Chapter 5. Simulation Results
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- Tabitha Jacobs
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1 Chapter 5 Simulation Results As TCP constitutes majority of the Internet traffic, an in-depth analysis of both loss-based and delay-based variants of TCP have been made in the earlier two chapters. Based on the analysis presented in those chapters, using NS-2 simulator [76], experimental study was conducted. Ensuing results obtained were presented in this chapter for further authenticating the pertinent variant of TCP over OBS networks. 5.1 Introduction Optical-bursts are generated at the ingress node, which acts as a router for all the IP packets that are assembled into a single burst. All the IP packets that acquire similar service are assembled into a single optical-burst. This process is called burstification of IP packets into optical-bursts. These optical-bursts traverse through the core network to reach the egress node. Here based on the destination address, optical-bursts are broken down into IP packets, which are sent into the access network for further transmission. Ingress-node and egress-node are together termed as edge nodes in an OBS network. Optical-burst at the ingress nodes is formed based on the burst assembly mechanism like quantity-threshold, time-threshold, and hybrid mechanism. Apart from burst assembly mechanism, ingress node also performs scheduling, routing, and control-packet generation. Every optical-burst formed at the ingress nodes is
2 preceded by a control packet that contains all the required information with respect to the optical-burst. This information is used by the intermediate nodes to process the optical-burst without any delay. Apart from providing information to the core nodes, control-packet also makes the necessary wavelength reservation along the route for the optical-burst transmission in the core network. In all the simulations performed in this work, JET signalling mechanism is used to reserve and relinquish the required wavelength at the core network for the transmission of the optical-burst. JET signalling mechanism is used as there is no extra overhead to relinquish the established wavelength at the core nodes after the transmission of optical-burst is completed. The reserved wavelength at the intermediate node is automatically released as soon as the optical-burst leaves a particular node. This mechanism assists in minimising contention problem that is very common in OBS network with bufferless intermediate nodes. Optical-bursts at the ingress node reach the egress node traversing the core network cutting through at-least four intermediate nodes. Based on the access network traffic, available bandwidth, and TCP flows, IP packets are accumulated into optical-bursts. TCP flows from access network can be classified into slow, medium, and fast. A TCP flow consists of the time taken for the transmission of window that contains IP packets from source to destination and an appropriate acknowledgement received from the receiver. This duration is also called as RTT. Based on this calculation, the size of the CW is modelled. While assuming Markov regenerative process, the size of the transmission window and RTT are considered to be independent of each other [77].
3 Another assumption made here is that the number of IP packets lost during one RTT is independent of other round. Thus a steady-state throughput of TCP flow can be written in terms of the loss ratio l r and the RTT calculated to obtain throughput in an OBS network is Throughput = Where B is the average burst size, RTT n is the roundtrip time in the network, B t is the optical-burst formation time, and ack is the number of acknowledged rounds before the window size is increased. If M w is the size of the maximum window advertised by the receiver, B s is the size of the optical-burst formed, in the time B t with the propagation delay P d. When the bandwidth of the access network is N a, and d l is the delay suffered due to aggregation of optical-bursts, with r b as the route followed by the IP packets is assumed to be Bernoulli distributed then TCP flows can be Slow-TCP-source = Medium-TCP-source = Fast-TCP-source =
4 In this work, fast TCP flows are assumed in case of TCP-Westwood, TCP- Westwood-Optic, TCP-Vegas, TCP-NewVegas, and FAST-TCP whereas medium flows are assumed to be in case of TCP-Reno, TCP-Newreno because of the underlying optical network. 5.2 Simulation Model In the simulation model used, NS-2 simulator, version 2.27 was used to evaluate TCP throughput over OBS network. NSF topology was implemented as shown in the figure 5.1 with the distance shown in Kilometres. Figure: 5.1- NSFNet Topology with 14 nodes.
