Security for Insider Attacks in Mobile Ad Hoc Networks
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Mobile ad hoc networks are collection of wireless mobile nodes forming a temporary network without the aid of any established infrastructure. Security issues are more paramount in such networks even more so than in wired networks. Despite the existence of well-known security mechanisms, additional vulnerabilities and features pertinent to this new networking paradigm might render the traditional solutions inapplicable. In particular these networks are extremely under threat to insider attacks especially packet dropping attacks. It is very difficult to detect such attacks because they comes in the category of attacks in mobile ad hoc networks in which the attacker nodes becomes the part of the network. In this research work we have proposed a two folded approach, to detect and then to isolate such nodes which become the part of the network to cause packet dropping attacks. First approach will detect the misbehavior of nodes and will identify the malicious activity in network, and then upon identification of nodes misbehavior in network other approach will isolate the malicious node from network. OMNET++ simulator is used to simulate and verify the proposed solution. Experimental results shows that E-SAODV (Enhanced Secure Ad hoc On Demand Distance Vector protocol) performs much better than conventional SAODV (Secure Ad hoc On Demand Distance Vector Protocol)
Mobile Ad-hoc networks are a new paradigm of wireless communication for mobile hosts. As there is no fixed infrastructure such as base stations for mobile switching. Nodes within each other's range communicate directly via wireless links while those which are far apart rely on other nodes to transmit messages. Node mobility causes frequent changes in topology. The wireless nature of communication and lack of any security infrastructure raises several security problems. The following flowchart depicts the working of any general ad-hoc network.
Features of Mobile Ad hoc Networks
Based on the characteristics, Mobile Ad hoc Networks has following main features.
- Because of the limited energy supply for the wireless nodes and the mobility of the nodes, the wireless links between mobile nodes in the Ad hoc Network are not consistent for the communication participants.
- Due to the continuous motion of nodes, the topology of the mobile ad hoc network changes constantly, the nodes can continuously move into and out of the radio range of the other nodes in the ad hoc network, and the routing information will be changing all the time because of the movement of the nodes.
- Lack of incorporation of security features in statically configured wireless routing protocol not meant for Ad hoc environments. Because the topology of the Ad hoc Networks is changing constantly, it is necessary for each pair of adjacent nodes to incorporate in the routing issue so as to prevent some kind of potential attacks that try to make use of vulnerabilities in the statically configured routing protocol.
Mobile Ad hoc Network Routing Protocols
Routing in Mobile Ad hoc Networks faces additional challenges when compared to routing in traditional wired networks with fixed infrastructure. There are several well-known protocols that have been specifically developed to cope with the limitations imposed by Ad hoc networking environments. The problem of routing in such environments is aggravated by limiting factors such as rapidly changing topologies, high power consumption, low bandwidth and high error rates . Most of the existing routing protocols follow two different design approaches to confront the inherent characteristics of Ad hoc networks namely Proactive Routing Protocols, Reactive Routing Protocols.
Proactive Routing Protocols
Proactive ad hoc routing protocols maintain at all times routing information regarding the connectivity of every node to all other nodes that participate in the network. These protocols are also known as Table-driven Ad hoc Routing Protocols. These protocols allow every node to have a clear and consistent view of the network topology by propagating periodic updates . Therefore, all nodes are able to make immediate decisions regarding the forwarding of a specific packet. Two main protocols that fall into the category of proactive routing protocols are Destination-Sequenced Distance-Vector (DSDV) protocol  and the Optimized Link State Routing (OLSR) protocol .
Reactive Routing Protocols
An alternative approach to the one followed by Proactive Routing Protocols also known as source-initiated on-demand routing, is Reactive Routing Protocols. According to this approach a route is created only when the source node requires one to a specific destination. A route is acquired by the initiation of a route discovery function by the source node. The data packets transmitted while a route discovery is in process are buffered and are sent when the path is established. An established route is maintained as long as it is required through a route maintenance procedure. The Ad hoc On-demand Distance Vector (AODV) routing protocol , Temporally Ordered Routing Algorithm (TORA)  and the Dynamic Source Routing protocol  are examples of this category of protocols.
Security issues in Mobile Ad hoc Routing Protocols
Any routing protocol must encapsulate an essential set of security mechanisms. These are mechanisms that help prevent, detect, and respond to security attacks. We can classify these major security goals into five main categories, which need to be addressed in order to maintain a reliable and secure ad-hoc network environment.
Confidentiality is the protection of any information from being exposed to unintended entities. In ad-hoc networks this is more difficult to achieve because intermediates nodes receive the packets for other recipients, so they can easily eavesdrop the information being routed.
Availability means that a node should maintain its ability to provide all the designed services regardless of the security state of it . This security criterion is challenged mainly during the denial-of-service attacks, in which all the nodes in the network can be the attack target and thus some selfish nodes make some of the network services unavailable, such as the routing protocol or the key management service.
Authentication assures that an entity of concern or the origin of a communication is what it claims to be or from. Without which an attacker would impersonate a node, thus gaining unauthorized access to resource and sensitive information and interfering with operation of other nodes.
