Checkpointing and Rollback Recovery Algorithms for Fault Tolerance in MANETs: A Review

Int. J. Advanced Networking and Applications
Volume: 6 Issue: 3 Pages: 2308-2313 (2014) ISSN : 0975-0290
2308
Checkpointing and Rollback Recovery
Algorithms for Fault Tolerance in MANETs:
A Review
Sushant Patial
Department of Computer Science, Himachal Pradesh University Shimla-5
Email: patialsushant @gmail.com
Jawahar Thakur
Department of Computer Science, Himachal Pradesh University Shimla-5
Email: jawahar.hpu @gmail.com
-------------------------------------------------------------------ABSTRACT--------------------------------------------------------------
Mobile Ad Hoc Networks (MANETs) are emerging as a major technology in mobile computing. A MANET is a
collection of mobile devices or nodes that communicate with each other using wireless links without availability of
any static infrastructure or centralized control. A node in such a network should be fault tolerable and failure free
execution of processes on the network nodes is vital. In order to make devices fault tolerant checkpoint based
recovery technique can be used. Checkpointing is a technique that can be used to make device or node fault
tolerant and reduce the recovery time in case of failure. It takes the snapshot of current application state of
process and stores it in some memory area and then using it to resume the computation from current checkpoint
instead of resuming it from the beginning. Some limitations of MANETs such as mobility, dynamic topology,
limited bandwidth of channel, limited storage space and power restrictions makes checkpointing as a major
challenge in mobile ad hoc networks. This paper presents the survey of some existing algorithms, which have been
proposed for making MANETs fault tolerant and implementing or deploying checkpointing in mobile ad hoc
network.
Keywords – Checkpointing, Dynamic topology, Fault tolerant, MANETs Mobile computing, Mobile Support
Station (MSS), Recovery.
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Date of Submission : September 16, 2014 Date of Acceptance : November 03, 2014:
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I. INTRODUCTION
Network is a collection of devices called nodes that
allow communication among users and shares the
resources using some set of rules also called as protocols.
Network can be broadly classified into two types. One is
the wired networks which are connected through a
physical medium or cables, such as Ethernet cables or
phone lines. And the other is the wireless networks, using
wireless networking cards that send and receive data
through the air with the help of radio waves. Wireless
networks are gaining much popularity these days since
they help in communication in areas where network
wiring is almost impossible. On the other hand a
distributed system consists of several processes that
execute on computers that are separated geographically
by some distance and coordinate via message-passing
with each other to achieve a common objective [1]. In a
traditional distributed system all hosts are stationary.
Advances in computers with wireless communication
interfaces and satellite services these days have made it
possible for mobile users to perform distributed
applications and to access information anywhere and at
anytime. A new computing environment in which some
hosts are mobile computers connected by wireless
communication networks and some are stationary
computers connected to a fixed network is called as
distributed mobile computing environment. Thus,
distributed systems can have a special type called
distributed mobile system where some of its hosts are not
stationary. A distributed mobile system is characterized
by the mobility and poor resource of mobile hosts.
Mobile ad-hoc network (MANET) is an autonomous ad
hoc wireless networking system which consists of
independent nodes that move frequently and changes the
network connectivity. MANETs are collection of self-
organizing mobile nodes with dynamic topologies and no
fixed infrastructure where nodes are autonomous and
independent wireless devices. From the fault tolerance
prospective the MANETs are highly vulnerable and
challenging, basically due to its complex system
infrastructure-less network where the wireless mobile
nodes are dynamically attached to temporary topology.
Nodes do not have to follow any constraint or rules.
Nodes can move freely in the network, it indicates that
host movement and topology changes frequently.
The advantages of ad hoc network are that they can be
easily deployed, their robustness, flexibility and they
inherently supports mobility of devices. The topology of
ad hoc network is very dynamic because of the host
mobility, so MANETs can be very useful where instance
communication is required in emergency like military
applications, mobile conferencing and inter vehicular
communication [2]. When a fault or failures of process

2309
occur, an application with mobile hosts must rollback to a
consistent global checkpoint as close as possible to the
end of the computation. The main constraints of ad-hoc
networks includes dynamic network topology, limited
bandwidth, variability of the links, low node capability in
terms of limited power or battery, no centralized control,
broadcasting nature of transmission and packet losses[2].
