CPSC 424/624 Exam 1 Solutions Spring 2016

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1 CPSC 424/624 Exam 1 Solutions Spring 2016 Name: You can use a single 8.5 by 11.0 sheet of paper (front and back) for notes. You can use a calculator. Please do not share your notes sheet or your calculator. Section 1 Short answer questions (40 points, 424 students 1 6 only, ) Q1.1 The following script will list files that exceed a certain size, beginning the search specified by the first parameter. find. -size k xargs ls -laah If I run this in my home directory, I see: jjm@jjm-virtualbox:~$ pwd /home/jjm jjm@jjm-virtualbox:~$ find. -size k xargs ls -laah -rw-rw-r-- 1 jjm jjm 137M Feb 20 19:02./courses.tar.gz How would you run redirecting standard out to append to the file myscript.out? How would you run redirecting both standard out and standard error to append to the file myscript.out? How would you run redirecting standard out to append myscript.out and standard error to the bitbucket? (i.e., so standard error messages end up not being saved) How would you run the command, redirecting standard out to append the file myscript.out, in a background mode? How would you run the command, redirecting standard out to append the file myscript.out and standard error to the bitbucket, asynchronously at regular times throughout a week? Explain exactly what you would do to run this at 8:00 am every day. find. -size k xargs ls laah >> myscript.out find. -size k xargs ls laah >> myscript.out 2>&1 find. -size k xargs ls laah >> myscript.out 2>/dev/null find. -size k xargs ls laah >>myscript.out & First, edit crontab by issuing crontab e mycrontab o Then add the crontab job: 1

2 o * 8 * 1 find. -size k xargs ls laah >> myscript.out 2>/dev/null Q1.2 The nice and renice commands allow you to do what? To change (increase or decrease) the priority of an application. Q1.3 You want a convenient way to change from the current directory on a Linux system to a directory you access frequently. In my case, it might be the location ~/courses/cpsc424. Identify two approaches you can take to code a shortcut (e.g., go424) to move your terminal session from the current directory to that directory. For one of the approaches, indicate the details of how you would implement it. 1. Edit.bashrc and add an alias 2. Edit a startup script and add a bash routine 3. Create a script and place in a directory that is in your path (~/bin) To setup an alias in./bashrc, add this line: alias go424='cd ~/courses/cpsc424 Q1.4 You try to run UDPServer using port 5000 but you see a Failure on bind. What is the likely problem? Assume the code is correct as you have seen it work in the past. Another UDP program is currently bound to port Q1.5 The Unix Time Sharing System paper indicates special files constitute the most unusual feature of the Unix file system. In the early Unix system described in this paper, What were special files intended for? Where did special files reside? Solution : Intended to provide an abstraction for I/O devices. These files were located in /dev 2

3 Q1.6 What class is the following TCP/IP v4 IP address? Since the first 2 bits are 0.this is a class A address. Q1.7 We have used the UDPEcho2.tar.gz package in our class. The UDPClient2.c represents the client program that interacts with the UDPServer2.c program. The client code includes the following two lines: char *servip; unsigned short echoservport; servip = argv[1]; echoservport = atoi(argv[2]); echoservaddr.sin_addr.s_addr = inet_addr(servip); /* Server IP address */ echoservaddr.sin_port = htons(echoservport); /* Server port */ The echoservaddr structure represents the servers TCP/IP v4 address. This code is filling in the information about the server prior to when the socket() system call is involved. The servip will hold the program parameter specifying the server s address and the echoservport will hold the program parameter specifying the server s port number. What does the inet_addr function return? What is the purpose of the function? What does the htons function doing? The inet_addr is translating the servip, which is a string, into an IP address. Specifically, inet_addr assumes the string is an IP address in dotted decimal format. If the server s address was input as a domain name, this routine returns a - 1. The htons is simply making sure that the echoservport, which is an unsigned int, is placed in the sin_port in network byte format. I.e., it is serving a host to network short function. 3

