[LAU] Physical distance performance with Linux?

Chris Caudle chris at chriscaudle.org
Wed May 6 20:03:00 CEST 2020

On Tue, May 5, 2020 2:43 pm, Christoph Kuhr wrote:
> Well, actually using the same ISP in the same city does not
> make a difference. My mate and me have both
> Unitymedia/Vodafone cable network access. We had the same RTT
> as with the other one at another ISP with ADSL.

That is a really interesting observation.  Not what I expected, so I am
curious why there is not more added latency from the routing out of your
ISP and  into the other ISP.
Perhaps the latency on the modems is high enough that the customer end
equipment dominates over the routing delays.

> It is worthwile noting that we are limited by the speed of light.

300km/ms speed of light, the lowest ping time I have ever seen from my
computer to a machine outside my house is about 20ms.  I know not all of
those machines were 6000km away, so I think speed of light is pretty low
on the list of factors influencing delay.  Yes for large distances, but
should not have been a factor in your connection inside the same city
through the same ISP.

> I read a paper a while ago which stated that most delay occurs in the
> access network, the last mile, due to shared media and signal
> multiplexing.

Also both DSL  and cable modem involve lots of pretty sophisticated DSP
operations, that all takes time.  I have not seen any information on time
delay between ethernet packet in and cable or DSL packet out, or the
reverse.  That may be a significant portion of the delay.

And for anyone not familiar with the term "bufferbloat" or "buffer bloat"
you should verify that you are using appropriate active queue management
at your home router.  There have been many papers, blogs, presentations,
etc. on how poorly most equipment manages network queues at the points
where network bandwidth changes (such as from your home 1Gb connection to
the usually much slower upstream connection).  If the queues are too deep
then many packets can be buffered in the network equipment waiting for
transmission, especially if you have multiple applications active and
sharing network bandwidth simultaneously.

Chris Caudle

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