Integrity guarantees the identity of the messages when they are transmitted. Integrity can be compromised through malicious and accidental altering. A message can be dropped, replayed or revised by an adversary with malicious goal, which is regarded as malicious altering while if the message is lost or its content is changed due to some failures, which may be transmission errors or hardware errors such as hard disk failure, then it is categorized as accidental altering.
Non-repudiation ensures that sending and receiving parties can never deny their sending or receiving the message.
In order to achieve the overall goal of Mobile Ad hoc Network security, above five mechanisms must be implemented in any ad-hoc networks so as to ensure the security of the transmissions along that network.
Secure Ad hoc Routing
As discussed earlier over the past decade, many Ad hoc routing protocols have been proposed in literature. Among them the most widely used are AODV (Ad hoc On Demand Distance Vector)  and DSR (Dynamic Source Routing)  which comes in the category of re-active routing protocols of Ad hoc Networks. All of these protocols have been studied extensively. But as there were no security considerations in the original design of these protocols, these protocols remain under threat from the attackers. The main assumption of these protocols was that all participating nodes do so in good faith and without maliciously disrupting the operation of the protocol. However the existence of malicious entities can not be disregarded in the systems especially the environment used for Ad hoc Networks. To overcome the security vulnerabilities in existing routing protocols, many security enhancements in these protocols have been proposed but unfortunately these secure Ad hoc Routing Protocols were either designed for a particular protocol or to address a specific problem operation of the protocol. For example SAODV (Secure Ad hoc On Demand Distance Vector Protocol)  was proposed to secure AODV (Ad hoc On Demand Distance Vector) protocol, Ariadne  was proposed to protect DSR (Dynamic Source Routing) protocol, ARAN  was proposed to protect the Ad hoc Routing in general while SEAD  was proposed to protect the DSDV (Destination Sequence Distance Vector Routing) protocol. The purpose of SAR  (Security Aware Routing) was also to protect the Routing in Ad hoc Networks.
Thus ongoing studies on MANETs pose many challenging research areas including MANETs security. Since MANETs are made up entirely of wireless mobile nodes, they are inherently more susceptible to security threats compared to fixed networks . Access to wireless links is virtually impossible to control thus adverse security events such as eavesdropping, spoofing and denial of service attacks are more easily accomplished. These security risks must be reduced to an acceptable level while maintaining an acceptable Quality of Service and network performance. However, in order to work properly, the routing protocols in MANETs need trusted working environments, which are not always available. There may be situations in which the environment may be adversarial. For example some nodes may be selfish, malicious, or compromised by attackers. Most of the work done regarding network security in MANETs focuses on preventing attackers from entering the network through secure key distribution and secure neighbor discovery ,. But these schemes become ineffective when the malicious nodes have entered the network, or some nodes in the network have been compromised. Therefore, threats from compromised nodes inside the network are far more dangerous than the attacks from outside the network. Since these attacks are initiated from inside the network by the participating malicious nodes which behave well before they are compromised, it is very hard to detect these attacks.
Keeping in view the security threats faced by MANETs we focus on Packet Dropping Attack which is a serious threat to Mobile Ad hoc Networks. Although many research efforts have been put on secure routing protocols but the attacks like packet dropping is not adequately addressed. We study the packet dropping attack in which a malicious node intentionally drops the packets they received. Unlike all previous researches which attempt to tolerate Packet Dropping Attacks, our work makes the first effort to detect the malicious activity and then identify the malicious or compromised nodes in the network.
The fundamental objective of this research is to discuss the security attacks faced by Mobile Ad hoc Networks specially insider attacks and to review the security in existing routing protocols especially secure routing protocols in MANETs. We particularly focus on packet dropping attack which is a serious threat to Mobile Ad hoc Networks. A novel security enhancement scheme to address packet dropping attack has been proposed.
Chapter 2 provides a brief introduction of security threats faced by Mobile Ad hoc Networks and secure routing to address these attacks. Chapter 3 discusses about the related work and flaws identified in the related work. Chapter 4 presents the possible solutions to address the packet dropping attack in Mobile Ad hoc Networks. Chapter 5 includes the implementation of proposed mechanisms and Results of the proposed mechanism and the thesis is concluded in Chapter 6.
Security Threats and Secure Ad hoc Routing Protocols
This chapter includes the threats and types of attacks faced by Mobile Ad hoc Networks. Secure Ad hoc routing protocols like SAODV  (Secure Ad hoc On Demand Distance Vector), SAR  (Security Aware Routing), and ARAN  (Authenticated Routing for Ad hoc Networks) etc and how these protocols are still vulnerable to attacks, are discussed in this chapter.
Types of attacks in MANETs
There are numerous kinds of attacks in the mobile ad hoc networks, almost all of which can be classified into two types, External Attacks and Insider Attacks.
External Attacks are those attacks, in which the attacker aims to cause congestion, propagate fake routing information or disturb nodes from providing services. External attacks are similar to the normal attacks in the traditional wired networks such that the adversary is in the proximity but not a trusted node in the network, therefore, this type of attack can be prevented and detected by the security methods such as authentication or firewall, which are relatively conventional security solutions.
