Lab 2 Solution. Jon Turner. CSE 473 Introduction to Computer Networks. (20 points) Place a copy of your source code for TcpMapServer here.

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1 CSE 473 Introduction to Computer Networks Lab 2 Solution Jon Turner (20 points) Place a copy of your source code for TcpMapServer here. This program implements a TCP MapServer that stores (key,value) strings. usage: MapServer [ ipaddress [ portnumber ] ] There are two optional arguments. The first is the ip address of the server's listen socket and the second is a port number for the listen socket. If the ip address is omitted, it defaults to the default address for the server. If the port number is omitted, it defaults to The server expects to receive TCP connections and then commands that store and retrieve (key,value) pairs, where both the key and the value are strings. There are three commands, get, put and get all. Get takes a single argument (the key) and put can take either one or two arguments (key,value). If only the key is provided, the mapping for that key is removed. If both key and value are provided, a new mapping is created, possibly replacing an existing pair. Each command is on a line by itself, with key and value strings enclosed in quotes. The get all command returns all (key,value) pairs on a single line, separated by double-colons (::). For example, to perform the operation put(foo,bar), the remote client would send the line put:foo:bar To perform the operation get(who hah) the client would send a packet containing get:who hah The reply to a successful put is the line ok The reply to a succesful get is ok:string where "ok:string" is the stored value. If the key to a get does not match any stored value, the server returns no match - 1 -

2 The response to a get all command is a set of pairs separated by a pair of colon characters. For example, foo:bar::who hah:whoop di doo::tish:tosh tosh On an error, the server returns unrecongizable input: copy of the offending input import sys from socket import * process command line arguments localip = '' localport = 6789 if len(sys.argv) > 1: localip = sys.argv[1] if len(sys.argv) > 2: localport = int(sys.argv[2]) open socket and bind to port (localip,localport) serversocket = socket(af_inet, SOCK_STREAM); serversocket.bind((localip, localport)) map = {} create an empty dictionary listen for incoming connections serversocket.listen(10) while 1: wait for, then accept incoming connection connsock, clientaddress = serversocket.accept() buffer = "" while 1 : add text to the buffer until we have a complete line exit loop if client closes connection newbuf = connsock.recv(1000); if len(newbuf) == 0 : break buffer = buffer + newbuf if buffer.find('\n') < 0 : continue get the next line of input, saving the rest in buffer chunks = buffer.partition('\n') command = chunks[0] buffer = chunks[2] if command == "get all" : reply = '' for key, val in map.iteritems(): reply += key + ":" + val + "::" reply = reply[:-2] else : chunks = command.partition(':') if chunks[1]!= ':': reply = "error :unrecognizable input:" + command - 2 -

3 else: if chunks[0] == "get": if chunks[2] in map: reply = "ok:" + map[chunks[2]] else: reply = "no match" elif chunks[0] == "put": subchunks = chunks[2].partition(':') if subchunks[1] == ':': map[subchunks[0]] = subchunks[2] else: del map[subchunks[0]] reply = "ok" else: reply = "error:unrecognizable input:" \ + command reply += '\n' send reply connsock.sendall(reply) close socket and do it again connsock.close() - 3 -

4 (10 points) Place a copy of your source code for TcpMapClient here. Simple client for use with a MapServer that stores (key,value) strings. usage: MapClient servername [ serverport ] The first argument is the name or IP address of a remote host, the second is a port number used by a MapServer on that remote host. Open a stream connection to the remote MapServer, then read commands from standard input (one per line) and send them to the server. Print the output returned by the server. A blank input line causes the client to close the connection and terminate. The client does no checking for malformed input. import sys from socket import * process arguments server = sys.argv[1] port = 6789 if len(sys.argv) > 2 : port = int(sys.argv[2]) open socket and connect to remote server clientsocket = socket(af_inet, SOCK_STREAM); clientsocket.connect((server, port)) buffer = '' while 1: get command from input and send it; stop if user types blank line command = raw_input("map server: ") if len(command) == 0 : break command += '\n' clientsocket.sendall(command) read until we get a newline, then print line while buffer.find('\n') < 0 : newbuf = clientsocket.recv(1000) if len(newbuf) == 0 : print "server closed connection prematurely" sys.exit(1) buffer += newbuf chunks = buffer.partition('\n') print chunks[0] buffer = chunks[2] print '' clientsocket.close() - 4 -

5 (10 points) Use the provided localscript to test your client and server. You may do this testing on any Unix (including MacOS) or Linux computer (shell.cec.wustl.edu or onl.wustl.edu). Paste a copy of the output below. :bar :hah :world map server: error :unrecognizable input:get map server: error:unrecognizable input:foo:who map server: no match :toast is tasty map server: foo:toast is tasty::goodbye:world::who:hah map server: - 5 -

