ETags and Conneg

[mnot@…]

I just received an interesting bug report on REDbot; <​https://github.com/mnot/redbot/issues/109>

""" When an ETag is marked weak with "W/", it need not change across different content encodings. "Weak" means the entity is semantically equivalent but not bit-equivalent. But Redbot complains that it doesn't change with different content encodings. """

The obvious question is whether two different *negotiated* representations of the same resource can (or should) have the same weak ETag.

[mnot@…]

Proposal:

1. add a MUST requirement for strong ETag uniqueness, and
2. clarify in prose that if you issue the same weak ETag for *any* two representations of a resource, you consider them interchangeable (possibly in the examples), and
3. clarify the definition of ETags WRT "differentiate"

[fielding@…]

Can you suggest prose to add near bottom of section 2.1? The current examples are all time-based. I don't think any other clarification is needed.

[mnot@…]

Insert before the last paragraph in 2.1:

Note that any two representations of a resource that share a weak validator can be considered equivalent when using the weak comparison function; this applies even if their other metadata (e.g., Content-Type) differs.

[fielding@…]

That doesn't fit with the new style of that section. I have added:

Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation metadata, such as when two different media types are available for the same representation data.

[fielding@…]

From [2136]:

note that simultaneous use of the same validator for different representation data would also make it weak; addresses #401

[mnot@…]

That reads like you're saying that a strong validator can be made magically into a weak one by associating it with two representations that have different bits.

[fielding@…]

Yes, that's intentional -- this section defines weak vs strong. A later section says that an entity-tag must be marked as weak unless it has all the characteristics of being strong.

[mnot@…]

(reading in context) OK. Thx.