For our measurements, we inserted
monitoring nodes (probes) into the BitTorrent network.
We define two types of measurements: active-start measurements,
in which probes initiate the contact with BitTorrent peers,
and passive-start measurements, in which probes wait
for an externally initiated contact from BitTorrent peers.
The need for the second approach is twofold. First, some
measurement platforms limit the number of contacts a user
can initiate during a fixed period of time. Second, the
first approach does not allow to contact peers behind firewalls,
while the second can overcome this limitation.
In active-start measurements, probes contact BitTorrent
trackers repeatedly, acquiring lists of peer contacts (the
GetPeers module, see Figure MP-1,
bullet 2). They subsequently contact the corresponding peers
using the BitTorrent protocol, and learn about what chunks
are owned by these peers (the PingPeers module, see
Figure MP-1, bullet 3). In passive-start
measurements, probes register themselves to a tracker, and
wait to be contacted by other peers (the ListenPeers
module, see Figure MP-1, bullet 4).
Discovery packets from multiple sources within the measurement
infrastructure are sent to the peers discovered in this
way, to characterize the Internet parts that are used by
BitTorrent (the TrackPeers module, see Figure
MP-1, bullet 5). We call this process multi-source