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Under the Surface of Azeroth:
A Network Baseline and Security Analysis
of Blizzard's World of Warcraft

The contents of the PatchSequenceFile.txt isn't seen by the end user, but its contents are used by the World of Warcraft client to determine the currently installed software version and the appropriate upgrade path for updates. If you open http://us.version.worldofwarcraft.com/update/PatchSequenceFile.txt with a browser, you'll get this relatively straightforward text file:

[GAME]
CurrentBuild=6898
6546=WoW-2.0.12.6546-2.1.0.6692-{PLATFORM}-{OS}-{LANG}
6692=WoW-2.1.0.6692-2.1.0.6729-{CPU}-{OS}-{LANG}

6729=WoW-2.1.0.6729-2.1.1.6739-{CPU}-{OS}-{LANG}
6739=WoW-2.1.1.6739-2.1.2.6803-{CPU}-{OS}-{LANG}
6803=WoW-2.1.2.6803-2.1.3.6898-{CPU}-{OS}-{LANG}

[PTR]
currentbuild=7051
6932=WoW-0.2.0.6932-0.2.0.6983-{CPU}-{OS}-{LANG}
6983=WoW-0.2.0.6983-0.2.0.7051-{CPU}-{OS}-{LANG}

[LAUNCHER]
CurrentBuild=193

The [GAME] section references the current released build of the World of Warcraft client, along with the path involved from getting from one version to another. These patches must be installed in order and can't be skipped, which means that a client installed from the original CD or DVD media often must undergo a number of upgrades before the most updated version is eventually installed.

The [PTR] section of the PatchSequenceFile.txt file uses a similar syntax, but it's referencing the "Public Test Realm" client. The Public Test Realm is used for end-user testing of patches and updates before going live, but it's not always available. The public test realm also requires that everyone use the same client version, so it uses this informational file to stay up-to-date.

The [LAUNCHER] section ensures that the World of Warcraft launcher is running the latest version. Although this isn't as critical as maintaining the latest version of the World of Warcraft client, having the latest version of software automatically updated is useful for staying up to date.

Windows users can find this version number embedded in the Launcher.exe file: