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go through nodes 5/6 in its local overlay. To enhance the communication e
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ciency, “vertical tunnels” can be used—e.g., a top level peer can communicate
directly with a bottom level peer using a tunnel [Qu et al., 2009].
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14
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7
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6
1
5
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FIGURE 3.9: A truncated pyramid P2P network architecture internetwork-
ing several local overlays [Qu et al., 2009].
3.3 Unstructured (Mesh) P2P Systems
As we have seen in Chapter 2, unstructured P2P systems simply involve
random and ad hoc connections among peers [Lv et al., 2002]. Indeed, there
is absolutely no central rule governing the formation of connections among
peers. In a sense, it is truly peer-to-peer in the communication aspects in that
connections are made in a purely autonomous manner. One important feature
of an unstructured network is that as time goes by, those long-lived nodes (i.e.,
nodes that stay on for an extended period of time) usually would have a larger
number of connections due to possibly their resourcefulness (e.g., in terms of
file chunks possessed over time). In some applications (e.g., Skype [Skype,
2009]), such nodes would even be designated as “Super Nodes”—nodes that
have more important responsibilities such as routing, traversal through NATs,
etc.
As to the construction of an unstructured P2P network, there are two ap-
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