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

[Huub van Helvoort <[e-mail]>]

Hello Steward,

You replied:

>>> 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.
>
> Huub,
>
> I get a terrible Heisenberg feeling when you explain this to me.

[Huub] I think Werner was referring to quantum mechanics...

> It is not clear whether or not including the EXER state and executing
> it from time to time poses a greater risk than assuming that :
> provided the daemon is running and the variables are as expected,
> the code will execute correctly.

[Huub] if there is a simple way to verify that the NR messages
received periodically are not caused by a malfunctioning
state-machine, the risk of of detecting a malfunctioning
state-machine at the moment an SF/SD event occurs is reduced considerably.

Regards, Huub.



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