5 The existing simulator has been patched with additional modules to incorporate optical features [78]. There is an additional module introduced to NS-2, Optical module that separates data-planes and control-planes while simulating TCP over OBS networks. This feature solves the complexity that arises in all-optical processing of an optical-burst along with its control packet. There are no opticalbuffers or fiber-delay-lines in OBS network. FTP and CBR connections are used to generate TCP traffic whose destination implements random uniform distribution algorithm. While routing optical-burst from ingress node to egress node, there are a minimum of four optical core nodes in the route. Source routing algorithm is used at the edge nodes while releasing the optical-burst into the core network. The parameters used at the edge nodes are the number of packets per optical-burst, number of burst assembly queues, off-set delay for the control packet, and the type of burstification mechanism to be implemented in the simulation. The Latest-available-unused-channel with Void-filling (LAUC-VF) [79, 80] and Minimum Sharing Void (Min-SV) [81] are the channel scheduling algorithms used for optimising the route in the core network. Minimum-hop-path scheduling is the algorithm used at the edge nodes while routing the optical-burst. There are 14 optical core nodes in our topology with 28 TCP/IP nodes and 10 TCP connections. The access links have bandwidth of 155Mbps with link propagation delay of 1μs. The way the connection is made is shown in section: 5.6. The IP packets are aggregated into optical-bursts at the edge nodes and transmitted all optically from source to destination. Packet processing in the core network is done by the optical-classifier in the optical module.
6 Optical-classifier is used for organizing and forwarding IP packets inside the optical core nodes. When an IP packet arrives, optical-classifier verifies the type of packet, and based on its routing table's address the packet is forwarded to the respective destination. The constraint here is that the working of the optical-classifier in core node and edge node is similar. The difference arises when optical-classifier verifies for the destination address of the packet. If the address is in the OBS domain, it is sent to the next core node based on the routing information. On the contrary, the existing node that holds this optical-classifier should act as edge node and transfer the packet to the optical-burst agent. It is the optical-burst agent that performs the act of burstification and deburstification.
7 Algorithm:4 //first set the timer /*Working of Optical classifier at the edge nodes*/ if(tmr_[queue][recv_destination].status() == TIMER_PENDING) { Tmr_[queue][recv_destination].cancel(); } //Insert the buffered packets into the burst - BURSTIFICATION p->allocdata(burstdh->packet_num*sizeof(packet*)); //send control packet send(cp, 0); //schedule and send optical-burst //s.schedule(target_, p, HOP_DELAY); //allocate control packet burst_cont[recv_destination][queue]= allocp_kt(); //Burst arrived to the destination. Start DEBURSTIFICATION hdr_burst* bursth = hdr_burst::access(pkt); /*If the received burst is an optical-burst and not a Control packet, and there is no occurrence of drop in that particular core node then*/ if((bursth->burst_type==1)&&(bursth->drop==0)) if (!bursth->ack) { /*save_burst_info_in_a_file */ }
8 The packets that are assembled in a single optical-burst during burstification process are defined in the variable burst size. During simulations, the size of the burst is varied to evaluate the performance of all the TCP variants. The value of the burst size ranges from a minimum of 10 packets per optical-burst to a maximum of packets per optical-burst. The hop delay between the control packet and optical-burst is 2μs. There are 100 assembly buffers at the ingress node. Burstification period is varied from and 1.0 to estimate its affect on throughput of TCP variants. To eliminate the problems in all optical processing of packet headers, the data plane and the control plane are separated in OBS simulator. MAX-PACKET- NUM is a variable used in the simulation to count the number of packets in an optical-burst. JET signalling mechanism is used in this network, where the control packet tries to reserve resources for the optical-burst just sufficient enough for transmission on each link it traverses. The control packet has all the vital information so that each intermediary optical switch in the core OBS network can transfer the optical-burst and also configures its switching matrix in order to switch the optical-burst all optically. The conversion of electrical-optical-electrical is taken care by the edge node in the core OBS network. They generate and forward the control packets followed by the optical-burst. There is no burst segmentation, fiber delay lines or deflection routing in the core network to handle contention. In this simulation environment using OBS over NS-2 with varying parameters, both loss-based variants and delay-based variants were simulated. The results and performance analysis is in the subsequent sections.
