An illustration of the hourglass Internet architecture showing the six layers, from top to bottom: specific applications, application protocols, transport protocols, network protocols, data-link protocols and physical layer protocols. Credit: Constantine Dovrolis |
In
the natural world, species that share the same ecosystem often compete
for resources, resulting in the extinction of weaker competitors. A new
computer model that describes the evolution of the Internet’s
architecture suggests something similar has happened among the layers of
protocols that have survived—and become extinct—on the worldwide
network.
Understanding
this evolutionary process may help computer scientists as they develop
protocols to help the Internet accommodate new uses and protect it from a
wide range of threats. But the model suggests that unless the new
Internet avoids such competition, it will evolve an hourglass shape much
like today’s Internet.
“To
avoid the ossification effects we experience today in the network and
transport layers of the Internet, architects of the future Internet need
to increase the number of protocols in these middle layers, rather than
just push these one- or two-protocol layers to a higher level in the
architecture,” said Constantine Dovrolis, an associate professor in the
School of Computer Science at the Georgia Institute of Technology.
The
research will be presented on August 17, 2011 at SIGCOMM, the annual
conference of the Special Interest Group on Data Communication, a
special interest group of the Association for Computing Machinery. This
research was supported by the National Science Foundation.
From top to bottom, the Internet architecture consists of six layers:
- Specific applications, such as Firefox;
- Application protocols, such as Hypertext Transfer Protocol (HTTP);
- Transport protocols, such as Transmission Control Protocol (TCP);
- Network protocols, such as Internet Protocol (IP);
- Data-link protocols, such as Ethernet; and
- Physical layer protocols, such as DSL
Layers
near the top and bottom contain many items, called protocols, while the
middle layers do not. The central transport layer contains two
protocols and the network layer contains only one, creating an hourglass
architecture.
Dovrolis
and graduate student Saamer Akhshabi created an evolutionary model
called EvoArch to study the emergence of the Internet’s hourglass
structure. In the model, the architecture of the network changed with
time as new protocols were created at different layers and existing
protocols were removed as a result of competition with other protocols
in the same layer.
EvoArch
showed that even if future Internet architectures are not built in the
shape of an hourglass initially, they will probably acquire that shape
as they evolve. Through their simulations, Dovrolis and Akhshabi found
that while the accuracy of the structure improved with time, the basic
hourglass shape was always formed—no matter what shape it started in.
“Even
though EvoArch does not capture many practical aspects and
protocol-specific or layer-specific details of the Internet
architecture, the few parameters it is based on—the generality of
protocols at different layers, the competition between protocols at the
same layer, and how new protocols are created—reproduced the observed
hourglass structure and provided for a robust model,” said Dovrolis.
The
model revealed a plausible explanation for the Internet’s hourglass
shape. At the top, protocols are so specialized and selective in what
underlying building blocks they use that they rarely compete with each
other. When there is very little competition, the probability of
extinction for a protocol is close to zero.
“In
the top layers of the Internet, many new applications and
application-specific protocols are created over time, but few things
die, causing the top of the hourglass to get wider over time,” said
Dovrolis.
Illustration showing the number and age of protocols in each layer of the Internet architecture. In the middle layers, there are only a few protocols that are old and conserved. Credit: Constantine Dovrolis |
In
the higher layers, a new protocol can compete and replace an incumbent
only if they provide very similar services. For example, services
provided by the File Transfer Protocol (FTP) and HTTP overlapped in the
application-specific layer. When HTTP became more valuable because of
its own higher layer products—applications such as web browsers—FTP
became extinct.
At
the bottom, each protocol serves as a general building block and shares
many products in the layer above. For example, the Ethernet protocol in
the data-link layer uses the coaxial cable, twisted pair and optical
fiber technologies in the physical layer. But because the bottom layer
protocols are used in an abundant way, none of them dominate, leading to
a low probability of extinction at layers close to the bottom.
The
EvoArch model predicts the emergence of few powerful and old protocols
in the middle layers, referred to as evolutionary kernels. The
evolutionary kernels of the Internet architecture include IPv4 in the
network layer, and TCP and the User Datagram Protocol (UDP) in the
transport layer. These protocols provide a stable framework through
which an always-expanding set of physical and data-link layer protocols,
as well as new applications and services at the higher layers, can
interoperate and grow. At the same time, however, those three kernel
protocols have been difficult to replace, or even modify significantly.
To
ensure more diversity in the middle layers, EvoArch suggests designing
protocols that are largely non-overlapping in terms of services and
functionality so that they do not compete with each other. The model
suggests that protocols overlapping more than 70% of their functions
start competing with each other.
When
the researchers extended the EvoArch model to include a protocol
quality factor—which can capture protocol performance, extent of
deployment, reliability or security—the network grew at a slower pace,
but continued to exhibit an hourglass shape. In contrast to the basic
model, the quality factor affected the competition in the bottom layers
and only high-quality protocols survived there. The model also showed
that the kernel protocols in the waist of the hourglass were not
necessarily the highest-quality protocols.
“It
is not true that the best protocols always win the competition,” noted
Dovrolis. “Often, the kernels of the architecture are lower-quality
protocols that were created early and with just the right set of
connections.”
Researchers
are also using the EvoArch model to explore the emergence of hourglass
architectures in other areas, such as metabolic and gene regulatory
networks, the organization of the innate immune system, and in gene
expression during development.
“I
believe there are similarities between the evolution of Internet
protocol stacks and the evolution of some biological, technological and
social systems, and we are currently using EvoArch to explore these
other hourglass structures,” said Dovrolis.
