Subject: Re: [Lsr] WG Last Call for draft-ietf-lsr-flex-algo

From: Shraddha Hegde <[email-masked]>
Date: Tue, 25 Aug 2020 15:27:38 +0000

Hi All,

draft-lsr-flex-algo-00 was created by combining
draft-hegdeppsenak-isis-sr-flex-algo-02 and draft-ppsenak-ospf-sr-flex-algo-00.
When draft-lsr-flex-algo-00 was published as a WG document, it included
a requirement for using te-app encodings that did not exist in either
draft-hegdeppsenak-isis-sr-flex-algo-02 and draft-ppsenak-ospf-sr-flex-algo-00.

Juniper's currently released implementation of flex-algo uses legacy encodings,
as opposed to te-app encodings.  I would like the following text added to
draft-lsr-flex-algo in order to record the history of these changes and to make
operators aware of possible inter-op problems that may arise due to the
non-backward compatible nature of mandating ASLA encodings.

=====
11.  Advertisement of Link Attributes for Flex-Algorithm

" Earlier versions of this draft did not mandate the use of ASLA TLVs for encoding the
link attributes. There may be implementations that depend on legacy encodings as defined in
RFC 5305, RFC 7810 , RC 3630 and RFC 7471. Implementations that look at only ASLA encodings
for flex-algo based on this version of the document will not interoperate with versions
that use legacy advertisements. "
========

Rgds
Shraddha

Juniper Business Use Only
From: [email-masked] <[email-masked]>
Sent: Thursday, August 20, 2020 7:56 PM
To: Peter Psenak <[email-masked]>; [email-masked]; Robert Raszuk <[email-masked]>
Cc: Christian Hopps <[email-masked]>; [email-masked]; Les Ginsberg (ginsberg) <ginsberg=[email-masked]>; [email-masked]; [email-masked]; Acee Lindem (acee) <[email-masked]>
Subject: Re: [Lsr] WG Last Call for draft-ietf-lsr-flex-algo

[External Email. Be cautious of content]

Peter,
Le 20/08/2020 à 14:12, Peter Psenak a écrit :
Hi Olivier,

On 20/08/2020 13:58, [email-masked]<mailto:[email-masked]> wrote:
Hi Peter,

Thank for the new version.

Le 19/08/2020 à 14:00, Peter Psenak a écrit :
Olivier,
[ ... ]
So, to speed up the deployment, I would prefer a reference to a delay value that could be advertise by means of RFC7471, RFC8570 and/or TE-App draft. It is then up to the operator to ensure the coherency of what it is announced in its network by the different routers.

I know you don't like the app specific link advertisement, but I'm afraid what you ask for is absolutely wrong.

We defined the ASLA encoding to address a real problems for advertising the link attributes. We allow the link attributes to be advertised in both legacy and ASLA advertisement for legacy application (RSVP-TE, SRTE) to address the backward compatibility. Flex-algo is a new application, there is absolutely no need to use the legacy advertisement. Doing so would just extend the problem to the flex-algo application.

Regarding the new version you provided, new section 5.1 (for IS-IS) and section 5.2 (for OSPF) mention respectively RFC 8570 and RFC 7471 for the definition of Min delay and TE metric which is fine for me. But, they also made reference to draft isis-te-app, respectively ospf-te-link-attr-reuse to encode these value.

that's what people were asking for. And it is right because we are mandating the usage of ALSA encoding for any flex-algo related link attributes.

Here, it is confusing.

I don't see how much more clear we can make it.

Indeed, RFC 8570 and RFC 7471 also define the way to encode TE metric and Min delay.

you have to distinguish between two things:

a)  where Min delay and TE metric were defined - RFC 8570 and RFC 7471
b)  how we encode it for flex-algo - isis-te-app,
ospf-te-link-attr-reuse

What I'm suggesting, is a clear reference to the RFC for TE metric and Min delay definition as well as the encoding (especially for the delay) while leaving open the door to how the router acquire these values: legacy a.k.a. RFC 8570 & 7471 or new draft a.k.a draft-isis-te-app & draft-ospf-link-attr-reuse.

no. This will not be done. We only allow ASLA advertisement for these metrics and other link attributes that are used for flex-algo. It is done for a reason and I have already explained that.
OK. Reading section 11 which clarify how metrics are convey, let me suggest to make a reference to section 11 in section 5.1 and 5.2 instead of reference to drafts.

In fact, in section 17.1.2, you mention only reference to RFC 8570 & RFC7471 for the IANA definition which is fine for me.

because in registry, we are defining a metric type, not how we are going to advertise it for the link.
OK.

I would suggest the same wording for section 5.1. and 5.2 leaving operator free about how it collect the values from the neighbour routers: legacy or new method.

please stop trying to make use of legacy RSVP-TE link advertisements for flex-algo - it will not be allowed.

This raise to me a simple question: Is it possible to use 2 different Flex Algo with delay metric, one for App A and another one for App B ? if yes, how can we link metrics advertise in ALSA A from metrics advertise in ALSA B ? The draft mention only one bit for Flex-Algo.