9 5.3 Loss-Based Variants - Simulations and Performance Evaluation Working of various loss-based TCP variants were discussed in chapter-3. While analysing their performance, a critical analysis of these variants working over OBS networks was made for evaluating the performance over OBS networks, TCP-Reno, TCP-Newreno, and TCP-Westwood was considered. In this work, TCP-Reno and TCP-Newreno were considered as they are the most prevalent protocols over Internet among existing TCP variants. Their counterpart TCP-Westwood was included for analysis in this work, as it is basically designed for high-speed networks. The other imperative reason for evaluating TCP-Westwood is that the performance of TCP-Westwood till date was not assessed over OBS network [82]. While simulating TCP-Westwood protocol for working over OBS networks modifications were made to the existing protocol, by adjusting the size of the CW and slow-start-threshold based on the parameter Average-rateassessment as shown in the equations 3.12 and 3.13 an enhanced performance of TCP-Westwood-Optic can be observed. In this section, an experimental evaluation of all the three loss-based variants under time-threshold, quantity-threshold and hybrid mechanism can verified. Simulation time of 30 minutes was considered for obtaining the throughput. To evaluate a superior variant under varying traffic conditions, simulations were done using both FTP and CBR type of traffic.
10 5.3.1 TCP-Reno TCP-Reno a popular loss-based variant of TCP that follows AIMD mechanism of congestion control. Experiments were done with varying opticalburst sizes. Simulations were conducted by varying traffic considering FTP and CBR. With CBR type of traffic, there was a negative delay after 500 packets and after 2000 packets there was a decline in BDR to minimum values. This can be seen in figure: 5.2. The minimum number of packets per optical-burst varied from 10 packets per burst to packets per burst. In case of optical-burst size of more than packets the throughput of TCP-Reno has reduced significantly. While implementing quantity-threshold mechanism of burstification, after 2000 packets we have experienced negative delay and end-to-end transmission of packets were delayed significantly. In case of time-threshold mechanism, it was observed that the throughput of TCP-Reno deteriorated with burstification time out (BTO) value of and 0.01.TCP-Reno performance is optimum when BTO value is more than 1μs and number of packets per optical-burst is less than 700.
11 Figure: 5.2 Throughput of TCP-Reno with CBR and FTP traffic when BTO is 1μs Figure: 5.3 Throughput of TCP-Reno with varying BTO
12 Figure: 5.4 Throughput of TCP-Reno with BTO 1μs and 2μs Figure: 5.3 shows the simulation results on TCP-Reno. It is a graph showing the performance of TCP-Reno when there is a variation in BTO value. While simulating in the value of y i as in the equn3.2. This performance has a direct impact on the value of x i therefore the size of the with lower time-out values in case of time-threshold mechanism, there were multiple retransmissions and there is deterioration congestion window W x has decreased persistently leading to a decline in the throughput of TCP-Reno over OBS networks. By increasing the upper limit on time and decreasing the number of packets per optical-burst to less than 1000 packets per optical-burst, there is an escalation in the performance of TCP-Reno. TCP-Reno was also simulated for a greater value of BTO ranging to a maximum of 12μs. There was insignificant change after 2μs this can be observed in figure: 5.4 the results pertaining to the same were presented for analysis TCP-Newreno
13 TCP-Newreno is another loss-based variant of TCP that augments its performance over its predecessor TCP-Reno in fast-recovery and fast-retransmit phases of AIMD mechanism of congestion control. During congestion control mechanism, TCP-Newreno is similar to TCP-Reno in its slow-start and congestion-avoidance phase. When time-threshold mechanism is implemented and time-out value of the optical-burst is limited to 0.01 μs, the throughput of TCP-Newreno is reduced to less than 70% and from 7000 packets onwards huge negative delay was experienced. The reason for this negative delay and decline in the throughput can be attributed to multiple triple duplicate acknowledgements leading to a considerable decline in the size of CW. So the value of NT(W) as in equn3.6 is deteriorated Since both the variants are actually designed for electronic networks their performance will be affected by BTO value also, apart from other factors like random contention and false-time-out, which are common in OBS networks. In figure5.5, the simulation results of TCP-Newreno are plotted with varying BTO values. There is a slight fall in the throughput of TCP-Newreno when the opticalburst size is more than 7000 and BTO 0.001μs owing to random contention in the core network. When time-out value of the optical-burst is more than 0.01 μs, there is a considerable raise in the throughput of TCP-Newreno as shown in figure: 5.5. It is observed that the throughput of TCP-Newreno remained unaffected after 1μs. As TCP-Newreno is a fast class variant of loss-based TCP, the input traffic from the access network will be comparatively higher than TCP-Reno. Hence after a threshold value of 1μs during burstification, there is a minimum impact on packet assembly of TCP-Newreno. Similar to TCP-Reno, TCP-Newreno also exhibited a decline in performance when number packets per optical-burst were above 700.
