Subject: Re: IPv6 w.g. Last Call on "IPv6 Host to Router Load Sharing"

From: Pekka Savola <[email-masked]>
Date: Thu, 17 Jan 2002 16:28:06 +0200 (EET)

On Thu, 17 Jan 2002, JINMEI Tatuya / [ISO-2022-JP] 神明達哉 wrote:
> I agre with Francis.  And, actually, our implementation (i.e. all *BSD
> variants) basically keeps the same router (of the same priority), so
> it will be non-compliant with the SHOULD.  I believe our
> implementation is not the only example of this "non-compliant" behavior.

True.
 
> If the SHOULD is really the consensus, I'll change the implementation
> so that it will be compliant again.  However, I'm still not fully
> convinced that the change is worth making (perhaps many) existing
> applications non-compliant.
> 
> As Francis pointed out, there are many cases of "several default
> routers with a better one."  In this case, picking up random routers
> (without the knowledge of router preference) will just be meangless,
> because we'll eventually converge on the better router with redirect
> messages.

I agree, as I commented on this when the draft first appeared.

IMO it's _much_ better practise have predictable behaviour.
Load-distribution is not that.  When there are two default routes of equal
weight, I want to only use one, and if the one I use fails, failover
nicely to the other.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

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---

From: Francis Dupont <[email-masked]>
Date: Thu, 17 Jan 2002 10:54:33 +0100

In your previous mail you wrote:

   >I have two concerns about this:
   >  - the random order is not defined (this is a formal concern because
   >    the intention is clear)
   
   I think most people will understand the intent of "random order".

=> as I've written this is a *formal* concern... (formal = on the form)

   >  - we moved from an implementation hint to a SHOULD, this will make
   >    old implementations not conformant and in some situations there
   >    can be far better solutions. So I prefer to get a MAY (enough to
   >    get this proposal implemented in the future by everybody without
   >    harm) and an explicit reference to the default router preference
   >    draft (first because I am in favor of this from the beginning,
   >    i.e. since many years, second because this is the best reply to
   >    the "special case" argument (not so special case because 100% of
   >    the IPv6 networks I used have several routers with a better one)).
   
   I disagree.  The intent of the document is to change the behavior from a 
   hint to a common behavior.  Hence the SHOULD.

=> if this is the intent the document is underspecified (i.e. it have to
remember when the default router selection is done (this is at the
beginning of 6.3.6)). There is too a real issue about the required
minimal default router list length (two issues in fact: do we allow
small implementations to use a short list (possibly shorter than
the real number of default routers)? What is the reply to the bad guy
who tries to give us 2^64 default routers?) Note there is something
about this in 5.3 GC and Timeouts and 6.3.4 Processing rcvd RAs.

   Note that the current "hint" for round robin is less than perfect.

=> I agree but my concern is more about the requirement level.
   
   I think the current implementations are covered by the SHOULD.

=> they are not compliant with the SHOULD, not a real problem for round
robin (as it can be looked at a bad random order) but implementations
which keep the same router have a very different behavior (than the
recommended one).

   If it was a MUST there might be a problem with current behavior.

=> I agree but a SHOULD in a draft standard (i.e. the intent) is
really quite strong.
   
   In Salt Lake City where the issue of combining this with the default router 
   preferences was discussed, the consensus was to keep this separate.

=> my concern is there are a lot of situations where some potential
default routers are better than others. Without the default router
preferences the only way to control the default router choice is
to use zero router lifetimes. This is far too drastic and can't
express any kind of nuance... So I'd really like to get preferences
before or at the same time than random selection.

   The plan was to also add this behavior to the default router
   preferences draft for the case where there are multiple routers at
   the same preference.
   
=> we agree but there is clearly a problem in priorities.

Regards

[email-masked]