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Comment by chrisseaton | Hacker News Reader

It’s weird I’d thought they use different terms for A’s two phrases, because it’s the same words but the first is an instruction and the second a statement! Different meanings but same words - why’d you do that in designing a protocol!

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Just like TCP, it’s a stateful protocol.

Seems like a terrible idea - what if you’re mistaken about the other person’s understanding of the current state?

I'm trying to think how this could possibly be misconstrued

What if A didn't hear B's response, and was reiterating their first instruction? B wouldn't respond because they think A is already at step 3 of the handshake, whereas A thinks they are at step 1. A thinks A has control of the plane, whereas B thinks B has control of the plane.

Even if A says the same words (he probably wouldn't) it definitely wouldn't be in the same tone. B would understand that it was a repeated command and not an acknowledgement.

That seems tricky. Climbing, I was always taught to make the commands unambiguous. You never say "take in the slack" because on a windy day that can sound awfully like "I need slack". Each command should be distinct.

Commands that can only be disambiguated via tone sounds insane to me.

Imagine being in an emergency, possibly listening to someone speaking while wearing an oxygen mask, in a smoke filled cabin, with alarms going off, and asking yourself - "wait did they say 'you ^have^ the controls', or 'you _have_ the controls'".

A is supposed to watch the other person and see their hands move to the controls.

Won't A just repeat until he gets a response? A doesn't want control, if they think they have it, they will probably retry the whole thing, meaning the confusion is probably brief.

He might also just see that B is making inputs and assume, if they are being lax, but I assume they wouldn't.

responding to sibling, in my experience it actually goes “i have controls, you have controls, i have controls”. always ending on someone saying “i have controls.” so if someone doesn’t hear a call or response, it ends with either two people on the controls (not a disaster), or the person relinquishing controls repeating his line.

I mean, TFA is about two people on the controls. So that sounds like an ambiguity to be avoided.

> two people on the controls (not a disaster)

Err… isn’t that exactly how Air France Flight 447 crashed? Both people thought they were flying, doing opposite things, the plane cancelled out their inputs and did nothing.

Then you send a letter to Byzantine and ask the Generals if they have any bright ideas?

TCP got it right tho:

    SYN

    SYN-ACK

    ACK

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