How long it takes to get there depends on two things: how soon a free slot comes up, and how far the distance is. But there's one more wrinkle: our packet might not make it all. It might fall off the train along the way! or it might have to wait too long for a slot and give up.
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So if you do those two things: make your window size big, and make sure selective acknowledgements are on, you can make a big difference to your performance! That video can get to you more quickly.
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Of course we do this kind of stuff ourselves for our own services, but if you're transferring data between your own machines or whatever, take a look!
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now let's extend the model: I said that a pipe or link is like a sushi belt, but there are lots of links interconnecting! So it's like a stadium full of sushi belts, with packets hopping belts. It's like Tim Burton and the Coen brothers made a movie together.
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when the belts interconnect, they might be moving at different speeds, or one might have less capacity than the other, so we have little holding areas, we call these "buffers".
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Generally packets enter and leave these buffers in order, but in some networks you can have priority lanes here, giving priority to some packets over others. At AWS, our belts move so quickly and there are so many free slots that we don't need to do this, it'd be pointless.
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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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