Mobile IP

Mobile IP
The Internet Protocol (IP) is the most successful network layer protocol in computing due to its many
strengths, but it also has some weaknesses, most of which have become more important as networks
have evolved over time. Technologies like classless addressing and Network Address Translation combat
the exhaustion of the IPv4 address space, while IPSec provides it with the secure communications
it originally lacked. The TCP/IP suite of protocols work well as long as all of the nodes in a network
stand still. Today, a conspicuous weakness of IP is that it was not designed with mobile computers
in mind. IP’s hierarchical addressing scheme assumes that once a node appears at a particular place
on the network, it remains there for good. While mobile devices can certainly use IP, the way that
devices are addressed and data is routed causes a problem when devices move from one network to
another, making it impossible for unmodified IP to allow a portable computer to maintain an “always
on” condition. At the time IP was developed, computers were large and rarely moved. Today, there
are millions of notebook computers and smaller devices, some of which even use wireless networking
to connect to the wired network. The importance of providing full IP capabilities for these mobile
devices has grown dramatically. To support IP in a mobile environment, a new protocol called IP
Mobility Support, or more simply, Mobile IP, was developed [496, 499, 500, 502, 515, 516].
Mobile computing and networking should not be confused with the portable computing and
networking in use today. In mobile networking, computing activities are not disrupted when the user
changes the computer’s point of attachment to the Internet. Instead, all the needed reconnection occurs
automatically, without interaction by the user. This means that users will have access to the Internet
at any time, anywhere, and not be bound to the locations of their offices and studies.
There are some technical obstacles that must be overcome before mobile networking can become
widespread. The most fundamental is the way the Internet Protocol routes packets to their destinations
according to IP addresses. These addresses are associated with a fixed network location much as a
nonmobile phone number is associated with a physical jack in a wall. When the packet’s destination
is a mobile node, this means that each new point of attachment made by the node is associated with
a new network number and, hence, a new IP address, making transparent mobility impossible [493].
The basic concept behind mobile IP is simple. A mobile device’s IP address must change as it
moves from network to network, and mobile IP allows it to do so. Applications require a constant IP
address, so it allows that too. The apparent conflict is resolved by maintaining two separate addresses
for each device [497].
Mobile IP enables the routing of IP data to mobile nodes. The mobile node’s home address
(assigned by Dynamic Host Configuration Protocol (DHCP) or some other mechanism) always identifies
the mobile node, regardless of its current point of attachment to the Internet or an organization’s
network. When away from home, a care-of address associates the mobile node with its home address
by providing information about the mobile node’s current point of attachment to the Internet or an
organization’s network. Mobile IP uses a registration mechanism to register the care-of address with a
home agent (HA). The HA redirects data from the home network to the care-of address by constructing
a new IP header that contains the mobile node’s care-of address as the destination IP address.
This new header then encapsulates the original IP datagram, causing the mobile node’s home address
to have no effect on the encapsulated datagram’s routing until it arrives at the care-of address. This type of encapsulation is also called tunneling. After arriving at the care-of address, each datagram is
de-encapsulated and then delivered to the mobile node [494].
A mobile node discovers its foreign and HAs via a process called agent discovery. During the
agent discovery phase, the home agent and foreign agent advertise their services on the network by
using the ICMP Router Discovery Protocol (IRDP). The mobile node listens to these advertisements to
determine if it is connected to its home network or a foreign network. The mobile node then registers
its current location with the foreign agent and HA during the registration process. The mobile node is
configured with the IP address and mobility security association (which includes the shared key) of its
HA. Thereafter, the mobile node sends packets using its home IP address, effectively maintaining the
appearance that it is always on its home network. Even while the mobile node is roaming on foreign
networks, its movements are transparent to correspondent nodes – this is sometimes called tunneling.
Mobile IP uses a strong authentication scheme for security purposes. All registration messages
between a mobile node and HA are required to contain the Mobile-Home Authentication Extension
(MHAE). The integrity of the registration messages is protected by a preshared 128-bit key between
a mobile node and HA [495].
The real charm of the Mobile IP solution is that most of the elements of the Internet do not need to
change. The server with which a mobile device is communicating does not need to do anything special.
Most of the protocol stack on the device itself can be blissfully unaware that the device is moving,
with the exception of the piece that negotiates with the foreign agent to establish the care-of address.
This plan works well in the mainstream of IP addressing, but complications are introduced by a
number of other protocols that have become commonplace on the Internet. DHCP, in particular, is an
example of an Internet protocol that needs modification to accommodate Mobile IP protocols. Right
now it is possible for a DHCP client to obtain an IP address and information about DNS servers,
gateway addresses, and resources on the local network. This capability is easily extended to support
dynamic discovery of available HAs. Foreign agents, however, must be discovered through different protocols, since the traveling device does not fully join the local network. A new option in DHCP
will do this.
Another Internet component that complicates Mobile IP is the network firewall. Mobile IP elegantly
routes packets from a mobile node directly to their destination, but a firewall that detects
a packet originating from within its network that has a return address from somewhere else may
become suspicious. In fact, many firewalls and border routers implement a feature called ingress
filtering, which blocks any packet that exhibits just these characteristics. Mobile IP can be modified
to encapsulate traffic from a mobile node and sent, as an intermediate step, to the HA, which can
then forward the data to the intended destination. However, this procedure puts a heavier load on the
network, and significantly increases the computational requirements of the HA. At this point in the
development of Mobile IP, either the new protocols or the standing firewalls will have to give in [497].
Figure 5.2 shows the principles of Mobile IP networking, illustrating a mobile node that is
connected to its home network, and hence has only a home address, and a mobile node that is
connected to a foreign network, with both a home and care-of address.