preconnect vs dns-prefetch resource hints

I've been researching the topic a bit lately and so far my (theoretical) conclusions are as follows:

Browser support difference is negligible as of mid-2018, when counting the real global usage of browsers (~73% vs ~74%)

dns-prefetch = DNS and preconnect = DNS + TCP + TLS. Note that DNS lookup is quite cheap to perform (a simple query-response to the DNS server, that is cached in the browser for a short amount of time), whereas TCP and TLS involves some server resources.

The practical difference is hence, if you know that a server fetch will happen for sure, preconnect is good. If it will happen only sometimes, and you expect huge traffic, preconnect might trigger a lot of useless TCP and TLS work, and dns-prefetch might be a better fit.

For example:

• if the page requests https://backend.example.com/giveMeFreshData on each load, and the response is not cacheable, preconnect is a good fit
• if the page only requests a static resource like https://statics-server.example.com/some-image.jpg or https://statics-server.example.com/some-css.css, and the resource is very likely to come from the user's browser cache (the very same resource(s) is used on many pages, and your user will trigger a lot of page loads like this with the warm cache -- and no other resources are fetched from that origin), then preconnect might be creating a lot of unnecessary TCP connections on your server (that will abandoned after a few seconds, but still, they were not necessary in the first place) and TLS handshakes (however in such case, preload might be an option if you know the exact URL and the resource is very important).
• If the traffic on your website is small though, it should not be impacted too much by this difference, so preconnect is probably a good fit for low-traffic websites, regardless of the things mentioned before.

As always, it's best to think about the use cases, deploy, measure, and fine tune.

1 Preconnect

The final resource hint we want to talk about is preconnect. Preconnect allows the browser to setup early connections before an HTTP request is actually sent to the server. This includes DNS lookups, TLS negotiations, TCP handshakes. This in turn eliminates roundtrip latency and saves time for users.

2 Prefetch

Prefetch is a low priority resource hint that allows the browser to fetch resources in the background (idle time) that might be needed later, and store them in the browser’s cache. Once a page has finished loading it begins downloading additional resources and if a user then clicks on a prefetched link, it will load the content instantly.

2.1 Link Prefetching

Link prefetching allows the browser to fetch resources, store them in cache, assuming that the user will request them. The browser looks for prefetch in the HTML or the HTTP header Link.

2.2 DNS Prefetching

DNS prefetching allows the browser to perform DNS lookups on a page in the background while the user is browsing. This minimizes latency as the DNS lookup has already taken place once the user clicks on a link. DNS prefetching can be added to a specific url by adding the rel="dns-prefetch" tag to the link attribute. We suggest using this on things such as Google fonts, Google Analytics, and your CDN.

2.3 Prerendering

Prerendering is very similar to prefetching in that it gathers resources that the user may navigate to next. The difference is that prerendering actually renders the entire page in the background, all the assets of a document.

More details: https://www.keycdn.com/blog/resource-hints/

Conclusion

Main Difference between dns-prefetch & preconnect

The difference between dns-prefetch and preconnect is dns-prefetch will only do the DNS lookup, while preconnect will do the DNS lookup, TLS negotiation, and the TCP handshake. This means that it avoids an additional 2 RTT once the resource is ready to be downloaded.

An important aspect is that support for dns-prefetch is much larger than support for preconnect.

You can find some concrete examples here: https://responsivedesign.is/articles/prefetch-preconnect-dns-priority/

<link rel="dns-prefetch preconnect"> doesn’t work in WebKit (Safari).

You can use <link rel="dns-prefetch"> and <link rel="preconnect"> in two separate elements, if you want to use preconnect (when available, browsers since end of 2015) with dns-prefetch (in browsers since 2008) as a fallback.