Re: [mpls] proposed drafts for aligning MPLS-TP PSC linear protection protocol to transport requirements
Malcolm.BETTS@zte.com.cn Wed, 24 July 2013 15:24 UTC
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Cc: "Huub helvoort (huub.van.helvoort@huawei.com)" <huub.van.helvoort@huawei.com>, "mpls@ietf.org" <mpls@ietf.org>, mpls-bounces@ietf.org
Subject: Re: [mpls] proposed drafts for aligning MPLS-TP PSC linear protection protocol to transport requirements
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Hi all, I agree with the points raised by Huub. One further consideration, in a typical network without the exerciser the APS will essentially sit in an idle state sending "keep alive" messages for years, until a fault is detected on the working path. At this time APS must "wake up" and execute a switch to protection. The purpose of the exerciser is to verify that the APS state machine has not suffered some obscure fault condition which otherwise would remain undetected until a second fault occurs. It is these "silent failures" cause service outages. Regards, Malcolm Huub van Helvoort <huubatwork@gmail.com> Sent by: mpls-bounces@ietf.org 22/07/2013 07:11 PM Please respond to huubatwork@gmail.com To "Eric Osborne (eosborne)" <eosborne@cisco.com> cc "Huub helvoort \(huub.van.helvoort@huawei.com\)" <huub.van.helvoort@huawei.com>, "mpls@ietf.org" <mpls@ietf.org> Subject Re: [mpls] proposed drafts for aligning MPLS-TP PSC linear protection protocol to transport requirements Hello Eric, You replied: > Inline with EO#, trimmed a bit. My response inline [Huub2] >> >> There was atwo week ITU-T plenary meeting, and after that I took >> (and still have) a holiday. > > If you're on holiday, why are you working? [Huub2] It is too hot outside, and I had dug out my logbooks from before 1984, I want to to return to storage as soon as possible ^_^ > I thought that was a uniquely American trait. [Huub2] maybe it is infectious... > Is this stuff that much fun to you? :) [Huub2] my wife does not like the piles of logbooks and I like to share my knowledge > ... > >>> i) can you explain EXER at a higher level? I'm not looking for a >>> description of the state machine changes, and I'm not looking for the >>> one line "It allows the FSM to be tested". We have all of that in >>> the draft and in the equivalent ITU specs. >>> >>> What I'd like to understand about EXER is where it came from. The >>> ITU specs that define it are pretty hard to follow, they seem to >>> assume the reader already knows what EXER is and what problem it >>> solves. It feels very much like a mechanism used to catch a very >>> specific implementation bug, back when transport gear was far less >>> debuggable than what we have today. >> >> [Huub] EXER was not designed/intended to be used for bug finding >> although it will detect problems with implementation. >> >> [Huub] EXER was designed to verify that the state-machine at the >> far end is able to respond to APS/PSC messages it receives from >> the local end. >> Even though state-machines should be tested extensively, there is >> no 100% warranty. It can still have stopped due to external >> circumstances, be in a deadlock due to unforseen order of events, >> etc. > > EO# I agree with your last two statements. To me, though, that > very text is a reasonable argument against EXER. > There is no mechanism within a protocol which can guarantee that > the entire state machine is 100% perfect. [Huub2] indeed > A node could be able to respond to EXER/RR but be unable to process > a real failure properly, either due to bug or to (as you indicate) > some unforseen combination of external events. [Huub2] in the case you describe the node has to send periodical NR _AND_ respond to the EXER while not responding to a real SF/SD, this would be a very complex state-machine fault. > The class of problems which EXER can catch but which will not be > detectable by other means (e.g. CC/CV, see below) seems pretty small. [Huub2] the EXER will detect the most common "stuck-at" problems. > In the transport world, what sort of problems does EXER _actually find_? > I'm not asking about things it _could_ find, but things that it *does*. [Huub2] stuck-at, deadlock >> [Huub] note that APS/PSC should continue to operate even if no >> control plane is available. > > EO# I agree, but this is not relevant to the discussion at hand. > Lots of work was done in TP to ensure that it would function without > the ability to forward IP packets (which I think is what you mean by > 'no control plane'). None of that has anything to do with the set > of messages and states in PSC. [Huub2] I did mean to say that the exercise request caanot be sent via the control plane, so it should be in the dataplane. Since the APS/PSC state-machine is exercised the EXER command should be part of the APS/PSC protocol. >> The EXER is to enable an operator to >> take corrective action before a protection switch request fails >> and the 50ms switch time is not met. >> >>> No other state machines that I'm familiar with (RSVP, LDP, BGP, OSPF, >>> ISIS) have explicit signaling