Downloading the Data
Once the TCP session has been established, the web browser can send its first HTTP request to download the HTML code for the specified web page. This request (yet again) incurs an RTT delay, as well as a serialization delay (see Figure 2).
Once the HTML code for the web page has been loaded, the browser analyses the web page and identifies additional components that have to be loaded to render the page:
- Cascading style sheets (CSS)
- Java applets
- XSL style sheets (if you perform the XML-to-HTML translation within the web browser)
The slow download of web page components also impacts modern web pages that use progressive enhancement techniques or Ajax. These pages often start enhancing the web page through the window.onload event, which fires only after the last page component (CSS, applet, image) has been loaded.
Takeaway point: Large numbers of web page components and high propagation delay can cause dismal performance even on high-speed links.