WebRTC relay path can reject the default 16 KiB framed message size

[yexiyue]

Thank you for maintaining the WebRTC transport.

We observed that the default 16 KiB WebRTC framing limit can exceed the effective SCTP/DataChannel message limit on a real browser -> webrtc-direct relay -> browser path.

Observed behavior

Connection establishment and control traffic succeed. Once transfer data starts, the relay reports:

outbound packet larger than maximum message size

The peers then observe:

connection is closed
unexpected end of file

For local validation, reducing libp2p_webrtc_utils::stream::MAX_MSG_LEN from 16 KiB to 8 KiB allowed the same browser-to-browser transfer to complete repeatedly.

Why I am opening an issue before a PR

The 16 KiB value follows the RFC 8831 guidance for non-interleaved messages. I do not think changing the global default to 8 KiB is necessarily the right fix: it could unnecessarily reduce throughput for native WebRTC users.

Could you advise on the preferred design direction?

1. Derive the framing limit from the effective SCTP/WebRTC maxMessageSize, where available.
2. Expose the maximum WebRTC frame/message size as transport configuration.
3. Use a conservative default only for webrtc-websys or relay paths.
4. Proactively fragment before the underlying SCTP/DataChannel layer rejects an oversized message.

I can provide full relay logs, browser console logs, and a minimal reproduction if useful. I would be happy to prepare a PR once there is agreement on the desired API and behavior.