// Internet Checksum as defined by RFC 791, RFC 793, RFC 1071, RFC 1141, RFC 1624
public static ushort Calcuate(ref byte buffer, int length)
{
    ref var current = ref buffer;
    ulong sum = 0;

    while (length >= sizeof(ulong))
    {
        length -= sizeof(ulong);

        var ulong0 = Unsafe.As<byte, ulong>(ref current);
        current = ref Unsafe.Add(ref current, sizeof(ulong));

        // Add with carry
        sum += ulong0;
        if (sum < ulong0)
        {
            sum++;
        }
    }

    if ((length & sizeof(uint)) != 0)
    {
        var uint0 = Unsafe.As<byte, uint>(ref current);
        current = ref Unsafe.Add(ref current, sizeof(uint));

        // Add with carry
        sum += uint0;
        if (sum < uint0)
        {
            sum++;
        }
    }

    if ((length & sizeof(ushort)) != 0)
    {
        var ushort0 = Unsafe.As<byte, ushort>(ref current);
        current = ref Unsafe.Add(ref current, sizeof(ushort));

        // Add with carry
        sum += ushort0;
        if (sum < ushort0)
        {
            sum++;
        }
    }

    if ((length & sizeof(byte)) != 0)
    {
        var byte0 = current;

        // Add with carry
        sum += byte0;
        if (sum < byte0)
        {
            sum++;
        }
    }

    // Fold down to 16 bits

    var uint1 = (uint)(sum >> 32);
    var uint2 = (uint)sum;

    // Add with carry
    uint1 += uint2;
    if (uint1 < uint2)
    {
        uint1++;
    }

    var ushort2 = (ushort)uint1;
    var ushort1 = (ushort)(uint1 >> 16);

    // Add with carry
    ushort1 = (ushort)(ushort1 + ushort2);
    if (ushort1 < ushort2)
    {
        ushort1++;
    }

    // Invert to get ones-complement result 
    return (ushort)~ushort1;
}