Is this specialized upstream TCP ACK handling, particularly the prioritization a general recommendation in all access technologies? Perhaps it should be, since otherwise up and downstream TCP flows interfere in a crazy queue oscillation that is typically misinterpreted by AQMs. Is this topic addressed in some RFC already?
Wolfram > -----Ursprüngliche Nachricht----- > Von: aqm [mailto:[email protected]] Im Auftrag von Greg White > Gesendet: Dienstag, 6. Oktober 2015 18:35 > An: Mikael Abrahamsson; [email protected] > Betreff: Re: [aqm] TCP ACK Suppression > > Mikael, > > Specialized upstream TCP ACK handling (which can include both > prioritization and suppression) is a recommended feature in the DOCSIS > specification. The details of the implementation are left to the > manufacturer, but I don't expect that it is actually done at dequeue (packet > processing at dequeue is expensive in cable modems). Rather, I expect that > devices identify ACKs at enqueue, and retain (separate from the main > service-flow queue) a single ACK for each TCP session. Then, upon receiving > a grant, the ACK queue is flushed first, followed by packets from the main > queue. > > The CM is not permitted to issue bandwidth requests for more data than it > has available to send, so bandwidth requests would need to already have > ACK suppression taken into account. For this reason (and the above), I > doubt that the CM would include suppressed ACKs in its queue depth and > queuing latency estimation. > > AQM in DOCSIS also happens at enqueue. The spec is silent on whether the > upstream TCP ACKs are subject to AQM packet drop, but it would be > compliant for them (i.e. the one ACK per session) to be protected. > > -Greg > > > On 10/6/15, 1:20 AM, "aqm on behalf of Mikael Abrahamsson" > <[email protected] on behalf of [email protected]> wrote: > > > > >Hi, > > > >after noticing that some TCP ACKs on my home DOCSIS connection were > not > >making it to their destination, I after some interaction with cable > >Internet people, I found this: > > > >http://www.cedmagazine.com/article/2006/12/docsis-sub-throughput- > optimi > >zat > >ion > > > >"TCP ACK Suppression (TAS)" > > > >"TCP ACK Suppression overcomes the TRGC limitation without actually > >affecting the DOCSIS specification or involving the CMTS. It improves > >downstream TCP transmissions by taking advantage of TRGC and only > >sending the last ACK it receives when its data grant becomes active. > >Thus, the number of TCP ACKs is fewer, but the number of bytes > >acknowledged by each TCP ACK is increased." > > > >So the DOCSIS modem basically looks at all the ACKs in the queue at the > >time of transmission (DOCSIS uses a "grant" system to tell a modem when > >it's allowed to transmit on the shared medium), and then basically > >deletes all the redundant ACKs (the ones who are just increasing > >linearly without indicating packet drop) and keeps the highest ACK > >only. > > > >Now, this kind of mechanism, how should it be treated when it comes to > >AQM? This mechanism is basically done at de-queue, when a number of > >packets are emptied from the queue at one time, which is then allowed > >to fill up again until the next transmit opportunity arises. > > > >Or is this a non-problem because it's likely that any AQM employed here > >would use the buffer fill right after a transmit opportunity has > >finished (for those that consider buffer fill as a variable), which > >would mean that most likely the TCP ACK purging had already occured so > >this mechanism doesn't influence the AQM in any significant manner > >anyway? > > > >Just as a data point from my home connection, I have 250/50 (down/up) > >and when downloading at 250 megabit/s, the upstream traffic is reduced > >by approximately 20x, so instead of sending 10 megabit/s (or so) of > >ACKs, I see approximately 500 kilobits/s of ACKs. > > > >-- > >Mikael Abrahamsson email: [email protected] > > > >_______________________________________________ > >aqm mailing list > >[email protected] > >https://www.ietf.org/mailman/listinfo/aqm > > _______________________________________________ > aqm mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/aqm
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