The recent developments of multipath data transport protocols such as Multipath TCP allow end-systems to explore and share available resources within networks. Through dynamic load balancing over subflows these protocols ensure high levels of robustness to network failures and traffic overloads. In this paper we use fluid models to study the benefits that accrue when load is shared across subflows. We combine insights gained from the fluid models with a precise description of the capacity

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... for the network and show that our models of multipath protocols approach the boundary of the capacity region as the intensity of the offered traffic approaches a critical value. We quantify the extent to which multipath protocols will make a network robust to unforeseen traffic mismatches and link failures and illustrate our results with parameterised models for random fluctuations in the offered traffic.