ethernet: MTU semantic meaning corrections, add FCS logic (#28)

ethernet/crc.go

@@ -0,0 +1,47 @@
+package ethernet
+
+import (
+	"encoding/binary"
+	"hash/crc32"
+)
+
+//
+// CRC API.
+//
+
+// crcTable is the IEEE CRC-32 table used for Ethernet FCS calculation.
+var crcTable = crc32.MakeTable(crc32.IEEE)
+
+// CRC32 calculates the Ethernet Frame Check Sequence (FCS) for the given data.
+// The CRC is computed using the IEEE 802.3 CRC-32 polynomial.
+// The input should be the frame data from destination MAC through payload,
+// excluding any existing FCS.
+func CRC32(data []byte) uint32 {
+	return crc32.Checksum(data, crcTable)
+}
+
+// CRC32Search searches for a valid CRC32 in data starting from minOffCRC.
+// It computes the CRC incrementally from minOffCRC, checking at each position
+// if the CRC matches the next 4 bytes (little-endian FCS).
+// Returns the offset where a valid CRC was found, or -1 if no valid CRC exists.
+// This is useful when the exact frame length is unknown but bounded by minOffCRC.
+func CRC32Search(data []byte, minOffCRC int) (foundOffOrNegative int) {
+	if minOffCRC < 0 {
+		minOffCRC = 0
+	}
+	if len(data) < minOffCRC+4 {
+		return -1
+	}
+	// Calculate CRC up to minOffCRC
+	crc := crc32.Checksum(data[:minOffCRC], crcTable)
+	// Incrementally extend and check at each position
+	for off := minOffCRC; off <= len(data)-4; off++ {
+		got := binary.LittleEndian.Uint32(data[off:])
+		if crc == got {
+			return off
+		}
+		// Extend CRC by one byte for next iteration
+		crc = crc32.Update(crc, crcTable, data[off:off+1])
+	}
+	return -1
+}