14 Figure: 5.5 Throughput of TCP-Newreno with BTO 0.01 μs and 1.0 μs When the size of the optical-burst was above 8000 packets per burst irrespective of the time-out value, there was delay in transmission and decline in throughput. Number of retransmission were considerably increased and lowering the performance even in fast-recovery phase where the throughput of W is RTT( +RTT This may be due to the fact that TCP-Newreno's fast-recovery phase performance is dependent on the expected number of packets lost in the current window. TCP-Newreno will respond to this by retransmitting one lost packet in each RTT thereby to retransmit n packets, TCP-Newreno needs n RTTs.
15 At this point, the performance of TCP-Reno and TCP-Newreno will have minimum disparity. Hence the steady state throughput of TCP-Newreno when log CWs<B is = TCP-Westwood-Optic TCP-Westwood-Optic differs from its counterpart in implementing congestion control mechanism while incrementing the size of the CW after a loss event is detected by the TCP-sender. The bandwidth estimation algorithm of TCP- Westwood-Optic uses the sender side window dynamics to identify the lost packets during a triple duplicate acknowledgement from the TCP-receiver, thereby retransmitting appropriately. This selective acknowledgement feature of TCP-Westwood-Optic supported by piggybacked acknowledgements from the receiver enhances the performance of the protocol over OBS networks. During a time-out caused due to loss of packets, instead to setting the size of CW to 1 directly as it is done in case of TCP-Reno and TCP-Newreno, TCP- Westwood-Optic makes use of RTT acq variable and swiftly adjusts the size of CW drastically to meet the existing bandwidth requirements. With these two modifications done to existing TCP-Westwood, a new modified TCP-Westwood-Optic is proposed to suit OBS networks. Implementing TCP-Westwood over OBS networks also demanded for some basic changes in the protocol as this variant of TCP was initially designed for IP networks. This work for the first time in literature presents implementation of TCP-Westwood and TCP-Westwood-Optic over OBS networks.
16 An additional component was added to TCP-Westwood after patching it over OBS networks to improve it from its previous versions. This extra component added includes a stack, which will temporarily store 5 RTT values and sequence numbers of the packets received from piggybacked acknowledgements. With these two additional values the performance of TCP-Westwood-Optic was enhanced to suit OBS networks. TCP working over barebone OBS network was considered for simulation as this study was mainly done for barebone OBS networks. Figure: 5.6- Evaluating the performance of TCP-Westwood-Optic
17 Figure: 5.7 Throughput of TCP-Westwood-Optic with varying BTO From the figure: 5.7 the performance of TCP-Westwood-Optic can be observed which shows an improvement over its previous version TCP-Westwood. The average throughput of TCP-Westwood was observed to be 86% whereas there was an enhanced throughput of 97% in case of TCP-Westwood-Optic. Impending simulation results shown in the next sections will be for analysing TCP-Westwood-Optic only. During the simulation of TCP-Westwood-Optic from figure: 5.7, it can be observed that there is not much variation in the throughput when there is change in BTO. Like TCP-Reno or TCP-Newreno, TCP-Westwood-Optic is not influenced by a change in BTO. It is also analyzed, that there is no degradation of performance when there is an increase in optical-burst size even with lower values of BTO. In case of time-out, the size of the CW is adjusted based on the assessed bandwidth, the steady state throughput of TCP-Westwood-Optic for the current window size is
18 = as discussed in section: Comparison of performance of Loss-based Variants The superior performance of TCP-Westwood-Optic can be seen in figure 5.9, 5.10 and Even after bursts there is a performance enhancement in throughput of TCP-Westwood-Optic in comparison with the result of TCP-Reno and TCP-Newreno, which show a decline in burst delivery ratio. Therefore modified TCP-Westwood-Optic has a superior performance when compared to other loss-based variants of TCP like TCP-Reno and TCP-Newreno. With higher BTO, the throughput of TCP-Westwood-Optic is unaltered. The decline in performance of TCP-Newreno after packets per optical-bursts can be attributed to false-time-out accounting to depreciated performance in fastretransmit phase. Figure: 5.8 Comparison of TCP-Reno, TCP-Newreno and TCP-Westwood-Optic with BTO μs
