[Make-wifi-fast] Fwd: Re: [iccrg] TCP behavior across WiFi pointers ?

Thu Nov 30 04:03:49 EST 2017

> On Nov 30, 2017, at 09:07, Dave Taht <dave.taht at gmail.com> wrote:
> 
> "We propose that receiving of an 802.11 ACK is a reliable hint that
> the corresponding TCP segments will eventually be processed by the
> receiver’s transport layer. On receipt of an 802.11 ACK from the
> wireless client, the AP will proactively generate a fake TCP ACK on
> behalf of the TCP receiver, forwarding it to the TCP sender. This
> results in the TCP ACKs arriving sooner at the sender, eliminating the
> delay variation induced by medium contention at the receiver. Since
> the receiver will still send a TCP ACK, these now-duplicate TCP ACKs
> should be suppressed by the AP. The TCP sender is thus shielded in the
> revers..."

Now, I admit I am no expert here, but that is crazy talk, making ack thinning look sane (note, I believe that thinning is _sane_ if explicitly opted-in by the end-user, less so if transparently and proactively performed by some hop along the path not under control of the end-users; I have no real opinion on its sanity as a measure against actual congestion.). But this is an active (crazy?) bet on the rest of the rest of the link being error free. In other words it is a gamble which will probably improve the average while also causing more/more extreme extremes, I wonder what measures they use to support their . 

Mildly funny side-note fast in german literally means "almost" in that sense the moniker "fastACK" looks actually quite fitting.

QUESTION:
"The receipt of an 802.11 ACK at the AP triggers it to send the corresponding fast ACK to the TCP sender, thereby moving it past the current sequence number. The client on the other hand would send a duplicate ACK for this lost segment. This is a problem since the TCP sender might not hold that packet anymore in its outgoing bu er, thereby disrupting the TCP session. To avoid this, the FastACK agent inserts each data packet into its local cache before forwarding it downstream. A duplicate ACK from the client then triggers local retransmissions from this cache, on behalf of the TCP sender. Second, local retransmissions are cheaper (and low-overhead) as compared to the end-to-end retransmissions. Further, it also avoids lowering the congestion window of the TCP sender, and maintaining the high end-to-end data ow."

Does that mean that the AP now needs to allow up to a full window of buffering for each TCP flow it fakeACKs?

I have a hunch that the _real_ solution would lie in making the 802.11 MAC have less per packet overhead, so that the need for excessive aggregation would go away, because in the end fastACK aims at lying to the endpoints that there is a relative stable link with nicely bound latency and latency variance when in reality the actual link does not show that, but what do I know...
Then again, since the wifi MAC seems unlikely to change this might be as good as it gets...

> 
> 
> On Thu, Nov 30, 2017 at 12:04 AM, Dave Taht <dave.taht at gmail.com> wrote:
>> wow. *median* of 7 interfering APs at 2.4ghz. 10% with more than 29 interferers.
>> 
>> On Wed, Nov 29, 2017 at 11:39 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>>> The folks over at Meraki had a paper at this year's IMC talking about
>>> TCP over WiFi.
>>> 
>>> -Toke
>>> 
>>> 
>>> 
>>> ---------- Forwarded message ----------
>>> From: Simon Barber <simon at superduper.net>
>>> To: Aaron Falk <aaron.falk at gmail.com>
>>> Cc: Toerless Eckert <tte+ietf at cs.fau.de>, iccrg at irtf.org, tsv-area at ietf.org, Michael Welzl <michawe at ifi.uio.no>, Mark Allman <mallman at icir.org>
>>> Bcc:
>>> Date: Wed, 29 Nov 2017 16:41:04 -0800
>>> Subject: Re: [iccrg] TCP behavior across WiFi pointers ?
>>> We’ve been doing some studies and work in this area at Meraki.
>>> 
>>> https://conferences.sigcomm.org/imc/2017/papers/imc17-final203.pdf
>>> 
>>> Simon
>>> 
>>> 
>>> On Nov 29, 2017, at 7:17 AM, Aaron Falk <aaron.falk at gmail.com> wrote:
>>> 
>>> Hey Mark-
>>> 
>>> Didn't you do some work on TCP over wifi a while ago?
>>> 
>>> --aaron
>>> 
>>> On Wed, Nov 8, 2017 at 1:33 PM Michael Welzl <michawe at ifi.uio.no> wrote:
>>>> 
>>>> I would think that 1) there are probably pointers, and 2) the people who have them should be on the ICCRG list, which I’m cc’ing.
>>>> 
>>>> I suggest for this to be the last email that includes tsvarea so that the thread entirely moves to ICCRG.
>>>> 
>>>> 
>>>>> On Nov 8, 2017, at 6:42 PM, Toerless Eckert <tte+ietf at cs.fau.de> wrote:
>>>>> 
>>>>> Any pointers to work analyzing the differences in behavior when TCP is run
>>>>> across WiFi as opposed to wired ? Especially with WiFi in the home ?
>>>>> 
>>>>> I am primarily thinking that there could be a higher demand for
>>>>> TCP (end-to-end) retransmissions when using WiFi because the L2/WiFi
>>>>> local retransmissions are insufficient. And if so, what the characteristics
>>>>> of those end-to-end retransmissions is (would assume they would be larger
>>>>> than N msec, where N is whatever the L2/wifi protection window is, which
>>>>> unfortunately i don't know).
>>>>> 
>>>>> Asking because we've got the poor "must-sit-in-back-of-the-bus" traffic
>>>>> called IP multicast that is not protected by L2/wifi retransmissions at
>>>>> all and now we're wondering if carrying it over TCP as a workaround
>>>>> could help, and therefore trying to educate myself on specific known
>>>>> issue left when running traffic over TCP over WiFi.
>>>>> 
>>>>> If any other TSV or other WG mailing list might be a better place to
>>>>> ask. pls. let me know.
>>>>> 
>>>>> Thank!
>>>>>   Toerless
>>>>> 
>>>> 
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>>> 
>>> 
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>> 
>> 
>> 
>> --
>> 
>> Dave Täht
>> CEO, TekLibre, LLC
>> http://www.teklibre.com
>> Tel: 1-669-226-2619
> 
> 
> 
> -- 
> 
> Dave Täht
> CEO, TekLibre, LLC
> http://www.teklibre.com
> Tel: 1-669-226-2619
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