Understanding the Transport Layer: A Beginner’s Guide

Key takeaways

• The transport layer is a layer in network architecture that ensures end-to-end data delivery by managing data flow, sequencing, traffic control, and data integrity between applications.

• Transport layer protocols include the transmission control protocol (TCP) and user datagram protocol (UDP).

• The three main responsibilities of the transport layer are ensuring smooth data transmission, data integrity and correctness, and data flow control.

The transport layer does tasks such as establishing and terminating connections, enabling reliable end-to-end communication, and supporting applications like web browsing, streaming, gaming, and remote logins. Learn more about transport layers in this article and gain a deeper understanding of the transport layer in computer systems to help you succeed in a computing career.

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What does the transport layer do? Transport layer protocols

Transport layers work with other layers of complex computer systems to ensure end-to-end data transfer across networks. Understanding the basics of computer networking requires knowledge of each layer in different models, including the open systems interconnection (OSI) model and the transmission control protocol/internet protocol (TCP/IP) model.

In both models, the transport layer services the sender and the receiver. It receives data from the sender’s application layer and readies it for delivery to the network layer. Then, the transport layer reassembles the delivered data from the network layer and sends it to the recipient’s application layer, ensuring the data is transmitted in the right sequence, free of errors, and without any duplicates.

For example, when you navigate to www.coursera.org, you want to see the website on your device's browser. For instance, it would be strange and unexpected if your computer displayed a binary DNS response as a series of numbers and letters. The transport layer ensures you get where you intend to be, which, in this case, is HTML-formatted data in the form of a website. The transport layer delivers your request to the web server and returns the web page to your browser, often instantaneously. Succinctly summarized, transport layers provide delivery services, flow and traffic control, and data integrity support in both models.

The transport layer in the OSI model

The transport layer comes fourth in the seven-layer OSI model. As the conduit between the session layer and network layer, the transport layer handles end-to-end delivery of messages.

At the sender’s end, the transport layer segments application data arriving at the session layer into smaller streams, manages the speed of data flow, corrects any errors, adds service point headers or port numbers, and hands over the data to the network layer to send to the right device. At the receiver’s end, the transport layer reassembles and sequences the data, checks for errors, retransmits data if necessary, and reads the headers or port numbers to route it to the session layer.

The transport layer in the TCP/IP model

You’ll find the transport layer first or second from the top in the four-layer TCP/IP model, or within the process layer or host-to-host layer. The layer it uses depends on whether or not you're using a connectionless user datagram protocol (UDP) transport layer protocol. In general, the transport layer sits between the application layer and network layer in the TCP/IP model.

The TCP/IP model provides a simplified view of the OSI model. TCP/IP transfers data from one device to another. Each TCP/IP stack layer serves a specific purpose, with the transport layer playing a pivotal role in ensuring that accurate, reliable data flows from the sender to the recipient.

What is TCP and UDP in transport layer?

TCP and UDP are the two internet protocols that occur in the transport layer. Each protocol provides different services in the transport layer depending on the requirements of the application. Depending on the application's needs, you may want to use the more reliable but slower TCP connection or the faster but less reliable UDP connection. TCP is connection-oriented, as in it verifies data receipt when a connection is established, while UDP is connectionless, meaning it doesn’t require the receiver to acknowledge data receipt. This makes TCP well-suited for the reliability required in the file-transfer protocol (FTP), while UDP is better suited for the needs of video and audio streaming, where packet loss is more tolerable.

Services that transport layers provide

The transport layer establishes, uses, and terminates the connection between sender and receiver applications and networks. Its protocols support smooth, reliable end-to-end communication through several essential services. By aligning network routing with system needs, the transport layer promotes smooth communication for applications such as web browsing, streaming content, online gaming, and remote logins.

Delivery

Put simply, when you get an email from a friend or coworker, that data has to go through the transport layer to get to you. Your friend writes the email on their application, and it goes through the transport layer to get out to the network and reach you. The transport layer segments the data.

You might have heard of data packets, the smaller components of overall communication. The transport layer makes those packets. It also adds source and destination data to help the data go where it needs to be. To ensure data reaches the correct application, the transport layer attaches port numbers to the sender and receiver, combining them with the IP address to give a unique address to each application. If something happens, it will retransmit that data until a successful transmission is acknowledged.

Read more: SSL/TLS Protocols: A Practical Guide to Secure Web Communication

Data integrity

Transport-layer protocols ensure data packets arrive accurately and in order. This layer accepts the packet data from the sender and reassembles it for the recipient. It performs sequencing, exchanges data receipt acknowledgments, identifies errors, and retransmits any missing or corrupted data packets.

Flow control

The individual devices in a network connection can’t know each other’s capabilities for data throughput. As a result, the transport layer takes on that responsibility. It can either send data faster if the receiving device proves capable, or slow the data flow if needed. By controlling the flow, the transport layer helps avoid network performance issues and manages data flow to work for both ends of the network connection.

Traffic control

As with flow control, the transport layer also tackles traffic control to avoid network congestion. With digital communications subject to bandwidth and processing speed restrictions, the transport layer identifies potential overloads and reduces flow rates accordingly.

Multiplexing

In the TCP/IP model, the transport layer also performs multiplexing duties. Imagine filling out a web form and sending emails on the same desktop. The computer must handle multiple communications simultaneously, a process known as multiplexing. It can accomplish this because of the transport layer’s work. By assigning unique port addresses to each application, the transport layer ensures multiple applications can use the same network at the same time.

What professions use transport layers?

Knowing data and network communication, particularly the transport layer, can help you better understand how different applications connect over networks, allowing you to optimize performance, troubleshoot, and design networks. You might work with network communication and transport layers in several professions, including computer programming, computer network architecture, and software development. Discover more about three such job titles, including median salary and how to start.

All salary information represents the median total pay from Glassdoor as of September 2026. These figures include both base salary and additional pay, which may represent profit-sharing, commissions, bonuses, or other forms of compensation.

Website developer

Median annual total pay (Glassdoor): $85,000 [1]

Job outlook (projected growth from 2025 to 2035): 5 percent [2]

As a web developer, you will help design and create websites. You may also help maintain existing sites or work with clients to add updates and functionality. In this role, you’ll work in a team and seek feedback from different departments to prototype and design an optimized website to help your company meet its goals.

Computer network architect

Median annual total pay (Glassdoor): $150,000 [3]

Job outlook (projected growth from 2025 to 2035): 8 percent [4]

As a computer network architect, you’ll use your expertise to establish IT infrastructure and ensure efficient, effective, and secure network communications. Demand for computer network architects continues to grow.

Software developer

Median annual total pay (Glassdoor): $123,000 [5]

Job outlook (projected growth from 2025 to 2035): 10 percent [6]

Understanding the transport layer can help your work as a software developer. In this role, you’ll work to build, test, and deploy the specific applications your clients desire, as well as their underlying systems and networks.

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