One of the features of HTTP/2 is the ability to push content to user agents before they request them.
| Contents |
|---|
| 1 Example |
| 2 Issues with Firefox |
| 3 Specifying resources to be pushed |
Example
Say someone visits icelk.dev. When Kvarn gets the request
to /, it knows the user agent (web browser) will request /icelk-logo.png,
/script.js, /style.css. Right after sending the response, Kvarn looks for
links on the responded web page. If it finds any, it pushes them to the user
agent. It generates requests and responses which are sent using the HTTP/2
protocol.
It would be wasteful to push resources on every page load, so the default implementation has a limit of pushing only every two minutes. You can however force Kvarn to always push content.
Issues with Firefox
Pushing content to Firefox-based user agents doesn’t work through Kvarn. The
reason is unknown. Kvarn checks for the user-agent header and ignores Firefox
browsers when pushing content.
This is odd, considering other push implementations work just fine, but Kvarn
passes more checks in the h2spec
testing tool. In fact, Kvarn passes all tests. It also works flawlessly with
all Chromium-based browsers, Safari browsers, and the nghttp2 tool. Running
nghttp https://localhost:8443 -nv gives all the push promises correctly.
Specifying resources to be pushed
Internally, the url_crawl crate
is used for getting URLs.
You can specify several push HTML elements (they are not in any spec and
therefore don’t have any semantic meaning) in the <head> of the document, with
the format as follows:
<!-- snip -->
<push src="my-dataset.csv" />
<!-- snip -->