Modeling and Performance Analysis of a Sliding Window Protocol

Transcription

1 Modeling and Performance Analysis of a Sliding Window Protocol Yong Deng 1 and Zhangqin Huang 2 1 Xi an Jiaotong University, Institute of Software, Xi an 7149, P.R. China yongde711@sina.com 2 Eindhoven University of Technology, Faculty of Electrical Engineering Section of Information and Communication Systems P.O. Box 513, 56 MB Eindhoven, The Netherlands Z.Huang@tue.nl Abstract The sliding window algorithm is widely used in many standard network protocols. It can ensure a correct data transfer over unreliable channels where packets may be duplicated, lost, or re-ordered. By now only few papers have presented the performance of the sliding window protocol. A number of parameters affecting the overall performance of the system still need to be investigated systematically. This paper introduces the sliding window protocol and related parameters. An abstract executable performance model of the protocol is created in POOSL. Several simulation results can be derived from this model in various network environments and different parameter settings. When we design adaptive algorithms or novel protocols with the sliding window algorithm, these results are very helpful to identify optimal parameter settings and to determine a number of key-parameters such as the window size, the timeout period and the packet size before the implementation of the protocols. Keywords sliding window protocol, system level model, performance analysis, POOSL I. INTRODUCTION The Sliding Window (SW) protocol has been widely used in many popular communication protocols such as TCP, HDLC and SPX. The protocol can ensure a correct data transfer over very poor quality communication channels where the packets may be duplicated, lost, or re-ordered [1,2,3,4]. A number of parameters affect the performance characteristics of the SW protocol. Usually, these performance characteristics cannot be determined analytically. However, we need to study these key performance characteristics and to identify optimal This research is supported by PROGRESS project EES522, Modeling and Performance analysis of telecommunication systems. parameter settings in the early design phases when designing an adaptive algorithm or novel protocols for specific computing environments. Such information is helpful for the designer to work out a product. In this paper we introduce the sliding window protocol generally, describe the communication procedure of the protocol, and conclude some related parameters affecting the overall performance of the system. In order to estimate the performance of the SW protocol systematically, as an exercise, an abstract executable performance model of the protocol using go back n is created in POOSL. The model allows us to present and analyze some performance characteristics. These performance characteristics include the throughput (the bandwidth utilization) of the protocol and the delay of the acknowledgement. Some key-parameters such as the window size, the timeout period, the round trip time, the packet size, and the probability of packets loss are investigated in several network environments and different parameter settings as well. The remainder of this paper is organized as follows. The next section gives an overview of the SW protocol generally. In section III, we will present an abstract performance model of the protocol in the modeling language POOSL. The simulation results derived from the model are illustrated in section IV. A simple performance analysis is also presented in the same section. Finally, conclusions are summarized in the last section. II. SLIDING WINDOW PROTOCOL DESCRIPTION Sender and receiver. In a SW protocol, there are two main components in the data link layer: the sender and the receiver. The sender obtains an input sequence of data frames from the network layer in our model. Each PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL

