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> I looked it up, though. It looks like you have to have an ETag to do a conditional GET

Nope, not the case. May I ask where you read that? It differs from my reading of RFC2616 and my experience.

If the server gave you an ETag though you're supposed to include it in future cache-conditional requests. Section 13.3.4 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13...) describes the interaction between ETags and modification dates.

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A blog post that's probably not worth linking to, since apparently it was wrong.

Am I correctly inferring that you need at least a Last-Modified in the response to do a cache-conditional request, though?

Either Last-Modified or Etag is sufficient. Most servers will add both, though.

Edit: The difference is that Last-Modified allows clients to use a heuristic to determine if the response should be cached for a certain duration (unless explicit Cache-Control or Expires headers are used). The heuristic isn't specified, but a 10% fraction of Date - Last-Modified is suggested, and in fact, that is what e.g. Internet Explorer does.

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