Subject: Re: Flow Label

From: "Steven M. Bellovin" <[email-masked]>
Date: Thu, 03 Jan 2002 12:34:07 -0500

In message <[email-masked]>, Randy Bush writes:
>>> The proposed new words for 2460 effectively say
>>> - you don't have to set it
>>> - you don't have to look at it
>>> - you mustn't change it
>>> I don't think encourages kludge significantly more than MBZ text, but it
>>> does allow for future usage.
>> as I see it, the proposed words let us get Flow Label off our
>> plates for now, which is good because we don't know what to do
>> with it anyway.  maybe someday someone wall make a convincing 
>> case for how to use it.
>
>and, if we do mbz, they can then write the rfc which changes that

The problem then is all the implementations -- or firewalls -- that 
will check that the bits really are zero upon receipt.  If you want 
MBZ, say "sender MUST set to zero; receiver and middle boxes MUST NOT 
check".

Then you have to hope that folks listen to that part.  Especially for 
firewalls, I wouldn't count on that.  (For precedent, so to speak, look 
at what happened with the ECN bits when a particular version of Linux 
started using them.)

		--Steve Bellovin, http://www.research.att.com/~smb
		Full text of "Firewalls" book now at http://www.wilyhacker.com

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---

From: Randy Bush <[email-masked]>
Date: Thu, 03 Jan 2002 09:37:13 -0800

> MBZ, say "sender MUST set to zero; receiver and middle boxes MUST NOT 

as i thought it was the stated mbz formulation, i was assuming this.

> at what happened with the ECN bits when a particular version of Linux 

hmmm.  i think i am starting to reconsider.  idiots are soooo creative.

randy

---

From: Keith Moore <[email-masked]>
Date: Thu, 03 Jan 2002 13:07:18 -0500

> MBZ, say "sender MUST set to zero; receiver and middle boxes MUST NOT

right.
 
> firewalls, I wouldn't count on that.  

nor I.  but clearly stating that routers and firewalls are supposed to
ignore Flow Label is probably the best we can do.  

Keith

p.s. I *wish* we had a mechanism for clearly pointing the finger at 
intermediaries that cause application failures by violating the specs -
it would be so wonderful if an application could say "you aren't 
receiving any incoming mail because your boneheaded network administrator 
installed a firewall that won't permit SMTP over TLS traffic to pass"
complete with a clickable button that says "Terminate Administrator"
(and another that says "...With Extreme Prejudice")
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