RFR: 8371475: HttpClient: Implement CUBIC congestion controller [v6]

[Daniel Fuchs]

On Fri, 21 Nov 2025 21:14:18 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 incrementally with one additional commit since the last revision:
> 
>   Add comment for negative K

Thanks for working on this! LGTM.

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Marked as reviewed by dfuchs (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28195#pullrequestreview-3499763026