Internal or Insider Attacks
Due to the invasive nature and open network media in the mobile ad hoc network, internal also known as insider attacks are more dangerous than the external attacks because the compromised or malicious nodes are originally the legitimate users of the Ad hoc network, they can easily pass the authentication and get protection from the security mechanisms. As a result, the adversaries can make use of them to gain normal access to the services that should only be available to the authorized users in the network, and they can use the legal identity provided by the compromised nodes to conceal their malicious behaviors. Therefore, more attention should be paid to the internal attacks initiated by the malicious insider nodes when we consider the security issues in the mobile ad hoc networks.
Internal or insider nodes when become part of the network can misuse the network in the following ways
A malicious node can attack at its level or at lower levels. Particularly in the context of Packet Dropping Attack, within a trust level, a malicious node or any other node which aims at saving its resources or intentionally launching a attack can successfully drop packets without being noticed and can get services from other nodes for forwarding its own packets.
An internal malicious node can prevent nodes from communicating with any other node.
A malicious node can break down an existing route or prevent a new route from being established.
An inside attacker adds itself between two endpoints of a communication channel.
Attacks based on modification
A very simplest way for a malicious node to disturb the operations of an ad-hoc network is to perform an attack based on modification. The only task the malicious or compromised node needs to perform is to announce better routes than the ones presently existing. This kind of attack is based on the modification of the metric value for a route or by altering control message fields.
There are various ways to perform this type of attacks; some of them are discussed below
Altering the Hop Count
This attack is more specific to the AODV  protocol wherein the optimum path is chosen by the hop count metric. A malicious node can disturb the network by announcing the smallest hop count value to reach the compromised node. In general, an attacker would use a value zero to ensure to the smallest hop count.
Changing the Route Sequence Number
When a node decides the optimum path to take through a network, the node always relies on a metric of values, such as hop count delays etc. The smaller that value, the more optimum the path. Hence, a simple way to attack a network is to change this value with a smaller number than the last "better" value.
Altering Routing Information
This type of attack leads network toward Denial of Service (DoS) attack. For example in a situation where a node M wants to communicate with node S. At node M the routing path in the header would be M-N-O-P-Q-R-S. If N is a compromised node, it can alter this routing detail to M-N-O-P. But since there exists no direct route from O to P, P will drop the packet. Thus, A will never be able to access any service from P. This situation leads the network towards a DoS attack.
Impersonation is also known as spoofing. In this type of attack the malicious node hides its IP address or MAC address and uses the addresses of other nodes present in the network. Since current ad-hoc routing protocols like AODV  and DSR  do not authenticate source IP address. By exploiting this situation a malicious node can launch variety of attacks using spoofing. For example in a situation where an attacker creates loops in the network to isolate a node from the remainder of the network, the attacker needs to spoof the IP address of the node he wants to isolate from the network and then announce new route to the others nodes. By doing this, he can easily modify the network topology as he wants.
Fabrication attacks can be classified into three main categories. Detection is very difficult in all of these three cases.
Routing table poisoning
Routing protocols maintain tables which hold information regarding routes of the network. In routing table poisoning attacks the malicious nodes generate and send fabricated signaling traffic, or modify legitimate messages from other nodes, in order to create false entries in the tables of the participating nodes. For example, an attacker can send routing updates that do not correspond to actual changes in the topology of the ad hoc network. Routing table poisoning attacks can result in selection of non-optimal routes, creation of routing loops and bottlenecks.
Route Cache Poisoning
This type of attack falls in the category of passive attacks that can occur especially in DSR  due to the promiscuous mode of updating routing tables. This type of situation arises when information stored in routing tables is deleted, altered or injected with false information. A node overhearing any packet may add the routing information contained in that packet's header to its own route cache, even if that node is not on the path from source to destination. The vulnerability of this system is that an attacker could easily exploit this method of learning routes and poison route caches by broadcast a message with a spoofed IP address to other nodes. When they receive this message, the nodes would add this new route to their cache and would now communicate using the route to reach the malicious node.
Rote Error Messages fabrication
This attack is very common in AODV  and DSR , because when nodes move these two protocols use path maintenance to recover the optimum path. The weakness of this architecture is that whenever a node moves, the closest node sends an error message to the other nodes so as to inform them that a route is no longer accessible. If an attacker can cause a DoS attack by spoofing any node and sending error messages to the all other nodes. As a result malicious node can separate any node quite easily.
Eavesdropping is another kind of attack that usually happens in the mobile ad hoc networks. The goal of eavesdropping is to obtain some confidential information that should be kept secret during the communication. This information may include the location, public key, private key or even passwords of the nodes. Because such data are very important to the security state of the nodes, they should be kept away from the unauthorized access.
Secure Ad hoc Routing Protocols
Many solutions have been proposed for secure routing in ad hoc networks, in order to offer protection against the attacks discussed earlier. These proposed solutions are either completely new stand-alone protocols, or in some cases incorporations of security mechanisms into existing ones (like DSR  and AODV ).
In order to analyze the proposed solutions and how they are still vulnerable to attacks we classified them into two main categories based on asymmetric cryptography and symmetric cryptography.
Asymmetric Cryptographic Solution
Protocols that use asymmetric cryptography to secure routing in mobile ad hoc networks require the existence of a universally trusted third party. This trusted third party can be either online or offline. The trusted third party issues certificates that bind a node's public key with a node's persistent identifier. Authenticated Routing for Ad hoc Networks ARAN  falls in this category of secure Ad hoc routing protocols; many of the other protocols presented in other categories that use asymmetric cryptography operate in a similar manner and have similar requirements.