2 data frame is composed of an embedded packet and some control information. Control information includes a number of fields such as the frame kind, the input sequence number and the acknowledgement. Each outbound frame is assigned a sequence number, ranging from up to some maximum number (usually 2 n -1, n>). The input sequence must be transmitted by the sender to the receiver via an unreliable channel. After receiving the frames from the network, the receiver should deliver them to the network layer in the same order in which they appear in the input sequence. Sending and receiving windows. At any instant of time, the sender maintains a sending window of consecutive sequence numbers corresponding to frames it is permitted to send. These sequence numbers within the sending window represent frames sent but not yet acknowledged. Similarly, the receiver maintains a receiving window, corresponding to the number of out of order frames it is permitted to accept. Any frames falling outside the receiving window are discarded without comment. In our model we give an assumption of the same window size in the sender and the receiver. Channels. The communication channels connect the sender and the receiver. Usually, the channels are unreliable and bi-directional. The packets may be duplicated, lost, or re-ordered in the channels, and will be delayed a time through the channels. In the SW protocol, the data frames are sent by the sender to the frame channel, and the acknowledgement frames are sent by the receiver to the another channel: the acknowledgement channel. Communication procedure. In the beginning of communication between the sender and receiver, the sending window and the receiving window are empty. The sender is starting to send data frames to the receiver via the frame channel. On the other side, the receiver is waiting for receiving data frames from the network. Whenever a new packet arrives at the sender from the network layer, the next highest sequence number according to the sending window is given, and the upper edge of the sending window is advanced by one. The sender puts the packet and the sequence number with some other control information into a frame, and then sends the frame to the frame channel. After transmitting a frame, the sender starts a new timer running for the corresponding to this frame in the sending window in our model. The timeout period of the timers must be chosen to allow enough time for the frame to get to the receiver, for the receiver to process it in the worst case, and for the acknowledgement frame to propagate back to the sender. After transmitting a frame and starting a timer, the sender will transmit the next data frame to the frame channel until the sending window is filled up. In the same time, the sender is waiting for the acknowledgement arriving from the network. There are three possibilities: an acknowledgement frame arrives undamaged, a damaged acknowledgement frame staggers in, and the timer goes off. If a valid acknowledgement comes in, the sender fetches the next packet from the network layer and put it in the buffer, overwriting the previous packet. It also advanced the input sequence number. If a damaged frame arrives or a timer goes off, a duplicate should be sent. In [5] many different policies for sending and resending of data frames exist. In this paper, as an exercise, we only focus on the SW protocol using go back n. The sender will send all unacknowledged frames in order, starting with the damaged or lost one. At the receiver, when a valid frame arrives, its sequence number is checked to see if it is the next one. If it is the next one, it is accepted, passed to the network layer, and an acknowledgement frame generated. Otherwise, it will be discard and is not passed to the network layer. Performance parameters. A number of parameters affect the performance of the SW protocol. Here we conclude some key-parameters in our model. Packet size (S): the size of the embedded packet in a frame. A packet is the unit of information exchanged between the network layer and the data link layer in the same machine, or between network layer peers. In our model, we give an assumption that the packet size is fixed. Maximum window size (w): window size is the number of the unacknowledged frames in the sending window or in the receiving window. The sender and the receiver are of the same maximum window size w in our model. Each of the sender and the receiver needs w buffers to hold the unacknowledged frames. Timeout period (T t ): a timer will go off within the timeout period. Bandwidth (B): it is the maximum amount of data passing the channel at a given time. Usually, it depends on the medium capacity of the communication channels itself. Process time of a packet (T s ): it is the time it takes for a packet to send/receive to/from the channel, so T s =S/B. Channel delay and round trip time (T r ): channel delay is the time it takes for data to travel across the channel. Round trip time is the time it takes for data to travel across the channel from the sender to the receiver and then back. In our model, we suppose that the channel PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL

3 delay of the data frames and acknowledgement frames have the same normal distribution with µ=t r /2 for various variance σ. Let σ has different values such as.1*t s,.2*t s or.5*t s. Probability of packets loss (P l ): it is the probability of packets loss in the data frame channel or in the acknowledgement frame channel. In our model, we will study various situations such as packets lossless and different probabilities of packets loss (e.g. 1%, 2% or 5%). III. MODELING OF THE SW PROTOCOL In this section, we create an abstract performance model of the sliding window protocol using go back n in POOSL. The POOSL language has a small set of very powerful language primitives that it allows us to represent a complex system in a concise and compact way with a mathematically defined semantics [6,7]. The remainder of this section is organized as follows. A framework of the model is presented in the next sub-section. A number of processors such as the network layer processor, the SW protocol processor, the timer processor and the physical layer processor in our model are introduced after sub-section B. A. Abstract Performance Model Figure 1 shows a model of the SW protocol in POOSL. The bigger window (above) in this figure presents a general frame of the protocol. The left is the side of the sender and the right is the receiver. Each side consists of three processors (or cluster), i.e. a network layer processor, a data link layer cluster and a physical layer processor. The data link layer cluster shown in the smaller window (below) in figure 1 contains a SW protocol processor and a timer processor. The model can present all the behavior of the protocol. The sender in the left data link layer cluster fetches a packet from the network layer processor via the channel NLtoDLL, put it to a frame with some control information such as framekind, seqnum and entertime, and then sends it to the physical layer processor via the channel DLLtoPL. In the mean time, the receiver in the right data link layer cluster is waiting for receiving frames from the physical layer processor. After receiving a frame, the receiver checks the seqnum of the frame, delivers the packet to the network layer processor via the channel DLLtoNL, and generates an piggybacking acknowledgement frame with a new packet from the receiver to reply to the sender if the frame is in the correct order. The whole communication procedure of the protocol is already described in section II. Figure 1. A performance model of the sliding window protocol in POOSL B. Network Layer Processor In our model, the network layer processor has two main functions. It sends and counts the packets to the data link layer cluster via a channel NLtoDLL. In the same time, it is waiting for receiving packets from the data link layer cluster through a port condll. We give a specification of the functions in a method, for example, startsendandreceive()() in POOSL. startsendandreceive()() p:packet /*Check enablenetworklayer Status before sending a packet */ condll?enablenetworklayer(enablesendstatus); sel /* Sending packets to the data link layer */ [enablesendstatus] condll!networklayerreadyenvet; condll!packet(new(packet)) setvalueto (packetinfo+counter printstring); counter:=counter+1; or /* Waiting for receiving packets from the data link layer */ condll?packet(p) abort delay acceptpackettime; les; startsendandreceiver()(). C. Sliding Window Protocol Processor The SW protocol processor is the core in our model. In this processor, we model the sender, receiver, the sending window and the receiving window described in section II. We conclude four main functions as follows. 1) Fetches/delivers packets from/to the network layer processor via the channel NLtoDLL ; 2) Forms data frames with some control information, and generates the acknowledgement frame; 3) Manages the sending/receiving windows in the SW PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL

