Older blog entries for apenwarr (starting at number 596)

13 Mar 2011 (updated 13 Mar 2011 at 23:05 UTC) »

The strange story of etherpad

I don't actually know this story - certainly no more of it than anyone who has read a few web pages. But luckily, I'm pretty good at making things up. So I want to tell you the story of etherpad, the real-time collaborative web-based text editor.

I find this story particularly fascinating because I have lately had an obsession with "simplifying assumptions" - individual concepts, obvious in retrospect, which can eliminate whole classes of problems.

You can stumble on a simplifying assumption by accident, by reading lots of research papers, or by sitting alone in a room and thinking for months and months. What you can't do is invent them by brute force - they're the opposite of brute force. By definition, a simplifying assumption is a work of genius, whether your own or the person you stole it from.

Unix pipes are the quintessential example of a simplying concept in computer science. The git repository format (mostly stolen from monotone, as the story goes) is trivial to implement, but astonishingly powerful for all sorts of uses. My bupsplit algorithm makes it easy and efficient to store and diff huge files in any hash-based VCS. And Daniel J. Bernstein spent untold effort inventing djb redo, which he never released... but he shared his simplifying ideas, so it was easy to write a redo implementation.

What does all this have to do with etherpad? Simple. Etherpad contains a few startling simplifying assumptions, with a surprising result:

Etherpad is the first (and still only) real-time collaborative editor that has ever made me more productive.

And yet, its authors never saw it as more than just a toy.

My first encounter with etherpad was when Paul Graham posted a real-time etherpad display of him writing an essay. I thought it looked cool, but pointless. Ignore.

Sometime later, I read about Google Wave. I thought that level of collaboration and noise in your inbox sounded like nothing anybody could possibly want; something like crack for lolcats. Ignore.

And then, a little while later, I heard about etherpad again: that it had been bought by Google, and immediately shut down, with the etherpad team being merged into the (technically superior, if you believe the comments at that link) Google Wave team.

Moreover, though, a lot of the commenters were aghast that etherpad had been shut down so suddenly - people were using it for real work, they said. Real work? Etherpad? Did I miss something? Isn't it just a Paul Graham rerun machine?

I couldn't find out, because it was gone.

The outcry was such that it came back again, though, a couple of days later, with a promise that it would soon be open sourced.

So I tried it out. And sure enough, it is good. A nice, simple, multi-user way to edit documents. The open source version is now running for free on multiple sites, including ietherpad and openetherpad, so you can keep using it. Here's what's uniquely great about etherpad:

  • Start a document with one click without logging in or creating an account.
  • Share any document with anyone by cut-and-pasting a trivial URL.
  • Colour coding, with pretty, contrasting colours, easily indicates who wrote what without having to sign your name every time (ie. better than wiki discussions).
  • Full document history shows what changed when.
  • Each person's typing shows up immediately on everyone's screen.
  • Documents persist forever and are accessible from anywhere.
  • No central document list or filenames, so there's nothing to maintain, organize, or prune.
  • Easy to import/export various standard file formats.
  • Simple, mostly bug-free WYSIWYG rich web text editor with good keybindings. (I never thought I would voluntarily use a WYSIWYG text editor. I was wrong.)
  • Just freeform text (no plugins), so it's flexible, like a whiteboard.
  • A handy, persistent chat box next to every document keeping a log of your rationale - forever.
  • A dense screen layout that fits everything I need without cluttering up with stuff I don't. (I'm talking to you, Google Docs.)
  • Uniquely great for short-lived documents that really need collaboration: meeting minutes, itineraries, proposals, quotes, designs, to-do lists.
Where's etherpad development now? Well, it seems to have stopped. All the open source ones I've seen seem to be identical to the last etherpad that existed before Google bought them. The authors disappeared into the Google Vortex and never came out.

A few months later, Google cancelled the Wave project that had absorbed etherpad. It was a failed experiment, massively overcomplicated for what it could do. Nobody liked it. It didn't solve anyone's problem.

And that could be just another sad story of capitalism: big company acquires little company, sucks life out of it, saps creativity, spits out the chewed-up remains.

But, you see, I don't believe that's what happened. I think what happened is much more strange. I think the people who made etherpad really believed Google Wave was better, and they still do. That's what fascinates me.

See, upon further investigation, I learned that etherpad was never meant to be a real product - it was an example product. The real product was AppJet, some kind of application hosting engine for developers. As an AppJet developer, you could use their tools to make collaborative web applications really easily, with plugins and massive flexibility and workflows and whatnot. (Sound familiar? That's what Google Wave was for, too.) And etherpad was just an example of an app you could build with AppJet. Not just any example: it was the simplest toy example they could come up with that would still work.

I get the impression that the AppJet guys were pretty annoyed at the success of etherpad and the relative failure of AppJet itself. Etherpad is so trivial! Where's the magic? Oh God, WHAT ABOUT EMBEDDED VIDEO? WILL SOMEONE PLEASE THINK ABOUT EMBEDDED VIDEO? Etherpad couldn't do embedded video; still can't. AppJet can. Google Wave can. Etherpad, as the face of their company, was embarrassing. It made their technology look weak. Google Wave was a massive testosterone-powered feature checklist from hell, and Etherpad was... a text editor.

No wonder they sold out as fast as they could.

No wonder they shut down their web site the moment they signed the deal.

They felt inferior. They wanted to get the heck out of this loser business as soon as humanly possible.

And that, my friends, is the story of etherpad.


But I'm expecting a sequel. Despite the Wave project's cancellation, the etherpad/appjet guys have still not emerged from the Google Vortex. Rumour has it that their stuff was integrated into Google Docs or something. (Google Docs does indeed now have realtime collaboration - but it's too much AppJet, too little Etherpad, if you know what I mean.)

When I had the chance to visit Google a few weeks ago, I asked around to see if anybody knew what had happened to the etherpad developers; nobody knew. Google's a big place, I guess.

I would love to talk to them someday.

Etherpad legitimized real-time web document collaboration. It created an entirely new market that Google Docs has been desperately trying, and mostly failing, to create. Google Docs is trying to be Microsoft Office for the web, and the problem is, people don't want Microsoft Office for the web, because Microsoft Office works just fine and Google Docs leaves out zillions of well-loved features. In contrast, etherpad targeted, and ironically is still targeting and progressively winning despite the project's cancellation, an actually new and very important niche activity.

The brilliance of etherpad has nothing to do with plugin architectures or database formats or extensibility; all that stuff is plain brute force. Etherpad's beauty is its simplifying assumption, that just collaboratively editing a trivial, throwaway text file is something a lot of people need to do every single day. If you make that completely frictionless, people will love you. And they did.

Somehow, the etherpad guys never recognized their own genius. They hated it. They wanted it dead, but it refuses to stay dead.

What happens next?


Pre-emptive commentary

I expect that as soon as anyone reads this article, I'll get a bunch of comments telling me that Google Wave is the answer, or Google Docs can now do everything Etherpad can do, or please try my MS Office clone of the week, etc. So let me be very specific.

First of all, look at the list of etherpad features I included above. I love all those features. If you want me to switch to a competing product for the times I currently use etherpad, I want all that stuff. I don't actually want anything else, so "we don't do X but we do Y instead, which is better!" is probably not convincing. (Okay, maybe I want inline file attachments and a few bugfixes. And wiki-like hyperlinks between etherpad documents, ooh!)

Specific things I hate about Google Wave (as compared to etherpad):

  • It's slower.
  • The plugins/templates make things harder, not easier.
  • Conversations are regimented instead of free-form; you end up with ThreadMess that takes up much more screen space than in etherpad, and you can't easily trim/edit other people's comments.
  • It has an "inbox" that forces me to keep track of all my documents, which defeats throwaway uses where etherpad excels.
  • Sharing with other users is a pain because they have to sign up and I have to authorize them, etc.
  • The Google Wave screen has more clutter and less content than the etherpad screen.
  • Google Wave has a zillion settings; etherpad has no learning curve at all.
  • Google Wave wants to replace my email, but that's simply silly, because I don't collaborate on my email.
  • Google Wave wants me to live inside it: it's presumptuous. Etherpad is a tool I grab when I want, and put down when I'm done.
Specific things I hate about Google Docs (as compared to etherpad):

  • It's slower.
  • The screen layout is very very crud-filled (menu bars, etc).
  • It creates obnoxious popovers all the time, like when someone connects/disconnects.
  • Its indication of who changed what is much clumsier.
  • Its limited IM feature treats conversation as transient and interruptive, not a valuable companion to the document.
  • The UI for sharing a document (especially with users outside @gmail.com) is too complicated for mere mortals, such as me, to make work. I'm told it can be done, but it's as good as missing.
  • I can't create throwaway documents because they clutter my personal "list of documents" page that I don't want to maintain.
  • I have to save explicitly. Except sometimes it saves automatically. Basically I have no idea what it's doing. Etherpad saves every keystroke and has a timeline slider; anybody can understand it.
  • It encourages "too much" WYSIWYG: like MS Word, it's trying to be a typewriter with paper page layouts and templates and logos and fonts and whatnot, and encourages people to waste their time on formatting. Etherpad has WYSIWYG formatting for bold/italic/etc, but it's lightweight and basic and designed for the screen, not paper, so it's not distracting.
There are probably additional things I would hate about Wave and Docs, but I avoid them both already because of the above reasons, so I don't know what those other reasons are. Conversely, I use etherpad frequently and love it. Try it; I think you will too.

Update 2011/03/13: In case you would like to know the true story instead of my made up one (yeah, right; that would be like reading the book instead of watching the TV movie), you can read a response by one of the etherpad creators. Spoiler: they have, at least physically, emerged from the Google Vortex.

Update 2011/03/13: Someone also linked to PiratePad, which is a modification of etherpad that includes #tags and [[hyperlinks]]. That means they accomplished one of my dreams: making it into a wiki!

Syndicated 2011-03-13 12:05:58 (Updated 2011-03-13 23:05:02) from apenwarr - Business is Programming

28 Feb 2011 (updated 28 Feb 2011 at 09:08 UTC) »

Insufficiently known POSIX shell features

I've seen several articles in the past with titles like "Top 10 things you didn't know about bash programming." These articles are disappointing on two levels: first of all, the tricks are almost always things I already knew. And secondly, if you want to write portable programs, you can't depend on bash features (not every platform has bash!). POSIX-like shells, however, are much more widespread.1

Since writing redo, I've had a chance to start writing a few more shell scripts that aim for maximum portability, and from there, I've learned some really cool tricks that I haven't seen documented elsewhere. Here are a few.

Update 2011/02/28: Just to emphasize, all the tricks below work in every POSIX shell I know of. None of them are bashisms.

1. Removing prefixes and suffixes

This is a super common requirement. For example, given a *.c filename, you want to turn it into a *.o. This is easy in sh:


You might also try OBJ=$(basename $SRC .c).o, but this has an annoying side effect: it *also* removes the /path/to part. Sometimes you want to do that, but sometimes you don't. It's also more typing.

(Update 2011/02/28: Note that the above $() syntax, as an alternative to nesting, is also valid POSIX and works in every halfway modern shell. I use it all the time. Backquotes get really ugly as soon as you need to nest them.)

Speaking of removing those paths, you can use this feature to strip prefixes too:


These are cheap (ie. non-forking!) alternatives to the basename and dirname commands. The nice thing about not forking is they run much faster, especially on Windows where fork/exec is ridiculously expensive and should be avoided at all costs.

(Note that these are not quite the same as dirname and basename. For example, "dirname foo" will return ".", but the above would set DIR to the empty string instead of ".". You might want to write a dirname function that's a little more careful.)

Some notes about the #/% syntax:

  • The thing you're stripping is a shell glob, not a regex. So "*", not ".*"
  • bash has a handy regex version of this, but we're not talking about bashisms here :)
  • The part you want to remove can include shell variables (using $).
  • Unfortunately the part you're removing *from* has to be just a variable name, so you might have to strip things in a few steps. In particular, removing prefixes *and* suffixes from one string is a two step process.
  • ##/%% mean "the longest matching prefix/suffix" and #/% mean "the shortest matching prefix/suffix." So to remove the *first* directory only, you could use SUB=${SRC#*/}.
2. Default values for variables

There are several different substitution modes for variables that don't contain values. They come in two flavours: assignment and substitution, as well as two rules: empty string vs. unassigned variable. It's easiest to show with an example:

	unset a b c d
	e= f= g= h=
	# prints 1 2 3 4 6 8
	echo ${a-1} ${b:-2} ${c=3} ${d:=4} ${e-5} ${f:-6} ${g=7} ${h:=8}
	# prints 3 4 8
	echo $a $b $c $d $e $f $g $h

The "-" flavours are a one-shot substitution; they don't change the variable itself. The "=" flavours reassign the variable if the substitution takes effect. (You can see the difference by what shows in the second echo statement compared to the first.)

