Check the header version on arrival and say what the error window actually spans

frame.go

@@ -11,6 +11,7 @@ const (
 	ethHdrLen = 14
 	hdrLen    = 20
 	hdrMagic  = 0x43424c54
+	hdrVer    = 1
 	minFrame  = ethHdrLen + hdrLen
 	maxFrame  = 9216
 
@@ -138,7 +139,7 @@ func headerCheck(h []byte) uint16 {
 func putHeader(buf []byte, patIdx int, stream uint16, seq uint64, payLen int) {
 	h := buf[ethHdrLen:]
 	binary.BigEndian.PutUint32(h[0:4], hdrMagic)
-	h[4] = 1
+	h[4] = hdrVer
 	h[5] = byte(patIdx)
 	binary.BigEndian.PutUint16(h[6:8], stream)
 	binary.BigEndian.PutUint64(h[8:16], seq)
@@ -176,6 +177,11 @@ func parseHeader(buf []byte) (parsed, hdrStatus) {
 	if binary.BigEndian.Uint16(h[hdrCheckOff:]) != headerCheck(h) {
 		return p, hdrDamaged
 	}
+	// Both halves are the same build, so the only way this differs is a checksum
+	// that happened to agree over damaged bytes.
+	if h[4] != hdrVer {
+		return p, hdrDamaged
+	}
 	p.patIdx = int(h[5])
 	p.stream = binary.BigEndian.Uint16(h[6:8])
 	p.seq = binary.BigEndian.Uint64(h[8:16])

main.go

@@ -541,8 +541,8 @@ const (
 	// Deliberately not tied to the refresh: letting a slow or blocked draw set
 	// the sampling clock would stretch the window it reports.
 	sampleInterval = 16 * time.Millisecond
-	// How far back the shown errors reach and how many buckets the median runs
+	// How far back the shown errors reach. The rate is not taken from this ring
-	// over, so a step in the rate lands half this late.
+	// but from the receive buckets, which are keyed by the mac's clock.
 	rateWindowSpan = time.Second
 )