I agree with Adrian, the domain-sequence should be indicated explicitly and kept consistency when passing through.
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. > 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. Regards, Peng On Mon, Mar 31, 2008 at 5:45 PM, Meral Shirazipour <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>: > > > > > 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. > > > > > > _______________________________________________ > > Pce mailing list > > [email protected] > > https://www.ietf.org/mailman/listinfo/pce > > > > > > > > _______________________________________________ > Pce mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/pce > _______________________________________________ Pce mailing list [email protected] https://www.ietf.org/mailman/listinfo/pce
