s2n-quic
An implementation of the IETF QUIC protocol
...It is built with configurability in mind—you can tune congestion control (like CUBIC), pacing, packet size discovery, and other advanced network behaviors. Extensive testing (unit, fuzz, interop) ensures protocol compliance and interoperability with other implementations. Because it is open-source under Apache 2.0, organizations can integrate it into services where low latency, multiple streams, or mobility (connection migration) matter.