Re: [Technical Errata Reported] RFC5884 (5085)

Greg Mirsky <gregimirsky@gmail.com> Tue, 15 August 2017 16:56 UTC

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From: Greg Mirsky <gregimirsky@gmail.com>
Date: Tue, 15 Aug 2017 09:56:30 -0700
Message-ID: <CA+RyBmVW6pG_YgpsmFy8QRCDWV3iUj43sF2uy7Sf=P8AnGF+dw@mail.gmail.com>
Subject: Re: [Technical Errata Reported] RFC5884 (5085)
To: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
Cc: Mach Chen <mach.chen@huawei.com>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com>, Tom Nadeau <tnadeau@lucidvision.com>, "mpls@ietf.org" <mpls@ietf.org>, "Kireeti Kompella (kireeti@juniper.net)" <kireeti@juniper.net>, Alia Atlas <akatlas@gmail.com>, "Reshad Rahman (rrahman)" <rrahman@cisco.com>, "rtg-bfd@ietf. org" <rtg-bfd@ietf.org>
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Hi Mach and Les,
thank you for your proposals.I think that the text provided by Les gives
very clear and detailed explanation. I have one suggestion to clarify
whether egress LSR MUST include BFD Discriminator TLV with local value in
the Echo reply. My understanding of the RFC 584 is that BFD Discriminator
TLV MUST be included in Echo reply.

Regards,
Greg

On Tue, Aug 15, 2017 at 5:16 AM, Les Ginsberg (ginsberg) <ginsberg@cisco.com
> wrote:

