Administratively assigning different priorities to different types of traffic is appealing, but impractical as a long-run solution to congestion costs due to the usual inefficiencies of rationing. However, there is an even more severe technological problem: it is impossible to enforce. From the network's perspective, bits are bits and there is no certain way to distinguish between different types of uses. By convention, most standard programs use a unique identifier that is included in the TCP header (called the ``port'' number); this is what NSFNET used for its priority scheme in 1987. However, it is a trivial matter to put a different port number into the packet headers; for example to assign the telnet number to ftp packets to defeat the 1987 priority scheme. To avoid this problem, NSFNET kept its prioritization mechanism secret, but that is hardly a long-run solution.
What other mechanisms can be used to control congestion? The most obvious approach for economists is to charge some sort of congestion price. However, to date, there has been almost no serious consideration of congestion pricing for backbone services, and even tentative proposals for usage pricing have been met with strong opposition. We will discuss pricing below but first we examine some non-price mechanisms that have been proposed.
Many proposals rely on voluntary efforts to control congestion. Numerous participants in congestion discussions suggest that peer pressure and user ethics will be sufficient to control congestion costs. For example, recently a single user started broadcasting a 350--450Kbps audio-video test pattern to hosts around the world, blocking the network's ability to handle a scheduled audio broadcast from a Finnish university. A leading network engineer sent a strongly-worded e-mail message to the user's site administrator, and the offending workstation was disconnected from the network. However, this example also illustrates the problem with relying on peer pressure: the inefficient use was not terminated until after it had caused serious disruption. Further, it apparently was caused by a novice user who did not understand the impact of what he had done; as network access becomes ubiquitous there will be an ever-increasing number of unsophisticated users who have access to applications that can cause severe congestion if not properly used. And of course, peer pressure may be quite ineffective against malicious users who want to intentionally cause network congestion.
One recent proposal for voluntary control is closely related to the 1987 method used by the NSFNET [Bohn et al. 1993]. This proposal would require users to indicate the priority they want each of their sessions to receive, and for routers to be programmed to maintain multiple queues for each priority class. Obviously, the success of this scheme would depend on users' willingness to assign lower priorities to some of their traffic. In any case, as long as it is possible for just one or a few abusive users to create crippling congestion, voluntary priority schemes that are not robust to forgetfulness, ignorance, or malice may be largely ineffective.
In fact, a number of voluntary mechanisms are in place today. They are somewhat helpful in part because most users are unaware of them, or because they require some programming expertise to defeat. For example, most implementations of the TCP protocols use a ``slow start'' algorithm which controls the rate of transmission based on the current state of delay in the network. Nothing prevents users from modifying their TCP implementation to send full throttle if they do not want to behave ``nicely.''
A completely different approach to reducing congestion is purely
technological: overprovisioning. Overprovisioning means maintaining
sufficient network capacity to support the peak demands without
noticeable service degradation.
This has been the most important mechanism used to date in
the Internet. However, overprovisioning is costly, and with both
very-high-bandwidth applications and near-universal access fast
approaching, it may become too costly. In simple terms, will capacity
demand grow faster than the decline in capacity cost?
Given the explosive growth in demand and the long lead time needed to introduce new network protocols, the Internet may face serious problems very soon if productivity increases do not keep up. Therefore, we believe it is time to seriously examine incentive-compatible allocation mechanisms, such as various forms of congestion pricing.