6 (15 points) In the remaining parts of the lab, you will be testing your application in ONL. Use the RLI to reserve an experimental network using the provided configuration file, cse473-lab2.onl (remember to first open an ssh connection to ONL with the tunnel required by the RLI), and commit your network. Open two separate ssh windows, one connecting to the host h4x2 and the other to the host h7x1 (remember to load the topology file first). First, start the server using the window for host h7x1. When starting the server, you will need to specify the host name (h7x1) or IP address ( ) for the host in the experimental network. Use the default port number. Run the command in the background by putting an ampersand (&) at the end of the line. This will allow you to use the window for command input, even while the server is running (read the job control section of the bash manual to learn more about running jobs in the foreground and background). Note that once you start the server, it will run forever until you stop it. One simple way to do this is to type kill %1. Note that if you have multiple jobs running in the background, you will need to substitute the appropriate job number for %1. See the bash manual for details. Now that your server is running in the background, type the following command in the window for h7x1. netstat -an grep 6789 and paste a copy of the output below. tcp : :* LISTEN Now, start the client on h4x2 (supplying the appropriate arguments) and then re-run netstat on h7x1 and paste the output below. tcp : :* LISTEN tcp : :51120 ESTABLISHED Explain the netstat output in the two cases. You should read the man page on netstat before answering this part (type man netstat to get the man page). The netstat command shows information about active sockets and the grep command filters out all lines but those containing the string So, in the first case, the output shows that there is a socket bound to port 6789 that is in the listening state; that is, it is waiting for incoming connections. The second also shows a connected socket to host h4x2 (which has the IP address ) using the remote port number This all makes sense, since before we start the client, the server is listening on the socket, while after we start the client, there is also an active connection to the client

7 Now, run the provided remotescript on h4x2. Paste the output from your run below. :bar :hah :world map server: error :unrecognizable input:get map server: error:unrecognizable input:foo:who map server: no match :toast is tasty map server: foo:toast is tasty::goodbye:world::who:hah map server: - 7 -

8 (10 points) In this part, you are to re-run the remotescript and take a screen capture of the two monitoring windows showing the traffic that results from running the remotescript. You can pause a monitoring window by selecting Stop from its Options menu. This makes it easier to do the screen capture. The charts should show a burst of traffic for some of the curves and no traffic for others. Which curves show a burst of traffic? Is this consistent with what you expect? Note that there are two possible routes between the two end hosts. Which of the two routes are used in this case? The top chart shows traffic leaving output 1 of router 1 and output 0 of router 2. This is consistent with traffic taking the direct link between the two routers (in both directions). The bottom chart shows traffic leaving host h4x2 and arriving at host h7x1. None of the other curves show any traffic. This is consistent with what we expect. The value on the curve shows a data rate of nearly 60 packets per second. The script only has 12 commands, so we would expect to see a total of about 12 packets in each direction. Because the burst occurs within a fraction of a second, the rate shown on the chart is a little bit misleading in this case

9 (10 points) In this next part, you are to run remotescript once again, but this time, you will be using the utility tcpdump to capture packets as seen at both hosts. Unfortunately we cannot use Wireshark for this purpose, because you cannot run Wireshark directly on the ONL computers. What we can do is to capture the packets using tcpdump, saving the results in a pcap file. We can then use Wireshark to examine the pcap file, just as we did when doing live captures in lab 1. Before going any further, read the man page for tcpdump. Then run tcpdump, on the machine where the server is running, by typing the following command into a shell window on h7x2. sudo /usr/sbin/tcpdump -i data0 -w servertrace.pcap tcp port 6789 Make sure you understand the purpose of all the arguments. Next, run tcpdump on the client machine, by typing sudo /usr/sbin/tcpdump -i data0 -w clienttrace.pcap tcp port 6789 To terminate tcpdump, type ctrl-c in the window where tcpdump is running (if it is running in the background, type the command fg first, then ctrl-c). You should now have two pcap files, which you can transfer to any computer with Wireshark installed, then you can use Wireshark to examine the captured packets

10 Open the file for the server-side capture and highlight the packet containing the command put:who:hah. Paste a copy of the Wireshark window below. How many microseconds pass between the arrival of the packet and the transmission of the reply? =27 µs

11 Open the client-side capture file in Wireshark and highlight the same packet. Paste a copy of the Wireshark window for the client below. How many microseconds pass between the time the client sends the packet containing the command and the time it gets the reply? What does this tell you about the network delay? =100 µs. If we subtract this from the difference at the server, we get 73 µs, suggesting a one-way delay of about 36 µs

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13 (10 points) In this part, you will measure the performance of your application in another way. Run the provided longscript on the client. This performs a large number of puts and gets. Make a screen capture of the two monitoring windows showing the packet traffic in the network. Paste those windows here. What does the traffic data tell you about the performance of the application? Compare this to what you learned above from the packet capture data. The charts above show a traffic rate of nearly 10,000 packets per second in each direction. That means there is about 100 µs between each successive pair of commands. This is consistent with the packet capture data at the client