The ":" rules affect both unassigned ("null") variables and empty ("") variables; the non-":" rules affect only unassigned variables, but not empty ones. As far as I can tell, this is virtually the only time the shell cares about the difference between the two.

Personally, I think it's *almost* always wrong to treat empty strings differently from unset ones, so I recommend using the ":" rules almost all the time.

I also think it makes sense to express your defaults once at the top instead of every single time - since in the latter case if you change your default, you'll have to change your code in 25 places - so I recommend using := instead of :- almost all the time.

If you're going to do that, I also recommend this little syntax trick for assigning your defaults exactly once at the top:

	: ${CC:=gcc} ${CXX:=g++}
	: ${CFLAGS:=-O -Wall -g}
	: ${FILES:=

The trick here is the ":" command, a shell builtin that never does anything and throws away all its parameters. I find this trick to be a little more readable and certainly less repetitive than:

	[ -z "$CC" ] || CC=gcc
	[ -z "$CXX" ] || CXX=g++
	[ -z "$CFLAGS" ] || CFLAGS="-O -Wall -g"
	[ -z "$FILES" ] || FILES="

3. You can assign one variable to another without quoting

It turns out that these two statements are identical:


...even if $b contains characters like spaces, wildcards, or quotes. For whatever reason, the substitutions in a variable assignment aren't subject to further expansion, which turns out to be exactly what you want. If $b was "chicken ls" you wouldn't really want the meaning of "a=$b" to be "a=chicken; ls". So luckily, it isn't.

If you've been quoting all your variable-to-variable assignments, you can take out the quotes now. By the way, more complex assignments like "a=$b$c" are also safe.

4. Local vs. global variables

In early sh, all variables were global. That is, if you set a variable inside a shell function, it would be visible inside the calling function. For backward compatibility, this behaviour persists today. And from what I've heard, POSIX actually doesn't specify any other behaviour.

However, every single POSIX-compliant shell I've tested implements the 'local' keyword, which lets you declare variables that won't be returned from the current function. So nowadays you can safely count on it working. Here's an example of the standard variable scoping:

		local Y=6
	echo $X $Y  # returns 1 2; parens throw away changes
	echo $X $Y  # returns 5 2; X was assigned globally

Don't be afraid of the 'local' keyword. Pre-POSIX shells might not have had it, but every modern shell now does.

(Note: stock ksh93 doesn't seem to have the 'local' keyword, at least on MacOS 10.6. But ksh differs from POSIX in lots of ways, and nobody can agree on even what "ksh" means. Avoid it.)

5. Multi-valued and temporary exports, locals, assignments

For historical reasons, some people are afraid of mixing "export" with assignment, or putting multiple exports on one line. I've tested a lot of shells, and I can safely tell you that if your shell is basically POSIX compliant, then it supports syntax like these:

	export PATH=$PATH:/home/bob/bin CHICKEN=5
	local A=5 B=6 C=$PATH
	A=1 B=2
	# sets GIT_DIR only while 'git log' runs
	GIT_DIR=$PWD/.githome git log

6. Multi-valued function returns

You might think it's crazy that variable assignments by default leak out of the function where you assigned them. But it can be useful too. Normally, shell functions can only return one string: their stdout, which you capture like this:


But sometimes you really want to get *two* values out. Don't be afraid to use globals to accomplish this:

		local X Y
		getXY 7 8
		echo $X-$Y
	X=1 Y=2
	test        # prints 7-8
	echo $X $Y  # prints 1-2

Did you catch that? If you run 'local X Y' in a calling function, then when a subfunction assigns them "globally", it still only affects your local ones, not the global ones.

7. Avoiding 'set -e'

The set -e command tells your shell to die if a function returns nonzero in certain contexts. Unfortunately, set -e *does* seem to be implemented slightly differently between different POSIX-compliant shells. The variations are usually only in weird edge cases, but it's sometimes not what you want. Moreover, "silently abort when something goes wrong" isn't always the goal. Here's a trick I learned from studying the git source code:

	cd foo &&
	make &&
	cat chicken >file &&
	[ -s file ] ||
	die "resulting file should have nonzero length"

(Of course you'll have to define the "die" function to do what you want, but that's easy.)

This is treating the "&&" and "||" (and even "|" if you want) like different kinds of statement terminators instead of statement separators. So you don't indent lines after the first one any further, because they're not really related to the first line; the && terminator is a statement flow control, not a way to extend the statement. It's like terminating a statement with a ; or & - each type of terminator has a different effect on program flow. See what I mean?

It takes a little getting used to, but once you start writing like this, your shell code starts getting a lot more readable. Before seeing this style, I would tend to over-indent my code, which actually made it worse instead of better.

By the way, take special note of the way we used the higher precedence of && vs. || here. All the && statements clump together, so that if *any* of them fail, we fall back to the other side of the || and die.

Oh, as an added bonus, you can use this technique even if set -e is in effect: capturing the return value using && or || causes set -e to *not* abort. So this works:

	set -e
	mv file1 file2 || true
	echo "we always run this line"

Even if the 'mv' command fails, the program doesn't abort. (Because this technique is available, redo always runs all its scripts with set -e active so it can be more like make. If you don't like it, you can simply catch any "expected errors" as above.)

8. printf as an alternative to echo

The "echo" command is chronically underspecified by POSIX. It's okay for simple stuff, but you never know if it'll interpret a word starting with dash (like -n or -c) as an option or just print it out. And ash/dash/busybox, for example, have a weird "feature" where echo interprets "echo \n" as a command to print a newline. Which is fun, except no other shell does that. The others all just print backslash followed by n.

