Reliability in IP-multicast forwarding becomes useful while operating in lossy environment of wireless sensor networks. RPL is a protocol for forming a multicast tree in sensor networks for multicast data forwarding. Trickle multicast uses RPL to provide reliability, however it has a high overhead and delay, especially when a radio duty cycle is used on the sensors. Stateless Multicast Forwarding (SMRF) optimizes for the radio duty cycle, reducing delay but also reducing reliability. This paper proposes iACK, a retransmission scheme on top of SMRF that uses implicit acknowledgements present in wireless broadcast to determine which packets a node should retransmit. We have implemented iACK in ContikiOS. Results show that iACK delay is about 5 times less than Trickle (and close to SMRF), and packet delivery ratio is about 80% (compared to between 20% and 60% for Trickle and SMRF). With a slight increase in memory requirements, iACK offers a valuable tradeoff compared to existing protocols. © 2014 IEEE.