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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 Feb 4
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      What happens when we send a large UDP response? Well as we saw earlier the UDP and IP headers both have length fields. The way it works out is that fragmentation actually happens at the IP layer ...

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    2. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Suppose we try to send a 4,000 byte UDP datagram over a 1500 byte MTU IP network, what happens is that it gets broken into 3 IP "fragments". The first contains the first part of the UDP message, including the UDP header itself, and the next packets follow on from there.

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    3. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Each fragment will have the same IP "identification" header (IPID) and different fragment offsets. It's the kernel's job to wait for all of the fragments to show up and pass them on to an application.

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    4. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Path MTU discovery works the same as with TCP. If the first fragment is too big, the sender will get an error. But since applications usually don't retry UDP responses, it can also just show up as that message being entirely dropped.

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    5. Colm MacCárthaigh‏ @colmmacc Feb 4
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      It also means that fragments of the message look strange to many parts of the network, like firewalls and switches. UDP packets without UDP headers! How are they supposed to know whether the packet is allowed or not? How is flow-switching supposed to work? The internet shrugs.

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    6. Colm MacCárthaigh‏ @colmmacc Feb 4
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      We often end up building in the ability to relate these packets to one another in many places, and since that's expensive, they also have to be rate-limited. It's deeply messy.

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    7. Colm MacCárthaigh‏ @colmmacc Feb 4
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      That's what's going on down there though, and understanding that full picture is key to diagnosing some common network mysteries. O.k. I have a team meeting now, more later!

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    8. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Meeting postponed! o.k. so what happens when the DNS server sends a 4K response and the MTU is 1200 bytes? Well the DNS server gets the error, and it fixes the *next* response, but that first is lost. Super annoying. So the client has to retry, but then it works.

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    9. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Some more fun: network paths don't have to be symmetrical, so MTUs don't either. If the client needs to send a large amount of data, this whole process happens for them too. A sender and a receiver can legitimately end up with different limits towards one another.

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    10. Colm MacCárthaigh‏ @colmmacc Feb 4
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      Because MTU discovery depends on state, and on ICMP messages being allowed, some folks do something like "MSS clamping" where for TCP they have the network actually meddle with the TCP connection (a little) to offer a different MSS to the other end.

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      Colm MacCárthaigh‏ @colmmacc Feb 4
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      It is surprising that the Internet works.

      11:39 AM - 4 Feb 2020
      • 27 Retweets
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      • kov Brendan Boyd Martin Müller Javier Beneito Super Cluster Nayeem Zen Benjamin Braithwaite Justin Reid Kristin Hill
      9 replies 27 retweets 157 likes
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        2. Torsten Curdt‏ @tcurdt Feb 4
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          Sometimes I am wondering how different protocols and infrastructure would look like if we'd start from scratch today.

          1 reply 0 retweets 1 like
        3. LionsPhil  ➡️ plush.city‏ @LionsPhil Feb 4
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          We'd put it over HTTPS and increase the complexity and overheads tenfold. Which, uh. Is a thing. Which is happening.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. Todd Lipcon‏ @tlipcon Feb 4
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          ... And aggravating when it stops working due to these abstractions leaking and losing composability... Eg I hit an issue last year where pmtu discovery didn't work inside a container with bridged networking, causing the gigabit network to download at a few k/sec

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        1. Garrett Wollman‏ @garrett_wollman Feb 4
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          Replying to @colmmacc

          The takeaway from Dave Clark's tutorial at Interop in 1997 (boy am I dating myself) was "Believe it or not, despite all of this, the Internet actually works most of the time." It hasn't gotten any less complicated, but people have certainly made heroic efforts to keep it working.

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        2. David Comay‏ @comay Feb 4
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          Replying to @colmmacc

          @threadreaderapp unroll s’il vous plaît.

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        3. Thread Reader App‏ @threadreaderapp Feb 4
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          Replying to @comay

          Hi, please find the unroll here: Thread by @colmmacc: Tuesday Technical Tweet Thread Time! Let's go on the roller coaster of what happens at a low level… https://threadreaderapp.com/thread/1224766402629795840.html …. See you soon. 🤖

          0 replies 0 retweets 1 like
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        1. Dirkjan Ochtman‏ @djco Feb 4
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          Replying to @colmmacc

          I'm hoping that DNS over QUIC will become a thing.

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        1. Charles Batchelor‏ @chazzyb Feb 4
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          Replying to @colmmacc

          I remember my old friend adjust-mms.

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        1. JuKap‏ @jukap Feb 5
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          Replying to @colmmacc

          This was the best thread I’ve read on Twitter in a semaphore time-out!

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