Early hint and QUIC session

[Debin Zhang]

Chromium checks the "Alt-svc" header in an HTTP response, such as alt-svc: h3=":443"; ma=86400, to detect if the server supports HTTP3. If yes, the subsequent URL_request will trigger a QUIC session setup. This means the earliest URL_request that can use a QUIC session is the third URL_request. (Chromium can also use the DNS server's dns-alpn-h3 response to detect the web server's HTTP3 support. But let's ignore that case for now). To speed up the QUIC session setup, I hope Chromium can determine the web server's HTTP3 support by checking the 103 Early Hint response header, suppose the 103 Early Hint response contains the alt-svc header.  However, it looks like the current Chromium code does not support this. The code that handles Early Hint responses and processes the HTTP3 Alt-svc header is in HttpNetworkTransaction::DoReadHeadersComplete(). This function processes the Early Hint response first here, then returns here, before hitting the code that processes the HTTP3 Alt-svc header here. So even if the 103 Early Hint response header contains the alt-svc header, it won't help the browser to set up a QUIC session sooner. 
Can anyone tell me why Chromium only checks the alt-svc header in the 200 responses; while ignoring the 103 Early Hint responses?

Thanks

Debin

[Ryan Hamilton]

I think it works the way it does because when the code was written, there were never alt-svc headers in 103 responses, so the code never needed to handle it. That being said, honoring the header in earl hints sounds plausible. If you'd like to send a CL, I'd be happy to review it.

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[Mark Gunn]

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[Mark Gunn]

I'm going to give up on this. I don't know what I'm doing and I get upset because of it all I wanted was to find the info on my my hacker to get him arrested and this isn't doing it for me good bye all and thanks

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[Debin Zhang]

Hi Ryan, 

I've created a CL to process Alt-svc headers in 103 Early Hints responses: https://chromium-review.googlesource.com/c/chromium/src/+/6149141

I tested this feature against timex.com using WebPageTest (WPT). The attached CDF chart shows the Largest Contentful Paint (LCP) times comparing current Chrome (baseline) vs. with Early Hints HTTP/3 support enabled. Each line represents 9 test runs. The results show improvement in LCP times with the feature enabled. 

The WPT waterfall charts demonstrate earlier QUIC session establishment: 

- Baseline: QUIC session is not available until the 53rd URL request 

- With Early Hints: QUIC session established by the 19th URL request, reducing latency for subsequent requests 

This improvement should benefit some sites that support http3 and serve Alt-svc headers in their Early Hints responses. 

Let me know if you need any additional information. 

Thanks, 

Debin