Side_1_360

• IP packets are routed based on the destination
  IP address.

By allowing the mobile node to use two IP add-
resses the mobility is supported by IP. Hence,
one of the addresses is fixed (indicating the
node’s home region), while the other address
may change at each point of attachment (called
acare-of-address).

Two types of care-of-address can be used: a
“foreign agent care-of-address” is the address of
the Foreign Agent(FA) where the mobile node
is registered, and a “co-located care-of-address”
being an externally obtained local address that
the mobile node has associated with its network
interface. Hence, the care-of-address refers to
the end point of the tunnel from the home agent.

Mobile IP consists of three basic mechanisms:

• Discovering agents and obtaining a care-of-
  address;


• Registering the care-of-address;


• Tunnelling to the care-of-address.

An illustration of mobile IP is given in Figure 13
where the mobile node Acommunicates with the
node denoted as correspondent node.

A mobile node would belong to a home
“domain”, controlled by a home agent. The
HA can control the forwarding of packets when
the mobile node is not connected to its home
domain. The mobile node also needs a Care-Of-
Address (COA) in the foreign domain it is con-
nected to. This is assigned by a foreign agent.
Foreign agents are defined for each area. When
the user turns up in an area, it makes its presence
known (by listening for foreign agents or check-
ing for one). The discovery messages applied are
quite similar to ICMP router discovery messages.

Then the foreign agent checks with the home
agent concerning authentication, etc. at the same
time as telling of the mobile user’s whereabouts.
A care-of-address can be allocated at the same
time, to be used for forwarding packets to the
user. When a packet destined for the mobile user
arrives at its home location area, the home agent
may forward it to the foreign agent (within a
tunnel), which sends it to the mobile user. At
the same time the home agent may inform the
sender of the packet of the new location of the
mobile user such that the sender can transmit
any subsequent packets directly to that area
(possibly in a tunnel to the foreign agent, by
using the care-of-address).

Most of the messages, as described in
[RFC2002], are carried by UDP.

There are multiple options for realising this sce-

nario, like all subsequent packets may also pass

through the home agent and the mobile user may

be assigned a temporary address allowing it to

receive all packets directly without further assis-

tance of the foreign agent. This is then using the

second type of care-of-address (co-located).

In order to avoid addresses being kept even after

the mobile user has left a foreign area, the regis-

tration is only valid for a given time interval,

requiring periodical refreshing. In case a number

of mobile users are following a common means

of transportation (like an aeroplane or train) con-

taining a router itself, several levels of tunnelling

can be used where one level refers to the on-

board router.

The binding between the home address (e.g. A)

of the mobile node and its COA (e.g. FA) is kept

by the home agent. The binding will likely be

attached with a validity duration, implying that

binding updates should be initiated. A central

function is for the mobile node to detect that it

has moved to another domain, like using router

discovery and neighbour unreachability detec-

tion.

In this way the mobile node will be accessible

via its home agent. When the mobile node is

connected to its home domain, normal routing is

used. When a packet arrives at HA for which a

valid binding is given, the HA tunnels the packet

to the COA. When a mobile node receives a new

COA, the information on this address is sent to

its home agent.

Routing from the mobile node to the correspon-

dent node will follow normal routes. In the

opposite direction, packets may initially pass the

home agent. When the mobile node receives the

packets, it will learn that the correspondent node

Figure 13 Mobile IP

illustration

Address Care-Of-Address

AFA

Address binding table

Home Agent

home network HA

Foreign Agent

FA

foreign network

Mobile

Node, A CorrespondentNode

tunnelling