Re: [mpls] proposed drafts for aligning MPLS-TP PSC linear protection protocol to transport requirements

["Eric Osborne (eosborne)" <[e-mail]>]

Hi Jeong-dong,

  Thanks for the reply.  Please see inline with EO#.

> -----Original Message-----
> From: Ryoo, Jeong-dong [mailto:[e-mail]]
> Sent: Monday, July 22, 2013 4:15 AM
> To: Eric Osborne (eosborne); D'Alessandro Alessandro Gerardo;
> [e-mail]
> Cc: Huub helvoort ([e-mail]); [e-mail]
> Subject: RE: [mpls] proposed drafts for aligning MPLS-TP PSC linear
> protection protocol to transport requirements
> 
> Hi, Eric.
> 
> Let me answer your 2nd question on SD.
> 
> SD detection methods defined or proposed for packet transport networks
> can be summarized as follows:
> - By OAM performance monitoring tool:
>   SD is raised if packet loss ratio exceeds a threshold during a
> measurement period.
>   Threshold value and measurement period are configured by an network
> operator.
>   This detection method is already defined in ITU-T G.8021 (Ethernet
> equipment spec.)


EO#  Where?  I'm not arguing that it's not in there, I'm just having a hard time finding it.  A search for ETH_CI_SSD doesn't yield much.  If I look for 'signal degrade' I see p. 131 which says that the algorithm is defined in G.8031.  G.8031 says ' How these defects are detected is the subject of the equipment Recommendations'.  I'm looking for something like "Signal Degrade is defined as $foo packet loss or CRC failure over $bar time"....what have I missed?


>   and the equipment spec for MPLS-TP can easily follow the same
> definition.
> - By server layer indication:
>   SD is raised if a server layer below MPLS-TP reports SD condition on
> its own layer.
> - By CCM packet counting:
>   SD is raised if the loss ratio of CCM packets exceeds a threshold
> during a measurement period.
> 
> Regardless of how to detect SD, any protection switching documents
> should describe the protection switching operation once such a SD is
> declared.
> 
...
> Regarding the multiple levels of SD:
> It is certainly possible to define multiple levels of SD.
> But, as far as the protection switching is concerned, it just needs to
> know if SD is signaled to protection switching process or not.
> It would be a network operator's choice at what level of SD he wants his
> network protection to switchover.
> In other words, what triggers protection switching is SD or no SD. It is
> yes or no decision.


EO#  I am not aware of any document which suggests that a server layer SD should be treated as a client layer SD.  In the IP world, if we have SD on a transport interface that is generally used to bring the interface down (i.e. SF).  This is the sort of thing I'd like to see in more detail, as SD in the packet world is a new concept and we can't just assume that it will work the same everywhere because we define state machine points for it.

The point about CCM is a good one.  Let's say we come up with a clever SD mechanism with two thresholds, call them major and minor.  For discussion purposes they could be simple error ratios, e.g. 1:10^6 and 1:10^9.  But they could be more powerful than that (flow type, flow length, error burst size, etc).

If we want to have SD-Major and SD-Minor inputs as separate triggers for PSC, we may want them at different points.  Perhaps  (leaving out the Working path for ease of reading)

LO
FS
SF-P
SD-P-Major
MS
SD-P-Minor


This seems like a perfectly reasonable thing to want.  


Even if we don't have multi-tier SD, even the single-tier SD needs to be defined before we can decide how to respond to it.

> The proposed draft covers SD-triggered protection no matter what kinds
> of SD detection methods are used.
> 
> 
> SF can also be viewed as having multiple levels of SF as the network
> operator can also make a choice on the period/interval of CCM messages.


EO# 
I'm not sure what that would look like.  'Fail' is a pretty binary thing.  'Degrade' is a continuous variable, as it can be anything from 'a little bad' to a 'a whole lot of bad but not quite fail'.

> If CCM is disabled, AIS from a server layer can be used as a trigger for
> protection switching. So and so forth.

