ETag field

Meaning and syntax

ETag provides the entity tag for the selected representation. The value is opaque: clients compare it under HTTP’s strong or weak rules rather than deriving version meaning from its characters.

ETag: "manual-v4-gzip"

Expected result: a client can store the strong entity tag with this selected representation and later send it in a conditional field such as If-None-Match.

Strong and weak tags

A tag beginning with W/ is weak and can identify semantically equivalent representations whose bytes differ. Strong comparison requires both tags to be non-weak and character-for-character equal.

Selected representation

Content negotiation and content coding can require different entity tags for different representations. Reusing one validator across byte-different variants can make range and cache behavior incorrect.