Checksum the header and refuse sequence numbers the sender never sent

frame.go

@@ -13,6 +13,8 @@ const (
 	hdrMagic  = 0x43424c54
 	minFrame  = ethHdrLen + hdrLen
 	maxFrame  = 9216
+
+	hdrCheckOff = 18
 )
 
 var crcTable = crc32.MakeTable(crc32.Castagnoli)
@@ -113,6 +115,26 @@ func (f *frameSpec) expectedCRC(patIdx, payLen int) (uint32, bool) {
 	return crc, ok
 }
 
+// A one's complement sum over the header, skipping the two bytes it is written
+// into. The payload checksum covers the payload alone, so without this the
+// sequence number is the one field on the wire that nothing checks, and a bit
+// flipped in it arrives looking exactly like a legitimate frame from far in the
+// future. Every single bit error changes the folded sum, which is the error a
+// cable going marginal actually produces.
+func headerCheck(h []byte) uint16 {
+	var sum uint32
+	for i := 0; i+1 < hdrLen; i += 2 {
+		if i == hdrCheckOff {
+			continue
+		}
+		sum += uint32(binary.BigEndian.Uint16(h[i:]))
+	}
+	for sum>>16 != 0 {
+		sum = sum&0xffff + sum>>16
+	}
+	return ^uint16(sum)
+}
+
 func putHeader(buf []byte, patIdx int, stream uint16, seq uint64, payLen int) {
 	h := buf[ethHdrLen:]
 	binary.BigEndian.PutUint32(h[0:4], hdrMagic)
@@ -121,7 +143,7 @@ func putHeader(buf []byte, patIdx int, stream uint16, seq uint64, payLen int) {
 	binary.BigEndian.PutUint16(h[6:8], stream)
 	binary.BigEndian.PutUint64(h[8:16], seq)
 	binary.BigEndian.PutUint16(h[16:18], uint16(payLen))
-	binary.BigEndian.PutUint16(h[18:20], 0)
+	binary.BigEndian.PutUint16(h[hdrCheckOff:], headerCheck(h))
 }
 
 type parsed struct {
@@ -131,21 +153,35 @@ type parsed struct {
 	payLen int
 }
 
-func parseHeader(buf []byte) (parsed, bool) {
+// Why a frame was turned away. A header that is not ours and a header of ours
+// that arrived damaged are different things to report: the first is somebody
+// else on the wire, the second is the fault being tested for.
+type hdrStatus int
+
+const (
+	hdrOK hdrStatus = iota
+	hdrForeign
+	hdrDamaged
+)
+
+func parseHeader(buf []byte) (parsed, hdrStatus) {
 	var p parsed
 	if len(buf) < minFrame {
-		return p, false
+		return p, hdrForeign
 	}
 	h := buf[ethHdrLen:]
 	if binary.BigEndian.Uint32(h[0:4]) != hdrMagic {
-		return p, false
+		return p, hdrForeign
+	}
+	if binary.BigEndian.Uint16(h[hdrCheckOff:]) != headerCheck(h) {
+		return p, hdrDamaged
 	}
 	p.patIdx = int(h[5])
 	p.stream = binary.BigEndian.Uint16(h[6:8])
 	p.seq = binary.BigEndian.Uint64(h[8:16])
 	p.payLen = int(binary.BigEndian.Uint16(h[16:18]))
 	if minFrame+p.payLen > len(buf) {
-		return p, false
+		return p, hdrDamaged
 	}
-	return p, true
+	return p, hdrOK
 }