People often think that a protocol is just the order of bits and fields and headers. But that shit is just the schema. You can have a bunch of well ordered bits and still manage to communicate absolutely nothing.

Protocols tell you how to do something in various situations in order to achieve orderly outcomes. Thus, a well-written communication protocol also tells you how to communicate, and what that communication is meant to achieve. Since two clients have adopted the same protocol, it can be assumed they want to achieve the same goals.

A protocol is thus also a contract.

I mean that all seems super obvious and straight forward, but then you look at a lot of modern protocols and they focus a lot on schema and leave very little to figuring out goals and means. They want to be everything-protocols, which is just counter productive. A contract without known goals has no enforcement mechanism, and no way if anything but extremely broad terms have been met. Has my message had the desired affect? Do I need to send more, wait for more? Hard to know. It's fine if you just want to send bytes back and forth, but its shit when you're trying to create an ocean for applications since they don't know what is going to be achieved broadly, they have to negotiate individually (...which they could have done anyways without a protocol).

And that's why you see app A fighting with App B fighting with ... to figure out what each has done and how to read each other's specific interpretations and ideas. Frankly, It's a fuckin' mess. I understand that'd It'd probably create a lot of friction and slowdown to do it right, but its also kinda silly that applications often have to identify and negotiate with each other despite sharing the same protocol/version.