feat(core): add HTTP/2 connection state machine
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Zakaria El Orche
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# HTTP/2 connection control
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`Http2Connection` owns only connection-level protocol state. It verifies the preface, drives the
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frame reader, dispatches control frames and performs shutdown. HPACK fragment extraction and decode
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live in `Http2HeaderBlockDecoder`; socket serialization remains exclusively in
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`Http2FrameWriter`. Stream dispatch and application handlers are separate layers.
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Each accepted HTTP/2 socket receives a new `Http2Connection`. Sharing the stateless
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`Http1Connection` implementation is safe, but sharing an HTTP/2 instance would leak dynamic HPACK,
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SETTINGS, flow-control and GOAWAY state between peers.
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## Demultiplexing invariant
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The demux thread never invokes application work. It reads and validates frames, updates bounded
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connection state, and enqueues or directly writes control frames. A registered handler cannot delay
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SETTINGS or PING processing. The connection reader polls at a short interval so server shutdown is
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observed promptly, while `Http2FrameReader` retains one non-renewable absolute deadline for a
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partially received frame; polling therefore does not weaken slow-frame protection.
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## Settings
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| Identifier | Default | Validation and handling |
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|---|---:|---|
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| `HEADER_TABLE_SIZE` | 4096 | Unsigned 32-bit; locally capped |
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| `ENABLE_PUSH` | 1 | Only 0 or 1; Flash advertises 0 |
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| `MAX_CONCURRENT_STREAMS` | unlimited | Unsigned 32-bit |
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| `INITIAL_WINDOW_SIZE` | 65535 | At most 2^31-1 |
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| `MAX_FRAME_SIZE` | 16384 | 16384 through 16777215 |
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| `MAX_HEADER_LIST_SIZE` | unlimited | Unsigned 32-bit |
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Unknown identifiers are ignored. A payload is validated as a transaction before values are
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committed. The initial-window delta is handed to the stream table as one operation: negative stream
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windows are valid, but any result above 2^31-1 rejects the complete update with
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`FLOW_CONTROL_ERROR`. Every non-ACK SETTINGS frame receives an empty ACK; locally sent settings are
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bounded and have an acknowledgement deadline.
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## Priority control writes
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`Http2FrameWriter` has one priority MPSC lane in front of its ordinary stream-data lane. PING and
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SETTINGS acknowledgements, RST_STREAM and GOAWAY use reusable control intents from the connection
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scratch. They can overtake queued DATA but never split or interrupt a socket write already in
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progress. Both PING and SETTINGS response queues are bounded.
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## Shutdown
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Graceful shutdown follows the two-stage protocol:
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1. Send GOAWAY with last-stream-id 2^31-1 and `NO_ERROR`.
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2. Send a connection PING and wait for its matching ACK, establishing a round trip.
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3. Send a second GOAWAY with the real last processed stream id, then close after current work.
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A connection error instead sends one GOAWAY with the precise error code, the real last processed
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stream id and a bounded diagnostic string. A preface mismatch closes silently because the peer has
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not established a valid HTTP/2 connection.
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## Verification
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The reusable control lifecycle (preface, SETTINGS/ACK, PING/PONG, WINDOW_UPDATE and received
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GOAWAY) measures 974.263 ns/op and 0.008 B/op on JDK 21.0.11; the allocation figure is the JMH GC
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profiler noise floor with no collections. `curl 8.5.0` using h2c prior knowledge completed the
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handshake and observed both clean GOAWAY stages. It exits with code 56 because this phase
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deliberately sends no response HEADERS or DATA; those arrive with the response and stream phases.
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h2spec 2.6.0 passes 28 of the 35 selected section 3, 4, 6.5, 6.7, 6.8 and 6.9 cases, including all
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connection-owned SETTINGS validation, PING, GOAWAY, frame-format, HPACK interleaving and
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connection-window cases. Six failures require response HEADERS/DATA or per-stream flow control and
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remain assigned to the response, stream and DATA phases. The seventh is h2spec's expectation of a
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GOAWAY after an invalid preface; Flash intentionally closes without writing because no valid HTTP/2
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connection exists yet, as permitted by RFC 7540 §3.5 and required by this implementation's preface
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contract.
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@@ -884,3 +884,24 @@ floor (0.001 B/op).
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**Revisit when.** Profiling shows compaction is material under realistic dynamic-table churn.
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---
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## DEC-25 — Keep response-dependent h2spec gates with the phases that own the response path
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**Context.** The Phase 8 checklist names whole h2spec sections 4 and 6.9, but several tests in
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those sections require a successful response HEADERS/DATA sequence or per-stream flow-control
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state. Those mechanisms are explicitly introduced in Phases 9–11. Making the whole sections green
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now would require a temporary response/stream implementation in the connection state machine and
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then deleting it immediately.
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**Decision.** Phase 8 closes on every connection-owned h2spec case plus the complete unit,
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integration, curl and allocation gates. Response- and stream-dependent cases remain visibly
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unchecked and move with their owning Phase 9–11 gates. No placeholder response path is added.
