our traveling consultant, however, needs to have two addresses; a normal
one and one that is used while he is away. Continuing our earlier
analogy, the Mobile-IP-equipped notebook
our consultant carries needs to have two addresses as well:
Mobile IP Care-Of Address Types
- Home Address: The normal,
permanent IP address assigned to the mobile node. This is the address
used by the device on its home network, and the one to which datagrams
intended for the mobile node are always sent.
- Care-Of Address: A secondary, temporary
address used by a mobile node while it is 'traveling away from
its home network. It is a normal 32-bit IP address in most respects,
but is used only by Mobile IP for forwarding IP datagrams and for administrative
functions. Higher layers never use it, nor do regular IP devices when
The care-of address is a slightly
tricky concept. There are two different types, which correspond to two
distinctly different methods of forwarding datagrams from the home agent
Foreign Agent Care-Of Address
This is a care-of address provided
by a foreign agent in its Agent Advertisement message. It is,
in fact, the IP address of the foreign agent itself. When this type
of care-of address is used, all datagrams captured by the home agent
are not relayed directly to the mobile node, but indirectly to the foreign
agent, which is responsible for final delivery. Since in this arrangement
the mobile node has no distinct IP address valid on the foreign network,
this is typically done using a layer two technology. This arrangement
is illustrated in Figure 129.
In our consultant analogy, this type
of care-of address is like forwarding from the London post office to
the Tokyo post office. The London personnel would take a letter for
John Smith sent to his London address, and repackage it for delivery
to John Smith, care of the Tokyo post office. The Tokyo
post office (or John Smith himself) would need to worry about the last
leg of the delivery.
Co-Located Care-Of Address
This is a care-of address assigned
directly to the mobile node using some means external to Mobile IP.
For example, it may be assigned on the foreign network manually, or
automatically using DHCP.
In this situation, the care-of address is used to forward traffic from
the home agent directly to the mobile node. This was the type of address
shown in Figure 128.
In our consultant analogy, this is
like John Smith obtaining a temporary address for his use while in Tokyo.
The London post office would forward directly to his Tokyo address.
They would not specifically send it to the Tokyo post office (though
of course that PO would handle the mail at some point).
Figure 129: Mobile IP Operation With A Foreign Agent Care-Of Address
This diagram is similar to Figure 128, except that instead of the mobile node having a co-located (distinct) IP address as in that example, here the mobile node is using a foreign agent care-of address. This means that the nodes care-of address is actually that of the foreign agent itself. Step #1 is the same as in Figure 128, but in step #2 the home agent forwards not to the mobile node directly, but to the foreign agent (since that router is the one whose IP address the mobile is using). In step #3 the foreign agent strips off the home agents packaging and delivers the original datagram to the mobile node. This is typically done using whatever layer two (LAN or wireless LAN) technology connects the mobile node and foreign agent together.
Key Concept: In Mobile IP, each mobile device uses a temporary, care-of address while on a foreign network. A co-located care-off address is one that is assigned directly to the mobile node, and enables direct delivery of datagrams to the node. The alternative is to use a foreign agent care-of address. In this situation the mobile node actually uses the IP address of the foreign agent; datagrams are sent to the foreign agent, which delivers them to the mobile node.
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