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

Greg Mirsky <gregimirsky@gmail.com> Thu, 08 August 2019 03:01 UTC

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From: Greg Mirsky <gregimirsky@gmail.com>
Date: Wed, 7 Aug 2019 20:01:12 -0700
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To: "Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net>
Cc: "bess-chairs@ietf.org" <bess-chairs@ietf.org>, "EXT - thomas.morin@orange.com" <thomas.morin@orange.com>, Robert Kebler <rkebler@juniper.net>, BESS <bess@ietf.org>
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Subject: Re: [bess] WGLC, IPR and implementation poll for draft-ietf-bess-mvpn-fast-failover
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Hi Jeffrey,
thank you for the most detailed responses and proposed updates. Please find
my followup notes and answers in-line tagged GIM4>>.
Also, attached are diff and the new working version of the draft. Much
appreciate your feedback on the changes and whether you find your questions
and concerns being addressed.

Regards,
Greg

On Fri, Aug 2, 2019 at 10:51 AM Jeffrey (Zhaohui) Zhang <zzhang@juniper.net>
wrote:

> Hi Greg,
>
>
>
> Sorry for the late response.
>
> Please see zzh2> in snipped text below.
>
>
>
> *From:* Greg Mirsky <gregimirsky@gmail.com>
> *Sent:* Saturday, May 11, 2019 3:24 PM
> *To:* Jeffrey (Zhaohui) Zhang <zzhang@juniper.net>
> *Cc:* bess-chairs@ietf.org; EXT - thomas.morin@orange.com <
> thomas.morin@orange.com>gt;; Robert Kebler <rkebler@juniper.net>et>; BESS <
> bess@ietf.org>
> *Subject:* Re: [bess] WGLC, IPR and implementation poll for
> draft-ietf-bess-mvpn-fast-failover
>
>
>
> Hi Jeffrey,
>
> thank you for your consideration and the detailed comments with great
> suggestions. Please find my answers below under GIM3>> tag. Attached is the
> diff to highlight the updates.
>
>
>
> Regards,
>
> Greg
>
>
>
> On Tue, May 7, 2019 at 7:43 AM Jeffrey (Zhaohui) Zhang <zzhang@juniper.net>
> wrote:
>
> Hi Greg,
>
>
>
> Most of changes are fine; though I suggest to replace the following:
>
>
>
>    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 to the same PE,
>
>    identified by the P-tunnel Attribute, are in Up state.
>
>
>
> With the following:
>
>
>
>    For P-tunnels of type P2MP MPLS-TE, the status of the P-tunnel is
>
>    considered up if the sub-LSP to this downstream PE is in Up state.
>
> GIM3>> Accept with one question. As this is the first sentence in the
> section, what is the PE we refer to as "this downstream PE"? Should we use
> "a downstream PE"?
>
>
>
> Zzh2> Not sure what the right word is, but the point is that it is from a
> particular downstream PE’s point of view (I remember often seeing text like
> “this router” in RFCs).
>
GIM4>> "this" it is. Thank you.

>
>
> Not all comments have been addressed, though. I trimmed some text below
> and highlighted the outstanding ones with “=============”. You may need to
> refer to my previous email for correlation/details.
>
>
>
> Jeffrey
>
>
>
> On Thu, Mar 14, 2019 at 3:04 AM Jeffrey (Zhaohui) Zhang <
> zzhang@juniper.net> wrote:
>
> Thomas, Bob,
>
>
>
> Some questions below for you. Some old, and some new.
>
>  ==============================
>
>
>
>
>
> Zzh> It’s not that the “rules … are not consistent”. It’s that by nature
> some PEs may think the tunnel is down while the others may think the tunnel
> is still up (because they’re on different tunnel branches), even when they
> follow the same rules. Traffic duplication in this case is also only with
> inclusive tunnels – so how about the following?
>
>
>
>    Because all PEs may arrive at a different
>
>    conclusion regarding the state of the tunnel,
>
>    procedures described in Section 9.1.1 of [RFC 6513] MUST be used
>
>    when using inclusive tunnels.
>
> GIM3>> Got it, thx. Would s/may/could/ be acceptable to avoid questions
> about RFC2119-like language?
>
>
>
> Zzh2> I think it should be a MUST – otherwise you get duplicates when
> different PEs pick different upstream PEs.
>
GIM4>> I've tried s/may/MUST/ and it doesn't read right:
   Because all PEs MUST arrive at a different
   conclusion regarding the state of the tunnel,
   procedures described in Section 9.1.1 of [RFC 6513] MUST be used
   when using inclusive tunnels.
I'll do s/may/could/ to have this text:
   Because all PEs could arrive at a different
   conclusion regarding the state of the tunnel,
   procedures described in Section 9.1.1 of [RFC 6513] MUST be used
   when using inclusive tunnels.

