RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowire Stitchi ng Function in draft-stein-pwe3-pwce2e-00.txt
"Busschbach, Peter B (Peter)" <busschbach@lucent.com> Wed, 12 November 2003 00:18 UTC
From: "Busschbach, Peter B (Peter)" <busschbach@lucent.com>
Subject: RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowire Stitchi ng Function in draft-stein-pwe3-pwce2e-00.txt
Date: Tue, 11 Nov 2003 19:18:35 -0500
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Cc: Alik Shimelmits <alik@AXERRA.com>, "Stewart Bryant (E-mail)" <stbryant@cisco.com>, "Prayson Pate (E-mail)" <prayson.pate@overturenetworks.com>, "PWE3 WG (E-mail)" <pwe3@ietf.org>, "David Sinicrope (E-mail)" <David.Sinicrope@Ericsson.com>, "Yaakov Stein (E-mail)" <yaakov_s@Rad.co.il>
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> -----Original Message----- > From: Sasha Vainshtein [mailto:Sasha@AXERRA.com] > Sent: Tuesday, November 11, 2003 5:57 PM > To: 'Busschbach, Peter B (Peter)' > Cc: Alik Shimelmits; Stewart Bryant (E-mail); Prayson Pate > (E-mail); PWE3 WG (E-mail); David Sinicrope (E-mail); Yaakov > Stein (E-mail) > Subject: RE: [PWE3] CE-to-CE PWs, Hierarchical VPLS and > Pseudowire Stitchi ng Function in draft-stein-pwe3-pwce2e-00.txt > > > Peter, > Thank you for a prompt response. > > I'll check your reference to hierarchical VPLS in > draft-ietf-l2vpn-signaling-00.txt > However, I do not think that Option 3 is really identical to > hierarchical VPLS because the PSF is service-agnostic > and does not look at any part of the native frame. > I would say it sounds like a hierarchical VPWS. My interpretation of HVPLS is that the N-PE (what you call PSF) performs a bridging function (i.e. a service-specific function), where as in DVPLS the N-PE merely splices PWs in a service-agnostic way. > > I would appreciate references to multi-hop LSPs if you can > provide them. RFC 3031, RFC 3209 and RFC 3212 :-) A multi-hop PW is essentially an regular LSP. You may need to extend existing specs to deal with FECs that may contain non-routable information, perhaps along the lines of Ina Minei's draft that we discussed this morning. Apart from that, I don't see anything special. > > Wrt interest - or lack thereof - based on Yaakov's draft, the crucial > question, IMO, is: what justifies support of Option 3 when Option 2 > does the same job as well? Saving one label stack entry on the CE-PE > packet clearly does not. And, from little I know about MPLS UNI > (David is the expert here), it allows you to obtain a valid transport > label without the need to learn the core topology. It is also > a clear-cut > control plane solution, without the need for a new function > (PSF) in the > high-speed data plane. > > Please note that I do not reject Option 3 out of hand. I simply try to > understand its advantages (and disadvantages:-). > > At the same time I feel pretty sure that Option 1 is a dead-end. > > -------------------------------------------------------------- > -------------- > -------- > With best regards, > Sasha Vainshtein > email: sasha@axerra.com <mailto:sasha@axerra.com> > phone: +972-3-7659993 (office) > +972-8-9254948 (home) > +972-58-674833 (cellular) > > > > -----Original Message----- > > From: Busschbach, Peter B (Peter) [mailto:busschbach@lucent.com] > > Sent: Wednesday, November 12, 2003 1:01 AM > > To: Sasha Vainshtein; Yaakov Stein (E-mail) > > Cc: Alik Shimelmits; Stewart Bryant (E-mail); Prayson Pate (E-mail); > > PWE3 WG (E-mail); David Sinicrope (E-mail) > > Subject: RE: [PWE3] CE-to-CE PWs, Hierarchical VPLS and Pseudowire > > Stitchi ng Function in draft-stein-pwe3-pwce2e-00.txt > > > > > > > > > > > -----Original Message----- > > > From: Sasha Vainshtein [mailto:Sasha@AXERRA.com] > > > Sent: Tuesday, November 11, 2003 3:02 PM > > > To: Yaakov Stein (E-mail) > > > Cc: Alik Shimelmits; Stewart Bryant (E-mail); Prayson Pate > > > (E-mail); PWE3 WG (E-mail); David Sinicrope (E-mail) > > > Subject: [PWE3] CE-to-CE PWs, Hierarchical VPLS and > > > Pseudowire Stitching Function in draft-stein-pwe3-pwce2e-00.txt > > > > > > > > > Yaakov and all, > > > I'd like to present the technical problem I have with the > > > current text of draft-stein-pwe3-pwce2e-00.txt and options > > > for solving this problem. > > > > > > My problem is with the following para in Section 3: > > > > > > <quote> > > > The ideal way of handling a PWCE2E packet is to > > > have the CE perform the service specific encapsulation > > > and to prepend the (inner) PW label, but no (outer) MPLS > > > transport labels. The PE, which participates in the provider > > > network signaling, then adds the appropriate MPLS labels > > > as required. > > > <end quote> > > > > > > IMO, it is easy to see why this will not work as written. > > > > > > Please consider the following use case: > > > > > > 1. There are three CEs (CE-1, CE-2 and CE-3) attached > > > to 3 PEs (PE-1, PE-2 and PE3 respectively). > > > 2. In each case the link between CE-i and PE-i is a > > > "single-hop" link (i.e., there are no LSRs in the > > > path, but there may be L2 switches - Ethernet, FR/ATM etc). > > > 3. You want to run two PWs: > > > a) PW-12 runs between CE-1 and CE-2 > > > b)PW-13 runs between CE-1 and CE-3. > > > > > > How can PE-1 (which both these PWs have to cross