End-to-end reliability in Delay Tolerant Networks (DTNs) is challenging and complicated because of high delay and absence of stable end-to-end path in the intermittently-connected mobile environment. Some existing acknowledgement mechanisms use active forwarding to provide the end-to-end reliability, while incurring excessive retransmissions or replications. The other passive mechanisms aim to reduce the storage overhead but may suffer a large delay. To improve the storage-delay tradeoff, we propose a Congestion Level based end-to-end ACKnowledgement (CL-ACK) mechanism, which adaptively adjusts the spread manner of ACK packets according to the ratio of drops over replications. Simulation results show that CL-ACK effectively controls resources consumption, reduces end-to-end delay, and achieves high message delivery rate.
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