Some other constraints to these networks are frequent
disconnections/partitions/joins of links of nodes, no stable
storage or limited stable storage, different mobility pattern
of nodes, devices are vulnerable to physical security
threats.
As fault-tolerance is an important design issue in building
a reliable Ad hoc network, MANETs must be fault
tolerant that is they must be able to recover even after a
failure occurs. Transient failures in system are the one
which stays for short duration time during operation only.
If the fault is recognized in the system, the fault tolerance
technique allows the system to resume the computation
from the last consistent state and thus reducing the
recovery time. There are various recovery scheme that
have been proposed to make the system fault tolerant such
as log based recovery, rollback recovery and
checkpointing. This paper has been organized into
different sections. Section II gives description about
checkpointing and its types. Section III describes about
the work done by various research scholars in the field of
checkpointing in MANETs. Finally the conclusion is
given in the Section IV.
II. CHECKPOINTING AND ITS TYPES
Checkpointing is a technique for inserting fault tolerance
into computing systems. It basically consists of taking a
snapshot of the current application state, storing it on
some memory area and later on using it for restarting the
execution from that particular point in case of failure. It is
a fault tolerant technique in which normal processing of a
process is interrupted specifically to preserve the status
information necessary and then to allow resumption of
processing at a later time. Computation may be restarted
from the current checkpoint instead of repeating it from
the beginning if a failure occurs. Checkpoint based
rollback recovery is being used as a technique in various
areas like scientific computing, mobile computing,
distributed database, telecommunication and critical
applications in distributed and mobile ad hoc networks.
Checkpoint-based rollback recovery restores the system
state to the most recent consistent set of checkpoints
whenever a failure occurs [3]. Checkpoint based rollback
recovery is not suited for applications that require
frequent interactions with the outside world, since such
interactions require that the observable behavior of the
system through failures. Checkpointing technique can be
basically classified into three categories that are:
uncoordinated checkpointing, coordinated checkpointing
(blocking and non-blocking) and communication-induced
check pointing.
• Uncoordinated Check pointing: It allows any
process can initiate checkpointing. Each process can
take a checkpoint in any critical state and does not
need to coordinate with other processes in the system.
[4].
• Coordinated Checkpointing: This type of
checkpointing simplifies recovery and with no domino
effect, since each process is restarted from its most
recent checkpoint not from the beginning. Coordinated
checkpointing requires that only one permanent
checkpoint is maintained on stable storage by each
and every process which helps in eliminating the need
for garbage collection and reducing storage
overhead[5].
• Blocking Checkpoint coordination: These
algorithms force all relevant processes in the system to
block their computation during checkpointing latency
and hence degrade system performance.
Checkpointing includes the time to trace the
dependence trees and to save the states of processes on
some stable storage, which may take some time.
Therefore, these algorithms may degrade the
performance of system [6].
• Non-blocking Checkpoint Coordination: In this
protocol, a checkpoint is taken by the initiator and
then a checkpoint request is broadcasted to all the
processes. When each process receives a request it
takes a checkpoint and before sending any application
message rebroadcasts the request to all processes. This
protocol works on assumption that the channels are
reliable and FIFO based [7].
• Checkpointing with Synchronized Clocks: A
process takes a checkpoint and waits for a period that
equals the sum of the maximum deviation between
clocks and the maximum time to detect a failure in
another process in the system. It can be assured to the
process that all the checkpoints belonging to the same
coordination session have been taken without the need
of exchanging any messages [7].
• Minimal Checkpoint Coordination: It is desirable to
reduce the number of processes involved in a
coordinated checkpointing session. This can be done
since only those processes that have communicated
with the checkpoint initiator either directly or
indirectly since the last checkpoint need to take new
checkpoints [8].
• Communication - induced Checkpointing: This type
of checkpointing avoids the domino effect while
allowing processes to take some of their checkpoints
independently [8]. It forces each process to take
checkpoints based on information piggybacked on the
application. However, process independence is
constrained to guarantee the eventual progress of the
recovery line and therefore processes may be forced to
take additional checkpoints. The checkpoints that are
taken by a process independently are called as local
checkpoints, while those that are taken by a process
forcibly are called forced checkpoints.
