public function setSiteControl($permittedCrossDomainPolicies = 'all') {
        if (!$this->validateSiteControl($permittedCrossDomainPolicies)) {
            throw new \UnexpectedValueException('Invalid site control set');
        }

        $this->_siteControl = $permittedCrossDomainPolicies;
        $this->_cacheValid  = false;

        return $this;
    }