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My team are now testing http/3 support in our app and backend. It looks strange to me how the behaviour is different between iOS 15.7.3 and 16.x (now 16.6). It is a default URLSession config and just a regular URLRequest, with no assumesHTTP3Capable flag set. I observe the protocol in use with both collecting URLSessionTaskMetrics and the Network instrument. On 15.7.3 the 1st request is done using h2, the network subsystem caches the Alt-Svc header (per host I suppose). Several seconds pass and then when I issue the same 2nd request a new connection is established and the protocol is upgraded to h3. This is all fine and as described in the documentation or wwdc videos. However, when I take the same steps with a device running e.g. iOS 16.5.1 I see that the protocol is never upgraded to h3. Neither a longer timeout nor an app relaunch make any difference. Interestingly, on my Ventura desktop the same url is also handled differently: h3 by Chrome but always h2 by Safari. In the mobile Safari on iOS 16, I'm also always shown HTTP/2 on e.g. the cloudflare status page for QUIC. What can be the reason for such behaviour on iOS 16 and Apple platforms in general? PS. I've tried running the app with CFNETWORK_DIAGNOSTICS but found no useful log messages to clarify problems with QUIC. Is there still a way to better diagnose such a problem?