This corollary is an important consequence of the ACPL, as the path
between a customer and the desired service is particularly sensitive
to the number and complexity of elements in the path. This is due to
the fact that the complexity "smoothing" that we find at high levels
of aggregation [ZHANG] is missing as you move closer to the edge, as
well as having complex interactions with backoffice and CRM systems.
Examples of architectures that haven't found a market due to this
effect include TINA-based CRM systems, CORBA/TINA based service
architectures. The basic lesson here was that the only possibilities
for deploying these systems were "Limited scale deployments (such) as
in Starvision can avoid coping with major unproven scalability
issues", or "Otherwise need massive investments (like the carrier-
grade ORB built almost from scratch)" [TINA]. In other words, these
systems had complex service delivery paths, and were too complex to
be feasibly deployed.
9. Conclusions
This document attempts to codify long-understood Internet
architectural principles. In particular, the unifying principle
described here is best expressed by the Simplicity Principle, which
states complexity must be controlled if one hopes to efficiently
scale a complex object. The idea that simplicity itself can lead to
some form of optimality has been a common theme throughout history,
and has been stated in many other ways and along many dimensions.
For example, consider the maxim known as Occam's Razor, which was
formulated by the medieval English philosopher and Franciscan monk
William of Ockham (ca. 1285-1349), and states "Pluralitas non est
ponenda sine neccesitate" or "plurality should not be posited without
necessity." (hence Occam's Razor is sometimes called "the principle
of unnecessary plurality" and " the principle of simplicity"). A
perhaps more contemporary formulation of Occam's Razor states that
the simplest explanation for a phenomenon is the one preferred by
nature. Other formulations of the same idea can be found in the
KISS (Keep It Simple Stupid) principle and the Principle of Least
Astonishment (the assertion that the most usable system is the one
that least often leaves users astonished). [WILLINGER2002] provides
a more theoretical discussion of "robustness through simplicity", and
in discussing the PSTN, [KUHN87] states that in most systems, "a
trade-off can be made between simplicity of interactions and
looseness of coupling".
When applied to packet switched network architectures, the Simplicity
Principle has implications that some may consider heresy, e.g., that
highly converged approaches are likely to be less efficient than
"less converged" solutions. Otherwise stated, the "optimal"
convergence layer may be much lower in the protocol stack that is
conventionally believed. In addition, the analysis above leads to
several conclusions that are contrary to the conventional wisdom
surrounding packet networking. Perhaps most significant is the
belief that packet switching is simpler than circuit switching. This
belief has lead to conclusions such as "since packet is simpler than
circuit, it must cost less to operate". This study finds to the
contrary. In particular, by examining the metrics described above,
we find that packet switching is more complex than circuit switching.
Interestingly, this conclusion is borne out by the fact that
normalized OPEX for data networks is typically significantly greater
than for voice networks [ML2002].
Finally, the important conclusion of this work is that for packet
networks that are of the scale of today's Internet or larger, we must
strive for the simplest possible solutions if we hope to build cost
effective infrastructures. This idea is eloquently stated in
[DOYLE2002]: "The evolution of protocols can lead to a
robustness/complexity/fragility spiral where complexity added for
robustness also adds new fragilities, which in turn leads to new and
thus spiraling complexities". This is exactly the phenomenon that
the Simplicity Principle is designed to avoid.
10. Security Considerations
This document does not directly effect the security of any existing
Internet protocol. However, adherence to the Simplicity Principle
does have a direct affect on our ability to implement secure systems.
In particular, a system's complexity grows, it becomes more
difficult to model and analyze, and hence it becomes more difficult
to find and understand the security implications inherent in its
architecture, design, and implementation.
11. Acknowledgments
Many of the ideas for comparing the complexity of circuit switched
and packet switched networks were inspired by conversations with Nick
McKeown. Scott Bradner, David Banister, Steve Bellovin, Steward
Bryant, Christophe Diot, Susan Harris, Ananth Nagarajan, Andrew
Odlyzko, Pete and Natalie Whiting, and Lixia Zhang made many helpful
comments on early drafts of this document.
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13. Authors' Addresses
Randy Bush
EMail: randy@psg.com
David Meyer
EMail: dmm@maoz.com
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