> On 11. Oct 2024, at 01:07, Alan Somers <asom...@freebsd.org> wrote: > > Can somebody please explain to me how the TCP measurement period > works? When does h_ertt decide to take a new measurement? > > Motivation: > I recently saw a long-distance connection that should've been capable > of 80+ MBps suddenly drop to < 1 MBps. Subsequent analysis of the > pcap file showed that while the typical RTT was 16.5 ms, there were a > few spikes as high as 380ms that coincided with the drop in > throughput. The surprising part was that even though RTT returned to > a good value, the throughput stayed low for the entire remaining > transfer, which lasted 750s. I would've expected throughput to > recover once RTT did. My theory is that h_ertt never made a new > measurement. However, I cannot reproduce the problem using dummynet > on a local VM. With dummynet, as soon as I return the RTT to normal, > the throughput quickly recovers, as one would expect. Which TCP stack and which CC module did you use? Which version of FreeBSD?
Best regards Michael > > Grateful for any insights. > -Alan >
