Re: [mpls] PSC: draft-dj-mpls-tp-exer-psc

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

Yaacov, hi,

You wrote:

> I would like to point out two things about the definition given in G.870
> for the EXER function, as given in the text quoted by Eric -
>
> 1. The definition clearly states that EXER is optional!

I have talked to the editor of G.870, he confirmed that this
statement is his interpretation of the following text in G.841
clause 7.1.2.1 (switch commands for MSP [Multiplex Section Protection])
"The exercise functionality may not exist in all MSP functions."

This is from the 1998 version of G.841, and is applicable to MSP
implementations that existed before 1998.

All other protection schemes in G.841 do not mention this exception.

Clause 7.2.4.1.2 even emphasizes the importance of EXER:
"the exerciser function is even more essential" (for ring protection).

> 2. It seems clear (at least to me) that the real reason for the command
> was to test the selectors that were present in the SDH hardware, (i.e.
> "switch is not actually completed, i.e., the selector is released by an
> exercise request") and therefore its relevance to packet switch networks
> seems questionable.

When you were editor of G.8032 (Ethernet ring protection) the
following text was written:
"Exercise Signal – Exercise of the R-APS protocol. The signal is
chosen so as not to modify the position of the blocked ring port."

This makes again clear that the reason for the EXER signal is to
exercise the protocol, without affecting traffic.

Regards, Huub.


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