CUBIC TCP

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CUBIC is a TCP congestion control algorithm designed to make connections faster and fairer, especially on long, high-latency networks. It builds on earlier ideas but aims to be simpler and more predictable. The first CUBIC implementation appeared in Linux in 2006 (kernel 2.6.13), and it has been the default TCP congestion control in Linux since version 2.6.19.

How it works
- The sending window (cwnd) grows according to a cubic function of time since the last congestion event. This means there are two growth phases: a quick rise back toward the previous window size, followed by a slower, then faster search for more bandwidth.
- There is a plateau period to let the network stabilize before trying to grab more bandwidth.
- Growth depends on time since the last loss, not on the round-trip time (RTT). This RTT-independence helps make flows with different RTTs behave more fairly, which is especially important in data centers, CDNs, and cloud environments.

Fairness and performance
- Because it doesn’t rely on RTT, CUBIC can be fairer when multiple data streams have different delays. Studies show it can be 3–5 times fairer than traditional Reno in networks with mixed RTTs.
- It is generally less aggressive and more predictable than older loss-based methods.

How it compares to other approaches
- CUBIC is an evolution of BIC TCP, designed to be simpler and more stable.
- Other approaches use sending rate rather than a window. A well-known example is Google’s BBR, which estimates available bandwidth and minimum RTT and uses a different cycle of operating modes. BBR can achieve high throughput in some networks, but it can be more aggressive and may dominate when competing with CUBIC flows.

Adoption around the world
- Linux: first released in 2006 (kernel 2.6.13); became the default in 2.6.19.
- macOS: adopted with OS X Yosemite in 2014 (Mavericks used the older TCP New Reno).
- Windows: included by default starting with the 2017 Fall Creators Update for Windows 10 and the 2016 1709 update for Windows Server.