19 Figure: 5.9 Comparison of TCP-Reno, TCP-Newreno and TCP-Westwood-Optic with BTO 0.01 μs Figure: 5.10 Comparison of TCP-Reno, TCP-Newreno and TCP-Westwood-Optic with BTO is 1.0 μs
20 It can be seen that the results obtained from simulation and analysis match the results closely. From the above simulation it can be concluded that TCP- Westwood-Optic is more suitable for OBS networks independent of the traffic conditions. 5.4 Delay-Based Variants - Simulations and Performance Evaluation Each variant of TCP is distinct from the other in the way they handle congestion control mechanism. TCP identifies network to be congested if there is a loss of data packets, or if TCP-sender received triple duplicate acknowledgements or if there is an extended delay in the expected RTT. In the present work, there is a detailed analysis of TCP variant that assume congestion in network when a loss of packet occurs, but variants like TCP-Vegas, TCP- NewVegas, and FAST-TCP assume congestion differently. Delay in transmission or out-of-order delivery of packets is considered as congestion in network by these variants and they initiate congestion control mechanism. Experimental study to analyse the throughput of delay-based variants was performed and the results in form of simulations were presented TCP-Vegas TCP-Vegas always try to adjust the size of CW to a value between α and β. When there is a multiple packet loss, TCP-Vegas based on the time-out value retransmits the lost packet without waiting for a triple duplicate acknowledgement. While considering fast flows of TCP-Vegas, it may be noted that there is an amount of delay encountered during burstification. This inference is made from the results obtained while simulating TCP-Vegas with BTO values 0.001μs,
21 0.01μs, 0.1μs, and 1μs. The throughput of TCP-Vegas improved when BTO was 1μs. When optical-burst size increased the performance of TCP-Vegas, it showed a constrained throughput with time-out value less than 1μs. After 10,000 packets there was a decrement in the performance of the protocol. Simulations were even run by increasing the time-out values, but after 1μs performance showed insignificant growth. Figure: 5.11-Throughput of TCP-Vegas with BTO 0.01 μs and 1.0 μs Figure-5.12 explains the simulation results on TCP-Vegas. It is a graph showing the performance of TCP-Vegas when there is a variation in BTO. While simulating with varying optical-burst sizes, the performance of TCP-Vegas is evaluated for BTO values varying from 0.001μs to 1.0μs. When the BTO is 1.0, there is an increase in throughput of TCP-Vegas when compared to BTO value 0.001μs. It can be inferred from this outcome that if BTO value is decreased, there will be degradation in the performance of TCP-Vegas. As TCP-Vegas is
22 principally designed for electronic networks, in OBS networks, it is implicit that bigger BTO will stabilize the performance of TCP-Vegas TCP-NewVegas TCP-NewVegas is an improved version of TCP-Vegas. During congestion control, with packet-pairing and rapid-window-convergence algorithms, the size of the CW is adjusted more rapidly than TCP-Vegas that implements AIMD mechanism of congestion control as shown in equn With the modifications made to the existing TCP-NewVegas algorithm like - Implementing selective acknowledgement to the piggybacked acknowledgements thereby retransmitting appropriately during packet-pairing mechanism and, Restoring the size of CW with the appropriate value based on the RTT assessment of the most recent sucessful acknowledgement from the TCP-receiver improves the performance of TCP-NewVegas over OBS networks. Simulations in figure: 5.13 show the results of TCP-NewVegas. The overall throughput of NewVegas is 90% and there is not much deterioration in its performance with varying BTO values. After a burst size of 500 packets and with BTO values equal to there is a decline in the performance of TCP- NewVegas. It can be concluded that the impact of BTO over TCP-NewVegas is very limited and the protocol performance has enhanced over its predecessor TCP- Vegas over OBS networks.
