Subject: Re: LLMNR interoperability with Rendezvous?

From: John Schnizlein <[email-masked]>
Date: Tue, 02 Sep 2003 06:25:49 -0400

At 12:21 PM 8/31/2003, Michael Richardson wrote:

>>>>>> "Ted" == Ted Lemon <[email-masked]> writes:
>    >> If I'm not mistaken, the BGP protocol originally used TTL=1, but 
>    >> now recommends using TTL=255 and checking that way instead.
>
>    Ted> Hm.  I was one of the people who thought that TTL=255 wasn't such a
>    Ted> great idea, all other things being equal.  However, nobody ever
>    Ted> presented any strong reason why we should use one over the other -
>    Ted> it was always a matter of preferences and fine distinctions.
>
>  TTL=1 depends upon the routers dropping at exactly the right time.
>  TTL=255 is locally checkable, but requires that link-local multicast works.
>
>  Both depend upon the router doing something correctly.

This interpretation depends on what you mean by "correct".
The correct behavior of a router getting a packet with TTL=1 has been
clear for a long time. The new proposal to use TTL as a flag to be
checked by (end-system) hosts is a radical change to the concept of
this field, which was defined for the health of the Internet. 

Changes to fundamentals of the Internet layer should not be made 
without significant and broad examination of their potential implications. 
The existence of an implementation is not a substitute for this analysis.
The use of TTL as a flag in multicast is also not a substitute.

This entire episode, including the reversal of the rough consensus
favoring TTL=1 for link-local traffic in the ZeroConf WG, brings to mind 
the freshman engineering example in which a column is removed from a 
room for aesthetic reasons, rendering the entire building weak.
Please do not mess with the foundation without compelling reasons!

John

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From: Ted Lemon <[email-masked]>
Date: Tue, 2 Sep 2003 15:45:41 -0400

> The use of TTL as a flag in multicast is also not a substitute.

Well, yes, John, but we *did* analyze this, at length.   The only reason that 
anybody came up with for not doing it the way Rendezvous does it was that it 
isn't how we normally use the TTL.   I didn't hear a single substantive 
objection.   You are simply re-raising this point.   It is a good point, but 
not a compelling one.   For example, to date nobody has pointed out any 
reason why this is similar to removing a supporting beam from a large 
structure for aesthetic reasons.   I would say, based on the discussion to 
date, that it is not similar.

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From: Rob Austein <[email-masked]>
Date: Tue, 02 Sep 2003 16:54:05 -0400

The one perhaps real difference I see has to do with how many discrete
entities would have to be buggy for LLMNR to leak beyond its intended
scope.  With TTL=255, only the LLMNR speakers would have to be buggy;
with TTL=1, the intervening routers would also have to be buggy.

How worriesome this is probably depends in large part on how
pessimistic one is about unplanned interactions in complex systems.

---

From: Masataka Ohta <[email-masked]>
Date: Wed, 3 Sep 3 6:50:25 JST

> pessimistic one is about unplanned interactions in complex systems.

A proper assumption should be that multicast does not work beyond
a subnet.

							Masataka Ohta