TCP

TCP (Transmission Control Protocol) is a connection-oriented, reliable, ordered transport-layer protocol, establishing connections with a three-way handshake and providing sliding-window flow control and ACK-based acknowledgment.

Purpose

It provides reliable data transfer that arrives complete and in order, fitting file transfers and the Web.

Mechanism

A three-way handshake sets up the connection (SYN, SYN-ACK, ACK); a four-way close tears it down. Data is sequence-numbered and acknowledged by the receiver. Flow control uses a sliding window; congestion control uses slow start and AIMD. Retransmission happens on timeout. The state machine has 11 states.

Use cases

HTTP/HTTPS web transport
SSH remote login
SMTP mail transfer
File transfer (FTP/SFTP)

Examples

Input
```
SYN seq=100
```
Output
```
SYN-ACK seq=200 ack=101
```
The three-way handshake

Common errors

TIME_WAIT buildup exhausts ports: Tune tcp_tw_reuse or use a connection pool

Related terms

UDP (User Datagram Protocol) is a connectionless, unreliable, unordered transport-layer protocol with low overhead and minimal delay, suited to real-time and broadcast traffic.

A socket is a network communication endpoint identified by (protocol, local IP, local port, remote IP, remote port), serving as the interface between the application and transport layers.

A port is a transport-layer 16-bit address (0-65535) that distinguishes multiple network applications on one host; combined with the IP address it forms a socket.

TLS (Transport Layer Security) provides encryption, integrity, and authentication at the transport layer; it replaced SSL and underpins HTTPS, SMTPS, and more.

Related tools

Tcp Header Parser

Tcp Header Parser — pure client-side tool. Privacy-first, no server round-trip.

Tcp State Reference

Tcp State Reference — pure client-side tool. Privacy-first, no server round-trip.

References