ACKs under 802.11 in general and 802.11p more specifically work as following (for a sender X and one or more recipients Y,Z)
- UNICAST: X sends a message and triggers a timer, Y receives the message and sends back an ACK, if the ACK is lost or not received before timer expiry then X retransmits the frame
- BROADCAST: X sends a message but does not trigger any time. Upon receipt, Y does not send an ACK. Indeed, in Broadcast mode, there are no Timers nor ACKs.