Part 2 · Lesson 8 · Intermediate

TCP: The Reliable Pipe

Watch TCP build a reliable pipe from unreliable packets.

In this interactive Infrastructure lesson from Stencel Academy you don't just read — you type real commands like sudo tcpdump -n host 203.0.113.10, curl -s https://api.stencel.ai/health and watch the machine respond on a live, animated map. It is part of the free How Infrastructure Works course.

Key takeaway: TCP numbers every byte and acknowledges every segment — a reliable pipe built on an unreliable network.

Before this lesson

Builds on: Packets & Routing.

What you'll practise

ss -tni state established — The kernel's live view of the pipe: rtt, cwnd, bytes acked.
sysctl net.ipv4.tcp_syn_retries — Lost SYNs are retried with backoff — reliability starts at packet one.
sudo tcpdump -n 'tcp[tcpflags] & tcp-syn != 0' — Filter to connection openings only.
curl --max-time 2 https://10.0.9.9/ — A SYN nobody answers: retransmit, retransmit, give up.

Terms you'll learn

TCPthree-way handshakeSYN / SYN-ACK / ACKsegmentretransmissionFIN

Check your understanding

Why three packets (SYN, SYN-ACK, ACK) before any data?

Both ends must agree on starting sequence numbers to number every byte
Encryption keys are exchanged in those three packets
The server needs time to wake up

Open the interactive lesson →