Authenticated Routing for Ad hoc Networks ARAN
The Authenticated Routing for Ad hoc Networks (ARAN) proposed in  is a standalone solution for secure routing in ad hoc networking environments. ARAN use digital certificates and can successfully operate in the managed open scenario where no infrastructure is pre-deployed. The basic mechanism used in ARAN is certification that is achieved through the existence of a trusted certification authority (CA). All nodes are supposed to know their public key from the certification authority and also the public key of server. Prior to entering into the network, each node has to apply for a certificate that is signed by the certificate server. ARAN accomplishes the discovery of routes by a broadcast message from source node which is replied in a unicast manner. This route discovery of the ARAN protocol begins with a node broadcasting to its neighbors a route discovery packet (RDP). The RDP includes the certificate of the initiating node, a nonce, a timestamp and the address of the destination node. Furthermore, the initiating node signs the RDP. Each node validates the signature with the certificate, updates its routing table with the neighbor from which it received the RDP, signs it, and forwards it to its neighbors after removing the certificate and the signature of the previous node (but not the initiator's signature and certificate). The signature prevents malicious nodes from injecting arbitrary route discovery packets that alter routes or form loops . The destination node eventually receives the RDP and replies with a reply packet (REP). The REP contains the address of the source node, the destination's certificate, a nonce, and the associated timestamp. The destination node signs the REP before transmitting it. The REP is forwarded back to the initiating node by a process similar to the one described for the route discovery, except that the REP is unicasted along the reverse path. The source node is able to verify that the destination node sent the REP by checking the nonce and the signature. Figure 2 illustrates the process of route discovery in ARAN. All messages are authenticated at each hop from source to destination as well as on the reverse path. Due to heavy computation involved with the certificates, ARAN is vulnerable to many attacks e.g. DOS attacks. In situation when there are no malicious nodes in the network the load involved in the routing process force the legitimate nodes to drop the packets in order to save their resources.
Symmetric Cryptography Solutions
Symmetric cryptographic solutions rely solely on symmetric cryptography to secure the function of routing in wireless ad hoc networks. The mechanisms utilized is hash functions and hash chains. A one-way hash function is a function that takes an input of arbitrary length and returns an output of fixed length . As hash functions are especially lightweight when compared to other symmetric and asymmetric cryptographic operations, they have been extensively used in the context of securing ad hoc routing.
Secure Ad hoc On-demand Distance Vector Protocol (SAODV)
The Secure Ad hoc On Demand Distance Vector (SAODV)  addresses the problem of securing a MANET network. SAODV is an extension of AODV routing protocol that can be used to protect the route discovery mechanism by providing security features like authentication, integrity and non-repudiation. It uses digital signatures to authenticate the non-mutable fields of the message, and hash chains to secure the hop count information (the only mutable field in message) in both RREQ and RREP messages. The SAODV scheme is based on the assumption that each node possesses certified public keys of all network nodes . In order to facilitate the transmission of the information required for the security mechanisms, SAODV defines extensions to the standard AODV message format. These SAODV extensions consist of the following fields. The hash function field identifies the one-way hash function that is used. The field max hop count is a counter that specifies the maximum number of nodes a packet is allowed to go through. The top hash field is the result of the application of the hash function max hop count times to a randomly generated number, and finally the field hash is this random number.
When a node transmits a route request or a route reply AODV packet it sets the max hop count field equal to the time to live (TTL) field from the IP header, generates a random number and sets the hash field equal to it, and applies the hash function specified by the corresponding field max hop count times to the random number, storing the calculated result to the top hash field. Moreover, the node digitally signs all fields of the message, except the hop count field from the AODV header and the hash field from the SAODV extension header. An intermediate node that receives a route request or a route reply must verify the integrity of the message and the hop count AODV  field. The integrity requirement is accomplished by verifying the digital signature. The hop count field is verified by comparing the result of the application of the hash function max hop count minus hop count times to the hash field with the value of the top hash field. Before the packet is re-broadcasted by the intermediate node the value of the hash field is replaced by the result of the calculation of the one-way hash of the field itself in order to account for the new hop. In SAODV route error messages (RERR) that are generated by nodes that inform their neighbors that they are not going to be able to route messages to specific destinations are secured using digital signatures. A node that generates or forwards a route error message cryptographically signs the whole message, except the destination sequence numbers.
Although SAODV provides reasonable security to MANETs routing, but it is still vulnerable to distance fraud attack  in which the forwarding node fails to increment the route metric because in SAODV there is no enforcement to do so. Further there is no method to detect the malicious nodes and DOS attacks because in SAODV it is assumed that DOS attacks are restricted to physical layer, but this assumption failed when colluding malicious nodes drop packets during the route discovery process.
