function mustRevalidate(req, cacheEntry) {
  var cacheHeaders = cacheEntry[1]
    , reqCC = utils.parseCacheControl(req.headers['cache-control'] || '')
    , cacheCC = utils.parseCacheControl(cacheHeaders['cache-control'] || '')
    , cacheAge = (new Date - new Date(cacheHeaders.date)) / 1000 || 0;

  if ( cacheCC['no-cache']
    || cacheCC['must-revalidate']
    || cacheCC['proxy-revalidate']) return true;

  if (reqCC['no-cache']) return true;

  if (null != reqCC['max-age']) return reqCC['max-age'] < cacheAge;

  if (null != cacheCC['max-age']) return cacheCC['max-age'] < cacheAge;

  return false;
}
Determine whether or not a cached value must be revalidated.

@param {Object} req
@param {Object} cacheEntry
@return {String}
@api private