Subject: Exception to "MUST NOT"

From: "Jiwoong Lee" <[email-masked]>
Date: Thu, 20 Sep 2001 15:56:25 +0900

All,

This is about the requirement level in the Internet specification.

If a statement has the requirement level of 'MUST NOT' while it has an exceptional case, and while 
the exceptional case does not elucidate its requirement level, what is the appropriate requirement level of this exceptional case ?

I guess the answer is 'MAY'. Are you agreeable to this ?

I think ICMPv6 specification has the similar statements in it, regarding to the requirement level of the generation of ICMPv6 packet in response to a multicast packet or to a link local multicast packet.

Jiwoong

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From: "Kurt D. Zeilenga" <[email-masked]>
Date: Thu, 20 Sep 2001 10:36:54 -0700

"MUST NOT" != "NOT REQUIRED"

See RFC 2119, but basically:

"MUST NOT" implies an absolute prohibition, not an optional requirement.

Kurt

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From: Fred Baker <[email-masked]>
Date: Fri, 28 Sep 2001 11:56:44 -0700

At 11:56 PM 9/19/2001, Jiwoong Lee wrote:
>the exceptional case does not elucidate its requirement level, what is the 
>appropriate requirement level of this exceptional case ?

in such a case, I should think the right thing to say is "should not".

My rule (mine personally, not the rule documented in 2119) is that "must" 
means "something will break if you fail to do this", and "must not" means 
"something will break if you do this." I try to say what breaks in such 
cases, and why.

"Should", from my perspective, means "I'd like to say 'must', but I can 
think of a case where it wouldn't apply". "should not" is the reflexive of 
that: "I'd like to say 'must not', but I can think of a case where you 
really should." This sounds like what you're describing.

I use "MAY" to mean "I really don't care, but someone was asking about 
this, so I should clarify."