[trill] Shepherd review of draft-ietf-trill-revilient-trees-05
Donald Eastlake <d3e3e3@gmail.com> Thu, 14 July 2016 02:26 UTC
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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Wed, 13 Jul 2016 22:26:23 -0400
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Subject: [trill] Shepherd review of draft-ietf-trill-revilient-trees-05
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I've review this draft and I have one big comment and number of smaller comments: Major Comment: -------------- Backwards compatibility: The biggest problem that I see has to do with backward compatibility. What happens if one or more RBridges in the campus do not implement resilient trees? Or even do not implement the Affinity sub-TLV? It seems like you need to be able to detect these conditions and either adapt to them or not try to do the resilient tree alternative distribution trees. This is related to the incremental deployment question. There is an Affinity sub-TLV support bit in the TRILL-VER sub-TLV but this does not seem to be mentioned in the document. Note that at the beginning of 3.2.1.1 it says "it is desirable that every RBridge independently computes Affinity Links for a backup DT across the whole campus" which would require every RBridge in the campus to implement resilient tress. So, while maybe some sort of local repair can be done when the local RBridge support, I think the global repair requires support throughout the campus. It seems to me that either the document needs to be changed to either (1) make support mandatory or (2) state that you don't try any of the resiliency features (or at least the global ones) unless the resilience is supported by all the RBridges (or at least all the RBridges other than singly connected leaves). In either case, there needs to be a capability bit (probably an Extended Capability bit) to indicate resilient trees support. And you really have to provide for "(2)" above because it will always be possible that you have a legacy RBridge in a campus. (Of course, as a practical matter, it will commonly be the case that all the RBridges in a campus support exactly the same capabilities.) I recommend that we NOT make resilient tree support mandatory. I think that would be a substantial change and require a new WG Last Call. Medium Comments: --------------- Section 3.2 and subsections: If you are going to provide for manually configured alternative tree and automatically computer alternative trees and allow the root for an alternative tree to be different from the root for the primary tree, it seem to me that some RBridge (presumably the highest priority tree root) needs to announce something like {primary tree nickname, alternative tree nickname, flags} where flags could indicate manually configured or automatically configured and have some spare bits for future uses. I don't think it makes sense to have two alternative algorithms, as specified in 3.2.1 and 3.2.2, for automatically computing a maximally disjoint alternative tree when the two algorithms are so similar. Maybe it would make sense to have two radically different algorithms if you could point to some circumstances under which each was superior to the other. But if you are going to have multiple algorithms, it seems like you need a way for some designated RBridge (the highest priority tree root?) to announce what algorithm is in use and you would need algorithm identifiers and a registry and ... I do not think it would be worth it to have all that mechanism. Of the algorithms in 3.2.1 and 3.2.2, I think I favor the one in 3.2.1 a little. (The one in 3.2.2 at least would need a maximum cost to avoid link cost overflowing and wrapping around.) I would suggest that the increment to link cost in 3.2.1 of the sum of the cost of all links in the campus is too conservative. I think it would be adequate for the increment to be 1 plus the total cost of the highest leaf-to-leaf cost in the primary tree. Section 4.1 on pruning: Maybe it is just me, but I don't quite understand why pruning for an alternative tree should be based on the pruned primary tree. What exactly happens when a TRILL Data packet is sent on an alternative tree? Presumably, the "egress nickname" field is changed from the nickname designating the primary tree to the nickname designating the alternative tree. So, generally, the packet will be forwarded down the alternative tree and it seems to me that it should be sent down any branch where there are RBridges interested in the packet VLAN/FGL further down that branch. What does the primary tree have to do with it except in the local repair case? Minor Comments: --------------- Title page: "Updates: RFC 6325" You are just supposed to list the RFC number without "RFC" before it in Updates and Obsoletes items on the front page. I guess the change required to RFC 6325 is in Section 5.3.1 about changing the egress nickname of a TRILL Data packet in flight. This should be mentioned in the Introduction (probably at the end of the Introduction). Section 1, page 5, at the top: It sounds a bit like the repair of the failed link itself. It's more like routing is being repaired by bypassing the failed link. Section 2.1, top of page 6: Should clarify that the Affinity Sub-TLV is distributed in an LSP. Section 2.1, at the end: I'm not really comfortable with saying you "MUST NOT" declare an Affinity Link between non-adjacent RBridges. Since such a declaration would just be ignored, it usually just means some wasted LSP space, so I think it should be "SHOULD NOT". Section 2.2: I think it might avoid confusion when you talk about "removing incoming links" to say that outgoing links/adjacencies are not removed. (The figure is clear.) Section 5.2.1: Although it is fairly obvious, I think you should point out that item 1 requires a very reliable and steady data packet stream. Section 5.3: The last two sentences should be deleted. It says pretty much the same thing in the first paragraph of Section 5. Section 5.3.3: This walkthrough starts talking about RBridges but does not indicate what Figure the reader should look at to follow what is being said. Section 5.4: The precision and the limits for the configuration of Ts should be given. Minor updates: The CMT draft has been published as RFC 7783. RFC 7180 has been obsoleted by RFC 7780. Thanks, Donald =============================== Donald E. Eastlake 3rd +1-508-333-2270 (cell) 155 Beaver Street, Milford, MA 01757 USA d3e3e3@gmail.com
- [trill] Shepherd review of draft-ietf-trill-revil… Donald Eastlake