Have you ever seen a diagram like the following?

TCP handshakeSystem ASystem BSYNSYN/ACKACK

I saw and drew a bunch of these things in my notebooks in college. They are useful for recording how protocols work, and they are easy to add to or adjust, which makes them good for note-taking.

It wasn’t until I wanted to put one of these in a proposal at work that I had to learn how to make one on the computer, and in order to do that I needed to know what they were called. It turns out they are called sequence diagrams or message sequence charts (there are minor differences between the two, but for my purposes either is as good as the other).

With that name, I was able to find lots of online tools for building sequence diagrams. My favorite is sequencediagram.org. It is easy to use, shows errors clearly, and exports to PNG and SVG. If you need to diagram any sort of communication between systems, I recommend giving it a try.