Re: [Pce] Remaining BRPC last call comment

[Meral Shirazipour <meral.shirazipour at polymtl.ca>]

Hi,
   Please see inline...

> I agree with Adrian, the domain-sequence should be indicated
> explicitly and kept consistency when passing through.

Again, I believe this is a plus but not a must.
Some PCCs might not even have the capability to come up with such a domain
sequence information.

> But I don't understand "each PCE along the path would decide of the
> "best next domain" to traverse next" , I suppose the domain-sequence
> is kind of 'global optimality' by all means; then how does a PCE
> decide  the 'best next domain', how do you define the term 'best', and
> it might be 'local optimality' in most times as Adrian's example
> indicates.
Of course the term "best" would be of local significance, if the PCE making the
decision does not have a global end to end view.

> >   On the other hand, I think it remains crucial to be able to indicate the
> >  domain(s) to exclude in the sequence of domains (for diversity or
> political
> >  reasons).
>
> If  the domain-sequence is decided by all means, then it might already
> exclude the unwanted domain(s) I guess.

Again  I don't agree here. Not all PCCs will be able to come up with the domain
sequence information. However, and mostly for diversity reasons, they will know
the domains/PCEs they want to avoid.

> Regards,
> Peng
>

Regards,
Meral


> On Mon, Mar 31, 2008 at 5:45 PM, Meral Shirazipour
> <meral.shirazipour at polymtl.ca> wrote:
> > Hi Adrian,
> >   I do not disagree with you that it would be a plus to be able to
> 'sometimes'
> >  indicate the sequence of domains we want the path to go through; however I
> >  understood that in most cases, each PCE along the path would decide of the
> >  "best next domain" to traverse next.
> >
> >   On the other hand, I think it remains crucial to be able to indicate the
> >  domain(s) to exclude in the sequence of domains (for diversity or
> political
> >  reasons).
> >
> >  Regards,
> >  Meral
> >
> >
> >
> >  Selon Adrian Farrel <adrian at olddog.co.uk>:
> >
> >
> >
> >  > Hi JP,
> >  > Thanks for convergence.
> >  >
> >  > There is just one remaining issue for discussion.
> >  > I've reproduced the thread here, but all the discussion is at the end.
> >  >
> >  > Cheers,
> >  > Adrian
> >  >
> >  > >>>> Section 5
> >  > >>>>
> >  > >>>> Is it assumed that PCE(i) knows which domain the requesting
> >  > >>>> PCE(i-1) represents?
> >  > >>>> The reverse is obviously true because PCE(i-1) must select
> >  > >>>> PCE(i) on this basis, but it seems to me that the PCReq could
> >  > >>>> useful identify the neighboring (upstream) domain. This would
> >  > >>>> be particularly useful in the case of a requesting PCE that
> >  > >>>> represents multiple domains since it will allow PCE(i) to know
> >  > >>>> which are the BN-en(i) nodes.
> >  > >>>>
> >  > >>>> Conversely, if PCE(i) has computation capabilities for multiple
> >  > >>>> domains, it will need to be told which for domain(i) it should act.
> >  > >>>> So we either need protocol extensions (requesting domain,
> >  > >>>> computation domain) or we need to be told how to use existing
> >  > >>>> protocol fields for this purpose.
> >  > >>>>
> >  > >>>> Furthermore :-( how although the sequence of domains is known
> >  > >>>> (a priori) by the PCC, we need some way to convey the sequence
> >  > >>>> in the PCReq so that PCE(i) knows that the next domain in the
> >  > >>>> sequence is domain(i+1).
> >  > >>>> If this can be achieved by existing protocol mechanism, you need
> >  > >>>> to describe the procedures. If it can't be done, we need protocol
> >  > >>>> extensions.
> >  > >>>
> >  > >>> We make the assumption that the sequence of domains is pre-
> >  > >>> determined or discovered by some means that is outside of the
> >  > >>> scope of this document.
> >  > >>> You're right that we do not say that the sequence of PCE is also
> >  > >>> predetermined or discovery by some means. Do you want us to
> >  > >>> explicitly add this assumption?
> >  > >>>
> >  > >>> Consider the two following cases:
> >  > >>> 1) Inter-area: obvious
> >  > >>> 2) Inter-AS: if the sequences of domain and related PCEs are known,
> >  > >>> there is no need for protocol extensions except if we want to
> enforce
> >  > >>> the sequence of PCEs, which can be done thanks to the PCE-ID
> >  > >>> object defined in
