code: def calculate_checksum(source_string):
      
      countTo = (int(len(source_string) / 2)) * 2
      sum = 0
      count = 0
  
      # Handle bytes in pairs (decoding as short ints)
      loByte = 0
      hiByte = 0
      while count < countTo:
          if (sys.byteorder == "little"):
              loByte = source_string[count]
              hiByte = source_string[count + 1]
          else:
              loByte = source_string[count + 1]
              hiByte = source_string[count]
          sum = sum + (ord(hiByte) * 256 + ord(loByte))
          count += 2
  
      # Handle last byte if applicable (odd-number of bytes)
      # Endianness should be irrelevant in this case
      if countTo < len(source_string): # Check for odd length
          loByte = source_string[len(source_string) - 1]
          sum += ord(loByte)
  
      sum &= 0xffffffff # Truncate sum to 32 bits (a variance from ping.c, which
                        # uses signed ints, but overflow is unlikely in ping)
  
      sum = (sum >> 16) + (sum & 0xffff)    # Add high 16 bits to low 16 bits
      sum += (sum >> 16)                    # Add carry from above (if any)
      answer = ~sum & 0xffff                # Invert and truncate to 16 bits
      answer = socket.htons(answer)
  
      return answer
docs: A port of the functionality of in_cksum() from ping.c
      Ideally this would act on the string as a series of 16-bit ints (host
      packed), but this works.
      Network data is big-endian, hosts are typically little-endian