RE: [PWE3] BFD for MPLS PWs

["Busschbach, Peter B (Peter)" <busschbach at lucent.com>]

Andy,

Thanks for your attempt to summarize our positions. I don't fully agree, but it clarifies the discussion.

Please see in-line.

Peter

> -----Original Message-----
> From: Andrew G. Malis [mailto:amalis at gmail.com]
> Sent: Tuesday, August 22, 2006 5:16 PM
> To: Luca Martini
> Cc: Busschbach, Peter B (Peter); Swallow George; 'Thomas D. Nadeau';
> Pignataro Carlos; Morrow Monique; pwe3 WG 
> ((((((((E-mail)))))))); Danny
> McPherson; Agarwal Rahul; Stewart (stbryant) Bryant
> Subject: Re: [PWE3] BFD for MPLS PWs
> 
> 
> Without trying to unwind the 20 levels of this email thread, I think 
> a key point is that Peter asserted (assuming I've followed this 
> thread correctly) that there was strong WG consensus for only using 
> BFD Status messages to signal AC and PW faults and that PW status 
> signalling is redundant.

That is not what I was trying to get across.

I wrote that there is strong consensus that the use of PW Status does not imply that PW Status must be used for *every* single AC and PW defect. E.g. in the case of ATM encapsulation, an AC defect will result in AIS insertion. On the other hand, in the case of FR encapsulation, an AC defect will result in a PW Status message. The OAM message mapping draft defines precisely when PW Status should be sent and when not.

I have not made any statements about the WG's view on the use of BFD. There are two options: (1) use BFD for fault detection only and use PW Status for fault notification (within the bounds of the OAM message mapping draft, i.e. recognizing that for specific defects AIS insertion will be used instead of PW Status); (2) use BFD for both fault detection and fault notification.

I am not aware of any WG consensus about a preferred approach. My personal preference is (1), because I believe that for protocols with well-defined OAM (i.e. ATM and Ethernet w/Y.1731), an AC failure will result in AIS insertion at which point BFD fault notification is unnecessary. However, in my opinion it is up to the service provider to decide which approach to use.

My only statement with regards to BFD was: IF (and only if) one decides to use BFD for both fault detection and notification, there is no need for additional PW Status fault notification. This is an obvious consequence of our general approach to OAM and it is stated as such in the OAM message mapping draft.


>  I don't see that consensus - for example, I 
> happen to agree with Luca that there may be cases where only IP will 
> be able to get through, and in those cases, PW status 
> signaling has value.

And what would that value be? If you run BFD and lose MPLS connectivity, both PEs will detect a forward PW defect and will perform the appropriate consequent actions. There is nothing in the PW Status message that adds any useful information over and beyond what the PE already knows.
> 
> I do agree that there has been an evolution of thinking over time in 
> the WG (seeing as we originally started with just withdrawing PW 
> labels in the original Martini draft), and what we probably need at 
> this point is a single clear overview in one place of the OAM and 
> status signaling functionality currently agreed upon in the WG. 

That already exists. See the OAM message mapping draft. This work has been going on for a couple of years. Furthermore, there is a companion document in L2VPN (draft-ietf-l2vpn-vpws-iw-oam-01). And finally, MFA Forum has produced a similar document that has already been approved: MFA Forum 13.0.0. 

> Once 
> we agree we have a clear consensus,  we can then make sure that the 
> specific documents (encapsulations, OAM-MAP, VCCV, etc.) are 
> consistent with that overview and with each other.

Great idea. Read the documents that exist and let us know how they can be approved.

