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Replay protection is one of the three vital security attributes, desirable in a link layer security protocol for a wireless sensor network (WSN). Whether to implement replay protection at the link layer or at the application layer, in the WSNs, remains a debatable issue. For, the popular WSN link security framework TinySec does not offer replay protection, whereas MiniSec at the other extreme, implements two different link layer anti-replay schemes separately, for unicast and broadcast communications.In this paper, we argue that anti-replay must be implemented at the link layer in WSNs. We also present a simple scheme for implementing replay protection for unicast communication, at the link layer security framework. We evaluate our scheme against the conventional schemes, for ensuring replay protection.Through the analysis and experimental evaluation in TinySec framework, we show that our bloom-filter based approach with multiple hash functions is simpler and optimal approach, for replay protection.