Determination of the parameters in the dynamic weighted Round-Robin method for network load balancing

Abstract

The purpose of this paper is to determine the values of the parameters in a new method (Dynamic Weighted Round-Robin, DWRR) developed for solving Internet traffic jam problems. Using the traditional Round-Robin (RR) method as a base, DWRR was developed to efficiently control loads in a multiple-link network. Unlike least-load algorithm, DWRR does not need to trace system loads continually, but achieves a far better load balancing than RR does. Mathematical functions are developed for predicting the optimal time interval of detection of line loads in this method, while the concept of variance in statistics is used as the criterion for evaluating the load balance level. A couple of related coefficients have also been determined by analyzing the simulation data. A centralized gateway with a multi-link-load-balancer is modeled for explaining the proposed algorithm. In addition, both theoretical and practical approaches are provided in this paper, along with performance comparisons between them. The results obtained from the computational experiments show that DWRR achieves a superior network loads balancing.

Introduction

Applications for the Internet are growing explosively, causing enormous of network loads. This high growth rate, however, leads to a decrease in Quality of Service (QoS) and performances of networks, both in network line bandwidth allocation and in server responding time. The solution currently widely used for this kind of Internet traffic problem is to build up a multiple-Internet-link or multiple-server distributed system to distribute the loads. Fig. 1 illustrates a multiple-Internet-link structure with three lines connecting outward to the Internet. Since a system with balanced loads among its multiple links or servers yields better system performance, this study focuses on finding a load balancing method to further improve its performance efficiency.

Quite a few load balancing techniques for the Internet have already been proposed [1], [2]. Some of them solved the problem by adding extra hardware to the system, such as Round-Robin DNS [2], others solved the problem by applying software to the controller. The former achieve approximate balancing but increases extra cost incurred through connecting additional systems, and are used in multiple web site servers. The latter are popularly applied in network devices using control algorithms directly and generally obtain fairly good results. Traditional control algorithms for load balancing include Random, Round-Robin (RR) [3], Weighted Round-Robin (WRR) [4], Least Load, Least Connections, and Fastest Response algorithms [5]. For the job of assigning loads, Random algorithm selects links or servers randomly, regardless of the load on each line. This can leave the system unsteady. The performance of Random algorithm for load balancing is worse than that of RR [6]. Round-Robin algorithm is the simplest one, in which links merely serve the system in turn. Logically, in a system with n links, if the current outgoing traffic goes through link_i, 1⩽i⩽n, then the next service line is $\text{link}{}_{\mathrm{<mtext>(i+1)</mtext><mspace\; sp="0.16"\; width="2px"\; linebreak="nobreak"\; is="true"></mspace><mtext>mod</mtext><mspace\; sp="0.16"\; width="2px"\; linebreak="nobreak"\; is="true"></mspace><mtext>n</mtext>}}$. RR was first used in network problems by Nagle [3] and compared with other algorithms in a lot of other research [6], [7], [8], [9], [10]. The benefit of RR lies in the omission of tracing the links, for the tracing work is known to add extra load to the system, therefore detracting from the performance. WRR has the same scheme as RR does, except that a fractional weight is given to each link according to the link's performance. Least Load algorithm detects the load of each server or link and allocates the new load to the least loaded one. Least Connections algorithm tracks the number of connected users on each server or link and assigns new traffic to the one with the fewest connections. Fast Response algorithm keeps track of the response time of each server and balances the loads by choosing the one with the fastest response one. Fastest Response algorithm can only be used on servers. Both Least Load and Least Connections algorithms continually detect, measure, and rank each link's utilization as the basis for line selection. For systems using these two algorithms, the load balancing devices are costly and can become a bottleneck for the whole system. It's suitable to apply RR to a system only when the servers have fixed performance levels, since they have no sense of line congestion.

There are variants of RR-related methods for flow controlling, but concentrate on fairness scheduling for lines: Deficit Round-Robin algorithm (DRR) [9] is proposed on the basis of RR. It uses a quantum concept to control the speed of packets traveling among flows. To do that, it continually traces the states of flows for measuring deficits in the lines. Bit-by-bit round-robin (BRR) [11] tries to perform Round-Robin algorithm bit by bit, but it's almost impossible to execute in a high speed network. Elastic Round-Robin (ERR) [12], [13] improves DRR using an un-fixed item, allowance, to avoid requiring information about packet length. Variably Weighted Round-Robin (VWRR) [14] observes the average packet length of each flow and calculates the weights for packets transferring within multiple flows. Still, both ERR and VWRR algorithms need a lot of computational and tracing work. These algorithms are good algorithms for controlling flow for fair scheduling among flows, but not for balancing loads as addressed in this paper. In fact, they are applied for job fairness on each outgoing link as illustrated in Fig. 1.

In other studies, Walraevens et al. [15], and Jiang et al. [16] tries to use priority scheduling discipline on buffers to reduce the delay-sensitive traffic and assess buffers performance at the ATM switch. Because an ATM system has fixed size of packets, cells, the priority scheduling method is not included in our study, as it is designed to handle different sized jobs. Furthermore, this research focuses on the load balancing problem, without concerning TCP methods, such as those in Sliding window [17] or packet error problems [18].

In this article, a new algorithm based on RR and named Dynamic Weighted Round-Robin (DWRR) for load balancing is developed, in which the need to measure, trace, rank, and calculate link loads is minimized. DWRR detects each link's load in the system at intervals and, following the detection of loads, a set of weights (the inverse ratio of link loads) is given to each link. The system allocates new loads to each link according to the set of weights.

The following sections will describe the system model to which DWRR is applied as in real network environments. In 2 The proposed method, 3 Determination of the detecting interval, we will introduce the details of the development of DWRR, including mathematical functions to determine the best detecting interval and other related coefficients. A simulation model for our study is given in Section 4. Next, in 5 Computational experiment, 6 Results, are simulation results and comparisons. Section 7 addresses the conclusions of the study.