Re: [Bloat] Apple WWDC Talks on Latency/Bufferbloat

[Matt Mathis <mattmathis@google.com>]

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Is there a paper or spec for RPM?

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.

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.

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
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