Security Aware Routing (SAR)
SAR  (Security Aware Routing) is an extension to existing on demand routing protocols and used where nodes are grouped on the basis of trust level. In SAR each node has different security level which assigns them different trust levels. Two nodes can only communicate with each other if they have equal or greater trust values. If a node has lower security level it simply discards the packet. In case there is no node in the network with the desired level then communication cannot take place or we can say that, that particular packet can't be forwarded unless its security level is lowered. By exploiting this condition a malicious node can attack at its level or at lower levels. Particularly in the context of Packet Dropping Attack, within a trust level, a malicious node or any other node which aims at saving its resources or intentionally launching a attack can successfully drop packets without being noticed and can get services from other nodes for forwarding its own packets. SAR also fails in the situations of secure routing in general because it only focuses on the situations in which certain groups are assumed to be trustworthy.
From the above discussion, we observe that all Secure Ad hoc routing protocols are still vulnerable to many attacks. Although proposed techniques provide security against external attacks, insider attacks are still an open issue in MANETs.
Many solutions have been proposed to prevent selfishness in MANETs. The main goal of all the schemes proposed in the literature is to make decisions regarding trustworthy entities and to encourage behavior that leads to increasing trust. In this section we discuss some of the solutions presented in the literature in order to detect the malicious nodes in the network in context of packet dropping attack.
Watchdog and Pathrater
In  Marti el al, proposed a mechanism called as watchdog and pathrater on DSR to detect the misbehavior of nodes in MANETs. Nodes in this scheme operate in a promiscuous mode. The watchdog monitors one hop neighbor by overhearing the medium to check whether the next neighbor forwards the packet or not. It also maintains a buffer of recently sent packets. If a data packet remains in the buffer too long, the watchdog declares the next hop neighbor to be misbehaving. Every node that participates in the ad hoc network employs the watchdog functionality in order to verify that its neighbors correctly forward packets. When a node transmits a packet to the next node in the path, it tries to promiscuously listen if the next node will also transmit it. Furthermore, if there is no link encryption utilized in the network, the listening node can also verify that the next node did not modify the packet before transmitting it . The watchdog of a node maintains copies of recently forwarded packets and compares them with the packet transmissions overheard by the neighboring nodes. Positive comparisons result in the deletion of the buffered packet and the freeing of the related memory. If a node that was supposed to forward a packet fails to do so within a certain timeout period, the watchdog of an overhearing node increments a failure rating for the specific node. This effectively means that every node in the ad hoc network maintains a rating assessing the reliability of every other node that it can overhear packet transmissions from. A node is identified as misbehaving when the failure rating exceeds a certain threshold bandwidth. The source node of the route that contains the offending node is notified by a message send by the identifying watchdog. As the authors of the scheme note, the main problem with this approach is its vulnerability to blackmail attacks. The pathrater selects the path with the highest metric when there are multiple paths for the same destination node. The algorithm followed by the pathrater mechanism initially assigns a rating of 1.0 to itself and 0.5 to each node that it knows through the route discovery function. The nodes that participate on the active paths have their ratings increased by 0.01 at periodic intervals of 200 milliseconds to a maximum rating of 0.8. A rating is decremented by 0.05 when a link breakage is detected during the packet forwarding process to a minimum of 0.0. The rating of -100 is assigned by the watchdog to nodes that have been identified as misbehaving. When the pathrater calculates a path value as negative this means that the specific path has a participating misbehaving node. The authors suggest that negative node ratings should be slowly incremented in order to avoid permanent isolation of nodes that suffer from malfunctions or overloads; however such a mechanism has not been implemented. But watchdog technique may fail to detect misbehavior in the presence of ambiguous collisions, receiver collisions , limited transmission power, false misbehavior and partial dropping .
In  Buchegger proposed a protocol CONFIDENT which also attempts to detect the malicious nodes in ad hoc networks. Monitor, Reputation System, Path Manager and Trust Manager are main components of CONFIDANT protocol. Monitor observes all of its neighbors for any malicious behavior. If any one of them is misbehaving, the reputation system is invoked. The Reputation System is responsible for managing a table which holds the entries for each node and its ratings. These ratings can change according to a function that assigns different rates to the type of behavior detected. If the rating of a malicious or misbehaving node exceeds from certain threshold value then Path Manager takes action against such node. Path Manger deletes this misbehaving node and generates an ALARM to Trust Manager. ALARM messages are forwarded to other friends and route initiators. The problems in CONFIDANT protocol comes with the Trust Manager that how he will maintain the friends list and what will be the criteria for threshold value. The attacker can also exploit this overall situation and can generate false ALARMS. Further there is no criteria if two friends declare each other misbehaving through ALARM messages. Moreover, CONFIDANT protocol only supports the negative experiences associated with a node and each entry in identified attackers list maintained by a node is associated with a timer. When this timer expires this node again becomes legitimate node in network.
From above discussion it is obvious that all the solutions proposed in the literature still lack deficiencies. Most of these approaches are either designed for a specific protocol or to address a particular problem. Further, node misbehavior in MANET environment is still an open issue. Thus, there is a strong need to pay attention in this area to mitigate node misbehavior.
Many defense mechanism have been proposed against malicious node behavior detection, however from previous discussion it is obvious that these approaches still lack some deficiencies. Thus with our observation that all the previously discussed protocols are incapable of protecting against insider attacks such as malicious packet dropping, we propose our solution to address these kind of attacks.. This chapter discusses our proposed security mechanism that how we can detect and then isolate these kind of misbehaving nodes from the network. A two folded solution has been proposed to first detect and then to isolate the misbehaving node from the network.
