Link level - if a frame never even arrives at a
receiver, a sliding window protocol is typically used.
At the socket endpoint (the transport protocol OR the
application)- might observe a gap in sequence numbers,
OR, a sender's transmission timer might pop
Error Recovery -
Over wireless where bit errors are common, redundant
data is added within frames so that the frame data can be
received in spite of some number of bit errors
Reliable data transfer (RDT) protocols are used to allow
a receiver to coordinate with a sender regarding what
might need to be retransmit.