code: def acknowledge(self, size, msg):
          
          stream = self.router.stream_by_id(msg.src_id)
          state = self._state_by_stream[stream]
          state.lock.acquire()
          try:
              if state.unacked < size:
                  LOG.error('%r.acknowledge(src_id %d): unacked=%d < size %d',
                            self, msg.src_id, state.unacked, size)
              state.unacked -= min(state.unacked, size)
              self._schedule_pending_unlocked(state)
          finally:
              state.lock.release()
docs: Acknowledge bytes received by a transfer target, scheduling new chunks
          to keep the window full. This should be called for every chunk received
          by the target.