13.3.2 Entity Tag Cache Validators

The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.

Entity Tags are described in Section 3.11. The headers used with entity tags are described in Sections 14.19, 14.24, 14.26 and 14.44.