> 
> Cheers,
> Andy
> 
> -----------
> 
> At 8/18/2006 16:14 -0600, Luca Martini wrote:
> >Busschbach, Peter B (Peter) wrote:
> >>To move the discussion forward, let me follow up on my own question.
> >>
> >>I believe that there is a broad concensus that insertion of AIS, as 
> >>mandated by draft-ietf-pwe3-atm-encap-11, implies that the PE will 
> >>NOT send an additional PW Status message to report the same defect.
> >>
> >>
> >Here I am assuming you mean insertion of an AIS alarm at the PE 
> >toward the local attachment circuit.
> >We should also send a status message with status "0x00000008 - Local 
> >PSN-facing PW (ingress) Receive Fault "  fault in this case.
> >Remember that the MPLS path might have gone down completely because 
> >somewhere a router was mis-configured , and can now only 
> forward IP packets.
> >This message would be immediate , and much faster then any VCCV path 
> >fault detection scheme.
> >
> >>RFC 4447 states "The PW status signaling procedures described in 
> >>this section MUST be fully implemented." The document itself, 
> >>however, specifies HOW to use PW Status signaling but is vague 
> >>about WHEN to use PW Status signaling. The latter is addressed in 
> >>the encapsulation drafts, such as atm-encap, and is fully defined 
> >>in OAM-MAP (draft-ietf-pwe3-oam-msg-map-04).
> >>
> >>
> >This is not my interpretation. When we say procedures described in 
> >this section MUST be fully implemented, implies that the protocol 
> >will send and receive status messages when appropriate. The trigger 
> >to send the status messages is attachment circuit specific, 
> and therefore is
> >described in the encapsulation drafts.
> >>The notion that RFC 4447 should be interpreted as a mandate to send 
> >>a PW Status message for every single defect is an incorrect 
> >>interpretation of the spirit of the standard, does not reflect WG 
> >>concensus and, if implemented, would lead to inefficient 
> implementations.
> >>
> >>
> >I disagree, and I have seen no indication that the WG interpreted 
> >the rfc4447 in this fashion.
> >>Therefore, I strongly disagree with the point of view that Tom and 
> >>Luca have formulated regarding the use of BFD for both fault 
> >>detection and status signaling. When this option is used, there is, 
> >>IMO, no need to send PW Status messages. The current text of 
> >>OAM-MAP is in line with this view.
> >>
> >>
> >I would suggest that you look at the e-mail archive. If I remember 
> >correctly there were strong opinions about making the PW status 
> >messages mandatory.
> >
> >The current text OAM-MAP needs to be changed.
> >
> >Luca
> >
> >>Peter
> >>
> >>
> >>>-----Original Message-----
> >>>From: Busschbach, Peter B (Peter) [mailto:busschbach at lucent.com]
> >>>Sent: Wednesday, August 16, 2006 4:40 PM
> >>>To: 'Luca Martini'
> >>>Cc: Swallow George; 'Thomas D. Nadeau'; Pignataro Carlos; Morrow
> >>>Monique; pwe3 WG ((((((((E-mail)))))))); Danny McPherson; 
> Agarwal Rahul;
> >>>Stewart (stbryant) Bryant
> >>>Subject: RE: [PWE3] BFD for MPLS PWs
> >>>
> >>>
> >>>Luca,
> >>>
> >>>Section 7.4 of the ATM Encapsulation draft 
> >>>(draft-ietf-pwe3-atm-encap-11.txt) mandates that upon LOS the 
> >>>ingress PE inserts F4 AIS for every affected VPC. Is it your 
> >>>opinion that because of RFC 4447 the ingress PE must send a PW 
> >>>Status message in addition to F AIS insertion?
> >>>
> >>>Peter
> >>>
> >>>
> >>>>-----Original Message-----
> >>>>From: Luca Martini [mailto:lmartini at cisco.com]
> >>>>Sent: Tuesday, August 15, 2006 6:29 PM
> >>>>To: Busschbach, Peter B (Peter)
> >>>>Cc: 'Thomas D. Nadeau'; Swallow George; Pignataro Carlos; Morrow
> >>>>Monique; pwe3 WG ((((((((E-mail)))))))); Danny McPherson; 
> Agarwal Rahul;
> >>>>Stewart (stbryant) Bryant
> >>>>Subject: Re: [PWE3] BFD for MPLS PWs
> >>>>
> >>>>
> >>>>Busschbach, Peter B (Peter) wrote:
> >>>>
> >>>>>In my opinion, the work on VCCV and OAM-MAP provides a
> >>>>further specification of RFC4447 and in fact overrules the 
> >>>>requirement that LDP Status signaling must be used.
> >>>>
> >>>>>
> >>>>>
> >>>>Peter, We agreed a long time ago that the LDP status messaging 
> >>>>was going to be mandatory , in fact people insisted I 
> make it mandatory.
> >>>>So I would have to say that the LDP status MUST always be used as 
> >>>>mandated in RFC 4447, and the BFD status messaging is an 
> >>>>optional, and , in my opinion, not a very useful option 
> when LDP is in use.
> >>>>
> >>>>Luca
> >>>>
> >>>>
> >>>>
> >>>>>A minor comment on your email:
> >>>>>I am not sure what you mean by the "de-facto status must
> >>>>always rely on LDP". Either you declare that that the PW is down 
> >>>>when (1) BFD OR LSP indicate a PW failure, or (2) based only on 
> >>>>the LDP status. In my mind "de-facto status" implies (2), whereas 
> >>>>the beginning of your email says that (1) is the proposed 
> procedure.
> >>>>
> >>>>>Peter
> >>>>>
> 

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