"Bless, Roland (TM)" <roland.bl...@kit.edu> writes: > Hi Luca, > > Am 27.11.18 um 10:24 schrieb Luca Muscariello: >> A congestion controlled protocol such as TCP or others, including QUIC, >> LEDBAT and so on >> need at least the BDP in the transmission queue to get full link >> efficiency, i.e. the queue never empties out. > > This is not true. There are congestion control algorithms > (e.g., TCP LoLa [1] or BBRv2) that can fully utilize the bottleneck link > capacity without filling the buffer to its maximum capacity. The BDP
Just to stay cynical, I would rather like the BBR and Lola folk to look closely at asymmetric networks, ack path delay, and lower rates than 1Gbit. And what the heck... wifi. :) BBRv1, for example, is hard coded to reduce cwnd to 4, not lower - because that works in the data center. Lola, so far as I know, achieves its tested results at 1-10Gbits. My world and much of the rest of the world, barely gets to a gbit, on a good day, with a tail-wind. If either of these TCPs could be tuned to work well and not saturate 5Mbit links I would be a happier person. RRUL benchmarks anyone? I did, honestly, want to run lola, (codebase was broken), and I am patiently waiting for BBRv2 to escape (while hoping that the googlers actually run some flent tests at edge bandwidths before I tear into it) Personally, I'd settle for SFQ on the CMTSes, fq_codel on the home routers, and then let the tcp-ers decide how much delay and loss they can tolerate. Another thought... I mean... can't we all just agree to make cubic more gentle and go fix that, and not a have a flag day? "From linux 5.0 forward cubic shall: Stop increasing its window at 250ms of delay greater than the initial RTT? Have it occasionally rtt probe a bit, more like BBR? > rule of thumb basically stems from the older loss-based congestion > control variants that profit from the standing queue that they built > over time when they detect a loss: > while they back-off and stop sending, the queue keeps the bottleneck > output busy and you'll not see underutilization of the link. Moreover, > once you get good loss de-synchronization, the buffer size requirement > for multiple long-lived flows decreases. > >> This gives rule of thumbs to size buffers which is also very practical >> and thanks to flow isolation becomes very accurate. > > The positive effect of buffers is merely their role to absorb > short-term bursts (i.e., mismatch in arrival and departure rates) > instead of dropping packets. One does not need a big buffer to > fully utilize a link (with perfect knowledge you can keep the link > saturated even without a single packet waiting in the buffer). > Furthermore, large buffers (e.g., using the BDP rule of thumb) > are not useful/practical anymore at very high speed such as 100 Gbit/s: > memory is also quite costly at such high speeds... > > Regards, > Roland > > [1] M. Hock, F. Neumeister, M. Zitterbart, R. Bless. > TCP LoLa: Congestion Control for Low Latencies and High Throughput. > Local Computer Networks (LCN), 2017 IEEE 42nd Conference on, pp. > 215-218, Singapore, Singapore, October 2017 > http://doc.tm.kit.edu/2017-LCN-lola-paper-authors-copy.pdf This whole thread, although diversive... well, I'd really like everybody to get together and try to write a joint paper on the best stuff to do, worldwide, to make bufferbloat go away. >> Which is: >> >> 1) find a way to keep the number of backlogged flows at a reasonable value. >> This largely depends on the minimum fair rate an application may need in >> the long term. >> We discussed a little bit of available mechanisms to achieve that in the >> literature. >> >> 2) fix the largest RTT you want to serve at full utilization and size >> the buffer using BDP * N_backlogged. >> Or the other way round: check how much memory you can use >> in the router/line card/device and for a fixed N, compute the largest >> RTT you can serve at full utilization. >> >> 3) there is still some memory to dimension for sparse flows in addition >> to that, but this is not based on BDP. >> It is just enough to compute the total utilization of sparse flows and >> use the same simple model Toke has used >> to compute the (de)prioritization probability. >> >> This procedure would allow to size FQ_codel but also SFQ. >> It would be interesting to compare the two under this buffer sizing. >> It would also be interesting to compare another mechanism that we have >> mentioned during the defense >> which is AFD + a sparse flow queue. Which is, BTW, already available in >> Cisco nexus switches for data centres. >> >> I think that the the codel part would still provide the ECN feature, >> that all the others cannot have. >> However the others, the last one especially can be implemented in >> silicon with reasonable cost. > _______________________________________________ > Bloat mailing list > Bloat@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/bloat _______________________________________________ Bloat mailing list Bloat@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/bloat