• Model-based Checkpointing: It relies on preventing
patterns of communications and checkpoints that
could result in inconsistent states among the existing
checkpoints.[8]

2310
• Index based Communication Induced
Checkpointing: This type of checkpointing works by
assigning monotonically increasing indexes to
checkpoints, such that the checkpoints having the
same index at different processes form a consistent
state [8].
• Hybrid Checkpointing: There might be some
situations where we require two or more
checkpointing schemes in one algorithm; such type of
checkpointing where combination of checkpointing
schemes is used is called as hybrid checkpointing.
III. ANALYSIS OF CHECKPOINTING ALGORITHMS
FOR MANETS
There are various checkpointing schemes or algorithms
that have been developed for reducing the time for
recovery if any failure occurs. The flexibility introduced
by mobile computing brings new challenges to the area of
fault tolerance. Failures become common which were rare
with fixed hosts, fault detection and message coordination
are made difficult by frequent host disconnection. Some
of the checkpointing algorithms developed for MANETs
are as follows:
Masakazu and Hiroaki [9] proposed an approach called
Checkpointing by flooding method. According to this
protocol ad hoc networks works without any stable
storage and enough communication bandwidth. Here,
flooding is used to deliver a checkpoint request message.
This message carries the state information of a mobile
computer and stored into neighboring mobile computers.
Intermediate mobile computer stores a candidate of a lost
message after its detection on its transmission route.
Singh and Jaggi [10] proposed a Concurrent
Checkpointing and Recovery scheme. They presented a
staggered approach in their work to avoid resources
simultaneous contention. The events which would
normally happen at the same time are forced to start or
happen at different times by staggering. This protocol
logs minimum number of messages and does not need any
FIFO channels. It successfully handles the overlapping
failures in ad hoc networks and supports concurrent
initiation of checkpoints.
Saluja and Kumar [11] in their work discussed a new
minimum process checkpointing procedure for mobile ad-
hoc networks which is based on the cluster based routing
protocol that reduces routing traffic and prohibitive of
flooding traffic in discovery of routes. A checkpoint can
be initiated by any of the process (MH) in this algorithm,
first it takes tentative checkpoint before sending message
and then sends request to CH and then on the behalf of
MH the CH coordinates checkpointing operation with
other processes. Only those process participate in
checkpointing operation with the initiator which are
present in the minimum processes set created with Z-
dependencies notion. This algorithm ensures that blocking
of processes does not take place and takes no useless
checkpoints as it maintains exact dependencies and
piggybacks checkpoint sequence number, dependency
vector onto the normal message communication.
Morita and Higaki [12] presented an approach to mission
critical application where the system can have both
mobile stations and fixed stations. Due to several
limitations of mobile stations checkpointing is recorded
asynchronously whereas fixed station take checkpointing
synchronously. During the recovery process mobile
station will get local state from consistent set along with
message logs stored in stable storage. Communication and
synchronization overheads are minimized as this
algorithm separates content and order of information.
Juang and Liu [13] provided with an independent
checkpointing and rollback recovery technique in
multihop communication environment. In the state
transition interval called interval index depends on
message received by the process and state of process, that
give way to the development of dependency matrix
considering both types of dependencies that are transitive
and direct dependencies. All the communication is
transmitted from cluster to cluster goes through the
clusterhead node CH which acts as a local coordinator of
transmissions within the cluster. CH maintains the
dependency matrix and message logs hence no additional
overheads are present on MH and also when process fails
this scheme covers resending of lost messages.
Biswas and Neogy [14] suggested a mobility aware
checkpointing and failure recovery algorithm for cluster
based mobile ad hoc networks (MANETs) in which
checkpoints of mobile nodes are saved on neighboring
nodes if the mobility of a node among the clusters crosses
the threshold value and if the failure occurs recovery of
node is done through the mobile cluster head. This
algorithm shows the minimum checkpoint and log
overhead per mobile host per checkpoint interval and no
orphan/lost messages.
Tuli and Kumar [15] introduced minimum process
coordinated checkpointing scheme for ad hoc network.
