private function filterRequestParameters(array $httpRequest)
    {
        //filter request for Sips parameters
        if (!array_key_exists(self::DATA_FIELD, $httpRequest) || $httpRequest[self::DATA_FIELD] == '') {
            throw new InvalidArgumentException('Data parameter not present in parameters.');
        }
        $parameters = array();
        $dataString = $httpRequest[self::DATA_FIELD];
        $this->dataString = $dataString;
        $dataParams = explode('|', $dataString);
        foreach ($dataParams as $dataParamString) {
            $dataKeyValue = explode('=', $dataParamString, 2);
            $parameters[$dataKeyValue[0]] = $dataKeyValue[1];
        }

        return $parameters;
    }