【My Study Note】The Transport Layer
The Transport Layer
The transport layer is responsible for lots of important functions of reliable computer networking. These include multiplexing and demultiplexing traffic, establishing long-running connections, and ensuring data integrity through error checking and data verification.
Mutiplex
The transport layer has the ability to multiplex and demultiplex, which sets this layer apart from all others.
Multiplexing in the transport layer means that nodes on the network have the ability to direct traffic toward many different receiving services. Demultiplexing is the same concept, just at the receiving end, it’s taking traffic that’s all aimed at the same node and delivering it to the proper receiving service.
The transport layer handles multiplexing and demultiplexing through ports. A port is a 16-bit number that’s used to direct traffic to specific services running on a networked computer.
Remember the concept of servers and clients? A server or service is a program running on a computer waiting to be asked for data. A client is another program that is requesting this data.
Different network services run while listening on specific ports for incoming requests. For example, the traditional port for HTTP or unencrypted web traffic is port 80. If we want to request the webpage from a web server running on a computer listening on IP 10.1.1.100, the traffic would be directed to port 80 on that computer.
Ports are normally denoted with a colon after the IP address. So the full IP and port in this scenario could be described as 10.1.1.100:80. When written this way, it’s known as a 「socket address」 or 「socket number」.
The same device might also be running an 「FTP (File Transfer Protocol Server)」. FTP is an older method used for transferring files from one computer to another, but you still see it in use today.
FTP traditionally listens on port 21, so if you wanted to establish a connection to an FTP server running on the same IP that our example web server was running on, you direct traffic to 10.1.1.100 port 21.
In a small business environment, a single server could host almost all of the applications needed to run a business. The same computer might host an internal website, the mail server for the company, file server for sharing files, a print server for sharing network printers, pretty much anything. This is all possible because of multiplexing and demultiplexing, and the addition of ports to our addressing scheme.