Harish Sethu

Fair and efficient packet scheduling using Elastic Round Robin

Parallel systems are increasingly being used in multiuser environments with the interconnection network shared by several users at the same time. Fairness is an intuitively desirable property in the allocation of bandwidth available on a link among traffic flows of different users that share the link. Strict fairness in traffic scheduling can improve the isolation between users, offer a more predictable performance and improve performance by eliminating some bottlenecks. This paper presents a simple, fair, efficient, and easily implementable scheduling discipline, called Elastic Round Robin (ERR), designed to satisfy the unique needs of wormhole switching, which is popular in interconnection networks of parallel systems. In spite of the constraints of wormhole switching imposed on the design, ERR is also suitable for use in Internet routers and has better fairness and performance characteristics than previously known scheduling algorithms of comparable efficiency, including Deficit Round Robin and Surplus Round Robin. In this paper, we prove that ERR is efficient, with a per-packet work complexity of O(1). We analytically derive the relative fairness bound of ERR, a popular metric used to measure fairness. We also derive the bound on the start-up latency experienced by a new flow that arrives at an ERR scheduler. Finally, this paper presents simulation results comparing the fairness and performance characteristics of ERR with other scheduling disciplines of comparable efficiency.

On achieving software diversity for improved network security using distributed coloring algorithms

It is widely believed that diversity in operating systems, software packages, and hardware platforms will decrease the virulence of worms and the effectiveness of repeated applications of single attacks. Research efforts in the field have focused on introducing diversity using a variety of techniques on a system-by-system basis. This paper, on the other hand, assumes the availability of diverse software packages for each system and then seeks to increase the intrinsic value of available diversity by considering the entire computer network. We present several distributed algorithms for the assignment of distinct software packages to individual systems and analyze their performance. Our goal is to limit the ability of a malicious node to use a single attack to compromise its neighboring nodes, and by extension, the rest of the nodes in the network. The algorithms themselves are analyzed for attack tolerance, and strategies for improving the security of the individual software assignment schemes are presented. We present a comparative analysis of our algorithms using simulation results on a topology obtained from e-mail traffic logs between users at our institution. We find that hybrid versions of our algorithms incorporating multiple assignment strategies achieve better attack tolerance than any given assignment strategy. Our work thus shows that diversity must be introduced at all levels of system design, including any scheme that is used to introduce diversity itself.

Fair, efficient and low-latency packet scheduling using nested deficit round robin

In the emerging high-speed integrated-services packet-switched networks, packet scheduling algorithms in switches and routers play a critical role in providing the quality of-service (QoS) guarantees required by many applications. We present a new scheduling discipline called nested deficit round robin (Nested-DRR), which is fair, efficient and in addition has a low latency bound. Nested-DRR splits each DRR round into one or more smaller rounds, within each of which we run a modified version of the DRR scheduling discipline. In this paper, we analytically prove that Nested-DRR results in a significant improvement in the latency bound in comparison to DRR, and in addition preserves the good properties of DRR such as the per-packet work complexity of O(1). Nested DRR also has the same relative fairness bound as DRR.

Cooperative topology control with adaptation for improved lifetime in wireless ad hoc networks

Topology control algorithms allow each node in a wireless multi-hop network to adjust the power at which it makes its transmissions and choose the set of neighbors with which it communicates directly, while preserving global goals such as connectivity or coverage. This allows each node to conserve energy and contribute to increasing the lifetime of the network. Previous work on topology control has largely used an approach based on considering only the energy costs across links without considering the amount of energy available on a node. Further, previous work has largely used a static approach where the topology is determined at the beginning of the networks life and does not incorporate the varying rates of energy consumption at different nodes. In this paper, we address these weaknesses and introduce a new topology control algorithm that dynamically adapts to current energy levels at nodes. The algorithm, called Cooperative Topology Control with Adaptation (CTCA), employs a game-theoretic approach that maps the problem of maximizing the networks lifetime into an ordinal potential game. This allows a node running the CTCA algorithm to make a sacrifice by increasing its transmission power if it can help reduce energy consumption at another node with a smaller lifetime. We prove the existence of a Nash equilibrium for the game. Our simulation results indicate that the CTCA algorithm extends the life of a network by more than 50% compared to the best previouslyknown algorithm.

A new distributed topology control algorithm for wireless environments with non-uniform path loss and multipath propagation

Each node in a wireless multi-hop network can adjust the power level at which it transmits and thus change the topology of the network to save energy by choosing the neighbors with which it directly communicates. Many previous algorithms for distributed topology control have assumed an ability at each node to deduce some location-based information such as the direction and the distance of its neighbor nodes with respect to itself. Such a deduction of location-based information, however, cannot be relied upon in real environments where the path loss exponents vary greatly leading to significant errors in distance estimates. Also, multipath effects may result in different signal paths with different loss characteristics, and none of these paths may be line-of-sight, making it difficult to estimate the direction of a neighboring node. In this paper, we present Step Topology Control (STC), a simple distributed topology control algorithm which reduces energy consumption while preserving the connectivity of a heterogeneous sensor network without use of any location-based information. The STC algorithm avoids the use of GPS devices and also makes no assumptions about the distance and direction between neighboring nodes. We show that the STC algorithm achieves the same or better order of communication and computational complexity when compared to other known algorithms that also preserve connectivity without the use of location-based information. We also present a detailed simulation-based comparative analysis of the energy savings and interference reduction achieved by the algorithms. The results show that, in spite of not incurring a higher communication or computational complexity, the STC algorithm performs better than other algorithms in uniform wireless environments and especially better when path loss characteristics are non-uniform.

