Assignment 4 Computer Science 441


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1 Objective
The objective of this assignment is to practice UDP socket programming and reliable data transfer. Specifically, you will implement a UDP-based program for reliable file transfer based on the
Stop-and-Wait protocol.
2 Specification
2.1 Overview
In this assignment, you will implement a simplified FTP client based on UDP called StopWaitFtp.
Since UDP does not provide any data reliability, you will implement your own reliability mechanism based on the stop and wait protocol. The simplified client only supports sending a file
to the server. Before the actual data transmission, the client and server go through an initial
handshake process to exchange control information about the file transfer. The handshake takes
place over TCP, while the actual file transfer is carried out over UDP.
2.2 Handshake
The handshake takes place over a TCP connection and initiated by the client. The host name
and port number of the server are provided to your program. Upon start, your program should
open a TCP socket (with the provided server name and port number) as well as a UDP socket
(using the no-argument constructor DatagramSocket()).
Use the TCP socket to exchange information about the file name to be sent, its length, the
initial sequence number and UDP port numbers on the client and server. All control messages during the handshake are in binary format. You can use the Java binary stream classes
DataInputStream and DataOutputStream to read from and write to the TCP socket in binary format. Specifically, follow the message sequence presented in Program 1 (in the exact
same order) to complete the handshake process:
Program 1 Handshake Message Sequence
1. Send the local UDP port number used for file transfer as an int value
2. Send the name of the file as a UTF encoded string
3. Send the length (in bytes) of the file as a long value
4. Receive the server UDP port number used for file transfer as an int value
5. Receive the initial sequence number used by the server as an int value
Do not forget to flush() the output stream to force TCP to send your data to the server at the
end of the send section of the handshake.
Assignment 4 CPSC 441
3 Implementation
3.1 High Level Structure
A high-level description of the internal operation of StopWaitFtp is presented in Program 2.
Program 2 StopWaitFtp
Open a TCP connection to the server
Open a UDP socket
Complete the handshake over TCP
while not end of file do
Read from the file and create a segment
Send the segment and start the timer
Wait for ACK, when correct ACK arrives stop the timer
end while
Close sockets and clean up
3.2 Sending File Content
A file name is given to your program as input. Your program should read the file chunk-bychunk and then encapsulate each chunk in a segment for transmission to the server. The class
FtpSegment is provided to you on D2L. Refer to the Javadoc documentation provided in the
source file for more information on how to use this class. Specifically, class FtpSegment provides methods for creating segments with a given sequence number and payload, creating a
UDP packet that encapsulates a segment, and creating a segment from a UDP packet, as demonstrated in method FtpSegment.main().
The maximum size of a segment payload is given by the constant FtpSegment.MAX_PAYLOAD_SIZE.
The sequence number for segments starts at the initial sequence number received from the server
during the handshake process, and is incremented per every segment transmitted. Once a segment is transmitted, you should wait to received an ACK for that segment. Your program should
listen for arriving ACK segments from the server using the same UDP socket that is used for
sending data segments to the server. The ACK segments received from the server carry the sequence number of the next expected segment at the server. That is, if the server receives a segment
with sequence number n, then it sends an ACK segment with sequence number n + 1, indicating
to the client to send segment n + 1 next. The ACK segments are regular segment objects that
have no payload.
Assignment 4 CPSC 441
3.3 Retransmission Timer
As soon as a segment is transmitted, the client should start a retransmission timer. The duration
of the timeout interval is given to the client program as an input parameter. Class Timer can
be used to schedule a recurring timer using method Timer.scheduleAtFixedRate(). To
use the Timer class, you need to define a timer task class as well. A timer task is similar to a
Thread class. The only difference is that it extends class TimerTask.
In your program, you should create one Timer object when the program starts. Then use the
timer object to start and stop recurring timer tasks. If an ACK arrives for the latest transmitted
segment, you should cancel the recurring timer task by calling method TimerTask.cancel().
Note that calling the cancel method does not cancel a timer task that is currently executing.
While this may result in an unnecessary retransmission of the segment that has been just ACKed,
it is an acceptable behavior in this assignment.
