Keep-alive not set on page resources that are used multiple times

This means that the URL in question is a resource URL, such as images or JavaScript files, that closes the connection and does not use keep-alive in the HTTP header.

Why is this important?

Enabling HTTP Keep-Alive or HTTP persistent connections allow the same TCP connection to send and receive multiple HTTP requests, thus reducing the latency for subsequent requests. Page resources that close the connection and do not use keep-alive in the HTTP header. If you have multiple requests to the same domain, you can use keep-alive headers so the connection is not closed until all the requests are made.

What does the Hint check?

This Hint will trigger for any internal resource URL that does not set a keep-alive header, or sets the connection to 'close'.

Examples that trigger this Hint:

Resource file URLs without a keep-alive header, and the connection set to 'close':

HTTP/... 200 OK
 
...
Content-Type => image/png
...
Connection => close

How do you resolve this issue?

There is basically no downside to enabling keep-alive, and on most web-servers it is actually enabled by default. This guide explains the details of setting keep alive up different servers. 

Further Reading

• Mozilla: Keep-Alive
• Improving Website Performance – Enabling Keep-Alive
• Gift of Speed: Enable keep-alive