程序代写代做代考 Hive Java CS1010 Programming Methodology
CS1010 Programming Methodology
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National University of Singapore
School of Computing
CS2105 Assignment 2 Semester 2 AY16/17
Learning Objective
In this assignment, we will implement the Go-Back-N protocol over an unreliable
communication channel. After solving this assignment you should know the Go-Back-N protocol
by heart.
Introduction
Alice wants to send a file to Bob. Unfortunately, in Alice’s network, all TCP traffic is blocked by a
firewall, i.e. she can neither send nor receive any data using TCP. After some exploration, Alice
realizes that the firewall does not block UDP traffic. Therefore she decides to send the file over
UDP but wants to ensure Bob can receive the file intact.
Help Alice by implementing a reliable transport on top of UDP. Alice should be able to send a
file successfully to Bob using UDP communication only. To achieve reliable transport you should
implement Go-Back-N.
No Group Work
This assignment should be solved individually. Group work is not permitted.
Submission Deadline
20th March 2017 (Monday), 6pm sharp. Please start early and submit early. You can always
upload a new version before the deadline. We strongly advice against submitting only 2
minutes before the deadline. 2 points penalty will be imposed on late submission (Late
submission refers to submission or re-submission after the deadline, even if it is only 5
minutes).
This programming assignment is worth 9 marks.
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Program Submission
1. Make sure each and every Java file contains your student number and name at the top.
2. Make sure the classes have the correct name and match the file name (e.g. Alice.java
should contain a class called Alice but not alice).
3. Ensure that the programs run correctly on sunfire using the given commands.
4. Please submit your files to CodeCrunch website:
https://codecrunch.comp.nus.edu.sg. You should submit at least Alice.java and
Bob.java. However, feel free to upload more Java files, e.g. in case you created extra
Java classes.
5. You can submit multiple Java files to CodeCrunch by pressing the
choosing programs to upload. Thus, you can upload all your files directly. Do not
compress or archive your files, i.e. do not use zip, rar, tar or similar.
6. Please note that 1 mark penalty will be imposed on all submissions that do not follow
the submission format.
7. Note that CodeCrunch is just used for program submission and no test case has been
mounted on it. Hence you may ignore the feedback from CodeCrunch regarding the
quality of your programs.
You are not allowed to post your solutions in any public domain in the Internet.
Plagiarism Warning
You are free to discuss this assignment with your friends. However, you should design and
write your own code. We employ a zero-tolerance policy against plagiarism. Confirmed breach
will result in zero mark for the assignment and further disciplinary action from the school. Do
not copy code from the Internet.
https://codecrunch.comp.nus.edu.sg/
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Grading Rubric
1. [2 points] You followed the submission format, e.g. you submit at least Alice.java and
Bob.java. Your program compiles without errors. You do not submit zips or rars or similar
archives.
2. [2 points] Programs can successfully transfer a file from Alice to Bob in a perfectly reliable
channel (i.e. no error at all). The received file is binary equivalent to the sent file. (Use cmp
or diff to test binary equivalence)
3. [2 points] Programs can successfully transfer a file from Alice to Bob in the presence of both
data packet corruption and ACK packet corruption using Go-Back-N.
4. [2 points] Programs can successfully transfer a file from Alice to Bob in the presence of both
data packet loss and ACK packet loss using Go-Back-N.
5. [1 point] Your implementation works correctly for different sender windows (different Ns in
Go-Back-N). N is passed to the program as a command-line argument.
During grading we do not care what messages you print on the screen. However, we do care
about binary equivalence, i.e. the received file must the exactly the same as the sent one.
Please use the cmp command to test for binary equivalence. Refer also to assignment 0 for
details.
If you struggle to solve this assignment, please post your questions in IVLE. Focus
on the low hanging fruits first, before you try the harder ones. If you still have
issues after a while, please approach the teaching staff.
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Overall Architecture
There are three programs in this assignment, Alice, Bob, and UnreliNET. Their
relationship is illustrated in Figure 1 below. Alice and Bob implement a one-way file transfer
application over UDP protocol. The UnreliNET program simulates the transmission channel
that randomly corrupts or loses packets. However, for simplicity, you can assume that this
channel always delivers packets in order.
Figure 1: UnreliNet Simulates Unreliable Network
The UnreliNET program acts as a proxy between the Alice and Bob. UnreliNET may
introduce bit errors to packets or lose packets randomly. It then forwards packets (if not lost) to
the destination.
The UnreliNET program is complete and given. Your task in this assignment is to develop the
Alice and Bob programs so that both sides will receive chat messages successfully in the
presence of packet corruption and packet loss.
UnreliNET Class
The UnreliNET program simulates an unreliable channel that may corrupt or lose packets
with a certain probability. This program is given and should not be changed.
To run UnreliNET on sunfire, type then following command:
java UnreliNET
For example:
java UnreliNET 0.3 0.2 0.1 0.05 50 150 9000 localhost 9001
listens on port 9000 and forwards all received data packets to Bob running on the same host at
port 9001. Packets send from Alice to Bob have a 30% chance of getting corrupted and 10%
chance of being lost. The UnreliNET program also forwards ACK packets back to Alice.
