"Frank Ellermann" <> Tue, 24 June 2008 21:27 UTC

Return-Path: <>
Received: from [] (localhost []) by (Postfix) with ESMTP id 310B43A69BC; Tue, 24 Jun 2008 14:27:17 -0700 (PDT)
Received: from localhost (localhost []) by (Postfix) with ESMTP id A41B93A69B4 for <>; Tue, 24 Jun 2008 14:27:16 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -4.026
X-Spam-Status: No, score=-4.026 tagged_above=-999 required=5 tests=[AWL=-0.427, BAYES_00=-2.599, RCVD_IN_DNSWL_LOW=-1]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id buHxaBlh8GNX for <>; Tue, 24 Jun 2008 14:27:16 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id C68193A6858 for <>; Tue, 24 Jun 2008 14:27:15 -0700 (PDT)
Received: from list by with local (Exim 4.43) id 1KBG2j-0003YD-NM for; Tue, 24 Jun 2008 21:27:13 +0000
Received: from ([]) by with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for <>; Tue, 24 Jun 2008 21:27:13 +0000
Received: from nobody by with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for <>; Tue, 24 Jun 2008 21:27:13 +0000
From: "Frank Ellermann" <>
Subject: Re: SHOULD vs MUST
Date: Tue, 24 Jun 2008 23:28:50 +0200
Organization: <>
Lines: 25
Message-ID: <g3ror8$2b9$>
References: <><><><> <>
Mime-Version: 1.0
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2800.1914
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1914
X-Mailman-Version: 2.1.9
Precedence: list
Reply-To: Frank Ellermann <>
List-Id: IETF Discussion <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit

Dave Cridland 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."

OTOH for a MUST X I'd want no qualifiers, MUST means "an
attempt to do not X can cause havoc."  

This is also about a readability, I had a serious case of
DEnglish some days ago, an article was tagged with a note:

"This article may contain no original research".  I read
this as "apparently there is no original research in this
article", which would be perfectly fine wrt the linked
guideline.  Only much later it occured to me that I got
the "may ... not ..." wrong.  

If Y is the *only* possible very good excuse your version
"MUST X or Y, SHOULD X" is clear.  But SHOULD X often has
an implicitly known very good excuse:  Any older software
written before the SHOULD X was approved.


IETF mailing list