Checksum the header and refuse sequence numbers the sender never sent

main.go

@@ -169,6 +169,7 @@ type sample struct {
 	rxFrames, rxBytes uint64
 	lost, late        uint64
 	crcErr, badMagic  uint64
+	badHdr            uint64
 	badLen            uint64
 	txErrs            uint64
 	rxErrs            uint64
@@ -204,6 +205,7 @@ func (d *direction) snapshot() sample {
 		s.rxBytes += r.bytes.Load()
 		s.crcErr += r.crcErr.Load()
 		s.badMagic += r.badMagic.Load()
+		s.badHdr += r.badHdr.Load()
 		s.badLen += r.badLen.Load()
 		s.rxErrs += r.rxErrs.Load()
 	}
@@ -279,10 +281,11 @@ type view struct {
 func errsBetween(b, n counterSet) errs {
 	return errs{
 		lost: n.s.lost - b.s.lost,
-		// Three ways of noticing one thing: a payload that does not match its
-		// checksum, a header that is not ours, and a length that cannot be.
-		corrupt: (n.s.crcErr - b.s.crcErr) + (n.s.badMagic - b.s.badMagic) +
-			(n.s.badLen - b.s.badLen),
+		// Four ways of noticing one thing: a payload that does not match its
+		// checksum, a header that does not match its own, a header that is not
+		// ours, and a length that cannot be.
+		corrupt: (n.s.crcErr - b.s.crcErr) + (n.s.badHdr - b.s.badHdr) +
+			(n.s.badMagic - b.s.badMagic) + (n.s.badLen - b.s.badLen),
 		// What the hardware reported. Nothing the host declined to send is here,
 		// so this one going red means the cable.
 		link: (n.nic - b.nic) + (n.s.rxErrs - b.s.rxErrs),
@@ -372,8 +375,15 @@ func (d *direction) primeCounters() {
 }
 
 func buildDirection(label string, tx, rx endpoint) (*direction, error) {
+	// Built before the windows, since each window judges sequence numbers against
+	// the frontier its own sender publishes.
+	txs := make([]*txStats, numStreams)
+	for i := range txs {
+		txs[i] = &txStats{}
+	}
 	d := &direction{
-		streams: newLossWindows(numStreams),
+		txStats: txs,
+		streams: newLossWindows(txs),
 		cable:   newCableStats(),
 	}
 	// Held open for the life of the run: the stats ioctl is issued five times a
@@ -399,7 +409,6 @@ func buildDirection(label string, tx, rx endpoint) (*direction, error) {
 			return nil, fmt.Errorf("%s tx socket: %w", label, err)
 		}
 		d.txFDs = append(d.txFDs, fd)
-		d.txStats = append(d.txStats, &txStats{})
 
 		fd, err = openRxSocket(rx.idx, et)
 		if err != nil {