Subject: [Isis-wg] Format of Restart TLV

From: "Matthew Noble" <[email-masked]>
Date: Thu, 14 Feb 2002 10:43:47 -0000

Mike

From your ISIS Restart draft, when a "helper" node responds to a Restart TLV
from a "restarter" node, it returns the RA bit set and the remaining time of
the adjacency holding time. This as it is is fine for p2p, but on a LAN
where there could be mutiple "restarter" nodes on the LAN, the info in the
broadcast IIH is specific to one "restarter" but will be received and
prcossed equally by all "restarter" nodes on that LAN.

Should the remaining holding time in the restart TLV be qualified by the
sysid of the "restarter" to which it refers ?

Omission of this qualification means that all the "restarter" nodes on the
LAN will use the minimum value of the remaining holding times sent out by
the "helper" nodes. This is not a disaster but reduces further the chances
that the "restarter" nodes will complete the restart process, and so
increase the possibility of churn in the routing tables.

What do you think ?

Thanks
Matt

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From: mike shand <[email-masked]>
Date: Thu, 14 Feb 2002 11:40:38 +0000

Matt,

Hmmm. I see your point, but I'm not at all keen on making what is already 
an embarrassingly complex design even more complicated.

A few points.

In the general case, with a holding multiplier of 3, the worst diversity we 
would expect to see in the holding times would we about 30% [actually that 
is not true, since it is possible that one router fails, and then a couple 
of hello times later, the second fails. Then both come up more or less 
together. However, I'm not sure we should go overboard trying to deal with 
what are essentially two partially overlapping, but unrelated failures.]

If there are multiple failures, and one router fails to come up in time to 
avoid a routing churn, then there is not much point in the other(s) 
persisting in trying to be stealthy, since chaos reigns anyway.

I'm almost tempted to say that if there ARE two coincident failures then we 
should just do a hard restart anyway.

There is a danger in trying to make this thing deal with ever more complex 
corner cases that we end up with something which is too complicated for its 
own good.

That said, if people think the addition of a "I am responding to a restart 
by X" field would be beneficial, it does cost us much packet real estate. 
But I think it WILL cost us a lot in specsmanship to make sure it all works 
how we expect it to :-)

         Mike

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From: "Matthew Noble" <[email-masked]>
Date: Thu, 14 Feb 2002 13:36:50 -0000

Mike

I agree with you that in many cases the addition of this extra sysid may be
overkill but I think there may be situations where every chance of reducing
the likelihood of churn should be taken.

To that end, is it worth making the sysid optional? We could state that all
zero's means "not defined - please ignore".

Another option would be to make the TLV variable length, so that if the
bytes are not there, they are not defined. This would save the space in
_every_ IIH being transmitted. If this is acceptable, then we could make it
so that if RR bit is set, then we dont include the (empty) space for the
remaining holding time as well.

Either way would allow the inclusion and/or processing of the sysid not to
be implemented where vendors choose not to.

Thanks
Matt

> -----Original Message-----
> From: mike shand [mailto:[email-masked]]
> Sent: Thursday, February 14, 2002 11:41 AM
> To: Matthew Noble
> Cc: 'Isis-Wg (E-mail)
> Subject: Re: Format of Restart TLV
>
>
> Matt,
>

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From: Alex Zinin <[email-masked]>
Date: Thu, 14 Feb 2002 21:13:15 -0800

Matthew,

 I tend to agree with Mike here.

 The discussion of whether multiple restarting routers are ok
 or dangerous would be subjective and most probably fruitless,
 as both opinions are acceptable. So, let's use the KISS
 principle as the tie-breaker. The doc should, of course,
 spell out the situation you described.

-- 
Alex Zinin

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From: "Matthew Noble" <[email-masked]>
Date: Fri, 15 Feb 2002 13:43:27 -0000

Alex/Mike

The more I think of the scenarios where this might occur the more I agree
with the KISS principle. Let's stick to how it is right now and just
highlight the situation in the draft.

Thanks
Matt

> From: Alex Zinin [mailto:[email-masked]]
> Sent: Friday, February 15, 2002 5:13 AM
> To: Matthew Noble
> Cc: 'mike shand'; 'Isis-Wg (E-mail)
> Subject: Re: [Isis-wg] RE: Format of Restart TLV
>
>
>
> Matthew,
>

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From: mike shand <[email-masked]>
Date: Fri, 15 Feb 2002 14:01:11 +0000

>highlight the situation in the draft.

Sounds good to me.

         Mike