I have discovered the source of the confusion. This is different, but similar to PUMA6 problems, I think.

Certain types of traffic are getting de-prioritized, especially small packets under load.

Comparison:

Normal ping:

Minimum = 17ms, Maximum = 734ms, Average = 202ms

Large 1472-byte ping

Minimum = 18ms, Maximum = 229ms, Average = 47ms

TCP-Based ping via psping.exe

Minimum = 14.15ms, Maximum = 40.39ms, Average = 25.88ms

For more fun:

ICMP in a VPN, ping set to 13-bytes

Minimum = 36ms, Maximum = 602ms, Average = 208ms

ICMP in a VPN, 1300 bytes

Minimum = 24ms, Maximum = 72ms, Average = 40ms

This explains why certain synthetic bufferbloat benchmarks were passing with flying colors but my basic ICMP ping was terrible under load. VPN does not seem to help, so it's not packet inspection... it's failure of tiny packets and success of larger, normal ones.

The problem only occurs during download stress, not upload stress. Anyway, I guess this is mostly an FYI.
Perhaps a script that does variable packet sizes for the generated small flow traffic could be useful to someone in the future.

On Thu, Mar 28, 2019 at 8:03 AM Sebastian Moeller <moeller0@gmx.de> wrote:

HI cloneman,

maybe have a look at http://www.dslreports.com/tools/puma6?

> On Mar 28, 2019, at 12:00, cloneman <bufferbloat@flamingpc.com> wrote:
>
> I'm getting a weird problem with this new modem , it's Docsis 3.1
>
> latency/jitter measured via ping is about ~300ms during download pressure of 3 http transfers. The latency doesn't appear on synthetic bufferbloat benchmarks, though. (dslreports ; 20ms ("A")
>
> question: trying to figure out why I'm seeing this particular behavior, and determine if it'd a real issue or just an anomalie with ICMP traffic
>
> I've not seen ping do this with other modems.
>
> Perhaps I need to try some other traffic generation/ capture tools?
>
>
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