A wireless sensor network typically consists of a dense deployment of sensor nodes to achieve higher resolution and better network coverage. A dense network also increases the fault-tolerance and robustness of the system. However, if not properly handled, it can lead to more collisions during transmission and network congestion. Furthermore, wireless communication is inherently unpredictable and error-prone. Hence, it is imperative to design an efficient medium access control (MAC) protocol that facilitates guaranteed delivery of data over unreliable wireless links. In this paper, we have designed an on-demand reliable MAC protocol (RMAC) that enables timely delivery of data. We have demonstrated its superior performance over existing reliability-enforcing approaches in terms of reliability, latency, scalability and energyefficiency.