What Is Transmission Control Protocol TCP?

You might not know it, but when you transfer a digital file, the transfer relies on TCP.

TCP, or Transmission Control Protocol, is the fundamental internet protocol that ensures files, webpages, emails, and other data are delivered as intended from sender to receiver, perfectly intact and without errors.

Our article explores how TCP diligently provides this data integrity and the features that make it possible. We also discuss its widespread applications and clarify its relationship with other foundational internet protocols, such as IP, and the TCP/IP model that underpins global network communication.

How Does TCP Work?

Let’s walk through the basic TCP working process step by step.

Establishing the connection

When you send an email, you’re transmitting a certain amount of data to a recipient. TCP’s rules require that the sender and receiver establish a connection before this data can bbreaks the data into smaller chunkse transmitted. TCP employs a process known as the 3-way handshake to form this connection.

SYN: The TCP client (sending device) sends a small control message, called a control segment, with the SYN (synchronize) flag set. This is to request a connection.
SYN-ACK: The server (receiving device) responds with a control message carrying both the SYN and ACK (acknowledgment) flags set.
ACK: The client replies with a segment that has the ACK flag set to confirm the TCP connection.

Segmenting the data

To start delivering your email, TCP breaks the data into smaller chunks called segments. Each segment is assigned a sequence number that allows the receiving device to reassemble the message in the correct order.

Data transmission, including error resolution

With the connection open, data flows in both directions. Each TCP segment carries the sequence number, advising the receiver on how to reassemble the data accurately.

Every TCP segment also includes a checksum, which is a form of error detection. If the checksum doesn’t match, the receiver knows the segment is corrupted and responds asking for a retransmission.

Each segment sent receives an acknowledgment. Any errors detected are fixed through retransmission. To prevent overloading devices or networks, TCP controls the traffic using flow and congestion management.

Closing the connection

Once the data transfer is complete, TCP closes the connection using a 4-step process:

FIN: The sender sends a FIN (Finish) message to start closing the connection.
ACK: The receiver acknowledges the FIN.
FIN: The receiver then sends its own FIN to indicate it has finished sending data.
ACK: The sender acknowledges the receiver’s FIN, completing the closure.

This ordered termination ensures that no data is lost or left hanging.

What is TCP Used For?

As a transport-layer protocol, TCP acts as the foundation for many application-layer protocols. The following protocols rely on TCP to ensure the reliable and complete delivery of data across the Internet:

File Transfer Protocol (FTP), Secure Shell (SSH), and Telnet for file sharing and remote access.
Internet Message Access Protocol (IMAP), Post Office Protocol (POP3), and Simple Mail Transfer Protocol (SMTP) to send and receive emails.
Hypertext Transfer Protocol (HTTP) and HTTPS for accessing websites and web services

Let’s explore essential everyday TCP use cases:

Email delivery (SMTP/POP3/IMAP)

TCP facilitates the connections between clients and servers for sending and receiving emails. It ensures messages, including attachments, are transmitted efficiently and accurately.

Chat and messaging

Many messaging and chat applications leverage TCP to ensure that every message sent is properly delivered. Its stringent protocols prevent dropped or jumbled communications.

Web browsing (HTTP/HTTPS)

When you load a webpage, TCP establishes the connection between your browser and the website’s server. It guarantees that all elements, including text, images, videos, and scripts, are delivered intact.

File uploads and downloads (FTP/SFTP)

Protocols like FTP and SFTP rely on TCP to facilitate the complete and error-free transfer of files, regardless of their size.

Database queries

Accessing databases over a network often depends on TCP. The protocol delivers a secure, dependable channel for transmitting sensitive queries and receiving complete database responses.

Remote access (SSH/Telnet)

TCP is utilized in remote access protocols like SSH and Telnet, which allow users to control a computer or network device securely. TCP governs the commands sent to the remote system and its responses. This is essential for managing servers, routers, and other systems off-site.

Virtual Private Networks (VPNs)

Many VPN protocols employ TCP to create secure and dependable encrypted tunnels. This supports reliable data flow for remote users connecting to private networks, particularly over potentially unstable internet connections.

Types of TCP

While TCP is a single protocol, it is sometimes categorized into types based on congestion control algorithms.

TCP’s ability to manage network traffic has evolved significantly over the years. This has led to more and better algorithms for managing congestion. Different versions work in distinct ways to intelligently adjust how quickly TCP transmits data to optimize performance, especially over unstable or high-latency internet connections.

Here are widely recognized variations:

TCP Reno: A traditional version used widely in early networks. Reno significantly improved upon its predecessor, TCP Tahoe, by introducing Fast Recovery. This allowed for quicker recovery from single packet losses without drastically slowing speeds.
TCP New Reno: This version improved on Reno. It’s designed to handle multiple packet losses in the same transmission window more effectively. It supports more efficient recovery, reducing the impact of such losses on overall throughput.
TCP Vegas: TCP Vegas detects congestion before packet loss. It uses changes in RTT (Round Trip Time) to slow down sending before queues overflow. Although Vegas is more stable and efficient than New Reno, it’s less widely adopted due to its more complex implementation and a lack of broad standardization.
TCP CUBIC: TCP CUBIC is the default congestion control algorithm in Linux systems. It’s optimized for high-speed, long-distance networks (often called “long fat networks”).
TCP BBR (Bottleneck Bandwidth and Round-trip propagation time): This variant was developed by Google to improve performance on modern internet paths.

