The Internet

In the coming weeks I'm going to be working on my HTTP server, so I've spent the last few days learning about HTTP and more generally about the components of the network that comprise the Internet. At first, I started reading about HTTP, but that led to TCP, which lead to IP, ... at which point I had 50 browser tabs open. So rather than writing about HTTP (hopefully in a later post), I first wanted to back up and try to answer for myself at a high level "What is the Internet?".

Cats? Clearly.

GIFs? Check.

But how does this all work? How does Google know where to send me when I search for "cats"? Also, once I find what I'm looking for how does it get sent back to my computer. How does my bank know that I'm who I say I am? What does it even mean to be on the Internet?

If you put me on the spot and asked what is the Internet, I'd probably have said something like, "A large network of computers", but if you'd have asked the logical follow-up question, "How do all of these computers, phones, printers and various other pieces of random hardware communicate with one another? Each piece of hardware is not only different, but is surely running completely different software." My response would have been along the lines of, "Ummm, let me get back to you on that ... ".

Like many complex systems that have become part of our daily lives, the complexity of transmitting large amounts of data, over a global network with billions of connections has long been removed from our day-to-day interactions with the Internet. However, the complexity is still there even if we don't see it.

Internetlandia

The first chapter of Java Network Programming has a really nice introduction to the basics of networking and uses the following pithy definition to describe the Internet

"It is simply a very large collection of computers that have agreed to talk to one another in a standard way.”

and for many computers on the Internet, HTTP is the "standard way" which they've agreed to communicate with one another. However, HTTP represents only the very top layer of the network, often called the application layer. The network comprising the Internet is actually composed of multiple layers of protocols, each with its own responsibilities and largely ignorant about the layers multiple levels above or below. The multilevel set of protocols used on the Internet is called the Internet protocol suite. The diagram from Wikipedia below describes the basic structure.

Using a multilevel architecture allows for upper levels of the network to ignore the details of transmitting data over physical hardware and instead focus only on communicating with other applications. At the same time, the hardware layer can ignore the specifics as to what exactly is being transmitted and only focus on moving data between point A and B. This multilevel approach also allows for a greater variety of protocols and hardware in the network, since each level doesn't need to know the details of levels multiple layers above or below.

Application layer

The application layer is the very top layer of the Internet protocol suite. This layer is used by applications to communicate over a network. An example of a typical application layer communication is a browser asking a server for an image. Such a request would be encoded as HTTP by the browser, and the server would also reply in HTTP. The messages would contain information about what image the browser was looking for, the format, and the server's address. So the application layer provides a specification for how messages over networks should be constructed, however it largely ignores the details of transmitting messaging over the network. Also, while HTTP is the most common protocol for communication over the Internet it's not the only one. Other examples of application layer protocols are FTP, IMAP, and BitTorrent.

Transport layer

The transport layer is the network layer immediately below the application layer. The transport layer is responsible for creating the channel of communication over which network communication occurs, along with error detection, and regulating the rate of data transfer. Again, as with the application layer, there are multiple transport layer protocols. However, TCP is the most common for HTTP traffic over the Internet. TCP is often preferred for HTTP traffic, because it is considered 'reliable', and assures data arrives in order, de-duplicated and with minimal error.

The transport layer protocol allows for the browser to largely ignore the details of transporting data over a network and instead just make sure that it writes the correct message to the TCP socket. At the same time, the TCP layer can just accept the data, also not really caring about the specifics of the messaging protocol and instead focus on ensuring that it gets to the correct location without loss or duplication.

Internet layer

The Internet layer is responsible for addressing (IP address), transmitting and efficiently routing data through the switches, routers and various subnetworks that comprise the Internet. The most commonly used protocol for this layer is IP (Internet Protocol). Interestingly, IP provides no guarantees regarding the ultimate delivery of the data. At first this might seem like a flaw, however by forcing the message originator to assume this responsibility the network can be more resilient to failures of individual nodes.

Link layer

At some point the IP packets have to be transported over fiber optic wire, satellite or whatever physical medium comprises the network. The link layer is responsible for formatting the IP packets (into frames) and then transmitting them over a physical network. The link layer is where all of the machine level protocols like Ethernet, WiFi and Bluetooth reside.

Once the data reaches its destination this entire process is reversed. The multilayer design allows each component to largely focus on its specific task, without worrying about the details of the other layers. Also, it allows for a network that is not based on one particular protocol or hardware.

And that's the Internet protocol suite. Every GIF, Facebook post and Google search all follow the same process in moving from the server to our devices.