Investigation of Multi-path Transmission Protocols for Congestion Control

Transcription

1 Investigation of Multi-path Transmission Protocols for Congestion Control Firat Tekiner & Santosh Kumar Battar Department of Computing, Engineering and Physical Sciences, University of Central Lancashire, PR1 2HE Abstract: In this paper, we compare multi-path transmission protocols TCP - Low Priority (LP), TCP Friendly rate control (TFRC), TCP Westwood (TCP-W) and Compound TCP (CTCP) with Traditional TCP for measurement of fairness. Presently internet is governed by TCP protocol stack which has been fundamentally broken causing extreme unfairness to other TCP flows where no user will be able to achieve his rightful share of bandwidth. Besides problem with transport layer routers use single path to route packets to destination which leads to underutilization of resources. Therefore so, this paper evaluates working of new transport protocols in conjunction with multipath routing for achievement of fairness while effectively utilizing the available resources. Here simulations are carried out under problematic conditions by taking into account the node and link failures. Although results show varying results and characteristics for different performance metrics TFRC is found to be working most effectively. I. Introduction In today s fast growing Internet, traffic conditions changes and failures occur at some parts of the network from time-totime, in unpredictable manner. The routing through single path to destination will not be able achieve the maximum throughput due to underutilization of resources. There is a possible existence of paths with higher bandwidth which are left unutilized and instead data is forced to take path with lower bandwidth. This single path to destination has more probability to create congestion on the internet and there are many techniques in existence that makes use of multipath [1]. The design and implementation part of the multipath TCP are of high importance because poor design and implementation can lead to inferior performance than traditional TCP i.e. unfairness to traditional TCP flows. Extreme unfairness of TCP towards other flows is the biggest problem being faced by the communication network where too much traffic is being placed on the network resulting in packet drops [2]. Internet is presently facing a serious congestion problem in internet causing excessive unfairness to other flows due to inefficiency of TCP congestion control algorithm. The congestion collapse is due to unresponsiveness to TCP flows by exploiting the fair share of bandwidth. The Van Jacobson client side stack patch is fundamentally broken and there is a need for development of new transport protocol [3]. The fairness among various TCP and UDP variants is studied under [2, 4, 5] which depicts the problem of extreme unfairness. The current routing protocols such as RIP, OSPF or IS-IS uses routing table to route the packets to its destination i.e. it holds a single path to destination. In reality there are multiple paths in existence and current routing mechanisms force the packet to follow a defined specific path to destination [6, 7]. This single path to destination has more probability to create congestion on the internet and there are many techniques in existence that makes use of multipath [13, 14]. The design and implementation part of the multipath TCP are of high importance because poor design and implementation can lead to inferior performance than traditional TCP i.e. unfairness to traditional TCP flows[7,8]. Extreme unfairness of TCP towards other flows leads to excessive placing of traffic on the network resulting in packet drops causing congestion collapse. In 1987 van Jacobson a contributor of TCP/IP stack created a client side patch that contains Additive increase multiplicative decrease (AIMD) congestion control algorithm that would reduce its congestion window on detection of packet loss in the network. This patch was able to address the problem of unfairness until 1999 before introduction of peer-to peer network were large number of parallel TCP streams would carry single application resulting in extreme unfairness to other flows since the more number of applications grown and number streams carrying through bottleneck also have grown which resulted towards the maintaining the bandwidth fairness [3]. [2] Depicts the problem of extreme unfairness caused by one TCP and three UDP and extends on looking into fairness among TCPs with different round trip times in packet switched gateway. In addition, [6] has clearly outlined advantages of using multiple routes to destination with experiments conducted in NS-2 [9]. [5, 10, 11, 12] outlines the scenarios of extreme unfairness in today s internet and proposes new congestion control algorithms with NS-2 based implementation to show fairness achievement. Multipath transfers offer several advantages to avail every opportunity good design and implementation of multipath TCP is a must. This paper uses the multipath routing based on distance vector routing algorithm which is already inbuilt in Network ISBN: PGNet