on their ways > > > to their peer CEs) know which transport labels to add to packets > > > received from CE-1? > > > > > > This decision CANNOT be based on the PW labels in these packets, > > > because these labels are independently allocated by CE-2 and CE-3 > > > respectively, so that nothing prevents them from being equal > > > (accidentally or else). > > > And inspection of the encapsulated data will not help either. > > > > > > (BTW, this is exactly the difference between MPLS and, say, > > > FR or ATM: in the latter cases, DLCI or VCI/VPI values are > > > unique on the attachment link and known to the PE, > > > in the former case they are not.) > > > > > > How can this be fixed? > > > > > > I see three ways to do so. > > > > > > 1. Limit this case to one PW per CE. IMO, this is > > > a very problematic limitation (one could say, that > > > it is ultimately non-scalable!) and does not > > > justify any action wrt the existing documents. > > > > > > 2. Allow the CE to push both the PW and transport labels > > > on top of the payload and PWE3 control info. > > > This would make it equivalent to the "normal" PW,and no > > > technical changes in the architecture doc are required. > > > The transport label can be obtained in many different ways, > > > including usage of the MPLS UNI (as suggested by David > > > Sinicrope). > > > > > > 3. Use PW stitching. This would introduce a new element > > > in the PWE3 architecture - the Pseudo-Wire > > > Stitching Function (PSF) - located in the PEs. > > > The PSF would operate like following: > > > a) Terminate a "usual" PW running between two PEs > > > adjacent to the CEs in question. > > > Note that the PEs must exchange their incoming > > > PW labels for this PW just as in the "normal" case > > > b) Terminate the "spoke" PW between the CE and its adjacent PE. > > > This PW would not require transport labels because > > the CE<-->PE > > > transport LSPs are just one hop, and PHP can be applied > > > to them. > > > The PW labels for the spoke PWs would be exchanged > > > between the CE and > > > its adjacent PE in the usual way. > > > c) Stitch the spoke PW with the PE-PE PW. Such a > stitching would: > > > * Leave the PW packet payload and control info (if > any) intact > > > * Swap the inner label > > > * In one direction (CE-->PE), push the transport > > > label(s) required for the PE-PE PW. > > > Opposite to the PW IWFs, one and only one PSF type is required. > > > The resulting architecture resembles the Hierarchical VPLS > > > defined in > > > draft-ietf-l2vln-vpls-ldp-01.txt (hence the term > "Spoke PW"). Its > > > advantage > > > stems from moving the service-specific IWF functionality > > > towards the CE > > > (and hence > > > may be of special importance for TDM PWs), while possibility of > > > misconfiguration > > > (e.g., connection of IWFs with different service > types) should be > > > considered > > > as a disadvantage. Another disadvantage, of course, is > > > introduction of a > > > new > > > (albeit simple) architectural element in the high-speed > > data plane. > > > > > > IMO, any technical changes in the architecture document > > > should be considered > > > only if we reach a conclusion that advantages of the PW > > > stitching option > > > outweigh its disadvantages. > > > > Sasha, > > > > I believe that option 3 is identical to the Distributed VPLS > > model described in section 5.5 of draft-ietf-l2vpn-signaling-00.txt. > > > > IMO, CE-CE PWs is just one example of situations that require > > multi-hop PWs. > > > > draft-ietf-l2vpn-signaling-00.txt talks about splicing PWs. > > You talk about stitching PWs. In both cases the solution > > sounds kludgy, to say the least. However, if you look at > > these multi-hop PWs as regular LSPs established along a > > multi-hop path, everything is pretty much straightforward. > > Setup of such multi-hop LSPs would require a protocol like > > CR-LDP or RSVP. RSVP is in my view the preferred solution, > > but CR-LDP would be closer to the current control draft. > > > > In any case, as I said in another email, I would prefer it if > > we would solve the multi-hop issue before progressing the > > control draft. The response to that email was that it is not > > clear that people are interested in the multi-hop solution. I > > would argue that Yaakov's contribution shows that there is interest. > > > > Peter > > > > > > > > Hope these notes would be helpful. > > > -------------------------------------------------------------- > > > -------------- > > > -------- > > > With best regards, > > > Sasha Vainshtein > > > email: sasha@axerra.com > > > phone: +972-3-7659993 (office) > > > +972-8-9254948 (home) > > > +972-58-674833 (cellular) > > > > > > _______________________________________________ > > > pwe3 mailing list > > > pwe3@ietf.org > > > https://www1.ietf.org/mailman/listinfo/pwe3 > > > > > >
- RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowir… Busschbach, Peter B (Peter)
- RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowir… Sasha Vainshtein
- RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowir… Busschbach, Peter B (Peter)
- RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowir… Sasha Vainshtein
- RE: CE-to-CE PWs, Hierarchical VPLS and Pseudowir… David Sinicrope