TCP Simulation Evaluations over Ad Hoc Wireless Networks

University of Bern & NCCR MICS Summer Internship 24 TCP Simulation Evaluations over Ad Hoc Wireless Networks Student: Derman Akcelik Supervisor: Ruy de Oliveira Professor: Torsten Braun dakcelik@eivd.ch

TCP Simulation Evaluations over Ad Hoc Wireless Networks Abstract The Transmission Control Protocol (TCP) represents the most deployed transport protocol used in the Internet so far. This popularity motivates its presence in wireless networks, including ad hoc networks. These networks are very promising since they make it possible communication among end systems directly without any infrastructure. However, TCP faces various problems when required to work in such environments. Because of that, it is very important to understand the behaviour of this protocol in wireless networks. This project evaluates TCP performance in ad hoc networks by means of simulation. We compare two TCP flavours, namely Newreno and SACK, under a variety of conditions. Specifically, we assessed throughput, retransmissions, and energy consumption features of TCP. We also evaluated the delayed acknowledgment option for TCP. The results provided some insight into this quite wide subject.

4 I. INTRODUCTION In today s world, wireless devices became an evident part of our life. These devices may provide seamless connectivity in environments where wired networks are not possible or desirable. One of the emerging wireless technologies is ad hoc networks. These networks are attractive for not depending on any fixed infrastructure to communicate. As a result, researches on ad hoc wireless networks have gained non-negligible popularity in recent years. Ad hoc networks consist of mobile hosts that communicate with each other over a wireless medium without central control. The utilisation of this medium develops the hidden-node problem and exposed node problems that are investigated in more detail in [1]. In an ad hoc network, the topology may change due to various medium constraints, forcing self-reconstruction adaptness. Because of the distance limit, multihop routing is used where the packets are forwarded towards the next node along a route to the receiver. Energy consumption is also a concern in these networks because the nodes are supposed to be battery powered. The IEEE proposed IEEE 82.11 as a standard MAC protocol for wireless multihop networks [2]. It defines the link and physical layer specifications. It minimizes the known hidden node problem using a local retransmission strategy with RTS/CTS (request-tosend/clear to send) control frames. It works efficiently for scenarios having at most 3 hops. Therefore, end-to-end communication protocols have to deal with the problem for the best use of network bandwidth. The popularity of the TCP in the Internet motivates its presence on the wireless networks. Since TCP was developed for wired environments, it has distinct behavior in ad hoc networks. In these networks, TCP will see every dropped packet as a congestion sign. It will reduce its sending rate, which means less throughput, more retransmissions and more spent energy. Packet error rate of the wireless channels and link interruptions are other reasons for packet losses. The interaction between TCP and the IEEE 82.11 MAC protocol is one of the most challenging topics in today s wireless communication systems. TCP Newreno and TCP SACK are two important flavors of TCP protocol. Their features are quite distinct though. TCP Newreno uses a new fast recovery mechanism for speeding up the recovery from losses. TCP SACK has a receiver that gives more information about the lost packets to the sender. A sender of TCP SACK uses this feature for recovering quickly in situation in which multiple packet losses within a single window occur. TCP SACK claim to be very good to deal with a packet loss because of its selective acknowledgement strategy. Nevertheless, adding extra information to be sent imply more spent energy. Both TCP flavors may work with the delayed acknowledgement (DA) option, whichmeans that the receiver acknowledges only one packet out of two two received. This is expected to reduce energy consumption. The aim of this work is to characterize, energy consumption, number of retransmissions and throughput for TCP Newreno and TCP SACK flavors, with and without Delayed Acknowledgement (DA). Because of the time limit, we focused on these two topical flavors. Additionally, we do not address mobility related issues, since the goal here is to evaluate the behavior of TCP over the IEEE 82.11 MAC protocol.

5 The remainder of this report is organized as follows. Section II gives general information about ad hoc networks. Section III describes the concepts that learnt by the student during this study. Section IV presents the simulation environments and the simulated scenarios. Section V contains the simulation results and their discussions. Finally, section VI summarizes our work. II. AD HOC An ad-hoc network is a local area network or other small network that is not dependent on any access point or any server. Fig. 1 shows a typical ad hoc network that communicates over a wireless medium. As the nodes are mobile its topology may change continuously. The devices can join and leave networks readily. Each device is immediately recognized as part of the network when is close enough to one of the network components. Fig. 1. Ad hoc networks In recent years, the number of commercial applications with ad hoc networks raised with the rapid spread of mobile telephone usage, personal digital assistants and the enlargement of inexpensive wireless solutions for monitoring tools. Accordingly, there are many ongoing distributed systems researches. Examples for the ad hoc applications are disaster relief, conferencing, home networking, habitat monitoring, warehouse inventory monitoring, and personal area networks. An ad-hoc network utilises multihop scenarios because of the limit of transmission range (see [1] for more details). The nodes works as a router and forward the packets to the other nodes. By virtue of the centralized entities absence, the commutation methods and routing algorithms are decentralized. They have to deal with the mobility of the participants and the variation in the connectivity. Thus, different routing protocols have been developed. AODV (Ad hoc On Demand Distance Vector routing protocol), DSR (Dynamic Source Routing protocol), DSDV (Highly Dynamic Destination-Sequenced Distance Vector routing protocol), and TORA (Temporally-Ordered Routing Algorithm routing protocol) are the examples of the routing protocols. AODV [3] and DSR [4] routing protocols are cited as the most prominent frameworks to be standardized in the future. The ad hoc networks devices are battery powered. Therefore, the amount of spent energy by the routing protocol affects the bandwidth utilisation. If a node,that is in the shortest

