Fast Retransmit and Fast Recovery
When a TCP segment arrives and the sequence number is not the next sequence number the receiver was expecting (a noncontiguous, out-of-order segment), an immediate ACK is sent with the Acknowledgment Number field set to the next sequence number the receiver was expecting. This ACK is a duplicate of an ACK that was previously sent and is not subject to the delayed acknowledgment behavior for new contiguous data received.
After receipt of this duplicate ACK, the sender cannot determine whether the duplicate ACK was sent by the receiver because of a TCP segment that arrived out of order or because a segment was lost.
If a TCP segment arrived out of order, the TCP segment that contains the next byte the receiver expects ...
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