Re: [RTG-DIR] [Detnet] Rtgdir last call review of draft-ietf-detnet-mpls-04

"Carlos Pignataro (cpignata)" <cpignata@cisco.com> Mon, 27 January 2020 18:19 UTC

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From: "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
To: Lou Berger <lberger@labn.net>
CC: Balázs Varga A <balazs.a.varga=40ericsson.com@dmarc.ietf.org>, Routing Directorate <rtg-dir@ietf.org>, "last-call@ietf.org" <last-call@ietf.org>, "draft-ietf-detnet-mpls.all@ietf.org" <draft-ietf-detnet-mpls.all@ietf.org>, "detnet@ietf.org" <detnet@ietf.org>
Thread-Topic: [Detnet] Rtgdir last call review of draft-ietf-detnet-mpls-04
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Date: Mon, 27 Jan 2020 17:49:45 +0000
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Subject: Re: [RTG-DIR] [Detnet] Rtgdir last call review of draft-ietf-detnet-mpls-04
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> 2020/01/27 午前11:59、Lou Berger <lberger@labn.net>のメール:
> 
> okay, will leave it -

Sounds good — 

> but we generally don't provide the motivation for every tradeoff / design decision in IETF standards.
> 

though this is not what was asked :-)

RFC 4385 says:

   To provide consistency between the
   designs of different types of PW, it SHOULD also use the following
   preferred format:

So explaining why not following a SHOULD from the relevant PS seems appropriate.

I agree, it would be counterproductive to provide the motivation for every tradeoff or design decision made. Please do not. The CW format, though, was asked a couple times.

Carlos.

> Lou
> 
> On 1/27/2020 10:08 AM, Carlos Pignataro (cpignata) wrote:
>> Hi, Lou,
>> 
>>> 2020/01/27 午前9:41、Lou Berger <lberger@labn.net>のメール:
>>> 
>>> Hi,
>>> 
>>> Balázs thank you for the clarification -- see below for one comment.
>>> 
>>> On 1/10/2020 8:56 AM, Balázs Varga A wrote:
>>>> Yes, but why not the Preferred CW?
>>>> 
>>>> <Balazs>/<Stewart> Sum of mailing + proposed fixing:
>>>> The PCW only supports a 16bit sequence number and it has the skip zero auto-signaling of active S/N feature.
>>>> This was a problem for DetNet because:
>>>> - We were worried about S/N rollover frequency in some applications and so we wanted the option of a larger S/N.
>>>> - We wanted to have the option to propagate the S/N from the payload to the transport to simplify the implementation
>>>> in some cases. These applications have a non-skip zero S/N. Skip zero is an irritation to implement and we should probably
>>>> have signaled in in PWs.
>>>> As you note in is only a preferred design for PWs, DetNet is not constrained by that and there were good reasons to adopt
>>>> this alternate approach.
>>>> We assume to fix this with adding above information to the text.
>>>> NEW text to be added in section 4.2.1:
>>>>     "This format of the d-CW was created in order (1) to allow larger S/N space to
>>>>     avoid S/N rollover frequency in some applications and (2) to allow non-skip
>>>>     zero S/N what simplifies implementation."
>>> While I completely agree with the rational and validity of the good question, I don't think such motivation belongs in the document.  We generally don't document every design decision in a specification.  I don't feel strongly about this so if others do, I'll defer to their opinion...
>>> 
>>> Balázs, Carlos, Do you think it should stay?
>>> 
>> I have no strong feelings either way, but I believe if this is a departure from a “preferred” format from a BCP, then a one-liner explanation would not hurt. I’d leave this one in.
>> 
>> Thanks,
>> 
>> Carlos.
>> 
>>> Thanks,
>>> 
>>> Lou (as contributor)
>>>