[mpls] Spencer Dawkins' Discuss on draft-ietf-mpls-tp-shared-ring-protection-05: (with DISCUSS and COMMENT)
Spencer Dawkins <spencerdawkins.ietf@gmail.com> Wed, 24 May 2017 20:10 UTC
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From: Spencer Dawkins <spencerdawkins.ietf@gmail.com>
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Date: Wed, 24 May 2017 13:10:09 -0700
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Subject: [mpls] Spencer Dawkins' Discuss on draft-ietf-mpls-tp-shared-ring-protection-05: (with DISCUSS and COMMENT)
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Spencer Dawkins has entered the following ballot position for draft-ietf-mpls-tp-shared-ring-protection-05: Discuss When responding, please keep the subject line intact and reply to all email addresses included in the To and CC lines. (Feel free to cut this introductory paragraph, however.) Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html for more information about IESG DISCUSS and COMMENT positions. The document, along with other ballot positions, can be found here: https://datatracker.ietf.org/doc/draft-ietf-mpls-tp-shared-ring-protection/ ---------------------------------------------------------------------- DISCUSS: ---------------------------------------------------------------------- I want to thank the authors for a very readable draft. It was a pleasure to review, and that's a high bar for the subject. I have loads of questions, but my first set of questions is an expansion of Alvaro's comment that I think rises to the level of a Discuss. Please note that I'm asking questions, not proposing text changes, so I really do want to discuss it. ---------- my first set of questions In this text, Three typical ring protection mechanisms are described in this section: wrapping, short wrapping and steering. All nodes on the same ring MUST use the same protection mechanism. I would like to understand what happens if they aren't - and I'm asking, mostly as a way of encouraging guidance for operators in debugging cases where they're not all using the same mechanism. I'm not asking for a full mesh of possible misconfigurations, only for a sentence or two ("If they aren't all using the same protection mechanism, the following things may happen"). More broadly, I'd like to understand why wrapping and short wrapping are both defined. It seems like the only functional difference is that short wrapping doesn't give you as much latency. Is that right? 24 pages in, I see this: o In rings utilizing the wrapping protection, each node detects the failure or receives the RPS request as the destination node MUST perform the switch from/to the working ring tunnels to/from the protection ring tunnels if it has no higher priority active RPS request. o In rings utilizing the short wrapping protection, each node detects the failure or receives the RPS request as the destination node MUST perform the switch only from the working ring tunnels to the protection ring tunnels. so I'm pretty sure there are differences beyond what I was seeing, earlier in the document. And, of course, I'm not sure what the effect of choosing steering over wrapping/short wrapping would be, for my users, but that can wait until we talk about wrapping and short wrapping ... At a minimum, I'd like to see guidance for operators in choosing among the three protection mechanisms. Why would they choose any one of the three? I also note that this MUST seems to be repeated using different words in section 5.1, as All nodes in the same ring MUST use the same protection mechanism, Wrapping, steering or short-wrapping. If that's saying the same thing, one MUST is all you need. ---------------------------------------------------------------------- COMMENT: ---------------------------------------------------------------------- ---------- all the other questions In this text, When the service LSP passes through the interconnected rings, the direction of the working ring tunnels used on both rings SHOULD be the same. For example, if the service LSP uses the clockwise working ring tunnel on Ring1, when the service LSP leaves Ring1 and enters Ring2, the working ring tunnel used on Ring2 SHOULD also follow the clockwise direction. I'm not understanding why this is a SHOULD, and not a MUST. If the direction of the working ring tunnels used on both rings is not the same, does this still work? If it still works, why does this matter? But, either way, you might usefully say something about why this isn't always the right thing to do, even if you just give one example. The point of SHOULD is that implementers make their own informed decisions, so providing information that will inform those decisions seems important. I wanted to call out Ring switches MUST be preempted by higher priority RPS requests. For example, consider a protection switch that is active due to a manual switch request on the given link, and another protection switch is required due to a failure on another link. Then an RPS request MUST be generated, the former protection switch MUST be dropped, and the latter protection switch established. MSRP mechanism SHOULD support multiple protection switches in the ring, resulting in the ring being segmented into two or more separate segments. This may happen when several RPS requests of the same priority exist in the ring due to multiple failures or external switch commands. as really good examples of the kind of text I think would help the places in this document ("For example", "This may happen when") where no examples are given. Thanks for providing those examples! Ouch. Do I understand from o Protection Switching Mode (M): This 2-bit field indicates the protection switching mode used by the sending node of the RPS message. This can be used to check that the ring nodes on the same ring use the same protection switching mechanism. The defined values of the M field are listed as below: +------------------+-----------------------------+ | Bits (MSB-LSB) | Protecton Switching Mode | +------------------+-----------------------------+ | 0 0 | Reserved | | 0 1 | Wrapping | | 1 0 | Short Wrapping | | 1 1 | Steering | +------------------+-----------------------------+ that you already have three protection mechanisms, and have only one possible codepoint to allocate for any future optimizations? Assuming that "0 0" can be unReserved ... Could you clarify what "anyway" means in this text? When multiple MS RPS requests exist at the same time addressing different links and there is no higher priority request on the ring, no switch SHOULD be executed and existing switches MUST be dropped. The nodes MUST signal, anyway, the MS RPS request code. I'm seeing that the commands like LP described in section 5.2.1.1 are used in the document before these (I'm serious) helpful and clear explanations appear. If it's possible to move section 5.2.1.1 up in the document, that would be great, but if it isn't possible, a forward pointer would be helpful to readers who don't already know what the command abbreviations mean. I'm really confused by this SHOULD: The PSC protocol [RFC6378] is designed for point-to-point LSPs, on which the protection switching can only be performed on one or both of the end points of the LSP. The RPS protocol is designed for ring tunnels, which consist of multiple ring nodes, and the failure could happen on any segment of the ring, thus RPS SHOULD be capable of identifying and handling the different failures on the ring, and coordinating the protection switching behavior of all the nodes on the ring. I suspect that's because it's not a 2119 SHOULD, but if people think it is, I wouldn't mind understanding why. Section 5.3, "RPS and PSC Comparison on Ring Topology" is really helpful, but it appears 43 pages in. Given that I'd expect people to be asking why they should implement a new protection switching protocol when they've already implemented PSC, I'd think this would be much more useful, early in the document. I'm somewhat confused about the code point allocation strategy in this text: The RPS Request Field is 8 bits, the allocated values are as follows: Value Description Reference ------- --------------------------- --------------- 0 No Request (NR) this document 1 Reverse Request (RR) this document 2 unassigned 3 Exercise (EXER) this document 4 unassigned 5 Wait-To-Restore (WTR) this document 6 Manual Switch (MS) this document 7-10 unassigned 11 Signal Fail (SF) this document 12 unassigned 13 Forced Switch (FS) this document 14 unassigned 15 Lockout of Protection (LP) this document 16-254 unassigned 255 Reserved My first question is, why the highest priority RPS value is 15, given that the field is 8 bits wide. If anyone ever needs to add a code point higher than the highest priority code point, will that work well? I can imagine code that says "if operation_priority is greater than highest_priority, it's an error", for example. I may have other questions depending on your answer, but let's start there.
- [mpls] Spencer Dawkins' Discuss on draft-ietf-mpl… Spencer Dawkins
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Huub van Helvoort
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Spencer Dawkins at IETF
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Alvaro Retana (aretana)
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Huub van Helvoort
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Spencer Dawkins at IETF
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Spencer Dawkins at IETF
- Re: [mpls] Spencer Dawkins' Discuss on draft-ietf… Dongjie (Jimmy)