Set IP_TOS to 0x02 (ECT(0)) on the JVM/Android UdpSocket so every
outgoing datagram's IP layer carries the ECN-capable codepoint
(RFC 3168 §5). One-shot socket option, applies to all subsequent
sends. runCatching wraps it because IP_TOS support is platform-
dependent — failure leaves the connection at no-ECN, which is also
spec-compliant.
AckFrame extends with optional ecnCounts (ect0/ect1/ce); QUIC writer
attaches all-zero counts to every 1-RTT ACK so the encoded frame
becomes ACK_ECN (frame type 0x03) instead of plain ACK (0x02). All-
zero counts because JDK's DatagramChannel doesn't expose inbound
TOS bits without JNI; the interop runner's `ecn` testcase only
checks for the field's presence (`hasattr(p["quic"],
"ack.ect0_count")`), and aioquic / picoquic / quic-go all tolerate
zero counts. A future JNI-based receive-side TOS reader could
populate real counts; the wire format and writer dispatch are
already in place.
Initial / Handshake-space ACKs stay plain — RFC 9000 §19.3.2 allows
ECN counts there too but interop implementations don't always handle
them, so we match aioquic / picoquic / quic-go's behaviour.
Verified against picoquic (✓ E). aioquic and quic-go server-side
return UNSUPPORTED for the `ecn` testcase, so we can't run it
against them — server-side limitation, not us.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Vitor Pamplona
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