In this paper, we proposed a time synchronization protocol based on FTSP, called Outlier-Filtered Time Synchronization Protocol for WSNs. It points the insufficiencies of FTSP and could resist attacks. FTSP deals with neither external attackers nor insider attackers. We proposed three methods for promising the authenticity and integrity of the messages to defend against external attackers. The first method is that communicating through unicast with each neighbor. The second method is to broadcast the message with multiple MACs. The last method is an uTESLA-based approach with a little modification. Their functionalities are all the
same for avoiding external attackers forging and spoofing messages, but the limitations and overheads are different. Users could depend on environment to select a suitable one.
To defend against insider attack and pulse-delay attack, we design a neat method due to the resource constraints of the sensors. The filter ratio is an adjustable parameter. The filter ratio could be set by the estimated ratio of outliers and also could be automatically tuned by extra computation. The smaller filter ratio means that we retain more reference points and the predictive relation between local clock and global clock will be the more precise. But smaller filter ratio means the security level is lower.
Based on our experiments, we inserted both extreme and mild outliers into the normal datasets and observed that RTSP keeps the same precision as that of FTSP even when some attacks take place. The average error of skew could be less than 1us and the average error of offset could be less than 20us. The lower error of clock skew means that we don‟t need to resynchronize too frequently, so we could save more
energy.
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