Homework 5: Reliable Communication

In this homework, you will implement reliable communication over an unreliable link, just like TCP.

You will be provided with code that simulates an unreliable link between sender and receiver. This link has a very constrained buffer (only two packets can be ‘in flight’ at a time), and can have arbitrary delay and loss rates. Your job will be to create and implement a protocol over this connection that correctly transfers data, in a reasonable amount of time.

Writing Your Solution

This repo contains several tools that will help you simulate and test your solution. You should not make changes to any file other than hw5.py. All other files contain code used to either simulate the unreliable connection, or code to help you test your your solution.

Your solution / hw5.py file will be tested against stock versions of all the other files in the repo, so any changes you make will not be present at grading time.

Your solution must be contained in the send and recv functions in hw5.py. You should not change the signatures of these functions, only their bodies. These functions will be called by the grading script, with parameters controlled by the grading script. Your solution must be general, and should work for any file.

Your task is to modify the bodies of these functions so that they communicate using a protocol that ensures that the data sent by the send function can be reliably and quickly reconstructed by the recv function. You should do so through a combination of setting timeouts on socket reads (e.x. socket.timeout(float)) and developing a system through which each side can acknowledge if / when they receive a packet.

Remember that the connection is bandwidth constrained. No more than two packets can be “on the wire” at a time. If you send a third packet while there are already two packets traveling to their destination (in either direction), the third packet will be dropped, so it is important that you get your timeouts and your acknowledgments right.

Testing Your Solution

You can use the provided tester.py script when testing your solution. This script uses the receiver.py, sender.py, and server.py scripts to simulate an unreliable connection, and to test your solution.

The tester.py script contains many parameters you can use to test your solution under different conditions, and to receive different amounts of debugging information to better understand the network. These parameters and options can be viewed by calling tester.py --help, and are also reproduced below.

usage: tester.py [-h] [-p PORT] [-l LOSS] [-d DELAY] [-b BUFFER] -f FILE
                [-r RECEIVE] [-s] [-v]

Utility script for testing HW5 solutions under user set conditions.

optional arguments:
-h, --help            show this help message and exit
-p PORT, --port PORT  The port to simulate the lossy wire on (defaults to
-l LOSS, --loss LOSS  The percentage of packets to drop.
-d DELAY, --delay DELAY
                        The number of seconds, as a float, to wait before
                        forwarding a packet on.
-b BUFFER, --buffer BUFFER
                        The size of the buffer to simulate.
-f FILE, --file FILE  The file to send over the wire.
-r RECEIVE, --receive RECEIVE
                        The path to write the received file to. If not
                        provided, the results will be written to a temp file.
-s, --summary         Print a one line summary of whether the transaction
                        was successful, instead of a more verbose description
                        of the result.
-v, --verbose         Enable extra verbose mode.

For example, to see how your solution performs when transmitting a text file, with a 5% loss rate, and with a latency of 100ms, you could use the following: python3 tester.py --file test_data.txt --loss .05 --delay 0.1.

Hints and Suggestions

  • A key part of this homework is determining how long to wait before resending a packet. You should estimate this timeout value using the EWMA technique for estimating the RTT, and use this in determining your timeout. With correctly tuned timeouts, lower RTT will result in higher throughput.

    A good way of determining the timeout to use is the “estimated RTT + (deviation of RTT * 4)”. You should check with your book for more details.

  • Use the included --verbose option to include very detailed information about what your code is sending over the network, and how the network is handling that data.

  • Use the included --receive option to see the results of your file transfer. By default, the testing script will store the results of your code to a temporary location. This option may be useful if you’re not sure how or why the received file does not match the sent file.

  • Make sure you try your solution under many different loss ratios and latencies by changing the parameters in the tester.py script.

  • Keep your packets smaller than or equal to homework5.MAX_PACKET (1400 bytes).

  • Pay attention to the end of the connection. Ensure that both sides of the connection finish without user assistance, even if packet losses occur, while guaranteeing that the entire file is transferred. Look at the FIN/FINACK/ACK sequence in TCP for ideas.


You solution will be graded by using it to transfer six different files, each under different simulated test conditions. For each test case, there is a minimum throughput requirement and a timeout for your program to exit. The timeout is set as 50% more than the corresponding required throughput.

Each test case will be scored accordingly:

Case Points Earned
File is not transmitted correctly 0
Transmission takes longer than the max time 0
Successful transmission, but low throughput 1
Successful transmission, fast throughput 2

If your program exits normally before the timeout, but the content of the received file is invalid, then zero points are awarded.

If your program doesn’t exit before the timeout, it will be terminated before completion, resulting in incorrect file content, and so 0 points.

If the program exits normally before the timeout and the received file’s content is valid but the throughput obtained is lower than the required minimum throughput then you receive 1 point.

If your program correctly transmits the file below the timeout, and with the required throughput, it will receive 2 points.

Code that earns at least 5 of the above points, and which is both “PEP 8” and “pylint” compatible will earn an additional 1 point.

There are 13 points possible on this assignment. Your solution will be graded out of 12 possible points.

This assignment is due April 10th at 3pm.