SeCQos Workshop, Athens, September 2005

1

New Approach for Selfish Nodes Detection in Mobile Ad hoc

Networks

Djamel Djenouri: Basic Software Laboratory, CERIST Center of Research, Algiers, Algeria.E mail: ddjenouri@mail.cerist.dz Nadjib Badache: Computer Science Department, USTHB University, Algiers, Algeria.E mail: badache@mail.cerist.dz

SeCQos Workshop, Athens, September 2005

2

Outline

1 Overview on Mobile Ad hoc Networks (MANETs) 2 Selfish behavior on packet forwarding problem 3 Watchdog4 Watchdog’s limitation5 New Solution6 Simulation results7 Perspectives8 Conclusion

SeCQos Workshop, Athens, September 2005

3

A MANET is a collection of Wireless mobile hosts making on the fly a temporary network, without relying on any fixed infrastructure. No central administration exists Mobile hosts cooperate to ensure roles ensured by the fixed infrastructure in traditional networks

MANET Overview (Definition)

SeCQos Workshop, Athens, September 2005

4

MANET Overview (Multi-Hop)

SeCQos Workshop, Athens, September 2005

5

• Dynamic Topology• Limited bandwidth• Limited physical Security• Infrastructurless• Limited energy resources

MANET’s features

SeCQos Workshop, Athens, September 2005

6

Selfish behavior

SeCQos Workshop, Athens, September 2005

7

Watchdog

• Proposed by Marti et al. In MobiCom 2000

• Many recent sophisticated solutions rely on it in their monitoring component 

SeCQos Workshop, Athens, September 2005

8

Watchdog’s principles

• Based on the promiscuous mode monitoring and implemented with a source routing protocol.

• When a node Na sends a packet to Nb to forward to Nc, it monitors it by simply overhearing the channel, it validates the forwarding iff it overhears the packet retransmitted by Nb.

• This process is generated for each couple of hops in the route

SeCQos Workshop, Athens, September 2005

9

WatchdogIf packet heard OK

Else increase Nc’s failure Tally

failure Tally > threshold the node misbehave

SeCQos Workshop, Athens, September 2005

10

Watchdog’s detection drawbacks

• Does not detect correctly and could cause false detections when the power control technique is employed

• Does not detect selfish nodes in many cases (collisions, partial dropping, collusions)

SeCQos Workshop, Athens, September 2005

11

SeCQos Workshop, Athens, September 2005

12

New solution (Two-hop ACK)

• We propose a new monitoring solution based on two-hop ACK.

• Assume A monitors B’s forwarding to C, this latter acknowledges each packet by sending a two hop ACK back to A via B.

• An efficient asymmetric encryption strategy is used to ensure authenticity of ACKs and to prevent the following vulnerability:

SeCQos Workshop, Athens, September 2005

13

New solution (hop ACK)

• B misbehaves and drops packets, to circumvent A it sends it a falsified ACK on behalf of C.

• The encryption strategy we used gets over this vulnerability and,

• it is relatively of low cost, since it merely encrypts short random numbers, instead of compute digital signature on the packets.

SeCQos Workshop, Athens, September 2005

14

New solution (hop ACK)

- Generates r- Encrypts it with C’s PK- Adds it to the packet

- Decrypts r- Encrypts it with A’s PK- Adds it to a 2 hop ACK packet

- Decrypts r- - Checks whether it matches the one it generates - If so, it validates the forwarding 

SeCQos Workshop, Athens, September 2005

15

Simulation results

SeCQos Workshop, Athens, September 2005

16

Two-hop ACKs Problem

• The obvious problem of our first solution is the important overhead it engenders, even if the nodes well behave. It requires a two-hop ACK for each data packet, which is costly.

SeCQos Workshop, Athens, September 2005

17

Perspectives

We plan to complete the proposal by • Reduce the overhead• Defining efficiently the threshold of

accusasion• Defining actions that have to be taken

when a node is accused as a selfish• and particulary by proposing a mechanism

allowing nodes to exchange their knowledge regarding nodes that behave selfishly.

SeCQos Workshop, Athens, September 2005

18

Random two-hop ACK

• To overcome the two-hop cost, we suggest that A would not continuously ask an ACK, but randomly decides whether to do so with a probability p, and maps the decision in the packet.

• This randomization and mapping prevent B from deducing which packet includes an ACK request, and let it motivated to forward all packets

SeCQos Workshop, Athens, September 2005

19

Random two-hop ACK

• P is updated according to B’s behavior, it is decreased each time B forward a packet until reaching its minimum value, and set to 1 upon a dropping detection.

• This way, more trust is accorded to well-behaving nodes and ACK ask is enforced after a dropping, which ensure the efficiency and decreases the cost especially when selfish nodes rate is low, as shown in the simulation results

SeCQos Workshop, Athens, September 2005

20

Conclusion

• Our solution is operational regardless the power control employment

• It reduces the false detection rate with low cost (energy and delay)

• The random two-hop ACK reduces dramatically the overhead

SeCQos Workshop, Athens, September 2005

21

?