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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    Rich Felker‏ @RichFelker 25 Dec 2017

    Fun puzzle: can you make a pure analog (bonus: unpowered) ethernet throttling knob?

    8:57 PM - 25 Dec 2017
    • 3 Retweets
    • 2 Likes
    • Austin Robinson ꜳꝛꝍꞥ Jonas Termansen
    2 replies 3 retweets 2 likes
      1. New conversation
      2. Robert Ou‏ @rqou_ 25 Dec 2017
        Replying to @RichFelker

        The "throwing star lan tap" deliberately puts capacitors on some data lines to force the network to fail 1000Base-T and fall back to 10/100 where passive taps are possible. Probably not what you had in mind though.

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker 25 Dec 2017
        Replying to @rqou_

        That's one ingredient but to get meaningful throttling you need more.

        1 reply 0 retweets 0 likes
      4. Robert Ou‏ @rqou_ 25 Dec 2017
        Replying to @RichFelker

        I don't think this is possible for 1000Base-T. That has a somewhat complicated physical layer that involves both directions transmitting on the same wires simultaneously (and hence why passive taps do not work).

        1 reply 0 retweets 0 likes
      5. Rich Felker‏ @RichFelker 25 Dec 2017
        Replying to @rqou_

        Thus why you start by eliminating it, forcing 10/100.

        2 replies 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker 25 Dec 2017
        Replying to @RichFelker @rqou_

        The problem then is how to throttle from 10/100 Mbit down to kbit rates. Basic idea: simulate collisions.

        1 reply 1 retweet 0 likes
      7.  🎃 unsafe { mem::transmute(@erincandescent) }  🎃‏ @erincandescent 26 Dec 2017
        Replying to @RichFelker @rqou_

        Collision detection is off if full duplex is negotiated

        1 reply 0 retweets 1 like
      8. Evan Plaice‏ @evanplaice 26 Dec 2017
        Replying to @erincandescent @RichFelker @rqou_

        Then force half-duplex by cutting 2 (ie 1 tx, 1rx) of the pairs. Specifically, the outer 2. An ethernet NIC should detect the change and fallback to 100MbE.

        1 reply 0 retweets 0 likes
      9.  🎃 unsafe { mem::transmute(@erincandescent) }  🎃‏ @erincandescent 26 Dec 2017
        Replying to @evanplaice @RichFelker @rqou_

        100Mbit still does full duplex

        1 reply 0 retweets 0 likes
      10. 1 more reply
      1. New conversation
      2. Evan Plaice‏ @evanplaice 26 Dec 2017
        Replying to @RichFelker

        1GbE uses all 4 pairs in an ethernet cable, 100MbE only uses 2. To force fast ethernet wire a 4P2T switch inline on the 2 outer pairs. This is a simple, no-power way to toggle 1000/100/100 ethernet down to 100/10.

        1 reply 0 retweets 0 likes
      3. Evan Plaice‏ @evanplaice 26 Dec 2017
        Replying to @evanplaice @RichFelker

        To mess with speed further requires screwing with the integrity of the signal. Ethernet uses balanced signaling, meaning the signal is sent across both sides of the pair and compared on the receiving end to cancel out noise. To force packet loss we need to screw with the balance.

        1 reply 0 retweets 0 likes
      4. Evan Plaice‏ @evanplaice 26 Dec 2017
        Replying to @evanplaice @RichFelker

        To do this we could probably screw with the signal integrity on one side of the pair. Maybe we can use inline capacitors to partially filter one side of the pair and screw with the balance. Maybe we could use a variable resistor inline to simulate logitudinal imbalance.

        0 replies 0 retweets 0 likes
      5. End of conversation

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