EO#  AIS from the server layer only gets you SD from the first hop of the underlying server path.




eric

> However, protection switching document does not define how to detect SF
> in anywhere.
> Similary, protection switching document does not define how manual
> switch and forced switch commands
> are initiated in a management system and signaled to protection
> switching process.
> 
> Again, in my opinion, the draft on SD protection can accommodate any SD
> detection methods.
> 
> Best regards,
> 
> Jeong-dong
> 
> 
> 
> 
> 
> 
> ________________________________
> 
> From : "Eric Osborne (eosborne)" <[e-mail]>
> Sent : 2013-07-20 02:48:28 ( +09:00 )
> To : D'Alessandro Alessandro Gerardo
> <[e-mail]>, [e-mail] <[e-mail]>
> Cc : Huub helvoort ([e-mail])
> <[e-mail]>, [e-mail]
> <[e-mail]>
> Subject : Re: [mpls] proposed drafts for aligning MPLS-TP PSC linear
> protection protocol to transport requirements
> 
> 
> Hi Alessandro-
> 
> Thanks for this; the threads I started some time back seem to have died
> down, it's good to get them going again.
> I have two things I never quite understood, can you clarify them for me?
> 
> 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.
> 
> 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. 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. 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?
> 
> 
> ii) Why the push to standardize the SD state changes before we've
> defined SD? I certainly agree that handling signal degrade is a good
> idea, but coming up with a definition for it has been challenging. What
> happens if we change the FSM to handle it, then come up with something
> more sophisticated (say, multiple levels of SD) that doesn't quite fit
> with the FSM changes?
> 
> 
> 
> thanks!
> 
> 
> 
> 
> 
> eric
> 
> 
> > -----Original Message-----
> > From: [e-mail] [mailto:[e-mail]] On Behalf
> Of
> > D'Alessandro Alessandro Gerardo
> > Sent: Wednesday, July 17, 2013 3:23 PM
> > To: [e-mail]
> > Cc: Huub helvoort ([e-mail]); [e-mail]
> > Subject: [mpls] proposed drafts for aligning MPLS-TP PSC linear
> > protection protocol to transport requirements
> >
> > Dear all,
> > we would like socializing the herebelow drafts that were submitted
> some
> > months ago with the aim to align PSC protocol (RFC 6378) to ITU-T
> > transport requirements. I would appreciate your comments about the
> > proposed mechanisms and behaviours.
> >
> > draft-rhd-mpls-tp-psc-priority-00
> > draft-cdh-mpls-tp-psc-non-revertive-00
> > draft-rhd-mpls-tp-psc-sd-00
> > draft-dj-mpls-tp-exer-psc-01 / draft-osborne-mpls-psc-alive-00
> >
> > The above drafts cover most of items highlighted in ITU-T liaisons
> about
> > PSC and they propose solutions in line with MPLS-TP transport
> > requirements.
> > A list of main liaisons exchanged between ITU-T and IETF with the aim
> to
> > align PSC behavious with ITU-T transport requirements for linear
> > protection are given below:
> > https://datatracker.ietf.org/liaison/1162/ (June 2012)
> > https://datatracker.ietf.org/liaison/1205/
> > (October 2012)
> > https://datatracker.ietf.org/liaison/1229/
> > (January 2013)
> > https://datatracker.ietf.org/liaison/1234/
> > (February 2013)
> > https://datatracker.ietf.org/liaison/1256/
> > (May 2013)
> >
> > Some details abou the proposed drafts for align PSC behaviour with
> > transport requirements:
> >
> > draft-rhd-mpls-tp-psc-priority-00 proposes swapping the priorities
> > between FS and SF-P (see section 4.3.2 of rfc6378).
> > Among the others, behaviors that will be fixed with the proposed
> update
> > are:
> > Use case A) At first, working path(WP) and protection path(PP) are
> > normal. Then, Forced Switch(FS) command is issued for maintenance on
> the
> > WP and the traffic moves from WP to PP. When Signal Fail occurs on PP,
> > service cannot recover and is interrupted. This could occur for
> example
> > as a result of accidentally un-plugging a PP fiber.
> > Use case B) If there is an existing signal fail on a protection path
> > (SF-P),and FS command is issued by accident the traffic on WP will
> move
> > to PP. This results in an interruption of service from which you will
