Programming Assignment 1: Socket Programming


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CSEE 4119: Computer Networks
Programming Assignment 1: Socket Programming

1. Introduction
In this assignment, starting from the simplest chat functionalities, you will develop more
comprehensive servers for messaging applications, including elementary security features and
current state of the chat­room. This will require that various processes in different machines
are able to work with each other, and recuperate in case of asynchronous messaging and
failure. This chat program is based on a client server model consisting of one chat server and
multiple chat clients over TCP connections. The server is mainly used to authenticate the chat
clients and direct the chat messages to one another. Besides, the server also has to support
certain commands that the clients can use. Detailed specifications of the functionalities are
given under Section 2.
2. Specifications
a. You should write your program in one of the following languages
• C/C++
• Java
• Python
b. You should write a server program (named for instance) and a client
program (named for instance). The server program will be run first
followed by multiple instances of the client program (Each instance supports one
client). They will be run from the terminals on the same and/or different hosts.
c. Server program
• You should have a file “user_pass.txt” that contains the valid combination of
usernames and passwords that it can use to authenticate the clients (or client
programs). In each line, the first term is the username and the second term is the
password. Use the following usernames­passwords to populate your file.
columbia 116bway
seas summerisover
csee4119 lotsofassignments
foobar passpass
windows withglass
google partofalphabet
facebook wastetime
wikipedia donation
network seemsez
• The server program will be invoked as
% Server <server_port_no
Ex: java Server 4119
python 4119
• When starting up, the server reads a list of username­password combinations from
“user_pass.txt” and then listens on a given port (4119 in the above example) and
waits for clients to connect.
• Authentication
o When a new chat client requests for a connection, the server should prompt the
client to input his username and password and should authenticate the user
(using the username­password combinations from “user_pass.txt”).
o If the combination is correct, the chat client should be “logged in” and a
welcome message may be displayed.
o If the password is incorrect, the server should ask the user to try again until
there are 3 consecutive failures (The number of consecutive failures should be
able to be changed as well). In this case, the server should drop this connection
and block access only for this user from the failed attempt IP address for 60
seconds. Please define 60 seconds using the variable “BLOCK_TIME” (we
will be changing this value while testing your code!).
• Commands
o After the client is logged in, the server should support the commands presented
in Table 1. An example is presented in the Appendix.
o If the server cannot recognize some command, an error message should be
Table 1: Commands
Command Functionality
whoelse Displays name of other connected users
wholast <number Displays name of those users connected within the last <number
minutes. Let 0 < number < 60
broadcast message <message Broadcasts <message to all connected users
broadcast user <user <user
… <user message <message
Broadcasts <message to the list of users
message <user <message Private <message to a <user
logout Log out this user.
d. Client program
• The client program will be invoked as
% Client <server_IP_address<server_port_no
Ex: java Client 4119
python 4119
• User should be able to connect to a given server and login themselves by entering
a valid username and password.
• For the sake of simplicity, prohibit concurrent duplicate users. Ex.: While the
user columbia is already logged in, make sure that columbia cannot log in from
another console.
• After logging in, users should be able to give any of the commands specified in
Table 1 to the server. Your program should be able to display what the server
responds at the terminal.
o A client must be able to view other users who are currently online
o A client must be able to view other users who were online anytime in the
past one hour (0­60 minutes).
o The client should be able to send and receive private messages with other
clients VIA the SERVER. If the user is not online, simply discard the
o A client can broadcast a message to all logged in users at any time. All
other logged in clients should be able to view the broadcast message at
their terminal instantly.
o Since this is a simple chat program, all messages may be displayed in same
console only. A separate console (or UI) for each private messaging is not
o When all work is done, user should be able to logout from the server.
o Also, if a client is inactive (i.e. the client has not issued any command) for
30 minutes, the server should automatically log this user out. Please define
30 minutes using the variable “TIME_OUT”.
o The client and server should accept the keyboard input ‘Control + C’ as a
signal to exit the program. The programs should exit gracefully upon this
o <username, <password shall not contain spaces or newlines, but
