東海大学紀要情報通信学部
Vol.11,No.1,2018,pp.7-14
Proc.
Schl. ITE Tokai Univ.

Paper

Paper

Vol. xx,No.xx ,20xx,pp.xxx -xxx

Ad hoc On-Demand Distance Vector Routing with Reachability
Checking as a Countermeasure against Black Hole Attacks
byThongcharoen JETTAPHAT *1 and Junichi MURAYAMA*2(received on Mar.29, 2018 & accepted on Jul.27, 2018)

Abstract
A mobile ad hoc network (MANET) attracts wide attention as a future device network. For achieving high mobility, ad hoc
on-demand distance vector (AODV) routing is suitable. However, an AODV-based MANET is vulnerable against a black holeattack. Secure AODV (SAODV) is the conventional countermeasure in order to bypass a black hole. However, it may cause
misrouting under the normal conditions. In order to solve this problem, we propose a novel countermeasure scheme. This
scheme comprises AODV, reachability checking function and reachability-information caching function. Reachability
checking suppresses misrouting effectively. Furthermore, reachability-information caching suppresses control overhead ofreachability checking. As the result, our proposed scheme is an attractive countermeasure against black hole attacks.
Keywords: MANET, AODV, Black hole attack, SAODV, Reachability check, Reachability-information cache

1. Introduction
A mobile ad hoc network (MANET) attracts wide
attention as an Infrastructure-less wireless network [1]. In
this network, communication devices behave autonomously
for connecting with each other. This feature seems suitable
for the base of Internet of things (IoT) [2]. Typical IoT
examples are sensor networks, vehicle networks [3] andemergency smartphone networks [4]. Thus, device mobility
is its important requirement.
In order to achieve high mobility, ad hoc on-demand
distance vector (AODV) routing [5] is suitable. In thisrouting, control packets are sent on demand and theirsource addresses are learned at each relay host. Thenpackets can be forwarded toward the address-learned
direction in a hop-by-hop scheme.
However, an AODV-based MANET is vulnerable against
a black hole attack [6]. In this attack, a black hole node
sends a malicious control packet for absorbing data packets.
At the black hole node, the absorbed data packets are
discarded. Consequently, communications in a MANET are
interfered.
As the conventional countermeasure, Secure AODV(SAODV) [7] has been proposed. It can establish a bypass
route using the secondary-received control packet.
However, it may not select an appropriate control packet
under the normal conditions. Here, the normal conditionmeans that a black hole node is not attacking. Accordingly,
data packets are not forwarded along the optimal routes.
In order to solve this problem, we propose a novel
countermeasure scheme. In this scheme, we combine*1 Graduate School of Information and Telecommunication Engineering, Course of Information andTelecommunication Engineering, Master's Program*2 School of Information and Telecommunication
Engineering, Department of Communication and
Network Engineering, Professor

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−1
7−

AODV routing with a reachability checking function and areachability-information caching function. Here, we checkreachability in order to determine whether an arrived
control packet is legitimate or malicious. This check
effectively suppresses misrouting in the normal conditions.
On the other hand, this check increases control overhead inrouting processing. In order to suppress this overhead, we
also cache checked reachability-information. This cachedinformation is reused for a short time. Consequently,
frequency of checking can be reduced.
Our scheme is evaluated by means of simulation. The
result has shown that our scheme is secure and achieves
good performance. Consequently, we can say that the
proposed scheme is an attractive countermeasure against
black hole attacks.
The rest of this paper is organized as follows: Sect. 2
introduces the base technologies as related works and Sect.
3 shows the conventional countermeasure against blackhole attacks. Then, Sect. 4 proposes the novel
countermeasure and Sect. 5 evaluates the proposed scheme.
Next, Sect. 6 discusses applicability of our scheme tomultipath conditions. Finally, Sect. 7 concludes this paperwith brief summary.

