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Intermediate Topic 21 of 50 Computer Networks

Three-Way Handshake

Exchange establishing a TCP connection.

How it works

TCP opens a connection with three messages: the client sends SYN with its initial sequence number, the server replies SYN-ACK with its own number and acknowledgement, and the client sends ACK. After this, both sides have agreed on starting sequence numbers and the connection is established. Closing uses FIN and ACK in each direction, with a TIME_WAIT state afterwards.

Step-by-step walkthrough

  1. The client sends SYN with its starting sequence number.
  2. The server replies SYN-ACK with its own number.
  3. The client sends ACK.
  4. Both sides now agree on sequence numbers.
  5. The connection is established and data can flow.

Where it is used

Every TCP connection, from a web page load to an SSH login, starts with it. It costs one round trip before data, which is why connection reuse, TLS session resumption and TCP Fast Open matter.

Common pitfall

SYN flood attacks fill the server's half-open connection queue; SYN cookies mitigate them. Firewalls dropping SYN-ACK cause timeouts. Latency from the extra round trip hurts short requests on long paths. Using keep-alive connections and pooling avoids repeated handshakes.

Where it fits in the course

Three-Way Handshake is topic 21 of 50 in Computer Networks, rated Intermediate. It follows UDP. The next topic is Flow Control.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Computer Networks quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Three-Way Handshake?
Exchange establishing a TCP connection. It belongs to Computer Networks and is an intermediate-level topic.
Where do people use Three-Way Handshake?
Every TCP connection, from a web page load to an SSH login, starts with it.
What is the most common mistake with Three-Way Handshake?
SYN flood attacks fill the server's half-open connection queue; SYN cookies mitigate them.
What should I learn before this topic?
Start with UDP, the topic just before it in Computer Networks.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Computer Networks quiz.
Is Three-Way Handshake hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Three-Way Handshake?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Three-Way Handshake?
UDP, Flow Control, TCP are closely connected. The full list is in the related topics section of this page.
What should I study after Three-Way Handshake?
Continue with Flow Control, the next topic in Computer Networks.

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