What is TCP? (Transmission Control Protocol)

TCP (Transmission Control Protocol) is a core communication protocol that ensures reliable and ordered delivery of data between applications over a network. It establishes a connection between the sender and receiver, verifies data integrity, and retransmits any lost or corrupted data.
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Related terms: IP address | SOCKS proxy | UDP

What is TCP?

TCP is one of the fundamental protocols that make the internet work. It provides a reliable way for applications to exchange data over a network, ensuring that data arrives correctly and in the right order.

TCP establishes a connection between the sender and receiver before transmitting data. It divides the data into packets, adds sequence numbers, and verifies their integrity upon arrival. If any packets are lost or corrupted, TCP retransmits them, and makes sure it correctly delivers all the data.

Unlike some other protocols that simply send data without checking for errors or ensuring correct order, TCP takes extra steps to guarantee data integrity and delivery. This makes it essential for applications where data accuracy and completeness are critical.

What is TCP used for?

You can use TCP for applications that require reliable data transfer, such as:

  • Web browsing
  • Email
  • File transfer
  • Remote access

Web browsing

When you access a website, your browser uses TCP to ensure that it correctly downloads all the web page elements (for example: text, images, and scripts) in the correct order, which ensures the page displays correctly.

Email

Email relies on TCP to reliable deliver messages to your recipient's inbox without any data corruption or loss.

File transfer

File transfer protocols like FTP use TCP to ensure that you can send files completely and without errors. This maintains the integrity of the files you transfer.

Remote access

Remote desktop applications and secure shell (SSH) use TCP for secure and reliable remote connections. This allows you to control remote computers without bugs.

How TCP works

TCP uses a series of steps to ensure reliable data transmission:

  1. Three-way handshake: The sender and receiver establish a connection by exchanging messages to synchronize their state and confirm they are ready to communicate.
  2. Data segmentation: Data is divided into smaller packets for transmission, making it easier to manage and transmit over the network.
  3. Sequence numbers: Each packet is assigned a sequence number, allowing the receiver to reassemble the data in the correct order, even if packets arrive out of order.
  4. Acknowledgement (ACK): The receiver sends acknowledgements to the sender, confirming the receipt of packets, so the sender knows the data has been successfully received.
  5. Retransmission: If a packet is lost or corrupted during transmission, the sender retransmits it until it's acknowledged by the receiver, ensuring no data is lost.
  6. Flow control: TCP manages the rate of data transmission to prevent the sender from overwhelming the receiver with too much data at once.
  7. Congestion control: TCP monitors network conditions and adjusts the transmission rate to avoid congestion and maintain a smooth flow of data.

TCP vs. UDP

TCP and UDP are both transport layer protocols, but they serve different purposes:

  • TCP: Provides reliable, ordered, and error-checked data delivery. It prioritizes accuracy and completeness.
  • UDP: Provides a faster but less reliable datagram service. It prioritizes speed and efficiency over guaranteed delivery.

Choosing between TCP and UDP depends on the specific needs of the application. For applications that require guaranteed delivery and data integrity, TCP is the preferred choice. For applications where speed and low latency are more important than absolute reliability, UDP might be suitable.

By understanding the fundamentals of TCP and its role in network communication, you can better appreciate how the internet ensures reliable data transmission for a wide range of applications.