RFR: 8371475: HttpClient: Implement CUBIC congestion controller [v5]

[Daniel Jeliński]

On Fri, 21 Nov 2025 10:10:16 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 more assertions

Thanks @dfuch for the review.

We didn't have any Reno-specific tests. I modified a few of the existing tests to also run with Reno now.

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PR Review: https://git.openjdk.org/jdk/pull/28195#pullrequestreview-3491960507