code: func (cln *Client) ExchangeMTU(mtu int) (int, error) {
  	// TODO: find the xpc command to tell OS X the rxMTU we can handle.
  	return cln.conn.TxMTU(), nil
  }
docs: // ExchangeMTU set the ATT_MTU to the maximum possible value that can be
  // supported by both devices [Vol 3, Part G, 4.3.1]