TCP

What is it?

TCP is a core internet protocol that delivers data reliably and in order between two computers, making sure nothing is lost or jumbled.

Explain like I'm 5

Why was it created?

Networks drop and reorder packets. TCP was created to give applications a dependable, ordered stream on top of an unreliable network.

Where is it used?

• Web (HTTP/HTTPS) traffic
• Email and file transfer
• Database connections
• Most reliable internet communication

Why should developers care?

Most internet traffic — web, email, APIs — rides on TCP, so understanding it underpins all networking knowledge.

How does it work?

TCP opens a connection with a handshake, splits data into numbered segments, acknowledges what's received, retransmits anything lost, and reorders segments so the application gets a clean, ordered stream.

Real-world example

Loading a web page opens a TCP connection that guarantees the page's bytes arrive complete and in order, even if some packets are lost in transit.

Common use cases

• Reliable data transfer
• Web and API communication
• File and email transfer
• Any case where every byte must arrive

Advantages

• Reliable, ordered delivery
• Handles retransmission automatically
• Controls congestion
• Universally supported

Disadvantages

• More overhead and latency than UDP
• Connection setup costs time
• Not ideal for real-time streaming where speed beats completeness

When should you use it?

When every byte must arrive correctly and in order.

When should you avoid it?

For latency-critical real-time data (video, games) where UDP's speed is preferred.

Alternatives

Related terms

Interview questions

Beginner

• What does TCP guarantee?
• How does TCP differ from UDP?

Intermediate

• What is the TCP three-way handshake?
• What is an acknowledgement?

Senior

• How does TCP congestion control work?
• Why might QUIC be preferred over TCP?

Common misconceptions

• "TCP and IP are the same" — IP routes packets; TCP adds reliability on top of IP.
• "TCP is always best" — for real-time media, UDP's lower latency often wins.

Fun facts

• TCP stands for Transmission Control Protocol.
• It's so often paired with IP that the combo is just called TCP/IP.

Timeline

• 1981 — TCP standardized as a core internet protocol

Learning resources

• MDN: TCP

Quick summary

TCP is the protocol that delivers data reliably and in order across the internet, underpinning the web, email, and most traffic.

Cheat sheet

• Reliable, ordered delivery
• Handshake + acknowledgements
• Retransmits lost data
• Heavier but dependable (vs UDP)

If you remember only one thing