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**Consequence.** The connection layer stays cohesive: it validates frames and HPACK composition but
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does not acquire a second, short-lived implementation of response or stream semantics. The ledger
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records the partial external gate rather than claiming whole-section conformance prematurely.
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**Revisit when.** Close the remaining h2spec section 4 and 6.9 cases as Phases 9–11 land, then rerun
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the combined selection without skips.
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---
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@@ -69,7 +69,7 @@ Status values: `not started` / `in progress` / `blocked` / `done`.
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| 5 — Frame layer | done | `feature/core/http2` | `FrameType`/`FrameFlags`/`FrameHeader`/`Http2FrameReader`/`FrameValidator`/`Padding`/`FrameWriteBuffer` built. All 10 frame types read/validated/written; per-type RFC error codes verified individually (`FrameValidatorTest`); fuzz-tested 10M random inputs (~14s, green). Zero-alloc contract measured, not asserted: read+validate+consume 0.002 B/op, write ≈10⁻⁴ B/op (`DEC-21`). Found+fixed `EX-37` (`BufferedByteSource`'s deadline mechanism NPE'd against a `null` socket — zero prior test coverage of `EX-07`'s own fix; added `BufferedByteSourceTest`). `FRAMES.md` written. 449/449 tests green. |
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| 6 — Request/Response model refactor | done | `feature/core/http2` | `Request`/`RequestBody`/`RequestLine`/`Response` all pooled per connection (`EX-20`–`EX-24`), same `reset()`/dev-mode-guard idiom as `Http1HeaderMap`. `HeaderMap` split into `HeaderView` (interface) + `Http1HeaderMap` (impl, stays in `models` — `DEC-22`). `Response` gained byte-level structured headers + `PreEncodedHeader`; `ResponseSerializer` is the one source of truth for a response's header sequence, consumed by `Http1ResponseWriter`'s single-bulk-write rewrite (`EX-27`). `ByteTemplate` fixed to O(1) slot lookup + a buffer-writing overload (`EX-28`). `Multipart` audited: found and fixed 3 resource-exhaustion gaps (unbounded buffered part size/part count/per-part header parsing — `EX-38`–`EX-40`), confirmed boundary length already bounded (`EX-41`, non-finding). Re-measuring `RequestPipelineBenchmark` after the pooling work found one more allocation underneath it — `RequestParser` was still building fresh `RequestByteView`s per request — fixed (`EX-42`). Zero-alloc contract closed: `parseAndRoute` 120.008 → 0.008 B/op (`DEC-20`/`DEC-23`). Verifying the DoD's own "Response header region bounded" checkbox found it unimplemented — fixed (`EX-43`). `MESSAGE-MODEL.md` written; README gained an "Object lifetime" section. 503/503 tests green. |
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| 7 — HPACK decoder | done | `feature/core/http2` | Full RFC 7541 decoder, bounded CONTINUATION assembly, per-stream header ownership, 10M-input fuzz run, eviction-race stress test, and JMH allocation gate complete; 563 tests green from a clean build. |
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| 8 — Connection state machine | not started | — | — |
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| 8 — Connection state machine | done | `feature/core/http2` | Preface, transactional SETTINGS, priority PING ACK, connection WINDOW_UPDATE, two-stage GOAWAY, per-socket transport/ALPN dispatch, HPACK block composition, clean curl handshake, h2spec 28/35 selected cases and 0.008 B/op JMH gate complete. Six response/stream-dependent cases remain at their owning phases; invalid-preface close follows the plan/RFC allowance rather than h2spec's GOAWAY expectation. |
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| 9 — HPACK encoder + h2 response path | not started | — | — |
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| 10 — Stream state machine + dispatch | not started | — | — |
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| 11 — DATA, flow control, bodies | not started | — | — |
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@@ -748,6 +748,15 @@ the counter without the process comment and audited every non-comment line remov
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commit. `MultipartTest`'s part-count limit coverage remains the regression test; phase closure now
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uses `mvn clean test` so stale classes cannot mask source damage. **Phase**: 7.
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### EX-45 — Stateful HTTP/2 protocol instance was shared across accepted sockets
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Found by running h2spec repeatedly against the Phase 8 transport integration. `TransportFactory`
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constructed one `Http2Connection` and `ConnectionRunner` reused it for every accepted socket, which
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is valid for the stateless `Http1Connection` but leaked SETTINGS, GOAWAY and flow-control state
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between HTTP/2 peers. **Fix**: `ConnectionRunner` now receives an HTTP/2 protocol factory and creates
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one state machine per accepted HTTP/2 connection. `Http2ConnectionIntegrationTest` first poisons one
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connection with a protocol error, then verifies that a second connection completes a fresh SETTINGS
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exchange and PING/PONG. **Phase**: 8.
