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colmmacc's profile
Colm MacCárthaigh
Colm MacCárthaigh
Colm MacCárthaigh
@colmmacc

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Colm MacCárthaigh

@colmmacc

AWS, Apache, Crypto, Irish Music, Haiku, Photography

Seattle
notesfromthesound.com
Joined April 2008

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    1. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      O.k., so let's take this model and use to see how TCP works and why! So TCP, the transmission control protocol, takes data like our movie and moves it from A to B in order and reliably.

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    2. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      It does this by breaking the movie into packets. Packets are better than sending the whole movie because if you're going to lose some data a long the way, it's better to resend only a small amount of it than the whole thing.

      1 reply 0 retweets 6 likes
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    3. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      Now we know our packets might not make it, so TCP says that packets have to be acknowledged, which just means that the other end says "yes I got your packet" when it gets a packet.

      1 reply 0 retweets 7 likes
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    4. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      So let's say we put a packet on the sushi belt, and it trundles along, and then it gets to the other end, and the other end takes it, and puts its own acknowledgment packet on the belt headed back to us. If we won't see that reply, we know to resend. O.k. great.

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    5. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      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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    6. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      The window says "hey, feel free to send me this many packets, even if I haven't acknowledged them yet". Ideally you want the window to be as big as the number of free slots between you and the recipient.

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    7. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      So TCP tries to find this value: it starts out slow, and increases until it detects a drop - a packet that didn't make it, which is assumes is because there aren't enough slots. When that happens it reduces (often by a lot) the size of the window. It sort of "homes in".

      1 reply 2 retweets 9 likes
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    8. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      On modern big fast networks, like AWS's, the default for this window size on common OS's is often too small, as is the number of "slots" in other layers. This is why increasing the TCP window size, and the TCP read/write buffers, and the ethernet queue lengths can be dramatic.

      2 replies 1 retweet 16 likes
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    9. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      O.k. the next thing our model shows is that acknowledging every packet is kind of dumb; sending back a sushi roll for every one that you receive is just wasteful. We can acknowledge what we got, and what we didn't get, in batches!

      1 reply 1 retweet 7 likes
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    10. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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      The modern form of this is selective acknowledgements (SACK) ... where we can basically just scribble a note back on the sushi belt that tells the sender "hey, here's what I got and didn't get". The sender than re-transmit only what it needs to.

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      Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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.

      3:51 PM - 22 Feb 2019
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      • Christian tinyrobots routermechanic vladdy Michał K. Feiler ❁ Otto Shrikanth Pranesh Prakash Garrett Wilkin
      1 reply 2 retweets 12 likes
        1. New conversation
        2. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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!

          1 reply 0 retweets 10 likes
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        3. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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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        4. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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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        5. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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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        6. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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/ …)

          2 replies 2 retweets 11 likes
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        7. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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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        8. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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.

          1 reply 0 retweets 5 likes
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        9. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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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.

          1 reply 1 retweet 11 likes
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        10. Colm MacCárthaigh‏ @colmmacc 22 Feb 2019
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          I'm going to meditate on better metaphors, so that's it for now :)

          13 replies 1 retweet 31 likes
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        11. End of conversation

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