Is the web browser performance rule “only 2 requests in parallel per hostname” still correct?

Question

In his book \"High Performance Websites\" Steve Souders wrote (2007) that browsers limit parallel requests to a domain/hostname to two at a time. Is this still valid today?<

Answer 1

The HTTP/1.1 spec suggests a cap of 2 simultaneous requests per host (With HTTP/1.0 a figure of 4 was more common).

It would be wise to assume that the client might not be able to have more than 2 simultaneous requests open at once.

Answer 2

Yes, there may be some variations, but that limitation is certainly still valid. Some browsers might allow more than two parallel requests to a host, but it's still only a few.

There may also be limitations on the server side. If you are requesting active pages, e.g. ASP/ASP.NET/PHP, they are usually limited to a single request at a time per user.

However, with bandwith increasing this limitation has a smaller impact than it had in 2007. Eventhough you can still only do a few requests in parallel, each request is faster so the limitation is not very noticable.

Answer 3

2 years later the same Steve Souders wrote in "Even Faster Web Sites":

IE8 and FF3 both increase the number of connections per server from two to six.

Answer 4

Firefox bumped up its maximum per-server connection count from 8 to 15 for Firefox 3 and later. The maximum is less for servers that support persistent connections:

• Firefox 2: 2
• Firefox 3: 6
• Opera 9.26: 4
• Safari 3.0.4: 4
• IE 7: 2
• IE 8: 6 (except on dial-up)

Answer 5

Almost incorrect today.

Most browsers have upgraded to 6 parallel connections. See Steve Souder's Browserscope > Network tab > Connections per Hostname. Older browsers still restrict to 2 connections - that may or may not be relevant to you depending on the browsers your application supports.