2. Related Works
This section shows the base technologies as related
works. We are assuming a network model composed of
MANET and AODV. In addition, a black hole attack is
assumed as an attack model. They are described in thissection.
2.1 MANETMANET is an infrastructure-less wireless network [1].
This network comprises only host devices. They areconnected with each other without any access points. For

Ad hoc On-Demand Distance Vector Routing with Reachability Checking as a Countermeasure against Black Hole Attacks

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achieving such behavior, they act autonomously. MANET
is designed as the base of IoT [2]. Thus, it is assumed to be
applied to such as sensor networks, vehicle networks [3]and emergency smartphone networks [4]. From thisbackground, device mobility is an important requirementfor MANET.

After those procedures, data packets are forwarded along
the fake route. Then, they are simply discarded at the blackhole node. Consequently, communications in a MANET are
interfered.
(4) Hunting the second RREPat the source(1) Flooding RREQ
from the sourceR

2.2 AODVIn order to achieve high mobility feature, routing is an
important issue. A well-known solution is to deploy AODV
routing [5]. This routing scheme is based on source-address
learning like Ethernet switches.
Fig. 1 shows this mechanism. (1) First, the source hostfloods a route request (RREQ) packet towards thedestination host. (2) Each relaying host learns the source
address of the flooding RREQ packet. (3) The destination
host that received the RREQ packet returns the route reply(RREP) packet to the source host. This RREP packet isunicasted to the source host hop-by-hop using learned
address. Each relaying host also learns the source addressof the returned RREP packet.
After this control procedure, the source host can senddata packets to the destination host by unicasting way.
Likewise, the destination host can also unicast data packets
to the source host.
This RREQ packet is sent on demand in order to achieve
high mobility feature.

(1) Flooding RREQ from the sourceRD(2) Address learningof RREQ and RREPS
R
(3) Unicasting RREP to the sourceSB(2) Fake RREP
from the black hole(3) Ignoring the first RREPat the source

Fig. 2 Overview of a black hole attack.
3. Conventional Countermeasure
SAODV [7] is the conventional countermeasure against
black hole attacks. This scheme is a kind of modified
AODV. It pays attention to the order of arrival RREP
packets. In many cases, a fake RREP packet arrives fasterthan a legitimate RREP packet. Here, the original AODV
uses only the first-arrived RREP packet for routing
processing. The next arrived RREP packets are discarded
simply. Consequently, a packet forwarding route ishijacked by a black hole route with high probability.
In order to solve this problem, SAODV uses only thesecond-arrived RREP packet for routing processing. Fig. 3
shows a mechanism of this countermeasure. (1) In thisscheme, according to the original AODV procedure, a
RREQ packet is first flooded. (2) Then, the fake RREP
packet is first-arrived with high probability. (3) This RREP
packet is simply discarded. (4) Next, the second-arrived
RREP packet is hunted and used for routing. RREP packets
arrived after the second one are simply discarded.
(4) Hunting the second RREPat the source(1) Flooding RREQ
from the source

Fig. 1 Overview of AODV.
2.3 Black hole attack
A black hole attack is a well-known attack targeted to an
AODV-based MANET [6]. Routing control in both mobile
and autonomous environment is vulnerable against such as
black hole attacks. This is because specifying malicious
control packets under such an environment is not easy.
Fig. 2 shows a mechanism of this attack. (1) First, thesource host floods a RREQ packet towards the destination
host. (2) A black hole node returns a fake RREP packet
when it receives a RREQ packet sent to another hosts. Then,
this fake RREP packet is used to establish a fake route
towards the black hole node. In many cases, a black holenode may locate nearer than destination hosts. (3)Consequently, this fake route is selected for data packet
forwarding. (4) Contrary, the legitimate route is ignored
despite that the legitimate RREP packet is arrived. This isdue to their arrival order.

――− 28 −

D

R

D

S
B
(2) Fake RREP
from the black hole(3) Ignoring the first RREPat the source

Fig. 3 Overview of SAODV.
This
simplemodificationmaybeeffective
countermeasure under the attacking conditions. However,under the normal conditions, its behavior seems a problem.
Fig. 4 shows a mechanism of SAODV under this conditions.
(1) A RREQ packet is first flooded. (2) Usually, only thesingle RREP may be returned from the destination host to
the source host. (3) This only one RREP is simply
discarded. Consequently, any route is not established. Thisbehavior seems fatal. Thus, this problem should be solved.

