Clarification on the use of ETag in 4.1. Conditional Retrieval

[madsfreek]

"4.1. Conditional Retrieval" reads:

Upon a successful response the responder MUST return an ETag header
and MAY return a Last-Modified header as well.  Requesters SHOULD use
either or both, with the ETag being preferred, in any subsequent
requests for the same resource.

RFC2616 says in 13.3.4:

If an entity tag has been provided by the origin server, MUST
use that entity tag in any cache-conditional request (using If-
Match or If-None-Match).

Ie. it is mandatory to use the ETag if provided and "2.5. Response Headers" mandates it's
provision.

So 4.1 should read:

"Upon a successful response the responder MUST return an ETag header and MAY return a Last-Modified header as well. Requesters SHOULD use the ETag or both, in any subsequent requests for the same resource."