I'm working on a new implementation of conditionals, and I'm a little confused about If-Unmodified-Since.

What should a server do if a client passed an If-Unmodified-Since header, but the target resource does not know its own modification date?

My intuition is that without a Last-Modified header, we should assume that the request is always fresh, and given that If-Unmodified-Since is used to protect against the 'lost update' problem, we should default to returning 412 if the last modification date could not be determined.

I don't think the current draft covers this case, but perhaps I've missed something.