Thongcharoen JETTAPHAT and Junichi MURAYAMA

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forwarding performance. Thus, in order to mitigate thisoverhead, we further deploy reachability-information cache.
In this scheme, multiple reachability check procedures
toward the same direction are omitted. Here, the first resultis shared between multiple check procedures for a while. Ifmany packets are forwarded in a short period, this scheme
behaves effectively. Consequently, data forwarding
performance can be kept high.

(2) Ignoring the first RREPat the source(1) Flooding RREQ
from the source

R

D

S
R

(3) Causing no packet forwarding route
under single path condition

Fig. 4 Problem of SAODV .

4. Proposed CountermeasureThis section shows a novel countermeasure against blackhole attacks as our proposal. First, the overview is shown.
Then, a reachability checking scheme is proposed in orderto solve misrouting problem. Finally, a reachabilityinformation caching scheme is proposed for reducing
control overhead of reachability check.
4.1 Overview
The problem of SAODV is that it does not use afirst-arrived legitimate RREP packet. In order to solve thisproblem, RREP authentication seems attractive. However,
it is difficult practically under the autonomous control
environment. Thus, we deploy reachability check in placeof direct RREP authentication.
The overview of this scheme is depicted in Fig. 5. This
scheme is designed based on the original AODV. The
difference is a reachability check procedure inserted after
arrival of a RREP packet. (1) First, a RREP packet isreturned to the source host. (2) Then, the source hostchecks reachability of the RREP-arrival direction to thedestination. If reachability is assured, in succession, data
packets are sent to this RREP-arrival direction. Else, thesource host waits the next RREP packet. Then, it rechecks
reachability of the next RREP-arrival direction.

R

R

S

D
R

R

R

(3) Packet forwarding
along the checked routeS

D

Ignored in original SAODV(1-1) Flooding RREQ from the source(1-2) Receiving the 1st RREP from the destination(2-1) Selecting the RREP arrival route(2-2) Requesting echo to the destination(2-3) Receiving echo reply from the destination(3-1) Packet forwarding to the destination(2) Reachability checking
along the notified route(1) Hunting
the arrived RREP

4.2 Reachability Check
Reachability check is deployed in place of direct RREP
authentication, as described above. Basically, this check isinserted after arrival of each RREP packet. Fig. 6 shows aprocedure of reachability check under the normal
conditions.
The first phase is the route-finding procedure based on
the original AODV. (1-1) The source host floods RREQ
packets to the destination host. (1-2) It receives thefirst-arrived RREP packet.
The second phase is the reachability check procedure
inserted into AODV. This is the core part of the proposal.
(2-1) The source host determines the packet forwarding
direction temporarily using the arrived RREP packet. (2-2)
It unicasts ICMP echo request to the destination host fromthe temporary direction. (2-3) It receives ICMP echo reply
from the destination host. Thus, reachability check is
succeeded.
The third phase is the packet forwarding procedure. (3-1)
The source host sends data packets to the destination hostfrom the checked direction.
Those processes effectively solve the problem of
SAODV misrouting under the normal conditions.

Fig. 6 Reachability checking under normal conditions.

: Echo Request: Echo Reply: RREP (AODV): IP Packet

Fig. 5 Overview of the proposed scheme.
Due to those procedures, a black hole node can bebypassed surely from the packet-forwarding route. In
addition, misrouting can be avoided in the normal
conditions.
However, this control overhead may affect data――
− 39 −On the other hand, Fig. 7 shows the procedure under theattacking conditions. The first phase is the same as that of
the normal conditions. (1-1) The source host floods a
RREQ packets to the destination host. (1-2) It receives thefirst-arrived RREP packet.
The second phase is little different from that of thenormal conditions. (2-1) The source host determines apacket forwarding direction temporarily. (2-2) It unicasts
ICMP echo request from this temporary direction to thedestination host. (2-3) It does not receive ICMP echo reply
from the destination host within a certain period. Thus,
reachability check is failed.
In this case, a process goes back to the first phase forAd hoc On-Demand Distance Vector Routing with Reachability Checking as a Countermeasure against Black Hole Attacks

