Please note that I am still learning about protocols, and I may be editing all or part of this document in the future.
I'm not sure if there is a right place to start. I'm going to start at a place that feels somewhat natural to me: talking about protocols. I'm sure that the fact that I have looked into this subject recently is the main reason I'm choosing it as a starting point. But the subject of protocols has some merits as a starting point beyond "Daniel read about it." The predominance of some protocols is tied to the history, and the future, of the Internet. Also, protocols are so important to telecommunications that, even though the details of protocols may be arcane, reminders of them pop up in front of the layman all the time.
Take a look at your browser address line. The address starts with four letters: HTTP. The 'P' stands for protocol. A cell phone retail chain in the UK recently changed its name to The WAP Store. I wonder if their customers know that that 'P' also stands for protocol. If you have been reading the history of this site, you'll recall that I FTP pages from my computer up to the Prodigy server. This 'P' too stands for protocol. (I use FTP as verb. As an old addage goes, "there is no noun that can't be verbed.")
Protocols are everywhere. They are part of communicating.
What is a protocol?
Simply put, a protocol is an agreement about mutual behavior. It's not really a computer term. Keep in mind that protocol was a word before modern computers and the Internet came into being. Techies use the word protocol pretty much in the same way as an ambassador or a military officer does. And while most people wouldn't refer to their behavior as being governed by protocols, their behavior is.
Usually, such protocol is implied. For instance, you wouldn't talk to your boss in front of clients they same way you would talk to a baby. In fact, you probably wouldn't talk to your boss in front of clients the same way you would talk to your boss after the clients have gone. ("Those guys were clowns. Did you see that one bozo's tie?") But despite these differences in how you expect yourself and others to behave, you generally don't need to stand outside the conference room before going in and ask your boss, "We're using the 'In Front Of Clients' protocol, right?"
Computers don't think (much) on their own, so they pretty much always have to be told which protocol to use. Humans sometimes do too. On party invitations, we might put "Black Tie". A restaurant review might say, "dress is casual."
"Black Tie Party" is a protocol. It dictates things about dress, activity, and speech. It says we'll wear tuxedoes and dresses. It says (pretty much) that nice alchoholic beverages will be available. It says that we won't greet each other with "How the f*ck have you been?", at least not until after the third nice alchoholic beverage. It does not say anything about what the party is about. It could be an awards ceremony, a spring formal, or a merger celebration.
A protocol in computer terms is the same thing: an agreement about how communications are going to be handled.
Why so many protocols?
Well, you could have a black tie backyard barbecue, with sauce-dripping ribs, or have a black tie funeral ("Ashes to ashes, dust to dust, we commit this body to the ground. <POP!> Who wants some bubbly?"), but you probably wouldn't. Often (but not always) there is no reason why you couldn't choose "Black Tie Party" protocol for those applications, but there are reasons why you wouldn't.
So, there are protocols for different types of activities. Some protocols could be for similar activities, say "Frat Party". And you could use Frat Party protocol for an awards ceremony ("And this year's winner is Sheila Johnson! Sheila, do a handstand on the keg for three minutes while we pump beer into your mouth!"), but you probably wouldn't.
Some activities, though, are just too different. Playing Blackjack in a casino, for instance, has a very specific protocol to efficiently handle a limited number of different tasks. Tap the table for a hit. Wave your hands over your cards to stand. Double your bet to double down. A table with a dealer and a group of players who know the protocol can play a lot of Blackjack very quickly without talking.
But the protocol is very specific to that application and wouldn't make any sense at all for a party. You wouldn't wave your hand over your date to indicate that you'll stand or tap your date, indicating you'll take another. (I suppose it's conceivable that you would increase your gamble and suddenly double down with a second drink if you thought you liked the way your date was going, but I'd still argue that a party wouldn't really be a feasible application of Casino Blackjack protocol.)
Why are there protocol layers? (By the way, there are protocol layers.)
Now "Black Tie" is a fairly high level protocol. There is already a lot of behavior and communication already taken absolutely for granted in even getting to the point where the idea of Black Tie can be communicated. For example: If there is a party and your presence is desired, there is going to be an invitation. You should respond to this invitation so that I know you've received it and whether or not you are coming. If there is no party (or you're not invited), you will not hear from me. That is, I will not be in the habit of sending you daily reminders that I am not having a black tie affair. So, in order for us to end up in a room together wearing tuxedoes and gowns and sipping fancy alchoholic beverages until cursing is allowed, there has to be some way to communicate that an event is happening and that Black Tie is the protocol that will be used.
To be continued...