Selective Repeat is one of the automatic repeat-request (ARQ) techniques. With selective repeat the sender sends a number of frames specified by a window size even without the need to wait for individual ACK from the receiver as in stop-and-wait. However receiver sends ACK for each frame individually, which is not like cumulative ACK as in go-back-n. The receiver accepts out-of-order frames and buffers them. The sender retransmits timeouted frames individually.