You may find it convenient to define two helper methods to start and stop the timer task. When
the entire file transmission is complete, make sure to shutdown the timer instance itself. This is
achieved by calling Timer.cancel() and Timer.purge() methods to cancel the timer object
and any timer tasks attached to it.
3.4 Handling Concurrency
Since the timer task runs concurrently with the main program, you should be careful when
accessing any shared data. In particular, the last transmitted segment may be accessed concurrently by the main program and the timer task (for retransmission), which could lead to memory
inconsistencies, e.g., retransmitting a segment by the timer task before it has been fully initialized by the main program. In general, such race conditions can be avoided by protecting access
to the shared data using Java synchronized blocks. In StopWaitFtp, since the shared data
is only one segment, a simple solution is to store a copy of the segment (to be transmitted when
a timeout happens) in the timer task object itself when creating a timer task for the segment.
3.5 Design Requirements
Your client design should be based on the following principles:
• Segment Payload: Create segments with the maximum payload size for good performance. Only the last segment from the client to the server may not carry sufficient data to
by of maximum size.
• Retransmission Timer: Using socket timeout option to handle retransmissions is not allowed, as it could lead to incorrect behavior with duplicate ACKs. Your implementation
must use the Timer class, as described above. Using a ScheduledExecutorService to
implement retransmissions is also acceptable.
Assignment 4 CPSC 441
4 Software Interfaces
4.1 Method Signatures
The required method signatures for class StopWaitFtp are provided to you in the source file There are two methods that you need to implement, namely a constructor and method send. Refer to the Javadoc documentation provided in the source file for more
information on these methods.
4.2 FtpSegment Class
This class defines the structure of the segments exchanged between the sender (i.e., client) and
receiver (i.e., server). Read the Javadoc documentation of the class for how to use it. Note that
both data packets and ACKs are of type FtpSegment. Segments that go from the sender to
receiver carry data, while segments that come from the receiver are ACKs that do not carry any
4.3 Exception Handling
Your implementation should include exception handling code to deal with all checked exceptions in your program. This includes catching the exceptions and printing exception messages
(or the stack trace) to the standard system output. After that, clean up, close all streams and
sockets and return from the send method.
4.4 Testing Your Code
A driver class named StopWaitDriver is provided on D2L to demonstrate how we are going
to run your code. Read the inline documentation in the source file for
detailed information on how to use the driver class. Moreover, you can download the server
program that implements the service side of the protocol from D2L. The server program is provided in a jar file named ftpserver.jar. The server comes with a README which includes
instructions on how to run it. Although you can check the transferred file in the working directory of the server for correctness, you should also correlate the output of your program with that
of the server to verify correct protocol implementation. For example, you can correlate packet
drops at the server with timeouts at the client, which should match.
4.5 Console Output
Add print statements (using System.out.println) in your code to print the following information on the console:
1. send – every time the client sends a segment to the server.
2. ack – every time the client receives an ACK.
Assignment 4 CPSC 441
3. retx – every time a segment is re-transmitted due to timeout.
4. timeout – every time a timeout happens at the client.
In the above, refers to the sequence number of the relevant data or ACK segment.
Do not directly print anything else to the console beyond exception messages and the above
information. For debugging purposes, you can use the global logger object defined in the
driver class, whose level can be set using the command line option -v. Refer to the driver class
source file for more information. The logger can be used to write messages to console during
code development.
4.6 Design Document
Prepare and submit a design document to describe the following aspects of your program:
• How did you implement retransmissions? Explain how you setup the timer and when
you start and stop the transmission timer.
• What happens in your implementation if a timeout occurs at the same time that an ACK
arrives from the server. Is there going to be a race condition in your program in that case?
Explain your answer.
• You are not allowed to modify the class and method signatures provided to you. However,
you can (and should) implement additional methods and classes as needed in your implementation. Any changes in StopWaitDriver and FtpSegment classes will be overwritten during marking.
• You have to write your own code for sending and receiving UDP packets. Ask the instructor if you are in doubt about any specific Java classes that you want to use in your