HTTP Headers - Cache Question

I am makin开发者_开发百科g 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
X-Powered-By:ASP.NET

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.

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.

To set the Cache-Control You have to specify it yourself. You can either do it in web.config , IIS Manager for selected folders (static, images ...) or set it in code. The HTTP 1.1 standard recommends one year in future as the maximum expiration time.

Setting expiration date one year in future is considered good practice for all static content in your site. Not having it in headers results in If-Modified-Since requests which can take longer then first time requests for small static files. In these calls ETag header is used.

When You have Cache-Control: max-age=315360000 basic HTTP responses will outnumber If-Modified-Since> calls and because of that it is good to remove ETag header and result in smaller static file response headers. IIS doesn't have setting for that so You have to do response.Headers.Remove("ETag"); in OnPreServerRequestHeaders()

And if You want to optimize Your headers further You can remove X-Powered-By:ASP.NET in IIS settings and X-Aspnet-Version header (altough I don't see in Your response) in web.config - enableVersionHeader="false" in system.web/httpRuntime element.

For more tips I suggest great book - http://www.amazon.com/Ultra-Fast-ASP-NET-Build-Ultra-Scalable-Server/dp/1430223839