Re: [mpls] Spencer Dawkins' Discuss on draft-ietf-mpls-tp-shared-ring-protection-05: (with DISCUSS and COMMENT)
Huub van Helvoort <huubatwork@gmail.com> Thu, 25 May 2017 10:02 UTC
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To: Spencer Dawkins <spencerdawkins.ietf@gmail.com>, The IESG <iesg@ietf.org>
Cc: draft-ietf-mpls-tp-shared-ring-protection@ietf.org, Eric Gray <Eric.Gray@Ericsson.com>, mpls-chairs@ietf.org, mpls@ietf.org
From: Huub van Helvoort <huubatwork@gmail.com>
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Date: Thu, 25 May 2017 12:02:06 +0200
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Subject: Re: [mpls] Spencer Dawkins' Discuss on draft-ietf-mpls-tp-shared-ring-protection-05: (with DISCUSS and COMMENT)
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Hello Spencer, Thank you for your review of our draft. Please find my response in-line [Huub] > Spencer Dawkins has entered the following ballot position for > draft-ietf-mpls-tp-shared-ring-protection-05: Discuss > > 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. [Huub] thank you! > 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. [Huub] OK, understood. > ---------- 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"). [Huub] if the MRPS protocol in any node detects RPS message with a mode that was not provisioned in that node a failure of protocol will be reported, and the protection mechanism will not be activated. > 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. [Huub] wrapping is a mechanism that can be used in case an LSP is dropped in several nodes (p-2-mp application). In this case the traffic will still have to reach every node in the ring. Short wrapping can be used only in p-2-p application. Now the traffic can be dropped at the moment the egress node is reached. Steering has the least additional propagation delay, but during a short time the traffic may be duplicated, which may not be desirable is some applications. > 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? [Huub] more explanatory text can be added, it could be in the introduction proposed by Alvaro. > 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. [Huub] OK, point taken. > ---------------------------------------------------------------------- > COMMENT: > ---------------------------------------------------------------------- > > ---------- all the other questions [Huub] I will answer your questions later (I am currently on holiday and risk a divorce if I don't stop now :-( ) Best regards, Huub. > > 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. > > -- ================================================================ Always remember that you are unique...just like everyone else...
- [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)