IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS. VOL 13. NO. 7, SEPTEMBER 1995 This approach has some important advantages. Most no. This clean separation is markedly different from the telephony tably, this approach does not require any change to the and cable infrastructures, which are more focused on a single service interface, Curren ly, applications can jus send their application and a uniform network technology. Experience packets without any negotiation with the network, and this suggests that violating the clean separation embodied in the will continue to be true with an extended service model IP layer by embedding application information in the network if the service is supplied implicitly; applications would not would inhibit the creation of new applications. Thus, the need to specify their desired service to the network, and the implicit approach, while certainly attractive in the short-term, network would, in turn, not need to describe the service- has serious long-term deficiencies to-be-delivered to the application. Also, since there is no The alternative is to have applications explicitly request the explicit commitment to a given service level, the mapping service they desire. That is, the network offers a set of service from application to service class, and the nature of the service classes and the applications indicate to the network which delivered to each service class, need not be uniform across service class they want. This approach has the advantage that all routers nor stable over time. Since applications would not it maintains the clean architectural interface between applica have to change, and the service given by the routers would tions and networks, so any future application can be serviced not have to be standardized, such an architecture could be in the desired service class. However, this approach does have deployed immediately and then modified incrementally ome unfortunate disadvantages. The first disadvantage is the Weighing against these powerful advantages are some prac- incentive issues that are raised, and the second is the lack of tical disadvantages. The implicit approach entails a fixed set flexibility in the service model. We address these two problems application classes: at any given time a router only knows separately about a certain set of applications and cannot properly service those applications about which it does not know. Moreover, this approach cannot accommodate individual or situational B. Incentives variations within a single application, For instance, if one Recall that in Section Iv we made two observations. First uses the same audio or video application for both interactive we observed that the optimal efficacythe maximal V--was (which needs low delay and low jitter)and lecture(which can only achieved when low priority service was given to the flow olerate large delays and jitter) settings, then there is no way with less stringent delay requirements. Second, we observed for the network to distinguish between these two cases: this that not all applications benefit directly from this increase in V leads to inefficiencies because the network must give service those asking for lower priority service received worse service appropriate for the interactive mode and thus the application In the explicit approach, this mapping betweenbetween the ap- gets better service than it needs when it is being used in lecture plication and service class is under control of the applications themselves and therefore ultimately under control of the user These two practical problems are symptoms of a fairly We can achieve optimal efficacy only if some applications ask mode. basic architectural flaw in the implicit approach. The implicit for lower quality service. What will motivate users to do this: approach can only work if the network knows something about why won always ask for the highest quality service the service requirements of each application. New applications no matter what their application requirements? 18Certainly cannot get the service they require if the network does not when the Internet had a small user population with a strong know what their service requirements are. Embedding appli- sense of tradition and comraderie, informal social conventions cation information into the network layer information violates would have been sufficient to induce users to behave properly he of the central design tenets of the Internet. The inter- However, in the public Internet of the future, as with ar network layer-the IP layer[351-was designed to provide a heavily utilized public facility, informal social conventions clean interface between networks and applications The fact will not be sufficient to discourage selfish behavior. Thus, that any application capable of running on top of IP could the network must provide some other system of incentives run on any network that supported IP encouraged diversity to encourage to request the proper service classes for both above and below the IP layer. a wide variety of net- their applications. Pricing of network services is one approach working(or, more correctly, subnetworking)technologies have Charging more for the higher quality service will ensure that emerged (e.g Ethernet, Token Ring, ATM, etc. )and a wide only the extremely performance-sensitive applications will and ever-increasing variety of applications have flourished. 7 request that service. As discussed in 14], [37], pricing can be sed to spread the benefits of increasing V to all applications user: is mucsauational variation. An individual variation would be for some applications, the reduction in the quality of service lay or jitter than another user is compensated by the reduction in price, and for others the re rather broad then this is unlikely to be that there es that the service request made by the general fashion: service requests are embedded into the application itself (much as the can be used as quality of service signals n the shor -term, is unlikely to be adopted as an architectural principle