Re: SHOULD vs MUST
John C Klensin <john-ietf@jck.com> Wed, 25 June 2008 12:24 UTC
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Date: Wed, 25 Jun 2008 08:24:22 -0400
From: John C Klensin <john-ietf@jck.com>
To: Scott Brim <swb@employees.org>, IETF Discussion <ietf@ietf.org>
Subject: Re: SHOULD vs MUST
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--On Wednesday, 25 June, 2008 07:59 -0400 Scott Brim <swb@employees.org> wrote: > On 6/25/08 5:37 AM, Fred Baker allegedly wrote: >> >> On Jun 25, 2008, at 5:28 AM, Frank Ellermann wrote: >> >>>> A SHOULD X unless Y essentially means "SHOULD (X or Y)" >>> >>> I'd read it as "do X, but if you have a very good excuse >>> not doing X might do. One known very good excuse is Y." >> >> That is more or less my definition of "should". I say >> something "must" be so when I can tell you an operational >> failure that would or could happen if it isn't. If I would >> like to say "must" but can think of a case in which it would >> not be appropriate I say "should", and am saying that if it >> is not so in someone's implementation they should be prepared >> to say what their reason was. > > ... and draft authors should include explanations in their > drafts of the reasons an implementor might legitimately have > for not implementing the "should". For example, an older > operating system that does not support a new capability. Scott, In principle, sure. But I note that you use a lower-case "should" in the first sentence above and that, like the incremental promotion of "these are available" to "MUST unless you receive a dispensation", this could easily be turned into a firm requirement by someone who was being zealous about rule-making. Do you really mean, e.g., ... where feasible and, in the author's judgment, appropriate, it is desirable to include explanations or illustrations of the exception cases in drafts that use SHOULD. ??? I've run into a number of situations over the years in which there are known edge cases that prevent a MUST but where those edge cases are rare and obscure enough that describing them would require extensive text... text that might indirectly end up providing guidance for bad behavior. For those situations, I'd prefer to see something like: In all of the common cases, the system SHOULD... Rather than The system SHOULD do A unless Y, in which case B SHOULD be done unless Z, in which case C SHOULD be done where each of X, Z, B, and C, might require a half-page explanation. That btw is part of the difficult with some of the discussion in this thread. The discussion has, as I've read it, concentrated on SHOULD do A unless Y and SHOULD do A but may do B where it would often be useful to say SHOULD do A unless Y and then SHOULD do B Note that the latter can often be rewritten as a MUST, e.g., MUST do A unless condition Y occurs, in which case MUST do B I believe that good sense and discretion are important here. I also believe that attempts to map case-by-case good sense into rules generally gets us into trouble and that such efforts should be examined carefully by the community. In addition, as Frank has noted, negative statements and words are often used quite differently than they are in English by languages that are otherwise reasonably similar to English. That calls for use of extreme care in use of negative statements in conformance clauses, a subject on which I would hope the RFC Editor (as well as authors and the IESG) would be very sensitive. john _______________________________________________ IETF mailing list IETF@ietf.org https://www.ietf.org/mailman/listinfo/ietf
- Review of draft-ietf-geopriv-http-location-delive… Eric Rescorla
- Re: Review of draft-ietf-geopriv-http-location-de… Hannes Tschofenig
- Re: Review of draft-ietf-geopriv-http-location-de… Eric Rescorla
- Re: Review of draft-ietf-geopriv-http-location-de… Hannes Tschofenig
- Re: Review of draft-ietf-geopriv-http-location-de… Eric Rescorla
- Re: Review of draft-ietf-geopriv-http-location-de… Eric Rescorla
- Re: [secdir] Review of draft-ietf-geopriv-http-lo… Richard Barnes
- RE: [Geopriv] [secdir] Review ofdraft-ietf-geopri… Tschofenig, Hannes (NSN - FI/Espoo)
- Re: [Geopriv] Review of draft-ietf-geopriv-http-l… Eric Rescorla
- RE: [Geopriv] Review of draft-ietf-geopriv-http-l… Tschofenig, Hannes (NSN - FI/Espoo)
- RE: Review of draft-ietf-geopriv-http-location-de… Mary Barnes
- Re: Review of draft-ietf-geopriv-http-location-de… Eric Rescorla
- Re: Review of draft-ietf-geopriv-http-location-de… TSG
- SHOULD vs MUST (was Re: Review of draft-ietf-geop… Lawrence Conroy
- Re: SHOULD vs MUST (was Re: Review of draft-ietf-… Eric Rescorla
- RE: [Geopriv] Review of draft-ietf-geopriv-http-l… Dawson, Martin
- Re: SHOULD vs MUST (was Re: Review of draft-ietf-… Dave Cridland
- Re: SHOULD vs MUST (was Re: Review of draft-ietf-… Joe Abley
- Re: SHOULD vs MUST Frank Ellermann
- Re: SHOULD vs MUST (was Re: Review of draft-ietf-… Iljitsch van Beijnum
- Re: SHOULD vs MUST Fred Baker
- Re: SHOULD vs MUST Scott Brim
- Re: SHOULD vs MUST John C Klensin
- Re: SHOULD vs MUST Fred Baker
- Re: SHOULD vs MUST Scott Brim
- Re: SHOULD vs MUST John C Klensin
- Re: SHOULD vs MUST Scott Brim
- Re: SHOULD vs MUST Dean Willis
- Re: SHOULD vs MUST Robert Sparks
- Re: SHOULD vs MUST Dave Crocker
- Re: SHOULD vs MUST Dave Cridland
- Re: SHOULD vs MUST Iljitsch van Beijnum
- SHOULD vs MUST case sensitivity Dave Crocker
- RE: SHOULD vs MUST case sensitivity Eric Gray
- Re: SHOULD vs MUST case sensitivity Julian Reschke
- Re: SHOULD vs MUST case sensitivity Keith Moore
- SHOULD vs MUST case sensitivity Dave Crocker
- RE: SHOULD vs MUST Eric Gray
- SHOULD vs MUST case sensitivity Dave Crocker
- Re: SHOULD vs MUST case sensitivity C. M. Heard
- Re: SHOULD vs MUST case sensitivity Iljitsch van Beijnum
- Re: SHOULD vs MUST case sensitivity Randy Presuhn
- Re: SHOULD vs MUST case sensitivity Dave Crocker
- Re: SHOULD vs MUST case sensitivity Dave Crocker
- Re: SHOULD vs MUST case sensitivity Randy Presuhn
- Re: SHOULD vs MUST case sensitivity Keith Moore
- Re: SHOULD vs MUST case sensitivity Dave Crocker
- RE: SHOULD vs MUST case sensitivity Eric Gray
- Re: SHOULD vs MUST case sensitivity Spencer Dawkins
- Re: SHOULD vs MUST case sensitivity Ralph Droms
- Re: SHOULD vs MUST case sensitivity Dave Crocker
- RE: SHOULD vs MUST case sensitivity Hallam-Baker, Phillip
- Re: SHOULD vs MUST case sensitivity John Levine
- RE: SHOULD vs MUST case sensitivity Hallam-Baker, Phillip
- Re: SHOULD vs MUST case sensitivity John Leslie
- RE: Review of draft-ietf-geopriv-http-location-de… Mary Barnes