Livt.Net

src/udp/UdpChecksum.lvt

namespace Livt.Net

/** IPv4 UDP checksum over addresses, ports, length and an InternetChecksum payload sum. */
class UdpChecksum
{
	/** Caller validates ports (0..65535) and UDP length (8..65515). Odd payloads are zero padded. */
	public static fn Calculate[](sourceIp: byte[4], destinationIp: byte[4],
		sourcePort: int, destinationPort: int, udpLength: int, payloadWordSum: int) int
	{
		var addressSum = NetworkOrder.Word(sourceIp[0], sourceIp[1])
			+ NetworkOrder.Word(sourceIp[2], sourceIp[3])
			+ NetworkOrder.Word(destinationIp[0], destinationIp[1])
			+ NetworkOrder.Word(destinationIp[2], destinationIp[3])
		var sum = addressSum + 17 + udpLength + sourcePort + destinationPort + udpLength
		var checksum = ChecksumArithmetic.Finish(sum + payloadWordSum)
		// Wire zero means omitted; a computed zero must be encoded as all ones.
		if (checksum == 0) { return 65535 }
		return checksum
	}
}