Upstream commit df6103ffdd ("truthful ECN reporting") replaced
`AckEcnCounts(0, 0, 0)` on 1-RTT ACK frames with
`ecnCounts = null`, on the rationale that hardcoded zero counts
were "lying" while we marked outbound ECT(0) but didn't read
inbound TOS.
That broke the interop runner's `ecn` testcase against quinn:
the runner's `_check_ack_ecn` requires at least one ACK_ECN
frame in the trace (`hasattr(p["quic"], "ack.ect0_count")`),
and explicitly says "we only check whether the trace contains
any ACK-ECN information, not whether it is valid". Without
the field present the runner reports "Client did not send any
ACK-ECN frames" and fails. 0/3 against quinn (was 19/22).
Re-emit `AckEcnCounts(0, 0, 0)` with corrected reasoning:
- We mark outbound packets ECT(0) (peers' tracking benefits).
- We don't read inbound TOS (JDK DatagramChannel needs JNI for
`IP_RECVTOS`, deferred). So we observe ZERO ECT-marked
inbound packets.
- Reporting 0 counts IS accurate to that observation. RFC 9000
§13.4.2 doesn't penalize a path that never sees marks; it
only requires the count be a "cumulative count of received
QUIC packets that were marked with the corresponding ECN
codepoint" — which is exactly 0 under our limitation.
The Initial / Handshake-space ACKs stay plain (RFC 9000 §13.4
forbids ECN on long headers); only application-space gets the
ECN counts.
Verified end-to-end:
- quinn ecn: 3/3 (was 0/3 post-rebase, 1/1 pre-rebase).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Vitor Pamplona
0bf0077aa5