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We in WiFi find UDP, while useful, also has severe limitations. The impact
to the TCP control loop matters a lot for things like aggregation.
Visualizations can be useful but also a bit limiting. We use stats
techniques such as PCA which is more mathematical and less visual.
We find syscall connect() times as a bit more relevant to user experience
than ICMP pings which are typically originated and terminated in kernel
space.
Bob
On Mon, Jul 12, 2021 at 1:32 PM Ben Greear <gree...@candelatech.com> wrote:
> UDP is better for getting actual packet latency, for sure. TCP is
> typical-user-experience-latency though,
> so it is also useful.
>
> I'm interested in the test and visualization side of this. If there were
> a way to give engineers
> a good real-time look at a complex real-world network, then they have
> something to go on while trying
> to tune various knobs in their network to improve it.
>
> I'll let others try to figure out how build and tune the knobs, but the
> data acquisition and
> visualization is something we might try to accomplish. I have a feeling
> I'm not the
> first person to think of this, however....probably someone already has
> done such
> a thing.
>
> Thanks,
> Ben
>
> On 7/12/21 1:04 PM, Bob McMahon wrote:
> > I believe end host's TCP stats are insufficient as seen per the "failed"
> congested control mechanisms over the last decades. I think Jaffe pointed
> this out in
> > 1979 though he was using what's been deemed on this thread as "spherical
> cow queueing theory."
> >
> > "Flow control in store-and-forward computer networks is appropriate for
> decentralized execution. A formal description of a class of "decentralized
> flow control
> > algorithms" is given. The feasibility of maximizing power with such
> algorithms is investigated. On the assumption that communication links
> behave like M/M/1
> > servers it is shown that no "decentralized flow control algorithm" can
> maximize network power. Power has been suggested in the literature as a
> network
> > performance objective. It is also shown that no objective based only on
> the users' throughputs and average delay is decentralizable. Finally, a
> restricted class
> > of algorithms cannot even approximate power."
> >
> > https://ieeexplore.ieee.org/document/1095152
> >
> > Did Jaffe make a mistake?
> >
> > Also, it's been observed that latency is non-parametric in it's
> distributions and computing gaussians per the central limit theorem for OWD
> feedback loops
> > aren't effective. How does one design a control loop around things that
> are non-parametric? It also begs the question, what are the feed forward
> knobs that can
> > actually help?
> >
> > Bob
> >
> > On Mon, Jul 12, 2021 at 12:07 PM Ben Greear <gree...@candelatech.com
> <mailto:gree...@candelatech.com>> wrote:
> >
> > Measuring one or a few links provides a bit of data, but seems like
> if someone is trying to understand
> > a large and real network, then the OWD between point A and B needs
> to just be input into something much
> > more grand. Assuming real-time OWD data exists between 100 to 1000
> endpoint pairs, has anyone found a way
> > to visualize this in a useful manner?
> >
> > Also, considering something better than ntp may not really scale to
> 1000+ endpoints, maybe round-trip
> > time is only viable way to get this type of data. In that case,
> maybe clever logic could use things
> > like trace-route to get some idea of how long it takes to get 'onto'
> the internet proper, and so estimate
> > the last-mile latency. My assumption is that the last-mile latency
> is where most of the pervasive
> > assymetric network latencies would exist (or just ping 8.8.8.8 which
> is 20ms from everywhere due to
> > $magic).
> >
> > Endpoints could also triangulate a bit if needed, using some anchor
> points in the network
> > under test.
> >
> > Thanks,
> > Ben
> >
> > On 7/12/21 11:21 AM, Bob McMahon wrote:
> > > iperf 2 supports OWD and gives full histograms for TCP write to
> read, TCP connect times, latency of packets (with UDP), latency of "frames"
> with
> > > simulated video traffic (TCP and UDP), xfer times of bursts with
> low duty cycle traffic, and TCP RTT (sampling based.) It also has support
> for sampling (per
> > > interval reports) down to 100 usecs if configured with
> --enable-fastsampling, otherwise the fastest sampling is 5 ms. We've
> released all this as open source.
> > >
> > > OWD only works if the end realtime clocks are synchronized using
> a "machine level" protocol such as IEEE 1588 or PTP. Sadly, *most data
> centers don't
> > provide
> > > sufficient level of clock accuracy and the GPS pulse per second *
> to colo and vm customers.
> > >
> > > https://iperf2.sourceforge.io/iperf-manpage.html
> > >
> > > Bob
> > >
> > > On Mon, Jul 12, 2021 at 10:40 AM David P. Reed <
> dpr...@deepplum.com <mailto:dpr...@deepplum.com> <mailto:
> dpr...@deepplum.com
> > <mailto:dpr...@deepplum.com>>> wrote:
> > >
> > >
> > > On Monday, July 12, 2021 9:46am, "Livingood, Jason" <
> jason_living...@comcast.com <mailto:jason_living...@comcast.com>
> > <mailto:jason_living...@comcast.com <mailto:
> jason_living...@comcast.com>>> said:
> > >
> > > > I think latency/delay is becoming seen to be as important
> certainly, if not a more direct proxy for end user QoE. This is all still
> evolving and I
> > have
> > > to say is a super interesting & fun thing to work on. :-)
> > >
> > > If I could manage to sell one idea to the management
> hierarchy of communications industry CEOs (operators, vendors, ...) it is
> this one:
> > >
> > > "It's the end-to-end latency, stupid!"
> > >
> > > And I mean, by end-to-end, latency to complete a task at a
> relevant layer of abstraction.
> > >
> > > At the link level, it's packet send to packet receive
> completion.
> > >
> > > But at the transport level including retransmission buffers,
> it's datagram (or message) origination until the acknowledgement arrives
> for that
> > message being
> > > delivered after whatever number of retransmissions, freeing
> the retransmission buffer.
> > >
> > > At the WWW level, it's mouse click to display update
> corresponding to completion of the request.
> > >
> > > What should be noted is that lower level latencies don't
> directly predict the magnitude of higher-level latencies. But longer lower
> level latencies
> > almost
> > > always amplfify higher level latencies. Often non-linearly.
> > >
> > > Throughput is very, very weakly related to these latencies,
> in contrast.
> > >
> > > The amplification process has to do with the presence of
> queueing. Queueing is ALWAYS bad for latency, and throughput only helps if
> it is in exactly the
> > > right place (the so-called input queue of the bottleneck
> process, which is often a link, but not always).
> > >
> > > Can we get that slogan into Harvard Business Review? Can we
> get it taught in Managerial Accounting at HBS? (which does address
> logistics/supply chain
> > queueing).
> > >
> > >
> > >
> > >
> > >
> > >
> > >
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> >
> > --
> > Ben Greear <gree...@candelatech.com <mailto:gree...@candelatech.com
> >>
> > Candela Technologies Inc http://www.candelatech.com
> >
> >
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> --
> Ben Greear <gree...@candelatech.com>
> Candela Technologies Inc http://www.candelatech.com
>
>
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