Checksum fixes (#24)

* Checksum 0x0000 is returned as 0xFFFF (mandatory for UDP, common for TCP)

* Both checksum zero values (0x0000 and 0xFFFF) are accepted when receiving

* Rename parameter in VerifySum16 to expectedSum16

---------

Co-authored-by: Pat Whittingslow <graded.sp@gmail.com>

crc.go

@@ -68,8 +68,7 @@ func (c *CRC791) AddUint8(value uint8) {
 	c.needPad = !c.needPad
 }
 
-// Sum16 calculates the checksum with the data written to c thus far.
+func (c *CRC791) sum16() uint16 {
-func (c *CRC791) Sum16() uint16 {
 	sum := c.sum
 	if c.needPad {
 		sum += uint32(c.excedent) << 8
@@ -80,5 +79,26 @@ func (c *CRC791) Sum16() uint16 {
 	return uint16(^sum)
 }
 
+// Sum16 calculates the checksum with the data written to c thus far.
+func (c *CRC791) Sum16() uint16 {
+	sum16 := c.sum16()
+	if sum16 == 0 {
+		// The zero value is always transmitted as 0xFFFF, as required by UDP (RFC 768), and still valid for TCP and IP.
+		sum16 = 0xffff
+	}
+	return sum16
+}
+
+// VerifySum16 verifies that the given checksum matches the data written to c thus far.
+func (c *CRC791) VerifySum16(expectedSum16 uint16) bool {
+	// as recommended by RFC 1624, this implementation supports both 0x0000 and 0xFFFF as zero value for the checksum
+	cc := *c
+	if cc.needPad {
+		cc.AddUint8(0)
+	}
+	cc.AddUint16(expectedSum16)
+	return cc.sum16() == 0
+}
+
 // Reset zeros out the CRC791, resetting it to the initial state.
 func (c *CRC791) Reset() { *c = CRC791{} }

internet/stack-ip.go

@@ -90,11 +90,12 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 		sb.handlers.error("ip:Demux.validate")
 		return err
 	}
-	gotCRC := ifrm.CRC()
+
-	wantCRC := ifrm.CalculateHeaderCRC()
+	// Incoming CRC Validation of common IP Protocols.
-	if gotCRC != wantCRC {
+	var crc lneto.CRC791
-		sb.handlers.error("StackIP:Demux:crc-mismatch", slog.Uint64("want", uint64(wantCRC)), slog.Uint64("got", uint64(gotCRC)))
+	ifrm.CRCWriteHeader(&crc)
-		return lneto.ErrBadCRC
+	if !crc.VerifySum16(ifrm.CRC()) {
+		sb.handlers.error("ip:demux.crc")
 	}
 	off := ifrm.HeaderLength()
 	totalLen := ifrm.TotalLength()
@@ -110,7 +111,7 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 		return lneto.ErrPacketDrop
 	}
 	// Incoming CRC Validation of common IP Protocols.
-	var crc lneto.CRC791
+	crc.Reset()
 	switch proto {
 	case lneto.IPProtoTCP:
 		ifrm.CRCWriteTCPPseudo(&crc)
@@ -119,7 +120,7 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 			return err
 		}
 		tfrm.CRCWrite(&crc)
-		if crc.Sum16() != tfrm.CRC() {
+		if !crc.VerifySum16(tfrm.CRC()) {
 			sb.handlers.error("ip:demux.tcpcrc")
 			return lneto.ErrBadCRC
 		}
@@ -130,7 +131,8 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 			return err
 		}
 		ufrm.CRCWriteIPv4(&crc)
-		if crc.Sum16() != ufrm.CRC() {
+		// checksums are optional in UDP: the field is set to zero in this case
+		if gotCrc := ufrm.CRC(); gotCrc != 0 && !crc.VerifySum16(gotCrc) {
 			sb.handlers.error("ip:demux.udpcrc")
 			return lneto.ErrBadCRC
 		}
@@ -210,7 +212,7 @@ func (sb *StackIP) recvicmp(carrierData []byte, offset int) error {
 		return err
 	}
 	cfrm.CRCWrite(&crc)
-	if crc.Sum16() != cfrm.CRC() {
+	if !crc.VerifySum16(cfrm.CRC()) {
 		return errors.New("ICMP CRC mismatch")
 	}
 	return nil

ipv4/frame.go

@@ -131,9 +131,13 @@ func (ifrm Frame) SetCRC(cs uint16) {
 // CalculateHeaderCRC calculates the CRC for this IPv4 frame.
 func (ifrm Frame) CalculateHeaderCRC() uint16 {
 	var crc lneto.CRC791
+	ifrm.CRCWriteHeader(&crc)
+	return crc.Sum16()
+}
+
+func (ifrm Frame) CRCWriteHeader(crc *lneto.CRC791) {
 	crc.Write(ifrm.buf[0:10])
 	crc.Write(ifrm.buf[12:20])
-	return crc.Sum16()
 }
 
 func (ifrm Frame) CRCWriteTCPPseudo(crc *lneto.CRC791) {