<message may contain anything except newlines
o When a user is blocked, this means we drop the connection and prevent
any new connections being made from the IP Address of the user for the
specified BLOCK_TIME for that user, but any other logins made from that
IP Address should be allowed.
o On a duplicate user login and we reject the login, this does not count as a
failed login attempt.
e. Any innovative and useful features can be developed. This is your chance for earning
some for extra credit! (One such example is implementing offline messaging. If the
receiving user of a private message is not online, the server can store the message and
deliver it to the receiver when the user comes online).
NOTE: All messages between clients MUST be sent VIA the SERVER.
Submission will be done on courseworks. Please post a single <UNI_< (Ex. file to the Programming Assignment 1 folder. The file should include the
• README.txt: This file should contain the following.
a. A brief description of your code
b. Details on development environment
c. Instructions on how to run your code
d. Sample commands to invoke your code
e. Description of any additional functionalities and how they should be
• Makefile: The makefile is used to build your application. Please try to keep this as
simple as possible. If you don’t know how to write a makefile, read this quick tutorial
• Source code and related files (like user_pass.txt). (Make sure your code is
4. Testing, Sample Run, and Grading
• Please make sure that your programs compile and run correctly. To grade your
program, we will extract the received zip file, type make and run the programs using
the same syntax given in Section 2. Please make sure that your submission
successfully passes this simple procedure.
• Name the executable programs as Server and Client, names being case sensitive.
• TA’s will compile and test the code you submit on CLIC machines. It is your
responsibility to make sure your code is able to be compiled and runnable on CLIC
The CLIC lab has following setup: Java 1.6, gcc version 4.6.3, python 2.7.3
• Do not simply submit the entire eclipse/your favorite project folder. Submit only the
relevant files.
• Although we mainly observe the functionality and correctness of your program, we
might examine your source code a little. Too poorly structured code or
human­unreadable code may result in a score penalty
Here is a sample run showing how we will be testing your code:
Terminal 1
java Server 4119 // You don’t have to use Java, and 4119 here is just an
// example. We may test on other unoccupied ports
Terminal 2
java Client 4119 // 4119 is the port that server listens on
// and is the IP address of the server
// program
Username: network // After connecting to server, your program should
// prompt “Username: ” and wait for user to put in
// the uername. Here our user is “network”
Password: seemsez // Again your program should prompt “Password: ” and
// wait for user to put in the password. Since this is a
// introductory assignment, the password displayed as
// plain text is acceptable
Welcome to simple chat server! // Welcome message from the server acknowledging
// that the user has logged in
Command: // From now on your server should respond to
// commands specified in Table 1
5. Appendix: Example
Consider the following scenario
4 clients ­ facebook, apple, wikipedia and network are currently logged in.
1 client ­ windows was logged in, but logged out more than an hour back.
1 client ­ google was logged in, but logged out half an hour back.
The following should happen on entering the given commands on the terminal of facebook.
• whoelse
Since whoelse should print all the other users currently logged in, the following
should be displayed on the terminal of facebook.
• wholast 32
Since wholast <number should print all the users who were logged in during the last
<number minutes, the following should be displayed on the terminal of facebook
• broadcast message hello world
The following should be printed on all the terminals of all the users who are currently
logged in, i.e., wikipedia, apple and network.
facebook: hello world
• broadcast user wikipedia apple message hello world
The following should be printed on the terminals of all the users who are listed, i.e.,
wikipedia and apple.
facebook: hello world
• message wikipedia hi
The following should be printed on the terminal of wikipedia
facebook: hi
• logout
facebook should be logged out and the client program supporting this user should
Grading rubric
Functionality Max Points awarded /
Basic client and server programs with following functionality as
per the specifications:
­ Successful log in
­ Multiple clients support (from multiple machines)
­ Implementation of whoelse
­ Implementation of logout
Correct implementation of wholast 10
Correct implementation of broadcast 10 (5+5)
Correct implementation of private messaging 15
Automatic “logout” of the client after 30 minutes of inactivity 10
Graceful exit of client and server programs using control + c 5
Code quality 5
Extra feature: The feature should be useful and grading will be
done on its innovation, utility and amount of effort.
Max 10 points per extra
feature. On TA discretion.
Total max extra feature points 20

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