# Liskov Substitution and Modularity in Network Design

Furthering the thoughts I’ve put into the forthcoming book on network complexity…

One of the hardest things for designers to wrap their heads around is the concept of unintended consequences. One of the definitional points of complexity in any design is the problem of “push button on right side, weird thing happens over on the left side, and there’s no apparent connection between the two.” This is often just a result of the complexity problem in its base form — the unsolvable triangle (fast/cheap/quality — choose two). The problem is that we often don’t see the third leg of the triangle.

The Liskov substitution principle is one of the mechanisms coders use to manage complexity in object oriented design. The general idea is this: suppose I build an object that describes rectangles. This object can hold the width and the height of the rectangle, and it can return the area of the rectangle. Now, assume I build another object called “square” that overloads the rectangle object, but it forces the width and height to be the same (a square is type of rectangle that has all equal sides, after all). This all seems perfectly normal, right?

Now let’s say I do this:

• declare a new square object
• set the width to 10
• set the height to 5
• read the area

What’s the answer going to be? Most likely 25 — because the order of operations set the height after the width, and internally the object sets the width and height to be equal, so the last value input into either field wins.

What’s the problem? Isn’t this what I should expect? The confusion is this — the square class is based on the rectangle class, so which behavior wins? But the result is pushing a button over here, and ending up with an unexpected result over there. Taking this one step further, what if you modified the rectangle class to include depth, and then added a function that returns volume? A user might expect the square class to represent a perfectly formed cube (all sides equal), based on the it’s behavior in the past — but that’s not what is going to happen. The solution, from a coding perspective, is to build a new class that underlies both the square and the rectangle — to find a more fundamental construct, and use that as a foundation.

In general, you want to find a foundation which will not change no matter what you build on it — in other words, you want to find a foundation that, when substituted for another foundation in the future, will not modify the objects sitting on top of the foundation.

Hopefully, you’ve tracked me this far. I know this is a bit abstract, but it comes back to network design in an important way. The simplest place to see this is in the data center, where you have an underlay and an overlay. To apply Liskov’s substitution principle here, you could say, “I want to build a physical underlay that will allow me to change it in the future without impacting the overlay.” Or, “I want to be able to change the overlay without impacting how the applications run on the fabric.” Now — take this concept and apply it to the entire network, wide area to data center fabric.

You should always strive to build a physical infrastructure that can be replaced without impacting the control plane. You should also strive to build a control plane that can be replaced without impacting the operation of the applications running on the network. Just like you should be able to replace the physical layer under IP, and not impact the operation of TCP on top in any meaningful way.

Now — the real world is always messier than the virtual worlds we build in our minds. Abstractions are always going to leak, and the interaction surface between any pair of underlying and overlying layers is always going to be deeper and broader than you think when you first look at the problem. None of this negates the end goals, however. Keep the interaction surfaces in a design shallow and narrow, and thinking through “what happens if I replace this piece with a new one later on?”

Hierarchical and modular design, by the way, already operate on these sorts of principles (in theory). They’re just rules of thumb, or design patterns, laid on top of the more foundational concepts. The closer we get to the foundational principles in play, the more we can take this sort of thinking and apply it along every interaction surface in a design, and the more we can move from black art to science in designing networks that work.