[mpls] FW: RE: [PWE3] Seeking feedback on I-D "MPLS-TPLinearProtectionApplicability to MS-PW"
Eric Gray <eric.gray@ericsson.com> Tue, 21 June 2011 14:54 UTC
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From: Eric Gray <eric.gray@ericsson.com>
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Date: Tue, 21 Jun 2011 10:54:34 -0400
Thread-Topic: RE: [mpls] [PWE3] Seeking feedback on I-D "MPLS-TPLinearProtectionApplicability to MS-PW"
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Subject: [mpls] FW: RE: [PWE3] Seeking feedback on I-D "MPLS-TPLinearProtectionApplicability to MS-PW"
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Forwarding in plain text - with ZTE disclaimer removed... ________________________________ From: ma.yuxia@zte.com.cn [mailto:ma.yuxia@zte.com.cn] Sent: Monday, June 20, 2011 4:31 AM To: Alexander Vainshtein Cc: Daniel Cohn; mpls@ietf.org; Sprecher, Nurit (NSN - IL/Hod HaSharon); pwe3@ietf.org Subject: ´ð¸´: RE: [mpls] [PWE3] Seeking feedback on I-D "MPLS-TPLinearProtectionApplicability to MS-PW" Hi Sasha, "Linear protection" is different from "Multi-homed CE redundancy" and has no reference with AC. Working path(s) and protection path(s) have same source and sink endpoints in linear protection type. IMHO, When S-PE fails, it is more simple to use "Linear protection" to protect it. Best Regards, Ma Alexander Vainshtein <Alexander.Vainshtein@ecitele.com> дÓÚ 2011-06-14 20:02:10: > Daniel, > Please see more inline (bold purple italics). I¡¯ve also stripped the > portions of the text that are not related to this round of comments. > > Regards, > Sasha > > From: Daniel Cohn [mailto:DanielC@orckit.com] > Sent: Tuesday, June 14, 2011 2:34 PM > To: Alexander Vainshtein > Cc: mpls@ietf.org; pwe3@ietf.org; Sprecher, Nurit (NSN - IL/Hod > HaSharon); ma.yuxia@zte.com.cn > Subject: RE: [mpls] [PWE3] Seeking feedback on I-D "MPLS- > TPLinearProtectionApplicability to MS-PW" > > Hi Sasha, thanks again and see inline with [DC]. > > From: Alexander Vainshtein [mailto:Alexander.Vainshtein@ecitele.com] > Sent: Monday, June 13, 2011 6:11 PM > To: Daniel Cohn > Cc: mpls@ietf.org; pwe3@ietf.org; Sprecher, Nurit (NSN - IL/Hod > HaSharon); ma.yuxia@zte.com.cn > Subject: RE: [mpls] [PWE3] Seeking feedback on I-D "MPLS- > TPLinearProtectionApplicability to MS-PW" > > Daniel and all, > Please see some comments inline below. > > Regards, > Sasha > > From: Daniel Cohn [mailto:DanielC@orckit.com] > Sent: Monday, June 13, 2011 5:55 PM > To: Sprecher, Nurit (NSN - IL/Hod HaSharon); Alexander Vainshtein; > ma.yuxia@zte.com.cn > Cc: mpls@ietf.org; pwe3@ietf.org > Subject: RE: [mpls] [PWE3] Seeking feedback on I-D "MPLS- > TPLinearProtection Applicability to MS-PW" > > Hi, > > Thanks all for the feedback. I believe we all agree that PW > protection is only required in the event of S-PE failure at an MS-PW > ¨C this is clearly stated in the draft. Now, both Sasha and Nurit > mention that the PW redundancy mechanism can meet the MPLS-TP PW > protection requirements. > [[[Sasha]]] ¡ snipped ¡ > E.g., the PW redundancy mechanisms take care of dual-homed CEs in > SS- and MS-PWs ¨C something that liner protection of PWs cannot do. > [DC] I¡¯m afraid I don¡¯t follow you here. Where does PW redundancy > take care of dual-homed CE? Actually PW redundancy draft explicitly > states: ¡°The method for dual-homing of CE1 to PE1 and to PE3 nodes, > and the protocols used, are outside the scope of this document¡±. > [[[Sasha]]] Quoting from the draft: > 15.2. Multiple Multi-homed CEs with single SS-PW redundancy > > |<-------------- Emulated Service ---------------->| > | | > | |<------- Pseudo Wire ------>| | > | | | | > | | |<-- PSN Tunnels-->| | | > | V V (not shown) V V | > V AC +----+ +----+ AC V > +-----+ | |....|.......PW1........|....| | +-----+ > | |----------| PE1|...... .........| PE3|----------| | > | CE1 | +----+ \ / PW3 +----+ | CE2 | > | | +----+ X +----+ | | > | | | |....../ \..PW4....| | | | > | |----------| PE2| | PE4|--------- | | > +-----+ | |....|.....PW2..........|....