Adds the client-side VN flow needed for the interop runner's
`versionnegotiation` testcase:
- `QuicConnection` accepts an `initialVersion` constructor parameter
(default `QuicVersion.V1`) and exposes a mutable `currentVersion`
the writer stamps into outbound long-headers. `start()` now caches
the ClientHello bytes for VN-driven re-emission.
- `applyVersionNegotiation(supportedVersions)` validates per §6.2
(anti-downgrade: reject if list contains the offered version),
picks v1 from the offered set, regenerates DCID, re-derives
Initial keys against the new DCID, resets the Initial level via
`LevelState.resetForVersionNegotiation`, re-enqueues the cached
ClientHello, and latches `vnConsumed` so a second VN is dropped.
Failure to find a mutually supported version closes the connection
with `QuicVersionNegotiationException`.
- `QuicConnectionParser.feedDatagram` detects `version == 0` long
headers BEFORE peekHeader (whose layout assumes v1) and dispatches
to a new `feedVersionNegotiationPacket` that parses the §17.2.1
shape and validates the echoed DCID.
- `QuicConnectionWriter` reads `conn.currentVersion` instead of the
hardcoded `QuicVersion.V1`.
- `QuicVersion.FORCE_VERSION_NEGOTIATION = 0x1a2a3a4a` for the
interop runner.
- `InteropRunner` honors `TESTCASE=versionnegotiation` (or
`-DinteropTestcase=`) and offers the force-VN version.
Regression coverage in `VersionNegotiationTest`:
- happy path: VN switches `currentVersion` to v1, regenerates DCID,
resets PN, and the next drain emits a v1 Initial on the wire.
- downgrade defense: VN listing the offered version is dropped.
- unsupported list: VN whose versions we can't speak fails the
handshake and closes the connection.
- second VN: post-consumption VN is ignored.
- DCID mismatch: spoofed VN with wrong echoed DCID is dropped.
- backward compatibility: default `initialVersion` keeps v1 behavior
for existing callers.
https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT
Claude
350387f7e0