Question

Describe and distinguish the "four layers of technology" that Dyer discusses. How do you see these four layers playing out in your own particular field of study or vocational goals?

Answer 1

1 — Link Layer:

The Internet is made up of end-hosts, links and routers. Data is delivered hop-by-hop over each link in turn. Data is delivered in packets. A packet consists of the data we want to be delivered, along with a header that tells the network where the packet is to be delivered, where it came from and so on.

The Link layer’s job is to carry the data over one link at a time. You have probably heard of Ethernet and WiFi — these are 2 examples of different Link layers.

2 — Network Layer:

The most important layer is the Network layer. It delivers packets end-to-end across the Internet from the source to the destination. A packet is an important basic building block in networks. A packet is the name we give to a self-contained collection of data, plus a header that describes what the data is, where it is going and where it came from.

Network layer packets are called datagrams. They consist of some data and a head containing the “To” and “From” addresses — just like we put the “To:” and “From” addresses on a letter. The Network hands the datagram to the Link Layer below, telling it to send the datagram over the first link. In other words, the Link Layer is providing a service to the Network Layer. Essentially, the Link Layer says:

At the other end of the link is a router. The Link Layer of the router accepts the datagram from the link, and hands it up to the Network Layer in the router. The Network Layer on the router examines the destination address of the datagram, and is responsible for routing the datagram one hop at a time towards its eventual destination. It does this by sending to the Link Layer again, to carry it over the next link. And so on until it reaches the Network Layer at the destination.

Notice that the Network Layer does not need to concern itself with *how* the Link Layer sends the datagram over the link. In fact, different Link Layers work in very different ways; Ethernet and WiFi are clearly very different. This separation of concerns between the Network Layer and the Link Layer allows each to focus on its job, without worrying about how the other layer works. It also means that a single Network Layer has a common way to talk to many different Link Layers by simply handing them datagrams to send. This separation of concerns is made possibly by the modularity of each layer and a common well-defined API to the layer below.

In the internet, the network layer is special: When we send packets into the Internet, we must use the Internet Protocol. It is the Internet Protocol, or IP, that holds the Internet together. IP provides a deliberately simple service. It is a simple, dumb, minimal service with four main features: It sends datagrams, hop-by-hop across the Internet. The service is unreliable and best-effort; there is no per-flow state making the protocol connectionless.

3 — Transport Layer:

The most common Transport Layer is TCP, or the Transmission Control Protocol.

TCP makes sure that data sent by an application at one end of the Internet is correctly delivered –in the right order -to the application at the other end of the Internet. If the Network Layers drops some datagrams, TCP will retransmit them, multiple times if need-be. If the Network Layer delivers them out of order –perhaps because two packets follow a different path to their destination — TCP will put the data back into the right order again.

Applications such as a web client, or an email client, find TCP very useful indeed. By employing TCP to make sure data is delivered correctly, they don’t have to worry about implementing all of the mechanisms inside the application. They can take advantage of the huge effort that developers put into correctly implementing TCP, and reuse it to deliver data correctly. Reuse is another big advantage of layering.

But not all applications need data to be delivered correctly. For example, if a video conference application is sending a snippet of video in a packet, there may be no point waiting for the packet to be retransmitted multiple times; better to just move on. Some applications just don’t need the TCP service.

If an application doesn’t need reliable delivery, it can use the much simple UDP — or user datagram protocol — instead. UDP just bundles up application data and hands it to the Network Layer for delivery to the other end. UDP offers no delivery guarantees.

In other words, an Application has the choice of at least two different Transport Layer services: TCP and UDP. There are in fact many other choices too, but these are the most commonly used transport layer services.

4 — Application Layer:

There are of course many thousands of applications that use the Internet. While each application is different, it can reuse the Transport Layer by using the well-defined API from the Application Layer to the TCP or UDP service beneath.

Applications typically want a bi-directional reliable byte stream between two end points. They can send whatever byte-stream they want, and Applications have a protocol of their own that defines the syntax and semantics of data flowing between the two end points.

For example, when a web client requests a page from a web server, the web client sends a GET request. This is one of the commands of the hypertext transfer protocol, or http. http dictates that the GET command is sent as an ASCII string, along with the URL of the page being requested. As far as the Application Layer is concerned, the GET request is sent directly to its peer at the other end –the web server Application. The Application doesn’t need to know how it got there, or how many times it needed to be retransmitted. At the web client, the Application Layer hands the GET request to the TCP layer, which provides the service of making sure it is reliably delivered. It does this using the services of the Network layer, which in turn uses the services of the Link Layer.