Subject: RE: [PWE3] Segmented PWs

From: [email-masked]
Date: Fri, 8 Jul 2005 15:45:13 +0100

Colleagues,

Regarding draft-ietf-pwe3-segmented-pw there is a definate peer
partition interworking scenario when different PW segments are
transported over different server layer technologies/networking modes
using different control protocols. I would therefore like to see some
text included which talks about the performance that the MS-PW is able
to receive (or provide) when the server layer partitons are not
identical (either technology/mode or control protocol wise).  Section 7
may be an appropriate place for such a section of text.

I include below some text that I am contributing to the next ITU-T Study
Group 13 meeting (Question 7) for inclusion into draft Recommendation
Y.ppi ("Principles and requirements for peer partition interworking") on
this issue which could be used as a baseline for some text to be
included in draft-ietf-pwe3-segmented-pw.  I understand that some of the
terminology may need to be changed and that not all of the text may be
appropriate for inclusion in draft-ietf-pwe3-segmented-pw.

Note that Y.ppi is a generic document that is not specific to any one
technolgy, so in the text below what is referred to as the client layer
maps to the MS-PW which is transported over various server layer
partitons (each of which map to either MPLS or L2TPv3/IP depending on
the MS-PW scenario).

I hope this is of use.

Ben

"In a client/server relationship the performance of the client layer is
equal to the performance of the server layer plus any impairments
introduced by the client layer itself.  Therefore it is not possible for
the client layer to provide better performance than the server layer
over which it is transported.

Therefore, it is necessary to carefully consider the order in which
different layer networks are stacked upon each other within a 'network
stack' in order to provide the topmost client layer (the 'service') with
the performance that it requires.  This performance inheritance within a
client/server relationship is vertical because the client layer is
vertically stacked upon its server layer.

Note: Due to this vertical performance inheritance and the different
performance provided by, and the characteristics of, each networking
mode it is generally advisable to stack modes that less efficiently
provide dedicated bandwidth/performance on top of modes that more
efficiently provide dedicated bandwidth/performance.

When performing peer partition interworking the client layer inherits
the performance of the server layer partition that provides the worst
performance of all the peered server layer partitions over which the
client layer is transported.  Therefore it is not possible for the
client layer to receive (or provide) better performance than the worst
performing of the peered server layer partitions over which it is
transported.

Therefore, it is necessary to carefully consider which server layer
modes (and/or technologies) it is appropriate to peer with one another
in order to provide the topmost client layer (the 'service') with the
performance that it requires.  This is a horizontal performance
relationship because the server layer partitions are peered with each
other horizontally."

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From: Stewart Bryant <[email-masked]>
Date: Fri, 08 Jul 2005 16:16:05 +0100

Benjamin

Yes, some text should be included to discuss this. Indeed it
we should have explored this area some more when writing
RFC 3985. We do talk about the emulation "being good
enough to do the job" rather than perfect, but do not
properly explore the impact of the PSN behavior.

However there is a related point worthy of further
exploration. In some instances the emulated service may be
better than client could have achieved in its own right,
for example by being capable of faster repair, repair against
a greater number of failures, lower error rate etc.

- Stewart

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From: Scott W Brim <[email-masked]>
Date: Fri, 08 Jul 2005 14:58:31 -0400

> over which it is transported.

Ben, as I think we discussed somewhere, "performance" is a service
issue, and ultimately the only measurement that matters is at the
application.  That is, some of the performance characteristics which
one might treasure in a low level transport technology might be
irrelevant to the services you are actually trying to deliver over the
higher layer.  If you want to put text like this in, let's be clear on
what it's important to measure.  It depends on the particular
performance metric(s) you care about in the client layer network.  As
Stewart points out, some of those performance metrics are better met
in a PW environment.  Does that fit with your premise?

> efficiently provide dedicated bandwidth/performance.

Not directly deducible from previous text.  Is the ability to provide
dedicated bandwidth the only interesting metric?

> performing of the peered server layer partitions over which it is

per-metric

> relationship because the server layer partitions are peered with each

This I like :-)

swb