Re: [w3ctag/design-reviews] WG Revision: WebTransport (Issue #1212)

toreini left a comment (w3ctag/design-reviews#1212)

Hi @wilaw, 

The WebTransport proposal is moving in a positive direction and demonstrates maturity, with multi-stakeholder support. This is a promising foundation for further development.

However, there are some concerns that need to be addressed:

- **Certificate Hash as Authentication:** It is unclear whether the certificate hash digest is used for authentication. Based on the context, it seems the digest serves as proof of certificate ownership for an **already established** channel, which justifies its use. The text explicitly states, 
   > certificate verification errors are considered critical (fatal) and cannot be bypassed. 

   Our concerns are:
  1. **Fallback Mechanism:** Can you please clarify which of these scenarios happen in case of certificate error?
     - A new TLS handshake (or equivalent) is re-initiated.
     - The same digest is resent until verification completes at the server-side through the same established TLS session.
  2. **Bi-Directional Authentication:** Given the bi-directional nature of communication, authentication could benefit from a bi-directional approach. While the state-persistence approach (relying on TLS session tickets) is fair, certificate verification errors could make client authentication (and state persistence) non-deterministic. Can you please provide more specifics on this?

- **Privacy Concerns with `WebTransportConnectionStats`:** The discussions on side-channel passive fingerprinting based on `WebTransportConnectionStats` are a reasonable starting point. However, the TAG encourages you to explore more privacy-preserving approaches for presenting fine-grained stats. For example, adopting a tier-based nomination of quantities, similar to the "performance Tiers" in the [CPU Performance API](https://wicg.github.io/cpu-performance/#tiers), could be a viable solution.


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