Replace Deprecated `mplex` with `yamux` for Improved Interoperability

[paschal533]

Description

While working on an interoperability test between py-libp2p and rust-libp2p for a simple ping exchange, I discovered that py-libp2p still relies on mplex as its default stream multiplexer (e.g., in libp2p/stream_muxer/mplex/ and used in TransportUpgrader). However, mplex has been deprecated across the libp2p ecosystem, with implementations like rust-libp2p and go-libp2p now using yamux as the standard multiplexer. This mismatch causes compatibility issues, as seen in my ongoing interop tests where py-libp2p fails to mux streams with rust-libp2p after the Noise handshake.

Impact

• py-libp2p cannot interoperate seamlessly with modern libp2p implementations that use yamux.
• The continued use of mplex risks falling further behind the libp2p spec and community standards.

Proposed Solution

• Replace mplex with yamux as the default multiplexer in py-libp2p.
• Update TransportUpgrader and related components to prioritize /yamux/1.0.0 over /mplex/0.0.1.
• Deprecate or remove mplex support to align with rust-libp2p and go-libp2p.

Current Work
I’m actively working on implementing yamux support in py-libp2p (see libp2p/stream_muxer/yamux/yamux.py). My current efforts include:

• Adapting yamux.py to use a 12-byte header per the spec.
• Integrating it into TransportUpgrader for Noise/Yamux interop with rust-libp2p.
• Resolving type and linting issues (mypy, flake8, pydocstyle) for a clean PR.

I plan to submit a PR as soon as possible once the interop test succeeds and the implementation is stable.

Request for Feedback

• Would maintainers prefer mplex deprecated gradually (e.g., dual support) or removed outright?

Related Efforts

• My WIP interop PR: WIP.
• rust-libp2p Yamux reference: https://github.com/libp2p/rust-libp2p/tree/master/muxers/yamux.

Looking forward to collaborating on this to bring py-libp2p up to date with the broader libp2p ecosystem