HTTP Entity Headers
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Specifies the media type and subtype of the entity, in a manner very similar to how this header is used in MIME. See the topic describing HTTP entities and Internet media types for a full discussion.
Specifies a date and time after which the entity in the message should be considered stale. This may be used to identify certain entities that should be held in HTTP caches for longer or shorter periods of time than usual.
This header is ignored if a Cache-Control header containing the max-age directive is present in the message.
Indicates the date and time when the server believes the entity was last changed. This header is often used to determine if a resource has been modified since it was last retrieved. For example, suppose a client machine already contains a copy of a very large file that was obtained two months ago, and its user wants to check if an update to the file is available. The client can send a HEAD request for the file, and compare the value of the returned Last-Modified header to the date of the copy of the file it already has. Then it needs only to request the entire file if it has changed.
Note the use of the word believe in the description above. The reason for this wording is that the server cannot always be certain of the time that a resource was modified. With files this is fairly simpleit is usually the last-modified time stored for the file by the operating system. For other more complex resources such as database records or virtual objects, however, it may be more difficult to ascertain when the last change occurred to a particular piece of information. In the case of dynamically-generated content, the Last-Modified date/time may be the same as that of the message as a whole, as specified in the Date field.
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