Hello experts.

There seems to be two retransmission nechanims supported in the 5G protocol stack. One HARQ at PHY MAC and another ARQ at RLC. I know HARQ works through retransmission of transport blocks based on ACK or NACK feedback. However I don’t know about ARQ at RLC. Does it work by retransmission of RLC SDU based on ACK NACK?
Thank you.

ARQ is discard the Bad Package, No need Acknowledgment from receiver.

Hello how does UE know what to discard at RLC? Is it discarding RLC SDUs? Thank you

the transmitting UM RLC entity shall discard the indicated RLC SDU if the RLC SDU or RLC SDU segment has not been mapped to a RLC data PDU yet.

Thank you for your time. In what mechanism does retransmission happen? How does UE know what to sdus to retransmit?