In this section, proposed solution to address the Packet Dropping Attack in MANETs is described. This proposed mechanism can be implemented on top of any source routing algorithm e.g. AODV  and SAODV . With the observation that SAODV  is unable of protecting insider node misbehavior such as packet dropping, we propose an enhancement to SAODV routing protocol. Since in SAODV, all RREQ, RREP and RERR packets are signed by the sender so malicious node cannot forge them. The only possible attack that will substantially affect propagation of these packets is to drop the packet.
Malicious activity detection and isolation
The proposed solution is two folded. Malicious activity detection and Identification of malicious node.
Proposed mechanism makes the first effort to detect the malicious activity in the network and then identify the malicious or compromised nodes in the network. This new mechanism does the malicious node misbehavior detection using information from the other neighboring nodes.
The assumptions of proposed scheme are,
- All the nodes in the network will be in promiscuous mode.
- Attackers do not work in the form of groups as it is assumed that there is no more than one misbehaving node on an active path.
Malicious activity detection
In order to find the malicious or compromised node in the network it has to be identified that is there any malicious activity took place in the network or not. Based on this malicious activity detection the other approach will identify that exactly which node is misbehaving.
In order to find a malicious or compromised node in the network we assume that there is an active route established between source node S and destination D using the intermediate nodes say N1, N2,N3 and N4 and all the necessary steps require to establish this route has been performed as shown in the figure below.
Now during packet forwarding every intermediate node is responsible for con?rming that the packet was received by the next hop. In order to ensure that every intermediate node has received data packet successfully a Data Acknowledgement mechanism has been introduced which we called here D-ACK.
Now when source node S forwards a packets for destination D through intermediate nodes N1,N2,N3 and N4 , source node S has no way of knowing that packet reached at destination D or not and if that packet is dropped by any intermediate node, exactly which intermediate node has dropped that packet.
In our proposed mechanism when source node S forwards any data packet for destination D, all intermediate nodes will send back a D-ACK packet to its source node. For example if at first step source node forwards a packet to N1, it will send back a D-ACK packet to source node S. Further when packet will reach at N2, it will also send back an D-ACK packet to Source node S through N1.
In an ideal situation when there is no malicious node present in the active route path , all intermediate nodes will send back their D-ACK packets to source node to inform that data packets has reached successfully up to that node. Thus source node S will remain updated until packet will successfully reach to its destination D. This operation is shown in figure 4.2.
Suppose in a situation, when packet reaches at intermediate node N1, it sends back its D-ACK to source node S informing that data packet has reached successfully. Source node thus source node S will never receive a D-ACK packet from N2. Upon not receiving a D-ACK packet source node S will retransmit that packet after a specific interval of time which we call here T-interval. Since node N2 is behaving maliciously, it will again drop the packet. Source node S will retransmit the packet again after T-interval up to a certain threshold which we call here Tmax (maximum threshold value). After this maximum threshold value Tmax, now source node S will broadcast a packet to declare the malicious activity in the network because until now source node S upon not receiving D-ACK packet comes to know that one of its intermediate nodes is misbehaving. Scenario in which N2 drops packets and not sending its D-ACK to source node S is shown in the figure 4.3.
In another situation in which when a packet is forwarded to its destination D by source node S through its intermediate nodes N1, N2, N3 and N4. When this packet reaches at N1, it forwards the packet to its next hope N2and sends back its D-ACK to source node S. Node N2 receives the packet, forward it and send back its D-ACK to source node S via N1. Now source node expects D-ACK from N3 but when this packet reaches at N3 , it sends back its D-ACK for source node, at this point say node N1 starts behaving as a malicious node , it will drop the D-ACK packet of node N3 which it receives from N2. As source node was expecting D-ACK from N3, upon not receiving D-ACK he will assume that node N3 is behaving as malicious or compromised node but actually node N1 is doing this activity. Although source node S cannot confirm that exactly which node is misbehaving, it can still observe an activity that one of its intermediate node is misbehaving. This whole scenario is shown in the figure below.
We have discussed three different scenarios in which a malicious or compromised node can drop a data packet or a D-ACK. Although source node S do not know that exactly which node is misbehaving , through this proposed activity detection mechanism source node S comes to know that there is some malicious activity took place in the network. Based on this activity, second approach will detect exactly which node is compromised or behaving intentionally as malicious node.
Identification of malicious node
Since a malicious activity has already been observed in the network, our second approach will detect exactly which node is misbehaving.
As all the nodes in the network are in promiscuous mode , nodes like N8, N9, N11, N19 lie in the transmission range of active route can hear all the packets coming into and out of the nodes of active route as shown in the figure below.
For example as shown in Fig, there is an active route between source node S and destination D with N1, N2, N3 and N4 as intermediate nodes. Some nodes near to the active path like N8, N9,N11, N19 lie in the intersection of ranges of node N1 and N2. These nodes can receive all packets sent by N1 to N2 because they are in transmission range of N1 and can also receive every packet forwarded by N2, since they also lie in the transmission range of N2. From our assumptions that all the nodes in the network lie in promiscuous mode and will be ready to receive any packet. These nodes count number of packets coming into and going out of N2. Every node in this same range maintains a list of sent and dropped packets. Once the total number of dropped packets reaches the maximum threshold (Tmax), the monitoring node will broadcast a packet to all their neighbors. This packet contains malicious nodes id, sender's id with sender's signature.
