Cross-layer Flow Control to Improve Bandwidth Utilization and Fairness for Short Burst Flows

Transcription

1 Cross-layer Flow Control to Improve Bandwidth Utilization and Fairness for Short Burst Flows Tomoko Kudo, Toshihiro Taketa, Yukio Hiranaka Graduate School of Science and Engineering Yamagata University Yonezawa, Japan Abstract Optical long distance and high speed networks require an effective flow control scheme better than the ordinary TCP. In this paper, we examine two cross-layer flow control schemes, XCP and XLBA especially for short burst flows. Cross-layer flow controls are essential means to optimize bandwidth utilization and fairness without severe control instability. We are proposing XLBA which eliminates feedback loops from the prospected XCP scheme. Our simulation results show that XLBA is better than XCP in comparing average throughput and fairness for most of the scenarios. Keywords-component; TCP, XCP, Cross-layer flow control, Bandwidth assignment, XLBA; I. INTRODUCTION In the cases of long-distance and high speed networks, TCP flow control fails to utilize the full bandwidth provided by networks such as the optical broadband networks because of the problems of control instability caused by the long control loops and convergence time relatively long compared to the flow s burst time. And also, it is impossible to secure fairness of bandwidth utilization between TCP flows of different round trip delays corresponding to the difference of the communication distance. Burst traffic is a natural characteristic of the Internet and is prevailing by expanding usage of video streams. Such burst flows cause buffer overflows and packet losses in network switches. In a general TCP flow control, packet losses greatly decrease TCP window size, and deteriorate transmission data rate[3]. New flow control techniques are proposed to improve bandwidth utilization and secure fairness between conflicting flows. FAST TCP controls window size by presuming the queue length of the bottleneck switch by measuring packets round trip time (RTT)[1]. However, FAST TCP may fail to presume the queue size on the bottleneck link or to set acceptable target queue size, the bandwidth utilization efficiency or the fairness between flows would not be attained. A cross-layer flow control scheme XCP [2] is proposed for overcoming the difficulties in presuming and setting by direct communication between flow controllers and the bottleneck switches. The bottleneck switches insert a feedback value in the XCP header, and the sources flow controller recalculate its window size using the feedback value. Unfortunately, XCP is a scheme to use the round trip path as a feedback control loop. Then, its dynamic performance is greatly influenced by the loop s delay, especially for short burst flows, which demands fast convergence to the optimum window size. We are proposing XLBA(Cross Layer Bandwidth Assignment)[3] to overcome the XCP s weakness. It is a technique for improving efficiency and fairness of bandwidth utilization by directly assigning the bandwidth to be used by the flow sources, when congestion occurs. In this paper, we examine the two flow control schemes, XCP and XLBA especially for short burst flows. Through simulation, we will evaluate their efficiency and fairness of the bandwidth utilization. II. CROSS-LAYER FLOW CONTROLS A. XCP(eXplicit Control Protcol) XCP [2,4,7-10] proposes a window based flow control which uses a cross-layer technique generalizing ECN (Explicit Congestion Notification). A bottle neck XCP router calculates and inserts a feedback value into the XCP packet header depending on the sign of the queue evaluation function, φ = ad( C T ) bq, (1) where a and b are constant control parameters (0.4, 0.226), C is the link capacity, T is the input traffic, Q is the queue size. Normally, a=0.4, b=0.226 are used [2], which are determined to stabilize the feedback loop. XCP flow source receives the XCP headers, and decreases or increases its window size by adding the feedback value. As XCP uses round trip control loop, its dynamic behavior depends on the loop delay and needs settling time to converge. B. XLBA(Cross-layer Bandwidth assignment) We are proposing another window based flow control scheme (XLBA) using cross-layer control technique[3], which secures efficiency and fairness of bandwidth utilization. XLBA controls window size of existing TCP algorithm (e.g. TCP New Reno) by notifying the flow source with the assigned bandwidth, which is calculated by dividing the bottleneck bandwidth with the number of flows passing through the bottleneck. A schematic diagram of agents is shown in Fig.1. A Watch Agent is attached to each link queue and a Flow Control Agent is attached to each source of TCP flow. The Watch /09/$ IEEE 651 ICSPCC 2011

