HTTP has a concept of conditional requests, where the result, and even the success of a request, can be controlled by comparing the affected resources with a validator.
These requests are useful for validating cached content, ensuring that it is only fetched if it differs from the copy that is already available to the browser.
Conditional requests are also useful for ensuring the integrity of a document when resuming a download, or preventing lost updates when uploading or modifying a document on the server....differently, depending on the value of specific headers. These headers...transmits a value describing the version. Such values are called...