23 Figure: 5.12 Throughput of TCP-NewVegas with BTO μs, 0.01 μs and 1.0 μs FAST-TCP This is another high-speed delay-based variant that considers queuingdelay as a parameter for assessing congestion in the network. FAST-TCP adjusts the size of CW to equilibrium based on the information obtained from estimation module. On a barebone OBS network where random burst loss may occur due to contention in the core network even at low network traffic, the performance of FAST-TCP may deter when quantity-threshold mechanism is implemented or when optical-burst size is increased.
24 Figure: 5.13 Throughput of FAST-TCP with BTO μs and 1.0 μs In case of Fast-TCP during simulation it is observed that its performance is high with higher BTO values with maximum throughput and minimum delay. Overall performance with the given parameters it is clearly understood that FAST-TCP shows nearly 91 percent throughput, though there is some amount of delay in end-to-end delivery after 6000 packets with BTO value less than 1ms Comparison of Delay-based Variants In order to identify a pertinent variant over OBS networks, the three delaybased variants were simulated and the figure: 5.14, figure: 5.15, and figure:5.16 epitomize the same. All the three variants belong to the same family of TCP- Vegas. TCP-Vegas implements AIMD mechanism during congestion control, thereby incrementing the size of CW linearly by 1 for every RTT.
25 Figure: 5.14 Comparison of TCP-Vegas, TCP-NewVegas and FAST-TCP when BTO is μs Figure: 5.15 Comparison of TCP-Vegas, TCP-NewVegas and FAST-TCP when BTO is 0.1 μs
26 Figure: 5.16 Comparison of TCP-Vegas, TCP-NewVegas and FAST-TCP when BTO is 1 μs TCP-NewVegas improves over TCP-Vegas by making several changes to the sender side implementation of congestion control. These changes are further modified in this work to suit OBS networks. With all these amendments, TCP- NewVegas shows a superior performance over TCP-Vegas in OBS networks. TCP-Vegas and TCP-NewVegas consider delay as a mode to identify congestion. FAST-TCP extends this feature and considers queuing-delay as a means to identify congestion in the network. This protocol constantly tries to maintain the size of CW close to the equilibrium value which is obtained from the most recent acknowledged transmission (latest RTT value). The figure: 5.14, 5.15, and 5.16 shows the simulation results of all the three delay-based variants with varying time-out values. We have considered CBR and FTP type of traffic. This core network in our simulation is modelled as a single network with 1Gbps bandwidth and 10μs propagation delay. The access links have bandwidth of 155Mbps with link propagation delay of 1μs.
27 compared with TCP-Vegas and TCP-NewVegas when BTO value is 1μs. With BTO value equal to 0.01μs, it is observed that TCP NewVegas and FAST-TCP performance is almost equal. From the above simulations, it is concluded that FAST-TCP performs better than its counterparts and is more suitable for OBS networks under medium traffic conditions. 5.5 Comparison of Loss-Based Variants and Delay Based Variants In this section, a cumulative comparison of loss-based and delay-based TCP variants is shown for obtaining an overall assessment with-respect-to performance of TCP over OBS networks. The results shown below will present a precise scrutiny about all the six variants of TCP. Various parameters considered for this comparison are The type of Congestion control mechanism implemented Optimum Size of the Optical-burst Burstification Time Out Impact of FTO Burst delivery ratio based on Burstification time-out
28 5.5.1 Comparison of TCP-NewVegas and TCP-Reno Figure: 5.17 Comparisons of TCP-NewVegas and TCP-Reno with BTO 0.01μs Figure: 5.18 Comparisons of TCP-NewVegas and TCP-Reno with BTO 0.1μs
29 Figure: 5.19 Comparisons of TCP-NewVegas and TCP-Reno with BTO 1μs When TCP-NewVegas is compared with TCP-Reno, the results are as follows: Throughput of both the variants can be observed in figure: 5.17, 5.18, TCP-NewVegas is a delay based variant of TCP and implements rapidwindow-convergence mechanism of congestion control whereas TCP- Reno is a loss based variant implementing AIMD method of congestion control The optimum optical-burst size for TCP-NewVegas is 100 packets per optical-burst whereas it is only between 20 to 50 packets for TCP-Reno. The optimum BTO for both the variants is 1μs TCP-NewVegas has a higher throughput than TCP-Reno.
