TCP/IP Model

What is the TCP/IP Model?

The TCP/IP model (also called the Internet model or DoD model) is the practical networking model used by the Internet. It was developed by DARPA in the 1970s. Unlike the OSI model (theoretical), TCP/IP is the actual implementation used in real networks.

The TCP/IP model has 4 layers (some sources say 5):

TCP/IP vs OSI Model

TCP/IP Layer	OSI Equivalent	Protocols
Application	Application + Presentation + Session (7,6,5)	HTTP, HTTPS, FTP, SMTP, DNS, DHCP, SSH, Telnet, SNMP
Transport	Transport (4)	TCP, UDP
Internet	Network (3)	IP (IPv4/IPv6), ICMP, ARP, RARP, OSPF, BGP
Network Access	Data Link + Physical (2,1)	Ethernet, Wi-Fi, PPP, Frame Relay

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented (3-way handshake)	Connectionless
Reliability	Reliable (guaranteed delivery)	Unreliable (best effort)
Ordering	Ordered delivery	No ordering guarantee
Error Checking	Yes (checksum + retransmission)	Checksum only (no retransmission)
Flow Control	Yes (sliding window)	No
Congestion Control	Yes	No
Speed	Slower (overhead)	Faster (less overhead)
Header Size	20-60 bytes	8 bytes
Use Cases	HTTP, FTP, SMTP, SSH (reliability needed)	DNS, DHCP, VoIP, video streaming, gaming

TCP 3-Way Handshake

Before data transfer, TCP establishes a connection using a 3-way handshake:

SYN (Synchronize): Client sends a SYN packet to the server with a random sequence number (ISN). "I want to connect, my sequence number is X."
SYN-ACK (Synchronize-Acknowledge): Server responds with SYN-ACK. Acknowledges client's SYN (ACK = X+1) and sends its own SYN with sequence number Y. "I acknowledge your X, my sequence number is Y."
ACK (Acknowledge): Client sends ACK (ACK = Y+1). "I acknowledge your Y. Connection established."

After the handshake, data transfer begins. The connection is full-duplex — both sides can send and receive simultaneously.

TCP Connection Termination (4-Way Handshake)

TCP uses a 4-way handshake to close a connection:

FIN: Client sends FIN to server. "I'm done sending data."
ACK: Server acknowledges the FIN. "Got it."
FIN: Server sends its own FIN. "I'm also done sending data."
ACK: Client acknowledges server's FIN. "Got it. Connection closed."

After the final ACK, the client waits in TIME_WAIT state (2×MSL) before fully closing, to ensure the server received the final ACK.

Port Numbers

Range	Type	Examples
0 – 1023	Well-Known Ports	HTTP(80), HTTPS(443), FTP(21), SSH(22), SMTP(25), DNS(53)
1024 – 49151	Registered Ports	MySQL(3306), PostgreSQL(5432), MongoDB(27017), Redis(6379)
49152 – 65535	Dynamic/Ephemeral Ports	Assigned temporarily to client connections

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