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finding another route again. (1-3) The source host waits foranother RREP packets and receives the second-arrived
RREP packet. Here, note that the second packets may havebeen caching if it arrived immediately after the first packet
arrival.
The third phase is the same as the second phase under thenormal conditions. (3-1) The source host determines thepacket forwarding direction temporarily. (3-2) It unicasts
ICMP echo request to the destination host from thetemporary direction. (3-3) It receives ICMP echo reply
from the destination host.
If reachability check is failed in (3-3), a process goesback to the first phase again and processes are repeated.
Else, a process goes to the fourth phase for data packet
forwarding. (4-1) The source host sends data packets
from the checked direction to the destination host.
Those processes solve effectively the problem of AODV
misrouting under black hole attacking. As the result, theproposed countermeasure behaves appropriately under bothnormal and anomaly conditions. Here, the anomaly
condition means that a black hole node is attacking.
S

BD(1-1) Flooding RREQ from the source(1-2) Receiving the 1st RREP from the black hole(2-1) Selecting the RREP arrival route(2-2) Requesting echo to the destination(2-3) Receiving no reply from the destination(1-3) Receiving the 2nd RREP from the destination(3-1) Selecting the RREP arrival route(3-2) Requesting echo to the destination(3-3) Receiving echo reply from the destination(4-1) Packet forwarding to the destination

Fig. 7 Reachability checking under anomaly conditions.
4.3 Reachability-information Cache
The proposed reachability check suppresses misrouting
effectively. However, its overhead seems large in thenormal conditions. In order to mitigate this overhead, we
also deploy a reachability-information caching scheme.
Fig. 8 shows a procedure of our reachability check under
the normal conditions. The overhead processes to beomitted are shown as the process (2-2) and (2-3).
In the initial stage, the whole processes should be
performed. Then, the result of the check (the process (2-2)
and (2-3)) is cached for a short time.
In the next stage, before the data packet transmission,
the source host checks whether the corresponding cache isexist or not. When the cache exists, it is used forreachability checking. Consequently, the ICMP echo
request/reply processes (the process (2-2) and (2-3)) areomitted. If the cache does not exit, the ICMP echo
request/reply processes are fully executed and its result iscached.

―
4 ―
− 10
−

S

D
(1-1) Flooding RREQ from the source(1-2) Receiving the 1st RREP from the destination(2-1) Selecting the RREP arrival route(2-2) Requesting echo to the destination(2-3) Receiving echo reply from the destination(3-1) Packet forwarding to the destination

Caching the result

Fig. 8 Processes to be omitted.
Fig. 9 shows how does cache work effectively. In thisfigure, the source host (S) and six destination hosts(D1-D4) are connected with each other using four relaying
nodes (N1-N2, R, B). Here, the node (B) is a black hole.
The number of destinations (D1-D4) seems large.
However, the number of the next-hop nodes (N1-N2) fromthe source (S) is small. This is because the packet
forwarding route from the source (S) to the destinations(D1-D4) is tree-shaped logically.
From the viewpoint of the source (S), the destinations(D1-D2) are located behind the black hole (B). RREQ
packets to those (D1-D2) are replied illegally by this blackhole (B). Those faked RREP packets are reached via thenext-hop (N1). Accordingly, received RREP packets via
this node (N1) are faked with high probability.
On the other hand, the destinations (D3-D4) reply for
RREQ packets directly. Those replied RREP packets arereached via the next-hop (N2). Consequently, received
RREP packets via this node (N2) are legitimate with highprobability.
According to those backgrounds, we can cache the resultof ICMP echo request/reply as the reachability check result.
In addition, we can use this result for the ensuingreachability checks directed to the same next-hop route.
Source(S)
Canceling packet forwarding
RREP
Past failed(fake)
Caching the result ofreachability checkS
via next-hop nodes
RREP
Past succeeded (legitimate)

Relay(N1-N2,R,B)

Destination(D1-D4)
D1

N1

B
D2

Next-hop
D3
N2

Packet forwardingR
D4

Fig. 9 Use of reachability-information cache.