30 5.5.2 Comparison of TCP-NewVegas and TCP-Newreno Figure: 5.20 Comparisons of TCP-NewVegas and TCP-Newreno with BTO 0.01μs Figure: 5.21 Comparisons of TCP-NewVegas and TCP-Newreno with BTO 0.1μs
31 Figure: 5.22 Comparisons of TCP-NewVegas and TCP-Newreno with BTO 1μs When TCP-NewVegas is compared with TCP-Newreno, the results are as follows: Throughput of both the variants can be observed in figure: 5.20, 5.21, and TCP-NewVegas is a delay based variant of TCP and implements rapidwindow-convergence mechanism of congestion control whereas TCP- Newreno is a loss based variant implementing AIMD method of congestion control The optimum optical-burst size for TCP-NewVegas is 100 packets per optical-burst whereas it is only between 5000 to 8000 packets for TCP- Newreno. The optimum BTO for both the variants is 1μs TCP-NewVegas has a higher throughput than TCP-Newreno when optical burst size is below 100 packets per burst. TCP-Newreno an improved counterpart of TCP-Reno exhibits consistent performance because of its Selective acknowledgements feature as discussed in section: 3.3.2
32 5.5.3 Comparison of FAST-TCP and TCP-Reno Figure: 5.23 Comparisons of FAST-TCP and TCP-Reno with BTO 0.0 1μs Figure: 5.24 Comparisons of FAST-TCP and TCP-Reno with BTO 0. 1μs
33 Figure: 5.25 Comparisons of FAST-TCP and TCP-Reno with BTO 1μs When FAST-TCP is compared with TCP-Reno, the results are as follows: FAST-TCP performs better than TCP-Reno when time-threshold mechanism is used. FAST-TCP a high-speed delay-based variant of TCP considers queuingdelay as a parameter for congestion where are TCP-Reno implements congestion control based on AIMD mechanism. Figure:5.23 and 5.24 shows a superior performance of delay-based FAST- TCP over its loss-based counterpart TCP-Reno In figure: 5.25 after 6000 packets when BTO is 1μs there is a trivial decline in throughput of FAST-TCP.
34 5.5.4 Comparison of FAST-TCP and TCP-Newreno Figure: 5.26 Comparisons of FAST-TCP and TCP-Newreno with BTO 0.01μs Figure: 5.27 Comparisons of FAST-TCP and TCP-Newreno with BTO 0.1μs
35 Figure: 5.28 Comparisons of FAST-TCP and TCP-Newreno with BTO 1μs When FAST-TCP is compared with TCP-Newreno, the results are as follows: Throughput of both the variants can be observed in figure 5.26, 5.27, and FAST-TCP a high-speed delay-based variant of TCP considers queuingdelay as a parameter for congestion where are TCP-Newreno implements congestion control based on AIMD mechanism. FAST-TCP has a higher throughput than TCP-Newreno when BTO is less than 1μs but both the variants exhibit similar throughput when BTO is 1μs.
36 5.5.5 Comparison of TCP-Westwood-Optic and FAST-TCP Figure:5.29, 5.30 and 5.31 displays the simulation results of TCP- Westwood-Optic and FAST-TCP. While analysing a superior loss-based variant (TCP-Westwood-Optic ) with a high-speed delay-based variant (FAST-TCP), with varying BTO values, under CBR and FTP traffic conditions the performance summary is as follows- TCP-Westwood-Optic throughput is in between 90 and 98 percent. FAST-TCP throughput varies from 98 to 80 percent. When BTO value is less, there is a steep decline in the performance of FAST-TCP after an optical-burst size value of 6000 packets. When there is an increase in optical-burst size TCP-Westwood-Optic throughput is considerably higher than FAST-TCP.
37 Figure: 5.29 Comparisons of TCP-Westwood-Optic and FAST-TCP with BTO 0.01μs Figure: 5.30 Comparisons of TCP-Westwood-Optic and FAST-TCP with BTO 0.1 μs
38 Figure: 5.31 Comparisons of TCP-Westwood-Optic and FAST-TCP when BTO 1μs Comparison of TCP-Westwood-Optic and TCP- NewVegas Algorithms of both TCP-Westwood-Optic and TCP-NewVegas were modified and a new adapted protocol was defined to suit OBS networks. Analysis made up to this point of research demonstrates a superior performance of TCP- Westwood-Optic over all its peers.