There's good news, though! It turns out the "printf" command is available everywhere nowadays, and its semantics are much more predictable. Of course, you shouldn't write this:

	# DANGER!  See below!
	printf "path to foo: $PATH_TO_FOO\n"

Because $PATH_TO_FOO might contain variables like %s, which would confuse printf. But you *can* write your own version of echo that works just how you like!

		# remove this line if you don't want to support "-n"
		[ "$1" = -n ] && { shift; FMT="%s"; } || FMT="%s\n"
		printf "$FMT" "$*"

9. The "read" command is crazier than you think

This is both good news and bad news. The "read" command actually mangles its input pretty severely. It seems the "-r" option (which turns off the mangling) is supported on all the shells that I've tried, but I haven't been able to find a straight answer on this one; I don't think -r is POSIX. But if everyone supports it, maybe it doesn't matter. (Update 2011/02/28: yes, it's POSIX. Thanks to Alex Bradbury for the link.)

The good news is that the mangling behaviour gives you a lot of power, as long as you actually understand it. For example, given this input file, testy.d (produced by gcc -MD -c testy.c):

	testy.o: testy.c /usr/include/stdio.h /usr/include/features.h \
	  /usr/include/sys/cdefs.h /usr/include/bits/wordsize.h \
	  /usr/include/gnu/stubs.h /usr/include/gnu/stubs-32.h \
	  /usr/lib/gcc/i486-linux-gnu/4.3.2/include/stddef.h \
	  /usr/include/bits/types.h /usr/include/bits/typesizes.h \
	  /usr/include/libio.h /usr/include/_G_config.h
	  /usr/include/wchar.h \
	  /usr/lib/gcc/i486-linux-gnu/4.3.2/include/stdarg.h \
	  /usr/include/bits/stdio_lim.h \

You can actually read all that content like this:

	read CONTENT <testy.d

...because the 'read' command understands backslash escapes! It removes the backslashes and joins all the lines into a single line, just like the file intended.

And then you can get a raw list of the dependencies by removing the target filename from the start:


Until I discovered this feature, I thought you had to run the file through sed to get rid of all the extra junk - and that's one or more extra fork/execs for every single run of gcc. With this method, there's no fork/exec necessary at all, so your autodependency mechanism doesn't have to slow things down.

10. Reading/assigning a variable named by another variable

Say you have a variable $1 that contains the name of another variable, say BOO, and you want to read the variable pointed to by $1, then do a calculation, then write back to it. The simplest form of this is an append operation. You *can't* just do this:

	# Doesn't work!
	$V="$$V appended stuff"

...because "$$V" is actually "$$" (the current process id) followed by "V". Also, even this doesn't work:

	# Also doesn't work!

...because the shell assumes that after substitution, the result is a command name, not an assignment, so it tries to run a program called "BOO=50".

The secret is the magical 'eval' command, which has a few gotchas, but if you know how to use it exactly right, then it's perfect.

		eval local tmp=\$$1
		tmp="$tmp $2"
		eval $1=\$tmp
	BOO="first bit"
	append BOO "second bit"
	echo "$BOO"

The magic is all about where you put the backslashes. You need to do some of the $ substitutions - like replacing "$1" with "BOO" - before calling eval on the literal '$BOO'. In the second eval, we want $1 to be replaced with "BOO" before running eval, but '$tmp' is a literal string parsed by the eval, so that we don't have to worry about shell quoting rules.

In short, if you're sending an arbitrary string into an eval, do it by setting a variable and then using \$varname, rather than by expanding that variable outside the eval. The only exception is for tricks like assigning to dynamic variables - but then the variable name should be controlled by the calling function, who is presumably not trying to screw you with quoting rules.

11. "read" multiple times from a single input file

This problem is one of the great annoyances of shell programming. You might be tempted to try this:

	(read x; read y) <myfile

But it doesn't work; the subshell eats the variable definitions. The following does work, however, because {} blocks aren't subshells, they're just blocks:

	{ read x; read y; } <myfile

Unfortunately, the trick doesn't work with pipelines:

	ls | { read x; read y; }

Because every sub-part of a pipeline is implicitly a subshell whether it's inside () or not, so variable assignments get lost.

A temp file is always an option:

	ls >tmpfile
	{ read x; read y; } <tmpfile
	rm -f tmpfile

But temp files seem rather inelegant, especially since there's no standard way to make well-named temp files in sh. (The mktemp command is getting popular and even appears in busybox nowadays, but it's not everywhere yet.)

Alternatively you can capture the entire output to a variable:


But then you have to break it into lines the hard way (using the eval trick from above):

		local INV=$1 OUTV=$2
		eval local IN=\$$INV
		local IFS=""
		local newline=$(printf "\nX") 
		[ -z "$IN" ] && return 1
		local rest=${IN#*$newline}
		if [ "$rest" = "$IN" ]; then
			# no more newlines; return remainder
			eval $INV= $OUTV=\$rest
			local OUT=${IN%$rest}
			eval $INV=\$rest $OUTV=\$OUT
	tmp=$(echo "hello 1"; echo "hello 2")
	nextline tmp x
	nextline tmp y
	echo "$x-$y"  # prints "hello 1-hello 2"

Okay, that's a little ugly. But it works, and you can steal the nextline function and never have to look at it again :) You could also generalize it into a "split" function that can split on any arbitrary separator string. Or maybe someone has a cleaner suggestion?

Parting Comments

I just want to say that sh is a real programming language. When you're writing shell scripts, try to think of them as programs. That means don't use insane indentation; write functions instead of spaghetti; spend some extra time learning the features of your language. The more you know, the better your scripts will be.

When early versions of git were released, they were mostly shell scripts. Large parts of git (like 'git rebase') still are. You can write serious code in shell, as long as you treat it like real programming.

autoconf scripts are some of the most hideous shell code imaginable, and I'm a bit afraid that a lot of people nowadays use them to learn how to program in sh. Don't use autoconf as an example of good sh programming! autoconf has two huge things working against it:

  • It was designed about 20 years ago, *long* before POSIX was commonly available, so they avoid using really critical stuff like functions. Imagine trying to write a readable program without ever breaking it into functions!
  • Because of that, their scripts are generated by macro expansion (a poor man's functions), so http://apenwarr.ca/log/configure is more like compiler output than something any real programmer would write.
autoconf solves a lot of problems that have not yet been solved any other way, but it comes with a lot of historical baggage and it leaves a bit of a broken window effect. Please try to hold your shell code to a higher standard, for the good of all of us. Thanks.


