Introduction
java，FileSender FileReceiver， ，CRC32 Checksum，
，
Requirement
Page 1 of 6
Assignment 2
Introduction
In this assignment, you will transfer a file over UDP protocol on top of an unreliable channel
that may either corrupt or drop packets randomly (but always deliver packets in order).
Writing Your Programs
You are free to write your programs on any platform/IDE that you are familiar with.
Grading
Your programs will be graded according to their correctness using a grading script.:
• [2 points] Programs are compilable on sunfire, program execution follows specified
Java commands (see sections below).
• [1 point] Programs can successfully transfer a file from sender to receiver when channel is
perfectly reliable (i.e. no error at all).
Page 2 of 6
• [1 point] Programs can successfully transfer a file from sender to receiver in the presence of
data packet corruption.
• [1 point] Programs can successfully transfer a file from sender to receiver in the presence of
ACK/NAK packet corruption.
• [1 point] Programs can successfully transfer a file from sender to receiver in the presence of
data packet loss.
• [1 point] Programs can successfully transfer a file from sender to receiver in the presence of
ACK/NAK packet loss.
• [1 point] Programs can successfully transfer a file from sender to receiver in the presence
of both packet corruption and packet loss.
To conclude a successful file transfer, received file must have identical content as the
sent one (use command cmp to check it on sunfire). Your program should work for
both text and binary files, and for both small files and large files (a few MBs).
Grading script. doesn’t care what messages your programs print on the screen. It just checks
if the received file is exactly the same as the sent one in respective test cases.
• [2 points] (Who runs faster?) Time taken for your programs to transfer an enormous file
(more than 10MB) in the presence of both packet corruption and packet loss will be noted
down. The fastest batch of student programs will receive 2 marks and slower programs that
are still faster than average will receive 1 mark. J
You may apply the knowledge gained from this or other courses as appropriate. However,
since the objective of this assignment is to design a fast network transmission protocol,
you are NOT allowed to play tricks (e.g., data compression) to gain unfair advantage
over your peers. Please consult the teaching team if you are unsure if certain techniques
can be used or not.
A Word of Advice
This assignment is complex and time-consuming. You are suggested to write programs
incrementally and modularly. For example, deal with data packet corruption first, then ACK
packet corruption, then data packet loss, etc. Test your programs after every single major
change. Take note that partial credit will be awarded even if your programs don’t meet all listed
requirements.
Page 3 of 6
Overall Architecture
There are three programs in this assignment, FileSender, UnreliNET and
FileReceiver. Their relationship is illustrated in Figure 1 below. The FileSender and
FileReceiver programs implement a file transfer application over UDP protocol. The
UnreliNET program simulates the transmission channel that transmits packets unreliably and
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 FileSender and FileReceiver.
Instead of sending packets directly to FileReceiver, FileSender sends all packets to
UnreliNET. UnreliNET may introduce bit errors to packets or lose packets randomly. It
then forwards packets (if not lost) to FileReceiver. When receiving feedback packets from
FileReceiver, UnreliNET may also corrupt them or lose them with certain probability
before relaying them to FileSender.
The UnreliNET program is complete and given. Your task in this assignment is to develop the
FileSender and FileReceiver programs so that a file can be successfully transferred
from sender to receiver in the presence of possible packet corruption and packet loss. The
received file should be identical to the file sent. You may need to implement techniques learnt
in lecture, including sequence number, acknowledgement, timeout and retransmission, to
ensure that packets are correctly delivered.
FileSender Class
The FileSender program is basically a file uploader that opens a given file and sends its
content as a sequence of packets to UnreliNet. UnreliNet would then corrupt/lose
packets with certain probability before relaying them to the FileReceiver program.
To run FileSender on sunfire, type command:
java FileSender
For example:
java FileSender ../test/cny.mp3 localhost 9000 gxfc.mp3
sends the file cny.mp3 from directory (relative path) ../test to UnreliNet running in
the same host at port 9000. UnreliNet will then pass the file to your FileReceiver
program to be stored as gxfc.mp3.
