Updating to mention a concern with ALPN implementation. HTTP/2 requires that a cipher with AEAD be negotiated. However, it also allows for offering a wider range of cipher suites: if an AEAD cipher is not present, this will allow fallback to HTTP/1.1.

There's some interplay between ALPN and cipher selection here. We'll want to ensure that either ALPN negotiation can inform cipher selection or vice-versa, or writing a Python HTTP/2 server will get tricky fast.