The detection of routing misbehavior in mobile ad hoc networks using the 2 ack scheme with olsr protocol

International Journal of Computer and Technology (IJCET), ISSN 0976 – 6367(Print),
International Journal of Computer Engineering Engineering
and Technology (IJCET), ISSN 0976May - June Print) © IAEME
ISSN 0976 – 6375(Online) Volume 1, Number 1,
– 6367( (2010),
ISSN 0976 – 6375(Online) Volume 1
IJCET
Number 1, May - June (2010), pp. 213-234 ©IAEME
© IAEME, http://www.iaeme.com/ijcet.html

THE DETECTION OF ROUTING MISBEHAVIOR IN
MOBILE AD HOC NETWORKS USING THE 2ACK
SCHEME WITH OLSR PROTOCOL
Mrs. S. A. Nagtilak
Department of Computer Engineering
S.C.O.E, Pune, E-Mail: saranagtilak@yahoo.co.in

Prof. U.A. Mande
Asst. Professors, Departments of Computer Engineering
S.C.O.E, Pune, E-Mail: uamande.scoe@sinhgad.edu

ABSTRACT
The operation of MANETs does not depend on preexisting infrastructure or base
stations. Network nodes in MANETs are free to move randomly. Therefore, the network
topology of a MANET may change rapidly and unpredictably. This paper presents the
existing methods to detect misbehavior in MANETs. Routing protocols used in such type
of networks generally based on the assumption that, all participating nodes will be fully
cooperative. But, due to the open structure node misbehavior may exist and packet loss
occurs. Among them one type of misbehavior is that some nodes will take part in routing
establishment processes but they do not respond to forward data packets and simply
dismiss the packets. The goal of this work is to simulate the 2ACK scheme in ad hoc
network. To reduce extra routing overhead, and packet loss, only a few of the received
data packets are acknowledged in the 2ACK scheme. It reduces the routing overhead. It
has been observed that by using the 2ACK scheme the packet delivery ratio is increased.
The result of the 2ACK-OLSR indicates that it reduces the packet loss and increase the
routing overhead.
KEYWORDS: 2ACK, SACK, S-TWOACK, beans, nuggets

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International Journal of Computer Engineering and Technology (IJCET), ISSN 0976 – 6367(Print),
ISSN 0976 – 6375(Online) Volume 1, Number 1, May - June (2010), © IAEME

I. INTRODUCTION
MANET mobile ad hoc network as the name suggests consists of a bunch of
entities called as nodes or hosts which communicate with each other exchanging
information with the help of intermediate devices called as routers. The links between
these so called nodes is generally invisible or called as wireless links. The structure of
such networks is not predefined or it does not depend on any base stations lonely but a lot
of other entities are also involved in the entire communication process. The topology of
such networks keeps on changing mainly due to a factor called as mobility.
Network clients in MANETs may move randomly. Therefore, the network
topology of a MANET can be change unpredictably and speedily. All network activities,
for instance forwarding data packets, and detecting the topologies which concern with
nodes themselves for execution either collectively or individually. Due to this changing
topology packet loss is a common phenomenon. Any wireless network consists of a lot of
nodes that interact with each other exchanging information continuously. As these nodes
have the flexibility of moving from one place to other, there may be cases wherein a
particular node which is a receiver for a particular packet, moves away from the range of
sender. However the sender is not aware of this scenario and it might still keep on
sending packets thus leading to packet and data loss.
The other case is a bit more interesting, whenever communication takes place
between any two nodes there are a lot of nodes involved in this communication process
acting as mediators. All these nodes agree to forward packets during the actual
communication process but one of them actually turns selfish during the data transfer,
this selfish node keeps on dropping packets as when received instead of forwarding it to
the next hop in the communication process. These selfish behavior results in packet loss
and also the source is unaware of such misbehaving node in the path towards the
destination. And there is no such mechanism to detect this misbehaving node.
The structure of a MANET totally depends on application and it can vary from
network to network. For e.g. from a small static network which is highly power
constrained to a large hugely dynamic network. Lots of techniques have been discovered
to avoid the misbehavior or selfishness among nodes.

