Mobile IP: Enabling Networks In Motion
BY KENT LEUNG
"Blood type AB, blood pressure's 88 over 58, pulse 130 and
dropping," the EMT relays to her partner as the ambulance races down
the narrow city streets with its sirens blasting. At a nearby hospital,
doctors in the emergency room prepare for the arrival of the patient.
Through a wireless Internet connection to the hospital network, the
ambulance medics can access the patient's complete health record while
keeping doctors in the ER apprised of any changes that could jeopardize
his chance of survival. When the doctors receive the patient ten minutes
later, they have received updates on his heart rate, breathing, and other
vital signs transmitted downloaded from the ambulance Internet in real
This scenario is not far from reality, as Mobile IP technologies are
enabling devices like patient monitors, voice over IP phones, and computer
laptops to communicate seamlessly while traveling between different
locations or networks. In an era when maintaining continuous connectivity
while roaming has become increasingly important to every day life, it has
become even more crucial for mission-critical fields such as the emergency
medical profession and the armed services. So, why Mobile IP? Why it is
important? How does it work? Furthermore, what are some of the future
applications for "networks in motion?"
WHY MOBILE IP?
Mobile IP, an open standard (RFC 2002) specified by the Internet
Engineering Task Force (IETF) in 1996, is the technology that enables
users to maintain connectivity while roaming between IP networks. IP
addressing and routing in networks are fixed at locations, so a device on
a network is reachable because it has an address on the network. The IP
address is analogous to a phone number associated with a telephone jack.
If a phone remains plugged into a particular jack, it is reachable via the
phone number assigned to that jack.
The problem occurs when a person having a phone conversation attempts
to move to a different location and plug the same phone into a new jack.
Because the circuit was designed to function with the original jack, the
conversation ceases. IP networks encounter the same challenge. When a
device is no longer associated with its former IP address on its home
network, its active sessions are dropped. Mobile IP was created to enable
users to retain the same IP address while traveling to a different
network, thus ensuring that a roaming individual could continue
communications without dropping the sessions or connection.
MOBILE IP: THREE-PART HARMONY
Mobile IP is comprised of three entities: The Mobile Node, the Home Agent,
and the Foreign Agent. A Mobile Node is a device capable of performing
network roaming. Examples of Mobile IP clients are cell phones, PDAs, or
laptops whose software enables roaming capabilities. The Home Agent is a
router on the home network serving as the anchor point for communications
with the Mobile Node; it tunnels packets to the roaming Mobile Node. The
Foreign Agent is a router that functions as the Mobile Node's point of
attachment when it travels to a foreign network, delivering packets from
the Home Agent to the Mobile Node.
Common terminology in Mobile IP also includes the Care-of Address and
Correspondent Node. The Care-of Address is the termination point of the
tunnel toward the Mobile Node when it is not on its home network, while
the Correspondent Node is the device that the Mobile Node is communicating
with, such as a Yahoo! Web server.
To facilitate an "always-on" connection for Mobile Nodes roaming
between different networks, Mobile IP begins with the process of agent
The Home Agent and Foreign Agent continuously advertise their services
on the network. The Mobile Node, upon receiving the advertisements,
discovers these agents and their offered services, and learns whether it
is at home or if it has moved to a foreign network. Within the
advertisement, the agent specifies whether it is a Home Agent, Foreign
Agent, or both; its Care-of Address; the types of services it will
provide, such as Reverse Tunneling; and the allowed registration lifetime
or roaming period, which may be extended for the Mobile Node.
When the Mobile Node hears a Foreign Agent advertisement, it detects
that it has moved outside of its home network and begins registration.
To formulate its registration request, the Mobile Node utilizes the
information captured within the Foreign Agent's advertisement, protecting
the integrity of this data with its Home Agent-shared key before sending
the request to the Foreign Agent. As the liaison between the Mobile Node
and the Home Agent, the Foreign Agent then processes and relays the
request to the Home Agent, which confirms the validity of the Mobile Node
using the same shared key for authentication. The Home Agent subsequently
constructs a mobility binding, which maps the Mobile Node to the Care-of
Address -- the location where the Mobile Node now actually resides within
the foreign network.
Mobile IP provides two options for obtaining a Care-of Address: The
Mobile Node can procure it from a Foreign Agent or, alternatively it may
directly acquire a collocated Care-of Address, which actually represents
its current position on the foreign network. A Mobile Node that obtains a
collocated Care-of Address will consume an address on the foreign network,
while a node that facilitates the process through a Foreign Agent can
share the address associated with that agent with other Mobile Nodes.
Because registration automatically expires on both the Home Agent and
Foreign Agent, the Mobile Node re-registers to maintain its attachment on
the foreign network. The Mobile Node can also de-register to explicitly
notify the agents that it is no longer roaming.