6 path, is not active because of the lack of the energy, a longer path will serve to the packet. The longer the transport time, the less the throughput and the more the energy consumption. The energy is one of the metric investigated in [5]. The energy consumption is induced by the well-known hidden node problem and its solution mechanisms. They are presented in [1]. Accordingly, many studies have been dedicated to analyze its characteristics and/or propose new routing methods (see, e.g., [5]). As mentioned in [6], due to the spatial reuse property of 82.11 [2], the nodes not only communicate but also interfere with other nodes. III. LESSONS LEARNT During the study, we worked with different utilities that I did not have the change to work with before. The LINUX as OS, Perl as text interpreter, ns2 as the simulator program and the Gnuplot as graph designer. I also tried to write a project report in English, which is not obvious at all. Working with LINUX was not only interesting but also hard. We worked with varius shell commands. In order to automate the tasks, we wrote several scripts. Their developments are hard but then changing the parameters for different executions becomes easier. Ns2 [7] is a well-known and developed simulation program. It was used to simulate the different scenarios. Is requires not only the ns2 commands knowledge but also networking concepts knowledge. For example, because of the headers that are added at every level, the physical layer data throughput value has to be bigger then the MAC layer throughput. When a file transfer is finished, we have to close the output files after enough time. This time is necessary to save eveything on the files. Happily we did not have installation problems. The only problem was the size of the traces files. They might be big enough to make impossible to work with a single PC. After certain numbers of trace file, the user might easily reach its hard disk limit. That is why we erased the trace files between each execution after getting the necessary information. Perl was used to obtain the desirable data from the trace files. There are different possible ways to get the same information. Developing the efficient one takes time because we have to have enough experience on it. Because of the lack of the time, and having lots to do, only a little introduction step on it was done. In our case, it was used to get the energy left on the sender node from the trace file. Besides the use of different tools, we studied the main concepts behind ad hoc networks, as explained in detail on section II, and the performance analysis of TCP Newreno and TCP SACK protocols with or without DA. The experiment is realized via three metrics. Those are energy consumption, number of the retransmission and throughput. IV. SIMULATION SETUP We used the ns2 [7] simulator to perform our evaluation. Fig. 2 shows the scenario used in our simulations. Each node is 2 meters apart from its closest neighbors and the wireless data rate is 2 Mbps. In the simulator, the effective transmission range is 25 meters while the interference range is 55 meters as recommended by the IEEE 82.11 standard [2]. In all evaluations, the throughput bw is computed as bw = seq 8 stime, where seq

7 is the maximum sequence number (in bytes) transmitted and acknowledged and stime is the simulated time. We focus our discussions on short chain of nodes containing at most 7 hops, because this is a reasonable limit for today s networks. The first node is the sender and the last node is the receiver of the packets. The sender transfers continuously a file (infinite ftp) to the receiver. For generating ten flows, we used ten ftp agents at the sender. At the receiver, depending on the flow number, we placed one or ten sink receiver agent. During the enery consumption analyses, we used an energy model that computes energy expenditure to transmitted and received packets only. Energy spent in idle state is computed. In this way, the energy related to only TCP operations are better evaluated. The measured energy is the one consumed by the sender. 1 2 3 4 n sender node receiver node Fig. 2. Chain topology The parameters settings are as follows. IEEE 82.11 is the MAC layer protocol. AODV is the routing protocol. The initial energy of nodes is 1 joules, a dynamic flat area computed as (number of hops * 2m + 1m) x (3m). The window limit (WL) for the TCP flavours is 3 packets. To take into account the hidden node problem, we simulated 5 and 6 hops for the measurements of the energy consumption and the throughput. Unless otherwise mentioned, the other parameters were kept as the default of the simulator and all simulation runs lasted 3 seconds. For the simulations with losses, a uniform distribution function is used as error pattern, and 5% of Packet Error Rate (PER) is simulated. A. Energy Consumption V. SIMULATION RESULTS In this section we evaluate the energy consumption pattern for TCP with varying packet sizes and 5 hops in Fig. IV. Energy consumption is a very important aspect for ad hoc networks as the nodes in place are supposed to be battery powered. Fig. 3(a) and Fig. 3(b) depict the results for packet sizes of 25, 5, 1, and 146 bytes under 1 and 1 flows, respectively. A general conclusion from these results, is that the larger the packet the less energy spent, as shown in both Figs. This is intuitive because with short packet size TCP sender has to perform more operations to send the whole data. Both Figs also show that by using the Delayed Acknowledgment (DA) option the two TCP flavours saved some energy. For the one flow evaluation, TCP SACK without DA spent up to 16% more energy than when using DA. Comparing TCP SACK with TCP Newreno, we did not get clear distinction between both performances. In our experiments with DA, TCP Newreno performed slightly better than the TCP SACK for short packet sizes.