Key Features of TCP

TCP’s core features and functionalities include the following:

Connection-oriented: TCP is defined as a connection-oriented protocol. TCP requires both the sender and receiver to establish a connection before data is transferred. This connection stays open until it is formally terminated after the transfer is complete.
Segment numbering system: TCP provides reliable and ordered data transfer by assigning sequence numbers to every data byte it sends. Each segment is identified by the sequence number of its first byte. The receiver sends back acknowledgment numbers, confirming receipt of segments and indicating the next expected byte.

The numbering system enables the receiver to reconstruct the data properly.
Retransmission of lost packets: If an ACK isn’t received within a certain time (due to packet loss), the sender retransmits the segment. This ensures reliability, even over unstable networks like Wi-Fi.
Congestion control: TCP adjusts its data transmission rate based on network congestion. If packet loss or delays occur, TCP slows the transfer to reduce pressure on the network.

Full-duplex communication. This means that data can flow in both directions simultaneously, from sender to receiver and vice versa. TCP’s efficient, continuous data exchange supports strong system communication.
Checksum: Every TCP data segment includes a checksum. This feature assists error detection and control. If the checksum doesn’t match, the receiver knows the segment is corrupted and asks for a retransmission.
Flow control (Windowing): TCP uses a sliding window protocol to regulate how much data is sent at once. The receiver signals a window size, advising the amount of data it can handle at one time. The flow control feature prevents overwhelming slower devices or networks.
TCP header: The TCP header is a block of control information added to the beginning of each TCP segment. It contains vital information used to manage and ensure proper delivery, like source and destination ports, sequence and acknowledgment numbers, flags (e.g., SYN, ACK), and window size.

When the TCP segment moves down to the network layer, it’s encapsulated in an IP header. It now becomes a packet (or more precisely, an IP packet).

TCP vs IP: What is the Difference?

Transmission Control Protocol (TCP) and Internet Protocol (IP) work together to enable communication across the Internet. Each has a distinct but complementary role in this process:

IP (Internet Protocol) is the set of rules that governs how data is transmitted across the Internet. IP assigns IP addresses to every online device. Each address is a unique identifier; think of it as the digital equivalent of a home address. These identifiers enable devices to communicate and exchange data with one another.

IP is responsible for determining how data should be routed between devices. Routers (the internet’s traffic controllers) within networks use IP addresses to forward data packets along the most efficient path toward their destination.

However, unlike TCP, IP is connectionless. It does not establish a link between the sender and receiver. This means it cannot guarantee reliable, intact delivery. It simply moves packets from one hop to the next until they (hopefully) reach their destination.

TCP’s role is to ensure reliable, ordered, and error-checked delivery of data (e.g., a web page) between client and server. It establishes connections with the 3-way handshake, breaks data into segments, and guarantees their arrival.

It ensures that any lost packets are retransmitted and reassembled in the correct order.

How Does Transmission Control Protocol (TCP)/IP Work?

When combined, TCP/IP forms the backbone of internet communication. IP defines where the data should go, ensuring proper addressing and routing. TCP guarantees that the data arrives as it was sent.

Here’s an analogy: Think of an IP address as a phone number assigned to a smartphone. It identifies the device on the network. TCP is like the system that makes the phone ring, sets up the call, ensures both sides can hear each other clearly, and prevents the conversation from becoming scrambled.

IP functions at the network layer. It directs data packets to their destination using unique IP addresses assigned to devices. It ensures that data finds the best possible path. The path typically includes several hops between routers as it navigates the network.

TCP operates at the transport layer, ensuring that data sent between applications is accurate and complete.

One of TCP’s key functions is sequencing. Packets don’t always arrive in the same order they were sent, because they may take different paths through the network. In such cases, TCP reorders them correctly before passing the data to the receiving application. If a packet is lost entirely, TCP handles its retransmission.

Its ultimate mission is to guarantee that the recipient gets the information (work file, message, or webpage) exactly as sent.

Working hand in hand, the powerful synergy of TCP/IP enables reliable online activities such as web browsing, email, file transfers, and real-time communication.

Layers of the TCP/IP Model

The TCP/IP model has four “layers”: Application, Transport, Internet, and Network Access. The idea of layers is to break down the complex task of sending information across the internet into smaller, manageable steps.

Every step is handled by a layer where each has a defined function, independent of the others, while contributing to the success of the whole.

Application Layer

The application layer is the top stratum, closest to the user. It’s like the “user interface” of the internet. All the common internet applications live here.

Function: It handles the specific rules for different types of applications, governing how applications interact with the network.

