The Expires header is understood by virtually all caches. The cached document will be retrieved again automatically once it has expired. HTTP 1.1 introduced the Cache-Control header, which provides more flexibility for telling caches how to handle the document. The Pragma header is generally ineffective because its meaning is not standardized and few caches honor it. However, just as with tricks that try to protect images from being saved , these tricks have very limited effectiveness and can cause various problems for law-abiding users.
Many browsers identify themselves when they request a document.
Keep in mind not all browsers identify themselves correctly. And of course, if a cache proxy keeps the Netscape enhanced document, someone with another browser will also get this document if he goes through the cache. For these reasons and others, it is not a good idea to play the browser guessing game. Although each request for a document is usually logged with the name or address of the remote host, the actual username is almost never logged as well. This is mostly because of performance reasons, as it would require that the server uses the ident protocol to see who is on the other end.