The Internet Encyclopedia (Volume 3)

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840 WIRELESSINTERNET

Fortunately, IPv6 was designed with flexibility and mobil-

ity in mind and, as such, can support mobility in a more

optimal/natural manner. Unfortunately, IPv6 is not being

adopted as quickly as expected, and is currently only be-

ing used in some small isolated locations and uses IPv4

tunnels for connectivity.

Below we will highlight some of the major features of

MIPv4.

MIP Terminology

In RFC 2002 describing IP mobility support (Perkins,

1996), the following entities were identified in conjunc-

tion with MIP:

Mobile node (MN)—A device that supports MIP and can

change its location without affecting its communica-

tion abilities so long as layer 2 connectivity is available.

Home agent (HA)—A device in the home network (i.e.,

the subnet to which the MN’s IP address belongs) of

the MN that keeps track of the location of an MN. It

tunnels packets destined for the MN to its new address.

The HA is typically a router on the home network.

Foreign agent (FA)—A device in the current foreign net-

work (i.e., the network that the MN is currently visit-

ing) of the MN that can forward packets sent to it for

the MN to the MN if it terminates the tunnel set up by

the HA. The FA is typically the default router on the

foreign network.

Care of address (COA)—IP address that defines the current

location of the MN. It is the address to which the HA

forwards all packets for the MN, thus terminating the

tunnel. There are two possible scenarios for COA:

1. The COA is located at the FA; i.e., it is the IP address
   of the FA. The FA then terminates the tunnel and for-
   wards the packets to the MN. This approach allows
   many MN to share one IP address.


2. The MN temporarily acquires a new IP address.
   For this scenario, the MN terminates the tunnel. Al-
   though a convenient approach, it does require that
   several IP addresses be made available for mobile
   devices, which may not always be the case. This is
   referred to as a colocated COA.
   Correspondent node (CN)—A device that communicates
   with the MN. It is unaware of the location of the MN
   and just simply uses the MN’s original IP address for
   packet forwarding.

From the above we see that a tunnel starts at a HA and

terminates at either the FA or the MN. The HA keeps track

of where the MN is. The FA is not always needed for MIP

functionality although it may be necessary for security

purposes.

Operation of MIP

The operation of MIP consists of three steps: agent dis-

covery, registration, and encapsulation/routing/tunneling.

Below, we discuss each step:

Agent discovery—Consists of broadcast messages used by

the MNs to detect whether they have moved. These

messages are sent out periodically by the default router

(FA) on a subnet. If the MN has not heard an advertise-

ment, it will solicit for one.

Registration—All MNs are required to register with the HA

and the FA (if used). As registrations expire, MNs must

re-register periodically. Any move to a new location re-

quires a new registration.

Encapsulation/routing/tunneling—All packets that arrive

for the MN on the home subnet are claimed by the HA.

The HA proceeds to encapsulate them to reflect the new

COA of the MN and then routes all the traffic to the MN

on the foreign network. The CN sends all messages to

the MN’s IP address, and the HA relays them, via the

tunnel, to the COA. The MN sends its messages directly

to the CN. This type of communication results in what

is referred to as triangular routing: CN to HA, HA to

MN, MN to CN. To improve the performance of MIP,

route optimization has been proposed, which allows

the CN to learn the COA of the MN and correspond

with it directly. This does mean that the CN must be

informed of any change in location of the MN and re-

quires additional authentication procedures.

Security in MIP

Security is one of the main concerns in any mobile en-

vironment. It is necessary that all devices involved with

data reception and forwarding be authenticated to ensure

their identities. MNs must register periodically with the

HA, which involves an authentication process. If an MN

moves, it must re-register at the new location. If route op-

timization is used, the CN must authenticate itself before

being capable of communicating directly with the MN.

TCP for Wireless Networks

The TCP protocol was not designed to operate over chan-

nels that are lossy in nature. It uses timers at the sender

to determine the state of congestion in the network. If an

ACK does not come back before the timer expiration, it

assumes that the link is congested and shuts itself down

by decreasing the congestion window size to one. It then

proceeds to retransmit the unacknowledged packet. TCP

does not take into account the possibility of a lost packet

due to channel conditions as a lost packet is directly in-

terpreted as congestion. Because of the proliferation of

wireless networks, it is imperative that the transport layer

understands the difference in performance of the wireless

link and not make erroneous assumptions as to the state

of the network. Several papers that propose modifications

to TCP to improve its performance over the wireless chan-

nels have appeared. We describe some of these approaches

below:

Indirect TCP (I-TCP) (Bakre & Badrinath, 1995)—It seg-

ments the TCP connection into two portions: one for

the wireline transmission, and one for the wireless seg-

ment. It uses the traditional TCP over the wireline seg-

ment, and a modified version for the wireless segment.

Assuming the use of MIP, the FA is the most likely can-

didate for acting as the proxy and acknowledging all

packets as well as terminating the connection. Over

the wireless segment, the proxy communicates with the