Links for 2013-08-24

  • Newest YouTube user to fight a takedown is copyright guru Lawrence Lessig

    This is lovely. Here’s hoping it provides a solid precedent.

    Illegitimate or simply unnecessary copyright claims are, unfortunately, commonplace in the Internet era. But if there’s one person who’s probably not going to back down from a claim of copyright infringement, it’s Larry Lessig, one of the foremost writers and thinkers on digital-age copyright. [..] If Liberation Music was thinking they’d have an easy go of it when they demanded that YouTube take down a 2010 lecture of Lessig’s entitled “Open,” they were mistaken. Lessig has teamed up with the Electronic Frontier Foundation to sue Liberation, claiming that its overly aggressive takedown violates the DMCA and that it should be made to pay damages.

    (tags: liberation-music eff copyright law larry-lessig fair-use)

  • TCP is UNreliable

    Great account from Cliff Click describing an interest edge-case risk of using TCP without application-level acking, and how it caused a messy intermittent bug in production.

    In all these failures the common theme is that the receiver is very heavily loaded, with many hundreds of short-lived TCP connections being opened/read/closed every second from many other machines.  The sender sends a ‘SYN’ packet, requesting a connection. The sender (optimistically) sends 1 data packet; optimistic because the receiver has yet to acknowledge the SYN packet.  The receiver, being much overloaded, is very slow.  Eventually the receiver returns a ‘SYN-ACK’ packet, acknowledging both the open and the data packet.  At this point the receiver’s JVM has not been told about the open connection; this work is all opening at the OS layer alone.  The sender, being done, sends a ‘FIN’ which it does NOT wait for acknowledgement (all data has already been acknowledged).  The receiver, being heavily overloaded, eventually times-out internally (probably waiting for the JVM to accept the open-call, and the JVM being overloaded is too slow to get around to it) – and sends a RST (reset) packet back…. wiping out the connection and the data.  The sender, however, has moved on – it already sent a FIN & closed the socket, so the RST is for a closed connection.  Net result: sender sent, but the receiver reset the connection without informing either the JVM process or the sender.

    (tags: tcp protocols SO_LINGER FIN RST connections cliff-click ip)

  • The ultimate SO_LINGER page, or: why is my tcp not reliable

    If we look at the HTTP protocol, there data is usually sent with length information included, either at the beginning of an HTTP response, or in the course of transmitting information (so called ‘chunked’ mode). And they do this for a reason. Only in this way can the receiving end be sure it received all information that it was sent. Using the shutdown() technique above really only tells us that the remote closed the connection. It does not actually guarantee that all data was received correctly by program B. The best advice is to send length information, and to have the remote program actively acknowledge that all data was received.

    (tags: SO_LINGER sockets tcp ip networking linux protocols shutdown FIN RST)

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