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There has been a lot of improvements in the field of computer and wireless
technologies therefore there has been a lot of development expected in mobile wireless
computing typical applications of which can be used in military scenes, rescue operations
or where it is almost difficult to rely on wired network. MANET's are self organizable
and configurable hence also known as multi hop wireless ad hoc networks, where the
topology of the network keeps on changing continuously.
A. PACKET LOSS DUE TO ROUTING MISBEHAVIORS
During data transfer in ad hoc networks, all the nodes in the network usually take
part in the communication process in order to increase or maximize the throughput.
Therefore the more the number of nodes greater is the bandwidth and smaller is the
network partition with smaller paths. However it might also happen that a node has
agreed to fully cooperate in the communication process but later refuses to do so resulting
in loss of packets because of its selfishness [14].
A selfish node which acts as selfish by dropping packets does so because it is
unwilling to spend its battery life and CPU cycles and save its bandwidth. Such a
misbehaving node launches a denial of service attack by simply dropping packets.
In some cases it might happen that the node which is dropping packets might have
fault in the software running at its end. So the need is to focus on how this misbehaving
node in order to decrease the packet loss can be detected [2]. There are a lot of schemes
available in order to detect the routing in MANET’s which are as follows:

Figure 1 Scenario for packet dropping and misrouting
i) The watchdog technique: In this scheme misbehaving nodes are detected by
overhearing the wireless medium. The path rater technique, which is based on the
watchdog’s output, allows nodes to avoid the use of the misbehaving nodes in any future

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route selections. However, the drawback with watchdog technique is that it depends
heavily on overhearing principal; therefore there is chance that it might fail to identify
misbehavior in routing or raise false alarms in cases where ambiguous collisions, receiver
collisions are present, it might also fail sometimes in cases for limited transmission
power.
ii) ACK and SACK schemes: They are used to measure the usefulness of the
current route and to take appropriate action. For example, congestion control is based on
the reception of the ACK and the SACK packets.[1]
iii) TWOACK Scheme: The 2ACK and the TWOACK schemes have the following
major differences: 1) the node which acts as receiver in the 2ACK scheme usually sends
2ACK packets for a small amount of data packet which it has received, whereas, in the
TWOACK scheme, TWOACK packets are acknowledged for every data packet received
at the receivers end. But, however it was observed that sending acknowledgement for a
fraction of data packets received, improves the performance of 2ACK scheme when it
comes to routing overhead.
iv) (S-TWOACK) scheme or Selective TWOACK: In this scheme every
TWOACK packet will acknowledge or reply the receipt for number of data packets,
whereas in the 2ACK scheme, a 2ACK packet only acknowledges one data packet.
Because of such a small change, the 2ACK scheme gains easy control on the trade-off
that appears between the network performance and the cost as compared to the S-
TWOACK scheme.
II. EXISTING METHODOLOGIES
Routing protocols for MANETs are designed based on the assumption that all
participating nodes are fully cooperative. Misbehaving nodes can be a significant
problem. The presence of selfish or malicious nodes degrades the efficiency of packet
relaying. It increases the packet delivery latency and the packet loss rate. Selfish nodes
lead to network partitioning. Various techniques have been proposed to prevent
selfishness in MANETs. These schemes can be broadly classified into two categories:
credit-based schemes and reputation-based schemes.

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A. CREDIT-BASED SCHEMES
The basic idea of credit-based schemes presented [6] is to provide incentives for
nodes to faithfully perform networking functions. In order to achieve this goal, virtual
(electronic) currency or similar payment system may be set up. Nodes get paid for
providing services to other nodes. When they request other nodes to help them for packet
forwarding, they use the same payment system to pay for such services. The concept of
nuggets (also called beans) is used for payments for packet forwarding. There are two
models which use nuggets are: the Packet Purse Model shown Figure 2 and the Packet
Trade Model shown in Figure 3. In the Packet Purse Model, nuggets are loaded into the
packet before it is sent. The sender puts a certain number of nuggets on the data packet to
be sent. Each intermediate node earns nuggets in return for forwarding the packet. If the
packet exhausts its nuggets before reaching its destination, then it is dropped. In the
Packet Trade Model, each intermediate node “buys” the packet from the previous node
for some nuggets and “sells” it to the next node for more nuggets. Thus, each
intermediate node earns some nuggets for providing the forwarding service and the
overall cost of sending the packet is borne by the destination.
In another implementation, each node maintains a counter termed the neglect
counter. The counter is decreased when the node sends packets of its own, but increased
when it forwards packets for the other nodes. The counter should be positive before a
node is allowed to send its packet. Therefore, the nodes are encouraged to continue to
help other nodes. Tamper resistant hardware modules are used to keep nodes from
increasing the neglect counter illegally.

