The ETag entity-header field defines the entity tag for the associated entity. The headers used with entity tags are described in sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used for comparison with other entities from the same resource (see section 13.3.2).
ETag = "ETag" ":" entity-tag
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""