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COMP 3331/9331
Assignment T1 2022
Online Discussion Forum
All details are in the specification
• Information about deadlines, file names, submission instructions,
marking guidelines, example interactions and various other specifics
are in the specification
• Choice of programming languages: C, Java, Python
• This talk provides a high-level overview
Client Server Architecture
• Server
• Always on
• Responsible for managing the discussion forum
• Maintains state about the forum
• Implements a request/response API for interacting with client(s)
• Prints informative updates at the terminal
• Client
• Allows a user to interact with the discussion forum
• Interaction is through the command line terminal
• Transport Protocol
• UDP (for most interactions) + TCP (only for implementing the file transfer aspects of
UPD and DWN commands)
• Client(s) and Server executed on the same machine in different terminals
• Server
• Command line arguments:
• Server port (use a value greater than 1023 and less than 65536)
• same port number for UDP and TCP as they are distinct ports
• Executed first – waits for client(s) to connect
• Forum is empty at start-up – no threads, no messages, no files, no users logged in
• Server will keep executing forever
• Client
• Command line arguments:
• Server port number (should match the first argument for the server)
• Let the OS pick an ephemeral port
• Client should send UDP segments to (“”, server port) and initiate TCP connection
(for file transfer) to (“”, server port)
Two Configurations
• One client at a time (14 marks – 70% of the assignment marks)
• At any given time only one client will interact with the server
• Simple sequential interaction between client and server
• Should not require multi-threading
• Multiple clients can connect sequentially – i.e., one client connects, executes some
commands, quits, second client connects, executes commands, quits, etc.
• You will have to define and manage a number of data structures (more on this later)
• You must design your own application layer protocol
• This includes the syntax/semantics of the messages exchanged between the client and server
and the actions to be taken upon receiving each message
• Non-CSE Students may chose to only implement functionality for this configuration
• MUST request approval – check spec for details
• Note that the server will not be stopped between tests for the two configurations
Two Configurations
• Concurrent multiple clients (6 marks – 30% of the assignment marks)
• Client program will very likely require no changes
• Server program must interact with multiple clients
• We recommend a multi—threaded approach
• Many implementation approaches possible – we do not mandate a particular approach
• Possible for multiple threads to communicate using a single UDP socket, so no need to create
multiple sockets
• The interactions with an individual client should still be sequential as in the first configuration
• However, the discussion forum state is now updated by multiple clients
• Client A adds a new message to a thread. Following this, when Client B reads the thread, client A’s message
should be viewable
• You may assume that each interaction (10 commands + authentication) with a client is “atomic”
• E.g.: Client A initiates an interaction to add a new message to a thread. While the server is processing this
interaction, it cannot be interrupted by a command from another client (B). Client B’s command will be
acted upon after the command from client A is processed
• For UPD and DWN – this means the server can either be sending/receiving a file to/from one client at a
given time, thus only needs to manage one TCP connection at any given time
• You must be particularly careful about managing multiple threads and how they access the
shared state (data structures, files, etc.) for the message board
User Authentication
• A credentials file will be available in the server working directory
(example is on the assignment page)
• The client will first authenticate the user
• Ask user to enter username
• If username matches with one of the entries in the credentials file, the user is
requested to enter the password
• If password matches, authentication successful
• If password doesn’t match, the user is asked to enter username again (no limit on
number of attempts)
• If username does not match it is assumed this is a new user and user is asked
to enter a new password
• User is authenticated and a new entry (username & password) is created in the
credentials file
Discussion Forum Operations
• Should support 10 commands
• User is prompted to enter one of the commands
• Basic error checking for syntax, appropriate arguments, etc. should be
• NOTE: We are more interested in observing if you have implemented the “normal”
functionality, so our tests for error checking will be limited and won’t include bizarre edge
• Some of these errors can be detected on the client-side and will not require communication
with the server
• 8 commands excluding UPD and DWN use UDP for communication
• UPD and DWN – only the file transfer should happen over TCP, other interactions
over UDP
• UDP segments may be lost, so your application must implement mechanisms to
