[Bloat] Apple WWDC Talks on Latency/Bufferbloat

Christoph Paasch cpaasch at apple.com
Thu Jun 17 19:43:34 EDT 2021


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

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.


> 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
> > _______________________________________________
> > Bloat mailing list
> > Bloat at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/bloat
> >

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