2 Simulator (NS2), by using multipath_1 routing function can be activated. II. Transport layer protocols A. TCP-LP (Low Priority) As it name suggests TCP-LP is a low priority algorithm that is designed to utilise unused network resources; the TCP-LP uses the knowledge about the available network bandwidth and competes fairly with the TCP for bandwidth, as it utilizes the unused resources. In order to provide lowpriority service TCP-LP must detect the congestion much earlier than TCP, in order to do this, TCP-LP employs packet delays as its congestion detection mechanism when compared to TCP packet loss. This packet delays are sensed earlier than packet loss, hence TCP-LP predicts congestions earlier than TCP even in scenarios of larger round trip times of TCP-LP [5]. B.TCP- TFRC (Friendly Rate Control Protocol) TCP-TFRC is a congestion control mechanism designed for operating in wide area communication networks such as internet. TFRC guarantees that other transport protocols achieve reasonable fairness in terms of bandwidth. TFRC is a very well suited protocol for applications like video conferencing and live streaming mediums due to the variation of time it employs. TCP-TFRC is designed to provide high reliability and performance during congestion in the network. TFRC makes sure that applications run smoothly even at the time of congestion maintaining the fair share with TCP [8, 15]. C.TCP Westwood TCP Westwood is a transport layer protocol with new congestion control algorithm which is implemented at sender side. Algorithm concentrates on modifying the congestion window with slow start settings based on traffic flows and reception of acknowledgements. The simple concept of TCP-W lies in computing available bandwidth at sender side by a minor modification on the TCP protocol stack. Bandwidth estimation factor identifies the amount of bandwidth used and alters the congestion window such that it does not exploit the Traditional TCP s share of bandwidth [10, 11]. D. Compound TCP(CTCP) Compound TCP is another congestion algorithm that employs both delay and loss as its congestion detection mechanism which is being modified at receiver side [12, 16]. Table 1 shoes the features of transport protocols in consideration in this paper. Table 1: Comparison of TCP Protocols CONGESTION DETECTION APPLYING PATCH MESSAGES PROTOCOLS MECHANISMS TCP-LP Delay Clientside ECN TCPWESTWOOD Ack. packets Serverside ELN COMPOUND TCP TFRC Loss and Delay Clientside ECN Loss event rate calculation Clientside Feedback packets III. Implementation Network Simulator (NS)-2 is a widely known simulation used in analysis of a number of network algorithms, protocols using different scenarios. NS-2 supports simulation of both wired and wireless networks. Hence it is used in this project to implement and evaluate four TCP multipath protocols. This paper evaluates the following transport layer protocols that use different congestion algorithms: 1. TCP-LP: uses delay as congestion indicator applied at client side 2. TFRC: uses loss event rate calculation as congestion indicator applied at client side 3. TCP-W: uses three duplicate acknowledgements as congestion indicator applied at sender side 4. CTCP : uses loss and delay as congestion indicator applied at clients side Multipath routing is used in conjunction with these new protocols for effectively utilizing the resources. The Network Scenario in Figure 1 is used to generate traffic from node 0 to node 11 via a number of intermediate nodes. Figure 1: Network Topology A study on TCP with respect to fairness shows that the congestion control algorithm employed does not focus on fairness aspect and a degree of unfairness on competing links is created. IV. Results A. TCP TFRC Comparison Figure 2 shows the throughput values were simulation is made to run for 100 seconds, the value of throughput is obtained at shared bottleneck (3-4) for two intermediate nodes and (10-11) for nine intermediate nodes. 10Mbps links are used between two nodes. Therefore, when two applications are competing for bandwidth each should share 5Mbps in average. Consider the case of two intermediate nodes were at node 0 the TFRC starts and sink node is at node 4. The TCP traffic is enabled at node 3 to node 4 and both applications started generating traffic at 0 and 5 seconds. From the figure we can see that TCP is trying to exploit the whole of the bandwidth for the initial 10seconds and uses 8.5Mbps of the available bandwidth. On the other hand

