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Hans-Werner Braun and K Claffy acknowledge support from the National Science Foundation for studying engineering and architectural issues of the National Research and Education Network.

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We will use the term bandwidth loosely, to include both transmission and switching. At present and in the near future, which factor is most constraining will depend on the environment: typically the bottleneck will be switching capacity for environments which can support high bandwidth, such as national backbones using large telecommunications pipes or local area networks using FDDI technology. In contrast the intermediate transmission between a LAN and national backbones is more often constrained by link or transmission bandwidth in campus or regional networks.

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The exact bandwidth requirements depend on compression techniques used and the motion within the video frame.

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UDP is a lesser-used transport protocol designed for higher volume traffic, with much less error checking. Unlike TCP, it does not automatically back off its transmission rate in the face of congestion, and can thus saturate a path.

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We admit that the growth of real-time applications may be self-limiting since each new user contributing to the congestion degrades performance for themselves as well as everyone else, and while degraded performance only makes e-mail late, it makes real-time video/audio streams useless. Nonetheless, we note that such a process will yield a rather high ``equilibrium'' level congestion.

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Whether lower priority queues starve when higher queues are non-empty is up for debate [6] [8]. We assume that they do in the numerical example of figure 1.

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Based on monthly data for September 1993 as maintained by Merit, Inc. on nis.nsf.net.

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We discarded the idea of setting the quota proportional to current usage, because that provides an incentive to flood the network with priority zero packets.

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In our data set, some types of traffic were burstier than others, leading to somewhat varying attrition rates under a FIFO scheme.