UnreliNET FileSender FileReceiver
Page 4 of 6
(Note: Windows system uses a different file separator ‘\’, e.g., ..\test\cny.mp3)
You may assume that during testing, your sender program will be supplied with the correct
path and filename (no more than 100 bytes long). No input validation is needed.
UnreliNET Class
The UnreliNET program simulates an unreliable channel that may corrupt or lose packets
with a certain probability. This program is given and shouldn’t be changed.
To run UnreliNET on sunfire, type command:
java UnreliNET
For example:
java UnreliNET 0.3 0.2 0.1 0.05 9000 localhost 9001
listens on port 9000 and forwards all received data packets to FileReceiver running on the
same host at port 9001, with 30% chance of packet corruption and 10% chance of packet loss.
The UnreliNET program also forwards ACK/NAK packets to FileSender, with 20% packet
corruption rate and 5% packet loss rate.
Packet Corruption Probability
The UnreliNET program randomly corrupts or loses data packets and ACK/NAK packets
according to the specified parameters P_DATA_CORRUPT, P_ACK_CORRUPT P_ACK_LOSS 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 file transmission).
If you have trouble getting your code to work, it might be advisable to set them to 0 first for
debugging purposes.
FileReceiver Class
The FileReceiver program receives a file from FileSender (through UnreliNET) and
saves it in the same directory as the FileReceiver program, with a filename specified by
FileSender.
To run FileReceiver on sunfire, type command:
java FileReceiver
For example:
java FileReceiver 9001
listens on port 9001 and dumps the bytes received into a file whose name is given by sender.
Page 5 of 6
Running All Three Programs
You should first launch FileReceiver, followed by UnreliNET in the second window.
Finally, launch FileSender in a third window to start data transmission. All programs should
receive user input from command-line argument only. Please always test your programs in
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 + c.
The UnreliNET program prints out information that is helpful in debugging your programs.
However, we will disable such outputs in speed test.
The sender and receiver programs shouldn’t communicate with each other directly – all traffic
should be forwarded by the UnreliNET program. The sender program should terminate
properly (e.g., no exception, no infinite loop) once transmission finishes. However, you may
leave the receiver program running infinitely (i.e., no need for receiver to detect end of
transmission and terminate, but DO remember to close the file output stream at the end).
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 FileSender sends should contain at most 1000 bytes of application
data (inclusive of self-defined header/trailer fields), or UnreliNET program will reject it.
Computing Checksum
To detect bit errors, FileSender should compute checksum for every outgoing packet and
embed it in the packet. FileReceiver needs to re-compute checksum to verify the integrity
of a received packet.
Please refer to Assignment 0 Exercise 3 on how to compute checksum using Java CRC32 class.
Timer and Timeout Value
Sender may have to maintain a timer for unacknowledged packet. You are suggested to use the
setSoTimeout() method of Java Socket class.
You shouldn’t set a timeout value that is larger than 200ms, or your program might be too slow
in transmitting data and thus get killed by the grading script.
Page 6 of 6
Reading/Writing Values to Header/Trailer Fields
The number of header/trailer fields and the sequence of their appearance in a packet is the
agreement between sender and receiver (i.e. an application layer protocol designed by you).
As discussed in tutorial 3, to give value to a header field of a 4-byte integer, you may consider
ByteBuffer class from java.nio package. An example is shown below.
At sender side:
int length = 1000;
// allocate a 4-byte array to store converted integer
byte[] pktLen = ByteBuffer.allocate(4).putInt(length).array();
// copy content of pktLen to the beginning of output buffer of pkt
System.arraycopy(pktLen, 0, buffer, 0, pktLen.length);
At receiver side:
// extract the first 4 bytes of a packet as the integer ‘length’
ByteBuffer wrapper = ByteBuffer.wrap(pkt.getData(), 0, 4);
int length = wrapper.getInt();