>
>
> ===============================
>
>  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?
>
>
>
> Zzh> It’s not that the alternative UMH selection algorithm change
> documented use of p2mp BFD. It’s the other way around – tunnel state
> changes the selection result. I guess hashing can still be used (this
> document only controls what goes into the candidate pool). For unicast
> based selection I thought it’d no longer work, but then I noticed the
> following:
>
>
>
>    o  second, the UMH candidates that advertise a PMSI bound to a tunnel
>
>       that is "down" -- these will thus be used as a last resort to
>
>       ensure a graceful fallback to the basic MVPN UMH selection
>
>       procedures in the hypothetical case where a false negative would
>
>       occur when determining the status of all tunnels
>
>
>
> Zzh> So this should still work, although Ideally, the PE advertising the
> next best route should be considered before going to the last resort (of
> using the PE advertising the best route but whose tunnel is down).
>
> GIM3>> I hope I've got the idea. Below is the updated text (second becomes
> third and your proposal - second):
>
> NEW TEXT:
>
>    o  Second, the PE advertising the next best route is to be
>
>       considered.
>
>
>
>    o  Third, the UMH candidates that advertise a PMSI bound to a tunnel
>
>       that is "down" -- these will thus be used as a last resort to
>
>       ensure a graceful fallback to the basic MVPN UMH selection
>
>       procedures in the hypothetical case where a false negative would
>
>       occur when determining the status of all tunnels.
>
>
>
> Zzh2> I checked the surrounding text in this draft and section 5.1.3 in
> RFC6513. I believe section 3 of this document, before its subsection 3.1
> should be re-written as following:
>
>
>
> 3.  UMH Selection based on tunnel status
>
>
>
>    Current multicast VPN specifications [RFC6513], section 5.1, describe
>
>    the procedures used by a multicast VPN downstream PE to determine
>
>    what the upstream multicast hop (UMH) is for a given (C-S, C-G).
>
>
>
>    The procedure described here is an OPTIONAL procedure that consists
>
>    of having a downstream PE take into account the status of P-tunnel
>
>    rooted at each possible upstream PEs. Because the PEs could arrive at
>
>    a different conclusion regarding the state of the tunnel, procedures
>
>    described in Section 9.1.1 of [RFC6513] MUST be used when using
>
>    inclusive tunnels.
>
>
>
>    For a given downstream PE and a given VRF, the P-tunnel corresponding
>
>    to a given upstream PE for a given (C-S, C-G) state is the S-PMSI
>
>    tunnel advertised by that upstream PE for this (C-S, C-G) and
>
>    imported into that VRF, or if there isn't any such S-PMSI, the I-PMSI
>
>    tunnel advertised by that PE and imported into that VRF.
>
>
>
>    There are three options specified in Section 5.1 of [RFC6513] for a
>
>    downstream PE to select an Upstream PE.
>
>
>
>    The first two options select the Upstream PE from a candidate PE set
>
>    either based on IP address or a hashing algorithm. When used together
>
>    with the optional procedure of considering the P-tunnel status as in
>
>    this document, a candidate upstream PE is included in the set if it
> either
>
>
>
>       A.  advertise a PMSI bound to a tunnel, where the specified tunnel
>
>           is not known to be down or
>
>
>
>       B.  do not advertise any x-PMSI applicable to the given (C-S, C-G)
>
>           but have associated a VRF Route Import BGP attribute to the
>
>           unicast VPN route for S (this is necessary to avoid
>
>           incorrectly invalidating a UMH PE that would use a policy
>
>           where no I-PMSI is advertised for a given VRF and where only
>
>           S-PMSI are used, the S-PMSI advertisement being possibly done
>
>           only after the upstream PE receives a C-multicast route for
>
>           (C-S, C-G)/(C-*, C-G) to be carried over the advertised
>
>           S-PMSI).
>
>
>
>    If the resulting candidate set is empty, then the procedure is repeated
>
>    without considering the P-tunnel status.
>
>
>
>    The third option simply uses the installed UMH Route (i.e., the "best"
>
>    route towards the C-root) as the Selected UMH Route, and its originating
>
>    PE is the selected Upstream PE. With the optional procedure of
>
>    considering P-tunnel status as in this document, the Selected UMH Route
>
>    is the best one among those whose originating PE's P-tunnel is not
> "down".
>
>    If that does not exist, the installed UMH Route is selected regardless
>
>    of the P-tunnel status.
>
>
>
> Zzh2> The reason is that for the candidate set is not ordered – it’s just
> a set to select from (either based on IP address or hashing).
>
GIM4>> Many thanks, Jeffrey! Please check the working verion or diff and
let me know if I've correctly applied the changes.