39 Figure: 5.32 Comparisons of TCP-Westwood-Optic and TCP-NewVegas with BTO 0.01μs Figure: 5.33 Comparisons of TCP-Westwood-Optic and TCP-NewVegas with BTO 0.1μs
40 Figure: 5.34 Comparisons of TCP-Westwood-Optic and TCP-NewVegas with BTO 1μs When TCP-Westwood-Optic is compared with TCP-NewVegas, the results are as follows: There a slight decline in throughput of TCP-NewVegas after an opticalburst size of 500 packets. The throughput of TCP-Westwood-Optic is consistent throughout the simulation. The overall throughput of TCP-NewVegas is between a maximum of 98 percent to a minimum of 80 percent. TCP-Westwood-Optic outperforms TCP-NewVegas.
41 5.5.7 Comparison of TCP-Westwood-Optic and TCP- Vegas Figure: 5.35 Comparisons of TCP-Westwood-Optic and TCP-Vegas with BTO 0.0 1μs Figure: 5.36 Comparisons of TCP-Westwood-Optic and TCP-Vegas with BTO 0.0 1μs
42 Figure: 5.37 Comparisons of TCP-Westwood-Optic and TCP-Vegas with BTO 1μs When TCP-Westwood-Optic is compared with TCP-Vegas, the results are as follows: TCP-Westwood-Optic performs much better TCP-Vegas. A consistent throughput of TCP-Westwood-Optic can be observed, whereas in case of TCP-Vegas there is decline in performance after 100 packets per optical-burst when BTO is less than 1μs. When burstification time is increased there is a consistent performance in TCP-Vegas performance but the overall throughput below 80%.
43 5.5.8 Performance of Loss-based TCP Variants Vs Delaybased TCP Variants Figure: 5.38, 5.39, and 5.40 represent an analysis of all the TCP variants that are being studied in this work. In all the three graphs, the performance of TCP-Westwood-Optic surpasses all its peer variants. While comparing the throughput of all the variants, TCP-Reno's throughput is between 60 and 70 percent when BTO is less than 1 μs and between 70 and 80 with 1μs BTO. Figure: 5.38 Comparison of six TCP variants with BTO 0.01μs
44 Figure: 5.39 Comparison of six TCP variants with BTO 0.1μs Figure: 5.40 Comparison of six TCP variants with BTO 1μs
45 TCP-Vegas exhibited a superior performance when compared to TCP- Reno, with an overall throughput of 85%. Throughput of TCP-Vegas was never below 70 percent and it can be inferred that TCP-Vegas performs better than TCP-Reno over OBS networks. One similarity between both the variants is that they implement AIMD congestion control mechanism. When a comparison is made between TCP-Vegas and TCP-Newreno, the latter protocol has a better throughput when compared to the former, but when TCP-Newreno is compared to TCP-NewVegas, TCP-Newreno has an improved throughput when BTO is 1μs. In case of lower time-out values, TCP-NewVegas performs better than TCP-Newreno. 5.6 Summary This chapter contains a detailed analysis of most popular TCP variants. Based on the method of congestion control, variants of TCP are classified into loss-based and delay-based. In principal, TCP is totally unaware of the underlying network. The semantics of TCP rely only on end-to-end communication, which infer TCP-sender accepts communication from TCP-receiver only. In most cases the underlying network is electronic; hence all the variants of TCP are designed for such networks. Therefore when TCP is used over OBS network, the basic algorithms, their congestion control mechanisms, and the size of transmission window growth after congestion will underutilise the existing bandwidth. Therefore a rigorous search was made in this research to identify an apt variant of TCP over OBS networks.
46 TCP-Westwood-Optic performance over OBS networks is significant and its throughput with the existing parameters showed 98 percent burst-deliveryratio. Hence in the next chapter a detailed summary, along with tabulated results, conclusions regarding the experiments conducted with a few research problems that are to be investigated for future work are presented.
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