old_contents: # Copyright (C) 2012 Nippon Telegraph and Telephone Corporation.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
  # implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  import struct
  
  from . import packet_base
  from . import packet_utils
  import ipv4
  
  
  class udp(packet_base.PacketBase):
      _PACK_STR = '!HHHH'
      _MIN_LEN = struct.calcsize(_PACK_STR)
  
      def __init__(self, src_port, dst_port, total_length=0, csum=0):
          super(udp, self).__init__()
          self.src_port = src_port
          self.dst_port = dst_port
          self.total_length = total_length
          self.csum = csum
          self.length = udp._MIN_LEN
  
      @classmethod
      def parser(cls, buf):
          (src_port, dst_port, total_length, csum) = struct.unpack_from(
              cls._PACK_STR, buf)
          msg = cls(src_port, dst_port, total_length, csum)
          return msg, None
  
      def serialize(self, payload, prev):
          if self.total_length == 0:
              self.total_length = udp._MIN_LEN + len(payload)
          h = struct.pack(udp._PACK_STR, self.src_port, self.dst_port,
                          self.total_length, self.csum)
          if self.csum == 0:
              ph = struct.pack('!IIBBH', prev.src, prev.dst, 0, 17,
                               self.total_length)
              f = ph + h + payload
              self.csum = packet_utils.checksum(f)
              h = struct.pack(udp._PACK_STR, self.src_port, self.dst_port,
                              self.total_length, self.csum)
          return h
  
new_contents: # Copyright (C) 2012 Nippon Telegraph and Telephone Corporation.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
  # implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  import struct
  
  from . import packet_base
  from . import packet_utils
  
  
  class udp(packet_base.PacketBase):
      _PACK_STR = '!HHHH'
      _MIN_LEN = struct.calcsize(_PACK_STR)
  
      def __init__(self, src_port, dst_port, total_length=0, csum=0):
          super(udp, self).__init__()
          self.src_port = src_port
          self.dst_port = dst_port
          self.total_length = total_length
          self.csum = csum
          self.length = udp._MIN_LEN
  
      @classmethod
      def parser(cls, buf):
          (src_port, dst_port, total_length, csum) = struct.unpack_from(
              cls._PACK_STR, buf)
          msg = cls(src_port, dst_port, total_length, csum)
          return msg, None
  
      def serialize(self, payload, prev):
          if self.total_length == 0:
              self.total_length = udp._MIN_LEN + len(payload)
          h = struct.pack(udp._PACK_STR, self.src_port, self.dst_port,
                          self.total_length, self.csum)
          if self.csum == 0:
              self.csum = packet_utils.checksum_ip(
                  prev, self.total_length, h + payload)
              h = struct.pack(udp._PACK_STR, self.src_port, self.dst_port,
                              self.total_length, self.csum)
          return h
  
subject: use checksum_ip()