5. Evaluation
This section shows evaluation using simulation. First, theoverview is shown. Then, reachability, latency andthroughput are compared between the conventional
schemes and the proposed schemes. Finally, the results are
summarized.

Thongcharoen JETTAPHAT and Junichi MURAYAMA

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5.1 Overview
Our reachability checking scheme and reachability-information caching scheme were evaluated by way ofsimulation. Here, we call the former and the latter as
AODV-RC and AODV-RC2, respectively. They arecompared with AODV and SAODV from the viewpoint ofreachability, latency and throughput. We used MATLAB as
the simulation tool. In this simulation, our reachability
checking function was implemented as shown in Table 1.
The simulated network model is shown in Fig.10. A circle
and the number in this figure show host location and hostidentification, respectively. Here, the black circle shows
the black hole. Every hosts dynamically move aroundwithin 1 square kilometer area. The reachable range ofradio wave is 250 meters. Other conditions are summarizedin Table 2.
Table 1 Reachability checking algorithm.
1. Check D by flooding RREQ from S
2. Select receiving RREP arrival route
3. Check reply by sending request echo to D
Then wait for replying from D(1) If received echo reply from D
Then packet forwarding to D
(2) If not received echo reply from
Then (go to 2)

Packet delivery ratio (percent)

100

12
19
14

0.5
1

3

16

11

4

15

18

10

17

70
60
50
40
30

9

0.5
Distance (km)1

Fig. 10 Network model.

10

Value
20-50 nodes
IEEE 802.11

Packet length

512 bytes

Simulation period

1,000 seconds

Number of simulation trials

100 times

25

30
35
40
Number of nodes

45

50

90
80
70
60
50
40

AODV-RCAODV-RC2
SAODV
AODV

30
20
10
0
20

Table 2 Simulation parameters.
Parameter
Number of nodes
Wireless protocol

AODV-RCAODV-RC2
SAODV
AODV

20

100

6

0
0

80

Fig. 11 Packet delivery ratio in normal conditions.

2

5
8
13

90

0
20

Packet delivery ratio (percent)Distance (km)1720

5.2 Packet Delivery Ratio
Packet delivery ratio was measured as the reachability
evaluation. It is a ratio of the number of received packets atthe destination to that of sent packets at the source in thewhole network. Fig. 11 and Fig. 12 show the measured
average ratio under the normal conditions and the anomaly
conditions, respectively. In both figures, X-axis and Y-axis
show the number of hosts in the network and average
packet delivery ratio, respectively.
In Fig.11, under the normal conditions, SAODV shows
the worst ratio. This is due to misrouting. Other schemes
show almost the same ratio because of the same shortest
routing.
On the other hand, in Fig.12, under the anomaly
conditions, AODV shows the worst ratio. This is due to theblack hole. SAODV shows better ratio than that of AODV.
This is because the black hole is bypassed effectively.
However, AODV-RC and AODV-RC2 show the best ratio
because of the optimal routing achieved by means ofreachability check.

25

30
35
40
Number of nodes

45

50

Fig. 12 Packet delivery ratio in anomaly conditions.
5.3 Latency
Latency here means the total delay of a routing procedure
and a data forwarding procedure. Fig. 13 and Fig. 14 show
the measured average latency under the normal conditions
and the anomaly conditions, respectively. In both figures,

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5 ―
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−

Ad hoc On-Demand Distance Vector Routing with Reachability Checking as a Countermeasure against Black Hole Attacks