> >  > >>> http://www.ietf.org/internet-drafts/draft-ietf-pce-monitoring-01.txt
> >  > >>> We could include the PCE-ID object definition in BRPC and add
> >  > >>> some text here.
> >  > >>> Thoughts?
> >  > >>
> >  > >> Well, I have no problems with "the sequence of domains is known a
> >  > >> priori." In fact, I strongly support it.
> >  > >>
> >  > >> However, to whom is this sequence known?
> >  > >>
> >  > >> Yes, if the ingress PCC or PCE knows the PCEs responsible for each
> >  > >> domain, then it could provide a list of such PCEs. But this is more
> >  > >> information than is implied in "the sequence of domains". My
> assumption
> >  > >> is that the default position is that PCE(i) will select PCE(i+1).
> >  > >
> >  > > This is what we referred to as ³discovered by some means² indeed.
> >  > >
> >  > >> But how does PCE(i) know that the next domain is domain(i+1)? How
> >  > >> is the a priori knowledge passed to PCE(i)?
> >  > >
> >  > > By some means out of the scope of the document.
> >  > > Example: inter-area + PCE on the ABR + PCE discovery
> >  > > (RFC 5088/5089).
> >  > > There are other mechanisms available but out of the scope of this
> >  > > document.
> >  > >
> >  > >> This is important, because knowing the sequence of PCEs is not enough
> >  > >> to know the sequence of domains. A PCE may serve more than one
> >  > >> domain.
> >  >
> >  > OK
> >  > The difference in our view point is, I think, what we mean by "the
> series of
> >  > domains is known a priori." Let's use ASes as the example, because it is
> >  > slightly more complex.
> >  > Here comes some ASCII Art (TM)
> >  >
> >  > <ascii-art>
> >  >                 ---------
> >  >                | AS-B    |
> >  >  ---------     |         |     ---------
> >  > | AS-A    |----|         |----| AS-D    |
> >  > |         |    |         |    |         |
> >  > |   PCC   |----|         |----|  Egress |
> >  > |         |     ---------     |         |
> >  > |         |       |   |       |         |
> >  > |         |     ---------     |         |
> >  > |         |----| AS-C    |----|         |
> >  > |         |    |         |    |         |
> >  > |         |----|         |----|         |
> >  >  ---------     |         |     ---------
> >  >                |         |
> >  >                 ---------
> >  > </ascii-art>
> >  > Here we have 4 ASes and we want to get from the PCC to the Egress.
> >  > The ASes are interconnected as shown.
> >  > Let's assume that there are five PCEs. PCE-A, PCE-B, PCE-C and PCE-D
> have
> >  > obvious scope.
> >  > PCE-E is special, it is capable of computing paths in AS-B and AS-C.
> >  >
> >  > So, in my book, "the series of domains is known a priori" means that it
> is
> >  > known before path computation starts. That means, either the PCC knows
> the
> >  > series of ASes (say, ABD) or the sequence is known by PCE-A.
> >  >
> >  > In the former case, we need a way for the PCC to pass that information
> to
> >  > PCE-A. Note that this is not a sequence of PCEs. It is a sequence of
> >  > domains.
> >  >
> >  > Let's suppose that PCE-A now knows the series of domains, ABD. Let us
> assume
> >  > that it selects PCE-B as the next PCE in the series. How does it
> communicate
> >  > to PCE-B that the series of domains is ABD and not ABCD? One possible
> way to
> >  > do this, would be to signal the series of PCEs (PCE-A, PCE-B, PCE-D).
> But
> >  > that has some implication of a one-to-one relationship between PCEs and
> >  > domains.
> >  >
> >  > So suppose PCE-A selects PCE-E as the next PCE in the series. How does
> PCE-E
> >  > know the pre-determined series of domains? How does it know that it has
> been
> >  > asked to find a path across AS-B and not AS-C? Do we suppose that this a
> >  > priori knowledge permeates the ether? Or is it passed along with the
> >  > request? Note that the sequence of PCEs PCE-A, PCE-E, PCE-D does not
> help
> >  > because we could still select a path through domains ABCD.
> >  >
> >  > So, I think that PCEP needs to be able to indicate the sequence of
> domains
> >  > that should be traversed. The additional ability to indicate the
> sequence of
> >  > PCEs is a great supplement.
> >  >
> >  >
> >  > _______________________________________________
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> >  >
> >  >
> >
> >
> >
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>



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