Modeling two-windows TCP behavior in differentiated services networks

Abstract

Two important issues in assured services within differentiated services architecture are bandwidth guarantee and fair sharing of unsubscribed bandwidth among TCP flows with and without bandwidth reservations. Although the subscribed bandwidth can be guaranteed by increasing network capacity or deploying strict admission control mechanisms, the costs of such solutions are very high. The issue of fair sharing of excess bandwidth is also not well solved. To address those issues, a modified TCP, named two-windows TCP, has been proposed. The performance of the protocol is evaluated by simulations. But its effectiveness is not validated theoretically under general network conditions, which is important for understanding the benefits and costs of using the protocol. In this paper, an analytical model is developed for the purpose. The model characterizes throughput of individual two-windows TCP flow as a function of contract rate, round trip time, loss rates of In and Out packets. Extensive simulations validate the analytical model. It's shown two-windows TCP is effective not only on solving the issues of bandwidth guarantee and fair sharing of unsubscribed bandwidth, but also on increasing the utilization of bottleneck link bandwidth. Moreover, its performance is robust to network conditions, which is important for wide deployment over Internet.

Introduction

Differentiated services (DiffServ) architecture receives extensive research interests in recent years [1], [2], [3], [4]. It is regarded as one of the most promising solutions for the issues of quality of service (QoS) encountered in today's IP networks [5], [6]. It relies on packet tagging and lightweight router support to provide premium services (PS) and assured services (AS) that extend beyond best-effort services (BE) [7], [8]. In DiffServ architecture, a source specifies a service class (e.g. PS, AS, or BE) and a service profile, which indicates the amount of traffic that the sender negotiates with the service provider to send its packets in the specified class. In particular, the class of AS is designed to give the customers the assurance of a minimum throughput (or target rate) during congestion, allowing flows to consume the remaining bandwidth in a fair manner when the network load is low. Thus, two major concerns for AS are bandwidth guarantee and fair sharing of unsubscribed (or excess) bandwidth [3]. Bandwidth guarantee means that each flow should receive its subscribed bandwidth in average. Fair sharing of unsubscribed bandwidth means that unsubscribed network bandwidth should be evenly shared among AS and BE flows.

Simulation results reported in Refs. [8], [9], [10] reveal that the quality of AS is affected by many factors, such as network bandwidth, service subscription and so on. Several recent studies [10], [11] also show that TCP flows with relatively higher bandwidth reservations may achieve lower throughput than subscribed while TCP flows with smaller reservations may realize throughput higher than their target. The intrinsic reason is that TCP is originally designed for best-effort services. The achievable TCP throughput is closely related to packet loss rate in the networks. In the profile of AS, packets from TCP flows with different bandwidth reservations are classified into In (profile) and Out (profile), resulting similar average packets loss rates of In and Out packets for flows passing the same path. As the losses of Out packets will cause the congestion window decrease, the overall TCP throughput will be largely affected by the losses of Out packets in the case of high bandwidth reservation rate. Thus, TCP flows with higher bandwidth reservation is more difficult to be guaranteed, causing issues on both bandwidth guarantee and bandwidth sharing.

A general idea to solve the issue of AS on bandwidth guarantee would be increasing the network capacity. Due to stirring increases in network capacity with advanced optical communications techniques, bandwidth in core network is likely to be abundant. But the last mile issue is still existing and the solution is very expensive. Deploying strict and accurate admission control mechanisms is another approach for bandwidth guarantee, but it is difficult to implement due to inaccurate and unpredictable traffic characteristics [12], [13]. And another issue of fair sharing of excess bandwidth is not well solved. Park investigate an modified RIO algorithm to improve the fair sharing of excess bandwidth, but the issue on bandwidth guarantee is not solved [14]. One the other hand, a modified TCP protocol, named two-windows TCP, is proposed to address both of those issues of AS [15]. The motivation of the protocol is that while service quality is almost impossible or extremely expensive to be guaranteed by only considering network designs and managements, it may be significantly improved with combined efforts from both networks and end users. Simulation results show that bandwidth guarantee and sharing can be partly improved with the use of the protocol [15]. However, the performance of two-windows TCP is not analytically evaluated, which is important to evaluate the benefits and costs for practical deployment. It's the motivation of the paper to propose an analytical model for the protocol, and examine the robustness of its performance to network conditions.

Although there are already wide studies on steady-state analytical models for TCP protocols [16], [17], few works investigate TCP behavior in DiffServ networks except [18], [19]. In this paper, conventional analytical approaches are modified to evaluate the performance of the two-windows TCP protocol in general DiffServed networks. The contributions of the work focus on modeling the protocol, understanding the effectiveness and limitations of the protocol on improving the quality of AS on bandwidth guarantee and sharing. The rest of this paper is organized as follows. Section 2 surveys the AS architecture and RIO (RED with In/Out) mechanism [20]. Two-windows TCP protocol is also described. Section 3 presents an analytical model for the protocol. Section 4 presents simulation results and analysis. Section 5 concludes the paper.