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 > <[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
