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Socket Programming Assignment 1: Web Server In this lab, you will learn the basics of socket programming for TCP connections in Python: how to create a socket, bind it to a specific address and port, as well as send and receive a HTTP packet. You will also learn some basics of HTTP header format.

You will develop a web server that handles one HTTP request at a time. Your web server should accept and parse the HTTP request, get the requested file from the server’s 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.

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Code Below you will find the skeleton code for the Web server. 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.

Running the Server 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., 128.238.251.26). From another host, open a browser and provide the corresponding URL. For example:

http://128.238.251.26:6789/HelloWorld.html

‘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 6789. 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.

What to Hand in You will hand in 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.

Skeleton Python Code for the Web Server #import socket module

from socket import *

serverSocket = socket(AF_INET, SOCK_STREAM)

#Prepare a sever socket

#Fill in start

#Fill in end

while True:

#Establish the connection

print ‘Ready to serve…’

connectionSocket, addr = #Fill in start #Fill in end

try:

message = #Fill in start #Fill in end

filename = message.split()[1]

f = open(filename[1:])

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()

Optional Exercises 1. 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.

2. Instead of using a browser, write your own HTTP client to test your server. Your client will connect to the server using a TCP connection, send an HTTP request to the server, and display the server response as an output. You can assume that the HTTP request sent is a GET method. The client should take command line arguments specifying the server IP address or host name, the port at which the server is listening, and the path at which the requested object is stored at the server. The following is an input command format to run the client.

client.py server_host server_port filename

  • Socket Programming Assignment 1: Web Server
    • Code
    • Running the Server
    • What to Hand in
    • Skeleton Python Code for the Web Server
    • Optional Exercises

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Programming Assignment 1: Building a Multi­Threaded Web Server In this lab we will develop a Web server in two steps. In the end, you will have built a multi­threaded Web server that is capable of processing multiple simultaneous service requests in parallel. You should be able to demonstrate that your Web server is capable of delivering your home page to a Web browser.

We are going to implement version 1.0 of HTTP, as defined in RFC 1945, where separate HTTP requests are sent for each component of the Web page. The server will be able to handle multiple simultaneous service requests in parallel. This means that the Web server is multi­threaded. In the main thread, the server listens to a fixed port. When it receives a TCP connection request, it sets up a TCP connection through another port and services the request in a separate thread. To simplify this programming task, we will develop the code in two stages. In the first stage, you will write a multi­threaded server that simply displays the contents of the HTTP request message that it receives. After this program is running properly, you will add the code required to generate an appropriate response.

As you are developing the code, you can test your server from a Web browser. But remember that you are not serving through the standard port 80, so you need to specify the port number within the URL that you give to your browser. For example, if your machine’s name is host.someschool.edu, your server is listening to port 6789, and you want to retrieve the file index.html, then you would specify the following URL within the browser:

http://host.someschool.edu:6789/index.html

If you omit “:6789”, the browser will assume port 80 which most likely will not have a server listening on it.

When the server encounters an error, it sends a response message with the appropriate HTML source so that the error information is displayed in the browser window.

Web Server in Java: Part A

In the following steps, we will go through the code for the first implementation of our Web Server. Wherever you see “?”, you will need to supply a missing detail.

Our first implementation of the Web server will be multi­threaded, where the processing of each incoming request will take place inside a separate thread of execution. This allows the server to service multiple clients in parallel, or to perform multiple file transfers to a single client in parallel. When we create a new thread of execution, we need to pass to the Thread’s constructor an instance of some class that implements the Runnable interface. This is the reason that we define a separate class called HttpRequest. The structure of the Web server is shown below:

import java.io.* ; import java.net.* ; import java.util.* ;

public final class WebServer {   public static void main(String argv[]) throws Exception   {     . . .   } }

final class HttpRequest implements Runnable {   . . . }

Normally, Web servers process service requests that they receive through well­known port number 80. You can choose any port higher than 1024, but remember to use the same port number when making requests to your Web server from your browser.http://www.rfc-editor.org/rfc/rfc1945.txt

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public static void main(String argv[]) throws Exception {   // Set the port number.   int port = 6789;

  . . . }

Next, we open a socket and wait for a TCP connection request. Because we will be servicing request messages indefinitely, we place the listen operation inside of an infinite loop. This means we will have to terminate the Web server by pressing ^C on the keyboard.

