User-space TCP/IP Stack

Establish the foundational environment by creating and configuring a TUN/TAP virtual network interface. This interface will serve as the bridge between the kernel's networking and your user-space application, allowing you to intercept and inject raw IP packets.

Define the fundamental data structures for all relevant network protocol headers (Ethernet, IP, ICMP, UDP, TCP) and create utility functions for handling network-to-host byte order conversions (endianness).

Implement the Internet Protocol (IP) layer. This involves parsing incoming IP packets, validating their headers, and preparing them for demultiplexing to the correct transport layer protocol (e.g., ICMP, UDP, TCP).

Implement the Internet Control Message Protocol (ICMP) to handle network diagnostics. The primary goal is to respond to ICMP Echo Requests, effectively making your stack 'pingable'.

Implement the User Datagram Protocol (UDP) layer. This connectionless protocol is simpler than TCP and involves parsing a UDP header and delivering the payload to a conceptual 'port'.

Begin the TCP implementation by focusing on connection management. This involves implementing the 3-way handshake (SYN, SYN-ACK, ACK) for establishing connections and the FIN/ACK exchange for graceful connection termination.

Implement the core data transfer mechanism for TCP. This includes managing sequence and acknowledgment numbers for reliable, in-order delivery, and implementing send and receive buffers for each connection.

Implement essential TCP timers and a basic congestion control algorithm. This includes the Retransmission Timer (RTO) to handle lost packets and the Slow Start algorithm to prevent network congestion.

Design and implement a user-facing 'socket-like' API. This will abstract the complexities of your TCP/IP stack, providing a familiar interface for application programmers to use your network implementation.

Build a simple application on top of your custom socket API to perform a full, end-to-end test of the stack. A basic HTTP server is an excellent choice as it tests connection handling and data transfer.