it is network management that is the cause.
It may well be network management - but network management by whom, and for what purpose? Not all network management comes down to the plusnet traffic shaping system (which is designed to place various limits on your connection, restricting individual protocol usage at various times of day).
The fact that shifting gateway can correct the problem does indeed suggest the problem does not come down to the traffic shaping system, but that the problem rests elsewhere.
I'd suggest the overall gateway is congested, thus the overall gateway network management is having to apply shaping to certain protocols to ensure that there is sufficient bandwidth is available to things that need it.
That could certainly be true - some leg/router is congested, and has started to employ congestion reduction techniques. That isn't shaping, in the same sense as shaping associated with your Plusnet account.
If it was congestion without network management then *everything* would have been slow,
Not necessarily. Weighted fair queuing techniques can affect long slow transfer more than short, fast ones.
Similarly, congestion detection avoidance techniques, such as RED (random early detection) can deliberately trigger early packet loss to slow down random TCP transfers; to the end user, it looks like some transfers are running slowly.
WRED can do the same, but with weighted traffic classes. The end-user experience will look very similar to traffic shaping, but is actually just plain network management and congestion prevention, although slightly more intelligent.
The kicker is that these impacts could be happening within the BTO and BTW domains, as well as within Plusnet. For one of the Plusnet users complaining about signs of congestion recently, the cause came down to congestion within certain BTW routes in Leeds.



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