On achieving fairness in the joint allocation of processing and bandwidth resources: principles and algorithms

The problem of achieving fairness in the allocation of the bandwidth resource on a link shared by multiple flows of traffic has been extensively researched over the last decade. However, with the increasing pervasiveness of optical networking and the occasional trend toward using over-provisioning as the solution to bandwidth congestion, a routers processor also becomes a critical resource to which, ideally speaking, all competing flows should have fair access. For example, achieving fairness in the allocation of processing resources can be part of an overall strategy of countering certain kinds of denial of service attacks (such as those based on an excessive use of the router processor by using unnecessary optional headers). In this paper, we investigate the issue of achieving fairness in the joint allocation of the processing and bandwidth resources. We first present a simple but powerful general principle for defining fairness in such systems based on any of the classic notions of fairness such as max-min fairness, proportional fairness, and utility max-min fairness defined for a single resource. We apply our principle to a system with a shared processor and a shared link with max-min fairness as the desired goal. We then propose a practical and provably fair packet-by-packet algorithm for the joint allocation of processing and bandwidth resources. We demonstrate the fairness achieved by our algorithm through simulation results using both synthetic and real gateway traffic traces. The principles and the algorithm detailed in this paper may also be applied in the allocation of other kinds of resources such as power, which is a critical resource in mobile systems.

On the latency bound of deficit round robin

The emerging high-speed broadband packet-switched networks are expected to simultaneously support a variety of services with different quality-of-service (QoS) requirements over the same physical infrastructure. Fair packet scheduling algorithms used in switches and routers play a critical role in providing these QoS guarantees. Deficit round robin (DRR), a popular fair scheduling discipline, is very efficient with an O(l) dequeuing complexity. Using the concept of latency-rate (/spl Lscr//spl Rscr/) servers introduced by Stiliadis and Varma (1996), we obtain an upper bound on the latency of DRR and prove that our bound is tight. Our upper bound is lower than the previously known upper bound. This illustrates that the DRR scheduler has better performance characteristics than previously believed, especially for real-time applications where the latency plays a role in the size of the playback buffer required.

Max-Min Fair Scheduling in Input-Queued Switches

Fairness in traffic management can improve the isolation between traffic streams, offer a more predictable performance, eliminate transient bottlenecks, mitigate the effect of certain kinds of denial-of-service attacks, and serve as a critical component of a quality-of-service strategy to achieve certain guaranteed services such as delay bounds and minimum bandwidths. In this paper, we choose a popular notion of fairness called max-min fairness and provide a rigorous definition in the context of input-queued switches. We show that being fair at the output ports alone or at the input ports alone or even at both input and output ports does not actually achieve an overall max-min fair allocation of bandwidth in a switch. Instead, we propose a new algorithm that can be readily implemented in a distributed fashion at the input and output ports to determine the exact max-min fair rate allocations for the flows through the switch. In addition to proving the correctness of the algorithm, we propose a practical scheduling strategy based on our algorithm. We present simulation results, using both real traffic traces and synthetic traffic, to evaluate the fairness of a variety of popular scheduling algorithms for input-queued switches. The results show that our scheduling strategy achieves better fairness than other known algorithms for input-queued switches and, in addition, achieves throughput performance very close to that of the best schedulers.

On the relationship between absolute and relative fairness bounds

The fairness of scheduling disciplines used in communication networks has frequently been evaluated using one of two fairness measures: the absolute fairness bound (AFB) and the relative fairness bound (RFB). We present a tight bounded relationship between these measures of fairness. The bounds established are exactly reached in the case of many real scheduling disciplines, thus leading to an easy conversion between these two measures. In the case of latency-rate (LR) servers, this also leads to an easy determination of the upper bound on the latency of a flow from the more tractable relative fairness bound of the scheduler. Our results also indicate that in many real contexts, the two measures of fairness approach each other as the number of flows increases, confirming the RFB as an adequate measure of fairness.

Low-latency guaranteed-rate scheduling using Elastic Round Robin

Packet scheduling algorithms in switches and routers will likely play a critical role in providing the Quality-of-Service (QoS) guarantees required by many real-time multimedia applications. Elastic Round Robin (ERR), a recently proposed fair scheduling discipline designed for best-effort traffic, is very efficient with an O(1) dequeuing complexity and, in addition, has better fairness characteristics than other algorithms of equivalent complexity. In this paper, we analyze ERR for guaranteed-rate services, and obtain an upper bound on its latency. We further show that the bound obtained in this paper is tight. Our analysis shows that ERR, in comparison to other scheduling disciplines of equivalent complexity, also has significantly better latency properties. The combination of fairness, efficiency and low-latency makes ERR an attractive scheduling discipline for both best-effort and guaranteed-rate services.