code: def read_packet(self, size=MTU):
          
          hdr = self.f.read(16)
          if len(hdr) < 16:
              return None
          sec,usec,caplen,wirelen = struct.unpack(self.endian+"IIII", hdr)
          s = self.f.read(caplen)[:MTU]
          return s, 0, (sec,usec,wirelen)
docs: return a single packet read from the file
          bytes, (sec, #timestamp seconds
                  usec, #timestamp microseconds
                  wirelen) #actual length of packet
          returns None when no more packets are available