// Establish the listen socket.        ?

// Process HTTP service requests in an infinite loop. while (true) {   // Listen for a TCP connection request.   ?

  . . . }

When a connection request is received, we create an HttpRequest object, passing to its constructor a reference to the Socket object that represents our established connection with the client.

// Construct an object to process the HTTP request message. HttpRequest request = new HttpRequest( ? );

// Create a new thread to process the request. Thread thread = new Thread(request);

// Start the thread. thread.start();

In order to have the HttpRequest object handle the incoming HTTP service request in a separate thread, we first create a new Thread object, passing to its constructor a reference to the HttpRequest object, and then call the thread’s start() method.

After the new thread has been created and started, execution in the main thread returns to the top of the message processing loop. The main thread will then block, waiting for another TCP connection request, while the new thread continues running. When another TCP connection request is received, the main thread goes through the same process of thread creation regardless of whether the previous thread has finished execution or is still running.

This completes the code in main(). For the remainder of the lab, it remains to develop the HttpRequest class.

We declare two variables for the HttpRequest class: CRLF and socket. According to the HTTP specification, we need to terminate each line of the server’s response message with a carriage return (CR) and a line feed (LF), so we have defined CRLF as a convenience. The variable socket will be used to store a reference to the connection socket, which is passed to the constructor of this class. The structure of the HttpRequest class is shown below:

final class HttpRequest implements Runnable {   final static String CRLF = “\r\n”;   Socket socket;

  // Constructor   public HttpRequest(Socket socket) throws Exception    {     this.socket = socket;   }

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  // Implement the run() method of the Runnable interface.   public void run()   {     . . .   }

  private void processRequest() throws Exception   {     . . .   } }

In order to pass an instance of the HttpRequest class to the Thread’s constructor, HttpRequest must implement the Runnable interface, which simply means that we must define a public method called run() that returns void. Most of the processing will take place within processRequest(), which is called from within run().

Up until this point, we have been throwing exceptions, rather than catching them. However, we can not throw exceptions from run(), because we must strictly adhere to the declaration of run() in the Runnable interface, which does not throw any exceptions. We will place all the processing code in processRequest(), and from there, throw exceptions to run(). Within run(), we explicitly catch and handle exceptions with a try/catch block.

// Implement the run() method of the Runnable interface. public void run() {   try {     processRequest();   } catch (Exception e) {     System.out.println(e);   } }

Now, let’s develop the code within processRequest(). We first obtain references to the socket’s input and output streams. Then we wrap InputStreamReader and BufferedReader filters around the input stream. However, we won’t wrap any filters around the output stream, because we will be writing bytes directly into the output stream.

private void processRequest() throws Exception {   // Get a reference to the socket’s input and output streams.   InputStream is = ?;   DataOutputStream os = ?;

  // Set up input stream filters.   ?    BufferedReader br = ?;

  . . . }

Now we are prepared to get the client’s request message, which we do by reading from the socket’s input stream. The readLine() method of the BufferedReader class will extract characters from the input stream until it reaches an end­of­line character, or in our case, the end­of­line character sequence CRLF.

The first item available in the input stream will be the HTTP request line. (See Section 2.2 of the textbook for a description of this and the following fields.)

// Get the request line of the HTTP request message. String requestLine = ?;

// Display the request line. System.out.println(); System.out.println(requestLine);

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After obtaining the request line of the message header, we obtain the header lines. Since we don’t know ahead of time how many header lines the client will send, we must get these lines within a looping operation.

// Get and display the header lines. String headerLine = null; while ((headerLine = br.readLine()).length() != 0) {   System.out.println(headerLine); }

We don’t need the header lines, other than to print them to the screen, so we use a temporary String variable, headerLine, to hold a reference to their values. The loop terminates when the expression

(headerLine = br.readLine()).length()

evaluates to zero, which will occur when headerLine has zero length. This will happen when the empty line terminating the header lines is read. (See the HTTP Request Message diagram in Section 2.2 of the textbook)

In the next step of this lab, we will add code to analyze the client’s request message and send a response. But before we do this, let’s try compiling our program and testing it with a browser. Add the following lines of code to close the streams and socket connection.

