The SLU sends Request Recovery to the PLU to request sequence number resynchronization or other recovery operations. In most cases, RQR is sent when the SLU discovers (1) a discrepancy in the sequence numbers of incoming requests or (2) a discrepancy between the sequence numbers it assigned to outgoing requests and the sequence numbers of the responses it is receiving to those requests. It might also send RQR if it loses or is forced to discard some incoming requests before it can process them. (For example, input buffers are too full to hold the incoming requests.)

For a description of the procedure used to resynchronize sequence numbers, see Controlling flow. For examples of the use of Request Recovery, see Figure 13, Figure 26, and Figure 27.