AN EFFICIENT FAULT-TOLERANT APPROACH FOR MOBILE IP IN WIRELESS SYSTEMS


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INTRODUCTION

Due to the rapid progress of wireless communication technology, there is a growing demand for accessing data by wireless systems .Mobility is one important characteristic of wireless systems .Each wireless user (mobile node) may change its location several times during the execution of its data service. To avoid interrupting the ongoing data session, The Internet Engineering Task Force (IETF) has defined Mobile IP as one which enables wireless users to maintain ongoing data sessions without interruption while changing locations.
To provide the functionality of Mobile IP in wireless system a number of mobility agents are required in the architecture of the wireless system. The mobility agents are classified into two types: Home agent (HA), Foreign agent (FA).The FA logically connects with several radio access networks (RANs) to form a wireless data serving area. The HA maintains the current addresses of mobile nodes(MNs).When an MN stats a data session, a data request is first sent to the located RAN of the MN, then to a serving FA and, finally ,to a corresponding application server. The application server processes the data requests and sends back the packets of the handling results to the MN. The packets send to the MN will be first intercepted by the MN’s corresponding HA. The HA looks up the current address of the MN and tunnels the packets to the FA serving the MN. Then, the FA detunnels the packets and forwards then to the MN. From this scenario of a wireless data session, we can know that, if a failure in a FA, all MNs located in its data serving area cannot perform wireless data sessions again. Likewise, if an HA crashes, the response packets of the data session can not be sent back to the corresponding MN.
This paper presents an efficient approach to providing fault-tolerant capability in the wireless system with Mobile IP functionality.

BACKGROUND:

This section describes the background materials of this paper. Firstly, an overview of the Mobile IP is given. Then the system model is described. Next, the fault assumption is made. Finally, related work is reviewed.

MOBILE IP Overview:

Mobile IP is designed to support node mobility by using two types of mobility agents: home agent (HA) and foreign agent (FA).Initially, each mobile node (MN) has a unique address to be managed by an HA on its home network. If an MN moves from its home network to foreign network, a “care-of-address” (COA) is allocated to the MN to reflect the MN’s current point of attachment. The FA on the foreign network also adds an entry corresponding to the MN in the visitor list. Then, a register message is sent to the MN’s HA to create or modify the MN’s mobility binding which associates the MN’s home address, the located FA, and the COA. Packets from a correspondent host (CH) to an MN or addressed to the home address of the MN. If the MN is away from its home network, the HA on the home network can intercept the packets and tunnel them to the located FA of the MN. Then, the FA decapsulates the packets and forwards them to the MN.


SYSTEM MODEL:

The system model considered in this paper refers to the architecture of a third generation (3G) wireless system. The system model contains three major components:
• Mobile node(MN)
• Radio access network(RAN)
• Core network
MN interacts with RAN to obtain radio resources for performing wireless data sessions.
The RAN provides the transmission across the air interface. With the core network, it is an IP based network and contains the following equipment:
• Foreign agent(FA)
• Home agent(HA)
• Intermediate routers
The FA and HA provide the wireless data session with Mobile IP functionality. The intermediate routers assist FA’s and HA’s to forward packets .In addition ,between RAN’s and FA’s there is an interconnection network to deliver the wireless data requests from MN’s to the core network and to send back the response packet from the core network to MN’s. The interconnection network can be a frame relay network, ATM network, or IP network.



FAULT ASSUMPTION:
In this paper, failures are only assumed to occur in mobility agents. To announce the presence of a mobility agent (FA or HA), the mobility agent periodically transmits an agent advertisement message on its located subnet. When a failure occurs in a mobility agent, the failure can be detected by not receiving an agent advertisement message within a period of time. The fail-stop assumption is also made so that the faulty mobility agent cannot send any advertisement message again.
As an FA fails, its in-processing data requests will be lost. For a faulty HA, its in-processing response packets are also lost. However, these lost data requests and response packets can be retransmitted by an end-to-end reliable Transport Control Protocol (TCP).

THE PROPOSED APPROACH:
This section presents a new approach to tolerating the failures of mobility agents. To avoid significant performance degradation, the approach dynamically selects multiple failure-free mobility agents to form a backup set for the faulty mobility agent. The work loads of the faulty mobility agent are redirected to the failure-free mobility agents in the backup set.


FAULT TOLERANCE OF FOREIGN AGENT
Upon detecting a failure in an FA, the MNs located in its serving area will not be able to execute wireless data sessions. The new arriving MNs are also affected. These MNs are called as FA_failure-affected MNs. To resume the data executive abilities of the FA_failure-affected MNs, a system initiated handoff is performed to dynamically select multiple failure-free FAs to organize a backup set for the faulty FA. Then, the FA_failure-affected MNs are virtually moved to the serving areas of the selected failure-free FAs.