Hi Gyan, Daniel, Peter, All, Thanks for sharing your insights and I agree mostly with your feedback
I agree and understand that summarization is needed to reduce the size of the LSDB. I also agree summarization good design practice, especially with IPv6 and SRv6 in mind. There never has been doubt about that. I am not sure I agree that UAP/UPA is ‘optimal-design’. Maybe it is the best we can do, however I have a healthy worry we could be suffering tunnel vision and that proposed solution may not be good enough. We should not be blind and believe that advertising UPA/PUA does not come without a cost. The architectural PUA/UPA usage complexity cost may not be worth the effort (none of the integration of using a PUA/UPA event triggers come for free). Do we really believe that PUA/UPA solve all the SID reachability problems for all IGP network design and SR use-cases elegantly? Maybe some use-case design constraints and assumptions should be documented to clarify architecturally where PUA/UPA is most beneficial for operators? Just stating “outside scope of the draft” seems unfair to operators interested in PUA/UPAs Let me give two examples where PUA/UPA benefit is unclear: (1) Multiple-ABRs I was wondering for example if a ingress router receives a PUA signaling that a given locator becomes unreachable, does that actually really signals that the SID ‘really’ is unreachable for a router? For example (simple design to illustrate the corner-case): ingressPE#1---area#1---ABR#1---area---ABR#2---area#3---egressPE#2 | | | | +--------area#1---ABR#3---area---ABR#4---area#3--------+ What if ABR#4 would loose connectivity to egressPE#2 and ABR#2 does not? In that case ABR#4 will originate a UPA/PUA and ABR#2 does not originate a PUA/UPA. How is ingressPE#1 supposed to handle this situation? The only thing ingressPE#1 see is that suddenly there is a PUA/UPA but reachability may not have changed at all and remains perfectly reacheable. (2) with sr-policy or SRv6 SRTE What if we have an inter-area/domain/level SRTE or sr-policy and suddenly there is a PUA/UPA for one of the SIDs in the sid-list of the path. will this impact the srte or sr-policy in any way? Will transit routers do anything with the UPA/PUA and drop packets. Will transit routers trigger fast-restoration? Can PCEs/controllers use the SID for crafting paths? Will all SRTE/sr-policy using the locator be pruned or re-signaled? Will ingress router do something with the PUA information? Should PUA/UPA draft give guidelines around this? Be well, G/ If there is an SRTE or sr-policy using a given SID somewhere in the SID list… and suddenly From: Gyan Mishra <hayabusa...@gmail.com> Sent: Thursday, June 16, 2022 6:12 AM To: Voyer, Daniel <daniel.voyer=40bell...@dmarc.ietf.org> Cc: Van De Velde, Gunter (Nokia - BE/Antwerp) <gunter.van_de_ve...@nokia.com>; draft-ppsenak-lsr-igp-ureach-prefix-annou...@ietf.org; draft-wang-lsr-prefix-unreachable-annoucement <draft-wang-lsr-prefix-unreachable-annoucem...@ietf.org>; lsr@ietf.org Subject: Re: [Lsr] Thoughts about PUAs - are we not over-engineering? Summarization has always been a best practice for network scalability thereby reducing the size of the RIB and LSDB. So in this case as Dan pointed out, the summary route is an abstraction of the area and so if a component prefix of the summary became unreachable we need a way to signal that the PE next hop is no longer reachable to help optimize convergence. We are just trying to make summarization work better then it does today so we don’t have to rely on domain wide flooding of host routes. Thanks Gyan On Wed, Jun 15, 2022 at 4:42 PM Voyer, Daniel <daniel.voyer=40bell...@dmarc.ietf.org<mailto:40bell...@dmarc.ietf.org>> wrote: Hi Gunter, Thanks for your comments, The idea, here, with summarization is to "reduce" the LSDB quite a lots and make a given backbone much more scalable / flexible and allow to simplify NNI's within that given backbones considerably. Summarization is "needed" for better scale and, in the context of IPv6, will help in preventing blowing up the IGP. With the size of an IPv6 prefix range (ex. /64) allocated per domain - summarization will help to contain the LSDB to that domain. What we are "highlighting" in draft-ppsenak-lsr-igp-ureach-prefix-announce-00, is an easy way to overcome the fact that PEs are hidden behind a summary route and need a fast way to notify other PEs when they become unreachable. I don't see "over-engineering" here, I see "optimal-engineering" instead. Thanks Dan On 2022-06-14, 4:59 AM, "Van De Velde, Gunter (Nokia - BE/Antwerp)" <gunter.van_de_ve...@nokia.com<mailto:gunter.van_de_ve...@nokia.com>> wrote: Hi All, When reading both proposals about PUA's: * draft-ppsenak-lsr-igp-ureach-prefix-announce-00 * draft-wang-lsr-prefix-unreachable-annoucement-09 The identified problem space seems a correct observation, and indeed summaries hide remote area network instabilities. It is one of the perceived benefits of using summaries. The place in the network where this hiding takes the most impact upon convergence is at service nodes (PE's for L3/L2/transport) where due to the summarization its difficult to detect that the transport tunnel end-point suddenly becomes unreachable. My concern however is if it really is a problem that is worthy for LSR WG to solve. To me the "draft draft-wang-lsr-prefix-unreachable-annoucement-09" is not a preferred solution due to the expectation that all nodes in an area must be upgraded to support the IGP capability. From this operational perspective the draft "draft-ppsenak-lsr-igp-ureach-prefix-announce-00" is more elegant, as only the A(S)BR's and particular PEs must be upgraded to support PUA's. I do have concerns about the number of PUA advertisements in hierarchically summarized networks (/24 (site) -> /20 (region) -> /16 (core)). More specific, in the /16 backbone area, how many of these PUAs will be floating around creating LSP LSDB update churns? How to control the potentially exponential number of observed PUAs from floating everywhere? (will this lead to OSPF type NSSA areas where areas will be purged from these PUAs for scaling stability?) Long story short, should we not take a step back and re-think this identified problem space? Is the proposed solution space not more evil as the problem space? We do summarization because it brings stability and reduce the number of link state updates within an area. And now with PUA we re-introduce additional link state updates (PUAs), we blow up the LSDB with information opaque to SPF best-path calculation. In addition there is suggestion of new state-machinery to track the igp reachability of 'protected' prefixes and there is maybe desire to contain or filter updates cross inter-area boundaries. And finally, how will we represent and track PUA in the RTM? What is wrong with simply not doing summaries and forget about these PUAs to pinch holes in the summary prefixes? this worked very well during last two decennia. Are we not over-engineering with PUAs? G/ ------------------------------------------------------------------------------ External Email: Please use caution when opening links and attachments / Courriel externe: Soyez prudent avec les liens et documents joints _______________________________________________ Lsr mailing list Lsr@ietf.org<mailto:Lsr@ietf.org> https://www.ietf.org/mailman/listinfo/lsr -- [http://ss7.vzw.com/is/image/VerizonWireless/vz-logo-email]<http://www.verizon.com/> Gyan Mishra Network Solutions Architect Email gyan.s.mis...@verizon.com<mailto:gyan.s.mis...@verizon.com> M 301 502-1347
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