Method calcule_checksum

IcmpPacket.py:82–109

I'm not too confident that this is right but testing seems to suggest that it gives the same answers as in_cksum in ping.c

(self)

    

    def calcule_checksum (self):
        """
        I'm not too confident that this is right but testing seems
        to suggest that it gives the same answers as in_cksum in ping.c
        """
        sum = 0
        countTo = (len(self.packet)/2)*2
        count = 0
        while count<countTo:
            thisVal = ord(self.packet[count + 1])*256 + \
                      ord(self.packet[count])
            sum = sum + thisVal
            sum = sum & 0xffffffff # Necessary?
            count = count + 2
     
        if countTo<len(self.packet):
            sum = sum + ord(self.packet[len(self.packet) - 1])
            sum = sum & 0xffffffff # Necessary?
     
        sum = (sum >> 16)  +  (sum & 0xffff)
        sum = sum + (sum >> 16)
        answer = ~sum
        answer = answer & 0xffff
     
        # Swap bytes. Bugger me if I know why.
        answer = answer >> 8 | (answer << 8 & 0xff00)
     
        self.checksum = answer