Broadcast message format
Format of the broadcast message to declare any misbehaving node as malicious node is shown below. This message contains sender ID (i.e. monitoring node), misbehaving node ID, and monitoring node signature.
The purpose of this notification packet is to advertise a malicious node in the network and hence to make this node as "Blacklisted" in routing table of all its neighbor nodes. This notification message is sent to neighbors of the nodes in range, which in this case are node N8, N11 and N19. Assume node N8 found node N1 crossing the threshold of dropped packets (Td), so it sends a notification packet to all its neighbors. Similarly node N11which is also in the same range as of N8 will see the same behavior from N1 and forward a same notification packet to their neighbors. When all the neighbors of N1 receive this notification packet signed by either N8 or N11, these neighboring nodes will break their links with N1 and isolate it from the network. In future all the nodes in the network before establishing a new route will consult their blacklist to add any node into active route.
Benefits of malicious activity detection
Now at this stage we can justify the benefit of using malicious activity detection before using the second approach which actually detects that which node is misbehaving. Suppose in a situation when there is no malicious node is present in the path of active route, any malicious node which actually is not the part of active route but it is in the transmission range of nodes of active route can broadcast a packet to declare a legitimate node as malicious node. In this situation source node S has no way to decide and check the authenticity of that node, but in the presence of malicious activity detection mechanism, source node already knows that there is some activity took place in the network, thus if source node did not notice such activity and secondly any other node in the same range of active route did not broadcast such packet , source node S and other nodes present in the network will consider that node as malicious node.
A novel approach to detect malicious nodes in the network has been proposed. This approach makes its best effort to detect malicious activity and then isolate the malicious node from the network. Our proposed mechanism can be deployed on top of any source routing algorithm like SAODV. As digital signature and hash chain mechanism is already developed in SAODV, this approach will mitigate the threats of insider attacks e.g. packet dropping attack in MANETs.
Results and Analysis
This chapter describes the implementation and effectiveness of our proposed scheme. The primary purpose of the simulation is to verify the contents of our proposed mechanism, described in Chapter 4. It is an important contribution for security issues in Mobile Ad Hoc Networks that will be used as a basis for further development work and experimentation on the behavior of malicious nodes present in MANETs.
Several simulators are available for simulation and testing of protocols and algorithms. We discuss some of them here.
Network Simulator2 (NS-2)
Ns-2 is probably the most popular and network simulator; however, it does need advanced skills to perform a meaningful simulation. It focuses on the ISO/OSI model simulation.
GloMoSim stands for Global Mobile Information Systems Simulation Library that currently provides only protocols for the simulation of pure wireless networks.
SENSE is a specific simulator for the simulation of sensor networks. The simulation capacity depends on the communication pattern of the networks, which could drop to an upper limit of 500 nodes.
Shawn is a new simulator developed for the simulation of large scale networks, which currently needs to be further developed, and provide more technical supports and contributed models.
OMNeT++ is a frequently used simulator for Mobile and Sensor networks, can run on both Microsoft Windows and Unix OS using various C++ compilers. The modular simulation model allows users to reflect the logical structure of an actual system in the flexible hierarchical modular structure. Various module parameters allow users to control every module using either the command line interface or the graphical user interface (GUI). The simulator also provides abundant base function modules called simple modules that are programmed in C++ using the simulation library. An advanced GUI makes the inside of modules and the process of simulation execution visible and controllable by users.
Components of OMNeT++ simulation
The following components are the basic elements to run an OMNeT++ simulation. The configuration file, usually called omnetpp.ini, is firstly processed by the program when a simulation is run. This file contains the settings needed for the execution of a simulation, values of model parameters, etc. Module and topology properties are described in .ned files, which are parameters, gates etc. Message definitions are user-defined and will be translated into C++ classes. Simple modules sources are programmed in C++ files, which are the functioning modules of a simulation. The decision to select OMNeT++ as the simulator in the present work is supported by the following considerations.
Object-oriented Language C++
C++ is probably the most popular object oriented language that just caters to the concept of modular simulation; in addition, C++ is one of the fundamental computer languages, easy to start and use.
Modular Simulation Structure
Modular structure is highly flexible; modules from different model packages can be modified and combined to function freely, as well as your own modules.
Excellent Technical Support
There are abundant models on OMNeT++ website contributed by the users all over the world. The forum and the news group of OMNeT++ provide effective interaction, help and support.
OMNeT++ is currently free for academic and non-profit use. The commercially supported licensed version, OMNEST, can be purchased from OMNEST Global, Inc.
We simulated SAODV routing protocol with the security enhancement of our proposed mechanism to protect intermediate packet dropping attack. Simulation is carried out with 50-200 mobile nodes moving in a 500 x 500 m flat area. Each node transmission range is 250 m. The IEEE 802.11 MAC layer is used. A random way point mobility model is assumed with a maximum speed of 20 m/sec. For the communication, CBR transfers between pair of nodes. Table below summaries the parameters used for simulation.
