13.2.6 Disambiguating Multiple Responses

Because a client may be receiving responses via multiple paths, so that some responses flow through one set of caches and other responses flow through a different set of caches, a client may receive responses in an order different from that in which the origin server sent them. We would like the client to use the most recently generated response, even if older responses are still apparently fresh.

Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response intentionally carries an earlier expiration time. However, the HTTP/1.1 specification requires the transmission of Date headers on every response, and the Date values are ordered to a granularity of one second.

When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older than the one for the existing entry, then the client SHOULD repeat the request unconditionally, and include

          Cache-Control: max-age=0

to force any intermediate caches to validate their copies directly with the origin server, or

          Cache-Control: no-cache

to force any intermediate caches to obtain a new copy from the origin server.

If the Date values are equal, then the client may use either response (or may, if it is being extremely prudent, request a new response). Servers MUST NOT depend on clients being able to choose deterministically between responses generated during the same second, if their expiration times overlap.