RFR: 8371475: HttpClient: Implement CUBIC congestion controller [v3]

[Daniel Jeliński]

On Wed, 12 Nov 2025 15:13:18 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

>>> CUBIC is a standard TCP congestion control algorithm that uses a cubic function instead of a linear congestion window increase function to improve scalability and stability over fast and long-distance networks. CUBIC has been adopted as the default TCP congestion control algorithm by the Linux, Windows, and Apple stacks.
>> 
>> This PR adds a new congestion controller algorithm. It reuses a large part of the QuicRenoCongestionController, which was refactored to two classes - QuicBaseCongestionController, containing the shared code, and QuicRenoCongestionController, containing only the code that is unique to Reno.
>> 
>> CUBIC is now the default congestion controller. Reno can still be selected by setting the system property `jdk.httpclient.quic.congestionController` to `reno`.
>> 
>> A new test was added to exercise the new congestion controller. Existing tests continue to pass.
>
> Daniel Jeliński has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 24 commits:
> 
>  - Update test comments
>  - Convert CubicTest to JUnit
>  - Merge declaration and assignment
>  - More aggressive target growth
>  - Merge remote-tracking branch 'origin/master' into quic-cubic
>  - Make classes final
>  - Rename system property to internal
>  - Add a system property to select congestion controller
>  - Implement fast convergence
>  - Add comments
>  - ... and 14 more: https://git.openjdk.org/jdk/compare/1f1f7bb4...195b0f89

Thanks @AlanBateman @vy for the reviews.

The property was not meant to be exposed; IMO CUBIC is superior to Reno in all aspects that matter. I renamed it to include `internal`.

We will probably need to revisit the configuration if and when we implement BBR, but we aren't quite there yet.

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PR Review: https://git.openjdk.org/jdk/pull/28195#pullrequestreview-3453025390