The SAODV standard provides many optional operations to make the protocol as adaptable and flexible as possible. However, implementing all of the operations is complicated and unnecessary. In our simulation we modified the basic options suitable for an ad hoc network.
HELLO messages are exchanged between neighbors to check the status. In this simulation, a self HELLO message is scheduled to initiate the simulation.
We call this ACK messages as DACK which is used to confirm the correct delivery of both DATA message, while SAODV standard uses it only for a RREP confirmation.
D-ACK Message Format
On successful reception of a data packet, an intermediate node replies with a DACK packet to its source node containing digital signature and sequence number informing that it has successfully received the DATA packet.
A blacklist is used to block misbehaving neighbor nodes. A node inserts a misbehaving node in the blacklist if it can not receive any acknowledgment after a number of tries. This blacklist is used as a reference in future to establish any new route.
Results and Analysis
We conduct several tests in order to analyze the effectiveness of proposed mechanism.
Total dropped packets
At first we carried out a simulation to determine the amount of packets that are dropped by malicious nodes from the total number of packets.
Packet drop ratio with no malicious nodes
In scenario-I, we simulated standard SAODV, with no malicious nodes present in the network. Simulation is carried out with the variation of different time intervals. Packet delivery ratio vs simulation time graph shows that almost all the packets delivered to their destination successfully.
Packet drop ratio with malicious nodes
In scenario-II, we simulated standard SAODV in the presence of malicious nodes present in the network. Traffic analysis was done for both TCP and UDP. UDP achieved better results as compared to TCP because there is no form of flow control or error correction.
Performance analysis of SAODV and E-SAODV
In scenario III , we compared percentage of packet loss after implementing our security enhancement to SADOV. Below graph shows the packets received after the proposed security mechanisms are incorporated in the routing protocol. Since routing protocol detects the misbehaving nodes and isolates them from the network, most of the active paths will be free of malicious nodes and hence packet transfer will be done effectively till the destination. There is a clear decrease in percentage of packet loss in E-SAODV (Enhanced SAODV) as compared to standard SAODV.
In scenario IV, comparison for throughput between standard SAODV with no malicious nodes present in the network, SAODV in the presence of malicious nodes and E-SAODV in the presence of malicious nodes is performed. Graph below shows that E-SAODV performs much better than standard SAODV in the presence of malicious nodes in term of throughput.
With above experiments, it can be seen that the performance of our proposed mechanism is more effective as compared to traditional SAODV in terms packet loss ratio and through put. The addition of control overhead is justified by successfully transmitting data packets. This also proves that these security extensions are not expensive to implement over a routing protocol.
Conclusion and Future Work
This research was aimed at to study the insider attacks in Mobile Ad hoc Networks. The objective was to address the packet dropping attack in MANETs and to modify current Ad hoc routing protocol like SAODV to make Ad hoc Networks more resilient against these types of attacks. This contribution has identified unique challenge offered by MANETs environment. A comprehensive analysis of Packet Dropping Attack was made to develop a security module that would meet the security goal.
In this thesis, effective and practicable improvements have been proposed to SAODV to challenge the packet dropping attack. Unlike other approaches proposed in the literature which attempt to tolerate the packet dropping attacks, a comprehensive approach to detect and then to isolate the malicious nodes present in the network have been proposed. Proposed DACK (Data Acknowledgement) mechanism successfully detects the malicious activity in the network and then upon detection of malicious activity, second approach with the collaboration of other nodes in the network detects that exactly which node was misbehaving. A black list is maintained, to keep track misbehaving nodes for future reference. Black list is checked before allowing any node to be a part of active route. We have successfully implemented our proposed mechanism in OMNET++ to show the effectiveness of our security enhancement against packet dropping attack. Previous chapter depicts different scenarios like performance of SAODV with and with out the presence of malicious nodes in the network. Further comparison of standard SAODV and E-SAODV (Enhanced SAODV) is performed to show the effectiveness of proposed mechanism. This research work presents initial work in detecting misbehaving nodes and isolating such nodes from ad hoc networks. The success of MANETs in many applications demand adequate security for routing protocols. This contribution has identified unique challenge offered by MANETs environment. A comprehensive analysis of Packet Dropping Attack was made to develop a security module that would meet the security goal.
The proposed mechanism has been tested on OMNET++. Due to higher node mobility and hardware costs, it is practically not possible to test these implementations on a large network testbed. However, the baseline protocol has vastly been researched for performance and scalability by many researchers. Secondly, being a novel approach E-SAODV (Enhanced SAODV) works with one malicious node per active route. Simulation results also shows that there is always possibility of false detection, consequently, a node can immediately accuse another node as selfish when detecting that a packet has been dropped. So the value of threshold must be chosen in a careful manner to avoid this situation.
During the course of this work a number of future directions have been identified. As perspective, we plan to improve the solution and decrease its overhead. We also plan to improve our research work for partial dropping and increasing the number of malicious nodes per active route. Further research can be completed by giving a rigorous definition to the tolerance threshold, defining actions that have to be taken when a node is accused as a selfish, and proposing a mechanism allowing nodes to exchange their knowledge regarding nodes that behave selfishly. More simulations are also planned in our future work to determine optimal values to increase throughput in different scenarios.
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