Implement the TCP 3-Way Handshake Using Raw Sockets

The Three-Way Handshake

TCP is connection-oriented. Before data flows to port 443, both sides complete a three-step dance. For example, a client with ISN 1000 connects to a server with ISN 5000:

1. SYN: client sends SEQ=1000, SYN flag set. State: CLOSED -> SYN_SENT
2. SYN-ACK: server replies SEQ=5000, ACK=1001, SYN+ACK flags. State: LISTEN -> SYN_RECEIVED
3. ACK: client sends SEQ=1001, ACK=5001, ACK flag only. Both sides reach ESTABLISHED

State Machines

Client: CLOSED -> SYN_SENT -> ESTABLISHED

Server: CLOSED -> LISTEN -> SYN_RECEIVED -> ESTABLISHED

Each side picks a random 32-bit ISN. The ACK number always equals the peer's sequence plus one because the SYN itself consumes one sequence number.

Why Three Steps?

The handshake confirms both sides can send and receive. A stale delayed SYN fails because the ACK number will not match any live connection. SYN flood attacks exploit the half-open SYN_RECEIVED state, which is why Linux ships SYN cookies as a defense.

Why This Exercise

This task asks you to simulate the handshake with printed segments and state transitions. The Linux kernel's tcp_v4_syn_recv() implements exactly this state machine, and nmap's SYN scan reads these same flag bytes without completing the handshake. Kevin Mitnick's 1994 ISN-prediction attack against Tsutomu Shimomura targeted the random sequence number chosen during step one of this exact exchange.