What is TCP and what does it provide on top of IP?

Question

Accepted Answer

TCP turns the unreliable, packet-based IP layer into a reliable, ordered, application-to-application byte stream.
What IP gives you:

Best-effort packet delivery between IP addresses
No guarantee of order, no guarantee of arrival, no flow control

What TCP adds:

Feature
What it solves

Ports
Multiplex multiple apps on one IP (port 80 for web, 25 for mail, 443 for HTTPS, …)

Sequence numbers
Reorder packets that arrive scrambled

Acknowledgments
Retransmit lost packets

Flow control
Sender doesn't overwhelm slow receiver (window size)

Congestion control
Sender adapts to network capacity

Connection state
Three-way handshake, four-way teardown

Where TCP lives:
Application  ← Your HTTP, SMTP, FTP, SSH
Transport    ← TCP (or UDP) ← provides ports, reliability
Network      ← IP            ← provides packet delivery
Link         ← Ethernet      ← provides frame delivery

The mental model:
IP is like the postal service — it delivers individual letters with no guarantee of arrival or order. TCP is like a manager who:

Numbers each letter
Asks the recipient to confirm each one
Re-sends lost ones
Pauses if the recipient says "I'm overwhelmed"

TCP vs UDP:

TCP
UDP

Reliable?
Yes
No

Ordered?
Yes
No

Connection?
Yes (handshake)
No

Overhead
Higher
Minimal

Use cases
HTTP, SSH, FTP, email
DNS, DHCP, VoIP, gaming

Tip: When choosing between TCP and UDP for a new protocol, ask: "Is reliability my problem to solve, or the transport's?" Real-time audio/video accept loss in exchange for low latency → UDP. File transfer cannot tolerate loss → TCP.
Go deeper:

Transmission Control Protocol (Wikipedia) — segment header, flags, flow/congestion control, and the full connection lifecycle.

Feature	What it solves
Ports	Multiplex multiple apps on one IP (port 80 for web, 25 for mail, 443 for HTTPS, …)
Sequence numbers	Reorder packets that arrive scrambled
Acknowledgments	Retransmit lost packets
Flow control	Sender doesn't overwhelm slow receiver (window size)
Congestion control	Sender adapts to network capacity
Connection state	Three-way handshake, four-way teardown

	TCP	UDP
Reliable?	Yes	No
Ordered?	Yes	No
Connection?	Yes (handshake)	No
Overhead	Higher	Minimal
Use cases	HTTP, SSH, FTP, email	DNS, DHCP, VoIP, gaming