Subject: Re: [bess] WGLC,IPR and implementation poll for draft-ietf-bess-mvpn-fast-failover

From: <[email-masked]>
Date: Fri, 1 Feb 2019 11:40:25 +0800 (CST)

Hi Greg, Jeffrey, co-authors,

About the questions provided by Jeffrey, I have some concerns, please see below with Sandy>.

And I have some other questions:

1. According to "draft-ietf-bfd-multipoint-19" and the function defined in this draft, IMO the BFD session should be demultiplexed by the combination of upstream peer address, the discriminator and the X-PMSI which is used for flow forwarding. IMO these content should be written in the draft clearly.

2. The P2MP BFD packet should be delivered in the X-PMSI tunnel. The BFD multicast packet MUST be encapsulated in associated tunnel. It seems like there is no specifiction for it. 

3. I am confused with section 3.1.1/3.1.2/3.1.3. IMO only the X-PMSI tunnel's state influence the BFD session, there is no need for other decision. 

4. For section 3.1.5, IMO the counter method has no relationship with the BFD function defined in this document. If the counter method will be used as a supplement for BFD session?

Thanks,

Sandy

原始邮件

发件人：GregMirsky <[email-masked]>
收件人：[email-masked] <[email-masked]>;
抄送人：[email-masked] <[email-masked]>;Thomas Morin <[email-masked]>;Robert Kebler <[email-masked]>;BESS <[email-masked]>;
日 期 ：2018年12月06日 02:38
主 题 ：Re: [bess] WGLC,IPR and implementation poll for draft-ietf-bess-mvpn-fast-failover

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Hi Jeffrey,thank you for the review, detailed questions and helpful comments. Please find my notes, answers in-line tagged GIM>>.

Regards,
Greg

On Fri, Nov 30, 2018 at 5:14 PM Jeffrey (Zhaohui) Zhang <[email-masked]> wrote:

Hi,

I have the following questions/comments:

   The procedure described here is an OPTIONAL procedure that consists

   of having a downstream PE take into account the status of P-tunnels

   rooted at each possible upstream PEs, for including or not including

   each given PE in the list of candidate UMHs for a given (C-S,C-G)

   state.  The result is that, if a P-tunnel is "down" (see

   Section 3.1), the PE that is the root of the P-tunnel will not be

   considered for UMH selection, which will result in the downstream PE

   to failover to the upstream PE which is next in the list of

   candidates.

Is it possible that a p2mp tunnel is considered up by some leaves  but down by some other leaves, leaving to them choosing different UMH? In that case, procedures described in Section 9.1.1 ("Discarding Packets from Wrong PE") of RFC 6513 must be used. I see that this is actually pointed out later in section 6 – good to have  a pointer to it right here.

GIM>> Would the following new text that follows the quoted text address your concern:
NEW TEXT: 
   If rules to determine the state of the P-tunnel are not
   consistent across all PEs, then some may arrive at a different
   conclusion regarding the state of the tunnel, In such a scenario,
   procedures described in Section 9.1.1 of [RFC 6513] MUST be used.

Sandy> I think Jeffrey means that a egress PE may choose a new UMH after the the "old" UMH fails. Then the egress PE may also receive (C-S, C-G) flows from old UMH p-tunnel, these flows MUST be discarded according to section 9.1.1 of RFC6513. 

Additionally, the text in section 3 seems to be more biased on Single  Forwarder Election choosing the UMH with the highest IP address. Section 5 of RFC6513 also describes two other options, hashing or based on “installed UMH route” (aka unicast-based). It is not clear how the text in this document applies to hashing based selection,  and I don’t see how the text applies to unicast-based selection. Some rewording/clarification are needed here.

GIM>> How would the use of an alternative UMH selection algorithm change documented use of p2mp BFD? Do you think that if the Upstream PE selected using, for example, hashing then defined use of BGP-BFD and p2mp BFD itself no longer applicable?

Sandy> Diffrent UMH selection methods don't influent p2mp BFD documented in this draft. IMO both of section 3 and section 5 need to be mentioned here in order to avoid confusion.

   For P-tunnels of type P2MP MPLS-TE, the status of the P-tunnel is

   considered up if one or more of the P2MP RSVP-TE LSPs, identified by

   the P-tunnel Attribute, are in Up state. 

Why is “one or more of …” used in the above text?

GIM>> Would s/one or more of/at least one of/ address your concern? 

Sandy> I am not sure there are the situations that  two or more LSPs are used to deliver a same (C-S, C-G). IMO only the LSP used by forwarding need to be mointor in egress PE. 