Regards,

Olivier

PS. I note a duplicate paragraph in section 12: "When computing the path for a given Flex-Algorithm, the metric-type that is part of the Flex-Algorithm definition (Section 5) MUST be used."

thanks,
Peter

Regards

Olivier

PS. We have a pre-alpha implementation of flex algo using the legacy metrics and I know that recent IOS-XR provided similar implementation of flex algo based on legacy metrics.

regards,
Peter

Regards

Olivier

Le 18/08/2020 à 19:02, [email-masked]<mailto:[email-masked]> a écrit :

Robert,

Thank you, exactly.

We just need a clarification of the document.  I don't understand why this is such a big deal.

Tony

On Aug 18, 2020, at 9:22 AM, Robert Raszuk <[email-masked]<mailto:[email-masked]> <mailto:[email-masked]><mailto:[email-masked]>> wrote:

Les,

I think this is not very obvious as Tony is pointing out.

See RFC 8570 says:

       Type    Description
       ----------------------------------------------------
        33     Unidirectional Link Delay

        34     Min/Max Unidirectional Link Delay

That means that is someone implementing it reads text in this draft literally (meaning Minimum value of Unidirectional Link Delay) it may pick minimum value from ULD type 33 :)

If you want to be precise this draft may say minimum value of Min/Max Unidirectional Link Delay (34) and be done.

That's all.

Cheers,
R.

On Tue, Aug 18, 2020 at 6:04 PM Les Ginsberg (ginsberg) <ginsberg=[email-masked]<mailto:ginsberg=[email-masked]> <mailto:[email-masked]><mailto:[email-masked]>> wrote:

    Tony -

    As an author of both RFC 8570 and I-D.ietf-isis-te-app, I am not
    sure why you are confused - nor why you got misdirected to code
    point 33.

    RFC 8570 (and its predecessor RFC 7810) define:

    34           Min/Max Unidirectional Link Delay

    This sub-TLV contains two values:

    "Min Delay:  This 24-bit field carries the minimum measured link
    delay

          value (in microseconds) over a configurable interval,
    encoded as

          an integer value.

       Max Delay:  This 24-bit field carries the maximum measured
    link delay

          value (in microseconds) over a configurable interval,
    encoded as

          an integer value."

    It seems clear to me that the flex-draft is referring to Min
    Unidirectional Link Delay in codepoint 34.

    I agree it is important to be unambiguous in specifications, but
    I think Peter has been very clear.

    Please explain how you managed to end up at code point 33??

       Les

    *From:* Lsr <[email-masked]<mailto:[email-masked]> <mailto:[email-masked]><mailto:[email-masked]>>
    *On Behalf Of [email-masked]<mailto:[email-masked]> <mailto:[email-masked]><mailto:[email-masked]>
    *Sent:* Tuesday, August 18, 2020 7:44 AM
    *To:* Peter Psenak (ppsenak) <[email-masked]<mailto:[email-masked]>
<mailto:[email-masked]><mailto:[email-masked]>>
    *Cc:* [email-masked]<mailto:[email-masked]> <mailto:[email-masked]><mailto:[email-masked]>; [email-masked]<mailto:[email-masked]>
<mailto:[email-masked]><mailto:[email-masked]>; Christian Hopps <[email-masked]<mailto:[email-masked]>
<mailto:[email-masked]><mailto:[email-masked]>>; Acee Lindem (acee) <[email-masked]<mailto:[email-masked]>
<mailto:[email-masked]><mailto:[email-masked]>>; [email-masked]<mailto:[email-masked]>
<mailto:[email-masked]><mailto:[email-masked]>
    *Subject:* Re: [Lsr] WG Last Call for draft-ietf-lsr-flex-algo

    Hi Peter,

        section 5.1 of the draft-ietf-lsr-flex-algo says:

        Min Unidirectional Link Delay as defined in
        [I-D.ietf-isis-te-app].

        We explicitly say "Min Unidirectional Link Delay", so this
        cannot be mixed with other delay values (max, average).

    The problem is that that does not exactly match "Unidirectional
    Link Delay" or "Min/Max Unidirectional Link Delay", leading to
    the ambiguity. Without a clear match, you leave things open to
    people guessing. Now, it's a metriic, so of course, you always
    want to take the min.  So type 33 seems like a better match.

        section 7.3. of ietf-isis-te-app says:

        Type   Description                          Encoding
                                                   Reference
        ---------------------------------------------------------
        34      Min/Max Unidirectional Link Delay    RFC8570

    And it also says:

    33      Unidirectional Link Delay RFC8570
<https://tools.ietf.org/html/rfc8570><https://urldefense.com/v3/__https:/tools.ietf.org/html/rfc8570__;!!NEt6yMaO-gk!WKuLWanfqtEwcGfdMcPi6zO93gLNz6GLtiLn6c7mmIPhPYuTAufXgyh1ir2QDxsg$>

    This does not help.

        So, IMHO what we have now is correct and sufficient, but I
        have no issue adding the text you proposed below.

    What you have now is ambiguous. We have a responsibility, as
    writers of specifications, to be precise and clear.  We are not
    there yet.

        BTW, before I posted 09 version of flex-algo draft, I asked
        if you were fine with just referencing ietf-isis-te-app in
        5.1. I thought you were, as you did not indicate otherwise.

    My bad, I should have pressed the issue.

        Anyway, I consider this as a pure editorial issue and
        hopefully not something that would cause you to object the WG
        LC of the flex-algo draft.

    I'm sorry, I think that this is trivially resolved, but important
    clarification.

    You also have an author's email that is bouncing, so at least one
    more spin is required.

    Sorry,

    Tony

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