- QUIC requires transmission of at least 1350 bytes of UDP packets on IPv6 (and 1370 bytes on IPv4).
- On Chrome/Chromium (and Vivaldi), if sending 1350-byte UDP packet fails on IPv6, fallback to HTTP/2 over TCP immediately occurs.
See the explanation in Japanese for the further details on the current implementation. Note well that this differs from the latest version of QUIC Internet-Draft: see draft-ietf-quic-transport-01 Section 8. Quote: