Revalidation (Conditional GET)
HTTP/1.1 304 Not Modified
GET /foo/index.html HTTP/1.1
Thu, 12 May 2005 21:08:50 GMT
Revalidate using Last-Modified time
The presence of If-Modified-Since header is what makes this a Conditional GET. Sometimes called an “IMS GET”.
If content had actually changed, server would simply reply with a 200 OK and send full content.