Figure 2 Packet purse model.

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Figure 3 Packet trade model.
Another credit-based scheme, termed Sprite, has nodes that keep receipts of the
received/forwarded messages. When the users have a fast connection to a Credit
Clearance Service (CCS), they report all of these receipts. The CCS then decides the
charge and credit for the reporting nodes. In the network architecture of Sprite, the CCS
is assumed to be reachable through the use of the Internet, limiting the utility of Sprite.
The main problem with credit-based schemes is that they usually require some kind of
tamper-resistant hardware and/or extra protection for the virtual currency or the payment
system.
B. REPUTATION-BASED SCHEMES
The second category of techniques to combat node misbehavior in MANETs is
reputation-based presented by [2]. In such schemes, network nodes collectively detect
and declare the misbehavior of a suspicious node. Such a declaration is then propagated
throughout the network so that the misbehaving node will be cut off from the rest of the
network. The two modules under this category are watchdog and path rater shown in Fig
4. Nodes operate in a promiscuous mode wherein the watchdog module overhears the
medium to check whether the next-hop node faithfully forwards the packet. At the same
time, it maintains a buffer of recently sent packets. A data packet is cleared from the
buffer when the watchdog overhears the same packet being forwarded by the next-hop
node over the medium. If a data packet remains in the buffer for too long, the watchdog
module accuses the next hop neighbor of misbehaving. Thus, the watchdog enables
misbehavior detection at the forwarding level as well as the link level. Based on the

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watchdog’s accusations, the path rater module rates every path in its cache and
subsequently chooses the path that best avoids misbehaving nodes. Due to its reliance on
overhearing, however, the watchdog technique may fail to detect misbehavior or raise
false alarms in the presence of ambiguous collisions, receiver collisions, and limited
transmission power.
The CONFIDANT protocol [7] is another example of reputation-based schemes.
The protocol is based on selective altruism and utilitarianism, thus making misbehavior
unattractive. CONFIDANT consists of four important components—the Monitor, the
Reputation System, the Path Manager, and the Trust Manager. They perform the vital
functions of neighborhood watching, node rating, path rating, and sending and receiving
alarm messages, respectively. Each node continuously monitors the behavior of its
first-hop neighbors. If a suspicious event is detected, details of the event are passed to
the Reputation System. Depending on how significant and how frequent the event is, the
Reputation
System modifies the rating of the suspected node. Once the rating of a node
becomes intolerable, control is passed to the Path Manager, which accordingly controls
the route cache. Warning messages are propagated to other nodes in the form of an Alarm
message sent out by the Trust Manager. The Monitor component in the CONFIDANT
scheme shown in Figure.5 observes the next hop neighbor’s behavior using the
overhearing technique. This causes the scheme to suffer from the same problems as the
watchdog scheme.

Figure 4 Watchdog & Path rater.

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C. END-TO-END ACKNOWLEDGMENT SCHEMES
There are several schemes that use end-to-end acknowledgments (ACKs) to detect
routing misbehavior or malicious nodes in wireless networks.
In the TCP protocol, end-to-end acknowledgment is employed. Such
acknowledgments are sent by the end receiver to notify the sender about the reception of
data packets up to some locations of the continuous data stream. The Selective
Acknowledgment (SACK) technique [3] is used to acknowledge out-of-order data blocks.

