It’s also possible to advertise support for different ALPNs (h2, h3) through DNS responses in SVCB/HTTPS records:
https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-https-10.html <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-https-10.html> In these cases, you can have a good hint that you should try H3 in parallel. Thanks, Tommy > On Jul 18, 2022, at 11:13 AM, Ryan Hamilton <[email protected]> > wrote: > > On Mon, Jul 18, 2022 at 2:05 AM Aditya Pratap Singh > <[email protected] <mailto:[email protected]>> wrote: > Hello everyone > I'm Aditya, a student at IIIT Delhi, India. I was looking into QUIC protocol > and have a query about its connection establishment. I noticed that browsers > start connections over TCP initially, and if the server advertises QUIC > support in the alt-svc header, then only a QUIC connection attempt is made. > However, considering QUIC is an improved protocol over TCP, why do we not > initially try QUIC first, and if it fails (due to the server not supporting > it or firewall blocking UDP), move back to TCP? > > That would be a performance penalty for most websites, since most web sites > do not yet support HTTP/3. Over time, that will change and eventually most > websites *will* support HTTP/3. At that point, we might see browsers making > different choices.
