  TCP and QUIC can both leverage ECN to avoid congestion loss and its
retransmission overhead. However, both protocols require support of their
remote endpoints and it took two decades since the initial standardization of
ECN for TCP to reach 80% ECN support and more in the wild. In contrast, the
QUIC standard mandates ECN support, but there are notable ambiguities that make
it unclear if and how ECN can actually be used with QUIC on the Internet.
Hence, in this paper, we analyze ECN support with QUIC in the wild: We conduct
repeated measurements on more than 180M domains to identify HTTP/3 websites and
analyze the underlying QUIC connections w.r.t. ECN support. We only find 20% of
QUIC hosts, providing 6% of HTTP/3 websites, to mirror client ECN codepoints.
Yet, mirroring ECN is only half of what is required for ECN with QUIC, as QUIC
validates mirrored ECN codepoints to detect network impairments: We observe
that less than 2% of QUIC hosts, providing less than 0.3% of HTTP/3 websites,
pass this validation. We identify possible root causes in content providers not
supporting ECN via QUIC and network impairments hindering ECN. We thus also
characterize ECN with QUIC distributedly to traverse other paths and discuss
our results w.r.t. QUIC and ECN innovations beyond QUIC.