1 Of course, finding a shell with POSIX compliance is rather nebulous. The reason autoconf 'configure' scripts are so nasty, for example, is that they didn't want to depend on the existence of a POSIX-compliant shell back in 1992. On many platforms, /bin/sh is anything but POSIX compliant; you have to pick some other shell. But how? It's a tough problem. redo tests your locally-installed shells and picks one that's a good match, then runs it in "sh mode" for maximum compatibility. It's very refreshing to just be allowed to use all the POSIX sh features without worrying. By the way, if you want a portable trying-to-be-POSIX shell, try dash, or busybox, which includes a variant of dash. On *really* ancient Unixes without a POSIX shell, it makes much more sense to just install bash or dash than to forever write all your scripts to assume it doesn't exist.

"But what about Windows?" I can hear you asking. Well, of course you already know about Cygwin and MSys, which both have free ports of bash to Windows. But if you know about them, you probably also know that they're gross: huge painful installation processes, messing with your $PATH, incompatible with each other, etc. My preference is the busybox-w32 (busybox-win32) project, which is a single 500k .exe file with an ash/dash-derived POSIX-like shell and a bunch of standard Unix utilities, all built in. It still has a few bugs, but if we all help out a little, it could be a great answer for shell scripting on Windows machines.

Syndicated 2011-02-28 04:21:46 (Updated 2011-02-28 09:08:39) from apenwarr - Business is Programming

Chip-and-pin is *not* broken

I've seen this article about the supposed security holes with chip-and-pin credit cards making the rounds lately. As with my previous article on smartcard PINs, I have just enough knowledge to be dangerous. Which in this case means just enough knowledge to tell you why this latest attack on chip cards is not a very exciting one.

In short, the attackers in the above article have revealed a simple way for anyone to use a stolen chip credit card, without knowing the PIN, regardless of whether or not the credit card reader is online at the time. This security hole is real. I'm not disputing that.

However, it's also not as bad as people are making it sound. Most importantly, chip cards are not even *close* to as insecure as the old ("magstripe") cards they were designed to replace. And while those old cards had annoyingly high levels of fraud, you as a consumer really didn't need to care, because big faceless megacorporations paid for it. In this case, it's still more secure than before, so you should care even less than before.

Here are three reasons why the current security hole is not very exciting:

1. You have to *physically steal the card*.

Most fraud on magstripe cards was from *copying*: anyone with a bit of technical skill can trivially copy a magstripe card just by buying a magstripe writer device for less than $200. Since magstripes are used all over for much more than just financial stuff, there's no regulation about who can buy such devices.

So a common form of attack on magstripes is social engineering. It works like this: go into a store where they swipe your card on a machine. The machine then says "card read error" or "denied" or whatever, and you give up and use a different card or cash. Or maybe they try again on a different terminal, and it mysteriously succeeds this time. But in the meantime, the first terminal has recorded the data on your magstripe - probably less than 1k of data per card. The criminal can pick up the data later, and use it to clone any card that has ever been read by the illegal reader.

Ah, but what about your signature on the back of the card? And how about the hologram of your face that's on some cards? How do they copy *that*, huh? Easy, they don't: they just rewrite the magstripe on a card with *their* picture and signature on it. When they take it to a store, the card is physically theirs, but the account number is yours, so they're spending your money, not theirs. Ouch.

Better still, they can simply re-rewrite the magstripe on their card back to their original identity later, leaving little evidence.

So anyway, be suspicious of any vendor who tries to scan your card but then it "fails," especially if they look like they were expecting it. If their reader was *that* unreliable, they would have stopped using it by now. Report them to your bank. It might help them find some fraudsters.

But anyway, that form of fraud is soon to disappear: your credit card still has a magstripe, but it's only for backwards compatibility with old magstripe-only readers. Chip cards are completely immune to this sort of attack, because the chip interface doesn't tell you the card number: it only gives the reader a one-time transaction authorization code, which you can't use to construct a new card.

Of course, someone who clones your card's magstripe can still copy and use it in a magstripe reader, so you're still susceptible to fraud. But it gets less valuable every day, and vendors who sell expensive stuff - the ideal fraud targets - are the first to upgrade their readers to support chip cards. Someday the magstripe "feature" will be safely turned off entirely.

(It *is* still possible to read the real account number and secret key information from a chip card and therefore clone it. But it requires extremely messy and unreliable equipment. As far as I know, there are no simple, reliable machines that can do this in a store setting without arousing suspicion. Moreover, any such machine would obviously have only-nefarious purposes, so you'll never be able to buy one for $200 like you can with general purpose magstripe devices.)

By the way, the extra "security code" on the back of your card is a partial protection against the card-copying attack: the security code isn't on the magstripe. So someone who copies your card can't use it for web purchases that need the security code, unless they manually write down the security code while stealing your card, and that would be too obvious. (Unless you're at a restaurant, and the server takes your card into a back room for "processing." But you wouldn't ever let them do that, right?)

2. You have to hack the *vendor's* card reading terminal.

Another handy thing about the old magstripe card attacks is they work at any store, on any credit card terminal. (Nowadays they only work at terminals that *don't* support chip cards, because the bank refuses magstripe transactions from chip-capable terminals on chip-capable cards. Getting better!)

That means the standard form of attack would be to steal your card number at a shady corner store or restaurant, then delay for an arbitrary amount of time - you don't want it to be too obvious which shady store stole the number, since they probably stole a bunch all at the same time. Then, take it to an expensive store like Future Shop and buy stuff... untraceably. It has your name on it, not the fraudster, and the vendor is Future Shop, not the fraudster. The perfect crime.

With the chip card hack we're discussing, your crime isn't so perfect anymore. Above, I said that you need to *steal* the card instead of copying it. That's hard enough to do, but it's doubly hard for another reason: the owner will probably notice and immediately call their bank to cancel it. That means you can't wait a few weeks or months before using the stolen card - you have to do it *fast*, before the owner notices. So even if you come up with a clever way of stealing cards in large quantities, it'll be a lot easier for the police to track down the theft just by using statistics.

