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

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

My intuition is that without aLast-Modified header, we should assume that the request isalways fresh, and given thatIf-Unmodified-Since is used to protect against the 'lost update' problem, we should default to returning412 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.