Re: [trill] Asymmetric link costs and D-Tree calculations

Olen Stokes <ostokes@extremenetworks.com> Fri, 01 March 2013 12:22 UTC

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From: Olen Stokes <ostokes@extremenetworks.com>
To: Donald Eastlake <d3e3e3@gmail.com>
Date: Fri, 1 Mar 2013 04:22:18 -0800
Thread-Topic: [trill] Asymmetric link costs and D-Tree calculations
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Subject: Re: [trill] Asymmetric link costs and D-Tree calculations
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Thanks for the feedback.

Cheers,
Olen

-----Original Message-----
From: Donald Eastlake [mailto:d3e3e3@gmail.com] 
Sent: Friday, March 01, 2013 2:21 AM
To: Olen Stokes
Cc: trill@ietf.org
Subject: Re: [trill] Asymmetric link costs and D-Tree calculations

Hi Olen,

On Thu, Feb 28, 2013 at 1:21 PM, Olen Stokes <ostokes@extremenetworks.com> wrote:
> Appendix A.1 of RFC 6325 discusses the fact that "two RBridges may 
> know they are connected but each sees the link as a different speed 
> from how it is seen by the other."

Yes, and asymmetry can also occur through manual cost configuration of costs.

> We have been looking at this and how it relates to D-Tree calculation 
> as discussed in Section 4.5.1.  In that section, it states, "Each 
> RBridge RBn independently calculates a tree rooted at RBi by 
> performing the SPF (Shortest Path First) calculation with RBi as the 
> root".  So, it appears that the D-Tree is calculated from the root RBridge outward.
>
> The following three paragraphs describe how to determine the potential 
> parents P for a node N.  There does not appear to be any mention of 
> direction so it would seem that all calculations are done in the 
> direction from the root RBi outward.

That was the intent. The tree is calculated through a Dijkstra SPF process starting at the Root and working outwards.

> The last paragraph in Section 4.5.1 states, "In other words, the set 
> of potential parents for N, for the tree rooted at R, consists of 
> those that give equally minimal cost paths from N to R and that have 
> distinct IS-IS IDs, based on what is reported in LSPs."  In this case, 
> costs are discussed as inward from node N towards root RBi.

That text in RFC 6325 is wrong, as far as I know.

> This change in direction of cost calculation could potentially be 
> confusing when there are asymmetric link costs as described in 
> Appendix A.1.  For any parent P found during the SPF calculation, it 
> is possible that the cost from N to P is different than the cost from 
> P to N.  It is also possible that none of the parents found during the 
> SPF calculation are found along any of the "equally minimal cost paths 
> from N to R".

Yes.

Although I think the asymmetric costs will be rarely encountered, it is critical that all RBridges calculate the same tree so the method must be unambiguous.

> Is the potential change of direction when calculating costs in the 
> last paragraph unintended?  Would the paragraph have been more clear 
> discussing "equally minimal cost paths from R to N"?

Yes, thanks for spotting this. It should have said "from R to N" or something like that. I'll file an Errata against RFC 6325.

> Is there agreement in the presence of asymmetric link costs that it is 
> possible that none of the parents found during the SPF calculation are 
> found along any of the "equally minimal cost paths from N to R" 
> calculated for known unicast traffic?

It seems to me that could happen.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street, Milford, MA 01757 USA  d3e3e3@gmail.com

> Cheers,
> Olen