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#271: SHOULD review in p4

From: Mark Nottingham <mnot@mnot.net>
Date: Fri, 22 Jun 2012 11:44:12 +1000
Message-Id: <D1DACF74-AE6D-4172-81CD-0F878DA37B90@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>

As per <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/271>, I'm reviewing our use of SHOULD in the documents; here I also pick on a few MAYs. Where I find issues, I've flagged with EDITORIAL or DESIGN as seems appropriate (I won't open issues for the design ones until we discuss; the editorial ones are considered attached to #271).

2.1

"However, if a resource has distinct representations that differ only in their metadata, such as might occur with content negotiation over media types that happen to share the same data format, then a server SHOULD incorporate additional information in the validator to distinguish those representations and avoid confusing cache behavior."

Seems OK, but not really testable.
EDITORIAL - change "a server" to "the origin server"

"A weak entity-tag SHOULD change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag SHOULD change whenever the origin server wants caches to invalidate old responses."

First one is OK, but not testable.

EDITORIAL - change first one to "An origin server SHOULD change a weak entity-tag whenever it considers..."
EDITORIAL - the second one should probably change to an "ought to", otherwise we're repeating ourselves.

2.2.1

"Origin servers SHOULD send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined, since its use in conditional requests and evaluating cache freshness ([Part6]) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service scalability and reliability."

As discussed, this is a social requirement, but justified.

"An origin server SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially if the representation changes near the time that the response is generated."

OK

2.3.1

"Origin servers SHOULD send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since the entity-tag's use in conditional requests and evaluating cache freshness ([Part6]) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability and reliability."

As discussed, this is a social requirement, so OK.

2.4

"""
HTTP/1.1 origin servers:

	 SHOULD send an entity-tag validator unless it is not feasible to generate one.
	 MAY send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags, or if it is unfeasible to send a strong entity-tag.
	 SHOULD send a Last-Modified value if it is feasible to send one.
"""

Repeating requirements above, but a nice summary. OK.

"""
HTTP/1.1 clients:

	 MUST use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided by the origin server.
	 SHOULD use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value has been provided by the origin server.
	 MAY use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent should provide a way to disable this, in case of difficulty.
	 SHOULD use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
"""

OK

"HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers SHOULD provide Last-Modified values. In those rare cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers SHOULD NOT provide one."

EDITORIAL - change SHOULD to "still ought to". (we're repeating ourselves)
DESIGN - What are the "rare cases" being talked about here, and how are servers supposed to detect them?

3.1

"The "If-Match" header field MAY be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations of the target resource."

EDITORIAL - change MAY to "can"

3.2

"The "If-None-Match" header field MAY be used to make a request method conditional on not matching any of the current entity-tag values for representations of the target resource."

EDITORIAL - change MAY to "can"

"If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation."

EDITORIAL - change MAY to "can"

"Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected representation that has a matching entity-tag."

OK

3.3

"The "If-Modified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has not been modified since the time specified in this field, then do not perform the request method; instead, respond as detailed below."

EDITORIAL - change MAY to "can"

"If the selected representation has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 (Not Modified) response."

OK

3.4

"The "If-Unmodified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server must not perform the requested operation and must instead respond with the 412 (Precondition Failed) status code."

EDITORIAL - change MAY to "can"

"If the selected representation has not been modified since the time specified in this field, the server SHOULD perform the request method as if the If-Unmodified-Since header field were not present."

OK (but why not MUST here?)

"If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or 412 status code, the If-Unmodified-Since header field SHOULD be ignored."

OK (but why not MUST here?)

EDITORIAL: "If the request.." -> "If a request..."
EDITORIAL: the language around ignoring headers / "were not present" here is needlessly divergent.

4.1

"Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, the response SHOULD NOT include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., future HTTP extensions)."

Good.

--
Mark Nottingham   http://www.mnot.net/
Received on Friday, 22 June 2012 01:44:40 GMT

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