3 TFRC traffic can only receive 1.4 Mbps. However, it can also be seen from the figure that after the initial 40 seconds both applications share the bandwidth equally TFRC continuously measures the loss event rate i.e. receiver feeds information to sender and sender continuously increases its congestion window thus increasing its sending rate. experience losses. When we compare the delay value it can be seen that TFRC s average packet delay is 3 times higher than TCP. Table 2: TCP traffic data for TCP-TFRC comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost Dropped pkts Table 3: TFRC traffic data for TCP-TFRC comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost(%) Dropped pkts Figure 2: Throughput comparison for TFRC and TCP In addition, it can be observed that TFRC throughput starts to increase from 0 to 4.8Mbps i.e. it continuously increases its sending rate without exploiting share of other flows. It also shows that after 40 seconds TFRC is setting its congestion window such that it nearly reaches to 5Mbps. It is noticeable that some values are little bit higher than 5Mbps i.e. maximum value is 5.217Mbps which is not effective as bandwidth consumption is under control. It should also be noted that fairness is undisturbed even with increase in number of nodes. When nine intermediate nodes are used share of bandwidth is 4.942Mbps between two. Therefore aggregation of different TFRC protocols is able to show extremely good amount of fairness with TCP flows Even though fairness is being the main concern of investigation it is important to compare the following parameters: average throughput, average delay, total packets received, packets dropped and percentage of loss. This is presented for TCP and TFRC in tables 2 and 3 respectively. The average throughput values are ranging from 4.3Mbps to 5.3Mbps which shows that TCP traffic and TFRC is utilizing 90 percent of its available bandwidth without exploiting each others bandwidth. Table depicts the number of packets received and number of packets dropped, router uses drop tail queuing technique which drops packet in event of congestion and number of packets dropped in this scenario ranges from 178 to 191. This is just 0.4 percent for TCP traffic and up to 2percent for TFRC traffic which is regarded as negative feature. The number of packets sent by TCP and TFRC traffic is almost same but relatively large number of packets is being dropped by TFRC traffic. The reason for this typical behaviour might be due improper allocation of packets to its intermediate paths causing some of the paths to B. TCP TCP-LP Comparison From the Figure 3 it can be seen that TCP-LP is so kind to allocate the required bandwidth to TCP and shares the remaining bandwidth. From the graph we can see extreme unfairness created by TCP which exploit the right share of TCP-LP. TCP-LP s bandwidth share ranges from 2Mbps to 4Mbps with the increase in multiple paths. The TCP traffic is found to be more aggressive with TCP-LP flows allocating very mere bandwidth. Figure 3: Throughput comparison for TCP-LP and TCP Table 4 and 5 presents further analysis on comparison between TCP-LP and TCP. Here TCP-LP shows inferior behaviour in consumption of bandwidth were it average value is ranging from 2.2 Mbps to 2.6 Mbps (i.e. it is utilizing only 50 percent of bandwidth) whereas TCP bandwidth is ranging from 7.2 Mbps to 7.7 Mbps. Th On the other hand, average delay observed for TCP is in range of 44 milliseconds and TCP-LP is ms. This is

4 due to, TCP-LP passes through intermediate nodes and its sink node is far from it whereas TCP s sink node is immediate neighbour node. Because of this TCP-LP has large delay value than TCP. Besides inferior performance in throughput by TCP-LP, number of packets dropped is relatively higher in comparison with number packets received. Table 4: TCP traffic data for TCP-LP comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost Dropped pkts Table 5: TCP-LP traffic data for TCP-LP comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost(%) Dropped pkts C. TCP CTCP Comparison Figure 4 shows throughput comparison for TCP and CTCP over 100 seconds. From the graph it is very much clear that instant throughputs are approaching 5Mbps within 25seconds. TCP and CTCP traffic is starting at 0 seconds. At the start, TCP tries exploiting whole of the bandwidth (i.e. up to 9Mbps). However, as the simulation progresses (after 25 seconds) both traffic shares the bandwidth equally. This is because CTCP with continuous monitoring of bandwidth expands its congestion window until it reaches to its fair share of bandwidth. Then, it maintaines a constant window making sure that both protocols share bandwidth equally. Fairness is undisturbed even with increase in multiple paths. Average delay for CTCP is more than TCP, this is obvious because node 0 is located at far more distance where it has to pass through two hops in between leading to much more delay than TCP. In overall the fairness aspect and delay issues are satisfactory but large numbers of packets are being dropped. Table 6: TCP traffic data for CTCP comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost Dropped pkts Table 7: CTCP traffic data for CTCP comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost(%) Dropped pkts D. TCP TCP-W Comparison TCP-W s new congestion control code concentrates at modifying the congestion window and slow start settings based on traffic flows and reception of acknowledgements. The simple concept of TCP-W lies in computing available bandwidth at sender side by slight modification at TCP protocol stack. The modification used in TCP-W is using duplicate acknowledgement as a indication of congestion in the network to sender. TCP-W computes the congestion window rather than directly reducing window to half. This results in fair share of bandwidth i.e. efficiently using the bandwidth. Figure 5 shows that when using TCP-W, TCP is sharing very little bandwidth initially and TCP traffic seems to be very aggressive exploiting total bandwidth of about 9 Mbps. At this stage TCP-W continuously estimates the bandwidth to find out the congestion and graph shows that within 15 seconds of time TCP-W is able to gain some bandwidth. Figure 4: Throughput comparison for CTCP and TCP