There are several occurrences of ((S, G)). I assume they should be  changed to (C-S, C-G).

GIM>> Indeed, globally replaced s/((S,G))/(C-S,C-G)/ 

   A PE can be removed from the UMH candidate list for a given ((S, G))

   if the P-tunnel for this (S, G) (I or S , depending) is leaf

   triggered (PIM, mLDP)

Perhaps either remove the (I or S , depending)or  move it to before the “for”.

GIM>> Moved before the "for". 

   This document defines the format and ways of usingr a new BGP

   attribute called the "BGP- BFD attribute". 

s/usingr/using/

GIM>> Yes, great catch. 

   o  MUST use [Ed.note] address as destination IP address when

      transmitting BFD control packets;

[Ed.note]?

GIM>> Replaced [Ed...note] to make it as follows:
    o  MUST use address in 127.0.0.0/8 range for IPv4 or in
      0:0:0:0:0:FFFF:7F00:0/104 range for IPv6 as destination IP address
      when transmitting BFD control packets;

   If tracking of the P-tunnel by using a p2mp BFD session is to be

   enabled after the P-tunnel has been already signaled, the the

   procedure described above MUST be followed.

What if the tracking is to  be enabled before the P-tunnel has been signaled? The text implies different behavior?

GIM>> Not really, I guess. I think that the second sentence is important:
   Note that x-PMSI A-D Route MUST be re-sent with exactly the same attributes as before and
   the BGP-BFD Attribute included.

s/the the/then the/

GIM>> Done. 

   … The dedicated p2mp BFD session MAY monitor the state of

   the Standby Upstream PE.

What does the above text mean? Do you mean “A different p2mp BFD session  …”?

GIM>> Yes, thank you for the suggested re-wording. Applied s/The dedicated/A different/ 

   When such a procedure is used, in the context where fast restoration

   mechanisms are used for the P-tunnels, leaf PEs should be configured

   to wait before updating the UMH, to let the P-tunnel restoration

   mechanism happen.  A configurable timer MUST be provided for this

   purpose, and it is recommended to provide a reasonable default value

   for this timer.

What does “such a procedure” refers to?

GIM>> Would s/When such a procedure is used/In such a scenario/

s/recommended/RECOMMENDED/?

GIM>> Great catch, thank you. Done. 

3.1.7.  Per PE-CE link BFD Discriminator

   The following approach is defined for the fast failover in response

   to the detection of PE-CE link failures, in which UMH selection for a

   given C-multicast route takes into account the state of the BFD

   session associated with the state of the upstream PE-CE link.

3.1.7.1.  Upstream PE Procedures

   For each protected PE-CE link, the upstream PE initiates a multipoint

   BFD session [I-D.ietf-bfd-multipoint] as MultipointHead toward

   downstream PEs.  A downstream PE monitors the state of the p2mp

   session as MultipointTail and MAY interpret transition of the BFD

   session into Down state as the indication of the associated PE-CE

   link being down.

Since  the BFD packets are sent over the P2MP tunnel not the PE-CE link, my understanding is that the BFD discriminator is still for the tunnel and not tied to the PE-CE link; but different from the previous case, the root will stop sending BFD messages when it detects  the PE-CE link failure. As far as the egress PEs are concerned, they don’t know if it is the tunnel failure or PE-CE link failure.

If my  understanding is correct, the wording should be changed.

GIM>> There are other than stopping transmission of BFD control packets ways to distinguish two conditions for the egress PE. For example, the MultipointHead MAY set the State to AdminDown and continue sending BFD control packets. If and when PE-CE link restored to Up, the MultipointHead can set the state to Up in the BFD control packet.

Sandy> I agree with Jeffrey. The BFD detection should be mapping to specific flow/flows associated with X-PMSIs, not the PE-CE link. The PE-CE link should influence the X-PMSIs and associated (C-S, C-G) flows. The AdminDown function defined in BFD works normally.

   …  If the route to the

   src/RP changes such that the RPF interface is changed to be a new PE-

   CE interface, then the upstream PE will update the S-PMSI A-D route

   with included BGP-BFD Attribute so that value of the BFD

   Discriminator is associated with the new RPF link.

If the RPF interface changes on the upstream PE, why should it update  the route to send a new discriminator? As long as there is a new RPF interface couldn’t the upstream PE do nothing but start tracking the new RPF interface?

GIM>> I'll defer this one to Thomas and Rob.

Sandy> I have the same question with Jeffrey. If RPF interface changes on the upstream PE, and a new route generated with a new BFD discriminator, a new P2MP BFD session need to be established and the network stability will be influenced. We need a function to guarantee the existed BFD session should not be influenced.

