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to a dynamically changing set of peers in an unstructured way (i.e., no tree
whatsoever). Consequently, a higher communication overhead would be in-
curred when packets are shared. Given their complementary nature, the two
extreme approaches are naturally combined to form a hybrid approach [Li
et al., 2008], which works by first using mesh pull to jump start the buffering
process, and then construct multicast trees with relatively stable peers (or
sometimes, peers with similar capabilities, as in the case of BitTorrent) to
enhance the e
ciency. Indeed, as pointed out by Zhang et al. [Zhang et al.,
2007], using a hybrid push-pull or even a traditional mesh pull mechanism
can lead to optimal performance in terms of peer upload capacity utilization
and system throughput even without intelligent scheduling and bandwidth
measurement.
Tracker
Peer 1
5. Connection
request
3. Get peers list
Peer 2
5. Connection
request
4. Return
peers list
1. Select channel
5. Connection
request
Login Server
Peer 3
Peer N
5. Connection
request
2. Send tracker
address
Peer 4
5. Connection
request
New Peer
FIGURE 2.4: A general information exchange process in a P2P video stream-
ing system.
In summary, the general process of a P2P video streaming session is illus-
trated in Figure 2.4. Initially, the new peer visits the so-called log-in server
(i.e., the Web site of the system) to select the channel or movie the user wants
to watch. The log-in server then redirects the new peer to a particular tracker
server which can furnish a list of peers currently watching the same channel
to the new peer. Usually the tracker server just randomly picks a subset of
peers to form a list for the new peer. The new peer then selects a subset from
this list so as to make connection requests. Such selection is, in current imple-
mentations, also based on randomization. After connections are established,
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