[Bloat] Apple WWDC Talks on Latency/Bufferbloat

Sebastian Moeller moeller0 at gmx.de
Tue Jun 29 03:58:02 EDT 2021

Hi Christoph,

one question below:

> On Jun 18, 2021, at 01:43, Christoph Paasch via Bloat <bloat at 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

> 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 at gmail.com> wrote:
>>>> On Jun 12, 2021, at 12:00 PM, bloat-request at 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 at 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
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