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socket programming and reliable data transfer
ASSIGNMENT OVERVIEW
The goal of this project is to practice your Application and Transport Layer skills by
implementing (i) the anonymizer, a client-server application that anonymizes user-specified
words from a text file and (ii) two reliable data transport protocols. The project will be
completed in two phases. In the first phase, all students will implement two versions of the
program: one that uses stock TCP1
for reliable data transfer; and one that implements stopand-wait
reliability at the application layer and uses UDP for transport. In the second phase,
students will be asked to evaluate and compare the stock TCP and the stop-and-wait
implementations using Wireshark.
Objectives: There are a number of objectives to this assignment. The first is to make sure you
have some experience developing a network-based socket application. Second, because you are
allowed to use any references you find on the Internet (including copies of existing code!), this
assignment will help you see just how many network programming aids are available. Third,
you will get a first-hand experience in comparative evaluation of protocol performance. Finally,
having just a bit of practical experience will put a lot of the protocol concepts we learn into
perspective.
Citing sources:
You may use code from the Internet to help you do this assignment (e.g. basic socket code).
However, this is just like citing a passage from a book, so if you copy code, you must cite it. To
do this, put a comment in the beginning of your code that explains exactly what you have
copied, who originally wrote it, and where it came from.
1 I.e. the TCP protocol implemented in the operating system.
The breakdown of points for this assignment is below. In addition to correctness, part of the
points count towards how well code is written and documented. Good code/documentation
does not imply that more is better. The goal is to be efficient, elegant and succinct!
Item Points Undergrads Grads
TCP implementation 40 Mandatory Mandatory
Stop-and-wait implementation 50 Mandatory Mandatory
Wireshark evaluation 10 Optional (Extra credit) Mandatory
Report 20 Optional (Extra credit) Mandatory
Code documentation 10 Mandatory Mandatory
submit a total of 13 files as follows:
• TCP server and client programs (2 files)
• Stop-and-wait (UDP) server and client programs (2 files)
• Report (1 file).
• Your pcap traces from Wireshark (8 files).
ASSIGNMENT DETAILS:
The Application:
Your application will use a client-server architecture to provide a text file anonymization
service. You will need to implement the anonymization function from scratch3
.
• Anonymization functionality: The application will allow a user to upload a text file of
arbitrary size along with a keyword to be anonymized. The file will then be loaded, read
and anonymized at the server and the redacted text will be stored in a new file. The
anonymization function will replace the keyword with the equal amount of the symbol
X. For example, if the target keyword is “networking” all instances of that word in a file
will be replaced with “XXXXXXXXXX”. Once the server is done anonymizing, it will issue a
message to the user indicating the output filename. The server will then allow the client
to download the output file.
• Supported commands: Your program should allow a user to upload and download text
files, to specify a keyword, and to quit the program. To this end, you will implement the
following four commands that the client can send to the server:
2 Unless undergrads have attempted the extra credit components, in which case undergrads should submit 13 files,
as detailed below.
3 If your code relies on external libraries your implementation will fail the grading on our server, which will not be
running custom libraries, and you will lose points.
Item File naming in Python
Client using TCP client_tcp.py
Server using TCP server_tcp.py
Client using UDP client_udp.py
Sever using UDP server_udp.py
User prompt conventions:
This assignment specification gives you several examples that illustrate the user
prompts and verbal feedback that your code should support. Please, study these
examples closely and implement your programs to follow the same prompt conventions.
Your code will be graded automatically, and we will be looking for these prompts in our
auto-grading scripts. You will lose points if your prompts do not follow our examples.
Assignment Turn-in:
• Copy a file from the client to the server (put), which takes as an input argument the
full path to a file on the client. Example execution:
put
• Copy a file from a server to a client (get), which also takes as an argument the full
path to a file on the server. Example execution:
get
• Keyword command that will allow the user to specify a keyword to be anonymized
and a target file, in which to anonymize. Example execution:
Keyword
• Quit the program per user request
quit
Starting the client; accepting user-specified commands:
fa21% client_udp.py
Enter command: put test.txt
Awaiting server response.
Server response: File uploaded.
Enter command: keyword Fall test.txt
Awaiting server response.
Server response: File test.txt anonymized. Output file is
test_anon.txt
Enter command: get test_anon.txt
File test_anon.txt downloaded.
Enter command: quit
Exiting program!
fa21%
Your application should behave the same way with both stock TCP and stop-and-wait reliability.
The Transport:
You will practice your understanding of the Transport Layer by implementing two versions of
reliable data transport: one that uses stock TCP (i.e. the TCP implementation that comes with
your operating system) and another one that implements stop-and-wait reliability at the
application layer and uses UDP for transport.
• Reliability over stock TCP: This version of your program will use stock TCP to implement
reliable transmission of the text file. Below are examples of how to start the server and
the client program, which will give you a sense of required input arguments and
formatting4
.