// Close streams and socket. os.close(); br.close(); socket.close();

After your program successfully compiles, run it with an available port number, and try contacting it from a browser. To do this, you should enter into the browser’s address text box the IP address of your running server. For example, if your machine name is host.someschool.edu, and you ran the server with port number 6789, then you would specify the following URL:

http://host.someschool.edu:6789/

The server should display the contents of the HTTP request message. Check that it matches the message format shown in the HTTP Request Message diagram in Section 2.2 of the textbook.

Web Server in Java: Part B

Instead of simply terminating the thread after displaying the browser’s HTTP request message, we will analyze the request and send an appropriate response. We are going to ignore the information in the header lines, and use only the file name contained in the request line. In fact, we are going to assume that the request line always specifies the GET method, and ignore the fact that the client may be sending some other type of request, such as HEAD or POST.

We extract the file name from the request line with the aid of the StringTokenizer class. First, we create a StringTokenizer object that contains the string of characters from the request line. Second, we skip over the method specification, which we have assumed to be “GET”. Third, we extract the file name.

// Extract the filename from the request line. StringTokenizer tokens = new StringTokenizer(requestLine); tokens.nextToken();  // skip over the method, which should be “GET” String fileName = tokens.nextToken();

// Prepend a “.” so that file request is within the current directory. fileName = “.” + fileName;

Because the browser precedes the filename with a slash, we prefix a dot so that the resulting pathname starts within the current directory.

Now that we have the file name, we can open the file as the first step in sending it to the client. If the file does not exist, the FileInputStream() constructor will throw the FileNotFoundException. Instead of throwing this

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possible exception and terminating the thread, we will use a try/catch construction to set the boolean variable fileExists to false. Later in the code, we will use this flag to construct an error response message, rather than try to send a nonexistent file.

// Open the requested file. FileInputStream fis = null; boolean fileExists = true; try {   fis = new FileInputStream(fileName); } catch (FileNotFoundException e) {   fileExists = false; }

There are three parts to the response message: the status line, the response headers, and the entity body. The status line and response headers are terminated by the character sequence CRLF. We are going to respond with a status line, which we store in the variable statusLine, and a single response header, which we store in the variable contentTypeLine. In the case of a request for a nonexistent file, we return 404 Not Found in the status line of the response message, and include an error message in the form of an HTML document in the entity body.

// Construct the response message. String statusLine = null; String contentTypeLine = null; String entityBody = null; if (fileExists) {   statusLine = ?;   contentTypeLine = “Content‐type: ” +      contentType( fileName ) + CRLF; } else {   statusLine = ?;   contentTypeLine = ?;   entityBody = “<HTML>” +      “<HEAD><TITLE>Not Found</TITLE></HEAD>” +     “<BODY>Not Found</BODY></HTML>”; }

When the file exists, we need to determine the file’s MIME type and send the appropriate MIME­type specifier. We make this determination in a separate private method called contentType(), which returns a string that we can include in the content type line that we are constructing.

Now we can send the status line and our single header line to the browser by writing into the socket’s output stream.

// Send the status line. os.writeBytes(statusLine);

// Send the content type line. os.writeBytes(?);

// Send a blank line to indicate the end of the header lines. os.writeBytes(CRLF);

Now that the status line and header line with delimiting CRLF have been placed into the output stream on their way to the browser, it is time to do the same with the entity body. If the requested file exists, we call a separate method to send the file. If the requested file does not exist, we send the HTML­encoded error message that we have prepared.

// Send the entity body. if (fileExists) {   sendBytes(fis, os);   fis.close(); } else {   os.writeBytes(?);

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}

After sending the entity body, the work in this thread has finished, so we close the streams and socket before terminating.

