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 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 > 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