This scheme allows minimum number of nodes to take
checkpoint and uses few control messages to produce
consistent global state. Cluster based routing protocol
used for the ad hoc network hence containing cluster head
and ordinary nodes, additionally cluster head sends
aggregated data information to base station which saves
cluster head state periodically, If some fault is detected or
a cluster head fails, then its failure is detected by the base
station (BS) and responsibility of the cluster head is
assigned to a new node in the cluster. If a transient fault
occur at the cluster head, the cluster can quickly recover
from it using checkpointing this approach addresses
recovery process for cluster head and ordinary nodes
without having any additional overheads.
Men et al. [16] presented a checkpointing and rollback
recovery scheme which is best suitable for the cluster-
based multi-channel ad-hoc wireless network
management where the MHs are controlled by the cluster
head to take the checkpoints in checkpoint beacon
intervals and in case of failure rollback to a consistent

2311
state. Every beacon interval consisting of different phases
depicts for checkpointing and recovery scheme capable of
handling ordinary host transient failures and also the crash
of gateway which are present between two neighboring
clusters. Beacon packet is used by CH which contains
clock data, traffic indication messages and data window
and also holds some other variables such as index of
ordinary node queue, checkpoint and reply messages.
There is no domino effect in the recovery scheme and the
recovery of the failure process can start from its latest
local consistent checkpoint then messages are restored
and repeated messages for rollback will be discarded to
make gateway consistent.
Bhalla [17] asserted global snapshot for host recovery that
helps in independent dependency tracking in a mobile ad-
hoc computing environment that without any message
overheads and delays finds the consistent global state.
The process to perform recovery computation is to inform
all other processes of its recovery state and then each
process verify their highest consistent state, if not
satisfied maps the processes to be rolled back to the
optimal recovery state. This algorithm assures for each
node failure n-1 messages are sent within the system of n
nodes. No orphan or no lost messages exist after the
failure recovery.
Cao and Singhal [18] introduced the concept of “Mutable
Checkpoint”. The Mutable checkpoint is neither a
tentative checkpoint nor a permanent checkpoint to design
efficient checkpointing algorithms for mobile computing
system. We can save these checkpoints anywhere (e.g. in
the main memory or local disk of MHs). The overhead of
transferring large amount of data to stable storage at
MSSs over the wireless network can be avoided by taking
a mutable checkpoint. This technique tries to minimize
the number of mutable checkpoints. This approach is a
non blocking algorithm which avoids the avalanche effect
and forces only a minimum number of processes to take
their checkpoints on the stable storage.
Neves and Fuchs [19] in their work described a
checkpoint protocol which is well adapted to the
characteristics of mobile environments. The protocol
saves consistent recoverable global states easily without
any need of exchanging messages. Whenever a local
timer expires a process creates a new checkpoint. The
checkpoint timers are kept approximately synchronized
by using a simple mechanism. Mobile host locally saves
soft checkpoints, and stable storage stores the hard
checkpoints. The protocol adapts itself and changes
behavior according to different networks by changing the
number of soft checkpoints that are created per hard
checkpoint.
Table1: Comparison of different checkpointing algorithms for Mobile Ad Hoc Network
Author
Checkpointing
Approach
Advantages Disadvantages Channel
Stable storage
Location
Ono Masakazu
and Higaki
Hiroaki in [9]
Uncoordinated
• Can be used in mission-critical
network applications
• Communication overhead for
taking
global checkpoint is reduced.
• It has additional overheads
and control messages
associated with it as
Checkpoint request message
is delivered by flooding.
FIFO
Neighboring
mobile
devices
A.K.Singh, P
.K. Jaggi, [10]
Uncoordinated
• Staggered approach to avoid
simultaneous contention for
resources.
• Successfully handles the
multiple failures.
• Suitable only for Small
sized message logs.
Non
FIFO
Own Memory
Saluja and
Kumar [11]
Coordinated
• Minimises useless checkpoints
by maintaining exact
dependencies among
processes.
• Ensures Zero blocking time.
• Piggybacks checkpointing
sequence number and
dependency vector on to the
normal messages.
• Dependency vector of
mobile hosts are maintained
at CHs so memory space of
Cluster head is wasted.
