[SOLVED] Is the per-host connection limit raised with HTTP/2?

Is the per-host connection limit raised with HTTP/2?

Browsers have a per-host limit regarding number of parallel XHR (about 6 nowadays).

Does this restriction apply to multiplexed HTTP/2 connections?

Solution

HTTP/1

When using HTTP/1.1, browsers impose a per-domain limit of 6-8 connections, depending on the browser implementation. This allows at most 6-8 concurrent requests per domain.

HTTP/2

With the HTTP/2 protocol, browsers have to open only 1 connection per domain. However, thanks to its multiplexing feature, HTTP/2 does not quite raise the limit, but simply removes the theoretical limit on the number of concurrent requests per domain.

It is virtually unlimited in the sense that browsers and servers may limit the number of concurrent requests via the HTTP/2 configuration parameter called SETTINGS_MAX_CONCURRENT_STREAMS.

In practice, typical limits are around 100 but could be larger (or, less commonly, smaller), depending on browser implementation and on the server you connect to. Examples:

Firefox's default value for network.http.http2.default-concurrent (previously network.http.spdy.default-concurrent) is 100.
NGINX's keepalive_requests defaults to 1000 since version 1.19.10, which implies a maximum of that many concurrent requests.

Expect these limits to vary over the years with the evolution and the more widespread usage of HTTP/2 (in the same way it happened with HTTP/1.1: browsers started with 2 connections, and ended up to 6-8 after years of usage, experience and tuning).

XMLHttpRequest

I don't think there is any difference between how a browser treats the number of connections and concurrent requests for normal browsing and for the usage of XHR, so the explanations above holds true for XHR as well.