[tsvwg] draft-ietf-tsvwg-nqb, more questions

Sebastian Moeller <moeller0@gmx.de> Mon, 04 November 2019 22:24 UTC

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From: Sebastian Moeller <moeller0@gmx.de>
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Date: Mon, 4 Nov 2019 23:11:58 +0100
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Subject: [tsvwg] draft-ietf-tsvwg-nqb, more questions
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Regarding https://datatracker.ietf.org/doc/draft-ietf-tsvwg-nqb/?include_text=1

7.3.  WiFi Networks

   WiFi networking equipment compliant with 802.11e generally supports
   either four or eight transmit queues and four sets of associated EDCA
   parameters (corresponding to the four WiFi Multimedia Access
   Categories) that are used to enable differentiated media access
   characteristics.  Implementations typically utilize the IP DSCP field
   to select a transmit queue, but should be considered as Non-
   Differentiated Services-Compliant Nodes as described in Section 4 of
   [RFC2475].  As a result this document discusses interoperability with
   WiFi networks, as opposed to PHB compliance.

   As discussed in [RFC8325], most existing implementations use a
   default DSCP to User Priority mapping that utilizes the most
   significant three bits of the DiffServ Field to select "User
   Priority" which is then mapped to the four WMM Access Categories.  In
   order to increase the likelihood that NQB traffic is provided a
   separate queue from QB traffic in existing WiFi equipment, the 0x2A
   codepoint is preferred for NQB.  This would map NQB to UP_5 which is
   in the "Video" Access Category.

   Systems that utilize [RFC8325], SHOULD map the NQB codepoint to UP_5
   in the "Video" Access Category.

   In order to preserve the incentives principle, WiFi systems SHOULD
   configure the EDCA parameters for the Video Access Category to match
   those of the Best Effort Access Category.


[SM] This last section is puzzling: if the wifi system configures AC_VI with EDCA parameters that match the AC_BE parameters, AC_VI ceases to be different from AC_BE, in that case picking a codepoint that automatically maps to CS0 and hence to AC_BE  seems much safer, simpler and straight forward to me. 
Especially since essentially none of the millions deployed WiFi APs out there will a) have this configured like proposed already and b) none of the consumer APs I know actually allow to easily adjust EDCA parameters at all. I guess I must be missing something and would be delighted to be shown why the proposed text is the right thing.
My take on this still is, if NQB traffic is sufficiently sparse using AC_VI can be justified, but without any rate limits this has the potential of being quite unfair to concurrent APs on the same channel (as well as the neighboring channels that overlap with the selected). 
I do not want to sound alarmist, but given the number of cable-ISP WiFi-APs (as indicated by a SSID containing the ISPs name) in my city, I believe making sure that those APs will not basically start hogging most airtime seems the prudent thing to do. If there are sufficient backstops in place (like rate limiting or automatic down-marking if the traffic is not sparse enough) to avoid the described situation, I am all for it.

The text probably should also openly discuss that in WiFi/WMM the four available queues by design have different priorities, and by moving NQB out of the default AC_BE while leaving QB flows in there, this effectively runs against  the following text in the draft: "The NQB queue SHOULD be given equal priority compared to queue-building traffic of equivalent importance." (leaving alone the question how an AP or a station is supposed to measure importance)


Sebastian