Dean, I am struggling constantly with 2119 as an AD, because if I take
the letter (and the spirit) of 2119 at face value, a lot of people are
doing this wrong. And 2119 is a BCP; it's one of our process documents.
So I'd like this to be cleared up as much as you. I think there is
active harm in the misuse we are seeing.
To Ned's points:
On 1/4/13 7:05 PM, ned+i...@mauve.mrochek.com wrote:
+1 to Brian and others saying upper case should be used sparingly, and
only where it really matters. If even then.
That's the entire point: The terms provide additional information as to
what the authors consider the important points of compliance to be.
We will like end up in violent agreement, but I think the above
statement is incorrect. Nowhere in 2119 will you find the words
"conform" or conformance" or "comply" or "compliance", and I think
there's a reason for that: We long ago found that we did not really care
about conformance or compliance in the IETF. What we cared about was
interoperability of independently developed implementations, because
independently developing implementations that interoperate with other
folks is what makes the Internet robust. Importantly, we specifically
did not want to dictate how you write your code or tell you specific
algorithms to follow; that makes for everyone implementing the same
brittle code.
The useful function of 2119 is that it allows us to document the
important *behavioral* requirements that I have to be aware of when I am
implementing (e.g., even though it's not obvious, my implementation MUST
send such-and-so or the other side is going to crash and burn; e.g.,
even though it's not obvious, the other side MAY send this-and-that, and
therefore my implementation needs to be able to handle it). And those
"even though it's not obvious" statements are important. It wastes my
time as an implementer to try to figure out what interoperability
requirement is meant by, "You MUST implement a variable to keep track of
such-and-so-state" (and yes, we see these in specs lately), and it makes
for everyone potentially implementing the same broken code.
The notion (that some have) that MUST means you have to do something
to be compliant and that a "must" (lower case) is optional is just
nuts.
You bet, Thomas!
In some ways I find the use of SHOULD and SHOULD NOT be to be more useful
than MUST and MUST NOT. MUST and MUST NOT are usually obvious. SHOULD and
SHOULD NOT are things on the boundary, and how boundary cases are handled
is often what separated a good implementation from a mediocre or even poor
one.
Agreed. Indeed, if you have a MUST or MUST NOT, I'd almost always be
inclined to have a "because" clause. If you can't given an explanation
of why I need this warning, there's a good chance the MUST is
inappropriate. (One that I've seen of late is "If the implementation
wants to send, it MUST set the <send> field to 'true'. If it wants to
receive, it MUST set the <send> field to 'false'." I have no idea what
those MUSTs are telling me. Under what circumstances could I possibly
want to send but set the <send> field to "false"?)
The idea that upper case language can be used to identify all the
required parts of a specificition from a
compliance/conformance/interoperability perspective is just
wrong. This has never been the case (and would be exceeding painful to
do), though (again) some people seem to think this would be useful and
thus like lots of upper case language.
At most it provides the basis for a compliance checklist. But such checklists
never cover all the points involved in compliance. Heck, most specifications in
toto don't do that. Some amount of common sense is always required.
And again, it's worse than incomplete. It also makes for brittle code. I
don't want you checking to see if you coded things the same way that I
did, which is what a compliance list gets you. I want you checking that
your *behavior* from the net interoperates with me. Insofar as you want
to call *that* compliance, well, OK, but I don't think that's what
people mean.
Where you want to use MUST is where an implementation might be tempted
to take a short cut -- to the detriment of the Internet -- but could
do so without actually breaking interoperability.
Exactly!
IMO, too many specs seriously overuse/misuse 2119 language, to the
detriment of readability, common sense, and reserving the terms to
bring attention to those cases where it really is important to
highlight an important point that may not be obvious to a casual
reader/implementor.
Sadly true.
And to the detriment of good code..
pr
--
Pete Resnick<http://www.qualcomm.com/~presnick/>
Qualcomm Technologies, Inc. - +1 (858)651-4478