| | +-----+ > AC +----+ +----+ AC > > > Figure 15-2 Multiple Multi-homed CEs with single SS-PW redundancy > > The application in Figure 15-2 makes use of the Independent mode of > operation. > > CE1 is dual-homed to PE1 and PE2. CE2 is dual-homed PE3 and PE4. The > method for dual-homing and the used protocols are outside the scope > of this document. Note that the PSN tunnels are not shown in this > figure for clarity. However, it can be assumed that each of the PWs > shown is encapsulated in a separate PSN tunnel. > > Assume that the AC from CE1 to PE1 is Active, from CE1 to PE2 is > Standby; furthermore, assume that the AC from CE2 to PE3 is Standby > and from CE2 to PE4 is Active. The method of deriving Active/Standby > status of the AC is outside the scope of this document. > > PE1 advertises the preferential status "Active" and operational > status "Pseudowire forwarding" for pseudowires PW1 and PW4 connected > to PE3 and PE4. This status reflects the forwarding state of the AC > attached to PE1. PE2 advertises preferential status "Standby" and > operational status "Pseudowire forwarding" for pseudowires PW2 and > PW3 to PE3 and PE4. PE3 advertises preferential status "Standby" and > operational status "Pseudowire forwarding" for pseudowires PW1 and > PW3 to PE1 and PE2. PE4 advertise the preferential status "Active" > and operational status "Pseudowire forwarding" for pseudowires PW2 > and PW4 to PE2 and PE1 respectively. Thus by matching the local and > remote preferential forwarding status of "Active" and operational > status of "Pseudowire forwarding" of pseudowires, the PE nodes > determine which PW should be in the Active state. In this case it is > PW4 that will be selected. > > On failure of the AC between CE1 and PE1, the forwarding state of > the AC on PE2 is changed to Active. PE2 then announces the newly > changed 'preferential forwarding' status bit of "active" to PE3 and > PE4. PE1 will advertise a PW status notification message indicating > that the AC between CE1 and PE1 is operationally down. PE2 and PE4 > match the local and remote preferential forwarding status of > "Active" and operational status "Pseudowire forwarding" and select > PW2 as the new active pseudowire to send traffic to. > > On failure of PE1 node, PE2 will detect it and will transition the > forwarding state of its AC to Active. The method by which PE2 > detects that PE1 is down is outside the scope of this document. PE2 > then announces the newly changed 'preferential forwarding' status > bit of "Active" to PE3 and PE4. PE2 and PE4 match the local and > remote preferential forwarding status of "Active" and operational > status "Pseudowire forwarding" and select PW2 as the new active > pseudowire to send traffic to. Note that PE3 and PE4 may have > detected that the PW to PE1 went down via T-LDP Hello timeout or via > other means. However, they will not be able to forward user traffic > until they received the updated status bit from PE2. > > Because each dual-homing algorithm running on the two node sets, > i.e., {CE1, PE1, PE2} and {CE2, PE3, PE4}, selects the active AC > independently, there is a need to signal the active status of the AC > such that the PE nodes can select a common active PW for end-to-end > forwarding between CE1 and CE2 as per the procedures in the > independent mode. > > Note that any primary/secondary procedures, as defined in sections > 5.1. and 5.2. , do not apply in this use case as the Active/Standby > status is driven by the AC forwarding state as determined by the AC > dual-homing protocol used. > > > I believe you¡¯ve taken your ¡°out of scope¡± reference from this text, > but IMHO you misinterpreted it. > What it means (or so I read it) is that specific dual-homing > protocol is out of scope as long as it meets certain assumptions. > Aside: It would be good if the authors of the PW redundancy drafts > could explicitly specify these assumptions. > > [[[Sasha]]] ¡ snipped ¡ > > Now let¡¯s turn our attention back to whether the PW redundancy > draaft can be used to meet MPLS-TP PW protection requirements. I can > identify the following reasons why in its current form it doesn¡¯t: > > - It explicitly (¡°outside the scope¡±) does not define > protection triggers and how to handle coexisting triggers, as > requested in RFC 5654 (MPLS-TP Requirements), reqs #75, #76 and #79 > [[[Sasha]]] So what? Definition of triggers is orthogonal to how > coordinated protection switching happens. > [DC] It is. But still it needs to be defined by the recovery > framework, e.g. what should the endpoint do when one