The Tor Project releases both Tor Browser Bundles, and Pluggable Transport Tor Browser Bundles.
How are these different? How should I decide which bundle to use?
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Tor Pluggable Transport bundles are the new name for the old 'Pyobfsproxy/Flashproxy' and 'Obfsproxy' Tor Browser Bundles . The PTB's include the python version of obfsproxy (not the old C version), and include more transport modes (see below).
The overall idea behind the PTB's attempt to disguise Tor traffic to look like traffic from other common services (eg. normal HTTPS traffic, Skype traffic, etc. Some of the pluggable transports also transform other aspects of Tor traffic; for instance, the Flashproxy version turns web browsers into relays using normal websockets, or StegoTorus (a fork of Obfsproxy) splits Tor traffic between multiple streams to prevent fingerprinting.
If you don't have a specific use for a pluggable transport bundle (eg. you live in a country that does deep packet inspection to detect or block Tor traffic), you can probably go with the normal TBB.
More information can be found in the docs.
The pluggable transport bundles include FlashProxy and Obfsproxy, to let users get around censors who are trying to block access to the Tor network.
You can read more about pluggable transports here: https://www.torproject.org/docs/pluggable-transports.html.en
The pluggable transport bundles come pre-configured with a handful of bridge addresses, which is good because you're up and running immediately, but bad because those are bottlenecks (and eventually a censor will pick up the list and block those bridges by address, even if it's hard to block them in fancier ways).
Eventually we want to merge the two into one big bundle, that ships with support for various pluggable transports but doesn't use them unless you specifically configure a bridge that needs one.
As for which one you should use, if you are in a censoring area (e.g. China, Syria, sometimes Iran, and some corporate networks), you will need the pluggable transport bundles. Otherwise you should use the normal bundle.