Selective Repeat

Selective Repeat

In GBN, one packet error can trigger many retransmissions, including packets that need not have been retransmitted. Selective Repeat retransmits only erroneous packets. The receiver buffers out-of-order packets within its window until the missing earlier packet arrives, then delivers the batch to the upper layer in order.

Individual ACKs, Buffering, and Timers

The receiver individually acknowledges each correctly received in-window packet. The sender records these ACKs but moves its window only when the left edge has been acknowledged; ACKs above a gap do not skip unacknowledged packets. Every unacknowledged packet has its own timer; timeout retransmits only that packet.

The receiver buffers in-window out-of-order packets and delivers a contiguous prefix after filling a gap. An old packet just outside the current receive window should be acknowledged again without redelivery, because its original ACK may have been lost.

Sequence Space Must Be at Least Twice the Window

With equal send and receive windows $N$ and sequence space $M=2^k$:

The receiver may have delivered a full window while lost ACKs leave the sender holding its old window. If old and new windows overlap in sequence numbers, the receiver cannot distinguish retransmitted old data from new data. Circular comparisons require modular arithmetic; the recovery lab uses logical IDs, while the sequence-ring lab isolates wraparound.