5 existing table based routing protocols, congestion algorithms was not able to compete fairly with TCP for bandwidth for second network. TCP-LP and TCP-W failed in evaluation because of improper placement of packets while moving packets over multiple paths. Bandwidth and delay affected the performance of transport protocols particulary TCP-W. On the other hand, CTCP was unaffected with changes in bandwidth but affected with changes in delay. One of the improvements that could be implemented as the future work is to send congestion packets when network is heavily loaded without being any need for TCP congestion detection. Transport layer could also be designed to work as single congestion control over multiple paths instead of having congestion control enabled on individual paths. Figure 5: Throughput comparison for TCP-W and TCP As simulation is made to run further, graph shows that TCP- W is bit aggressive exploiting some of the fair share of TCP. However TCP-W is able to show considerable amount of fairness. TCP-W is designed to more robust where it detects congestion much earlier than TCP and reduces its congestion window more quickly compared to TCP making sure that both share bandwidth equally. Tables 8 and 9 shows that relatively more packets are received by TCP-W and as a consequence more packets are dropped. This is the common trend with all the ransport protocols examined here. As the number of paths increases more packets are dropped. This is due to improper distribution of packets across multiple paths which leads to more packets congested at certain nodes. Table 8: TCP traffic data for TCP-W comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost Dropped pkts Table 9: TCP-W traffic data for TCP-W comparison when multiple paths Recv pkts Avg throughput (kbps) Avg delay Pkts lost(%) Dropped pkts V. Conclusion An evaluation of a number of TCP protocols with respect to fairness is done using the network simulator NS 2 in conjunction with multipath routing for effectively utilizing resources. The studies indicate that TFRC is better protocol which employes undisturbed algorithm with changes in delay as well as bandwidth. However due to limitation of the VI. References 1: H Han, S Shakkottai, CV Hollot, R Srikant, D Towsley, "Multi-Path TCP: A Joint Congestion Control and Routing Scheme to Exploit Path Diversity in the Internet", Networking, IEEE/ACM Transactions on, Vol. 14, No. 6. (2006), pp : S. Floyd, K. Fall, "Promoting the use of end-to-end congestion control in the Internet", IEEE ACM Transactions on Networking, Vol. 7, No. 4. (1999), pp : George Ou Fixing the unfairness of TCP congestion control 24th march 2008, 4: D. Bansal, H. Balakrishnan, S. Floyd, S. Shenker, "Dynamic Behavior of Slowly-Responsive Congestion Control Algorithms", SIGCOMM : Aleksandar kuzmanovic and Edward W. Knightly TCP- LP: A Distributed Algorithm for low priority data transfer, Infocom 2003, pp : Thomas J. Hacker, Brian D. Noble and Brian D. Athey Improving Throughput and maintaining fairness using parallel TCP, Infocom 2004, pp : Y. Lee, I. Park, and Y. Choi, "Improving TCP performance in multipath packet forwarding networks," Journal of Communications and Networks, vol. 4, no. 2, pp , June : Handley, M Floyd, S Pahdye,. J & Widmer J TCP Friendly Rate Control (TFRC): Protocol Specification Jan 2003, 06.pdf 9: Network simulator NS2, 10: Ren Wang; Yamada, K.; Sanadidi, M.Y.; Gerla, M., "TCP with sender-side intelligence to handle dynamic, large, leaky pipes", IEEE Journal on Communications, Volume 23, Issue 2, Feb Page(s): : A. Zanella, G. Procissi, M. Gerla, and M. Y. Sanadidi. TCP Westwood: Analytic Model and Performance Evaluation. In Proceedings of IEEE Globecom, pages , : Kun Tan Qain Zhang murari sridharan A Compound TCP Approach for High-speed and Long Distance Networks, Infocom 2006, pp

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