Re: [Idr] [Technical Errata Reported] RFC4271 (5001)

"John G. Scudder" <jgs@juniper.net> Fri, 21 April 2017 14:33 UTC

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From: "John G. Scudder" <jgs@juniper.net>
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Date: Fri, 21 Apr 2017 10:33:45 -0400
CC: "Alvaro Retana (aretana)" <aretana@cisco.com>, "idr@ietf.org" <idr@ietf.org>
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To: Bruno Decraene <bruno.decraene@orange.com>
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Subject: Re: [Idr] [Technical Errata Reported] RFC4271 (5001)
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On Apr 19, 2017, at 2:45 PM, bruno.decraene@orange.com wrote:
> 
> IMHO, OPTIONAL may not be what was intended.
> As per RFC 2119 " the adjective "OPTIONAL", mean that an item is
>   truly optional.  One vendor may choose to include the item because a
>   particular marketplace requires it or because the vendor feels that
>   it enhances the product while another vendor may omit the same item."
> 
> Here, I think that the original intention was to say that the attribute MAY be sent if appropriate and MAY be omitted if appropriate. IOW, it is _not_ REQUIRED to always send it.
> 
> As an example, quickly parsing RFC 4171, it does not seem to indicate whether the LOCAL_PREF attribute is mandatory or discretionary. But given that it is well-known, as per §5, it can only be mandatory or discretionary. Given that it usually does not appear in EBGP session, I would argue that it is not mandatory, hence discretionary. But I don't think that we could say that sending the attribute LOCAL_PREF is OPTIONAL given that RFC 4271 mandates its use in IBGP (plus not sending it on some IBGP sessions would create forwarding loops).

In short: there is no good fit between the RFC 4271 concept of "discretionary" and any RFC 2119 keyword at all. Agreed. (Actually there is not even a good match between the RFC 4271 concept of "discretionary" and the plain-English definition of "discretionary", more's the pity.)

> I may propose the follow text: Others are discretionary and may or may not be sent in a particular UPDATE message
> Or: Others are discretionary and are not required to be sent in all UPDATE message

I would be OK with either of these options, or with simply truncating the sentence: "Others are discretionary." (It's clear from context that they aren't required to be sent with every update, the previous sentence being the "exception that proves the rule".)

Thanks for the feedback.

--John