Subject: Re: Flow label drafts updated

From: Ran Atkinson <[email-masked]>
Date: Tue, 10 May 2011 08:38:42 -0400

Earlier, Brian Carpenter wrote:
> I suggest squaring this circle by retaining the MUST NOT (which IMHO
> is the present consensus) but making it clear in the security
> considerations that there will be exceptions. The problem here is that
> the RFC 2119 language doesn't provide MUST NOT UNLESS..., which is
> what we really need. SHOULD NOT is not perceived as strong enough.

My understanding is that the "...MUST NOT..., unless ..." language
is acceptable procedurally -- PROVIDED the unless is crisply and
narrowly defined.  RFC-2119 does not prohibit that language.
Certainly I understand that language has been used in the past 
by this community, albeit not frequently.  I've checked with a few 
other folks, who have the same view on that language being
acceptable (provided the exceptions are crisply/narrowly defined),
if infrequent, usage.

I'm quite happy with the compromise you put forward above,
and since the exceptions are quite narrow and precise,
I propose that the I-D(s) use the "MUST NOT ... unless" construct.

Here is a starting point on some text.  Please edit to taste:

	The Flow Label MUST NOT be changed in transit, unless
	(a) a router is changing a zero value Flow Label to
	a non-zero value Flow Label compliant with this RFC
	or (b) a security gateway for operational security
	reasons changes a non-zero value Flow Label to a 
	different non-zero value Flow Label compliant with this RFC.

I believe that security gateways overtime will become compliant
with this new specification if we provide them the instructions
above.

Yours,

Ran
[email-masked]

---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 11 May 2011 09:40:32 +1200

> with this new specification if we provide them the instructions

Ran, this looks fine to me, subject to wordsmithing when I see
it in context, and to adding a real discussion of covert channels
in the Security Considerations. Of course I will have to seek the
agreement of my co-authors and resolve the other recent comments (mainly
from Thomas), so expect a few more days before we have a new draft.

Thanks for your help in getting this right.

    Brian