code: def _checksum(self, packet):
          '''calculate the XOR checksum of a packet in string format'''
          xorsum = 0
          for s in packet:
              xorsum ^= ord(s)
          return xorsuf _checksum(self, packet):
          '''calculate the XOR checksum of a packet in string format'''
          xorsum = 0
          for s in packet:
              xorsum ^= ord(s)
          return xorsum
docs: calculate the XOR checksum of a packet in string format