Checksum simplification proposal (#29)

* Simplified checksum calculation and verification

* Fixed tests

* UDP: using NewBoundedFrame to create a Frame instance limited to the actual frame size.

* Minor: renamed some functions

* Commented and made more readable consecutive SetCRC calls.

* Fixed icmp checksum calculation after rebase

* Replaced NewBoundedFrame with explicit validation

x/xnet/stack-async.go

@@ -567,6 +567,5 @@ func addr4(addr [4]byte, ok bool) netip.Addr {
 
 func hash(b []byte) uint16 {
 	var csum lneto.CRC791
-	csum.Write(b)
-	return csum.Sum16()
+	return csum.PayloadSum16(b)
 }

x/xnet/xnet_dns_test.go

@@ -205,7 +205,10 @@ func buildDNSResponsePacket(t *testing.T, txid uint16, dstPort uint16, hostname
 	ifrm.SetProtocol(lneto.IPProtoUDP)
 	*ifrm.SourceAddr() = srcIP.As4()
 	*ifrm.DestinationAddr() = dstIP.As4()
-	ifrm.SetCRC(ifrm.CalculateHeaderCRC())
+	// Zero the CRC field so its value does not add to the final result.
+	ifrm.SetCRC(0)
+	crcValue := ifrm.CalculateHeaderCRC()
+	ifrm.SetCRC(crcValue)
 
 	// UDP header.
 	udpStart := ipStart + ipHdrLen
@@ -215,7 +218,8 @@ func buildDNSResponsePacket(t *testing.T, txid uint16, dstPort uint16, hostname
 	}
 	udpFrame.SetSourcePort(dns.ServerPort)
 	udpFrame.SetDestinationPort(dstPort)
-	udpFrame.SetLength(uint16(udpHdrLen + len(dnsPayload)))
+	udpLen := udpHdrLen + len(dnsPayload)
+	udpFrame.SetLength(uint16(udpLen))
 
 	// Copy DNS payload before calculating checksum.
 	dnsStart := udpStart + udpHdrLen
@@ -223,9 +227,11 @@ func buildDNSResponsePacket(t *testing.T, txid uint16, dstPort uint16, hostname
 
 	// Calculate UDP checksum using pseudo header.
 	var crc lneto.CRC791
-	ifrm.CRCWriteUDPPseudo(&crc)
-	udpFrame.CRCWriteIPv4(&crc)
-	udpFrame.SetCRC(crc.Sum16())
+	ifrm.CRCWriteUDPPseudo(&crc, uint16(udpLen))
+	// Zero the CRC field so its value does not add to the final result.
+	udpFrame.SetCRC(0)
+	crcValue = crc.PayloadSum16(udpFrame.RawData())
+	udpFrame.SetCRC(crcValue)
 
 	return pkt, nil
 }