deal with occasional packet loss
• Keep it simple, use ideas discussed in the reliable data delivery lecture
• CRT – Create thread
• CRT threadtitle
• Create a new thread – represented as a text file (threadtitle), update state
information about active threads and thread creators
• MSG – Post message
• MSG thread message
• Add message to the thread – update the thread file (message number,
username, message)
• DLT – Delete message
• DLT thread messagenumber
• A message can only be deleted by the original creator (error if condition not
• Delete message – update the thread file (remove the message and update
message numbers if required)
• EDT – Edit message • EDT threadtitle messagenumber message
• A message can only be edited by the original creator (error if condition not met)
• Edit the thread – update the thread file (only update message)
• LST – List threads • LST
• List all active threads
• Read data structure that stores this information and send it to client
• RDT – Read thread • RDT threadtitle
• Display all messages contained in thread
• Read the corresponding thread file and send relevant information to client
• RMV – Remove thread
• RMV threadtitle
• A thread can only be removed by the original creator (error if condition not
• Remove all state associated with the thread (thread file, uploaded files)
• XIT – Exit
• User quits
• Update state about active users
• UPD – Upload file • UPD threadtitle filename
• Use UDP for all communication except for file transfer which should use TCP
• Create filename in server working directory
• Server opens a welcome socket (@ server port – UDP and TCP ports are distinct)
• Client should initiate TCP connection with server
• Transfer file over TCP connection
• Close TCP connection
• All file data received from client stored in the file in the serve working directory
• Update thread file with information about file (username uploaded filename)
• DWN – Download file • DWN threadtitle filename
• Create filename in client working directory and store all file data received from server
• As with UPD use UDP for all communication except file transfer which should use TCP
• Follow the steps similar to UPD
Data Structures
• Server must maintain certain state information about the forum
including number of threads, thread creators, messages per thread,
message creators, online users, etc.
• Server will also have to manage several files – one for each thread
and files uploaded by users
• Proper design of data structures is essential to ensure all functionality
can be achieved
• Be careful about accessing the data structures across multiple threads
• Avoid arbitrary upper limits for variables
• Dynamic memory allocation is recommended
How to start and progress
• Focus on first configuration
• Get the client and server to exchange UDP segments
• Implement authentication
• Implement one command, test it thoroughly, move to the next
• Suggest the following order: XIT, CRT, LST, MSG, RDT, EDT, DLT, RMV
• Get the client to setup a TCP connection with the server and test data transmission over the
• Implement UPD, DWN with the file transfer happening over TCP
• Once all individual commands are done and tested, then make sure you test extensive interaction
with a single client (meaningful combination of commands)
• Next progress to second configuration – multi-threading
• Added benefit – CSE accounts are backed up
• Program design
• Data structures
• Details of the application layer protocol
• Trade-offs considered
• Point out issues if program does not work under certain
• Refer to all borrowed code
• Test, Test, Test
• Server and client(s) executing on same machine but different working directories
• Emphasis on correct behaviour
• Basic error checking
• MUST Test In VLAB environment and through command line
• If we cannot run your code, then we cannot award you any marks
• Your assignment will be MANUALLY marked by your tutors
• Client and server must display meaningful messages at the terminal
• Extensive examples are available at the end of the spec
• You are NOT required to use the exact same text
• Detailed marking rubric is available in specification
• Both parties involved are considered guilty
HOSTING SERVICE (e.g., Github)
• If caught
• You will receive zero marks (and there may be further repercussions if this is
not your first offence)
• Your name will be added to the school plagiarism register
• Every term I have had to do the above for a small group of students
• Would be nice to not have to do it in this term
Non-CSE Students
• Must be enrolled in a non-CSE program (double degree that includes
a CSE major does not qualify)
• Can opt for non-CSE option
• MUST request permission to do so by 5pm, 18th March by emailing
• Many program snippets are on the web page
• Including multi-threading code snippets
• Your socket programming experience in Labs 2 and 3 will be useful
• Repository of resources is here
Seeking Help
• Assignment specific consults (for all 3 programming languages) from
Weeks 7-10
• Schedule to be announced in a few days
• Course message forum
• Read posts from other students before posting your question
• Read the spec – very often your answer will be in there

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