Re: [tsvwg] NQB versus WIFI access classes Gedankenexperiment
Toke Høiland-Jørgensen <toke@toke.dk> Mon, 09 September 2019 12:00 UTC
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From: Toke Høiland-Jørgensen <toke@toke.dk>
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To: Sebastian Moeller <moeller0@gmx.de>, tsvwg IETF list <tsvwg@ietf.org>
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Date: Mon, 09 Sep 2019 13:00:04 +0100
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Subject: Re: [tsvwg] NQB versus WIFI access classes Gedankenexperiment
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Sebastian Moeller <moeller0@gmx.de> writes: > Dear List, > > > since it seems clear to me that my objections against using AC_VI or > AC_VO for indifferently for NQB marked packets, mainly falls on deaf > ears. > > > Let me introduce a small Gedankenexperiment to demonstrate why this a > patently problematic idea especially in the light of the stated goals > of the NQB marking and L4S in general. > > Let me propose the following scenario (sticking somewhat to Bob's numbers): > > An dualQ type AQM on the ISP side of the access link which tries to > split (a gross-rate of) 120 Mbps between a QB and a NQB queue more or > less equally*. So full saturation steady state will be: > 60 Mbps QB traffic > 60 Mbps NQB traffic > > Now let's look exactly at that saturating-load situation that is > surprisingly common on end-user internet access links (otherwise an > NQB mark would not really be needed) > > The exact number of flows does not matter for my argument, but just to > make things simple assume each flow takes of 10 Mbps of the > gross-rate. So we have > 6 * NQB flows @10Mbps each (say paced video streams), dscp marked 0x2A > 6 * QB flows @10Mbps average (say non-paced bulk data flows), dscp "marked" 0x00 > > On the end user side this data is flowing over wifi links from the CPE > to the actual endpoints. > > As long as wifi rates stay >> 120 Mbps, things will just work out as > intended by NQB more or less independent of which AC is assigned to > NQB traffic. > > But now the neighbors come home and there is going to be channel > contention in the RF-medium and achievable data rates for our example > household drop to 60 Mbps. > > What the NQB /L4S approach seems to intend in that situation would be > a grace-full reduction in rates for both NQB and QB flows ot an > aggregate of ~30 Mbps each. > > But with NQB mapping to AC_VI/AC_VO what is going to happen ist, that > the NQB flows will secure almost all of the tx-ops and hence starve > out the QB-flows almost completely (doubly so, since the NQB airtime > hogging will make it hard for QB data to reach the endpoints but also > equally hard for ACKs from there to go back to the senders). The > upstream dualQ AQM will see more or less NQB~60Mbps, QB~0Mbps, and > will not help at al, since 60/0 is a valid traffic split. It's worse than this: The maximum aggregation size for AC_VI and AC_VO is way smaller than for AC_BE, so even without interference, the capacity for NQB traffic is going to be smaller, so it doesn't take that much traffic before it starts congesting the WiFi network... -Toke
- [tsvwg] NQB versus WIFI access classes Gedankenex… Sebastian Moeller
- Re: [tsvwg] NQB versus WIFI access classes Gedank… Toke Høiland-Jørgensen