MSDN Magazine - November 2008 - (Page 74)

you’ll see very little traffic for the first three seconds. In Figure 5, the orange file at the top-left corner of the chart is an SSL handshake. Inside the rectangles are indications about the packet flow that I will discuss in more detail later. If you scan the figure from left to right and count the number of files actively downloading at any one time, you’ll see it’s not until three seconds into the download that more than one file loads concurrently. The SSL starts out before another two red HTML files are loaded in series, then finally six ports are used to load nine files. 2. limit the Number of Small Files to Be Downloaded It’s hard to get full utilization of bandwidth across a wide area network (WAN) when shipping many small files. Figure 4 illustrates this throughout the page load but especially after the tenth second. Each of these icon image files is less than 500 bytes. If the server is 200ms RTT from the user and each file is one round-trip, then only five of these files could be loaded per second (5 files/sec × 500 Bytes/file × 8 bits/Byte = 20Kbps per TCP port). Consolidating small files into larger ones allows more bytes to be sent per round-trip. For images, this is called image clustering or using sprites. A single file is downloaded with many images positioned side-by-side, which are then cropped out and placed on the page. Here’s an example of this concept at work: a while back when we shipped Windows 2000, we had reports that it took six hours to download the upgrade to Germany from Redmond. My analysis found that 30,000 little files were being transferred one Client Ports 1) 2377 2) 2367 3) 1026 4) 2378 5) 2379 6) 1026 7) 1025 8) 2380 9) 2381 10) 2382 11) 2383 at a time. After we zipped them all into a single file, the download completed in 35 minutes instead. 3. load JavaScript Files outside of the JavaScript Engine Browsers running Internet Explorer 7, and earlier versions, block on JavaScript files. Normally they download files in what’s called speculative mode as quickly as possible. But when encountering JavaScript, the browser drops out of this mode and focuses on downloading the JavaScript. If you were to view this situation in VRTA you would see that none of the JavaScript files overlap with other JavaScript files and very few files of other types are loading at the same time. One workaround is to move the JavaScript files to the end, but that misses the point that there is plenty of bandwidth to use. A better solution is to use a document.write of the files so they load outside of the JavaScript engine in the browser, as you see here: function AsyncLoad() { var l = arguments.length; for (var i=0;i<l;i++) { document.write(" "); } } AsyncLoad( "file1.js", "file2.js", "file3.js"); We are anticipating that Internet Explorer 8.0 will finally resolve this problem. Domain SSL mail.live.com DNS bl140w.blu140.mail.live.com bl140w.blu140.mail.live.com bl140w.blu140.mail.live.com DNS gfx6.hotmail.com DNS gfx7.hotmail.com gfx6.hotmail.com gfx6.hotmail.com gfx6.hotmail.com gfx6.hotmail.com 2 4 6 Seconds Kbps 922 738 553 369 184 2 4 6 191 Kbps Seconds Figure 5 Concurrent Ports 74 msdn magazine Visual Round Trip Analyzer

Table of Contents Feed for the Digital Edition of MSDN Magazine - November 2008

Contents MSDN Magazine - November 2008 Toolbox CLR Inside Out Data Points Cutting Edge Security Briefs Test Your Security IQ Agile SDL Access Control Utility Spotlight RIA Test Run Wicked Code Foundations Team System End Bracket

MSDN Magazine - November 2008

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