But even worse, to execute the chip-card-without-PIN hack, you have to break the card *reader* device and make it lie to both the card and the bank. That's not so hard to do, if you're an excellent programmer. Much harder than just copying a magstripe, which any halfway competent techie could do with off-the-shelf equipment. But yes, as with any DRM, it's crackable, so somebody can do it. Based on the "Chip and PIN is Broken" paper, someone already has.

What's *much* harder, though, is getting a store to *use* your hacked card reader on their merchant account. You can't just walk into Future Shop and hand them a card and a special card reader and say, "Yeah, charge it to this." No, you'll have to *infiltrate* a Future Shop and get them to use your special card reader. Not only is that much more traceable - suddenly there are people at Future Shop who *know* who the criminal is - but it's also not much easier than just stealing the stupid TV in the first place. I mean, if you can sneak into Future Shop, then you can sneak out of Future Shop, right?

Or you could get your own merchant account, I guess, and charge the money through that. But no... not likely. What could be more traceable than opening a merchant account at a bank and then running fraudulent transactions through it? I'm sure there's some way to use a fake identity or something, but again, that's *way* harder than walking into a chain store, buying something, and walking out again.

3. The only reason it even works is for backwards compatibility.

The third reason this attack is rather boring is that it exploits an *optional* feature of the EMV specification. Essentially, they independently convince the card, and the bank, that they really don't need a PIN today. In a simpler world, that would be impossible; the card would demand a PIN, and the bank would demand a PIN-authenticated transaction. But because of backwards compatibility and maybe some too-flexible specifications, it's possible to convince the card that the bank has authenticated the PIN, *and* convince the bank that the card has authenticated the PIN, all at the same time.

Yes, it's a real security hole caused by specification bugs; that attack simply shouldn't be possible to do. And yes, because of that attack, a physically stolen card can be used on a hacked card reader without knowing the PIN, so banks should worry about fraud.

But from my reading of the EMV specification (the spec is available for free online, by the way; google it), the particular modes that make this attack possible are optional. You can just turn them off. Banks just don't want to, because in some situations, those modes let you do a transaction (ie. spend money, thus earning the bank money) where you otherwise couldn't.

If I understand correctly - and maybe I don't, as I didn't read the exact form of the attack too carefully - the main point of confusion is that PINs can be verified either online (by the bank) or offline (by the card), and which one we use is determined by a negotiation between the card, the reader, and the bank. (Interestingly, exactly this was the focus of my previous article on chip cards.) If the reader lies to the card and says the bank isn't available right now (offline mode), and lies to the bank and says the card has requested offline PIN verification (maybe the card is set to *require* offline verification; that's one of the options in the spec), then the transaction can go through.

Moreover, the bank will store a "PIN Verified" flag that supposedly means the transaction is known to be much safer than an unverified (eg. magstripe) transaction. That might tell a bank's auto-fraud-detection algorithms to relax more than they should, which is probably the *real* story here. (If you're a bank, you should care about this security hole. If you're a normal person, probably not.)

Bonus trivia: incentives

By the way, here's a bit of information I ran into that I found interesting:

In the magstripe days, most fraud was by default considered the responsibility of the *vendor*. That is, if a store accepted a fraudulent card and you later reversed the transaction, it was the store who lost the money, not the bank. This seems really cruel to store owners, but the idea was that if you don't give them an incentive to reject faked cards, then they won't be careful. For example, a store has no reason to check your signature or your photo id if it's not them who'll lose in case of fraud. In fact, they have an incentive to *not* check those things, since checking them slows them down, costs money, and discourages you from shopping there. (Plus, a store can make their own decisions about how careful they want to be. Do we want a fast checkout line in exchange for higher fraud, or a slow checkout line with less fraud? What saves us the most money in the long run?)

Remember, even without professional magstripe fraud, there was still the even more trivial low-tech kind: a teenager steals a credit card from their parents' wallet, walks into a store, and buys something. Signature/photo checks are really hopelessly weak, but at least they reduce *that*.

Now, putting all the blame on the vendor was supposedly the default, but I suspect it wasn't what actually happened. I'm guessing that, officially or unofficially, if it was proven that a real forged card was used - as opposed to the vendor just not checking the signature carefully enough - that the bank would take on some of the loss. Maybe half, maybe all of it, who knows.

As magstripe fraud got more and more out of control, the industry started switching to chip cards. The problem is, you can't switch the entire world from magstripe to chip all in one day. It's a chicken-and-egg problem. So how do you make it worthwhile for banks to start issuing chip cards - since magstripe fraud isn't their problem - *and* for stores to upgrade to chip readers?

Someone somewhere (in government, perhaps?) came up with a neat solution: we'll change the liability rules. Nowadays it works like this: if a vendor has a chip-capable reader, card fraud is the bank's fault, so they pay for it. If a vendor only has an old magstripe reader, the vendor pays for it. And this is true whether or not the particular credit card is chip capable, because if it's not, then it's the bank's fault for not issuing a newer card.

I find this to be an extremely clever social hack. It aligns everybody's best interests toward reducing fraud, but requires no fines, laws, agreed-upon industry-wide schedules, deprecation periods, or enforcement.


The "Chip and PIN is Broken" attack:

  • only works if your card is physically stolen;
  • only works if the criminal uses a traceable vendor with a modified terminal;
  • can be disabled someday by turning off some optional protocol features;
  • is a liability issue for banks and/or vendors, but not consumers like you.
My advice:

  • If your card is stolen, report it immediately.
  • If a vendor has trouble reading your card and doesn't look surprised, consider reporting it to your bank.
  • Yes, you still need to monitor your bank statement for incorrect transactions (not just for fraud; incompetence remains rampant too).
  • If anyone asks you whether chip cards are more secure than the alternatives, say YES YES OH GOD YES, PLEASE PLEASE LET THE OLD SYSTEM DIE NOW.
I hope that clears things up.

Syndicated 2011-02-22 12:56:37 from apenwarr - Business is Programming

10 Feb 2011 (updated 10 Feb 2011 at 05:04 UTC) »

Daring Fireball linked to me

Oh wow, not only am I internet famous, but now Daring Fireball used my uninformed speculation to inform their uninformed speculation! This, after my original uninformed speculation was inspired by theirs!