PW is in SF and > the other is in SD, or when both are in SD. Operators expect well- > defined behavior in these and other scenarios, and PW redundancy > does not define them (because it was not in their scope). > [[[Sasha]]] I wonder if you have followed the discussion regarding > ability to define SD condition for LSPs and PWs on the MPLS-TP list? > IMO it is not possible to define it in a meaningful way. Hence I do > not see a proposal that does not address a scenario that does not > exist in reality as a flawed one. > > - It does not support the ability to distinguish between > different types of triggers (i.e. one end doesn¡¯t know why the other > end triggered switch), as requested in RFC 5654 (MPLS-TP > Requirements), req #77 > - It does not define revertive/nonrevertive behavior, as > requested in RFC 5654 (MPLS-TP Requirements), req #64 > - It does not define holdoff support, which is especially > important to avoid race conditions with LSP protection when it exists > - It doesn¡¯t support 1+1 mode, as requested in RFC 5654 > (MPLS-TP Requirements), req #65 > [[[Sasha]]] All these claims are correct ¨C and this should not be a > surprise, because MPLS-TP requirements have been defined much later > than the PW redundancy mechanism. > But I do not think that this justifies co-existence of two different > mechanisms. > [DC] So how do you propose to meet the PW protection requirement? > - It¡¯s a two-phase protocol, with the consequent impact on timing > [[[Sasha]]] Could you please elaborate? [DC] In draft-ietf-mpls-tp- > linear-protection, each endpoint will immediately switch traffic to > the other path upon identifying an SF condition in a path, without > waiting for the far end to acknowledge the switch (1-phase). In PW > redundancy, an endpoint detecting an SF condition in a path will not > switch until the far end has acknowledged the switch (2-phase). > Needless to say, recovery is slower in a 2-phase protocol. > [[[Sasha]]] To the best of my understanding, 1-phase protection is > only possible in 1+1 unidirectional architectures. And yes, I know > that MPLS-TP requires a mechanism to support this; whether anybody > would really do that in the packet-switching network is not clear to > me. And I acknowledge that the PW redundancy drafts do not support > 1+1 unidirectional scheme. > > - It doesn¡¯t define retransmission of protection > coordination messages, so loss of a single PDU can result in > switchover not taking place, thus not supporting sub-50 ms recovery > in this case > [[[Sasha]]] The PW redundancy protocol runs either on top of LDP > (which benefits from TCP retransmissions) or on top of static PW > status messages (where retransmission is defined). > [DC] True ¨C but static PW status defines slow retransmission (¡°will > be transmitted twice at an initial interval of one second¡±) while > draft-ietf-mpls-tp-linear-protection specifies fast retransmission > when required for fast recovery (section 3.1.4). > > In summary, PW redundancy was not designed with TP requirements in > mind, and as such does not meet the TP requirements. Of course > modifications may be introduced, but why reinvent the wheel when > there is a protocol (draft-ietf-mpls-tp-linear-protection-06) in the > standards track that supports all the above requirements and can be > applied to MS-PW protection with minor modifications? > [[[Sasha]]] As I said, because the applicability scope is by far too > narrow to justify a dedicated protocol. > [DC] If we were discussing designing a new protocol, I might agree > with you. But from a practical standpoint, the PW protection draft > is not a dedicated protocol ¨C it¡¯s an applicability statement to an > existing protocol. Both from the implementation and the operational > point of view it defines a new use case for an existing protocol and concept. > > > Regards, > > Daniel > > > From: mpls-bounces@ietf.org [mailto:mpls-bounces@ietf.org] On Behalf Of > Sprecher, Nurit (NSN - IL/Hod HaSharon) > Sent: Monday, June 13, 2011 4:02 PM > To: ext Alexander Vainshtein; ma.yuxia@zte.com.cn > Cc: mpls@ietf.org; pwe3@ietf.org > Subject: Re: [mpls] [PWE3] Seeking feedback on I-D "MPLS- > TPLinearProtection Applicability to MS-PW" > > Hi, > I would like to second Sasha. > End-to-end PW protection (with diverse paths) does not