Network Working Group R. Fielding Request for Comments: 2616 ...

match.In HTTP/1.1, a conditional request looks exactly the same as a normalrequest for the same resource, except that it carries a specialheader (which includes the validator) that implicitly turns themethod (usually, GET) into a conditional.The protocol includes both positive and negative senses of cachevalidatingconditions. That is, it is possible to request either thata method be performed if and only if a validator matches or if andonly if no validators match.Fielding, et al. Standards Track [Page 85]RFC 2616 HTTP/1.1 June 1999Note: a response that lacks a validator may still be cached, andserved from cache until it expires, unless this is explicitlyprohibited by a cache-control directive. However, a cache cannotdo a conditional retrieval if it does not have a validator for theentity, which means it will not be refreshable after it expires.13.3.1 Last-Modified DatesThe Last-Modified entity-header field value is often used as a cachevalidator. In simple terms, a cache entry is considered to be validif the entity has not been modified since the Last-Modified value.13.3.2 Entity Tag Cache ValidatorsThe ETag response-header field value, an entity tag, provides for an"opaque" cache validator. This might allow more reliable validationin situations where it is inconvenient to store modification dates,where the one-second resolution of HTTP date values is notsufficient, or where the origin server wishes to avoid certainparadoxes that might arise from the use of modification dates.Entity Tags are described in section 3.11. The headers used withentity tags are described in sections 14.19, 14.24, 14.26 and 14.44.13.3.3 Weak and Strong ValidatorsSince both origin servers and caches will compare two validators to