The truth is, however, that while protocol designers may talk about these things, and network designers study them, very few networks today are built using any of these models. What is often used instead is what might be called the Infinitely Layered Functional Indirection (ILFI) model of network engineering. In this model, nothing is solved at a particular layer of the network if it can be moved to another layer, whether successfully or not.
There are many times in networking history, and in the day-to-day operation of a network, when an engineer has been asked to do what seems to be impossible. Maybe installing a circuit faster than a speeding bullet or flying over tall buildings to make it to a remote site faster than any known form of conveyance short of a transporter beam (which, contrary to what you might see in the movies, has not yet been invented).
sarcasm warning—take the following post with a large grain of salt
A thousand years from now, when someone is writing the history of computer networks, one thing they will notice—at least I think they will—is how we tend to reduce our language so as many terms as possible have precisely the same meaning. They might attribute this to marketing, or the hype cycle, or… but whatever the cause this is clearly a trend in the networking world. Some examples might be helpful, so … forthwith, the reduced terminology of the networking world.
Software Defined Networking (SDN): Used to mean a standardized set of interfaces that enabled open access to the forwarding hardware. Came to mean some form of control plane centralization. Now means automated configuration and management of network devices, centralized control planes, traffic engineering, and just about anything else that seems remotely related to these.
Fabric: Used to mean a regular, non-planar, repeating network topology with scale-out characteristics. Now means any vaguely hierarchical topology (not a ring) with a lot of links.
DevOps: Used to mean applying software development processes to the configuration, operation, and troubleshooting of server and network devices. Now means the same thing as SDN.
Clos: Used to mean a three stage fabric in which every device in a prior stage is connected to every device in the next stage, all devices have the same number of ports, all traffic is east/west, and having a scale-out characteristics. Now means the same thing as fabric, and is spelled CLOS because—aren’t all four letter words abbreviations? Now external links are commonly attached to the “core” of the Clos, because… well, it kindof looks hierarchical, after all.
Hierarchical Design: Used to mean a network design with a modular layered design, and specific functions tied to each layer of the network. Generally there were two or three layers, with clear failure domain separation through aggregation and summarization of control plane information. Now means the same thing as fabric.
Cloud: Used to mean the centralization and abstraction of resources to support agile development strategies. Now means… well… the meaning is cloudy at this time, but generally applied to just about anything. Will probably end up meaning the same thing as DevOps, SDN, and fabric.
Network Topology: Used to mean a description of the interconnection system used in building a network. Some kinds of topologies were hub-and-spoke, ring, partial mesh, Clos, Benes, butterfly, full mesh, etc. Now means the same as fabric.
Routing Protocol: Used to mean the protocol, including the semantics and algorithm or heuristic, used to calculate the set of loop-free paths through a network. Includes instances such as IS-IS, EIGRP, and OSPF. Now means BGP, as this is the only protocol used in any production network (except SDN).
Router: Used to mean a device that determines the next hop to which the packet should be forwarded using the layer 3 address, replacing the layer 2 header in the process of forwarding the packet. Now means the same thing as a switch.
Switch: Used to mean a device which determined which port through which a packet should be forwarded based on the layer 2 header, did not modify the packet, etc. Now means any device that forwards packets; has generally replaced “router.”
Security: Used to mean thinking through attack surfaces, understanding protocols and their operation, and how to build a system that is difficult to attack. Now means inserting a firewall into the network.
We used to have a rich set of terms we could use to describe different kinds of topologies, devices, and ways of building networks. We seem to want to insist on merging as many terms as possible so they all mean the same thing; we are quickly reducing ourselves to fabric, switch, SDN, and cloud to describe everything.
Which makes me wonder sometimes—what are they teaching in network engineering classes now-a-days?