Writing your own TCP/IP stack may seem like a daunting task. Indeed, TCP has accumulated many specifications over its lifetime of more than thirty yea

Let's code a TCP/IP stack, 1: Ethernet & ARP

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2021-06-27 19:00:05

Writing your own TCP/IP stack may seem like a daunting task. Indeed, TCP has accumulated many specifications over its lifetime of more than thirty years. The core specification, however, is seemingly compact1 - the important parts being TCP header parsing, the state machine, congestion control and retransmission timeout computation.

The most common layer 2 and layer 3 protocols, Ethernet and IP respectively, pale in comparison to TCP’s complexity. In this blog series, we will implement a minimal userspace TCP/IP stack for Linux.

The purpose of these posts and the resulting software is purely educational - to learn network and system programming at a deeper level.

To intercept low-level network traffic from the Linux kernel, we will use a Linux TAP device. In short, a TUN/TAP device is often used by networking userspace applications to manipulate L3/L2 traffic, respectively. A popular example is tunneling, where a packet is wrapped inside the payload of another packet.

The advantage of TUN/TAP devices is that they’re easy to set up in a userspace program and they are already being used in a multitude of programs, such as OpenVPN.