Common examples:

Web browser (HTTP/HTTPS): When you type a website address into your browser, HTTP (Hypertext Transfer Protocol) or HTTPS (Secure HTTP) is activated. The protocols enable your browser to request and display web pages.
Email (SMTP/POP3/IMAP): Protocols such as SMTP (Simple Mail Transfer Protocol) for sending email, and POP3/IMAP for receiving mail, operate in the application layer.
File transfer (FTP/SFTP): When you upload or download files, FTP (File Transfer Protocol) or SFTP (Secure File Transfer Protocol) facilitates the process.

Transport Layer

The transport layer handles the end-to-end communication between applications. Think of it as an information organizer, preparing data to move from an application on your device to an application on the target device.

Function: It breaks your data into smaller chunks, or segments. It also decides whether to prioritize speed or reliability for that data.

Key protocols:

TCP: As discussed, TCP is the “reliable delivery” service. It makes sure all segments arrive at their destination, in the correct order, and without errors. TCP is the preferred protocol where application and completeness are critical. Not surprisingly, it’s widely employed for web browsing, email, and file transfers.
UDP (User Datagram Protocol): UDP is a less reliable service. It supports “fast but no guarantees” delivery. It sends segments without checking if they arrive or whether they’re in order. This lack of diligence is also an advantage. Why? Because it makes the transfer faster. UDP is widely used where a delay is more problematic than a lost packet. Video/audio streaming, online gaming, and voice calls (VoIP) are examples of applications that benefit from UDP.

Internet Layer

The internet layer is the “addressing and routing” level. It’s responsible for routing data packets across different networks to the correct destination anywhere on the internet.

Function: It adds the IP addresses to each data segment, making them IP packets. It then uses routers to determine the best path for these packets to travel across various networks.

Key Protocol: IP (Internet Protocol) is the core protocol for addressing and routing data across the vast networked system that is the modern internet. Without IP, data wouldn’t know where to go.

Note: The Network Access Layer is sometimes split into two layers: the Data Link Layer and the Physical Layer to reflect the roles of both hardware and media access in transmitting data. In this scenario, the TCP/IP model is described as having five layers.

Network Access Layer (also called Link Layer or Network Interface Layer)

The network access layer is the stratum closest to the hardware. It handles the actual transmission and reception of data over your local network. Various technologies are used to transfer data over physical media such as cables or wireless signals.

Function: The layer defines how data packets are physically transmitted over network media. It converts IP packets into frames (raw digital signals) that can travel over Ethernet cables, Wi-Fi signals, fiber optics, and other transmission technologies. It also manages access to the shared network medium, preventing conflicts.

Common technologies employed:

Ethernet: For wired connections.
Wi-Fi: For wireless connections.
Fiber optic: For high-speed, long-distance data transfer with minimal signal loss.
Device drivers: Software that lets your computer’s operating system communicate with its network hardware (like Wi-Fi adapters).

What Is My TCP/IP Address?

An IP address may be required to configure a network or a local connection.

When people refer to a “TCP/IP address,” they usually just mean “IP address,” the unique numerical label assigned to each device on a network.

To find your public IP address:

Google “what is my IP address”. The search engine will usually display it prominently near the top of the results.
You can also visit sites like WhatIsMyIP.comand IPinfo.io to find your public IP address.

To find your private IP address (on most devices):

Windows 10 and 11: Go to Settings, Network & Internet, then Wi-Fi. Look for the Properties of your connected Wi-Fi, and you should see your IP.
macOS: Go to System Settings (or System Preferences). Select Network and click on your active connection. This should show your IP.
iPhone: Go to Settings and then Wi-Fi. Tap on the “i” icon next to your connected Wi-Fi network and look for “IP Address”.
Android: Go to Settings, Network & internet, and then Wi-Fi. Tap on your connected Wi-Fi network and look for “IP address” or similar. Some newer Android versions may place the IP address under Advanced Settings in the Wi-Fi section.

FAQs

What is the difference between TCP vs UDP?

TCP and UDP are both core transport layer protocols, but differ in several respects.

TCP prioritizes reliability, using connection setup, acknowledgments, and retransmissions to ensure data is delivered accurately and in order. It’s well-suited for situations where complete delivery matters, like web browsing or file transfers.
UDP is a lightweight, connectionless protocol that sends data without establishing a link or checking for errors. While UDP is less reliable, it is faster and more resource-efficient. It is well-suited for time-sensitive applications such as DNS lookups, VoIP calls, and streaming media, where speed outweighs perfect delivery.

Is HTTP a TCP protocol?

No, HTTP is not a TCP protocol. It is an application layer protocol that enables users to access and interact with websites. HTTP uses TCP as its transport layer to deliver website content.

What is the difference between TCP and OSI?

The Open Systems Interconnection (OSI) model is a conceptual framework that helps us understand how communication takes place over the Internet. It explains how different systems communicate efficiently by breaking down the process into seven layers.

TCP/IP is the foundational set of communication protocols that allows computers to send and receive data over the Internet reliably and accurately.

A fundamental difference between the two is that the OSI model is a theoretical reference model, while TCP/IP is a practical implementation that is widely used in real-world networking. The OSI model is useful for understanding and teaching networking principles, whereas TCP/IP powers the modern internet.

The OSI model’s seven layers are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

TCP/IP is usually defined by four layers: Network Access, Internet, Transport, and Application.