In order to transport packets between the Mobile Node and its home
network, the Home Agent creates a tunnel to the Care-of Address, and then
sends a registration reply back to the Foreign Agent. The Foreign Agent --
or Mobile Node, in the collocated Care-of Address case -- also constructs
a tunnel to the Home Agent for successful registrations, and relays the
reply to the Mobile Node. Finally, the Mobile Node authenticates this
reply, confirming the agent's awareness that it is roaming. Registration
sets up the routing for transporting packets to and from the Mobile Node,
a process that is accomplished using tunneling.
The Mobile Node sends packets using its home IP address, effectively
maintaining the appearance that it is always on its home network. Thus,
even while the Mobile Node is roaming on foreign networks, its movements
are transparent to Correspondent Nodes.
Data packets addressed to the Mobile Node are routed to its home
network, where the Home Agent now intercepts and tunnels them to the
Care-of Address toward the Mobile Node. Tunneling consists of two primary
functions: Encapsulation of the data packet to reach the tunnel endpoint,
and decapsulation when the packet is delivered at that endpoint.
Typically, the Mobile Node sends packets to the Foreign Agent, which
routes them to their final destination, the Correspondent Node. However,
this data path is topologically incorrect because it does not reflect the
true IP network source for the data -- rather, it reflects the home
network of the Mobile Node. Because the packets show the home network as
their source inside a foreign network, an access control list on the
routers in the network called ingress filtering drops the packets instead
of forwarding them. A feature called Reverse Tunneling solves this problem
by having the Foreign Agent tunnel packets back to the Home Agent when it
receives them from the Mobile Node.
NETWORKS IN MOTION
When deployed by an IP device running the necessary software, Mobile IP
enables users to seamlessly roam between networks while they benefit from
an "always-on" connection. A Mobile Network takes this concept a
step further by allowing a cluster of devices on a network to roam without
requiring that each device have Mobile Node software.
The Mobile Router can be defined as a router that supports a Mobile
Network. This concept of networks in motion is specified in IETF RFC 2002.
The Mobile Router functions similarly to to a Mobile Node; the key
difference, however, is the fact that the Mobile Router is able to
maintain connectivity for an entire network rather than for just a single
Mobile IP client. Agent discovery operation remains the same. During
registration, the Home Agent creates an additional tunnel to the Mobile
Router and the Mobile Router creates a reciprocal tunnel.
When a packet is sent to a Mobile Network, the Home Agent encapsulates
the packet twice and sends it to the Foreign Agent. The Foreign Agent then
decapsulates and forwards the encapsulated packet to the Mobile Router,
which decapsulates the packet and delivers it to the appropriate node for
which it is currently maintaining connectivity. To the rest of the
network, the node still appears to be located on the Home Agent; however,
it exists physically on the Mobile Network of the Mobile Router.
In order to continue communications while roaming, IP devices on the
Mobile Network send packets first to the Mobile Router, which typically
forwards the packets to the Foreign Agent for routing. Again, ingress
filtering on routers in the network will cause packets to be dropped
because the data path is topologically incorrect. To prevent this from
occurring, the Mobile Router utilizes Reverse Tunneling as well,
encapsulating packets from IP devices and sending them to the Foreign
Agent, which encapsulates packets again before routing them toward the
Home Agent. The Home Agent decapsulates twice before forwarding the
original packets to their destinations.
IP NETWORKS UNPLUGGED
Now recalling our ambulance example, the Home Agent resides within the
hospital, while the Foreign Agents are attached to wireless access points
deployed throughout the city streets, providing wireless connectivity to
the ambulance. Inside the ambulance is the Mobile Router, which is
attached to a wireless access interface and maintains connections for
several different devices on its Mobile Network.
The ambulance's Mobile Network appears to be at the Home Agent, but the
network resides physically at the Mobile Router.
This setup enables doctors in the ER to monitor the patient's vital
signs without disruption. To access updated statistics from the ambulance,
the computer in the ER sends a request through the Home Agent, which
forwards the request to the Foreign Agent. The Foreign Agent then
transmits the information to the Mobile Router, which finally communicates
with the end device -- the monitor, electrocardiogram, or other equipment
being used in real time to record the patient's latest status. As the
ambulance races toward the hospital, the Mobile Router attaches to new
Foreign Agents and sessions continue.
The Mobile Router is especially useful for mission-critical
applications because it maintains "always-on" connectivity for
multiple devices that users may want to deploy within the same network. It
also ensures that any additional devices added to the network will
automatically support roaming capabilities.
As Mobile IP technology advances, it will be utilized in a wide range
of applications, enabling continuous connectivity not only to single
mobile users, but also in vehicles such as ships, trains, buses, and
airplanes. By allowing us to be connected anywhere, at any time we choose,
Mobile IP and its enhancements will enable us to harness the power of an
enormous resource -- the Internet -- anywhere we happen to travel.
Kent Leung is a senior software engineer in the IOS Technologies
Division (ITD) at Cisco Systems. Cisco
Systems is the worldwide leader in networking for the Internet. Cisco's
networking solutions connect people, computing devices, and computer
networks, allowing people to access or transfer information without regard
to differences in time, place, or type of computer system.
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