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If you want to observe the behavior of your browser and the HTTP requests it generates, Fiddler is the ultimate HTTP debugging tool. It's implemented as an HTTP proxy, so it can monitor all requests made by any browser. (As long as you configure the proxy parameters in the browser; integration with Internet Explorer is seamless.) It also logs the requests and allows you to inspect request and response headers, response body (text, images, XML, or raw response), and caching-related headers.

Because Fiddler intercepts all the HTTP requests and responses, you can modify them on the fly, change the HTTP requests sent by the browser to test alternate sets of parameters, or even build HTTP requests by hand (or clone and modify existing requests).

When analyzing your website performance, the best reports to use are the "Statistics" and "Timeline" reports. Fiddler doesn't group HTTP requests into related bundles, so you have to select all related HTTP transactions manually. After that, you can open the Statistics tab to see the web page statistics as well as estimated response time from various places around the world for server access based on the U.S. west coast (see Figure 10).

Fiddler's Timeline tab gives you a graph that's very similar to the one produced by Page Detailer, the only difference being that Fiddler's graph includes request paths (without the hostname), as shown in Figure 11.

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