I am actually part of an internet circle jerk. Wow. This is too awesome. I love you guys. <snif>

Update 2011/02/09: And The Guardian linked to me too, apparently.

Syndicated 2011-02-10 03:12:33 (Updated 2011-02-10 05:04:03) from apenwarr - Business is Programming

10 Feb 2011 (updated 10 Feb 2011 at 03:09 UTC) »

A co-op job posting for the 21st century

Zak at Upverter recent wrote a post titled Interns: How to hire Porn Stars that had a few links to my old articles on hiring interns (which we Waterloovians1 call "co-op students"). I had totally forgotten about Try not to *need* to advertise for employees, which I recommend as background for the following.

Since my series of articles is about four years old now, the Upverter post reminded me that maybe I should make an update with some of my observations since then. Here's what I've learned.

Silly job titles have become cliches.

I may have been the person who started the recent trend of ridiculous job titles, starting with NITI's old Human Cannonball and Evil Death Ray co-op jobs. In its prime, NITI became infamous among Waterloo co-op students, and those co-op students (ie. spies) have since spread everywhere. I heard fairly reliable rumours that Research in Motion was seriously considering their own silly job descriptions at one point, using ours as a direct model. And I heard that Akoha's original "Python Charmer" video job ad was inspired by one of our co-op students who worked there. As far as I can tell, the "Ruby Ninja Pirate" knockoff job descriptions all find their roots in Akoha's video job ad initiative.

Here's the bad news: Akoha partly missed the point, and everybody after that *really* missed the point. The point of these job ads was to do something unique that would get people's attention and be really memorable, while accurately representing our work environment. They *don't work* once they stop being unique, and they *don't work* if they don't accurately represent your work environment.

We now live in a world where stupid boring companies think they can fool people by using Ninja Python Rock Pornographer job descriptions. It doesn't work; in fact, as far as I'm concerned, stupid job descriptions now have a negative correlation with the kind of company I'd want to work for. If you don't understand why you're being silly, then you don't understand your own fundamental human motivations, so your company has a pretty high probability of failure. That's not sexy.

The silliness was never the point.

I covered this in my earlier Job Titles and Roles article, but it bears repeating: the reason we didn't use *standard* job titles was that standard job titles come with preconceptions. Preconceptions mean instant inflexibility. Meaningless job titles mean that nobody really knows what their job is, and that's a good thing, because startups live or die by the flexibility of their people.

But silliness and standardization are orthogonal. (Just look at IPsec.) The more famous nonstandard job titles, like "Just a Programmer" or "Member of Technical Staff," also achieve these goals without the distracting silliness.

More rumours from my spies: after a while, Akoha supposedly switched away from silly video job descriptions, because they discovered that the silliness was selecting more for silly people than for good programmers. The Human Cannonball / Evil Death Ray job descriptions were subtly balanced in a way that silly video ads aren't. (I admit that this "subtle balance" was more accidental than purposeful.) For example, you have to be literate to understand the Human Cannonball ad. You even have to use your imagination a bit to understand what a human cannonball has to do with writing software at a startup. (Hint: think of a bull in a china shop.) But you don't even need a working brain to hear, "Porn rocker hermit laser monkey!!" and say "Ooga booga! Me want!"

The people who want to pad their resumes are *right*.

Zak's article links to another article that suggests people are somehow wrong to want to work at Google to make their resume look more impressive when applying for a "real" job after graduation. Instead, the advice is to get a job at a startup, because you'll get better experience there.


Let me badly paraphrase some ancient wisdom, to wit: Respect your enemies, or they will beat the pants off you.

As a company trying to attract students, you are *competing* with Google. All the rules of business competition apply, including this one: if you badmouth your competitors, it just makes you look like a sore loser, with emphasis on the "loser."

Google is, from all reports I've ever heard (and oh boy, do a lot of my spies^H^H^H^H^H former co-op students work at Google), a really excellent place to work. Furthermore, it's widely acknowledged that investors are, on average, happier to give money to a company founded by "former Googlers" than by other random people. And you know what? If I was hiring someone, even a student, for *my* company, all else being equal, I'd absolutely prefer someone who's worked at Google or Apple versus someone who hasn't. Facebook, Twitter, Amazon? The same, maybe not as strongly.

Why does Google make great resume padding? Simple: because they're a great pre-filter. They have a really complicated job interview process and acknowledged high standards. That you managed to sneak by their filters doesn't automatically mean you're great, but it sure means you're great enough that you should get by my (by comparison, stone aged) initial resume filter and probably phone screen. We might as well fly you in right away; we already know Google did and *then* they were happy enough to give you a job.

Maybe your noname startup has great screening/hiring practices too; we sure did. But a random interviewer at a random company won't know that. They *will* know about Google.

So anyway, if you're a startup and you want to attract students, that's what you're dealing with. Every student absolutely, positively should try to pad their resume with a job like that. Even if they sat at Google and twiddled their thumbs and stared at the screen in shock and fear and tried not to get noticed for four months, that doesn't matter; they can just lie on their resume and in your interview. It won't work every time, but it's sure better than not having the option.

Will you try to tell students that working for a startup is more valuable than that? If so, instant fail. You'll be lying. They'll know it.

Working for a startup is still valuable experience.

Okay, so working at Google is great for the resume, and as a bonus, if you put some work into it, you'll probably also learn something. (I don't know, I've never worked there, but spies confirm this too.)

But don't get me wrong: if you *really* want to learn stuff, there is nothing like working at a startup. Every student who wants to get wide experience - as every student should - ought to work for a startup, at least for a while. This is not either/or. You should work for Google or a big name company. You should work for a startup. If you don't do both, you're doing it wrong.

Furthermore, Google recruiters are smart. If you have experience working at a startup, *they* know what that means. So let's say Google rejected you this time around. If you work at a startup, all that new experience should increase your chances of getting the Google offer next time.

