Hi Christoph, one question below:
> On Jun 18, 2021, at 01:43, Christoph Paasch via Bloat > <[email protected]> wrote: > > Hello, > > On 06/17/21 - 11:16, Matt Mathis via Bloat wrote: >> Is there a paper or spec for RPM? > > we try to publish an IETF-draft on the methodology before the upcoming IETF > in July. > > But, in the mean-time please see inline: > >> There are at least two different ways to define RPM, both of which might be >> relevant. >> >> At the TCP layer: it can be directly computed from a packet capture. The >> trick is to time reverse a trace and compute the critical path backwards >> through the trace: what event triggered each segment or ACK, and count >> round trips. This would be super robust but does not include the queueing >> required in the kernel socket buffers. I need to think some more about >> computing TCP RPM from tcp_info or other kernel instrumentation - it might >> be possible. > > We explicitly opted against measuring purely TCP-level round-trip times. > Because > there are countless transparent TCP-proxies out there that would skew these > numbers. Our goal with RPM/Responsiveness is to measure how an end-user would > experience the network. Which means, DNS-resolution, TCP handshake-time, > TLS-handshake, HTTP/2 Request/response. Because, at the end, that's what > actually matters to the users. > >> A different RPM can be done in the application, above TCP, for example by >> ping-ponging messages. This would include the delays traversing the kernel >> socket buffers which have to be at least as large as a full network RTT. >> >> This is perhaps an important point: due to the retransmit and >> reassuebly queues (which are required to implement robust data delivery) >> TCP must be able hold at least a full RTT of data in it's own buffers, >> which means that under some conditions the RTT as seen by the application >> has be be at least twice the network's RTT, including any bloat in the >> network. > > Currently, we measure RPM on separate connections (not the load-bearing > ones). We are also measuring on the load-bearing connections themselves > through H2 Ping frames. But for the reasons you described we haven't yet > factored it into the RPM-number. > > One way may be to inspect with TCP_INFO whether or not the connections had > retransmissions and then throw away the number. On the other hand, if the > network becomes extremely lossy under working conditions, it does impact the > user-experience and so it could make sense to take this into account. > > > In the end, we realized how hard it is to accurately measure bufferbloat > within a reasonable time-frame (our goal is to finish the test within ~15 > seconds). [SM] I understand that 10-15 seconds is the amount of time users have been trained to expect an on-line speedtest to take, but experiments with flent/RRUL showed that there are latency affection processes on slower timescales that are better visible if one can also run a test for 60 - 300 seconds (e.g. cyclic WiFi channel probing). Does your tool optionally allow to specify a longer run-time? Thinking of it, to keep everybody on their toes, how about occasionally running a test with longer run-time (maybe after asking the users consent) and store the test duration as part of the results? Best Regards Sebastian > > We hope that with the IETF-draft we can get the right people together to > iterate over it and squash out a very accurate measurement that represents > what users would experience. > > > Cheers, > Christoph > > >> >> Thanks, >> --MM-- >> The best way to predict the future is to create it. - Alan Kay >> >> We must not tolerate intolerance; >> however our response must be carefully measured: >> too strong would be hypocritical and risks spiraling out of >> control; >> too weak risks being mistaken for tacit approval. >> >> >> On Sat, Jun 12, 2021 at 9:11 AM Rich Brown <[email protected]> wrote: >> >>>> On Jun 12, 2021, at 12:00 PM, [email protected] wrote: >>>> >>>> Some relevant talks / publicity at WWDC -- the first mentioning CoDel, >>>> queueing, etc. Featuring Stuart Cheshire. iOS 15 adds a developer test >>> for >>>> loaded latency, reported in "RPM" or round-trips per minute. >>>> >>>> I ran it on my machine: >>>> nowens@mac1015 ~ % /usr/bin/networkQuality >>>> ==== SUMMARY ==== >>>> Upload capacity: 90.867 Mbps >>>> Download capacity: 93.616 Mbps >>>> Upload flows: 16 >>>> Download flows: 20 >>>> Responsiveness: Medium (840 RPM) >>> >>> Does anyone know how to get the command-line version for current (not >>> upcoming) macOS? Thanks. >>> >>> Rich >>> _______________________________________________ >>> Bloat mailing list >>> [email protected] >>> https://lists.bufferbloat.net/listinfo/bloat >>> > >> _______________________________________________ >> Bloat mailing list >> [email protected] >> https://lists.bufferbloat.net/listinfo/bloat > > _______________________________________________ > Bloat mailing list > [email protected] > https://lists.bufferbloat.net/listinfo/bloat _______________________________________________ Bloat mailing list [email protected] https://lists.bufferbloat.net/listinfo/bloat
