protected function _isCacheValid()
    {
        if (null === $this->_etag) {
            // lmTime is copied to ETag, so this condition implies that the
            // server sent neither ETag nor Last-Modified, so the client can't
            // possibly has a valid cache.
            return false;
        }
        $isValid = ($this->resourceMatchedEtag() || $this->resourceNotModified());
        if ($isValid) {
            $this->_headers['_responseCode'] = 'HTTP/1.0 304 Not Modified';
        }

        return $isValid;
    }