In this paper, we develop a general model, called Latency-Rate servers (LR-servers), for the analysis of traffic scheduling algorithms in broadband packet networks. The behavior of an LR scheduler is determined by two parameters --- the latency and the allocated rate. We show that several well-known scheduling algorithms, such as Weighted Fair Queueing, VirtualClock, Self-Clocked Fair Queueing, Weighted Round Robin, and Deficit Round Robin, belong to the class of LR-servers. We derive tight upper bounds on the end-to-end delay, internal burstiness, and buffer requirements of individual sessions in an arbitrary network of LR- servers in terms of the latencies of the individual schedulers in the network, when the session traffic is shaped by a leaky bucket. Thus, the theory of LR-servers enables computation of tight upper-bounds on end-to-end delay and buffer requirements in a heterogeneous network, where individual servers may support different scheduling architectures, and under diff...