TCP vs UDP — Packet.School

The transport layer's two main protocols: TCP chooses reliability, UDP chooses speed. Knowing which application uses which is half of reading network traffic.

01Core differences

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	No guarantee
Ordering	In-order delivery	Unordered
Speed	Slower	Fast
Overhead	High	Low

02TCP — like a phone call

First a connection is established, you confirm the other side is listening, then you talk.

Its features

3-way handshake — connection setup
Sequence numbers — pieces are put back in order
Acknowledgments — every packet is confirmed
Retransmission — lost packets are sent again
Flow control — slows down to the receiver's pace

TCP connection setup

code

Client         Server
  |--- SYN --->|
  |<-- SYN+ACK-|
  |--- ACK --->|
  [Connection established]

TCP data transfer

code

Data 1 →→→ ACK 1
Data 2 →→→ ACK 2
Data 3 →→→ LOST!
Data 3 →→→ ACK 3   (noticed, retransmitted)

03UDP — like a letter

Drop the envelope in the box and move on. Did it arrive? In order? UDP doesn't care.

code

Data 1 →→→ arrived
Data 2 →→→ arrived
Data 3 →→→ lost (nobody noticed)
Data 4 →→→ arrived

That indifference isn't a flaw — it's a deliberate trade: a protocol that never waits for confirmation keeps latency at a minimum.

04When to use which?

Use TCP — when every byte matters

Websites (HTTP/HTTPS)
Email (SMTP, IMAP)
File transfer (FTP)
SSH connections

Use UDP — when speed is everything

Online games
Video streaming and live broadcasts
VoIP (voice calls)
DNS queries
DHCP

An online game uses both at once: your character's position flows over UDP (a lost frame is corrected by the next one), while a purchase goes over TCP (it absolutely must arrive).

05Try it yourself

See what latency and packet loss actually look like by measuring them:

Sandbox · CommandPing & TracerouteCompare latency, packet loss, and hop counts across different destinations.Open tool

06Summary

TCP = reliable but costly: web, email, files
UDP = fast but no guarantees: games, video, DNS
One application can use both at the same time
HTTP/3 (QUIC) builds its own reliability on top of UDP

01Core differences

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	No guarantee
Ordering	In-order delivery	Unordered
Speed	Slower	Fast
Overhead	High	Low

02TCP — like a phone call

First a connection is established, you confirm the other side is listening, then you talk.

Its features

3-way handshake — connection setup
Sequence numbers — pieces are put back in order
Acknowledgments — every packet is confirmed
Retransmission — lost packets are sent again
Flow control — slows down to the receiver's pace

TCP connection setup

code

Client         Server
  |--- SYN --->|
  |<-- SYN+ACK-|
  |--- ACK --->|
  [Connection established]

TCP data transfer

code

Data 1 →→→ ACK 1
Data 2 →→→ ACK 2
Data 3 →→→ LOST!
Data 3 →→→ ACK 3   (noticed, retransmitted)

03UDP — like a letter

Drop the envelope in the box and move on. Did it arrive? In order? UDP doesn't care.

code

Data 1 →→→ arrived
Data 2 →→→ arrived
Data 3 →→→ lost (nobody noticed)
Data 4 →→→ arrived

That indifference isn't a flaw — it's a deliberate trade: a protocol that never waits for confirmation keeps latency at a minimum.

04When to use which?

Use TCP — when every byte matters

Websites (HTTP/HTTPS)
Email (SMTP, IMAP)
File transfer (FTP)
SSH connections

Use UDP — when speed is everything

Online games
Video streaming and live broadcasts
VoIP (voice calls)
DNS queries
DHCP

An online game uses both at once: your character's position flows over UDP (a lost frame is corrected by the next one), while a purchase goes over TCP (it absolutely must arrive).

05Try it yourself

See what latency and packet loss actually look like by measuring them:

Sandbox · CommandPing & TracerouteCompare latency, packet loss, and hop counts across different destinations.Open tool

06Summary

TCP = reliable but costly: web, email, files
UDP = fast but no guarantees: games, video, DNS
One application can use both at the same time
HTTP/3 (QUIC) builds its own reliability on top of UDP