8 Energy Spent (joule *e-7/bits) 2 16 12 Energy spent vs. Paket Size (1 flows) DA DA 8 (a) 1 flow Energy Spent (joule *e-7/bits) 2 16 12 Energy spent vs. Paket Size (1 flows) DA DA 8 (b) 1 flows Fig. 3. Energy Consumption B. Energy Consumption Including Packet Error Rate of 5% Owing to observe the energy efficiency of TCP Newreno and TCP SACK over an ad hoc network including losses, we simulated the same network that is used in the previous section. There is no congestion but only a packet error rate of 5%. Figs. 4(a) and 4(b) illustrate the results for packet sizes of 25, 5, 1, and 146 bytes under 1 and 1 flows, respectively. We obtained the same general conclusions that we had for the case without error. Yet the energy consumption increased with the packet error constraint. DA option reduced the spent energy under losses as well. Fig. 4(b) shows that TCP SACK with DA performed

9 Energy Spent (joule *e-7/bits) Energy spent vs. Paket Size (1 flows / 5% error) 32 28 24 2 16 12 DA DA (a) 1 flow Energy Spent (joule *e-7/bits) Energy spent vs. Paket Size (1 flows/ 5% error) 32 28 24 2 16 12 DA DA (b) 1 flows Fig. 4. Energy Consumption with Packet Error Rate of 5% the best in terms of energy consumption in most cases. Comparing Figs. 4(a) and 4(b) one can say that for small packet sizes TCP Newreno outperformed TCP SACK for the scenario with one flow. The opposite happened for the ten flows case. For large packet sizes we did not notice any difference between the two flavours. In short, the only effective conclusion from these results is that DA is helpfull for energy consumption benefits, even for noisy environments. C. Retransmissions The idea here is to observe the typical behavior of the retransmissions in the ad hoc networks. For that, we simulated flows with packet size of 25, 5, 1, and 146 bytes.

1 As in the previous simulations, we considered the DA option a well. The scenario is also the chain topology of Fig. 2 with 5 and 6 hops. The results are shown in Figs. 5 and 6, which include evaluations for 1 or 1 flows, respectively. Retransmissions vs. Paket Size (1 flow) Number of Retransmissions 8 7 6 5 4 3 2 1 DA DA (a) 5-hop Number of Retransmissions 8 7 6 5 4 3 2 1 Retransmissions vs. Paket Size (1 flow) DA DA (b) 6-hop Fig. 5. Number of retransmissions for 1 flow For the scenario facing one single flow, we can see in Fig. 5(a) and Fig. 5(b) that the number of retransmissions rise with the increase of the packet size. Furthermore, we obtained the best performances for both flavours with DA option enabled. On the other side, the results for the evaluations with 1 flows exhibited the opposite as illustrated in Figures 6(a) and 6(b). That is, the larger packet size the less retransmissions.

11 Number of Retransmissions 1 9 8 7 Retransmissions vs. Paket Size (1 flows) DA DA 6 (a) 5-hop Number of Retransmissions 1 9 8 7 Retransmissions vs. Paket Size (1 flows) DA DA 6 (b) 6-hop Fig. 6. Aggregate number of retransmissions for 1 flows Presumably, this is a result of the high congestion caused by the competing flows, which becomes worse as the packet size increases. In terms of comparison between Newreno and SACK, the results confirm again that their performance are variable in most cases. There are cases in which the former outperforms the latter and vice versa, but no one is definitely better the other.

