[Ecn-sane] comments: "Enabling ECN for Datacenter Networks with RTT Variations"

[Dave Taht]

this ecn# & codel in p4 paper had probably of most interest to me four points:

1) Codel like algorithm on a tofino switch, implemented in p4
2) with some source code published... Not the p4 code, darn it...
unless I missed it?
   but a detailed description of how they used the timestamp field was neat
3 Honestly I'd never thought about how much rtt variation a dc had,
nor do I know if this model is correct
3) Not really clear what parameters the used for codel

https://resource.hpisys.com/seminar-papers/2019/paper/20191108_CoNEXT2019_Enabling%20ECN%20for%20Datacenter%20Networks%20with%20RTT%20Variations.pdf

"Despite being successful, prior ECN-based transports have an
important drawback: they adopt a fxed RTT value in calculating
instantaneous ECN marking threshold while overlooking the RTT
variations in practice. In this paper, we reveal that the current
practice of using a fxed high-percentile RTT for ECN threshold
calculation can lead to persistent queue buildups, signifcantly
increasing packet latency. On the other hand, directly adopting lower
percentile RTTs results in throughput degradation. To handle the
problem, we introduce ECN!, a simple yet efective solution to enable
ECN for RTT variations. At its heart, ECN! inherits the current
instantaneous ECN marking (based on a high-percentile RTT) to achieve
high throughput and burst tolerance, while further marking packets
(conservatively) upon detecting long-term queue buildups to eliminate
unnecessary queueing delay without degrading throughput. We implement
ECN! on a Barefoot Tofno switch"

-- 
Make Music, Not War

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-435-0729