Systems & CLI · Pro
Build a TCP/IP Stack
Implement a userspace TCP/IP stack over a TUN/TAP device that speaks Ethernet, ARP, IPv4, and ICMP and drives a real TCP connection through its full state machine.
You build a networking stack in C that attaches to a virtual TUN/TAP interface, parses raw Ethernet frames off the wire, answers ARP requests, routes IPv4 datagrams, replies to ICMP echo requests, and walks a TCP connection from SYN through ESTABLISHED to the FIN/TIME_WAIT teardown. It is worth building because almost no engineer ever works below the socket API, so implementing the three-way handshake, retransmission, and the sliding window yourself turns 'TCP is reliable' from a slogan into something you can reason about byte by byte. It is resume-defining because it proves bit-level protocol parsing, finite state machine design, and the discipline to debug live packet captures in Wireshark. You finish able to explain exactly what happens between two hosts that a job description only abbreviates as 'strong networking fundamentals'.
What you build
- Reads and writes raw frames through a TUN/TAP device
- Parses and emits Ethernet II framing with correct MAC handling
- Implements ARP request/reply and a cached neighbor table
- Validates and routes IPv4 headers including checksums and fragmentation basics
- Answers ICMP echo so the host can ping the stack
- Drives a TCP state machine covering handshake, data transfer, and teardown
- Handles retransmission timers, sequence/acknowledgement numbers, and a receive window
What it teaches
- Bit-level protocol parsing and checksum computation
- The TCP connection state machine and reliable delivery
- ARP, IPv4, and ICMP packet semantics
- Working with TUN/TAP virtual interfaces
- Debugging with Wireshark and tcpdump
- Timers, sequence math, and flow control with the sliding window
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Take it further
- Add congestion control with slow start and congestion avoidance.
- Expose a minimal sockets-style API so a sample app can connect through your stack.
- Support a basic retransmission and out-of-order reassembly queue under packet loss.
