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

[Stewart Bryant <[e-mail]>]

On 22/07/2013 12:19, Huub van Helvoort wrote:
>
>>
>> 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.

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.

Stewart