tcp: bugfix for missing ack for window rejected segments (#45)

tcp/control.go

@@ -445,6 +445,14 @@ func (tcb *ControlBlock) validateIncomingSegment(seg Segment) (err error) {
 		err = errRequireSequential
 	}
 	if err != nil {
+		// RFC 9293 §3.4: If segment not acceptable, send ACK (unless RST).
+		switch err {
+		case errSeqNotInWindow, errLastNotInWindow, errRequireSequential, errZeroWindow:
+			if !flags.HasAny(FlagRST) {
+				tcb.challengeAck = true
+				tcb.pending[0] |= FlagACK
+			}
+		}
 		return err
 	}
 	if flags.HasAny(FlagRST) {