Look the expected checksum up instead of carrying it in the frame

frame.go

@@ -9,7 +9,7 @@ import (
 const (
 	etherBase = 0x88b5
 	ethHdrLen = 14
-	hdrLen    = 24
+	hdrLen    = 20
 	hdrMagic  = 0x43424c54
 	minFrame  = ethHdrLen + hdrLen
 	maxFrame  = 9216
@@ -88,7 +88,7 @@ func newFrameSpec(dst, src [6]byte, etherType uint16, sizes []int) *frameSpec {
 		fill(ref)
 		crcFor := make(map[int]uint32, len(sizes))
 		for _, s := range sizes {
-			crcFor[s] = crc32.Checksum(ref[:s-minFrame], crcTable)
+			crcFor[s-minFrame] = crc32.Checksum(ref[:s-minFrame], crcTable)
 		}
 		f.refs = append(f.refs, ref)
 		f.crcFor = append(f.crcFor, crcFor)
@@ -103,7 +103,17 @@ func (f *frameSpec) prefill(buf []byte, patIdx int) {
 	copy(buf[minFrame:], f.refs[patIdx])
 }
 
-func putHeader(buf []byte, patIdx int, stream uint16, seq uint64, payLen int, crc uint32) {
+// Looked up rather than read from the frame: a checksum travelling beside the
+// bytes it covers is only ever compared against itself.
+func (f *frameSpec) expectedCRC(patIdx, payLen int) (uint32, bool) {
+	if patIdx < 0 || patIdx >= len(f.crcFor) {
+		return 0, false
+	}
+	crc, ok := f.crcFor[patIdx][payLen]
+	return crc, ok
+}
+
+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
@@ -112,7 +122,6 @@ func putHeader(buf []byte, patIdx int, stream uint16, seq uint64, payLen int, cr
 	binary.BigEndian.PutUint64(h[8:16], seq)
 	binary.BigEndian.PutUint16(h[16:18], uint16(payLen))
 	binary.BigEndian.PutUint16(h[18:20], 0)
-	binary.BigEndian.PutUint32(h[20:24], crc)
 }
 
 type parsed struct {
@@ -120,7 +129,6 @@ type parsed struct {
 	stream uint16
 	seq    uint64
 	payLen int
-	crc    uint32
 }
 
 func parseHeader(buf []byte) (parsed, bool) {
@@ -136,7 +144,6 @@ func parseHeader(buf []byte) (parsed, bool) {
 	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]))
-	p.crc = binary.BigEndian.Uint32(h[20:24])
 	if minFrame+p.payLen > len(buf) {
 		return p, false
 	}