Question

I am making a a request to an image and the response headers that I get back are:

Accept-Ranges:bytes
Content-Length:4499
Content-Type:image/png
Date:Tue, 24 May 2011 20:09:39 GMT
ETag:"0cfe867f5b8cb1:0"
Last-Modified:Thu, 20 Jan 2011 22:57:26 GMT
Server:Microsoft-IIS/7.5
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Note the absence of the Cache-Control header.

On subsequent requests on Chrome, Chrome knows to go to the cache to retrieve the image. How does it know to use the cache? I was under the impression that I would have to tell it with the Cache-Control header.

Answer 1

You have both an ETag and a Last-Modified header. It probably uses those. But for that to happen, it still needs to make a request with If-None-Match or If-Modified-Since respectively.