review articles

I M A G E B Y T I E R N E Y M J

IN 1992, THE explosive growth of the World Wide Web began. The architecture of the Internet was commonly described as having four layers above the physical media, each providing a distinct function: a “link” layer providing local packet delivery over heterogeneous physical networks, a “network” layer providing best-effort global packet delivery across autonomous networks all using the Internet Protocol (IP), a “transport” layer providing communication services such as reliable byte streams (TCP) and datagram service (UDP), and an “application” layer.

In 1993, the last major change was made to this classic Internet architecture; 11 since then the scale and economics of the Internet have precluded further changes to IP. 12

A lot has happened in the world since 1993. The overwhelming success of the Internet has created many new uses and challenges that were not anticipated by its original architecture:

• Today, most networked devices are mobile.

• There has been an explosion of security threats.

• Most of the world’s telecommunication infrastructure and entertainment distribution has moved to the Internet.

• Cloud computing was invented to help enterprises manage the massive computing resources they now need.

• The IPv4 32-bit address space has been exhausted, but IPv6 has not yet taken over the bulk of Internet traffic.

• In a deregulated, competitive world, network providers control costs by allocating resources dynamically, rather than provisioning networks with static resources for peak loads.

Here is a conundrum. The Internet is meeting these new challenges fairly well, yet neither the IP protocol suite nor the way experts describe the Internet have changed significantly since 1993. Figure 1 shows the headers of a typical packet in the AT&T backbone, 19 giving us clear evidence that the challenges have been met by mechanisms well outside the limits of the classic Internet architecture. In the classic description, the only headers between

The
Compositional
Architecture
of the Internet

DOI: 10.1145/3226588

A new model for describing the Internet
reflects today’s reality and the future’s needs.
BY PAMELA ZAVE AND JENNIFER REXFORD
key insights

˽ For the past 25 years, the Internet has been evolving to meet new challenges by composition with new networks that were unanticipated by the original architecture. New networks make alternative trade-offs that are not compatible with the general-purpose Internet design, or maintain alternative views of network structure.

˽ In an architecture composed of self-contained networks, each network is a microcosm with all the basic network mechanisms including a namespace, routing, a forwarding protocol, session protocols, and directories. The mechanisms are specialized for the network’s purposes, membership, geographical span, and level of abstraction.