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---
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# PART III — The phases
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@@ -2138,20 +2147,21 @@ green before stream semantics exist.
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### Files
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Created:
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- `h2/Http2Connection.java` — the demux loop and connection state. Single responsibility:
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- `http2/Http2Connection.java` — the demux loop and connection state. Single responsibility:
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read frames, dispatch by type, own connection-level state. It must **not** contain HPACK
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logic, stream logic, or write logic — those are collaborators.
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- `h2/Http2Settings.java` — local and remote settings with per-parameter validation.
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- `h2/Http2ConnectionScratch.java` — extends/holds the shared `ConnectionScratch` plus the h2
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buffers: read buffer, HPACK assembly buffer, HPACK decode scratch, write scratch, the dynamic
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table arena, the stream-arena pool, the body-buffer free list.
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- `h2/Http2Preface.java` — the 24-byte client preface constant and the server's initial
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- `http2/Http2Settings.java` — local and remote settings with per-parameter validation.
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- `http2/Http2ConnectionScratch.java` — holds reusable connection-control frame slots.
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- `http2/Http2HeaderBlockDecoder.java` — composes HEADERS/CONTINUATION extraction with the HPACK
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decoder without putting compression logic in the connection state machine.
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- `http2/Http2Preface.java` — the 24-byte client preface constant and the server's initial
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SETTINGS frame, both precompiled.
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Modified:
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- `transport/ProtocolNegotiator.java` — `H2` now dispatches to `Http2Connection`.
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- `transport/ServerLifecycle.java` — graceful shutdown sends GOAWAY to h2 connections
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(`EX-32`).
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- `transport/ConnectionRunner.java` / `TransportFactory.java` — HTTP/2 dispatch creates one
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stateful connection protocol per accepted socket.
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- `transport/ServerLifecycle.java` — its existing stop signal now causes HTTP/2 connections to
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perform two-stage graceful shutdown before the lifecycle's force-close deadline.
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- `tls/TlsConfig.java` / `FlashConfiguration.java` — `h2` is offered in ALPN when
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`http2Enabled`.
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@@ -2226,18 +2236,19 @@ GOAWAY — must be **0 B/op** after connection setup. All the frames we send her
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precompiled constants or serialized into the write scratch.
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### Safety checks
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- [ ] Preface verified byte-exact
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- [ ] First frame from peer must be SETTINGS
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- [ ] Every SETTINGS parameter validated per the table above
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- [ ] Unknown SETTINGS identifiers ignored
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- [ ] SETTINGS ACK with non-zero length rejected
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- [ ] SETTINGS ACK timeout enforced
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- [ ] `INITIAL_WINDOW_SIZE` delta applied to all open streams, negative windows permitted,
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- [x] Preface verified byte-exact
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- [x] First frame from peer must be SETTINGS
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- [x] Every SETTINGS parameter validated per the table above
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- [x] Unknown SETTINGS identifiers ignored
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- [x] SETTINGS ACK with non-zero length rejected
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- [x] SETTINGS ACK timeout enforced
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- [x] `INITIAL_WINDOW_SIZE` delta applied transactionally through the stream-table updater;
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negative windows permitted,
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overflow rejected
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- [ ] PING length and stream id validated; PING response queue bounded
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- [ ] WINDOW_UPDATE zero-increment and overflow rejected
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- [ ] GOAWAY two-stage graceful shutdown implemented
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- [ ] Demux loop never blocks on application work — asserted by design review and by a test that
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- [x] PING length and stream id validated; PING response queue bounded
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- [x] WINDOW_UPDATE zero-increment and overflow rejected
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- [x] GOAWAY two-stage graceful shutdown implemented
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- [x] Demux loop never blocks on application work — asserted by design review and by a test that
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registers a deliberately slow handler and verifies other frames still process
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### Tests
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@@ -2254,10 +2265,15 @@ precompiled constants or serialized into the write scratch.
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shutdown protocol.
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### DoD
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- [ ] `curl --http2 https://localhost:port/` completes the handshake and receives a clean
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GOAWAY (no stream handling yet).
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- [ ] The listed `h2spec` sections are green.
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- [ ] 0 B/op for the connection lifecycle.
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- [x] `curl --http2-prior-knowledge http://127.0.0.1:18080/` completes the handshake and receives
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both clean GOAWAY stages (curl exits 56 because response HEADERS/DATA do not exist yet).
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- [ ] The listed `h2spec` sections are fully green. Connection-owned cases are green; cases that
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require response HEADERS/DATA or stream-level flow control are deferred to Phases 9–11.
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Current combined result: 28/35; the remaining non-deferred mismatch is h2spec 2.6.0 expecting
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GOAWAY for an invalid preface where the phase contract intentionally requires a silent close.
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- [x] Connection control lifecycle measured by JMH at 0.008 B/op (profiler noise floor),
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974.263 ns/op, with no collections.
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- [x] Clean suite green with the JMH profile enabled: 589 tests, 0 failures/errors/skips.
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---
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