In 2003, the world of network engineering was far different than it is today. For instance, EIGRP was still being implemented on the basis of its ability to support multi-protocol routing. SONET, and other optical technologies, were just starting to come into their own, and all optical switching was just beginning to be considered for large scale deployment. What Hartley says of history holds true when looking back at what seems to be a former age: “The past is a foreign country; they do things differently there.”
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In the midst of this change, the Association for Computing Machinery (the ACM) published a paper entitled “Will IP really take over the world (of communications)?” This paper, written during the ongoing discussion within the engineering community over whether packet switching or circuit switching is the “better” technology, provides a lot of insight into the thinking of the time. Specifically, as the author say, the belief that IP is better:
…is based on our collective belief that packet switching is inherently superior to circuit switching because of the efficiencies of statistical multiplexing, and the ability of IP to route around failures. It is widely assumed that IP is simpler than circuit switching, and should be more economical to deploy and manage.
Several of the reasons given are worth considering in light of the intervening years between the paper being written and today. Section 2 of the paper suggest four myths that need to be considering in the world of IP based packet switching.
The first of these myths is that IP is more efficient. Here the authors point out an early argument used by packet switching advocates: when a circuit is idle, reserved bandwidth is unused. Because packets does not reserve bandwidth in this way, packet switching makes more efficient use of the available resources. The authors counter this argument by observing packets switched networks are not that heavily utilized, and the exploring why this might be so. The reasons they give are that packet switched networks become unstable if they are overutilized, and the desire to drive delay down as much as possible. The authors say:
But simply reducing the average link utilization will not be enough to make users happy. For a typical user to experience low utilization, the variance of the network utilization needs to be low, too. Reducing variations in link utilization is hard; today we lack effective techniques to do it. It might be argued that the problem will be solved by research efforts on traffic management, congestion control, and multipath routing. But to-date, despite these problems being understood for many years, effective measures are yet to be introduced.
The second of these myths is that IP is stable. Here the authors point out the many potential problems with packet switched networks, particularly in the area of in band signaling.
The third myth the authors consider is that IP is simpler. Here the argument is that circuit switched networks have fewer moving parts, each of which can be more regimented in their implementation. This means circuit switched networks are generally simpler than their packet switched counterparts.
The fourth myth the authors consider is that real time traffic will be able to run over IP based networks. They argue that because of the various quality of service problems, near real time traffic, such as voice and video, can never be carried over an IP network.
What did the authors do get right, and what did they get wrong? Looking back over the intervening years, some of these problems seem to have been largely solved. For instance, between research into new quality of service solutions, virtualization technologies, and research into and deployment of new methods of managing traffic flows over a packet switched network (for instance, as implemented in QUIC), have largely eased the problems the authors discuss.
Other problems in their list seem to have simply fallen by the wayside. For instance, the authors are largely correct on the stability of IP routing. The Internet, as it exists today, is not really stable. The default free zone, the Internet core, never does really converge. But, on the other hand, it turns out that it might not be all that important. So long as reachability exists, and the applications running over the network can deal with the variable delay, then it does not really matter. It is clear that IP networks can be used for near real time traffic at this point, as well.
The third myth is, perhaps, the most interesting—IP is more complex. Circuit switching generally relies on a centralized control plane, rather than a distributed one. Is this truly simpler? Advocates of Software Defined Networks have argued recently that it is, but I am not convinced. The problem here, it seems to me, is the conflation of reachability and policy into a single “thing” that must be solved using a single subsystem of the network as a system. I tend to think that we will have a much healthier discussion around control plane design and operation once we split the various purposes the control plane serves up into different pieces, and discuss each one as a separate problem and solution space.
What did the authors get right? They were right in surmising that circuit switched networks, particularly in the area of optical switching, would continue to play a role in the global Internet. In fact, you could argue that packet and circuit switched networks often live side-by-side in the global Internet today, with many long haul and metropolitan area links being good examples of circuit switched networks underlying a packet switched overlay. MPLS, of course, also provides a sort of hybrid mode between circuit and packet switching which enables many of the solutions deployed today.
Much as we might like to look back in derision at papers such as this, the reality is the authors were accurately pointing out many of the problems with packet switched networks, some of which still have not been solved today. These objections should not be taken as a relic of the past, but rather as a pointer to what yet remains to be solved, where problems may crop up again, and, finally, where we might look for solutions that may, ultimately, solve the problem of carrying data at scale.