1. A method for assigning communication sessions in a communication network in which a network controller is configured to communicate with nodes that are coupled by communication links and at least some of the communication links support communication on discrete multiplexed channels, the method comprising: at the network controller, accepting a specification of a communication capacity for each of a plurality of communication sessions between nodes of the network to be assigned to paths formed by links and multiplexed channels on said links through the network according to a cost function that depends on said assignment; and computing a routing configuration by the network controller, including formulating a continuous optimization problem in terms of a set of optimization variables and an objective function that depends on the optimization variables, wherein the optimization variables are subject to a set of constraints that define a continuous convex multidimensional region and wherein each assignment of the communication session to paths formed by links and multiplexed channels through the network corresponds to a different point in the continuous region, solving the continuous optimization problem, including determining optimal values for each of the optimization variables that optimize the objective function, and determining the assignment of the communication sessions to paths formed by links and multiplexed channels through the network such that it corresponds to the optimal values obtained in solving the continuous optimization problem, wherein said assignment optimizes the cost function.