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CS 168 Spring 2025

Project 3: Transport

In this project, you will implement a subset of the TCP protocol. Your implementation will provide reliability, though it wonʼt include congestion control.

Lectures needed for this project: Lecture 11 (Reliability), 12 (TCP Implementation).

You can work on this project alone, or with one partner. You can keep the same partner from an earlire project, or choose a different partner. Reminder: Our course policies page has a collaboration policy that you must follow.

Setup

Operating System Installation

This project has been tested to work on Linux and Mac.

If youʼre on Windows, we recommend using WSL.

Python Installation

This project has been tested to work on Python 3.7. (It probably works on newer versions of Python as well, but use at your own risk.)

If you run python3 --version or python --version in your terminal and you see output of the form Python 3.7.* , youʼre all set.

Starter Code

Download a copy of the starter code here.

In your terminal, use cd to navigate to the cs168-sp25-proj3-transport/ext/cs168p2 directory. All of the Python commands should be run from this directory.

To check that your setup works, in your terminal, run:

If you see this message, everything should be set up correctly:

You should only edit ext/cs168p2/student_socket.py . There are comments clearly indicating the places where you should fill in code.

Guidelines:

Donʼt modify any other files.
Donʼt add any new files.
Donʼt add any imports.
Donʼt edit any code outside the sections indicated by the comments.
Donʼt add any hard-coded global variables.
Adding helper methods is fine.
In general, if we havenʼt told you to use something, you probably donʼt need it. Our goal is not to trick you!

New Spec

Note: This spec has been rewritten for Fall 2024 to be clearer and provide a few extra hints. It may contain bugs.

If you would like to work off the previous version of the spec, you can find it here.

Project Overview

This project is split into 9 stages.

In Project 3A, youʼll need to complete Stages 1–5.
In Project 3B, youʼll need to complete Stages 6–9.

Each stage has its own unit tests, provided to you locally. Your grade will be determined only by these unit tests (no hidden tests).

To run a unit test, navigate to cs168-sp25-proj3-transport/ext/cs168p2 , and run one of these commands, replacing 5 with the number of the stage you want to test:

python autograder.py s5 # Runs unit tests for Stage 5.
python autograder.py all # Runs all unit tests.
python autograder.py all 5 # Runs all unit tests up to and including Stage 5.

If a test fails, youʼll see some output like this:

test_s1_t1 (__main__.test_s1_t1.test_s1_t1) ...

Running test: ./tests/s1_t1.cfg

Tracefile: ./trace/s1_t1/2024-08-22T16:09:52.593234

../../pox.py config=./tests/s1_t1.cfg tcpip.pcap --node=r1 --no-tx --filename=./trace/s1_t1/20

FAIL

To see nicely-formatted output for that specific test, take the last line, just before the “FAIL,” drop the --filename argument, and run that command in your terminal:

../../pox.py config=./tests/s1_t1.cfg tcpip.pcap --node=r1 --no-tx

Code Overview

You will be implementing the StudentUSocket class in student_socket.py , which represents a TCP socket.

student_socket.py also contains other useful methods, which weʼll describe below

Modular Arithmetic

Sequence numbers are 32 bits long, and they can wrap around (e.g. 0xFFFFFFFF + 1 = 0x00000000）.

Whenever you perform arithmetic operations on sequence numbers, use these operators:

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CS 168 Spring 2025 Project 3: Transport