4 Section 2 Bash Scripting (30 points) Q 2.1 In HW1, the hw1.c program prints a line for each iteration as follows (we show three iterations): X8005d X8005d X8005d. The first field represents the amount of time it took the checksum calculation to compute a checksum (the second field). Assume the results are contained in a data file called hw1.dat. And that there are lines (or samples) in the file. What standard filter would be used to count the number of lines in the file? Show the command (don t worry if the output contains additional fields of data). (Hint: pipe the contents of the file to a filter that does word counts!!) Next, again in a single command line, use awk to count and display the number of lines. cat hw1.dat wc cat hw1.dat awk ' {lc++;} END {print lc} ' Q 2.2 Consider the following bash script. Assume you run the script in a directory with 4 files : hw1.dat myscript1.sh myscript2.sh myscript3.sh 1. #!/bin/bash 2. # specific conversion script to convert sh files to./tmp/script 3. LIST="$(ls *.sh)" 4. echo $LIST for i in "$LIST"; do 7. echo i 8. done if [! -d "./tmp" ]; then 11. mkdir tmp; 4

5 12. fi path=*.sh for f in $path; 17. do 18. cp "./$f" "./tmp/$f" 19. done On line 4, what would you expect to see displayed as a result of the echo? There is one bug that we purposely added somewhere in lines 6, 7, or 8. Identify and fix it. What is line 10 looking for? Once the script runs, what exactly will have been done? myscript1.sh myscript2.sh myscript3.sh Line 7 should be: echo $i If./tmp exists. If copies *.sh to the subdirectory./tmp Section 3 Networking (40 points) 624 students must do all 3 questions, 424 students must do only 2.if a 424 student does all 3, we won t grade Q3.3 All questions in Section 3 are based on the following Network You were given the following network: Host Network Router Network Host Network 1 has a fixed data rate of 10 Mbps and a propagation delay of 10 ms. Network 2 has a fixed data rate of 10 Mbps and a prop delay of 10 ms. 5

6 Assumptions: Assume the client message size is 1000 bytes. Assume no IP options are used (we will explain those later in the class) - which means the IP datagram is going to be exactly 1028 bytes (1000 bytes data + 8 bytes UDP header + 20 bytes IP header). Assume the frame overhead is 26 octets - which means all frames sent over the wire are exactly 1054 bytes) Assume the network characteristics are bi-directional. So the data rates and prop delays are the same in both directions. Assume there is no competing traffic Assume the hosts and routers do not add their own system delays Q3.1 We run 1 iteration of the UDPEcho client from Host 1, specifying a destination service of and port The message size is 1000 bytes. What is the expected round trip time observed by Host 1? 6

7 4* ( (1054*8)/ ) seconds Q3.2 We run the UDPEcho client and server at Host 1 and Host 2 respectively. The iteration delay is 1.0 seconds. We also run iperf3 client at Host1 which interacts with iperf3 server at Host 2. We configure iperf3 to establish a UDP session that send data at a rate of 20 Mbps. Based on the network shown above, estimate the throughput observed by the iperf3 server. Explain your thinking. Solution : 10 Mbps. The available bandwidth is only 10 Mbp. Because this is UDP, Host 1 sends data (or tries to send data) at 20 Mbps. Since the first hop is limited to 10Mbps, roughly 50% of the traffic will be dropped at the outbound interface on Host 1. Q3.3 We run the UDPEcho client and server at Host 1 and Host 2 respectively. The iteration delay is seconds. We record the arrival time of each packet arriving at Host 2. The interarrival time is computed for each packet that arrives, and is the difference between the current time and the time the last packet arrived. Assume ideal conditions (no delays other than the transmission times and propagation delays). Plot the distribution associated with the interarrival samples. Make sure to label axis s. Since there are no delays, we expect packets to arrive at fixed interval of second. The probability distribution would have a single mode centered at second. PDF (seconds) 7