@@ -572,9 +572,16 @@ To see a simple implementation, generate the ETag as the md5 of the content::
572572 }
573573
574574The:method: `Symfony\\ Component\\ HttpFoundation\\ Response::isNotModified `
575- method compares the ``ETag `` sent with the ``Request `` with the one set
576- on the ``Response ``. If the two match, the method automatically sets the
577- ``Response `` status code to 304.
575+ method compares the ``If-None-Match `` sent with the ``Request `` with the
576+ ``ETag `` header set on the ``Response ``. If the two match, the method
577+ automatically sets the ``Response `` status code to 304.
578+
579+ ..note ::
580+
581+ The ``If-None-Match `` request header equals the ``ETag `` header of the
582+ last response sent to the client for the particular resource. This is
583+ how the client and server communicate with each other and decide whether
584+ or not the resource has been updated since it was cached.
578585
579586This algorithm is simple enough and very generic, but you need to create the
580587whole ``Response `` before being able to compute the ETag, which is sub-optimal.