Article 28 research and environment notes, checked 2026-09-23.

Primary sources:
- RFC 9114, HTTP/3, IETF, June 2022:
  https://www.rfc-editor.org/rfc/rfc9114.html
  Checked connection discovery, ALPN h3, TCP-based fallback advice, request streams,
  critical unidirectional streams, SETTINGS placement and HTTP frame mapping.
- RFC 9204, QPACK, IETF, June 2022:
  https://www.rfc-editor.org/rfc/rfc9204.html
  Checked Required Insert Count, blocked decoding, encoder/decoder streams,
  SETTINGS_QPACK_BLOCKED_STREAMS and flow-control deadlock guidance.
- RFC 9000, QUIC: A UDP-Based Multiplexed and Secure Transport, IETF, May 2021:
  https://www.rfc-editor.org/rfc/rfc9000.html
  Checked stream-local ordered delivery and stream/connection flow control.
- RFC 9002, QUIC Loss Detection and Congestion Control, IETF, May 2021:
  https://www.rfc-editor.org/rfc/rfc9002.html
  Used to retain the connection-path congestion-control boundary from article 27.
- curl project, docs/HTTP3.md, accessed 2026-09-23:
  https://github.com/curl/curl/blob/master/docs/HTTP3.md
  Checked current build backends and the need for an HTTP/3-capable build.

Environment:
- Python 3.12.3.
- curl 8.5.0 with nghttp2/1.59.0; feature list does not contain HTTP3.
- OpenSSL 3.0.13.
- No nghttp3-client, h3client or quiche-client found on PATH.

Claims retained:
- HTTP/3 request/response pairs use client-initiated bidirectional QUIC streams.
- Each endpoint has one HTTP control stream; SETTINGS is its first frame.
- QPACK uses two additional unidirectional streams.
- A field section blocks when Required Insert Count exceeds the decoder Insert Count.
- QPACK dependency is distinct from stream byte gaps, QUIC flow control and shared
  congestion control.
- A failed QUIC attempt can lead a client to a new TCP-based HTTP connection; it is
  not an in-place conversion of a QUIC connection.

NOT_RUN:
- Mature HTTP/3 client/server exchange, qlog and packet capture.
- Fixed-condition HTTP/2 versus HTTP/3 loss comparison.
- Real Alt-Svc caching and UDP-blocked fallback timing.