We still need to code the two methods that we have referenced in the above code, namely, the method that determines the MIME type, contentType(), and the method that writes the requested file onto the socket’s output stream. Let’s first take a look at the code for sending the file to the client.

private static void sendBytes(FileInputStream fis, OutputStream os)  throws Exception {    // Construct a 1K buffer to hold bytes on their way to the socket.    byte[] buffer = new byte[1024];    int bytes = 0;

   // Copy requested file into the socket’s output stream.    while((bytes = fis.read(buffer)) != ‐1 ) {       os.write(buffer, 0, bytes);    } }

Both read() and write() throw exceptions. Instead of catching these exceptions and handling them in our code, we throw them to be handled by the calling method.

The variable, buffer, is our intermediate storage space for bytes on their way from the file to the output stream. When we read the bytes from the FileInputStream, we check to see if read() returns minus one, indicating that the end of the file has been reached. If the end of the file has not been reached, read() returns the number of bytes that have been placed into buffer. We use the write() method of the OutputStream class to place these bytes into the output stream, passing to it the name of the byte array, buffer, the starting point in the array, 0, and the number of bytes in the array to write, bytes.

The final piece of code needed to complete the Web server is a method that will examine the extension of a file name and return a string that represents it’s MIME type. If the file extension is unknown, we return the type application/octet‐stream.

private static String contentType(String fileName) {   if(fileName.endsWith(“.htm”) || fileName.endsWith(“.html”)) {     return “text/html”;   }   if(?) {     ?;   }   if(?) {     ?;   }   return “application/octet‐stream”; }

There is a lot missing from this method. For instance, nothing is returned for GIF or JPEG files. You may want to add the missing file types yourself, so that the components of your home page are sent with the content type correctly specified in the content type header line. For GIFs the MIME type is image/gif and for JPEGs it is image/jpeg.

This completes the code for the second phase of development of your Web server. Try running the server from the directory where your home page is located, and try viewing your home page files with a browser. Remember to include a port specifier in the URL of your home page, so that your browser doesn’t try to connect to the default port 80. When you connect to the running web server with the browser, examine the GET message requests that the web server receives from the browser.

Computer Networking MCIS 6163 Project 1

Simple Web Server & Client

Instructor: Sajib Datta

Fall 2021

“What I cannot create, I do not understand.” Richard P Feynman

Objectives (A) To understand Client-Server communication via sockets

(B) To gain exposure to the basic operations of a Web Server and Client

(C) To explore basic structures of HTTP messages

Due Date

November 5, 2021 11:59 PM1

Project Description (A) You will be developing a multi-threaded Web server which interacts with

any standard Web Clients ( You may use any web browser of your choice to test the functionality however you should also submit the a client as given in (B) below ). The Web server and Web client communicate using a text-based protocol called HTTP (Hypertext Transfer Protocol)

(B) Build a single threaded Web Client on your own which interacts with your Web Server, and downloads a file from the server

(C) Display the essential connection parameters of connection for both the Web client ( on the server side ) and for the Web Server ( on the client side )

1 All Submissions should be completed through BlackBoard

1

Guidelines and Requirements MCIS 6163 Project 1

Specification

Specifications – Server The server being multi-threaded, should be able to handle multiple requests concurrently. The main thread ( server ), listens to a specified port like the standard port for HTTP (8080). Upon receiving a HTTP request, the server sets up a TCP connection to the requesting client and serves the request in a separate thread using a new port. After sending the response back to the client, it closes the connection. For this exercise you may choose any browser of your choice for testing. ( Internet Explorer or FireFox or Chrome )2. However you should submit a client program as per the the Section Specifications – Client.

The server is assumed to work with HTTP GET messages. If the requested file exists at the server, it responds with a “HTTP/1.1 200 OK” together with the requested page to the client, otherwise it sends a corresponding error message, “HTTP/1.1 404 Not Found” or “HTTP/1.1 400 Bad Request”.

• If running the server program using command line, the syntax should be

server_code_name < port_number >

• You must test your Web server implementation on your local machine using a Web browser. You need to specify the used port number within the URL. If omitting the port number portion, i.e., 8080, the browser should use the default port 8080. To cite an example,

http ∶ //localhost ∶ 8080/index.html

• You should display/log the request and header lines of request messages on the server for the purpose of debugging.