Acknowledgements send from Bob to Alice have a 20% chance of being corrupted and a 5%
chance of being lost. Furthermore, the propagation delay is between 50 and 150 ms.
Alice
UnreliNET
Bob
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Packet Error Rate
The UnreliNET program randomly corrupts or loses data packets and ACK packets according
to the specified parameters P_DATA_CORRUPT, P_DATA_LOSS, P_ACK_CORRUPT, and
P_ACK_LOSS. You can set these values to anything in the range [0, 0.3] during testing (setting a
too large corruption/loss rate may result in a very slow transmission).
If you have trouble getting your code to work, it might be advisable to set them to 0 first for
debugging purposes.
Alice Class
The Alice program is a simple one-way file transfer program. It opens a given file and sends
its content as a sequence of packets to UnreliNet. UnreliNet then forwards the packets
to Bob.
To run Alice on sunfire, type command:
java Alice
For example:
java Alice 10 ../test/cs2105.zip 9000 notes.zip
sends the file cs2105.zip from directory (relative path) ../test to UnreliNet running
in the same host at port 9000. UnreliNet will then forward the file to Bob to be stored as
notes.zip. The sender window size (the N in Go-Back-N) is 10.
Notes:
1. For this assignment, we will always run UnreliNET, Alice and Bob programs on the
same host.
2. Alice should terminate after (1) reading all data from the file and (2) forwarding it
successfully to the Bob.
3. Use a Maximum Segment Size of 512 bytes or your packets may be rejected by the
UnreliNET program.
4. Do not hard code the target file name “notes.zip” in the Bob program. During testing we
will use different file names.
5. Again, be very careful about binary equivalence.
6. During grading the supplied file will exist, the path is correct, and the file name is
smaller than 256 bytes.
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Bob Class
The Bob program receives packets from Alice (through UnreliNET) and store the received
file in the current working directory, with a filename specified by Alice.
To run Bob on sunfire, type command:
java Bob
For example,
java Bob 9001
listens on port 9001 and stores the bytes received into a file whose name is given by Alice.
Bob does not have to terminate, i.e. it can run infinitely and does not need to detect end of
transmission and terminate. (This simplifies the implementation. Otherwise you would have to
implement a full TCP teardown which is quite hard) You can assume that after Alice is
terminated, Bob will not be reused for another communication attempt.
However, even though Bob does not have to terminate, Bob has to make sure that all the
received data is stored on disk. A good way to achieve this is to flush any buffers and close the
file properly once the last data is successfully received.
Running All Three Programs
You should first launch Bob, followed by UnreliNET in the second window. Finally, launch
Alice in a third window to start data transmission. Please note that Alice and Bob take the
ports (
described above. Please always test your programs on localhost to avoid the interference
of network traffic on your programs.
The UnreliNET program simulates unreliable communication network and runs infinitely.
Once launched, you may reuse it in consecutive tests. To manually terminate it, press
The Alice and Bob programs should not communicate with each other directly – all traffic has to
go through the UnreliNET program
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Tips and Hints
Reading Data from File
Please refer to Assignment 0 Exercise 3 on how to read data in batch. You should not read the
entire file into an array as java.lang.OutOfMemoryError may be thrown when a huge
file is used in testing. Especially, do not use the code that was provided in the skeleton of
assignment 1.
Self-defined Header/Trailer Fields at Application Layer
UDP transmission is unreliable. To detect packet corruption or packet loss, you may need to
implement reliability checking and recovery mechanisms at application layer. The following
header/trailer fields are suggested though you may have your own design:
Sequence number
Checksum
Note that each packet Alice sends should contain at most 512 bytes of application data
(inclusive of user-defined header/trailer fields), or UnreliNET will reject it.
Computing Checksum
To detect bit errors, Alice should compute checksum for every outgoing packet and embed it
in the packet. Bob needs to re-compute checksum to verify the integrity of a received packet.
Please refer to Assignment 0 Exercise 2 on how to compute checksum using Java CRC32 class.
Timer and Timeout Value
Alice may have to maintain a timer for unacknowledged packet. You may want to use the
setSoTimeout() method of Java Socket class.
You should use a timeout value of 2000ms.
Reading/Writing Values to Header/Trailer Fields
The number of application layer header/trailer fields and the sequence of their appearance in a
packet have to be agreed by sender and receiver before data transmission (i.e. an application
layer protocol designed by you).
As will be discussed in the week 8 tutorial, you may use ByteBuffer class from the
java.nio package to form a packet containing various header/trailer and application
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message.
Common Pitfalls
The received file has to be binary equivalent to the sent file. A good way to test this is to use
the cmp command (please refer to assignment 0 for more details). A bad way to test is to
compare file sizes (especially if the file sizes are in KB). Another bad way is to transfer a file and
then test if it can be opened (many applications have built-in error correction).
A Word of Advice
This assignment is complex and time-consuming. You are suggested to write programs
incrementally and modularly. For example, deal with error-free transmission first, then data
packet corruption, ACK packet corruption, etc. Test your programs after every single major
change. Take note that partial credit will be awarded even if your programs do not meet all
listed requirements.
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 applicable.
