Description

Group Work
This assignment should be solved individually. However, if you struggle, feel free to form a team with another student (max. team size is 2). Group submissions are subject to 2 points penalty for each student.
Grading
We will test and grade your program on the sunfire server. Please make sure that your programs run properly on sunfire. Moreover, you are allowed to use libraries installed in public folders of sunfire (e.g. /usr/lib) only.
We accept submission of Python 3 (3.7), Java, or C/C++ program, and we recommend that you use Python 3 for your assignments. Programming languages other than Python 3, Java, and C/C++ are not allowed. For Python 3, we use the python3 program installed in folder /usr/local/Python-3.7/bin on sunfire for grading. If you use Java or C/C++, we will compile and run your program for grading using the default compilers on sunfire (java 9.0.4 installed in /usr/local/java/jdk/bin, or gcc 4.8.4 installed in /usr/local/gcc-4.8/bin). For C/C++ users, we will link your program with zlib during compilation. The grading script infers your programming language from the file extension name (.py, .java, .c). Therefore, please ensure your files have the correct extension names. For a Java program, the class name should be consistent with the source file name, and please implement the static main() method so that the program can be executed as a standalone process after compilation.
We will grade your program based on its correctness only. A grading script will be used to test your program and no manual grading will be provided.
Program Submission
For individual submission, please submit a zip file that contains source programs and submit it to the Assignment_2_student_submission folder of LumiNUS Files. Please name your zip file as <Student number>.zip, where <Student number> refers to your matric number that 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++, replace “.py” with “.java”, “.c”, or “.cpp”, respectively. Do not put your programs under any subfolders (or package paths).
For 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 student numbers of two members and be named as <Student number 1>-<Student number 2>.zip. An example would be A0165432X-A0123456Y.zip.
We will deduct 1 mark for every type of failure to follow instructions (e.g. wrong program name, wrong zip file name, folder structure).
Grading Rubric
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. The grading script infers your programming language from the file extension name (.py, .java, .c). Therefore, please ensure your files have the correct extension names. 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.
1. [2 points] Programs are free from syntax errors and can be successfully executed (or compiled, for Java/C/C++). Program execution follows the specified commands exactly. Source files are correctly named and there are no additional subfolders inside the zip file.
2. [1 points] Programs can successfully send chat messages from Alice to Bob in a perfectly reliable channel (i.e. no error at all).
3. [2 points] Programs can successfully send messages from Alice to Bob in the presence of packet corruptions.
4. [2 points] Programs can successfully send messages from Alice to Bob in the presence of packet losses.
5. [1 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 total 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 10 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.
CAUTION: 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.
Question and Answer
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

Figure 1: UnreliNET Simulates Unreliable Network
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
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 listen on port 9000 of localhost and forwards all received data packets to Bob running on the same host with 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
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 send them to UnreliNET (which will then forward to Bob as appropriate). The chat messages contain ASCII characters only and there is no empty message (i.e. blank line). There’s no delay regarding the input to Alice.
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
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 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
To test your program, please use your SoC UNIX ID and password to log on to sunfire as instructed in Assignment 0 paper. To make the python3 alias permanent, and thus avoid typing the alias command every time you log in, you can run the following command once to store the shortcut into the configuration file: echo alias python3=/usr/local/Python-3.7/bin/python3 >> ~/.bash_profile
If you test your server manually, please select a port number greater than 1024, because the OS usually restricts usage of ports lower than 1024. If you get a BindException: Address already in use (or similar errors for other languages), please try a different port number.
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 <unreliNetPort> < input.txt
You can replace input.txt with another file name if you wish.
In the same way, you can let Bob print to a file (rather than print on screen – just for the convenience of testing):
python3 Bob.py <rcvPort> > 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.
We also release a comprehensive set of grading scripts to you. There are no hidden test cases during grading. However, passing all the test cases does not guarantee that you will get full marks. We will detect fraudulent cases such as hard-coding answers, or Alice communicates with Bob in any means other than via UnreliNET.
To use the grading script, please upload your programs along with the test folder given in the package to sunfire. Make sure that your programs and the test folder are in the same directory. Then, you can run the following command to test your programs:
bash test/RunTest.sh
By default, the script runs through all test cases. You can also choose to run a certain test case by specifying the case number in the command:
bash test/RunTest.sh 2
To stop a test, press <Ctrl> + c. If pressing the key combination once does not work, hold the keys until the script exits.
Self-defined Header/Trailer Fields at Application Layer
1. Sequence number
2. Checksum
You are reminded again that each packet Alice or Bob sends should contain at most 64 bytesofpayloaddata(inclusiveofuser-definedheader/trailerfields), 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