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Re: draft-zinin-microloop-analysis-01.txt - times

Subject: Re: draft-zinin-microloop-analysis-01.txt - times
From: Alex Zinin
Date: Mon, 6 Jun 2005 16:39:40 -0700
Stewart,

I suspect there's a confusion here. See inline below, pls:

> In the general case we have to assume that every router has been configured
> with a different value for the time that it's operator thinks it will need.
> So the problem is how to make sure that the network has a consistent view
> of the max value.

> My thought was that since the set of LSPs that were used to calculate the
> topology had to be consistent over the network (or the network would be
> unstable), then if the delay parameter was imbedded in the LSPs it would
> be consistent amongst the routers that were converging.

I meant the same announcement mechanism in my message--stick a TLV in an
LSP/LSA.

> By using the value extracted from the current topology info, you avoid
> a whole load of messy protocol associated with routers joining, or leaving
> and wanting to change the value - it just happens automatically.

I assume you mean the standard link-state database synchronization
mechanisms here. Yes, I meant to use the same in my message.

> By using historical values - as you seem to propose, you need to consider
> how you propose to synchronize the addition or removal of routers from
> the max-time algorithm. I am proposing that this happen automatically
> as part of the convergence calculation, whilst your approach needs to
> consider quasi asynchronous removal of a router that has the net max
> value, just as the topology changes. Extracting the info from the LSPs
> makes such actions a synchronous event.

This is where you lost me.

Injection of info from newly joined routers and removing stale information
from unreachable routers is a solved problem. Have I missed something?

> If I look at your second para:
 >>  While this would work algorithmically, I would instead prefer finding the
 >>  max among all available announcements, whether the corresponding nodes are
 >>  momentarily reachable or not.
 >>  This would be more robust,

> What can be more robust than extracting the parameter from the set of
> topology info that EVERYONE is using for THIS transition?

 >>  easier to debug,

> Not convinced, you know which routers are in the net, you know what the
> time should be. The other way you get strange bugs due to the
> asynchronous aging of LSPs

 >>  and save extra fluctuations when the network topology changes.

> I am not sure how much of a problem this is in practice.

 >>  If any of
 >>  the routers becomes unreachable for an extended period of time, its
 >>  LSAs/LSPs would finally age out and hence taken out of consideration.

> In the past this was a garbage collection activity with no topology
> significance, and did not need to be synchronized. We are now making
> it a topology significant event that needs to be synchronized so that
> max-time is synchronously removed across the network.

I think above you're assuming that every router would age out stale
LSPs/LSAs independently. Both IS-IS and OSPF have mechanisms to flush an
LSP/LSA from the domain once it has aged in one of the routers.

Alex


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