> I tend to agree with Mach – and I think what Mach states is also
> reinforcing the point that Carlos has made – which is that echo reply
> procedures are defined by RFC 8029 – not by RFC 5884.
>
>
>
> However, the current text suffers from much more than the ambiguity
> regarding Echo Reply.
>
>
>
> 1)Second paragraph of Section 6 goes back and forth between discussing BFD
> Control packets, then Echo Reply, then BFD Control Packets
>
>
>
> 2)Third paragraph of Section 6 has an inappropriate use of “,” in the
> sentence:
>
>
>
> “The BFD Control packets from the
>
>    ingress to the egress LSR MUST set the local discriminator of the
>
>    egress *LSR, in *the Your Discriminator field.”
>
>
>
> 3)Section 6.1 defines when the BFD Discriminator TLV MUST be sent and when
> it is optional in LSP ping. There is actually no need for Section 6 to say
> anything in this regard.
>
>
>
> I propose revised text below – which is much more extensive in its changes
> than what has been proposed thus far, but I think it is necessary to
> eliminate all ambiguity.
>
> That said, there is no question that the current text is subject to
> multiple interpretations – so any change in text runs the risk of
> introducing new interoperability issues. On balance it is probably
> necessary to take this risk as there is no guarantee that implementations
> are interoperable today, but the WG should still consider this point
> carefully.
>
>
>
> The text below replaces current paragraphs 2 and 3 of Section 6.
>
>
>
> *<new text start>*
>
> *On receipt of the LSP Ping Echo request message, the egress LSR MUST*
>
> *   send a BFD Control packet to the ingress LSR, if the validation of*
>
> *   the FEC in the LSP Ping Echo request message succeeds.  This BFD*
>
> *   Control packet MUST set the Your Discriminator field to the*
>
> *   discriminator received from the ingress LSR in the LSP Ping Echo*
>
> *   request message.    The local discriminator assigned by the egress LSR*
>
> *   MUST be used as the My Discriminator field in the BFD session packets*
>
> *   sent by the egress LSR.*
>
>
>
> *   The ingress LSR follows the procedures in [BFD] to send BFD Control*
>
> *   packets to the egress LSR in response to the BFD Control packets*
>
> *   received from the egress LSR.  The BFD Control packets from the*
>
> *   ingress to the egress LSR MUST set the local discriminator of the*
>
> *   egress LSR in the Your Discriminator field.  The egress LSR*
>
> *   demultiplexes the BFD session based on the received Your*
>
> *   Discriminator field.  As mentioned above, the egress LSR MUST send*
>
> *   Control packets to the ingress LSR with the Your Discriminator field*
>
> *   set to the local discriminator of the ingress LSR.  The ingress LSR*
>
> *   uses this to demultiplex the BFD session.*
>
>
>
> *  The egress LSR follows the procedures defined in [RFC 8029] to
> determine when to respond with an LSP Ping Echo  reply message.*
>
> *<new text end>*
>
>
>
>     Les
>
>
>
>
>
> *From:* Rtg-bfd [mailto:rtg-bfd-bounces@ietf.org] *On Behalf Of *Mach Chen
> *Sent:* Tuesday, August 15, 2017 12:56 AM
> *To:* Carlos Pignataro (cpignata); Greg Mirsky
> *Cc:* Tom Nadeau; mpls@ietf.org; Kireeti Kompella (kireeti@juniper.net);
> Alia Atlas; Reshad Rahman (rrahman); rtg-bfd@ietf. org
> *Subject:* RE: [Technical Errata Reported] RFC5884 (5085)
>
>
>
> Hi all,
>
>
>
> IMHO, the point is not about whether the Echo Reply is optional for a
> normal LSP Ping, where the echo reply is totally controlled by the reply
> mode.
>
>
>
> For RFC5884, since the reply mode is not specified, based on the current
> text, it can be interpreted as the following two ways:
>
> 1)      it implies a new “mode” introduced, it’s actually a “special” LSP
> Ping,  the process is just as what is currently described in the RFC: an
> Echo Reply is OPTINAL, whether and when to send Echo Reply is up to the
> egress LSR, and the Ingress LSR should not assume an Echo reply will be
> returned;
>
> 2)      the echo reply is still controlled by the reply mode, and given
> that there is a “Do not reply” mode, the current text seems right, but not
> that clear.
>
>
>
> I incline to think way (2) is more nature, if so,  the proposed “Corrected
> Text” may not work if the Sender set the reply mode to “Do not reply”.
>
>
>
> I’d suggest:
>
>
>
> Original Text
> -------------
> The egress LSR MAY respond with an LSP Ping Echo
> reply message that carries the local discriminator assigned by it for
> the BFD session.
>
>
>
> NEW:
>
> The egress LSR MAY respond with an LSP Ping Echo
> reply message that carries the local discriminator assigned by it for
> the BFD session. Whether to send an LSP Ping Echo reply message is
>
> determined by the reply mode carried the received Echo request message.
>
>
>
> Best regards,
>
> Mach
>
>
>
> *From:* Rtg-bfd [mailto:rtg-bfd-bounces@ietf.org
> <rtg-bfd-bounces@ietf.org>] *On Behalf Of *Carlos Pignataro (cpignata)
> *Sent:* Tuesday, August 15, 2017 8:17 AM
> *To:* Greg Mirsky <gregimirsky@gmail.com>;
> *Cc:* Tom Nadeau <tnadeau@lucidvision.com>;; mpls@ietf.org; Kireeti
> Kompella (kireeti@juniper.net) <kireeti@juniper.net>;; Alia Atlas <
> akatlas@gmail.com>;; Reshad Rahman (rrahman) <rrahman@cisco.com>;;
> rtg-bfd@ietf. org <rtg-bfd@ietf.org>;
> *Subject:* Re: [Technical Errata Reported] RFC5884 (5085)
>
>
>
> Greg,
>
>
>
> This is my final email on this topic, since the arguments are now just
> silly and not technically constructive.
>
>
>
> 1. It's not about understanding English. It's about understanding specs!
> The "(if any)" that you quote means there are situations in which there's
> no echo reply. As I already explained to you, that's for example the case
> with Reply-mode: No-reply. However, the "(if any)" does not mean an Echo
> Reply is OPTIONAL. !! Or that you choose when a reply is not sent!!
>
> 2. RFC 8029 obsoleted 4379. But to my recollection, nothing changed
> relevant to this Errata.
>
>
>
> BFD for MPLS could have updated LSP ping behavior -- it just didn't.
>
>
> Sent from my iPad
>
>
> On Aug 14, 2017, at 2:12 PM, Greg Mirsky <gregimirsky@gmail.com>; wrote:
>
> Hi Carlos,
>
> thank you for sharing your view on how LSP Echo request with BFD
