In this paper we analyze the hit performance of cache systems that receive file requests with general arrival distributions and different popularities. We consider timer-based (TTL) policies, with differentiated timers over which we optimize. The optimal policy is shown to be related to the monotonicity of the hazard rate function of the inter-arrival distribution. In particular for decreasing hazard rates, timer policies outperform the static policy of caching the most popular contents. We provide explicit solutions for the optimal policy in the case of Pareto-distributed inter-request times and a Zipf distribution of file popularities, including a compact fluid characterization in the limit of a large number of files. We compare it through simulation with classical policies, such as least-recently-used and discuss its performance. Finally, we analyze extensions of the optimization framework to a line network of caches.

This paper considers the problem of allocating exchange rates in peer-to-peer dissemination, which must consider the dual objectives of throughput efficiency and reciprocity between peers, the latter essential to cooperation incentives. This question has been studied in prior research for wired networks under an upload constraint, where the focus is on achieving reciprocity through decentralized peer interactions. We consider here a wireless network substrate, for which link capacities are non-uniform according to the peering choice, and exchanges may be subject to interference. A convex optimization problem is formulated that trades off efficiency and reciprocity, and various schemes are investigated to achieve a decentralized solution.

This paper studies network resource allocation between users that manage multiple connections, possibly through different routes, where each connection is subject to congestion control. We formulate a user-centric Network Utility Maximization problem that takes into account the aggregate rate a user obtains from all connections, and we propose decentralized means to achieve this fairness objective. In a first proposal, cooperative users control their number of active connections based on congestion prices from the transport layer to emulate suitable primal-dual dynamics in the aggregate rate; we show this control achieves asymptotic convergence to the optimal user-centric allocation. For the case of noncooperative users, we show that network stability and user-centric fairness can be enforced by a utility-based admission control implemented at the network edge. We also study stability and fairness issues when routing of incoming connections is enabled at the edge router. We obtain in this case a characterization of the stability region of loads that can be served with routing alone and a generalization of our admission control policy to ensure user-centric fairness when the stability condition is not met. The proposed algorithms are implemented at the packet level in ns2 and demonstrated through simulation.

IEEE 802.11n, the latest version of the widely used standard for wireless LANs, promises significant increases in speed by incorporating multiple enhancements at the physical layer. In this paper we demonstrate that, on the contrary, the straightforward deployment of 802.11n in conjunction with TCP over a simple, single access-point network, can dramatically underachieve the promised speeds. Part of the deficiency is due to overheads and can be improved by the technique of packet aggregation present in the standard. However more subtle problems are identified, in particular the downward equalization of throughputs that occurs under physical rate diversity, or the unreasonable portion of resources taken by uplink flows when competing with the more numerous downlink connections. These difficulties are demonstrated and their causes explained through a sequence of experiments with the ns3 packet simulator. Our analysis leads us to propose a desirable resource allocation for these situations of competition, and an architecture for control in the access-point to achieve it. Our solution involves a combination of packet aggregation, multiple queues and TCP-ACK isolation, compatible with the standard and where all the control resides at the AP. We demonstrate analytically and through extensive simulation that our method is able to provide significant enhancements in performance under a variety of traffic conditions.

We develop a protocol through which multipath enabled IP routers collectively engage in dynamic traffic engineering, to optimize performance in concert with legacy TCP congestion control. We build on recent theory which shows a globally optimum resource allocation across the TCP/IP layers can be achieved through control of rates and multipath routing fractions following a consistent congestion signal. In this work we fully develop, in an ns2 simulation environment, the required multipath IP layer consistent with prevailing loss-based TCP protocols, with the additional requirement that individual TCP connections should be routed through single paths. The solution involves a generalization of distance vector protocols where routing metrics reflect loss probabilities. We demonstrate through simulations the stability and performance of the resulting protocol in combination with TCP, and we comment on the complexity of the implementation.

Methods for network resource allocation have mainly focused on establishing fairness among the rates of individual flows. However, since multiple TCP connections in one or many paths can serve a common user, we advocate in this paper a user-centric notion of fairness, which we formulate in the Network Utility Maximization (NUM) framework. In particular, we develop control laws for the number of connections identified with a certain user, which can include single-path, multipath or more general aggregates of flows, and prove convergence to the optimal resource allocation. This theory applies directly to the case of cooperative users. In the case where connections are generated exogenously by possibly non-cooperative users, we develop admission control policies that ensure both network stability and user-centric fairness.

