[neuralkoi]

As per the article, CCAs aim to maximize data transfer by inferring the "available bandwidth" of the network. CUBIC relies primarily on packet loss as a congestion signal. For recovery, CUBIC's window size is a cubic function of time since the last congestion event.

After the initial packet loss triggered purposefully the first two seconds in this experiment, the only thing which could cause loss is the network queue (i.e. a simple tail drop, fq-codel, etc) which cannot process packets faster than they can arrive. At this point the link is saturated. The loss becomes a signal for CUBIC to reduce its window. This causes the oscillations you pointed out.

Unlike CUBIC, BBR [0] uses a model-based approach that estimates the available bandwidth and leaves some headroom kind of like you suggest to achieve higher throughput, and doesn't react as aggressively to loss as CUBIC.

[0] https://datatracker.ietf.org/meeting/104/materials/slides-10...