code: def start(self, protocol):
          
          if self._protocol is not None:
              raise TransportError('already started')
          self._protocol = protocol
          self._protocol.connection_made(self)
          if self._readable:
              self.resume_reading()
          if self._writable:
              self._writing = True
              self._can_write.set()
docs: Bind to *protocol* and start calling callbacks on it.