Re: [Technical Errata Reported] RFC5884 (5085)

"Carlos Pignataro (cpignata)" <cpignata@cisco.com> Sat, 12 August 2017 02:53 UTC

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From: "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
To: Greg Mirsky <gregimirsky@gmail.com>
CC: Jeff Haas <jhaas@pfrc.org>, "kireeti@juniper.net" <kireeti@juniper.net>, Tom Nadeau <tnadeau@lucidvision.com>, "rtg-bfd@ietf.org" <rtg-bfd@ietf.org>, "Reshad Rahman (rrahman)" <rrahman@cisco.com>, Alia Atlas <akatlas@gmail.com>
Subject: Re: [Technical Errata Reported] RFC5884 (5085)
Thread-Topic: [Technical Errata Reported] RFC5884 (5085)
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Date: Sat, 12 Aug 2017 02:53:47 +0000
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References: <20170811053550.27303B81263@rfc-editor.org> <20170811173930.GJ24942@pfrc.org> <CA+RyBmWYRAT3g=zCN+ot9mFDYuPCOGCwyPQJp-+9AfA6oJjttA@mail.gmail.com>
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Greg,

> On Aug 11, 2017, at 2:12 PM, Greg Mirsky <gregimirsky@gmail.com>; wrote:
> 
> Re-sending to the corrected list (apologies for duplicates).
> 
> Dear All,
> I suggest to reject this proposal. The current text is clear and the mechanics of bootstrapping BFD session over MPLS LSP is well understood - remote peer MUST start sending BFD control packets first and BFD peer MAY send Echo Reply with its Local Discriminator as value in BFD Discriminator TLV.
> 

This seems to repeat the text in 5884 without explaining why you feel a particular interpretation is the correct technical one.

The text you include:
	“MAY send Echo Reply with its Local Discriminator as value in BFD Discriminator TLV”

suffers from the ambiguity that this Errata is trying to clarify. Which one is it?
* (MAY send Echo Reply with its Local Disc)
* (MAY send Echo Reply), with its Local Disc.

The actual text is:
   The egress LSR MAY respond with an LSP Ping Echo
!  reply message that carries the local discriminator assigned by it for
   the BFD session. 

And NOT:
   The egress LSR MAY respond with an LSP Ping Echo
!  reply message, which carries the local discriminator assigned by it for
   the BFD session. 


Based on restrictive versus non-restrictive clause, I feel it is correct to accept the errata.


And by the way, RFC 5884 is not say what happens if the LSP Ping Reply has a different discriminator value!

Thanks,

Carlos.

> Regards,
> Greg
> 
> 
> On Fri, Aug 11, 2017 at 10:39 AM, 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.
> >
> > 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.
> 
> 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.
> 
> -- Jeff
> 
> 

—
Carlos Pignataro, carlos@cisco.com

“Sometimes I use big words that I do not fully understand, to make myself sound more photosynthesis."