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4
The Generative Pattern
Anyone can design new applications to operate over the Internet.
Good applications can then be adopted widely while bad ones are ignored.
The phenomenon is part of the Internet’s “hourglass architecture”
(Figure 4.1).
The hourglass portrays two important design insights. First is the
notion that the network can be carved into conceptual layers. The exact
number of layers varies depending on who is drawing the hourglass
and why, 1 and even by chapter of this book. 2 On one basic view
the network can be understood as having three layers. At the bottom
is the “physical layer,” the actual wires or airwaves over which data will
flow. At the top is the “application layer,” representing the tasks people
might want to perform on the network. (Sometimes, above that,
we might think of the “content layer,” containing actual information
exchanged among the network’s users, and above that the “social
layer,” where new behaviors and interactions among people are enabled
by the technologies underneath.) In the middle is the “protocol
layer,” which establishes consistent ways for data to flow so that the
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