2 Agent monitors the queue size, and it considers that congestion occurred when the queue size exceeds the notification threshold (x% of the buffer size). Then it notifies the Flow Control Agents of traffic sources with the assigned bandwidth utilization for each TCP flow BW/n, where n is the number of TCP flows and BW is the bandwidth of the bottleneck link. Such notification messages are delivered through the same paths as for the data flows. Therefore, the dynamic characteristics of the cross-layer control will be influenced by the corresponding propagation delay. Flow Control Agent Set up window size TCP0 TCPn n0 n1 n2 notification Watch Agent Figure 1. Structure of Agents. notification threshold x% SINKn Maximum window size maxw is calculated from the expression (2) by using notified BW /n. The transmission of the TCP flow is governed by its congestion window size limited by the maxw. basertt * BW max W = + MTU, (2) n where n is the number of TCP flows, BW is the bandwidth of the bottleneck link in bps, basertt is the minimum RTT in sec, and MTU is the Maximum Transmission Unit in bits. III. SIMULATION We compared the performance of XCP and XLBA by using the packet-level simulator ns-2(version2.34)[11]. We also examined the relation between the performance and the threshold, by which XLBA determines whether congestion occurred or not. A. Network Topology Figure 2 shows the network topology used in the simulation. It is a typical dumbbell topology with only three ftp flow sources as the basic performance testing. The TCP Flows are FTP0 to SINK0 through TCP0, FTP1 to SINK1 through TCP1, and FTP2 to SINK2 through TCP2. SINK0 FTP0 L1[ms] 2[ms] L1[ms] TCP L2[ms] L2[ms] 5 TCP1 TCP2 FTP1 FTP2 n3 n4 n5 SINK0 SINK1 SINK2 BufferSize :100[segments] Figure 2. Simulated topology. The bandwidth of all links was 100Mbps. The delays for the links were set as in TABLE I, while the delay of the bottleneck link was set to be 2ms. TABLE I. DELAY OF LINKS L1[ms] L2[ms] (a) TCP0,1,2 basertt 12ms 2 2 (b) TCP0,1,2 basertt 44ms (c) TCP0 basertt 12ms 2 10 TCP1,2 basertt 44ms (d) TCP0 basertt 44ms TCP1,2 basertt 12ms 10 2 B. Simulation Scenario In our simulation scenario, TCP flows were started and stopped as in Fig.3. A TCP0 flow (from node 0 to node 1) occurred from the beginning to the end of the simulation (duration is t_all). On the way, flows of TCP1 and 2 (from node 1 to node 5) occurred at the time specified in Fig.3 with a constant duration (t_int) as the period for single flow and also the period for overlayed flows. The constant t_int was set as 5, 2 and 1 sec for testing the three cases of short burst flows(table II). t_int TABLE II. Scenario TCP1 TCP0 t_all Figure 3. Simulation time chart. SIMULATION SCENARIOS. t_all[sec] (simulation time) TCP2 t_int[sec] (interval) Scenario(25sec) 25 5 Scenario(10sec) 10 2 Scenario(5sec) 5 1 C. Evaluation Indexes The results of the control schemes, XCP and XLBA, were evaluated for the efficiency and stability of throughput, and also, for the fairness of bandwidth utilization. The average throughput of the bottleneck link was calculated over the simulation duration (t_all). The stability of throughput was mainly evaluated by the convergence time after the number of the flows had changed. The fairness F was calculated by the following definition [12], t 652

3 F n 2 ( xi ) i= 1 : =, n 2 ( n x ) i= 1 i where x is the average throughput for each flow i in the i duration(t_int) of overlapped flow period, and n is the number of flows. It can be said that the throughput of each TCP flows are fair when F is one. (3) IV. RESULT The simulation results are shown for dynamic characteristics of stability and convergence time, and then, efficiency and fairness of the bandwidth usage of the bottleneck. A. Dynamic characteristics To examine dynamic characteristics of XCP and XLBA, their throughput changes are shown in Fig. 4 and 5. The both figures are the cases of Scenario(10sec). In Fig. 5 the throughput of XLBA are shown for notification threshold of 50% as the safe intermediate value. In these graphs, thick lines (Node2-3 Throughput) indicate the throughput of bottleneck link and others show the throughput of individual TCP flows. Table III shows the convergence time in which two flow's throughput entered within 10% from the fair usage of 50 Mbps. (a) TCP0,1,2 basertt 12ms (b)tcp0,1,2 basertt 44ms TABLE III. CONVERGENCE TIME. Convergence (a) (b) (c) (d) Time[sec] XCP XLBA(x=50%) The both of XCP and XLBA need acquisition time of RTT before the calculation of the window size and the notification delay from the bottleneck monitor. Furthermore, XCP needs a settling time of the feedback control before to reach the fair throughput. When the number of flow increased to two, the vibration of throughput is seen in Fig.4(b) and Fig.5(b), when the RTT of the both TCP flows are large. However, XLBA s throughput vibration is around 8Mbps and smaller than XCP s throughput vibration around 11Mbps. In Fig. 5(c) and 5(d), packet losses are observed. Those are the cases of mixed traffic of large RTT and small RTT TCP flows. In the TCP s slow start phase, window size increases exponentially and cause packet losses if the window size control is not done in time. Generally, dynamic behavior of XLBA was better than that of XCP for short burst flows, because XLBA does not use feedback settling to the target throughput. B. Average Throughput and Fairness Figure 6 shows average throughput of XCP and XLBA (notification threshold x=10-70%) in all scenarios. Figure 7 shows the fairness results. (c)tcp0 basertt 12ms TCP1,2 44ms (d)tcp0 basertt 44ms TCP1,2 12ms Figure 4. XCP Throughput in Scenario(10sec) 653

