[tsvwg] "Pacing" requirement is lost in L4S

[Vasilenko Eduard <[e-mail]>]

Hi Experts,
I see L4S as the "Congestion Control Next Generation from IETF" (that is actually in competition with "Congestion Control Next Generation from Google").
Then I see the important requirement that is missing in L4S.

The primary requirement for CC is that it "should not grow the buffer on the bottleneck link".
It is very disputable: is "the Scalable" requirement about it or not? Let's pretend that it is about it.

Then the next critical requirement is "pacing" which is missed in L4S completely.
Kudos to Google because I understood its importance after one local (but big) company tested and investigated BBRv1 (then implemented it).
After tests, they concluded that pacing is even more important than low latency. (I doubt, probably latency is more important)
Imagine that the server would increase the window sharply. The Server may have a 100GE interface. It could generate 10us of traffic as a burst (or even more).
Intermediate links could be 100GE or even bigger (highly probable), and the burst would travel as it is (without any spreading).
Then this burst could arrive at 10Mbps subscriber (good performance for shared public WiFi). 0.01ms burst would become 100ms burst.
It would create many negative consequences for the bottleneck link:
- tail drop if buffers are not enough
- guaranteed huge latency
Hence, we should completely forget about W (window) while discussing CC, we have to use T (time between packets).
CC next generation "should avoid bursts regulating inter-packet time, not the information permitted in transit". 

Best Regards
Eduard Vasilenko
Senior Architect
Network Algorithm Laboratory
Tel: [phone]