Assignment 2 Solution

You are free to write your programs using any editor/IDE. However, you can only write your program in Python 3, C/C++, or Java.

You are responsible to ensure that your programs compile and run properly on sunfire because

we will only test and grade your programs on sunfire. The screenshot on the next page shows

respective language/compiler version on sunfire.


Program Submission

For this assignment, you are required to submit one zip file that contains source programs for the sender and the receiver. Please name your zip file as <Student number>.zip, where <Student number> refers to your matric number which starts with letter A. Note that the first and last letters of your student number should be capital letters. One example file name would be A0112345X.zip. Your zip file should only contain two source programs, Alice.py and Bob.py. If you use Java, C, or C++, please replace the “.py” extension name with “.java”, “.c”, or “.cpp”, respectively. Submit this zip file to IVLE folder “Assignment2_submission”. 2 points will be deducted if you fail to follow the submission format, including wrong file naming format, folder structure, file names or extensions (for example, .rar is not acceptable).

In case of group work, please designate one person to submit the zip file, instead of submitting the same file twice by two different persons. The file name should contain the matric numbers of two members and be named as <Student number 1>-<Student number 2>.zip. An example would be A0165432X-A0123456Y.zip.

We strongly recommend that you use Python 3 for the assignment. If you use Java or C/C++, we will compile and run your program for grading, using the default compilers on sunfire (g++ 4.8.4 or java 9.0.4). For C/C++ users, we will link your program with zlib during compilation.

The grading script will infer the programming language from your file extension (e.g. .py, .java, etc). For a Java program, the class name should be consistent with the source file name, and please implement the public static main() method so that the program can be executed as a standalone process after compilation. Do not put your class under any package paths.


Grading

We will grade your programs based on their correctness only. A grading script will be used to test your programs and no manual grading will be provided.

    • [2 points] Programs can be successfully compiled on sunfire without any error. Program execution follows the specified commands exactly (see sections below). Source files are correctly named and there are no additional subfolders inside the zip file.

    • [2 points] Programs can successfully send chat messages from Alice to Bob in a perfectly reliable channel (i.e. no error at all).

    • [2 points] Programs can successfully send messages from Alice to Bob in the presence of packet corruptions.

    • [2 points] Programs can successfully send messages from Alice to Bob in the presence of packet losses.

    • [2 points] Programs can successfully send messages from Alice to Bob in the presence of both packet corruptions and packet losses.

The chat messages received by Bob must be identical to the ones sent by Alice, to claim a successful transmission. The size of input messages to Alice is guaranteed to be no larger than 5,000 bytes (including newline characters). During grading, Alice and Bob processes will be forced to terminate 60 seconds after Alice is started. The grading script will then compare the output of Bob against the input to Alice. Please ensure your programs are efficient enough to meet the above criteria.

CAUSION: The grading script grades your program according to what Bob prints out on the screen. Thus, make sure that you remove all debug output before you submit your solution. In each test case, no points will be awarded if your output does not conform with the expected output.


A Word of Advice

This assignment can be time-consuming. We suggest that you start writing programs early, incrementally and modularly. For example, deal with error-free transmission first, then data packet corruption, ACK packet corruption, and etc. Test your programs frequently and make backup copies before every major change. Frequent backups will allow you to submit at least a partially working solution that is worth some marks.


Do not post your solution in any public domain on the Internet or share it with friends, even after this semester.








Overall Architecture

There are three programs involved in this assignment, Alice, UnreliNET and Bob (see Figure 1 below). Alice will send chat messages to Bob over UDP (and Bob may provide feedback as necessary). The UnreliNET program is used to simulate the unreliable transmission channel (in both directions) that randomly corrupt or drop packets. For simplicity, you can assume that UnreliNET always delivers packets in order.

Alice






UnreliNET

Bob












Figure 1: UnreliNet Simulates Unreliable Network


Instead of sending packets directly to Bob, Alice will send all packets to UnreliNET. UnreliNET may introduce bit errors or lose packets randomly. It then forwards packets (if not lost) to Bob. When receiving feedback packets from Bob, UnreliNET may also corrupt them or lose them with certain probability before relaying them to Alice.

The UnreliNET program is complete and given. Your task is to develop the Alice and Bob programs so that Bob will receive chat messages successfully in the presence of packet corruption and packet loss.


UnreliNET Program

The UnreliNET program simulates an unreliable channel that may corrupt or lose packets with tunable probability. This program is complete and provided as a Java class file. There’s no necessity to view and understand the source code of UnreliNET to complete this assignment.

