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