Figure 5 CONFIDANT Scheme.
The 2ACK technique differs from the ACK and the SACK schemes in the TCP
protocol in the following manner: The 2ACK scheme tries to detect those misbehaving
nodes which have agreed to forward data packets for the source node but refuse to do so
when data packets arrive. TCP, on the other hand, uses ACK and ACK to measure the
usefulness of the current route and to take appropriate action.
In order to identify malicious routers that draw traffic toward them but fail to
correctly forward the traffic, the secure trace route protocol is proposed. The normal trace
route protocol allows the sender to simply send packets with increasing Time-To- Live
(TTL) values and wait for a warning message from the router at which time the packet’s
TTL value expires. The secure trace route protocol authenticates the trace route packets
and disguises them as regular data packets.
In secure trace route scheme, binary search is initiated on faulty routes.
Asymptotically, log (n) probes are needed to identify a faulty link on a faulty n-hop route.
This technique only works with static misbehaviors and needs to disguise the probing

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messages as regular routing control packets. Once a link is identified as faulty, the link
weight is increased so that future link selections will avoid this link.
The Best-effort Fault-Tolerant Routing (BFTR) scheme [16] also employs end-to-
end ACKs. The BFTR scheme continuously monitors the quality (i.e., packet delivery
ratio) of the path in use. This is compared with the predefined expected behavior of good
routes. If the behavior of the route in use deviates from the behavior of good routes, it is
marked as “infeasible” and a new route is used. Since BFTR throws out the entire route
before detecting the misbehaving nodes, the newly chosen route may still include the
same misbehaving nodes. Even though the new route will be detected as infeasible by the
source after a period of observation time, data packet loss will occur in traffic flows when
using protocols such as UDP. Such a repeated detection process is inefficient. In contrast
with BFTR, it is try to identify such misbehaving links in this work. Therefore, more
accurate information on routing misbehavior can be obtained in the 2ACK scheme.
III. COMPARISON WITH THE EXISTING SCHEMES
Compared with the above schemes, the 2ACK scheme doesn’t depend on end-to
end acknowledgment. Instead, the 2ACK scheme tries to detect misdemeaning links as
the links are being used. Such a proactive detection approach results in quicker detection
and identification of misdemeaning links. In such a combined scheme, the Multi-Hop
transmission and the monitoring processes are turned on only when routing carry outface
degrades. It will further reduce the routing overhead of the 2ACK scheme.
A scheme to choose routes based on the reliability index of each outgoing
neighbor has each node maintaining a table of reliability indices of its neighbors. This
type of reliability index indicates the previous success or failure experience of packet
transmissions through neighboring nodes. For example, a successful point-to-point
transmission will give output in an increase of the reliability index of the neighbor
associated with the route. When selecting path for data transmissions, nodes prefer those
rooted at the neighbors with higher reliability indices. Since a sender searches all possible
path from its immediate neighbors, the overall reliability of the selecting path depends on
how the neighbors select the rest of the route.

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As compared to the watchdog, the 2ACK scheme has the following
advantages:
Ambiguous Collisions: Ambiguous or doubtful collisions may occur at node N1.
When a well behaved node N2 forwards the data packet toward N3, it is possible that N1
cannot overhear the transmission due to another concurrent transmission in N1's
neighborhood. The 2ACK technique solves this problem by requiring N3 to send a Multi-
Hop packet explicitly.
Receiver Collisions: Receiver or acceptor collisions take place in case of overhearing
techniques when N1 overhears the data packet being forwarded by N2, but N3 fails to get
the packet due to collisions in its neighborhood. The data packets will not be
retransmitted by a misbehaving N2 because retransmission requires extra energy. Again,
due to the explicit Multi-Hop packets our 2ACK scheme overcomes this problem.
Limited Transmission Power: A misbehaving N2 may engineer its transmission
power in such a way that N1 can overhear its transmission but not N3 such that, this
problem matches with the Receiver Collisions problem. It goes to a level of threat only
when the distance between N1 and N2 is less than the distance between N2 and N3. The
2ACK scheme does not suffer from limited transmission power problem.
Limited Overhearing Range: In order to transmit data to N3 a well-behaved N2
could apply low level of power transmission. Due to N1's limited overhearing range, it
will not overhear the transmission successfully and will thus infer that N2 is
misbehaving, causing a false alarm. Both this problem occur due to the potential
asymmetry between the communication links. The 2ACK scheme is not affected by
limited overhearing range problem.
The 2ACK scheme of detecting routing misbehavior is different from the
TWOACK and SACK schemes present in the TCP protocol in the following manner:
The 2ACK technique identifies misbehaving nodes which had agreed to forward
data packets originating from the source node but later refuse to do so during actual data
transfer. On the other side, the TCP protocol uses SACK and ACK to find the benefit of
the current route and to take the required action. 2ACK scheme does not depend on end-
to-end acknowledgment. Such a 2ACK scheme may not exist in some traffic flows for

