After completing this lesson, you will be able to confidently handle the following three things.

• ▸✅ TCP 3-way handshake / 4-way close
• ▸✅ Criteria for choosing TCP vs UDP
• ▸✅ TCP flow control + congestion control

Keep the learning objectives as a checklist and close the lesson only once you can answer all of them.

TCP (Transmission Control Protocol):

• ▸Connection-oriented (3-way handshake)
• ▸Reliability (retransmission, ordering, deduplication)
• ▸Flow and congestion control (adjusts speed based on network conditions)
• ▸Use cases: HTTP, SSH, email, DB

UDP (User Datagram Protocol):

• ▸Connectionless (no handshake)
• ▸Fast (low overhead)
• ▸Reliability ❌ (the application is responsible)
• ▸Use cases: DNS, VoIP, gaming, real-time video, QUIC

Comparison:

QUIC (HTTP/3) — reliability over UDP:

• ▸Developed by Google, RFC 9000 (2021)
• ▸Avoids TCP+TLS overhead
• ▸Excels in mobile and real-time environments
• ▸Adopted by YouTube, Google, and Cloudflare

3-way Handshake (Connection):

Client                              Server
  │   ── SYN, seq=x         ──▶     │
  │                                  │
  │   ◀── SYN-ACK, seq=y, ack=x+1   │
  │                                  │
  │   ── ACK, ack=y+1       ──▶     │
  │                                  │
  │       [Connection established — data transfer begins]

1. SYN — Client: "Connection request (seq=x)"
2. SYN-ACK — Server: "Accepted + my seq=y"
3. ACK — Client: "Confirmed (ack=y+1)"

Bidirectional data transfer follows.

4-way Handshake (Termination):

Client                              Server
  │   ── FIN              ──▶       │
  │   ◀── ACK              ─        │   (server is wrapping up data)
  │                                  │
  │   ◀── FIN              ─        │
  │   ── ACK              ──▶       │
  │       [TIME_WAIT — waits 2*MSL]

1. FIN — Client: "I am closing"
2. ACK — Server: "Got it. Just a moment (finishing remaining data)"
3. FIN — Server: "I am done too"
4. ACK — Client: "Confirmed"

TIME_WAIT (2 × MSL):

• ▸Needed to retransmit the final ACK if it is lost
• ▸Prevents confusion with old packets when the same port is reused
• ▸Linux default: 60 seconds

Why 4 steps? Termination requires both sides to finish their data. Unlike connection setup, SYN and ACK cannot be combined.

Flow Control — matches the receiver's processing speed:

• ▸The receiver advertises its Window Size
• ▸The sender transmits only up to that amount
• ▸When the receive buffer is full, Window Size = 0 → sender pauses

Sliding Window:

Data sent          Can be sent        Cannot be sent
[──ACKed──][──Sent but not yet ACKed──][──Outside Window──]
                   └─── Window Size ───┘

Congestion Control — matches the network's condition:

4 algorithms:
1. Slow Start — doubles the window every RTT (exponential growth)
2. Congestion Avoidance — linear increase after the threshold (+1 per RTT)
3. Fast Retransmit — retransmits immediately upon receiving 3 duplicate ACKs
4. Fast Recovery — goes to Congestion Avoidance instead of Slow Start

Modern algorithms:

RTT (Round-Trip Time) — a key network metric:

• ▸Seoul → Seoul: 1-5 ms
• ▸Seoul → Tokyo: 30 ms
• ▸Seoul → California: 150 ms
• ▸Seoul → Europe: 250 ms

> 💡 Mobile networks have highly variable RTT → delay-based algorithms like BBR have an advantage.

Once you understand the concepts in this lesson, you can give AI specific instructions. Instead of a vague "fix this," you can make requests with vocabulary — that is the starting point for saving tokens.

• ▸"Diagnose whether TCP or UDP is the right fit for this task"
• ▸"Simulate a TCP 3-way handshake in Wireshark capture format"

Why this reduces tokens

Without understanding the concepts, you have to ask "What does that mean?" after every AI response. Those follow-up questions burn tokens. Learn the concepts once and the conversation ends in one exchange.