scale, and > put hard restrictions on the utilization of the resources. > MPLS-TP PWs are carried across the network inside MPLS-TP LSPs. > Therefore, an obvious way to provide protection for a PW is to > protect the LSP that carries it. > If the PW is a multi-segment PW, then LSP recovery can only protect > the PW in individual segments. This means that a single LSP > recovery action cannot protect against a failure of a PW switching > point (an S-PE). > When protecting against an AC or T/S-PE failure by dual > connectivity, PW redundancy mechanisms provide means for the PEs to > coordinate over which LSP the traffic of the PW is carried. > I also doubt why there is a need for additional mechanism. > Best regards, > Nurit > > From: pwe3-bounces@ietf.org [mailto:pwe3-bounces@ietf.org] On Behalf Of > ext Alexander Vainshtein > Sent: Monday, June 13, 2011 3:43 PM > To: ma.yuxia@zte.com.cn > Cc: mpls@ietf.org; pwe3@ietf.org > Subject: Re: [PWE3] [mpls] Seeking feedback on I-D "MPLS-TP > LinearProtection Applicability to MS-PW" > > Dear Ma and all, > Adding the PWE3 WG to my response. > > The PW redundancy mechanism supports linear protection of MS-PWs as > one of many additional application use cases: > Appendix A of the PW redundancy Bit draft describes 5 application > uses cases in addition to MS-PW with single-homed CEs (which is > listed there as use case 5). > And it is equally applicable to IP/MPLS and MPLS - with the help of the > Static PW Status Messages draft( if, for whatever reason, you do not > want to, or cannot, use RFC 4447). > > Hence I doubt the need for yet another PW redundancy mechanism with > narrow scope of applicability. > > Regards, > Sasha > > From: mpls-bounces@ietf.org [mailto:mpls-bounces@ietf.org] On Behalf Of > ma.yuxia@zte.com.cn > Sent: Monday, June 13, 2011 3:25 PM > To: mpls@ietf.org > Subject: Re: [mpls] Seeking feedback on I-D "MPLS-TP Linear > Protection Applicability to MS-PW" > > Hi all, > > The linear protection mechanism for LSP and PW(including MS-PW) > should be the same and it is valuable to describe it clearly. > > BTW, there is a typo, it is "T-PE Z" instead of "T-PE B". > > " > Figure 1 illustrates such a scenario, where two MS-PWs are > established between T-PE A and T-PE B, over S-PEs 1-2 and 3-4 > respectively. Each PW segment is established over an LSP (e.g. PW- > s12 over LSP12). > " > -----Original Message----- > From: Daniel Cohn > Sent: Tuesday, May 17, 2011 4:14 PM > To: mpls > Subject: Seeking feedback on I-D "MPLS-TP Linear Protection > Applicability to MS-PW" > Importance: High > > Hi MPLSers, > > I uploaded "MPLS-TP Linear Protection Applicability to MS-PW" I-D > (http://tools.ietf.org/html/draft-cohn-mpls-tp-pw-protection-00) > > The abstract goes: > > One of the requirements of the MPLS transport profile [RFC 5654] is > to provide linear protection for transport paths, which include both > LSPs and PWs. The functional architecture described in [SurvivFwk] > is applicable to both LSP and PWs, however [LinearProt] does not > explicitly describe mechanisms for PW protection in MPLS-TP. > > This document extends the applicability of the linear protection > mechanism described in [LinearProt] to MPLS-TP segmented PWs > (MS-PWs) as defined in [RFC 6073]. > > Could you please review it and send feedback to the mailing list or > directly to the author? > > Looking forward to your feedback, > > Daniel > This e-mail message is intended for the recipient only and contains > information which is CONFIDENTIAL and which may be proprietary to > ECI Telecom. If you have received this transmission in error, please > inform us by e-mail, phone or fax, and then delete the original and > all copies thereof. > This e-mail message is intended for the recipient only and contains > information which is CONFIDENTIAL and which may be proprietary to > ECI Telecom. If you have received this transmission in error, please > inform us by e-mail, phone or fax, and then delete the original and > all copies thereof. > This e-mail message is intended for the recipient only and contains > information which is CONFIDENTIAL and which may be proprietary to > ECI Telecom. If you have received this transmission in error, please > inform us by e-mail, phone or fax, and then delete the original and > all copies thereof. ©mz¹Ú??br>