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instance UDP protocol. Such a proactive approach of detection results in faster detection
of misbehaving links.
IV. THE 2ACK SCHEME
The watchdog finding technique has a very small overhead. Unfortunately, the
watch dog technique going to various problems i.e. equivocal collisions, recipient
collisions, and low transmit energy. The primary issue is the event of successful packet
reception can only be accurately calculated at the receiver of the next-hop link, but the
watchdog technique only observer the transmission from the sender of the next-hop link.
Not a good behavior client can either be sender or receiver of the next-hop link,
concentration is given on the problem of finding a not good behavior links rather of not
good behavior client. In the next-hop link, a not good behavior source or destination has
alike contrary effect on the data packet: It will not be sent on further. The outcome is that
this link will be marked. 2ACK scheme importantly simplifies the detection mechanism.
A. DETAILS OF THE 2ACK SCHEME
The 2ACK scheme is a network-layer technique to find links and to extenuate
their effects. It can be implemented as an add-on to existing path protocols for MANETs,
such as OLSR and any other routing protocols. The 2ACK scheme finds a good behavior
through the use of a new type of acknowledgment bundle, termed 2ACK. A 2ACK
bundle is assigned a fixed path of two hops (three nodes) in the contrary direction of the
data traffic path.
The Figure 6 shows the operation of the 2ACK scheme. Suppose that N1, N2,
and N3 are three successive clients (triplet) along a path. The path from a source client, S,
to a destination client, D, is established from routing table information though HELLO
and TC control messages exchange. When N1 sends a data bundle to N2 and N2 sent it to
N3, it is not clear to N1 whether N3 receives the data bundle correctly or not. Such an
ambiguity exists even when there is a good behavioral client present. The problem
becomes much more severe in open MANETs with potential not good behaving client.

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Figure6 The 2ACK Scheme

The 2ACK scheme needed an external acknowledgment to be sent by N3 to
advice N1 of its successful receipt of a data bundle. When client N3 got the data bundle
successfully, it sends out a 2ACK above two hops to N1, with the ID of the matching
data bundle. The three client N1→ N2→ N3 is calculated from the path of the main data
traffic.
Such three clients are utilized by N1 to the link N2→N3. For gadget of displays,
it is termed that N1 in the three client N1→ N2→N3 the 2ACK bundle destination or the
remarking client and N3 the 2ACK source. Such a 2ACK packet sending takes place for
every set of three clients along the path. Hence, only the first path from the sender will
not do as a multi hop bundle destination. The final path just before the receiver and the
receiver will not do as multi hop destinations.
To find misbehavior, the multi hop bundle source controls a records of IDs of data
bundles that have been sent out but have not been acknowledged. For e.g., later N1 sends
a data bundle on a actual path, say, N1→ N2 → N3. In diagram it sums the data ID to
LIST this is, on its records matching to N2N3. A counterpunch of sent on data bundle,
Cpkts, is incremented all the same time.

The details execution procedure is as follows:

DUID DST Cpkts 2ACK 2ART R-CNT Cmis Rmis DTime

Figure7 Data structure maintained by the observing node

DUID (Data pkt Unique ID): This is used to record the unique packet id of the sent Data
packet.
DST (Data pkt Sent Time) : This records the time at which Data packet is sent.

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Cpkts: This gives the total number of Data packets that are sent.
D2ACK: This records the Data packet ID for which the observing node has received the
2ACK packet i.e., 2ACK Packet is received for this Data packet.
2ART (2ACK Receive Time): This records the time at which the 2ACK packet is
received.
R-CNT: It counts the total number of 2ACK packets that are received by the observing
node.
Cmis: It counts the total number of Data packets for which the 2ACK packet is not
received.
Rmis: It is the ratio of the Cmis to the total number of Data packets sent, i.e., Cmis / Cpkts.
Diff-Time: It is the difference of the time intervals 2ART and DST.
At N1, each ID will remain on the record for τ seconds, the respite for 2ACK
reception. If 2ACK bundles matching to this ID arrive in front the timer exits, the ID will
be took out from the records. Other than, the ID will be taken out at the last of its look out
time separation and a counter called Cmis will be incremented.
If N3 receives a data bundle, then calculated whether it wants to send a 2ACK
bundle to N1. In order to cut down the extra path overhead reason by the 2ACK outline,
only a divide the data bundle will be acknowledged verses multi hop bundle. Such a
divide termed the acknowledgment proportion, Rack. By changing Rack, we can
dynamically tune up the overhead of Many-Hop bundle transmissions.
The Data structure maintained by Data packet receiving node i.e., by Node N3 is as
follows:
DUID D-Total R-Status R2ACK_Total Rack