in them just to ask the neighbor >>> whether it *would* be broken if if were, in the future, to be given a >>> particular input. >> >> [Huub] All these rely on a control plane, some of then include >> "keepalive" messages to see if the far end responds. > > EO# PSC has keepalives; see section 4.1 of rfc6378 - "The purpose of > the continual messages is to verify that the PSC session is still alive." [Huub2] it only proves that the state-machine can send NR priodically, it does not prove that it wil respond to any external/remote events. The EXER will give much more confidence of being alive. > The next sentence says "If no valid PSC message is received, over a > period of several continual messages intervals, the last valid > received message remains applicable." As an implementor, what that > means to me is that I time out only after the loss of a few > retransmissions. [Huub2] yes, this is part if the PDU validation process >>> Part of my reluctance to get behind EXER has been >>> that I don't feel comfortable with the idea of keeping a 30-year-old >>> workaround in a protocol. >> >> [Huub] it is NOT a workaround, it is an essential part of the >> protocol. > > EO# In a TDM world, I can see this point. APS is carried in the > frame header, so the receipt of an APS message doesn't mean that > there's any intelligence behind it as it could just be the hardware > repeating the last APS overhead that it sent. If a PSC message is > sent it must have been sent deliberately, as during steady state > we have periodic retransmissions. Do you agree? [Huub2] similarly hardware (or software) can contiouously send NR messages. So I agree that there is no difference between TDM and packet APS/PSC. >>> Is there more to it than that? Have I >>> misread and misunderstood EXER? Does modern transport gear ever >>> actually detect a problem via EXER/RR that wasn't obvious to the >>> operator using other means? >> >> [Huub] if there is no control plane I have no other means. >> What means are available to verify if a state-machine that is >> in a stable state is still functioning? > > EO# Periodic retransmission of current state by the remote side. > This performs the exact same function as a keepalive in any other > protocol, and I think we agree that the keepalive function in > protocols such as OSPF is sufficient to ensure the sanity of the > remote end. [Huub2] see above for the possibilty of stuck-at repeating only the NR message. > Many, many protocols use keepalives as a sort of belt-and-suspenders > failure detection mechanism. In the IP world these mechanisms are > far, far less useful than they used to be as we now have BFD. [Huub2] I would consider the "hello" message to have the same purpose as the EXER message: check if the remote BFD session is still up. > That brings us to CC/CV. Lots of work was done to ensure that it > did not require IP to function. I cannnot believe that any TP > implementation would ship without some sort of CC/CV, and isn't > that a strong enough mechanism to detect the failure of the remote > end? [Huub2] CC/CV only checks the conductivity and connectivity of a path It has no relation at all with the APS/PSC state-machine other than in case CC/CV causes a signal fail defect the resulting SF event triggers the APS/PSC state-machine. Regards, Huub. -- ***************************************************************** 请记住,你是独一无二的,就像其他每一个人一样 _______________________________________________ mpls mailing list mpls@ietf.org https://www.ietf.org/mailman/listinfo/mpls
- [mpls] proposed drafts for aligning MPLS-TP PSC l… D'Alessandro Alessandro Gerardo
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Eric Osborne (eosborne)
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Ryoo, Jeong-dong
- Re: [mpls] proposed drafts for aligning MPLS-TP P… S. Davari
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Ryoo, Jeong-dong
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Huub van Helvoort
- Re: [mpls] proposed drafts for aligning MPLS-TP P… D'Alessandro Alessandro Gerardo
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Stewart Bryant
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Eric Osborne (eosborne)
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Eric Osborne (eosborne)
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Huber, Thomas J.
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Eric Osborne (eosborne)
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Huub van Helvoort
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Ryoo, Jeong-dong
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Ryoo, Jeong-dong
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Huub van Helvoort
- [mpls] 回复: proposed drafts for aligning MPLS-TP P… Larry
- Re: [mpls] 回复: proposed drafts for aligning MPLS-… Yaacov Weingarten
- Re: [mpls] 回复: proposed drafts for aligning MPLS-… Yaacov Weingarten
- Re: [mpls] 回复: proposed drafts for aligning MPLS-… Ryoo, Jeong-dong
- Re: [mpls] 回复: proposed drafts for aligning MPLS-… Malcolm.BETTS
- Re: [mpls] proposed drafts for aligning MPLS-TP P… Malcolm.BETTS