12 D. Retransmissions Including Packet Error Rate of 5% In this section, we investigate the behaviour of the retransmission in ad hoc networks with losses. We used the same topology evaluated in section V-D containing packet error rate of 5%. The results for 2 and 5 hops are shown in Figs. 7 and 8, which includes evaluations for 1 or 1 flows, respectively. Number of Retransmissions Retransmissions vs. Paket Size (1 flows / 5%) 12 8 4 DA DA (a) 1 flow 2 hops Number of Retransmission Retransmissions vs. Paket Size (1 flows / 5%) 12 8 4 DA DA (b) 1 flows 5 hops Fig. 7. Number of retransmissions for 1 flow with packet error rate of 5% Figs. 7 and 8 show that it is not always possible to get the best results with DA option. Once again the results do not allow us to make clear distinction between TCP Newreno and TCP SACK flavors. It was expected that TCP SACK would perform better under losses,

13 mainly for large packet sizes, since its receiver provides better information to the sender about losses. That was observed for the largest packet size in Figs. 7, 8, 8(a) but not in Fig. 8(b). However, this tendency was not observed for the smaller packet sizes. Number of Retransmissions Retransmissions vs. Paket Size (1 flows / 5% ) 9 8 7 6 5 4 3 2 1 DA DA (a) 1 flows 2 hops Number of Retransmission Retransmissions vs. Paket Size (1 flows / 5% ) 8 7 6 5 4 3 2 1 DA DA (b) 1 flows 5 hops Fig. 8. Aggregate number of retransmissions for 1 flows with packet error rate 5% E. Throughput In this section, we investigate the bandwidth utilisation not only with different number of hops but also different number of concurrent flows in the chain topology of Fig. 2. The

14 results are shown in Fig. 9, which includes the results for 1, 1 flows and using or not the DA option. It is well known that the throughput decreases as the number of hops go high. The primary reason is that the longer the path the higher delay for the packets to arrive at the destination. Additionally, there is the hidden node problem that becomes worse for larger number of hops. So, the probability of packet loss is significantly higher for large number of hopes. More flows, on the other side, means more congestion which will decrease the throughput as well. Throughput (kbits/s) 16 14 12 1 8 6 4 2 Throughput vs. Number of hops (1 flow) DA DA 1 2 3 4 5 6 7 8 Number of hops (a) 1 flow Throughput (kbits/s) 16 14 12 1 8 6 4 2 Throughput vs. Number of hops (1 flows) DA DA 1 2 3 4 5 6 7 8 Number of hops (b) 1 flows Fig. 9. End-to-end throughput

15 Fig. 9(b) shows that the aggregate throughput, which is the capacity of the network, is nearly the same of that obtained in Fig. 9(a) in which only one flow is crossing the network. The difference between the two flavors, using or not DA, is too small to see from the figures. Once again, the DA option provided better result for both flavors. Fig. 9 shows improvement of up to 7%, which is not very significant. Among the TCP flavors the difference is completely negligible. VI. CONCLUSIONS In this work, we have investigated the performance of TCP Newreno and TCP SACK over ad hoc networks. ns2 was used for simulating the different scenarios. We used three metrics: energy consumption, throughput and retransmissions with or without delayed acknowledgment option DA. We observed the following: The energy consumed at the sender was inverseley proportional to the packet size regardless of the flavor. The energy consumption increased with losses for both flavors as well. The losses increased the retransmissions number in all cases. The two TCP flavors with DA option spent in general less energy and achieved better throughput in many cases. However, the DA option did not guarantee the best result in all circumstances. The difference between the TCP flavors performances were more perceptive in the simulations with losses. TCP SACK performed slightly better than TCP Newreno for large packet sizes. Because of the lack of time, we evaluated only TCP Newreno and TCP SACK protocols with or without DA option. In general the results did not show us meaningful differences between the two TCP flavors. Although TCP SACK uses the selective acknowledgment strategy, it did not outperformed TCP Newreno in most cases as expected. Therefore, the general outcome is that the two flavors work quite the same in the evaluated scenarios, and delayed acknowledgment is indeed helpful in ad hoc networks without high level of bit errors. For future work, more simulation runs and different scenarios with other metrics could be considered. REFERENCES [1] R. Oliveira and T. Braun. Tcp in wireless mobile ad hoc networks. University of Bern, Technical Report IAM-2-3, July 22. [2] IEEE. Wireless lan medium access control (mac) and physical layer (phy) specifications - std 82.11. The Institute of Electrical and Electronics Engineers, 1999. [3] IETF. Rfc 3561 - ad hoc on-demand distance vector (aodv) routing. The Internet Engineering Task Force, July 23. [4] Maltz Johnson. Internet draft - the dynamic source routing protocol for mobile ad hoc networks. The Internet Engineering Task Force, Apr 23. [5] H.Singh and S.Singh. Energy consumption of tcp reno, newreno and sack in multi-hop wireless networks. California, USA, July 22. ACM SIGMETRICS. [6] R. Oliveira and T. Braun. A dynamic adaptive acknowledgement strategy for tcp over multihop wireless networks. July 24. [7] Kevin Fall and Kannan Varadhan. editors. ns notes and documentation. San Francisco, USA, November 1997. The VINT Project, UC Berkeley, LBL, USC/ISI, and Xerox PARC.