SHENKER: FNDAMENTAL DESIGN ISSUES FOR THE FL'TLRE INTERNET developments.none of which we can accurately foretell. the least efficient network design is to build separate networks Moreover, the amount of bandwidth needed to offset the each with a different application class, and that describes th benefits of extending the service model depends in detail on the current situation utility functions of applications and the service model being Our analysis of this simple example also reveals two other offered. Evaluating this trade-off requires making judgements important points about extending the service model. First egarding future developments about which little is known and not every client gains directly from the increase in efficacy vary widely compare two service allocations and Nonetheless, despite this uncertainty, at the core of this v(s)>v(5)does not imply that U (s, )>Ui(8? )for all bandwidth versus mechanism trade-off is the central fact i, For instance, in the simple example with just two clients, that the timescales of the service requirements of real-time U2 F0=2 but Up ority= 4/3 even though Priority applications are much smaller, by several orders of magnitude, VFIFo. Efficacy in heterogeneous networks is gained by han those of, say, FAX or electronic mail. One cannot operate shifting resources from applications that are not extremely a network at reasonable utilizations while delivering to all performance-sensitive to those that are: the performance traffic a service suitable for real-time applications: yet the sensitive clients gain from offering more sophisticated service extreme elasticity of FAX and e-mail would be able to utilize models, but the less-performance sensitive clients lose; the the significant amount of leftover bandwidth if the service overall efficacy increases because the losses are smaller than model could just keep it out of the way of the real-time traffic. the gains. For this reason alone it seems plausible, if not probable, that Second, in order to achieve the additional efficacy with an the payoff in terms of the bandwidth saved from offering these extended service model, the mapping between service classes multiple classes of service will more than outweigh the costs and applications must reflect the application requirements. In of the extra mechanism our simple example, the increased efficacy of priority service Another alternative is to use separate networks for can only be realized if the network can recognize which client various applications, each with its own single service is more delay-sensitive and assign it the appropriate service his is similar to what we do now with separate telephony, class. Assigning flow I to the lower priority class and fiow 2 video, and data network infrastructures. We can use our to the higher priority class would result in a lower value for imple example to examine this possibility. If we double the V: in fact, it yields V= 4/ 3(which is 1/2 of the optim linespeed. /=2, and double the number of applications, value ). We will return to these two points in Section V. the performance numbers become: I'prorty 32 /3 and 10, Now consider two networks, with separ andwidths /(1 + 2=2. each carrying two applications. If we V. WHO CHOOSES THE SERVICE FOR A FLOW divide the clients so that each network carries one applicatio At this point, we have argued that the network should offer of each type. and the bandwidths are split evenly (which is a service model that includes more than just the single class optimal here). then we revert back to our original case and the of best-effort service This service model could be as simple most efficient thing is to use priority on each link and achieve as two priority levels, or as complicated as the multiple delay a total efficacy of 16/3. If we partition the clients so that one bounded service classes described in [3]. However, we have network carries two delay-sensitive applications and the other yet to address one fundamental question which applies to all carries the two less sensitive applications, then the optimal of these possible service models: how does the architecture arrangement is to use FIFO on each network and to split the decide which service to give a particular flow? There are bandwidth. as /lr 086 and /2=V2-1/2 essentially two possible answers to this question: the flow car .9I4. which yields an efficacy of 10-lv2 2 4. 313. Thus, pick the service, or the network can pick the service. We now combining the networks into a single infrastructure yields a contrast these two options. I-4 much higher value for v than using separate networks: in fact. in this simple example the value of V is doubled by combining A."ImplicitlY Supplied"versus"Explicitly Requested the networks. This increased efficiency due to statistically sharing a resource is one of the central design principles of If the network chooses the service class, then we say that the data networks. When considering separate networks, there is service is implicitly supplied; the application sends its packets greater efficacy when the application types are mixed. Thus, without saying anything about its service requirements and the network then classifies these ts into some service class the near and handles them accordingly. For instance, the network might oubiful that. when considering the network as a whole bandwidth will be divide all traffic into the categories of asynchronous bulk mportant to note that if indeed interactive bulk, interactive burst, and real-tim port number. and then deliver to ace category ag For instance, it might then be preferable to mere add bandwidth end the service rnodel appropniate service Using the formulas for delay +{8 throughout this paper. in this section the larceny ially egregious The od by which the categorization is done is not important and the fact that m +g2=2. we can solve for the optimal value of AI and here. we care only that the categorization is under the control of the network and not the applicatio