msh3 as the third h3 backend

With the brand new merged support for the msh3 library, curl now supports no less than three different HTTP/3 backends. It was merged into curl’s git repository on April 10.

When you build curl, you have the option to build it with HTTP/3 support enabled. The HTTP/3 support in curl is still considered experimental so it is still not enabled by default.

The HTTP/3 support in curl depends on the presence and support from third party libraries. You need to select and enable a specific HTTP/3 backend when you build curl. It has previously been doing HTTP/3 using either quiche or ngtcp2 + nghttp3. Starting now, there is yet another option to consider: the msh3 library.

The msh3 library itself uses msquic for doing QUIC. This is a multi platform library that uses Schannel for TLS when on Windows and OpenSSL/quictls for other platforms. The Schannel part probably makes solution this particularly interesting for curl users on Windows.