The Transport Layer: Multiplexing, Demultiplexing, and Ports

📚 Overview

The transport layer is responsible for many important functions in reliable computer networking. These include multiplexing and demultiplexing traffic, establishing long-running connections, and ensuring data integrity through error checking and data verification. In this lesson, we will explore these concepts and more.

Learning Objectives

By the end of this lesson, you will be able to:

• Describe multiplexing and demultiplexing and how they work

• Identify the differences between TCP and UDP

• Explain the three-way handshake and the use of TCP flags

• Understand the basics of how firewalls keep networks safe

Multiplexing and Demultiplexing

🔀 Directing Traffic

The transport layer has the unique ability to multiplex and demultiplex, setting it apart from other layers. Multiplexing allows nodes on a network to direct traffic towards different receiving services, while demultiplexing delivers traffic aimed at the same node to the proper receiving service. This is achieved through the use of ports.

🔌 Understanding Ports

A port is a 16-bit number used to direct traffic to specific services running on a networked computer. Servers or services are programs waiting to be asked for data, while clients are programs requesting this data. Different network services listen on specific ports for incoming requests.

For example, the traditional port for unencrypted web traffic (HTTP) is port 80. If we want to request a web page from a web server running on IP address 10.1.1.100, the traffic would be directed to port 80 on that computer. Ports are denoted by a colon after the IP address, such as 10.1.1.100:80, known as a socket address or socket number.

Similarly, an FTP (File Transfer Protocol) server traditionally listens on port 21. To establish a connection to an FTP server running on the same IP address as our web server example, traffic would be directed to port 21: 10.1.1.100:21.

🏢 Multiplexing in Business Environments

In small business environments, a single server can host multiple applications needed to run the business. This is made possible through multiplexing and demultiplexing, along with the use of ports. For example, a server can host an internal website, a mail server, a file server for sharing files, a print server for network printers, and more.

Conclusion

🎉 The transport layer plays a critical role in computer networking by enabling multiplexing and demultiplexing of network traffic using ports. Understanding these concepts is essential for ensuring efficient and reliable communication between network applications.

🔜🌐 In the next lesson, we will explore the differences between TCP and UDP protocols. See you there!

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