Http headers used to maintain coherence

Last-Modified:

• identifies a version of a page

If-Modified-Since:

• (also called conditional GET)
• conditional GET only returns the page if it has been modified since the date given
• definitory behavior is strict: always pass to home server
• practice is liberal: cache can satisfy the request

Date:

• says when the page was last known to be fresh
• unused for coherence by either W3C or Netscape

Pragma:no-cache

• makes the request fall through to the original site (RELOAD button)
• very expensive

Expires:

• let's the page authority specify how long the page is valid
• rarely used