Testing IE8's Connection Parallelism

Ryan Breen: A few weeks ago, I discussed IE8's improved connection parallelism, specifically the increase from 2 concurrent connections per host to 6. One open question was the total number of connections allowed — my speculation was that the IE team would stick with a max of 6 rather than triple that value as well. […]

Ryan Breen: A few weeks ago, I discussed IE8's improved connection parallelism, specifically the increase from 2 concurrent connections per host to 6. One open question was the total number of connections allowed — my speculation was that the IE team would stick with a max of 6 rather than triple that value as well.

I was wrong. The new max is an astonishing 18 (!) concurrent connections:

That is some serious parallelism, and it has significant implications for application performance.

In December of 2006, I discussed the CNAME trick for circumventing browser connection limits, using 3 hostnames to serve images to trick the browser into using all available connections. At the time, that was 6 for IE. The above capture from IBM Page Detailer confirms 18 concurrent connections in IE8.

As expected, IE8’s handling of the unoptimized version, where only one hostname is used, is comparable to the performance of the optimized page in previous IE versions:

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Internet Explorer 8, IE8, Testing, Parallelism