FIFO
Local mobile
support
stations
(MSS) at
cluster head
(CH)
Morita and
Higaki [12]
Hybrid(Coordi
nated and
uncoordinated)
• Supports both mobile and
fixed
stations.
• Reduced communication and
synchronization overheads.
• Overheads may be incurred
due to large amount of
processing as it involves
two different types of
checkpointing schemes.
FIFO
Local Mobile
support
station(MSS)
Juang and Liu
[13]
Uncoordinated
• Asynchronous Recovery and
an efficient rollback algorithm.
• The mobile hosts need to
rollback only once and can
immediately resume operation
without waiting for any
• It has to resend some of the
lost messages after finishing
the recovery algorithm
which can lead to wastage
of time and resources.
FIFO Cluster Head

2312
coordination message from
other mobile hosts.
Biswas and
Neogy [14]
Uncoordinated
• Random mobility of cluster
members and cluster heads is
considered.
• Reduce storage overhead of
cluster head and supports for
efficient recovery.
• Threshold value is defined to
take checkpoint if MH crosses
from its cluster.
• If the node fails, the data has
to be searched and retrieved
for recovery along with last
saved checkpoint.
• This search and retrieval
cost increases with
increasing ‘cluster-change-
count’ and is added to total
recovery cost of a failed
mobile node.
FIFO
Neighboring
nodes
Tuli and Kumar
[15]
Coordinated
• It does not consider useless
checkpoints
• The energy consumption and
recovery latency are reduced
when a cluster head fails.
• Checkpoint is taken by the
minimum number of
processes.
• As it takes checkpoint for
minimum number of
processes, it is difficult and
time consuming to decide
which process should take
the checkpoints.
FIFO Cluster Head
Men et al [16] Coordinated
• Cluster-basedmulti-channel
management protocol.
• Local consistent checkpoint-
two consecutive beacon
interval.
• Rollback recovery in one
beacon interval.
• Additional power
consumption and memory
overhead is incurred.
FIFO
Mobile
supporting
stations
(MSS) at
cluster head
(CH
Bhalla [17] Independent
• Uses a modified cumulative
dependency tracking approach
for the recovery process.
• And also for the generation of
global snapshot.
• For recovery one message
needs to be sent to each
connected station to inform
them about the occurrence
of a failure which leads to
wastage of time and
bandwidth.
FIFO
Nodes own
stable
memory
Cao and
Singhal [18]
Coordinated
(Non blocking)
• Checkpoints can be saved
anywhere.
• Overhead of transferring
checkpoint information over
the network to the stable
storage in Mobile Support
Stations is minimized.
• May result in an
inconsistency as the number
of useless checkpoints in
may be exceedingly high in
some situations.
FIFO
Anywhere in
the main
memory or
local disks of
Mobile
host(MH)
Neves and
Fuchs[19]
Coordinated
(Indirect)
• Uses two different types of
checkpoints to adapt to the
current network
characteristics.
• Uses time to indirectly
coordinate the creation of
recoverable consistent
checkpoints.
• Saves consistent recoverable
global states without any need
to exchange messages.
• As it saves two types of
checkpoints there is wastage
of some memory resource.
FIFO
Soft
checkpoint
saved locally
in the mobile
host, hard
checkpoints in
the stable
storage
IV. CONCLUSION
Fault tolerance is a major research area in the Mobile Ad
Hoc Networks. No doubt MANETs have a great advantage
of being usable in remote areas where the wired
communication media cannot reach but still there are
many important issues in MANETs to be handled like
network stability, low communication bandwidth, power
consumption of mobile nodes, time and memory
overheads, large stable storage constraints, frequent node
disconnections/join and traffic load with the cluster, which
makes implementation of fault tolerance techniques
difficult in them as compared to distributed system since

2313
they do not have constraints like MANETs in them. So the
algorithms are developed for less overhead, reducing
number of checkpoints for saving both time and memory
space by using different approaches. It can also be done
that making the techniques implementable on the
distributed systems also implementable in MANETs by
making some negotiations. We can use a better approach
for node arrangement for checkpointing process or a
hybrid checkpointing strategy can be used which is a
combination of two or more checkpointing schemes.
