PROBLEMS P20 P21 P22 P23 P24 b Repeat a but this time choose one city in France and another city in Germany c Pick a city in the United States and perform traceroutes to two hosts each in a different city in China How many links are common in the two traceroutes Do the two traceroutes diverge before reaching China Consider the throughput example corresponding to Figure 1 20 b Now suppose that there are M client server pairs rather than 10 Denote Rs Rc and R for the rates of the server links client links and network link Assume all other links have abundant capacity and that there is no other traffic in the network besides the traffic generated by the M client server pairs Derive a general expression for throughput in terms of Rs Rc R and M Consider Figure 1 19 b Now suppose that there are M paths between the server and the client No two paths share any link Path k k 1 M consists of N links with transmission rates R1k R2k RNk If the server can only use one path to send data to the client what is the maximum throughput that the server can achieve If the server can use all M paths to send data what is the maximum throughput that the server can achieve Consider Figure 1 19 b Suppose that each link between the server and the client has a packet loss probability p and the packet loss probabilities for these links are independent What is the probability that a packet sent by the server is successfully received by the receiver If a packet is lost in the path from the server to the client then the server will re transmit the packet On average how many times will the server re transmit the packet in order for the client to successfully receive the packet Consider Figure 1 19 a Assume that we know the bottleneck link along the path from the server to the client is the first link with rate Rs bits sec Suppose we send a pair of packets back to back from the server to the client and there is no other traffic on this path Assume each packet of size L bits and both links have the same propagation delay dprop a What is the packet inter arrival time at the destination That is how much time elapses from when the last bit of the first packet arrives until the last bit of the second packet arrives b Now assume that the second link is the bottleneck link i e Rc Rs Is it possible that the second packet queues at the input queue of the second link Explain Now suppose that the server sends the second packet T seconds after sending the first packet How large must T be to ensure no queuing before the second link Explain Suppose you would like to urgently deliver 40 terabytes data from Boston to Los Angeles You have available a 100 Mbps dedicated link for data transfer Would you prefer to transmit the data via this link or instead use FedEx overnight delivery Explain 75 PROBLEMS the protocol stack Is there an equivalent notion of header information that is added to passengers and baggage as they move down the airline protocol stack P31 In modern packet switched networks including the Internet the source host segments long application layer messages for example an image or a music file into smaller packets and sends the packets into the network The receiver then reassembles the packets back into the original message We refer to this process as message segmentation Figure 1 27 illustrates the end to end transport of a message with and without message segmentation Consider a message that is 8 106 bits long that is to be sent from source to destination in Figure 1 27 Suppose each link in the figure is 2 Mbps Ignore propagation queuing and processing delays a Consider sending the message from source to destination without message segmentation How long does it take to move the message from the source host to the first packet switch Keeping in mind that each switch uses store and forward packet switching what is the total time to move the message from source host to destination host b Now suppose that the message is segmented into 800 packets with each packet being 10 000 bits long How long does it take to move the first packet from source host to the first switch When the first packet is being sent from the first switch to the second switch the second packet is being sent from the source host to the first switch At what time will the second packet be fully received at the first switch c How long does it take to move the file from source host to destination host when message segmentation is used Compare this result with your answer in part a and comment Message a Source Packet switch Packet switch Destination Packet b Source Figure 1 27 Packet switch Packet switch Destination End to end message transport a without message segmentation b with message segmentation 77 78 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET d In addition to reducing delay what are reasons to use message segmentation e Discuss the drawbacks of message segmentation P32 Experiment with the Message Segmentation applet at the book s Web site Do the delays in the applet correspond to the delays in the previous problem How do link propagation delays affect the overall end to end delay for packet switching with message segmentation and for message switching P33 Consider sending a large file of F bits from Host A to Host B There are three links and two switches between A and B and the links are uncongested that is no queuing delays Host A segments the file into segments of S bits each and adds 80 bits of header to each segment forming packets of L 80 S bits Each link has a transmission rate of R bps Find the value of S that minimizes the delay of moving the file from Host A to Host B Disregard propagation delay P34 Skype offers a service that allows you to make a phone call from a PC to an ordinary phone This means that the voice call must pass through both the Internet and through a telephone network Discuss how this might be done Wireshark Lab Tell me and I forget Show me and I remember Involve me and I understand Chinese proverb One s understanding of network protocols can often be greatly deepened by seeing them in action and by playing around with them observing the sequence of messages exchanged between two protocol entities delving into the details of protocol operation causing protocols to perform certain actions and observing these actions and their consequences This can be done in simulated scenarios or in a real network environment such as the Internet The Java applets at the textbook Web site take the first approach In the Wireshark labs