To run UnreliNET on sunfire, type the following command:

java UnreliNET <P_DATA_CORRUPT> <P_DATA_LOSS> <P_ACK_CORRUPT> <P_ACK_LOSS> <unreliNetPort> <rcvPort>

For example:

java UnreliNET 0.3 0.2 0.1 0.05 9000 9001

The above command makes UnreliNET listens on port 9000 of localhost and forwards all received data packets to Bob running on the same host port 9001, with 30% chance of packet corruption and 20% chance of packet loss. The UnreliNET program also forwards ACK/NAK packets back to Alice, with a 10% packet corruption rate and a 5% packet loss rate.

Note that UnreliNET only accepts packets with a maximum payload of 64 bytes (inclusive of user-defined header/trailer fields). It will drop any packet beyond this size limit and show an error message on the screen.





Packet Error Rate

The UnreliNET program randomly corrupts or drops data packets and ACK/NAK packets according to the specified parameters P_DATA_CORRUPT, P_DATA_LOSS, P_ACK_CORRUPT, and P_ACK_LOSS. Please choose these values in the range [0, 0.3] during testing (setting a too large corruption/loss rate may result in a very slow transmission). The grading script also chooses parameters in the range [0, 0.3].

If you have trouble getting your code to work, better set the parameters to 0 first for debugging purposes.


Alice Program

Alice will read chat messages from standard input and sends them to UnreliNET (which will then forward to Bob as appropriate). The chat messages contain ASCII characters only.

To run Alice on sunfire, type the following command (suppose python3 alias is set up):

python3 Alice.py <unreliNetPort>

<unreliNetPort> is the port number UnreliNET is listening to. An example command line would be,

python3 Alice.py 9000


Bob Program

Bob receives chat messages from Alice (via UnreliNET) and prints them to standard output. It may also send feedback packets to Alice (also via UnreliNET) as you deem appropriate. The command to run Python version of Bob is:

python3 Bob.py <rcvPort>

For example,

python3 Bob.py 9001

With above command, Bob listens to port 9001 of localhost and prints all received messages to the standard output.

Some tips are given below.

    1. Bob should only print messages that are correctly received from Alice, no more, no less.

    2. Make sure you do not print anything irrelevant to standard output. In particular, remember to remove all debug output before submission.







Running All Three Programs

For this assignment, you will always run UnreliNET, Alice and Bob programs on the same host. You should launch Bob first, followed by UnreliNET in the second window. Finally, launch Alice in a third window to start data transmission.

The UnreliNET program simulates a unreliable network and runs infinitely. Once launched, you may reuse it in consecutive tests if you do not want to change its parameters. To manually terminate it, press <Ctrl> + c.

The Alice and Bob programs should not communicate with each other directly – all traffic has to go through the UnreliNET program. Alice should terminate once all input is read from user and properly forwarded (i.e. the input stream is closed and everything in the input stream is successfully received by Bob).

There is no need for Bob to detect end of transmission and terminate. If you need to manually terminate it, press <Ctrl> + c.


Testing your programs

For the convenience of testing, you can use the file redirection feature to let Alice read from a file (rather than from keyboard input).

For example, you can run Alice as follows:

python3 Alice.py < input.txt

You can replace input.txt with another file name if you wish.

In the same way, you can let Bob prints to a file (rather than print on screen - just for the convenience of testing):

python3 Bob.py > output.txt

Remember to flush output buffer so that data will be written to output.txt.

Finally, you can compare input.txt to output.txt by issuing the following command:

cmp input.txt output.txt

This line compares two files byte by byte and prints the differences if found. If the two files are binary equivalent, nothing will be printed on the screen. This method is useful to detect unexpected outputs such as hidden characters.

A sample input.txt is provided for your testing.












Self-defined Header/Trailer Fields at Application Layer

UDP transmission is unreliable. To detect packet corruption or packet loss, you need to implement reliability checking and recovery mechanisms by yourself. The following header/trailer fields are recommended though you may choose a different design:
    • Sequence number

    • Checksum

You are reminded again that each packet Alice or Bob sends should contain at most 64 bytes of payload data (inclusive of user-defined header/trailer fields), or UnreliNET will drop it.


Computing Checksum

To detect bit errors, Alice should compute checksum for every outgoing packet. Please refer to Assignment 0 exercise 2 on how to compute checksum.


Timer and Timeout Value

Alice may have to maintain a timer for unacknowledged packet. You should set a timeout value of 50ms. Socket timeout can be set using Python function socket.settimeout(). Other languages provide similar feature which can be found online.


Question & Answer

If you have any doubts on this assignment, please post your questions on IVLE forum or consult the teaching team. We will not debug programs for you. However, we may help to clarify misconceptions or give necessary directions if required.


Plagiarism Warning

You are free to discuss this assignment with your friends. However, ultimately, you should write your own code. We employ zero-tolerance policy against plagiarism. If a suspicious case is found, we will award zero points and may take further disciplinary action.
















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