Link-level retry
Beginner
If a burst of data gets garbled on one link, the two ends of that link quietly resend it before anyone else notices.
Novice
Error recovery done by the two ends of a single link: the receiver checks each block with a checksum (CRC) and asks for a resend if it fails, without involving software.
Expert
Hop-by-hop replay from a retry buffer (LLR). Keeps the bit-error rate seen by memory-semantic traffic low enough without end-to-end retransmission, at the cost of buffer area and a little latency; combined with forward error correction on high-speed SerDes.
Explained in Scale-up fabrics (Systems).