Validation failures

[mnot]

4.3.3 says:

However, if a cache receives a 5xx (Server Error) response while attempting to validate a response, it can either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, the cache may send a previously stored response (see Section 4.2.4).

I think that MAY should be prose, and should also list retrying the request as an option.