Regardless which way (the currently described way and my imagined  way), some text should be added to discuss how the downstream would not switch to another upstream PE when the primary PE is just going through a RPF change.

GIM>>  Would appending the following text be acceptable to address your concern:
NEW TEXT:
   To avoid unwarranted switchover a downstream PE MUST gracefully handle the
   updated S-PMSI A-D route and switch to the use of the associated BFD
   Discriminator value.

4.  Standby C-multicast route

   The procedures described below are limited to the case where the site

   that contains C-S is connected to exactly two PEs. The procedures 
   require all the PEs of that MVPN to follow the single forwarder PE

   selection, as specified in [RFC6513].   

 

Why would it not work with more than two upstream PEs?

Why is it limited to single forwarder selection? What about unicast  based selection?

GIM>> Again, asking for Thomas and Rob to help. 

Sandy> I agree with Jeffrey. There is no limition for advertising same flows through more than two PEs. Maybe the text should be modify to the UMH and the next best UMH.

This route, that has the semantics of being a 'standby'

   C-multicast route, is further called a "Standby BGP C-multicast

   route", and is constructed as follows:

   o  the NLRI is constructed as the original C-multicast route, except

      that the RD is the same as if the C-multicast route was built

      using the standby PE as the UMH (it will carry the RD associated

      to the unicast VPN route advertised by the standby PE for S)

Since you mention RD, you might as well mention it carries a Route  Target derived from the standby RE’s UMH route’s VRF RT Import EC.

GIM>> Woud the following be acceptable:
NEW TEXT:
   o  the NLRI is constructed as the original C-multicast route, except
      that the RD is the same as if the C-multicast route was built
      using the standby PE as the UMH (it will carry the RD associated
      to the unicast VPN route advertised by the standby PE for S and a
      Route Target derived from the standby PE's UMH route's VRF RT
      Import EC)

   If at some later point the local PE determines that C-S is no longer

   reachable through the Primary Upstream PE, the Standby Upstream PE

   becomes the Upstream PE, and the local PE re-sends the C-multicast

   route with RT that identifies the Standby Upstream PE, except that

   now the route does not carry the Standby PE BGP Community (which

   results in replacing the old route with a new route, with the only

   difference between these routes being the presence/absence of the

   Standby PE BGP Community).

Additionally the LOCAL_PREF should also change?

GIM>> Like normative SHOULD? 

4.3.  Reachability determination

   The standby PE can use the following information to determine that

   C-S can or cannot be reached through the primary PE:

Shouldn’t this be 4.2.1 instead of 4.3?

GIM>> Yes, agree. Thank you.  

5.  Hot leaf standby

   The mechanisms defined in sections Section 4 and Section 3 can be

   used together as follows.

This section is a little confusing to me. It seems that it really  should be how a leaf should behave when hot root standby is used, not that there is a “hot leaf” mode. A leaf is just a leaf, not a cold/warm/hot/primary/standby leaf.

GIM>> Would re-naming the section to "Use of Standby C-multicast Route" better reflect the content of the section?

Thanks.

Jeffrey

From: BESS <[email-masked]> On Behalf Of [email-masked]
 Sent: Thursday, November 22, 2018 2:54 AM
 To: [email-masked]
 Cc: [email-masked]
 Subject: [bess] WGLC, IPR and implementation poll for draft-ietf-bess-mvpn-fast-failover

Hello Working Group,

This email starts a two-week Working Group Last Call on draft-ietf-bess-mvpn-fast-failover-04  [1]

This poll runs until *the 6th of December*.

We are also polling for knowledge of any undisclosed IPR that applies to this Document, to ensure that IPR has been disclosed in compliance with IETF IPR rules (see RFCs 3979, 4879, 3669 and 5378 for more details).

If you are listed as an Author or a Contributor of this Document please respond to this email and indicate whether or not you are aware of any relevant undisclosed IPR. The Document won't progress without answers from all the Authors  and Contributors.

Currently two IPRs have been disclosed against this Document.

If you are not listed as an Author or a Contributor, then please explicitly respond only if you are aware of any IPR that has not yet been disclosed in conformance with IETF rules.

We are also polling for any existing implementation as per [2].

    Thank you,

    Stephane & Matthew

    [1] https://datatracker.ietf.org/doc/draft-ietf-bess-mvpn-fast-failover/

    [2] https://mailarchive.ietf.org/arch/msg/bess/cG3X1tTqb_vPC4rg56SEdkjqDpw

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