  Tor, one of the most popular censorship circumvention systems, faces regular
blocking attempts by censors. Thus, to facilitate access, it relies on
"pluggable transports" (PTs) that disguise Tor's traffic and make it hard for
the adversary to block Tor. However, these are not yet well studied and
compared for the performance they provide to the users. Thus, we conduct a
first comparative performance evaluation of a total of 12 PTs -- the ones
currently supported by the Tor project and those that can be integrated in the
future.
  Our results reveal multiple facets of the PT ecosystem. (1) PTs' download
time significantly varies even under similar network conditions. (2) All PTs
are not equally reliable. Thus, clients who regularly suffer censorship may
falsely believe that such PTs are blocked. (3) PT performance depends on the
underlying communication primitive. (4) PTs performance significantly depends
on the website access method (browser or command-line). Surprisingly, for some
PTs, website access time was even less than vanilla Tor.
  Based on our findings from more than 1.25M measurements, we provide
recommendations about selecting PTs and believe that our study can facilitate
access for users who face censorship.
