由於行動隨意網路(MANET)具有不需要連線基礎設備以及動態拓撲等特性，使得各裝置間的時間同步成為了一個非常重要的議題。本研究針對IEEE 802.11標準上所制定的時間同步協定進行安全防禦，並提出中位數演算法(Median Method)來解決一種可破壞時間同步機制的攻擊 － Hit-and-Run攻擊。
我們利用收集各節點的時間資訊來計算一個校正值，並且利用這個校正值來預測出所收到較快節點的時間，再利用校正值與預測出的時間來過濾惡意節點。此外，本研究採取了一部分的Automatic Self-time-correction Procedure(ASP)來解決MANET上的規模性問題。我們也計算了各時間點中各節點時間差異的標準差來更加精確的觀察中位數演算法的防禦效果。
最後，我們使用了Network Simulator version 2(NS2)網路模擬器來測試中位數演算法防禦Hit-and-Run攻擊的效果。經過模擬實驗後的結果顯示，當攻擊者不超過總節點個數30%時，中位數演算法可完全防禦Hit-and-Run攻擊。在網路沒有攻擊者狀況下，各節點單位時間內時間差異的標準差值落在[5.31, 10.02]之間，而當總節點數為400，惡意節點個數為120時，此為模擬實驗中最差的結果，其標準差值仍落在[6.79, 11.69]之間。同時，在10秒內即可讓各節點無法同步的現象消失。

The dynamic topology feature of wireless ad hoc network (MANET) makes time synchronization an important issue to this kind of infrastructure-less netwrok. On the other hand, the IEEE 802.11 standard specifies a clock synchronization protocol for MANET, but it is designed without taking into consideration of possible security issues. In this paper, we introduce Median Method, which is developed to defend a feasible malicious attack to IEEE 802.11 TSF: the Hit-and-Run attack.
In Median method, we calculate a calibration value by collecting the clock information of each mobile hosts, then use the calibration value to predict the time of faster nodes and to filter out malicious nodes. In addition, we solved the scalability issue of MANET by adapting a part of Automatic Self-time-correction Procedure(ASP). We also calculated standard deviation of clock difference of every node at each time to observe the defensive performance of Median Method more precisely.
Simulations of Median Method defending Hit-and-Run attack are conducted in ns2 network simulator, and the simulation results show that the proposed method is able to completely defend the attack given the percentage of attackers no more than 30%. The standard deviation of clock difference, when there is no attack launched, is between 5.31 to 10.02.
On the other hand, our worst case happens when the numbers of legitimate nodes and the attacker node are 280 and 120 respectively. In this scenario, standard deviation still remains in the range of 6.79 to 11.69, and the status of lack of synchronism caused by the Hit-and-Run attack vanishes within 10 seconds.

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