> > not automatically recover, because PSC should not have switched the
> > traffic from WP to PP.
> > Discussion about this draft led to the proposal to modify RFC 4427
> that
> > was "written correctly though lacking in detail causing mis-
> > interpretation" that led to the current PSC set of priority that the
> > above draft is proposing to modified and to align to the required
> > transport behavior. draft-helvoort-ccamp-fs-priority-00 has been
> > submitted to CCAMP for clarifying the definitions related to Manual
> > Switch and Forced Switch and their usage relative to priorities.
> > The way this behavior has to be incorporated into the PSC has to be
> > discussed. The text proposes to replace the current behavior with the
> > new one. If there is consensus to procede in that way this can bring
> to
> > a simple and effective way to operate the protocol.
> >
> > ----------------------------------------------------------------------
> > draft-cdh-mpls-tp-psc-non-revertive-00 contains the updates to RFC6378
> > to change non-revertive operations to behaves in the same way
> > irrespectively of the trigger of protection switching (fault or
> operator
> > command FS, MS). Consequently an operator command, Manual Switch to
> > Working (MS-W) a.k.a "Manual switch-over for recovery LSP/span" is
> also
> > added to enable this behavior. From an operational point of view, MS
> to
> > working path has also to be supported to be able to initially align at
> > both sides in case of non-revertive switching mode. MS to working path
> > is defined in RFC 5654, requirement 83.
> >
> > The proposed MS-W command is of equal priority to the existing MS-P
> > command, and there is text to handle the simultaneous or sequential
> > occurrence of two equal-priority commands. This behavior, already
> > adopted in other transport network protection switching protocol, can
> be
> > used for other addition to the protocol in the future.
> >
> > ----------------------------------------------------------------------
> > draft-rhd-mpls-tp-psc-sd-00 provides extensions to the PSC state
> > machine to handle Signal Degrade (SD). It does not define SD or
> provide
> > scope around where or how SD may be used similarly as it already
> happen
> > in the draft in handling other defects like SF (Signal Failure).
> > In MPLS-TP survivability framework [RFC6372], a fault condition
> > includes both Signal Fail (SF) and Signal Degrade (SD) that can be
> used
> > to trigger protection switching.
> > While the standardization lack of an SD definition and detection
> > mechanisms, the relevant behaviors in terms of protection actions may
> > already be defined.
> >
> > ----------------------------------------------------------------------
> > draft-dj-mpls-tp-exer-psc-01 proposes adding the EXER/RR commands to
> > test if the APS communication is operating correctly. In other words
> > both APS process logic including state machine and APS channel on
> > protection path, without service disruption and without affecting any
> > protection operation, unless the protection transport entity is in
> use.
> > This command is documented in R84 of [RFC5654] and it is part of ITU-T
> > transport requirements.
> > An alternative proposal is documented in the Appendix B of RFC6378
> that
> > utilizes the Lockout of Protection (LO) or Forced Switch (FS) in
> > combination of OAM functionalities. However, it has some functional
> > limitation and has a potential risk of losing traffic as a signal
> > failure might occur during the exercise operation. In that case, LO or
> > FS has to be canceled to allow the PSC protocol to provide proper
> > switching.
> > A further alternative proposal is documented in draft-osborne-mpls-
> psc-
> > alive-00 that anyway show some functional limitations because cannot
> > validate the PSC state machine status and probably the Local Request
> > logic.
> >
> >
> > The authors encourage the IETF experts to comment on these drafts,
> > eventually proposing other options/mechanisms that can satisfy the
> same
> > requirements.
> > Best regards,
> > Alessandro, Huub, Jeong-dong, Taeksid
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