Starting the server:
icsi416-fa21% server_tcp.py
Starting the client:
icsi416-fa21% client_tcp.py
Enter command:
4 Input arguments are specified with <>. An actual run might look like: client_tcp.py 169.226.65.98 2222
• Stop-and-wait reliability over UDP: This version of your project will implement the text
file exchange using stop-and-wait reliability over UDP. As a reminder, UDP provides
best-effort packet delivery service; you will have to implement reliability checks on top
of UDP to ensure that your data is successfully transmitted between the server and the
client. Since we are working at the application level, we will implement our stop and
wait reliability at the level of message chunks. We will discuss this functionality in terms
of sender and receiver. Note that depending on whether you are performing get or
put your sender and receiver will switch places in the client-server architecture (i.e. for
get, your sender will be the server and your receiver will be the client, whereas for
put, the sender will be the client whereas the receiver will be the server). The reliable
data transfer should function identically for both get and put. Your reliable protocol
will function as follows:
o First, the sender calculates the amount of data to be transmitted and sends a
“length” message to the receiver, letting them know how many bytes of data to
expect. The length message should contain the string LEN:Bytes.
o Second, the sender splits the data into equal chunks of 1000 bytes each, and
proceeds to send the data one chunk at a time. Note that the last chunk might
be smaller than 1000 Bytes and that is OK. Your programs should be able to
handle arbitrary text file sizes. After transmitting each chunk, the sender stops
and waits for an acknowledgement from the receiver. To this end, the receiver
has to craft and send a special message containing the string ACK.
o Finally, once the receiver receives all expected bytes (as per the LEN message),
the receiver will craft a special message containing the string FIN. This message
will trigger connection termination.
o Timeouts. Note that there are a few points in the sender-receiver interaction
where a timeout might occur. The below description specifies how your program
should behave in a timeout.
§ Timeout after LEN message. If no data arrives at the receiver within one
second from the reception of a LEN message, the receiver program
should terminate, displaying “Did not receive data.
Terminating.”
§ Timeout after a data packet. If no ACK is received by the sender within
one second from transmitting a data packet, the sender will terminate,
displaying “Did not receive ACK. Terminating.”
§ Timeout after ACK. If no data is received by the received within one
second of issuing an ACK, the receiver will terminate, displaying “Data
transmission terminated prematurely.”.
You will want to test your system with large enough files to confirm that it works
correctly. Ideally, you should test with the test files provided with this assignment:
file1.txt, file2.txt, file3.txt and file4.txt.
Below are a few example executions of the server and the client program, which will
give you a sense of required input arguments and formatting. Note that the user
interface is identical between the TCP and the stop-and-wait programs, so you can reuse
it. You have to devise the full list of client-server interactions for the get and put
commands based on the description of stop-and-wait reliability over UDP above.
Starting the server:
fa21% server_udp.py
Starting the client; accepting user-specified command:
fa21% client_udp.py
Enter command:
Phase 2: Wireshark Evaluation:
In this phase you will perform a comparative evaluation of your implementations in terms of
overall delay and achieved throughput. We define overall delay as the relative time difference
between the last and the first packet exchanged within a single program invocation. We define
the achieved throughput as the total sum of bits exchanged within a single program invocation
divided by the overall delay for that invocation. You will run your server and client
implementations on different physical machines in order to account for a realistic Internet
scenario. Specifically, you will run your server program on our course VM (icsi416-
fa21.its.albany.edu) and your client program on your personal computer. You will also
run Wireshark on your personal computer to be able to record a packet trace for each program
invocation. You will need to record four packet traces for each of the TCP and UDP
implementation (so eight altogether) for the provided with this assignment (file1.txt,
file2.txt, file3.txt and file4.txt). Once you have collected the Wireshark traces,
you need to process them offline and determine the overall delay and achieved throughput for
each invocation. You need to fill out the results in the tables below.
Delay File 1 (16KB) File 2 (32KB) File 3 (48KB) File 4 (62KB)
TCP (sec)
UDP (sec)
Throughput File 1 (16KB) File 2 (32KB) File 3 (48KB) File 4 (62KB)
TCP (bps)
UDP (bps)
You will use your ‘get ’ command implementation in order to transmit anonymized
files from the server to the client and capture Wireshark Traces in this evaluation phase. The
four files against which you should evaluate are provided as a part of this assignment.
Note: You should run your experiments from the same network. For example, if you run your
UDP experiments from campus and your TCP experiments from home, different delay
characteristics of the campus and your home network will skew your results.
Preparing your report:
You need to submit a brief report (not more than 4 pages) on your evaluation
and findings. Your report should also include:
• Your name and email address.
• A description of your methodology. How did you process the Wireshark traces to
calculate the above metrics? Did you use a program, or did you do it manually?
• Two tables, using the same format as the ones above, with filled out values for overall
delay and achieved throughput, calculated in your Wireshark analysis.
• A description of the trends you see in your results along with a justification of these
trends.

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