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

Greg Mirsky <gregimirsky@gmail.com> Wed, 05 December 2018 18:37 UTC

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From: Greg Mirsky <gregimirsky@gmail.com>
Date: Wed, 5 Dec 2018 10:37:30 -0800
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To: zzhang@juniper.net
Cc: Thomas Morin <thomas.morin@orange.com>, Robert Kebler <rkebler@juniper.net>, bess-chairs@ietf.org, 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 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 <zzhang@juniper.net>
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.

>
>
> 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?

>
>
>    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?

>
>
> 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.

>
>
>    …  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.

>
>
> 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.

>
>
> 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 <bess-bounces@ietf.org> *On Behalf Of *
> stephane.litkowski@orange.com
> *Sent:* Thursday, November 22, 2018 2:54 AM
> *To:* bess@ietf.org
> *Cc:* bess-chairs@ietf.org
> *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/
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__datatracker.ietf.org_doc_draft-2Dietf-2Dbess-2Dmvpn-2Dfast-2Dfailover_&d=DwMFAg&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=f7wsLGcfzAWDNS6XNTBZwj_OLAOsZZqdrR2IDAzeZqE&m=21UeMvv2ofELpScacCIlRV64tml5G3zQ3NN5NqhC90s&s=ZKwzFkFZdTKGHJdgRZ6PExBQcl1Ck5CGjhXDxYQYvvI&e=>
>
>
>
>     [2]
> https://mailarchive.ietf.org/arch/msg/bess/cG3X1tTqb_vPC4rg56SEdkjqDpw
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__mailarchive.ietf.org_arch_msg_bess_cG3X1tTqb-5FvPC4rg56SEdkjqDpw&d=DwMFAg&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=f7wsLGcfzAWDNS6XNTBZwj_OLAOsZZqdrR2IDAzeZqE&m=21UeMvv2ofELpScacCIlRV64tml5G3zQ3NN5NqhC90s&s=fR1eK_EmnRha7QRf37WKaJmt1F5OLq7ynG7afcmPhM0&e=>
>
>
>
>
>
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