The Internet protocol suite

As we've seen over the course of this unit, there are many protocols that power the Internet. Each protocol operates at a different layer, building functionality on top of the layer below it.

The layers of Internet protocols are often visualized in a diagram like this:

That diagram is by no means complete. There are many more protocols in the Internet protocol suite—especially at the application layer—such as SMTP for sending email and FTP for uploading files.

Let's review protocols at each layer and their contributions to the Internet.

At the bottom layer, two computing devices need a physical mechanism to send digital data to each other. They send electromagnetic signals either over a wired or wireless connection and interpret the signal as bits. The type of physical connection affects the bit rate and bandwidth.

Once a network is bigger than two computers, we need addressing protocols to uniquely identify who is sending data and who should receive the data. Every node on the Internet is identified with an IP address.

The route between any two computers on the Internet isn't just a straight path from A to B. The data must pass from router to router until it finally reaches its destination, a strategy that comes from the Internet routing protocol.

Data needs to be broken up into small packets, which are then reassembled at the destination. The Transmission Control Protocol (TCP) is used to ensure reliable transport of those packets, with sequencing, acknowledgement, and retries. A faster but less reliable transport protocol is the User Datagram Protocol (UDP).

There are many uses for the data flowing around the Internet, such as sending emails, uploading files, or chatting online. The most common use of the Internet is the World Wide Web, with its millions of publicly viewable websites, all made possible due to the HyperText Transfer Protocol (HTTP). We can visit a website by typing a domain name in the browser address bar, since the browser knows how to turn the domain into an IP address using the Domain Name System (DNS).

When the data contains private information, it needs to be transported securely from the sender to the destination. The Transport Layer Security (TLS) protocol uses algorithms to encrypt the data, while certificate authorities help users trust the encryption.

When a message is sent through the Internet, it doesn't use every protocol in the suite. It does use at least one protocol from every layer, however.

When you're loading a webpage from a domain your browser has never visited before, your browser may need to make a DNS request. This stack of protocols is used when a DNS request is sent through the Internet:

Then your browser will make an HTTP request to fetch the webpage. This protocol stack is used when an HTTP request is sent through the Internet:

If the webpage is served over HTTPS, then the stack includes multiple protocols at the application layer (both HTTP and TLS):

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