4 (a) TCP0,1,2 basertt 12ms In most of the cases, the performance of XLBA is better than that of XCP. However, the both performance deteriorated with the decrease of burst time of the overlapping flow. In the small delay cases of Fig.6(a) and Fig.7(a), average throughput and fairness of XCP deteriorated more than that of XLBA as the overlapping duration (t_int) of the TCP flows decreased. The average throughput and fairness deteriorated for the cases of notification threshold 10% and 70% in XLBA in Fig.6(b) and Fig.7(b) when flow s RTT are large. The reason for 10% case deterioration may be too-early detection of congestion. In TCP s slow start phase, a large traffic will be generated for long RTT network, and it may cross the threshold momentarily. This indicates that we should avoid small values for the congestion detection threshold. The reason for the 70% case deterioration is naturally understood that the queue size was growing near to the buffer limit and the TCP s flow control was too late. In Fig.6(c) and Fig.7(c) where the later joining TCP flow (TCP1,2) had large RTT, XLBA also caused packet losses. The influence of packet losses grows by decreasing the overlapping duration of flows. (b)tcp0,1,2 basertt 44ms (c)tcp0 basertt 12ms TCP1,2 44ms (d)tcp0 basertt 44ms TCP1,2 12ms Figure 5. XLBA Throughput ( x=50%) in Scenario(10sec) V. CONCLUSION In this paper, XCP and XLBA are compared for stability, convergence time, efficiency and fairness of bandwidth utilization for the cases of short burst flows. Proposed XLBA has a straight forward approach and showed a good performance in dynamic characteristics. It can be said that the control technique of XLBA is more effective than XCP. XLBA regulates directly the flow sources, while XCP takes time to settle to the fair throughput owing to the feedback control time. When the RTT is large, the vibration of throughput of XCP is larger than XLBA. However, packet losses may be generated in the cases of XLBA by the characteristics of the used conventional TCP flow controller. It may be dissolved by modifying the Flow Control Agent to do a rate based flow control in place of the described window based flow control. XLBA limits bandwidth utilization by setting the maximum window size to suppress the congestion when congestion is detected. However, there is no explicit control mechanism in XLBA to raise the window when bandwidth surplus exists. TCP s flow control is sufficient for raising the window when RTT is small. It is too weak to raise the window size in a short time needed for short burst flows. It will be necessary to device the mechanism for raising the window size in the cases of large RTT flows. In this paper, we assumed that the flow sources are eager to use the full bandwidth. Practically, flow sources would be influenced by other resources such as CPU, memory and devices, and may not fully use the assigned bandwidth. In such situation, we may need a discounted estimation of the number of flows for the utilization efficiency of the bottleneck bandwidth. As for the practicality, XCP is not thought to be easy for deployment because it needs XCP applications. XLBA s Flow Control Agent is designed to use ordinary window based TCPs and to modify the maximum window size parameter which ordinary TCPs have in their internals. Such software can be 654

5 implemented as an add-in software. Therefore, XLBA scheme is easier for deployment than XCP. (a) TCP0,1,2 basertt 12ms (a) TCP0,1,2 basertt 12ms (b) TCP0,1,2 basertt 44ms (b) TCP0,1,2 basertt 44ms (c) TCP0 basertt 12ms TCP1,2 44ms (c) TCP0 basertt 12ms TCP1,2 44ms (d) TCP0 basertt 44ms TCP1,2 12ms Figure 6. Average throughput. (d) TCP0 basertt 44ms TCP1,2 12ms Figure 7. Fairness. 655

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