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>> wrote:
On Monday, July 12, 2021 9:46am, "Livingood, Jason" <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>
Candela Technologies Inc http://www.candelatech.com
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