From: Sebastian Moeller <moeller0@gmx.de>
To: Christoph Paasch <cpaasch@apple.com>
Cc: Matt Mathis <mattmathis@google.com>, bloat <bloat@lists.bufferbloat.net>
Subject: Re: [Bloat] Apple WWDC Talks on Latency/Bufferbloat
Date: Tue, 29 Jun 2021 09:58:02 +0200 [thread overview]
Message-ID: <EC9D2550-897D-49A1-A814-DDBC92C9A78E@gmx.de> (raw)
In-Reply-To: <YMveJhEsnRi0OF2O@MacBook-Pro.local>
Hi Christoph,
one question below:
> On Jun 18, 2021, at 01:43, Christoph Paasch via Bloat <bloat@lists.bufferbloat.net> wrote:
>
> Hello,
>
> On 06/17/21 - 11:16, Matt Mathis via Bloat wrote:
>> Is there a paper or spec for RPM?
>
> we try to publish an IETF-draft on the methodology before the upcoming IETF
> in July.
>
> But, in the mean-time please see inline:
>
>> There are at least two different ways to define RPM, both of which might be
>> relevant.
>>
>> At the TCP layer: it can be directly computed from a packet capture. The
>> trick is to time reverse a trace and compute the critical path backwards
>> through the trace: what event triggered each segment or ACK, and count
>> round trips. This would be super robust but does not include the queueing
>> required in the kernel socket buffers. I need to think some more about
>> computing TCP RPM from tcp_info or other kernel instrumentation - it might
>> be possible.
>
> We explicitly opted against measuring purely TCP-level round-trip times. Because
> there are countless transparent TCP-proxies out there that would skew these
> numbers. Our goal with RPM/Responsiveness is to measure how an end-user would
> experience the network. Which means, DNS-resolution, TCP handshake-time,
> TLS-handshake, HTTP/2 Request/response. Because, at the end, that's what
> actually matters to the users.
>
>> A different RPM can be done in the application, above TCP, for example by
>> ping-ponging messages. This would include the delays traversing the kernel
>> socket buffers which have to be at least as large as a full network RTT.
>>
>> This is perhaps an important point: due to the retransmit and
>> reassuebly queues (which are required to implement robust data delivery)
>> TCP must be able hold at least a full RTT of data in it's own buffers,
>> which means that under some conditions the RTT as seen by the application
>> has be be at least twice the network's RTT, including any bloat in the
>> network.
>
> Currently, we measure RPM on separate connections (not the load-bearing
> ones). We are also measuring on the load-bearing connections themselves
> through H2 Ping frames. But for the reasons you described we haven't yet
> factored it into the RPM-number.
>
> One way may be to inspect with TCP_INFO whether or not the connections had
> retransmissions and then throw away the number. On the other hand, if the
> network becomes extremely lossy under working conditions, it does impact the
> user-experience and so it could make sense to take this into account.
>
>
> In the end, we realized how hard it is to accurately measure bufferbloat
> within a reasonable time-frame (our goal is to finish the test within ~15
> seconds).
[SM] I understand that 10-15 seconds is the amount of time users have been trained to expect an on-line speedtest to take, but experiments with flent/RRUL showed that there are latency affection processes on slower timescales that are better visible if one can also run a test for 60 - 300 seconds (e.g. cyclic WiFi channel probing). Does your tool optionally allow to specify a longer run-time?
Thinking of it, to keep everybody on their toes, how about occasionally running a test with longer run-time (maybe after asking the users consent) and store the test duration as part of the results?
Best Regards
Sebastian
>
> We hope that with the IETF-draft we can get the right people together to
> iterate over it and squash out a very accurate measurement that represents
> what users would experience.
>
>
> Cheers,
> Christoph
>
>
>>
>> Thanks,
>> --MM--
>> The best way to predict the future is to create it. - Alan Kay
>>
>> We must not tolerate intolerance;
>> however our response must be carefully measured:
>> too strong would be hypocritical and risks spiraling out of
>> control;
>> too weak risks being mistaken for tacit approval.
>>
>>
>> On Sat, Jun 12, 2021 at 9:11 AM Rich Brown <richb.hanover@gmail.com> wrote:
>>
>>>> On Jun 12, 2021, at 12:00 PM, bloat-request@lists.bufferbloat.net wrote:
>>>>
>>>> Some relevant talks / publicity at WWDC -- the first mentioning CoDel,
>>>> queueing, etc. Featuring Stuart Cheshire. iOS 15 adds a developer test
>>> for
>>>> loaded latency, reported in "RPM" or round-trips per minute.
>>>>
>>>> I ran it on my machine:
>>>> nowens@mac1015 ~ % /usr/bin/networkQuality
>>>> ==== SUMMARY ====
>>>> Upload capacity: 90.867 Mbps
>>>> Download capacity: 93.616 Mbps
>>>> Upload flows: 16
>>>> Download flows: 20
>>>> Responsiveness: Medium (840 RPM)
>>>
>>> Does anyone know how to get the command-line version for current (not
>>> upcoming) macOS? Thanks.
>>>
>>> Rich
>>> _______________________________________________
>>> Bloat mailing list
>>> Bloat@lists.bufferbloat.net
>>> https://lists.bufferbloat.net/listinfo/bloat
>>>
>
>> _______________________________________________
>> Bloat mailing list
>> Bloat@lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/bloat
>
> _______________________________________________
> Bloat mailing list
> Bloat@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/bloat
next prev parent reply other threads:[~2021-06-29 7:58 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-12 16:11 Rich Brown
2021-06-17 18:16 ` Matt Mathis
2021-06-17 23:43 ` Christoph Paasch
2021-06-18 0:17 ` Matt Mathis
2021-06-18 1:03 ` Christoph Paasch
2021-06-18 3:33 ` Matt Mathis
2021-06-28 22:54 ` Christoph Paasch
2021-06-29 7:58 ` Sebastian Moeller [this message]
2021-07-06 18:54 ` Christoph Paasch
2021-07-06 19:08 ` Sebastian Moeller
-- strict thread matches above, loose matches on Subject: below --
2021-06-11 19:14 Nathan Owens
2021-06-11 21:58 ` Jonathan Morton
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