That's actually enough to get some reliability, but it's really slow. We'd really like to fill the belt with packets, and get much more data across in the same amount of time. To do this, TCP has something called "windows".
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But if there is congestion, and slots are busy, it's because senders are sending too much; it's key that they know quickly, so we make these buffers small. The problem of having these buffers too big is called buffer bloat (https://www.bufferbloat.net/projects/bloat/wiki/What_can_I_do_about_Bufferbloat/ …)
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This can all be very confusing without the right mental model. Because we generally want one kind of buffer - the window size - to be big, but another kind of buffer - the buffers between links - to be small.
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But if you see the network as train-cars or sushi on a belt, what you can see is that what we *really* want is to fill as many slots as we can when we're sending data! That's really all that's going on.
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One problem with the metaphor: packets don't actually go in loops, they come at the other end, so unlike a sushi belt, there's a kind of off-ramp at each end. Also packets only enter and exit at the ends. There's really no perfect metaphor.
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I'm going to meditate on better metaphors, so that's it for now :)
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End of conversation
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