[Bloat] Little's Law mea culpa, but not invalidating my main point

Bob McMahon bob.mcmahon at broadcom.com
Mon Jul 12 16:04:41 EDT 2021


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 <greearb at 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 <dpreed at deepplum.com
> <mailto:dpreed at deepplum.com>> wrote:
> >
> >
> >     On Monday, July 12, 2021 9:46am, "Livingood, Jason" <
> Jason_Livingood at comcast.com <mailto:Jason_Livingood at 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).
> >
> >
> >
> >
> >
> >
> >
> > This electronic communication and the information and any files
> transmitted with it, or attached to it, are confidential and are intended
> solely for the use of
> > the individual or entity to whom it is addressed and may contain
> information that is confidential, legally privileged, protected by privacy
> laws, or otherwise
> > restricted from disclosure to anyone else. If you are not the intended
> recipient or the person responsible for delivering the e-mail to the
> intended recipient,
> > you are hereby notified that any use, copying, distributing,
> dissemination, forwarding, printing, or copying of this e-mail is strictly
> prohibited. If you
> > received this e-mail in error, please return the e-mail to the sender,
> delete it from your computer, and destroy any printed copy of it.
>
>
> --
> Ben Greear <greearb at candelatech.com>
> Candela Technologies Inc  http://www.candelatech.com
>
>

-- 
This electronic communication and the information and any files transmitted 
with it, or attached to it, are confidential and are intended solely for 
the use of the individual or entity to whom it is addressed and may contain 
information that is confidential, legally privileged, protected by privacy 
laws, or otherwise restricted from disclosure to anyone else. If you are 
not the intended recipient or the person responsible for delivering the 
e-mail to the intended recipient, you are hereby notified that any use, 
copying, distributing, dissemination, forwarding, printing, or copying of 
this e-mail is strictly prohibited. If you received this e-mail in error, 
please return the e-mail to the sender, delete it from your computer, and 
destroy any printed copy of it.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/cerowrt-devel/attachments/20210712/12f0e280/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 4206 bytes
Desc: S/MIME Cryptographic Signature
URL: <https://lists.bufferbloat.net/pipermail/cerowrt-devel/attachments/20210712/12f0e280/attachment-0001.bin>


More information about the Cerowrt-devel mailing list