litterTCP

A lightweight TCP/IP stack with ARP, ICMP, UDP, simplified TCP, and socket APIs—built for teaching, research, and simple communication with Linux servers and clients.

Tutorial

English: Let’s write a TCP/IP Stack from Scratch(Part1: Ethernet Frame I/O) | by Skaiuijing | Medium

RUN

We will open three terminals:

1. One for the Linux TCP server
2. One for the litterTCP client
3. One for tcpdump to capture packets

Clone the project

git clone https://github.com/skaiui2/litterTCP.git

Create the virtual network interface

Inside the project directory, run the script I prepared:

cd litterTCP/
chmod +x do.sh
./do.sh

This will create a TAP interface for our TCP/IP stack.

Build the project with CMake

mkdir build
cd build/
cmake ..

Then go back to the project root:

cd ..

Start the Linux TCP server

cd linux_tcp
gcc tcp_handle.c -o handle
./handle

This program listens for incoming TCP connections from our litterTCP stack.

Open another terminal and start tcpdump

First, disable unrelated IPv6 traffic to keep the output clean:

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.ens33.disable_ipv6=1

Then capture packets on our TAP interface:

sudo tcpdump -i tap0

You should now see raw Ethernet frames flowing through the interface.

Run the litterTCP client

Open a third terminal:

cd ..
chmod +x k.sh
./k.sh

This will start the litterTCP TCP client, which connects to the Linux TCP server.

Result

On the Linux TCP server

You will see the messages sent by the litterTCP client, and the server will respond accordingly.

On the litterTCP side

You will see the incoming packets from the Linux server

On tcpdump

You will see the complete TCP exchange captured at the Ethernet frame level.

Everything works perfectly — a full TCP session implemented by our own TCP/IP stack.