REFERENCES
[1] Zhonghua Yang, Chengzheng Sun, Abdul Sattar, and
Yanyan Yang, Consistent Global States of
Distributed Mobile Computations, Proceedings of
International Conference on Parallel and Distributed
Processing Techniques and Applications, LasVegas,
Nevada, USA, 1998.
[2] Andrea J. Goldsmith, Stephen B. Wicker, Design
Challenges for energy constrained ad hoc wireless
networks, IEEE wireless Communications, 2002.
[3] Randall B., System Structure for Software Fault
Tolerance, IEEE Trans On Software Engineering,
1(2), 1975, 220-232.
[4] Y. Wang and W.K. Fuchs, Lazy Checkpoint
Coordination for Bounding Rollback Propagation,
Proc. 12th Symp. Reliable Distributed Systems, 1993,
78-85.
[5] Tamir Y., Sequin C.H., Error Recovery in
Multicomputers using Global Checkpoints, In
Proceedings of the International Conference on
Parallel Processing, 1984, 32-41.
[6] Guohong Cao, Mukesh Singhal, On Coordinated
Checkpointing in Distributed Systems, IEEE
Transactions on parallel and distributed systems,
Vol. 9, No. 12, 1998.
[7] E.N. Elnozahy, L. Alvisi, Y.M. Wang and
D.B.Johnson, A Survey of Rollback-Recovery
Protocols in Message-Passing Systems, ACM
Computing Surveys, 34(3), 2002, 375-408.
[8] Franco Zambonelli, On the Effectiveness of
Distributed Checkpoint Algorithms for Domino Free
Recovery, IEEE Proceeding of HPDC-7, Chicago,
1998.
[9] Masakazu Ono, Hiroaki Higaki. Consistent
Checkpoint Protocol for Wireless Ad-hoc Networks,
The International Conference on Parallel and
Distributed Processing Techniques and Applications,
Las Vegas, Nevada, USA, 2007, 1041-1046.
[10] A. K .Singh, P. K. Jaggi, Staggered Checkpointing
and Recovery in Cluster Based Mobile Ad Hoc
Networks, International Conference on Parallel,
Distributed Computing technologies and
Applications Springer Proceedings 2011.
[11] K. Saluja and Praveen Kumar. Transitive
Dependencies Tracking in Minimum-Process
Checkpointing Protocol for Mobile Ad hoc
Networks, International Journal of Computing
Science and Communication Technologies, Vo. 4,
No. 1 (ISSN 0974-3375) , 2011.
[12] Y. Morita and H. Higaki, Hybrid Checkpoint Protocol
for Supporting Mobile-to-Mobile Communication,
Information Networking, Proceedings 15th
International Conference on, 2001, 529 – 536.
[13] T.Y.T. Juang and M. C. Liu, An Efficient
Asynchronous Recovery Algorithm In Wireless Mobile
Ad Hoc
Networks, Journal of Internet Technology, Vol. 3, No.
2, 2002, 143-152.
[14] S. Biswas and S. Neogy, Checkpointing and Recovery
using Node Mobility among Clustersing Mobile Ad Hoc
Network, Advances in Intelligent Systems and
Computing, Vol. 176, 2012, 447-456.
[15] R. Tuli and P. Kumar, Minimum process
coordinated Checkpointing scheme for ad hoc
Networks, International Journal on AdHoc
Networking Systems, Vol.1, No.2, 2011, 51-63.
[16] C. Men, Z. Xu and X. Li, An Efficient Checkpointing
and Rollback Recovery Scheme for Cluster-based
Multi-channel Ad-hoc Wireless Networks,
Proceedings of the IEEE International Symposium on
Parallel and Distributed Processing with
Applications, IEEE Computer Society Washington,
DC, USA, 2008, 371-378.
[17] S. Bhalla, Independent Dependency Tracking in a
Mobile Adhoc Computing Environment.
Communication System Software and Middleware,
First International Conference on Comsware, 2006,
1-4.
[18] G. Cao and M. Singhal, Mutable Checkpoints: A
New Checkpointing Approach for Mobile
Computing Systems, ACM Symposium on Principles
of Distributed Computing, 1999.
[19] N. Neves and W.K. Fuchs, Adaptive Recovery for
Mobile Environments, Communications of the ACM,
vol. 40, no. 1, 1997, 68-74.

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