Figure 8 Data structure maintained by the data packet receiving node (Node N3), i.e. 2ACK packet sender side

DUID (Data pkt Unique ID) : This records received Data packet unique ID.
D-Total: It counts the total number of Data Packets that are received.
R-Status: This records the status of Data packets for which 2ACK packet is sent.
R-status = 1 means for this DUID the receiving node has sent 2ACK packet and
viceversa.

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R2ACK_Total: This field is used to count the total number of 2ACK packets that are
sent.
Rack: This field gives the ratio of the total number of data packets acknowledged to the
total number of Data packets received, i.e., R2ACK_Total/D-total.
Client N1 remarks the behavior of link N2→N3 for a session of time Tobs. At the
last of the session, N1 determines the proportion of losing 2ACK bundles as Cmis / Cpkts
and compare it with a threshold Rmis. If the proportion is greater than Rmis, link N2→N3
is announced misbehaving and that particular link is being removed from the routing
table. The Data structure format of 2ACK is in the Figure 4.5. Since only a divide of the
get data bundle are acknowledged, Rmis could simplify Rmis > 1 - Rack in order to neglect
false alerts reason by such a partially acknowledgment technique.
Every client getting such a 2ACK packet remarks the link N2→N3 as
misbehaving and sums it to the black records list of such misbehaving links that it
controls. When a client begins its own data traffic after, it will avoid using such
misbehaving connects as a part of its path.
DataPkt Src ID Data Pkt Dst ID Data Pkt ID

Figure 9 Data structure of the 2ACK packet.

B. TIMEOUT FOR MANY-HOP RECEPTION, Τ
The argument timeout, τ, is need to set up a timer for 2ACK reception. If the
timer dies before the expected 2ACK bundle is received, the absent 2ACK counter, Cmis,
will be added. Hence, corresponding value of τ is significant for the successful work of
the 2ACK scheme.
It is shows that false alarms achieved if τ is too small. On the other base, if τ is too
big, the waiting client will have to adjust big records, wanting a big memory space.
Therefore, set a value which is large sufficient to the occurrence of temporary link fails
(for e.g. the unsuccessful sending due to client wireless or local traffic congestion).

τ > 4 * (single-hop transmission delay). (1)

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Where a single-hop transmission delay adds bundle sending delay, random back-down
delay at the Medium Access Control (MAC) layer, data processing delay, and potential
resending delay.
C. OBSERVATION PERIOD, TOBS, AND DYNAMIC BEHAVIOR
The 2ACK technique different link misbehavior and temporary link fails by
detecting the reception of 2ACK bundles on a certain session of time, termed the
detection session, Tobs. Since the impermanent link fails don’t use end long, such
assumption is able to unlike impermanent link fails from link misbehavior. The value of
Tobs should be large enough so that several 2ACK packets are sent from the multiple
source to the destination client. This is particularly significant when the acknowledgment
proportion, Rack, is light. For e.g., when Rack= 0.1, one 2ACK bundle will be sent for all
10 data bundles that are received. Even so, the watching session should not be too very
short. A very long watching session means that the watching client takes big time to
watch the behavior of the another-hop link in front not good behavior is stated. Data
bundles may be felled over this lengthy session of time and the effective of the not good
behavior detection algorithm is cut down.
The 2ACK source has to send 2ACK packets for the total data session (on
platform the acknowledgment proportion, Rack). Such continuous watching activity will
help in the detection of not good behaving client which have active behavior depend on
their power layers. When such client are good-behaved, the links closed with them will
be treated as normal links and utilized. Once such clients misbehave, the links closed
with them will be observed as not good behaving and other clients will stop using them.
D. ACKNOWLEDGMENT RATIO, RACK
The adding path overhead stimulated by the sending of the 2ACK bundles can be
handled by the argument acknowledgment proportions, Rack, at the 2ACK bundle sender.
With the use of the argument Rack in the 2ACK technique, only a divide of the received
data bundles will be acknowledged. Hence, the parameter Rack supplies a technique to
turn on the overhead. The decrease of overhead comes with a cost: the reducing of the
range over which Rmis can get cost. When only one Rack of the data bundles gotten are

