Problem Set 4 Due: Start of class, October 5

Transcription

1 CS242 Computer Networks Handout # 8 Randy Shull September 28, 2017 Wellesley College Problem Set 4 Due: Start of class, October 5 Reading: Kurose & Ross, Sections 2.5, 2.6, 2.7, 2.8 Problem 1 [4]: Free-riding Suppose Bob joins a BitTorrent torrent, but he does not want to upload any data to any other peers (so called freeriding). a [2]: Bob claims that he can receive a complete copy of the file that is shared by the swarm. Is Bob s claim possible? Why or why not? b [2]: Bob also claims that he can make his free-riding more efficient by using a collection of multiple computer (with distinct IP addresses) in the computer lab in his department. ow can he do that? Problem 2 [2]: UDP versus TCP The UDP server described in lecture and the text needed only one socket, whereas the TCP server needed two sockets. Why? If the TCP server were to support n simultaneous connections, each from a different client host, how many sockets would the TCP server need? Problem 3 [5]: Sockets Image that you compiled and installed the programs 1 TCPClient and UDPClient on one host and TCPServer and UDPServer on another host. a [2]: Client first TCP Suppose you run TCPClient before you run TCPServer. What happens? Why? b [2]: Client first UDP Suppose you run UDPClient before you run UDPServer. What happens. Why? c [1]: Port numbers What happens if you use different port numbers for the client and server sides. 1 As discussed in lecture. 1

2 Problem 4 [4]: clientsocket.bind Suppose that in the UDPClient.py, after we create the socket, we add the line: clientsocket.bind((, 5432)) Will it become necessary to change the UDPServer.py? What are the port numbers for the sockets in UDPClient and UDPServer? What were they before making this change? Problem 5 [12] Develop a web server in Python that handles one HTTP request at a time. Your web server should accept and parse a single HTTP request, get the requested file from the servers file system, create an HTTP response message consisting of the requested file preceded by header lines, and then send the response directly to the client. If the requested file is not present in the server, the server should send an HTTP 404 Not Found message back to the client. Skeleton code for the Web server is given below. You are to complete the skeleton code. The places where you need to fill in code are marked with Fill in start and Fill in end. Each place may require one or more lines of code. Put an HTML file (e.g., HelloWorld.html) in the same directory that the server is in. Run the server program. Determine the IP address of the host that is running the server (e.g., ). From another host, open a browser and provide the corresponding URL. For example: HelloWorld.html is the name of the file you placed in the server directory. Note also the use of the port number after the colon. You need to replace this port number with whatever port you have used in the server code. In the above example, we have used the port number The browser should then display the contents of HelloWorld.html. If you omit :6789, the browser will assume port 80 and you will get the web page from the server only if your server is listening at port 80. Then try to get a file that is not present at the server. You should get a 404 Not Found message. Hand in a copy of the complete server code along with the screen shots of your client browser, verifying that you actually receive the contents of the HTML file from the server. 2

3 Skeleton Python Code for the Web Server #import socket module from socket import * # Create a TCP server socket # (AF_INET is used for IPv4 protocols) # (SOCK_STREAM is used for TCP) serversocket = socket(af_inet, SOCK_STREAM) # Prepare a server socket # Fill in start # Fill in end while True: #Establish the connection print Ready to serve... # Set up a new connection from the client connectionsocket, addr = #Fill in start #Fill in end # If an exception occurs during the execution of try clause # the rest of the clause is skipped # If the exception type matches the word after except # the except clause is executed try: # Receives the request message from the client message = #Fill in start #Fill in end # Extract the path of the requested object from the message # The path is the second part of HTTP header, identified by [1] filename = message.split()[1] # Because the extracted path of the HTTP request includes # a character \, we read the path from the second character f = open(filename[1:]) 3

4 # Store the entire contenet of the requested file in a temporary buffer outputdata = #Fill in start #Fill in end #Send one HTTP header line into socket #Fill in start #Fill in end #Send the content of the requested file to the client for i in range(0, len(outputdata)): connectionsocket.send(outputdata[i]) connectionsocket.close() except IOError: #Send response message for file not found #Fill in start #Fill in end #Close client socket #Fill in start #Fill in end serversocket.close() A simple HTML page If your HTML is a bit rust, here s a very simple web page that will get the job done. 2 <HTML> <HEAD> <TITLE> My first HTML document </TITLE> </HEAD> <BODY> Hello world! </BODY> </HTML> 2 And you don t need much more cause the server will only serve up a single file. See the extra credit exercise. 4

5 Problem 6 [4]: Extra Credit Currently, the web server handles only one HTTP request at a time. Implement a multithreaded server that is capable of serving multiple requests simultaneously. Using threading, first create a main thread in which your modified server listens for clients at a fixed port. When it receives a TCP connection request from a client, it will set up the TCP connection through another port and services the client request in a separate thread. There will be a separate TCP connection in a separate thread for each request/response pair. 5