>
>
> ========================================
>
>
>
> zzh> BTW, the same applies to 3.1.7 as well.
>
> GIM>> Agree
>
>
>
> ==================================
>
>
>
> 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.
>
> ===================== this needs more discussion =====
>
> ===== should be clear on which way is done – stop sending BFD message or
> use AdminDown
>
> ===== an PMSI may be used for many flows, which may use different PE-CE
> interfaces on the ingress PE. A downstream PE would not know which
> interface it should track for a particular flow.
>
> GIM3>> Thank you for helping me to understand the problem with PE-CE and
> p2mp BFD. I've updated the paragraph is 3.1.7, I've found the better method
> to indicate the PE-CE link failure to the downstream. Also, stress that
> though it is likely that PE-CE association be 1:1, it is outside the scope
> of the draft. Please let me know if the new text addresses your questions:
>
> NEW TEXT:
>
>    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.
>
>    According to section 6.8.17 [RFC5880], failure of a PE-CE link MAY be
>
>    communicated to the downstream PE by setting the bfd.LocalDiag of the
>
>    p2mp BFD session associated with this link to Concatenated Path Down
>
>    and/or Reverse Concatenated Path Down.  The mechanism to communicate
>
>    the mapping between the PE-CE link and the associated BFD session is
>
>    outside the scope of this document.
>
>
>
> Zzh2> Because you still want to track the tunnel state (in addition to
> pe-ce interface state), you would need at least two discriminators – one
> for the tunnel and one for the PE-CE link. However, the new “BGP- BFD
> attribute” defined in this spec only accommodates one discriminator (and my
> understanding is that you can’t have more than one of the same attribute).
>
GIM4>> It is implied that the PE-CE link is monitored by p2p BFD session,
most likely as described in RFC 5881 for single-hop BFD. That would not
require bootstrapping.

> Zzh2> The simplest solution is that just use the same discriminator (vs.
> per PE-CE link discriminator). With that, the ENTIRE section 3.1.7
> (including its subsections) become the following:
>
GIM4>> I'm confused by "use the same Discriminator". The root advertises
its Discriminator to the downstream PEs. The value is only locally unique
for the root, not for any downstream PE. For a PE-CE link, if BFD is used,
each PE must pick its locally unique value to use it as My Discriminator.
CE uses that value in Your Discriminator field and thus the PE
demultiplexes p2p sessions using its locally unique value in the Your
Discriminator field. Note that p2mp BFD session among the root and the
downstream PEs is such that PEs receives BFD control packets with the value
of Your Discriminator field zeroed, and PEs use a different mechanism to
demultiplex p2mp BFD sessions (as described in RFC 8562).

>
>
> 3.1.7 Tracking upstream PE-CE link status
>
>
>
>    In case the PE-CE link on an upstream PE failed, even though the
> provider tunnel is still up,
>
>    It is desired for the downstream PEs to switch to a backup upstream PE.
> To achieve that,
>
>    If the upstream PE detects that its PE-CE link fails, it SHOULD set the
> bfd.LocalDiag of the
>
>    p2mp BFD session to Concatenated Path Down and/or Reverse Concatenated
> Path Down,
>
>    unless it switches to a new PE-CE link immediately (in that case the
> upstream PE will start tracking
>
>    the status of the new PE-CE link).
>
>    When a downstream PE receives that bfd.LocalDiag code, it treats as if
> the tunnel itself
>
>    failed and tries to switch to a backup PE.
>
GIM4>> Would the downstream PE be switching to the backup Provider Tunnel,
not to a backup PE? If yes, that option already listed in section 3.1.7.2

>
>
>    …  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.
>
> ===========================================
>
> Zzh> I re-read section 3.1.6 and 3.1.7 and have more questions 😊
>
> Zzh> 3.1.6 seems to be about tracking tunnel itself while 3.1.7 is about
> tracking PE-CE interfaces. From an egress point of view, (how) does it know
> if the discriminator is for the tunnel or for PE-CE interface 1 or PE-CE
> interface 2? Does it even care? It seems to me that an egress PE would not
> need to care. If so, why are there different procedures for 3.1.6/3.1.7 (at
> least for the egress PE behavior)? Even for the upstream PE behavior,
> shouldn’t 3.1.6.1 apply to 3.1.7 as well?
>
> GIM>> Added the following text to the first paragraph of section 3.1.7:
>
> NEW TEXT:
>
> The mechanism to communicate the mapping between the PE-CE link
>
> and the associated BFD session is outside the scope of this document.
>
>
>
> =============== the above added text does not address my questions
>
> Zzh2> The above was still outstanding, but with my proposed new 3.1.7
> section it is fine.
>
>
>
> 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.
>
> ================= how that is done needs to be discussed
>
> GIM3>> I think that this is implementation issue and we just point to the
> recommended behavior without prescribing what steps must be taken to
> achieve it.
>
> Zzh2> What is specified is not enough for even vendor-dependent
> implementation. On the other hand, it is no longer a problem with my newly
> proposed section 3.1.7.
>
> Zzh2> One more zzh2> comment below.
>
>
>
> 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.
>
> Zzh2> I suggest we remove this paragraph. OK I see it’s already removed.
>
> Zzh2> Jeffrey
>
> Juniper Internal
>
>
>
> Juniper Business Use Only
>
>