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acknowledged via the 2ACK packets, 1 - Rack of them are not acknowledged. So 1 - Rack
of all data bundles are not acknowledged, Rmis >= 1 -Rack. i.e.,

Rmis > =1 – Rack (2)

Hence, the different between Rmis and 1 – Rack as the buffer to control fells. i.e.
Rack lowers the voltage extra space to fells down.
E. PARTIAL DATA FORWARDING
A misbehaving node may forward data packets partially by forwarding a fraction
of the packets and try to cheat the monitoring system. Such a behavior will be detected by
the 2ACK scheme. The triplet N1→ N2 → N3 is used as an example for explanation.
Assume a misbehaving node N2 receives ND data packets from N1 successfully
and only forwards a fraction of the data packets, say, Rpart (0 <= Rpart <= 1), of ND toward
N3. Further assumption is that all data packets forwarded by N2 are successfully received
by N3. Thus, N3 receives Rpart _ND data packets and only Rack _ Rpart _ ND of them will be
acknowledged by 2ACK packets sent from N3. Therefore, in order to cheat the system, a
misbehaving node N2 has to make sure that 1 - Rack. Rpart < Rmis. As the gap between 1 -
Rack and Rmis shrinks, the feasible value of Rpart approaches 1. Therefore, the 2ACK
scheme effectively guards against partial forwarding.
Rpart > 1 – Rmis/Rack (3)
Thus, by increasing 1-Rmis/Rack, force N2 to forward more data packets. The
disadvantage of such an approach is the loss of protection from false alarms.
F. OLSR PROTOCOL
OLSR is a table-driven, link-state routing protocol that periodically advertises the
links in the network. OLSR optimizes the link advertisement process by reducing the
amount of advertised links and the number of nodes advertising them. OLSR also
optimizes the message broadcasting mechanism by limiting message forwarding to Multi
Point Relays (MPRs) only. [22]
HELLO AND TC MESSAGES TRANSMISSION
OLSR nodes become aware of one-hop and two-hops neighbors by continuously
exchanging HELLO messages with their one-hop neighbors. MPR nodes are selected by

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each node in the network (called MPR Selector) as the minimum set of one-hop
neighbors that allow reaching every two-hop neighbor via a node in the MPR set. MPR
nodes optimize broadcasting and support path calculation. MPRs are the only nodes
generating Topology Control (TC) messages and are also responsible for forwarding
them. TC messages advertise the links between MPRs and MPR Selectors. The shortest
path algorithm uses these links to construct paths for every MPR Selector. MPR selection
process is as follows.
MPR SELECTION
MPRs form the core optimization in OLSR. The idea of MPRs is to minimize the
overhead of flooding messages in the network by reducing the number of redundant
retransmissions. Each node in the network selects a minimum set of nodes in its
symmetric 1-hop neighborhood which retransmit its messages. This set is selected such
that every symmetric 2-hop node can be reached via a node in this set. MPR set of
selected neighbor nodes is called the MPR set of that node. The neighbors of the node
which are not in its MPR set, receive and process broadcast messages but do not forward
them. The smaller a MPR set, the lesser the control traffic overhead of the routing
protocol.
V. PERFORMANCE EVALUATION
In the simulations, we used a version of Network Simulator (ns-2.34) [19] that
includes wireless extensions developed by the CMU Monarch project group. We installed
and modified the OLSR protocol in ns-2 to simulate misbehaving nodes. Unless specified
otherwise, the 2ACK scheme used Rack=0.2. The IEEE 802.11 MAC was used 15
mobile nodes were randomly distributed in a 1500 m by 1500 m flat area. The source and
the destination nodes were randomly chosen among all nodes in the network. The total
simulation time was 100 seconds UDP traffics have been simulated to evaluate the
performance of 2ACK. Constant Bit Rate (CBR) traffic was used. We used the
following metrics to measure the performance of the 2ACK scheme with respect to UDP
traffic:. Packet Delivery Ratio, PDR: the ratio of the number of packets received at the
destination and the number of packets sent by the source. Routing Overhead, RO : the
ratio of the amount of routing related transmissions (HELLO, TC, 2ACK and CBR) to

