We examine in this paper the tradeoff between application complexity, network complexity, and network efficiency. We argue that the design of the current Internet reflects a tradeoff between lower network complexity (no state in the network, no signalling) and higher application complexity (rate and error control mechanisms to obtain an adaptive application) assuming a unicast service model. For such a service model, a design methodology that leans heavily towards application complexity has proven very successful. However, we also argue that this tradeoff changes radically for a multicast/multilayer service model. There are several reasons for this. First, implementing a multicast/multilayer service requires per-flow state. The incremental cost of deploying a slightly more elaborate service model that takes into account flow dependence is much smaller than in the case of unicast. Second, several end-to-end functions, such as channel estimation and error control, are considerably more difficult for multicast/multilayer applications in a large-scale and heterogeneous environment than for unicast applications. Third, the operating point of a pure best-effort network is dictated largely by elastic applications (such as those based on TCP). Unfortunately, this operating point tends to be undesirable for multicast/multilayer applications, as they have for example to use FEC to protect high-priority layers. Other choices similarly lower the network efficiency. These insights motivate a new service model which slightly departs from the best-effort model, and which trades off a slightly higher network complexity for much lower application complexity and higher network efficiency. We describe this service model and the associated network protocols. The protocol complexity is only marginally higher than that of a simple multicast routing protocol with receiver-initiated join/leave capabilities. The dependencies between multilayer flows are established and maintained as soft state; therefore, no explicit session signalling to establish and tear down flow dependence state is necessary.
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