2 ANÁLISIS DE MERCADO
2.3 ESTRUCTURA DEL MERCADO
The presented results confirm that TTHCA pragmatically fulfils the research Objective 1 to accurately and consistently detect HM/PM O-B and PM I-B wormholes with no false positive detection occurring. Any route infected by either a HM/PM O-B or PM I-B wormhole is straightforward to distinguish from a healthy route since it leads to a significantly higher PTT/HC due to the high ∆Twh value which leads eq. (5.8) to be upheld. The high detection rate is not always met however, when the MANET is infected by a PM O-B wormhole and the wormhole link is short, i.e. less than 6 hops in the test simulation environment. This is true because ∆Twh = 0 for such a wormhole and therefore a long route with a short wormhole means that Lemma 5.2 is not fulfilled and the wormhole is undetected. Though the detection rate is higher than the defined baseline comparator ground truth (70%) when the wormhole length ≥ 4 hops.
From a computational and network complexity perspective, TTHCA offers a low overhead solution as all the operations have linear complexity O(HC). However, the new routing packet RREPTTHCA introduces an extra delay in the route discovery procedure and some
additional packet processing at both the intermediate and destination nodes is incurred because TTHCA requires two reply packets instead of one in the AODV protocol.
A summary of the high level aims to fulfil Objective 1 and how the proposed TTHCA algorithm fulfils these goals when nodes have identical hardware and are in LOS is presented in Table 5.3.
Table 5.3: A summary of desired goal settings for TTHCA and how they were fulfilled.
Desired goal settings
TTHCA Objective
achieved? Summary
100% wormhole detection Partially
Detection rate is 100% for HM/PM O-B and PM I-B wormholes but < 100% for short PM O-B wormholes.
Network topology independent Partially A short PM O-B wormhole in relation
to the route HC is not detected.
No FP detection Yes
Assuming accurate RTT and ∆Ti
measurements eq. (5.3) can never be true for a healthy route.
Low computational overheads Yes All computational operations have order
of complexity O(HC)
Negligible bandwidth load Partially
New routing packet RREPTTHCA causes a minor delay on the routing discovery procedure and bandwidth overheads on intermediate nodes.
Using the fixed threshold in eq. (5.3) has proven to work well provided all nodes have same
R and are located in a LOS environment. However, the wormhole detection rate degrades
once the aforementioned assumptions are relaxed, such as for instance, high variability in radio coverage, which can be experienced due to different MANET node hardware and/or variability in network surroundings.
To illustrate the effect of relaxing the LOS environment assumption on TTHCA wormhole detection performance, consider the 5 HC route example in Figure 5.8, where A and D are the source and destination nodes respectively, M1 and M2 are malicious PM O-B wormhole
nodes, and B as well as C are legitimate intermediate nodes.
Figure 5.8: An example of a route infected by a PM O-B wormhole.
Assuming identical node hardware with R = 100 m, and all ri,i+1 values being close to the lower bound eq. (5.6) in Lemma 5.1, such as 51 m. If the wormhole link is 3R (3 hops) then
the corresponding PTT for that route will be ∑ 𝑟𝑖,𝑖+1+
𝐻𝐶−1
𝑖=1 𝑟𝑤ℎ
𝑆 = 1680 ns and the wormhole
will in a LOS environment be detected, but only within the very narrow window (𝑃𝑇𝑇𝐻𝐶 = 336𝑛𝑠 > 𝑅
𝑆= 333𝑛𝑠) from eq. (5.3). If however, there are physical obstacles between two or
more nodes on the A to D path, the lower bound for ri,i+1 between these nodes will inevitably become lower than eq. (5.6) and as a result there is a higher likelihood the wormhole will not be detected. For example, if rA,B = 44 m due to an obstacle between A and B, while all remaining ri,i+1 = 51 m, PTT/HC will then = 332 ns which is less than R/S so the wormhole will not be detected using the static threshold in eq. (5.3).
The impact of relaxing the assumptions on network environment and node hardware is rigorously analysed in Chapter 6 where a new extended version of TTHCA, called TTpHA is presented which integrates a dynamic threshold mechanism which is able to adapt to the prevailing network conditions and thus provides improved wormhole detection performance in challenging environments where there are high radio range variations.
5.4. Summary
This Chapter has presented a new wormhole detection algorithm, TTHCA, that is based on PTT/HC analysis. Similarly to the RTT based approach DelPHI, TTHCA analyses the delay of a route in relation to its HC for identifying a wormhole infected route, however unlike DelPHI and other RTT-based solutions, it reduces all node packet processing delays from the RTT measurement to get the PTT. PTT more accurately reflects the distance of a route compared to RTT and is therefore significantly more robust for detecting wormholes.
Simulation results showed that TTHCA works well when assuming LOS environments and identical hardware on all nodes and therefore TTHCA fulfils Objective 1 to a satisfactory standard. TTHCA wormhole detection and false positive performance was also significantly
better compared to DelPHI and MHA. However, if the route is infected by a short PM O-B wormhole in relation to the route HC there is a risk that TTHCA wormhole detection will fail. High fluctuations in radio ranges further increase the risk that TTHCA will not detect PM O-B wormholes. TTHCA uses a fixed threshold for PTT/HC validation which has two main limitations, firstly it does not adapt to prevailing network conditions meaning that the wormhole detection performance in an indoor environment is poor since such an environment requires a lower threshold than a LOS environment. Secondly, in a heterogeneous network where network nodes are using dissimilar wireless communication technologies the threshold for the maximum permissible PTT/HC cannot be based on R since it may be highly variable even in a LOS environment.
To address the identified limitations of TTHCA in node radio range variability and fulfill research Objective 2, a new extended version of TTHCA, called TTpHA is proposed in the next Chapter.