* concurrently readable hash table. The map supports non-blocking reads and concurrent writes
   * across different segments.
   *
   * The page replacement algorithm's data structures are kept casually consistent with the map. The
   * ordering of writes to a segment is sequentially consistent. An update to the map and recording

   * X-DNS-Prefetch-Control}</a> header controls DNS prefetch behavior. Value can be "on" or "off".
   * By default, DNS prefetching is "on" for HTTP pages and "off" for HTTPS pages.
   */
  public static final String X_DNS_PREFETCH_CONTROL = "X-DNS-Prefetch-Control";
  /**
   * The HTTP <a href="http://html.spec.whatwg.org/multipage/semantics.html#hyperlink-auditing">

     * IllegalArgumentException, since the call could succeed with a different argument. Those
     * exceptions' docs suggest that either is acceptable. Google's Java Practices page recommends
     * IllegalArgumentException here, in part to keep its recommendation simple: Static methods
     * should throw IllegalStateException only when they use static state.
     *