ETag

The ETag response-header field provides the current value of the entity tag for the requested variant. The headers used with entity tags are described in sections 14.24, 14.26 and 14.44. The entity tag MAY be used for comparison with other entities from the same resource (see section 13.3.3).

      ETag = "ETag" ":" entity-tag

Examples:

      ETag: "xyzzy"
      ETag: W/"xyzzy"
      ETag: ""