> Discriminator used to bootstrap a BFD session over MPLS LSP. I'm surprised
> that you refer to RFC 8029 as normative reference when commenting on RFC
> 5884. But even if we look into RFC 8029, it still has the same texts I've
> quoted in the previous note that suggest that echo reply is optional.
> Consider one of them "The Sender's Handle is filled in by the sender and
> returned unchanged by the receiver in the echo reply (if any)." Though
> English is my third language, I interpret "if any" in that sentence as
> clear indication that the echo reply may not be sent ever.
>
>
>
> Regards,
>
> Greg
>
>
>
> On Fri, Aug 11, 2017 at 7:45 PM, Carlos Pignataro (cpignata) <
> cpignata@cisco.com>; wrote:
>
> Jeff, WG,
>
>
>
> I believe there is one additional consideration — please see inline.
>
>
>
> On Aug 11, 2017, at 1:39 PM, Jeffrey Haas <jhaas@pfrc.org>; wrote:
>
>
>
> [Note that I have adjusted the addresses in the headers to try to catch the
> RFC authors' current accounts.]
>
>
> The 5884 interop issue keeps bubbling up.  Balaji submitted an errata,
> which
> provides us with a good place to start technical discussion.
>
> Please note I also spent some time off-list discussing this errata with
> Balaji.
>
>
> On Thu, Aug 10, 2017 at 10:35:50PM -0700, RFC Errata System wrote:
>
> Section: 6
>
> Original Text
> -------------
> The egress LSR MAY respond with an LSP Ping Echo
> reply message that carries the local discriminator assigned by it for
> the BFD session.
>
> Corrected Text
> --------------
> The egress LSR MUST respond with an LSP Ping Echo reply message that
> MAY carry the local discriminator assigned by it for the BFD session.
>
>
> Notes
> -----
> It is not clear from the original text which of the following is optional:
>  -  The egress MUST send a reply, but the discriminator in the reply is
> optional
>  -  The reply itself is optional
>
> Technically, the reply cannot be optional, because the egress needs to
> report LSP-Ping verification status to the ingress.
>
>
>
> This is correct — but even more so, technically, it is not up to RFC 5884
> to define when an LSP-Ping reply is optional or not.
>
>
>
> That’s’ up to https://tools.ietf.org/html/rfc8029#section-4.4
>
>
>
> Lacking a Reply Mode set to "Do not reply" (https://tools.ietf.org/html/
> rfc8029#page-12) the RFC 8029 procedures dictate a response be sent,
> independent of whether the RFC 5884 procedures use that information or not.
>
>
>
> More below.
>
>
>
>
> The proposed text recommends to include BFD discriminator in the reply.
> This was the intent of the original text.
>
>
> My opinion follows:
>
> In section 6 -
>
> :    On receipt of the LSP Ping Echo request message, the egress LSR MUST
> :    send a BFD Control packet to the ingress LSR, if the validation of
> :    the FEC in the LSP Ping Echo request message succeeds.  This BFD
> :    Control packet MUST set the Your Discriminator field to the
> :    discriminator received from the ingress LSR in the LSP Ping Echo
> :    request message.  The egress LSR MAY respond with an LSP Ping Echo
> :    reply message that carries the local discriminator assigned by it for
> :    the BFD session.  The local discriminator assigned by the egress LSR
> :    MUST be used as the My Discriminator field in the BFD session packets
> :    sent by the egress LSR.
>
> In the text above, I consider it quite clear that the receipt of the BFD
> packet contains sufficient state to bring up the BFD session.  The receipt
> of the same Discriminator in the LSP Ping Echo Reply is optional.
>
> This makes sense partially because the reply may be dropped and we want the
> BFD session to come up as fast as possible.
>
>
>
> Yes, especially because the first sentence says that the egress sending a
> BFD Control packet implies FEC validation passed. However,
> https://tools.ietf.org/html/rfc8029#section-4.4 does more than FEC
> validation.
>
>
>
>
> The point of contention appears to be what to do if we *never* get such
> replies.  It's worth pointing out additional text in RFC 5884, section 3.2.
>
> :    Hence, BFD is used in conjunction with LSP Ping for MPLS LSP fault
> :    detection:
> :
> :       i) LSP Ping is used for bootstrapping the BFD session as described
> :          later in this document.
> :
> :      ii) BFD is used to exchange fault detection (i.e., BFD session)
> :          packets at the required detection interval.
> :
> :     iii) LSP Ping is used to periodically verify the control plane
> :          against the data plane by ensuring that the LSP is mapped to
> :          the same FEC, at the egress, as the ingress.
>
> iii above reminds us that the LSP may be torn down because LSP Ping fails.
> Thus, it seems problematic that we do not get a reply ever.
>
> However, with the BFD session in the Up state, we have information proving
> that the LSP is up.  Thus we have contradictory intent.
>
> ---
>
> My opinion is that the MAY in the RFC 5884 procedures is intended to have
> the BFD session come up by the most expedient means.  I do not believe the
> likely intent was to say "don't send Echo Reply".  Among other things, that
> seems contrary to the intent of the general LSP Ping procedures.
>
> Having given my personal observations, we now get to the business of the
> Working Group: Debating intent and related text.
>
>
>
> My individual opinion is that, as written, RFC 5884 cannot mean any other
> thing that “ The egress LSR MUST respond with an LSP Ping Echo reply
> message that
>
> MAY carry the local discriminator assigned by it for the BFD session”.
>
>
>
> In other words, I support this errata.
>
>
>
> This is because RFC 5884 did not update RFC 4379’s procedures. And thus a
> response is needed based on 8029 irregardless of whether 5884 uses it.
>
>
>
> That said, it is debatable whether that LSP Ping response is useful or
> not. If it is not sent, it does not comply to 8029. But if the WG wants for
> it to be not send, a new spec is needed.
>
>
>
> Thanks,
>
>
>
> -- Jeff
>
>
>
> —
>
> Carlos Pignataro, carlos@cisco.com
>
> *“Sometimes I use big words that I do not fully understand, to make myself
> sound more photosynthesis."*
>
>
>
>
>
>