[tsvwg] draft-ietf-tsvwg-nqb-23 inconsistency of advice

Sebastian Moeller <moeller0@gmx.de> Wed, 29 May 2024 06:53 UTC

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From: Sebastian Moeller <moeller0@gmx.de>
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Date: Wed, 29 May 2024 08:53:22 +0200
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Subject: [tsvwg] draft-ietf-tsvwg-nqb-23 inconsistency of advice
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Dear list,

draft-ietf-tsvwg-nqb-23 contains IMHO incongruent advice:

https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-23#name-guidance-for-lower-rate-lin
Alternatively, operators of networks with lower rate links MAY choose to disable NQB support (and thus aggregate traffic marked with the NQB DSCP with Default traffic) on these lower rate links. For links that have a data rate that is less than ten percent of "typical" path rates, it is RECOMMENDED that the NQB PHB be disabled and for traffic marked with the NQB DSCP to thus be carried using the Default PHB. However, the NQB DSCP SHOULD NOT be re-marked to the Default DSCP (0).

So the advice is to follow general IETF recommendation for unhandled DSCPs and keep the DSCP value but treat them as default forwarding.


While https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-23#name-interoperability-with-exist contains:
    • For application traffic that originates outside of the Wi-Fi network, and thus is transmitted by the Access Point, the choice of DSCP 45 does create a potential for abuse by non-compliant applications. But, opportunities exist in the network components upstream of the Wi-Fi Access Point to police the usage of the NQB DSCP and potentially re-mark traffic that is considered non-compliant, as is recommended in Section 4.4.1. Furthermore, it is a common practice for residential ISPs to re-mark the Diffserv field to zero on all traffic destined to their customers' networks, and any change to this practice done to enable the NQB DSCP to pass through could be done alongside the implementation of the recommendations in Section 4.4.1.


But here we relay on actual DSCP remapping to 0  as a safety mechanism... even though a few section above we explicitly advised against doing that.
So if that is supposed the be a back-stop for WiFi, then we need to change the "6.1. Guidance for Lower-Rate Links" section to reference to the corner case of legacy WiFi.

In addition I would like to see the claim "it is a common practice for residential ISPs" substantiated quantitatively. The last two ISPs (both big European Telcos) I was /am customer of do not generally remark DSCPs to zero (I took packet captures of my link and looked at the DSCPs of incoming packets, if the claim would be true all packets would be DSCP 0, but that is not what I observed and e.g. ).
If we consider that remarking to zero is against IETF recommendations and at best done inconsistently, we should remove this argument from "7.3.1. Interoperability with Existing Wi-Fi Networks".

Regards
	Sebastian

P.S.: The solution to all of this is still rather obvious, instead of making the side effect of NQB on WiFi opt-out, change the desired special treatment of NQB over WiFi to be opt-in with the default behaviour to stick to AC_BE: et voila instant safety by design and no having to rely on circumstantial safety via expected DSCP re-mapping behaviour of non participating parties.