HTTP headers used for coherence checks

HTTP defines several headers to support caching which are listed below :

1.

If-Modified-Since: date can be appended to a GET message to obtain a conditional GET. It is being replied to by returning the requested page only if it has been modified since date.

2.

Pragma: no-cache can be attached to a GET message to indicate that the page must be retrieved from the remote server irrespective of whether it is in some cache on the way up, or whether it was modified or not.

3.

Last-Modified: date is returned with every message requested by a GET to indicate when was the last time the page was modified.

4.

Date: date is returned with every requested page to indicate the last time the document was considered to be fresh. This date, in combination with the `¨Last-Modified`¨} date give the user an idea of how stale the data may be.

5.

Expires: date may be returned with a document to indicate for how long the document is not going to be modified.