This paper studies the problem of allocating network capacity through periodic auctions. We impose the following conditions: fully distributed solutions over an arbitrary network topology, and the requirement that resources allocated in a given auction are reserved for the entire duration of the connection, not subject to future contention. Under these conditions, we study the problem of selling capacity to optimize revenue for the operator. We first study optimal revenue for a single distributed auction in a general network. Next, the periodic auctions case is considered for a single link, modelling the optimal revenue problem as a Markov decision process (MDP); we develop a sequence of receding horizon approximations to its solution. Combining the two approaches we formulate a receding horizon optimization of revenue over a general network topology, that yields a distributed implementation. The proposal is demonstrated through simulations.

We propose to add a new dimension to existing wireless multimedia systems by enabling autonomous stations to dynamically compete for communication resources through adjustment of their internal strategies and sharing their private information. We focus on emerging spectrum agile wireless networks, where developing an efficient strategy for managing available communication resources is of high importance. The proposed dynamic resource management approach for wireless multimedia changes the passive way stations are currently adapting their joint source-channel coding strategies according to available wireless resources. Each wireless station can play the resource management game by adapting its multimedia transmission strategy depending on the experienced channel conditions and user requirements. The resource allocation game is coordinated by a network moderator, which deploys mechanism-based resource management to determine the amount of transmission time to be allocated to various users on different frequency bands such that certain global system metrics are optimized. Subsequently, the moderator charges the various users based on the amount of resources it has allocated to them, in order to discourage them from being dishonest about their resource requirements. We investigate and quantify both the users' and the system performance when different cross-layer strategies, and hence users' levels of smartness, are deployed by wireless stations. Our simulations show that mechanism-based resource management outperforms conventional techniques such as air-fair time and equal time resource allocation in terms of the obtained system utility. They also provide insights that can guide the design of emerging spectrum agile network protocols and applications

This paper is aimed at designing a congestion control system that scales gracefully with network capacity, providing high utilization, low queueing delay, dynamic stability, and fairness among users. The focus is on developing decentralized control laws at end-systems and routers at the level of fluid-flow models, that can provably satisfy such properties in arbitrary networks, and subsequently approximate these features through practical packet-level implementations. Two families of control laws are developed. The first "dual" control law is able to achieve the first three objectives for arbitrary networks and delays, but is forced to constrain the resource allocation policy. We subsequently develop a "primal-dual" law that overcomes this limitation and allows sources to match their steady-state preferences at a slower time-scale, provided a bound on round-trip-times is known. We develop two packet-level implementations of this protocol, using 1) ECN marking, and 2) queueing delay, as means of communicating the congestion measure from links to sources. We demonstrate using ns-2 simulations the stability of the protocol and its equilibrium features in terms of utilization, queueing and fairness, under a variety of scaling parameters.

The paper concerns TCP Westwood, a recently-developed modification of TCP, in combination with RED queue management. We develop a fluid-flow model of the protocol, and use it to study both equilibrium and dynamic features. On the equilibrium side, we identify the scaling of the congestion window with loss-probability, and compare it to TCP NewReno. We also use the model to find the boundary of stability, beyond which we see large oscillations; we find that the stable region of TCP Westwood is enhanced with respect to TCP NewReno. Furthermore we show preliminary evidence that oscillations, when they occur, have a limited impact on network throughput.

This paper is aimed at designing a congestion control system that scales gracefully with network capacity, providing high utilization, low queueing delay, dynamic stability, and fairness among users. In earlier work we had developed fluid-level control laws that achieve the first three objectives for arbitrary networks and delays, but were forced to constrain the resource allocation policy. In this paper we extend the theory to include dynamics at TCP sources, preserving the earlier features at fast time-scales, but permitting sources to match their steady-state preferences, provided a bound on round-trip-times is known. We develop two packet-level implementations of this protocol, using (i) ECN marking, and (ii) queueing delay, as means of communicating the congestion measure from links to sources. We discuss parameter choices and demonstrate using ns-2 simulations the stability of the protocol and its equilibrium features in terms of utilization, queueing and fairness. We also demonstrate the scalability of these features to increases in capacity, delay, and load, in comparison with other deployed and proposed protocols.

We demonstrate that the dynamic behavior of queue and average window is determined predominantly by the stability of TCP/RED, not by AIMD probing nor noise traffic. We develop a general multi-link multi-source model for TCP/RED and derive a local stability condition in the case of a single link with heterogeneous sources. We validate our model with simulations and illustrate the stability region of TCP/RED. These results suggest that TCP/RED becomes unstable when delay increases, or more strikingly, when link capacity increases. The analysis illustrates the difficulty of setting RED parameters to stabilize TCP: they can be tuned to improve stability, but only at the cost of large queues even when they are dynamically adjusted. Finally, we present a simple distributed congestion control algorithm that maintains stability for arbitrary network delay, capacity, load and topology.