Handling of If-Unmodified-Since with unknown modified dates

[evert]

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.

[mnot]

It's a fair question.

I think we should explicitly state that an origin server can ignore this header if the resource does not have a concept of modification time.

Furthermore, I suspect that I-U-S should change this requirement:

An origin server that receives an If-Unmodified-Since header field must evaluate the condition prior to performing the method (Section 8.2.1).

to something like:

An origin server that receives an If-Unmodified-Since header field must evaluate the condition (Section 8.2.1) prior to performing an unsafe method (Section 7.2.1).

[mnot]

Hm. In Evaluation, we say:

Likewise, a server &MUST; ignore the conditional request header fields
defined by this specification when received with a request method that does
not involve the selection or modification of a
<x:ref>selected representation</x:ref>, such as CONNECT, OPTIONS, or TRACE.

It seems like we need to reference that if we're placing requirements around methods later on.

(now #358)