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X-axis and Y-axis show the number of hosts in the networkand latency, respectively.
In Fig. 13, under the normal conditions, AODV shows
the minimum latency, while SAODV shows the maximum
latency. This is because AODV route is the shortest, while
SAODV route is longer. The latency of AODV-RC isalmost the same as that of SAODV. However, AODV-RC
route is the shortest. This means that overhead ofreachability check is large. The latency of AODV-RC2 isalmost the same as that of AODV. This means that thereachability check overhead is mitigated effectively bymeans of reachability-information caching.
The features of Fig. 14 are almost the same as that of Fig.
13. This is because the latency can be measured only if data
packets are forwarded successfully.
5

7

3

6

2
AODV-RCAODV-RC2
SAODV
AODV

1
0
20

Throughput (kilobyte)

Latency (ms)4This is because AODV achieves optimal routing, while
SAODV causes misrouting. Although AODV-RC and
AODV-RC2 also achieve optimal routing, their throughput
is less than that of AODV. This is due to overhead ofreachability checking. Since this overhead is mitigated in
AODV-RC2, throughput of AODV-RC2 is better than thatof AODV-RC.
On the other hand, in Fig.16, under the anomaly
conditions, AODV shows the worst throughput due to theblack hole. SAODV shows better throughput than that of
AODV. This is the effect of bypassing the black hole.
However, AODV-RC and AODV-RC2 show further better
throughput because of their optimal routing achieved bymeans of reachability checking. In addition, AODV-RC2
shows the best throughput. This means that the reachability
check overhead is mitigated effectively even under the
anomaly conditions by means of reachability-informationcaching.

30
40
Number of nodes

50

Fig. 13 Latency in normal conditions.

5
4
3
2

5

0
20

4

30
40
Number of nodes

50

Fig. 15 Throughput in normal conditions.
7

3

6

2
AODV-RCAODV-RC2
SAODV
AODV

1
0
20

Throughput (kilobyte)

Latency (ms)AODV-RCAODV-RC2
SAODV
AODV

1

30
40
Number of nodes

50

5
4
3
2

Fig. 14 Latency in anomaly conditions.

1

5.4 Throughput
Throughput here means the total amount of data thatdelivered successfully in the network during the simulation
period. Fig. 15 and Fig. 16 show the measured averagethroughput under the normal conditions and the anomaly
conditions, respectively. In both figures, X-axis and Y-axis
show the number of hosts in the network and throughput,
respectively.
In Fig. 15, under the normal conditions, AODV andSAODV show the best and worst throughput, respectively.

0
20

―
6 ―
− 12
−

AODV-RCAODV-RC2
SAODV
AODV
30
40
Number of nodes

50

Fig. 16 Throughput in anomaly conditions.
5.5 Summary
The most important evaluation index is throughput
because of the low-power and high throughput requirementfor MANET. AODV is the best under the normal conditions,while AODV-RC2 is the best under the anomaly conditions.
AODV-RC2 is also the second best under the normal

Thongcharoen JETTAPHAT and Junichi MURAYAMA

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conditions. In addition, the throughput difference is small.
Consequently, we can say that AODV-RC2 is the current
best countermeasure against black hole attacks on MANETs.
This conclusion is also supported from the results ofreachability evaluation and latency evaluation.

6. Discussions
In basic AODV routing, a RREP packet is unicasted fromthe destination host to the source host. Consequently, apacket forwarding route is determined uniquely. However,this feature is fatal against black hole attacks. This isbecause multipath routes are indispensable for bypassing a
black hole node. In this section, we discusses about
multipath routing.
6.1 Multipath routing in original AODV
Relay hosts may know the route to the final destination.
In AODV routing, such hosts can independently send a
RREP packet in place of the destination. Consequently,
multiple RREP packets may be returned to the source hostfrom the different directions. Those RREP packets areuseful to achieve multipath routing.
The mechanism of AODV multipath routing is
schematically shown in Fig. 17. In this mechanism, first,
the source host (S) floods a RREQ packet toward thedestination host (D) via relay hosts (R1-R5). The flooded
RREQ packets are reached to relay hosts (R2, R5). Thosehosts are located near the destination (D) and thus they (R2,
R5) know the route to the destination (D) with highprobability. (1) Then, they (R2, R5) respond to this RREQ
packet using a RREP packet independently. (2)Accordingly, multiple RREP packets are returned to thesource (S) via the different routes.
The source (S) that receives multiple RREP packets
selects an appropriate RREP packet for data packet
forwarding. Typically, a RREP packet corresponding to theshortest route is selected. In addition, another RREP
packets can be also selected to use the alternative routes.
(1) Unicasting RREP from the relay node
who know the route to the destination(2-1) First arrived RREP
(Shorter route)S(2-2) Next arrived RREP(Longer route)

