code: func (net *Network) DidDisconnect(one, other enode.ID) error {
  	net.lock.Lock()
  	defer net.lock.Unlock()
  	conn := net.getConn(one, other)
  	if conn == nil {
  		return fmt.Errorf("connection between %v and %v does not exist", one, other)
  	}
  	if !conn.Up {
  		return fmt.Errorf("%v and %v already disconnected", one, other)
  	}
  	conn.Up = false
  	conn.initiated = time.Now().Add(-DialBanTimeout)
  	net.events.Send(NewEvent(conn))
  	return nil
  }
docs: // DidDisconnect tracks the fact that the "one" node disconnected from the
  // "other" node