Specifications – Client • The client should be able to initiate a connection to the server, via a socket

and request any page on the server. Upon receipt of the response message from the server, the client extracts and displays/logs the message status3, and then retrieves the page content from the corresponding message body.

• The requested file need not be HTML, even a text file would suffice 4.

• You may execute the client program using command line, with the follow- ing syntax,

2 Caveat: Some of the browsers need some additional setting changes for enabling complete functionality

3 ’HTTP 200 OK’ or ’404 Bad Request’ 4 But the format of the request should strictly be HTTP as discussed in class

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Guidelines and Requirements MCIS 6163 Project 1

client_code < server_IPaddress >< port_no >< requested_file_name >

(a) Server_IPaddress: The IP address or name of the Web server, e.g., 127.0.0.1 or localhost for the server running on the local machine.

(b) port_no: The port on which the server is listening to contnections from clients. If the port number is not entered, the default port 8080 should be used.

(c) requested_file_name: The name of the requested file, which may include the path to the file.

Specifications – Connection Parameters You should be able to extract the following information from the connection objects,

(a) Calculate and Display RTT for the client request5.

(b) Print the relevant server details on client side. The examples could be Host Name of the server, socket family, socket type, protocol, timeout and get peer name 6.

(c) Print the relevant client details on server side. The examples could be Host Name of the client, socket family, socket type, protocol, timeout and get peer name 7.

Notes (a) This is an individual project.

(b) You can use the programming language of your choice 8.

(c) You may use the skeleton code for the server provided in the textbook’s companion website for reference. You may also want to refer to the text- book, chapter 2, section 2.2.3, for more details on HTTP message format and section 2.7, for socket programming.

(d) The source codes should be well documented to make it easier for the GRADER to follow.

5 Refer Slide 2 – 25 of class lecture 6 Print a minimum of 4 out of 6 7 Print a minimum of 4 out of 6 8 You may get more help with Java or Python. Our best choice for you will be Python

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Guidelines and Requirements MCIS 6163 Project 1

Submission Guidelines • Submit a single zipped file with the naming convention,

< your_SAU_id > _ < your_name > .zip

• Your submission should have the following items to be considered for eval- uation,

(a) Source codes of the Web server and client (b) Any additional files required to run your codes (c) –Very Important– readme.txt file with instructions on how to

compile and run your codes. You must mention the IDE as well as any packages that are required to run the codes.

(d) –Very Important– Provide ample amount of comments in the code to make it more readable and sustainable.

• Do NOT include any runnable executable (binary) program.

• Make sure your name and your SAU ID are also listed in the readme file and in comments at the beginning of your source files.

• Make sure that submissions of the zipped file is through BlackBoard9.

• No Late submission will be accepted.

Additional Requirements/Instructions (a) Please email your Instructor for any doubts and clarifications regarding the

project 1.

(b) Complete documentation and instructions for running the codes are rec- ommended.

(c) If you are using any code from some external source or book, you MUST mention it explicitly in the codes as well as the readme file. Otherwise, it will be considered plagiarism and your project will not be evaluated.

9 Please strictly follow the naming convention of the zipped file

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Guidelines and Requirements MCIS 6163 Project 1

(d) You can discuss with other classmates on steps/algorithms to implement the project. However, the source codes must be written by your- self .

Grading Rubric (25 points) (i) The server works correctly with requests from a Web browser (3.5 points)

(ii) The server can serve multiple requests at the same time (multithreaded implementation) (4 points)

(iii) The client sends/receives messages to/from the server correctly (5 points)

(iv) The client extracts the status and content of messages from the server correctly (4 points)

(v) Extracting and displaying connection parameters (1.5 points)

(vi) Calculate and Display Round Trip Time (RTT). (2.5 points)

(vii) Proper closing of the ports with exception handling. (2 points)

(viii) Display/log of proper messages on the server as well as on the client. (1.5 points)

(ix) Code documentation and Readme file. (1 points)

Wish you all a good luck

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