4 protocol processor; 4) Sends and resends the data/acknowledgement frames using go back n to the physical layer processor via the channel DLLtoPL. D. Timer Processor After the sender transmits a frame, the timer processor receives an event of starting a new timer from the SW protocol processor, and then it starts the new timer running. When a timer goes off, the timer processor in our model produces a timeout message to inform the SW via an internal channel. E. Physical Layer Processor We model the physical layer processor as two directional channels. One channel is used to send the data frames from the sender to the receiver. Another one is for the piggybacked acknowledgement frames from the receiver to the sender. The packets in these channels will be delay a time. We model the delay time as a normal distribution with µ=t r /2 for various variance σ. We set σ as a different values such as.1*t s,.2*t s or.5*t s. The packets may be also lost in these channels. We model the different loss probabilities as a normal distribution with parameter µ and σ. Here µ is set to be a value of 1%, 2% or 5%. IV. PERFORMANCE ANALYSIS We illustrate simulation results derived from the performance model in this section. Our experiments concentrate on the impacts of a number of parameters of the protocol on the bandwidth utilization and on the delay of the acknowledgements. These parameters include the medium bandwidth, the window size, the timeout period, the packet size and the probability of packets loss. A. Medium Bandwidth In order to present the performance of the SW protocol, we ignore the possibility of data error in the beginning. Figure 2, 3 and 4 illustrate three simulation results in different medium bandwidth environments. If we do the same with the SW protocols, the behavior of the protocol is determined entirely by the window size w in the sender and the receiver. In figure 2, we draw the curves of the bandwidth vs. the window size on a 1 Mbps Ethernet LAN. We suppose that the packet size S=124 bytes, the round trip time T r =1 ms, the timeout period T t =12 ms and no packet loss. We can learn some experiences from this simulation as follows. Bandwidth (M) Figure 2. Simulation results of the bandwidth utilization on a 1Mbps Ethernet LAN: impact on the bandwidth utilization as a function of the window size with S=124 bytes, T r =1 ms, T t =12 ms and no packet loss Bandwidth(M) Figure 3. Simulation results of the bandwidth utilization on a 1Mbps Ethernet LAN with S=124 bytes, T r =1 ms, T t =12 ms and no packet loss Bandwidth (M) transmitted acknowledged LAN 1M LAN 1M transmitted acknowledged transmitted acknowledged Wireless channel Figure 4. Simulation results of the bandwidth utilization on a wireless channel with B=5 kbps, S=1 bytes, T r =5 ms, T t =6 ms and no packet loss 1) In order to keep the pipeline busy, we can set the maximum window size as w =T r /T s = T r /(S/B)=122. When the window size is round 122, the throughput of the SW protocol is growing to maximum. 2) When the window size increases to PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL

5 w =T t /T s = T t /(S/B)=146, the throughput will be dropped rapidly. 3) Due to the receiver uses the policy of the piggybacked acknowledgement in our model, when a data frame arrives to the receiver, the acknowledgement cannot generated immediately and should wait to the next data frame which is sent by the receiver. The half-bandwidth capacity is wasted in this case when we set the window size not bigger enough. Figure 3 presents the simulation results on a 1 Mbps Ethernet LAN. In this case we can also get w =122 and w =146. Figure 4 presents some results on the wireless channel. The medium bandwidth capacity is 5 kbps, the packet size S=1 bytes, the round trip time T r =5 ms, the timeout period T t =6 ms and no packet loss. In the same way, we can get w=31 and w =38. This exercise is shown that we can derive similar performance results in different medium bandwidth environments with no packet loss. It is significant for the high performance network. In many network, errors are very rare so these simulation results is close to reality. B. Window size Figure 5 describes the curves of the bandwidth utilization vs. the window size in the sender and receiver. In the same figure, we show the impacts of the different probabilities of packets loss on the bandwidth utilization. delay of acknowledgements. The probabilities of packets loss will affect the delay of acknowledgements dramatically. Acknowledgement Delay(µs) Lossless Loss 1% Loss 2% Loss 5% Figure 6. The delay of the acknowledgements vs. the window size curves on a 1M Ethernet LAN with S=124 bytes, T r =1 ms, T t =12 ms C. Timeout period Timeouts play a crucial role in the network performance. Figure 7 illustrates the impacts of timeout periods on the bandwidth utilization in a 1M Ethernet LAN. For reliable communication channels, a high timeout value doesn t appear to reduce throughput. But for unreliable channels, a high timeout period will affect the performance unfavorably. Bandwidth(M) transmitted Lossless Loss 1% Loss 2% Loss 5% WindowSize Figure 5. The bandwidth utilization vs. window size curves, and impact of different probabilities of packets loss in the channels on the bandwidth utilization in a 1M Ethernet LAN with S=124 bytes, T r =1 ms, T t =12 ms We can get some conclusions from this exercise. In no packet loss channels, the window size have a dramatic effect on the throughput of the protocol as we described in sub-section A. However, with the increase of the probabilities of packets loss in the channel, the throughput becomes insensitive about the window size. If we set the window size exceeding a number which is smaller than the maximum window size, the throughput is not growing. Figure 6 can give us an overview of the Bandwidth(M) Transmitted Lossless Loss 1% Loss 2% Loss 5% Timeout Period (ms) Figure 7. Impact of the timeout periods on the bandwidth utilization in a 1M Ethernet LAN with S=124 bytes, T r =1 ms, T t =12 ms and the different probabilities of packets loss in the channels D. Packet size The simulation results of curves of the bandwidth utilization vs. the packet sizes shown in figure 8 are derived on a 1Mbps Ethernet LAN. A bigger enough packet generally gives better performance. However, too bigger packet size fills up the pipeline, the performance will be drop off. PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL

6 Bandwidth (M) Transmittedl Lossless Loss 1% Loss 2% Loss 5% Packet Size (bytes) [7] L.J. van Bokhoven. Constructive Tool Design for Formal Languages: From Semantics to Executing Models. PhD thesis, Eindhoven University of Technology, Department of Electrical Engineering, 22. Figure 8. Impact of the packet sizes on the bandwidth utilization in a 1M Ethernet LAN with S=124 bytes, T r =1 ms, T t =12 ms and the different probabilities of packets loss in the channels V. CONCLUSIONS In this paper we have presented an abstract executable performance model of the sliding window protocol in the modeling language POOSL. Some simulation results can be derived from the model. These results illustrated a number of key-parameters affecting the overall performance of the protocol. We have discussed that the impacts of the key-parameters on the bandwidth utilization and the delay of the acknowledgement in several network environments and different parameter settings. It is shown that the window size, the timeout period and the probability of packets loss are effect on the performance characteristics of the protocol dramatically. For our future work, we would like to extend the model to the sliding window protocol using selective repeat. More parameters and complex network environments should be investigated in our model. We are also interested in the comparison between the simulation results and the measurements in a real network. REFERENCES [1] N.V. Stenning. A data transfer protocol. Computer Networks, 1(2):99-11,1976. [2] Eric Madelaine and Didier Vergamini. Specification and Verification of a Sliding Window Protocol in LOTOS. FORTE '91, Sydney, Australia, November 1991, pages [3] Mark A. Smith and Nils Klarlund. Verification of a Sliding Window Protocol Using IOA and MONA. FORTE/PSTV 2, Pisa, Italy, October 2, pages [4] Dmitri Chkliaev, Jozef Hooman and Erik de Vink. Formal Verification of an Improved Sliding Window Protocol. In proceeding of the PROGRESS 22, Utrecht, The Nethelands, October 22, pages [5] A.S. Tanenbaum. Computer Networks. Prentice-Hall International, Inc., [6] P.H.A.van der Putten and J.P.M. Voeten. Specification of Reactive Hardware/Software Systems. PhD thesis, Eindhoven University of Technology, Department of Electrical Engineering, PROGRESS/STW 23, ISBN OCTOBRE 22, 23, NBC NIEUWEGEIN, NL