Judge a frame against the stream its socket receives rather than the one its header claims

rx.go

@@ -65,12 +65,13 @@ func (s *rxStats) bucket(e int64) (frames, bytes uint64) {
 }
 
 type rxWorker struct {
-	fd      int
-	batch   int
-	spec    *frameSpec
-	stats   *rxStats
-	streams []lossWindow
-	ready   *sync.WaitGroup
+	fd     int
+	batch  int
+	stream uint16
+	spec   *frameSpec
+	stats  *rxStats
+	loss   *lossWindow
+	ready  *sync.WaitGroup
 }
 
 func (w *rxWorker) run(done *atomic.Bool) {
@@ -115,11 +116,10 @@ func (w *rxWorker) run(done *atomic.Bool) {
 				w.stats.observe(ts, uint64(len(buf)))
 			}
 
-			// 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) {
+			// The ethertype this socket is bound to already says which stream the
+			// frame belongs to, so a header naming another one is damaged, as is a
+			// sequence number the sender never reached.
+			if p.stream != w.stream || !w.loss.observe(p.seq) {
 				w.stats.badHdr.Add(1)
 				continue
 			}