D
R3

R4

R5

(1) Ignored route notified
by the first RREPR

RS
(2) Used route notified by
the second RREP

D
R

R

R

Shorterroute
Longerroute

Fig. 18 SAODV multipath routing.
6.3 Multipath routing in proposed countermeasure
The proposed countermeasure under multipath shown in
Fig. 19. (1) In our scheme, at the source (S), thefirst-arrived RREP packet can be used even in the normal
conditions. (2) This is enabled by way of reachability
checking. (3) Thus, data packets can be forwarded along
the shortest route. However, this RREP packet is discarded
when reachability check is fail. In this case, thesecond-arrived RREP packet is hunt to use the alternativeroute. This means that a black hole node can be bypassed
effectively. Accordingly, data forwarding latency ismaintained low and the whole network throughput is kept
high.
(2) Reachability checking
along the notified route(1) Hunting the firstarrived RREP
R

R

S

D
R

R

(3) Packet forwarding
along the shortest routeR: Echo Request: Echo Reply: RREP (AODV): IP Packet

Alternative route(for bypassing black hole)Fig. 19 Proposed multipath routing.

: RREQ: RREP

R2

R1

is hunted and used for data packet forwarding. Then, data
packet can be forwarded to the destination even in thenormal conditions.
However, the route determined by the second-arrived
RREP packet is not the shortest but the second shortest.
Consequently, data forwarding latency increases and thewhole network throughput decreases.

Known
route

Fig. 17 AODV multipath routing.
6.2 Multipath routing in conventional countermeasure
The RREP packet corresponding to the shortest route isarrived first with high probability as shown in Fig. 18. (1)This RREP packet is simply discarded at the source (S) in
SAODV routing. (2) Next, the second-arrived RREP packet

―
7 ―
− 13
−

6.4 Countermeasure against advanced attacks
Once our proposal has been applied, black hole nodes
may evolve so as to return fake ICMP echo replies. To
solve this problem, host applications also need to checkreachability. Accordingly, every arrived RREP packet ischecked their reachability by the received host regardlessof the check results. Then, a host will replace the failurecommunication route to the available one if the hostapplication detects an initial-communication failure. In
order to achieve this solution, the socket interface between
the network function and application functions needs to beextended so as to support failure notifications.
Some black hole nodes may further evolve to act as

Ad hoc On-Demand Distance Vector Routing with Reachability Checking as a Countermeasure against Black Hole Attacks

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phishing servers. To solve this problem, certification of thedestination host is necessary. A public and private key pairseems attractive for achieving this solution. However, in
this paper, balancing both good convenience and strong
security optimally in MANETs is out of scope. It is an
important future issue.

7. Conclusion
An AODV-based MANET is attractive as a future device
network because of its high mobility feature. The essenceof MANET is economical routing in order to achieve
low-power and high-throughput features. The biggest threat
for such features is a black hole attack that interferes
optimal routing. The conventional countermeasure iseffective against such an attack, while it does not work
well under the normal conditions.
The proposed countermeasure solves this problem bymeans of a reachability checking scheme. This scheme isbased on ICMP echo request/reply and supports optimal
AODV routing. However, this simple solution may loose
low-power feature due to its large control overhead. Thenour countermeasure also deploys a reachability-informationcaching scheme. In our scheme, reachability check using
ICMP is executed once and the result is cached. This cache
is used to check reachability for the ensuing packet routing.
Consequently, the cache effectively mitigates overhead of
ICMP-based reachability checking.
As the result, our countermeasure maintains low-powerand high-throughput features against black hole attacks.

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