code: func (p *Client) ExchangeMTU(mtu int) (int, error) {
  	p.Lock()
  	defer p.Unlock()
  	return p.ac.ExchangeMTU(mtu)
  }
docs: // ExchangeMTU informs the server of the client’s maximum receive MTU size and
  // request the server to respond with its maximum receive MTU size. [Vol 3, Part F, 3.4.2.1]