Checksum the header and refuse sequence numbers the sender never sent

rx.go

@@ -12,6 +12,7 @@ type rxStats struct {
 	frames   atomic.Uint64
 	bytes    atomic.Uint64
 	badMagic atomic.Uint64
+	badHdr   atomic.Uint64
 	badLen   atomic.Uint64
 	crcErr   atomic.Uint64
 	rxErrs   atomic.Uint64
@@ -99,9 +100,13 @@ func (w *rxWorker) run(done *atomic.Bool) {
 		}
 		for i := 0; i < n; i++ {
 			buf := bufs[i][:int(hdrs[i].len)]
-			p, ok := parseHeader(buf)
-			if !ok {
-				w.stats.badMagic.Add(1)
+			p, st := parseHeader(buf)
+			if st != hdrOK {
+				if st == hdrForeign {
+					w.stats.badMagic.Add(1)
+				} else {
+					w.stats.badHdr.Add(1)
+				}
 				continue
 			}
 			w.stats.frames.Add(1)
@@ -110,8 +115,13 @@ func (w *rxWorker) run(done *atomic.Bool) {
 				w.stats.observe(ts, uint64(len(buf)))
 			}
 
-			if int(p.stream) < len(w.streams) {
-				w.streams[p.stream].observe(p.seq)
+			// A sequence number the sender never reached got past the header
+			// checksum, so the frame is damaged whatever its payload says. Counted
+			// here rather than left to the payload check, which would report the
+			// same frame twice or, if only the header was hit, not at all.
+			if int(p.stream) < len(w.streams) && !w.streams[p.stream].observe(p.seq) {
+				w.stats.badHdr.Add(1)
+				continue
 			}
 
 			want, ok := w.spec.expectedCRC(p.patIdx, p.payLen)