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the amount of data transmissions. The amounts are in bytes. Both forwarded and
transmitted packets are counted.
A. PACKET DELIVERY RATIO
PDR: It is the ratio of the number of packets received at the destination and the number
of packets sent by the source.
In the Figure10 the Rack is 0.2
Table 1 The Relative Throughput Supported by 2ACK-OLSR and OLSR
Pm 0.1 0.2 0.3 0.4
OLSR 0.895 0.775 0.655 0.536
2ACK- 0.950 0.950 0.950 0.950
OLSR

In Table1, the relative throughput, normalized number of packets received is
presented, when the 2ACK-OLSR scheme and the OLSR scheme are used. Based on
Table1, the relative throughput reduces when pm increases due to higher chances of using
routes with misbehaving links and longer time being spent to switch to good routes.
From the table, it can be observe that the 2ACK-OLSR scheme outperforms the OLSR
scheme in terms of relative throughput, especially in the networks with larger pm. It
represents packet delivery ratio (PDR), the total number of data packets that are received.

Figure 10 Packet Delivery Ratio of OLSR and 2ACK-OLSR at different misbehavior ratio.

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B. ROUTING OVERHEAD
In the Figure11 the Rack is 0.2
Table 2 The Routing overhead by 2ACK-OLSR and OLSR
0.1 0.2 0.3 0.4

OLSR 0.280 0.294 0.307 0.318
2ACK- 0.700 0.700 0.700 0.700
OLSR

In the Table 2 the routing overhead is increased as a misbehavior ratio increased
in the case of OLSR. The routing overhead is calculated by using the formula as given
HELLO + TC/CBR for different Pm. The routing is calculated for the 2ACK-OLSR by
using the formula HELLO+TC+2ACK/CBR. The higher routing overhead in the 2ACK-
OLSR scheme is due to the transmission of extra acknowledgment packets.
VI. CONCLUSIONS
The 2ACK scheme which helps detect misbehavior by a two hop
acknowledgement. The 2ACK scheme is a network-layer technique to detect
misbehaving links and to mitigate their effects. The 2ACK scheme detects misbehavior
through the use of a new type of acknowledgment packet, termed 2ACK.

Figure 11 Routing overhead of OLSR and 2ACK-OLSR at different misbehavior ratio.
A 2ACK packet is assigned a fixed route of two hops (three nodes N1, N2, N3),
in the opposite direction of the data traffic route. This technique identifies misbehaving
nodes which had agreed to forward data packets originating from the source node but
later refuse to do so during actual data transfer and it helps to reduce the routing
overhead. The 2ACK technique is based on a simple 2-hop acknowledgment packet that

231


is sent back by the receiver of the next-hop link. The 2ACK scheme can be used as an
add-on technique to routing protocols such as OLSR in MANETs. One advantage of the
2ACK scheme is its flexibility to control overhead with the use of the Rack parameter.
VII. FUTURE WORK
In this work, focus is given only on link misbehavior. The more troublesome task
is to determine the characteristics of a single node. The main reason behind this is the
communication is only between two nodes, and is not any node’s sole act. So, any nodes
associated with the misbehaving links should be punished carefully. In case of a link
misbehaving, then any one of the two co-related nodes may be misbehaving in the
association. In order to find the characteristics and punish a node, the behavior of links
around that node should be analyzed. A second case may arise wherein Node N1 floods
Node N2 with packets thus causing N2 to drop packets. Hence it is imperative that before
declaring a node to be selfish on the basis of the number of packets dropped, compute the
ratio of number packets received against the number of packets dropped should be
computed.
VIII. REFERENCES
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[19] The Network Simulator – ns-2 - http://www.isi.edu/nsnam/ns/
[20] Tutorial for Ns-2 - http://www.isi.edu/nsnam/ns/tutorial/
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[22] Unik olsrd homepage http://www.olsr.org

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