So with all that in mind, here's how I might write a co-op job ad for the 21st century:


I know what you're thinking. You're thinking, "Hmm, maybe I'd better at least apply for a few other jobs in case Google doesn't want me. I don't know how to cook. I really need that free gourmet cafeteria food if I want to survive... but maybe, worst case, with another job, I could work something out. Pay someone to make food for me, somehow. Can you even do that? Or maybe mooch off a co-worker."


If you're smart, you'll make sure to have at least one brand-name company on your resume by the time you graduate. But that's not all you should have: you should have real-life experience working in every aspect of product development and release, from building office furniture and installing your operating system to coding features, fixing bugs, answering support calls, and substituting for the CEO while he's away on his biweekly bender.

This job isn't easy. You'll need every skill you have, plus more. But that's okay, because one of our full-time developers will be your personal mentor and help you develop those skills.

We work reasonable hours, except when we don't want to, and then we work unreasonable hours. Our co-op students release more code in an afternoon than Google co-ops release in a whole work term, and sure, their code works and ours doesn't, but we have PRIDE, dammit, PRIDE, and we'll fix the bugs tomorrow before anybody notices.

We can't hire as many people as the big companies, which means you're going to have to be the best of the best if you want to work here. But that also means we'll give you more responsibility than you'll get anywhere else, which means you'll learn more than you would learn anywhere else. This company was founded by Waterloo co-op students, so we know co-op students can be trusted with responsibility. We also know exactly what we wished we knew when we were your age, duckies, and we'll tell you so you can ignore us and go learn it the hard way.

Here are some skills you should have:

  • (list of skills)
And here are some projects we'll be working on:

  • (list of projects)
If you do your job right, next time around, you'll have so much experience that you'll get that Google job for sure.

If we do our job right, you won't want it.

Bonus Interview Tip from People Who've Done This Before

When you wear a suit to your interviews, the only jobs it helps you get are the ones where they care how you dress more than how you code.


1For those who don't know, the University of the Waterloo is the university in Canada with the "best reputation." (Yeah, I know, it's not as good as being objectively best, but we take what we can get.) It also has, by a very very very large margin, the best co-op/internship programme of any Canadian university I'm aware of. Students in the co-op programme, which is most of them, have six different four-month work terms before they graduate, at up to six different companies. NITI hired students almost exclusively from U.Waterloo simply because it was consistently easy and the students were consistently high quality, and if you want word-of-mouth to spread among students, it helps if they're all in the same classes.

Syndicated 2011-02-10 02:03:46 (Updated 2011-02-10 03:09:25) from apenwarr - Business is Programming

Sshuttle VPN 0.51: now with DNS forwarding and a MacOS GUI

I just released version 0.51 of my Sshuttle VPN. Normally I don't re-announce my projects here unless something really interesting happens, but if you have a Mac, then I think this counts as really interesting:

Sshuttle now has a fancy MacOS GUI!

There's not too much to it. Other than the menubar icon, there's just a preferences window:

For those just joining us, what's so interesting about sshuttle?

  • It's easier to install than any other VPN software in the history of the universe.
  • It forwards over a plain ssh session, which means your server doesn't need sshuttle installed and you don't need server admin access.
  • Authentication is just ssh authentication; there's nothing new to learn.
  • It avoids the tcp over tcp problem that's infamous among simple-minded VPNs.
  • You don't have to change any SOCKS settings.
  • It's more reliable than ssh's port forwarding, which freezes randomly (at least for me).
  • It has latency controls to avoid interactive slowness caused by bufferbloat in ssh.
  • You can choose to forward only a subset of your traffic (ie. the subnets that exist on the remote end).
  • If you have multiple offices or clients, you can connect to more than one remote network at once.
  • You can choose to forward *all* your TCP traffic to protect yourself from things like FireSheep.
  • Since Sshuttle 0.50, you can also capture all DNS requests and send them over the tunnel.
  • Since Sshuttle 0.51, we now have a workaround for stupid MacOS 10.6 network-dropping-dead kernel bugs. (Man, Apple should hire me as a QA tester. I'll just write open source software and reveal serious kernel bugs. Of course, they don't ever fix them, so...)
To download the source code (for Linux, MacOS, and FreeBSD), visit the sshuttle page on github.

If you want to just download the precompiled MacOS GUI app, I made a github tag for that: download Sshuttle VPN.app here. (The resulting filename is really stupid, because github's auto-generated download filenames are really stupid. It seems obvious to me that a tag named 'sshuttle-0.51-macos-bin' should result in a file called 'sshuttle-0.51-macos-bin.tar.gz', but no, the generated filename is full of crap. If this upsets you, complain to the github people. I already did. They told me I was wrong.)

Will this be going into the fancy newfangled Mac App Store?

No. Sshuttle needs root access (on your client machine, not on the server), which disqualifies it. Oh well. You'll have to go through the rather trivial process of downloading and extracting the zip file instead.

Syndicated 2011-02-05 06:45:28 from apenwarr - Business is Programming

Template for apenwarr's political opinions

It's that time of year again apparently. Rather than fill in the template for you this time, I'll just give you the general outline of what I always think of your pointless and uninformed politically-motivated petition. That way you can guess, in the future, what I will say, and I won't have to actually say it.

First of all, cut it out with the ad-hominem attacks. Stick to the actual issues. If you don't like the other guy's TV ads, and you loudly say so, then *you* are the one not talking about the issues, and it's *your fault*. Be better than that. When you rise above the sludge, people will notice. You won't have to tell them.

Pay attention to people's biases. If there's a web site lobbying for something, *figure out who runs it*. If you agree with a web site lobbying for something, be *especially* suspicious, because while you agree with their main point, they might be using your general agreeableness to slip by some absolute lies. They're lobbyists! Lobbyists lie! If you don't see any lies on that lobbyist site, YOU ARE A VICTIM OF MANIPULATION.

Remember that most branches of the government are not *directly* controlled by elected officials. Elected figureheads are at the top of the pyramid, but they swap out frequently and rarely have time to learn every detail. Thus, when an unelected regulator makes a decision, it's rarely motivated by re-election or party sponsors or whatever, because the person making the decision usually doesn't care about those things. The decision might still be wrong for many reasons, but it almost always isn't political reasons. Thus, never trust someone who simplistically tells you otherwise. Find out the real reasons a decision was made.

Try to imagine that the person making a law or regulation is actually a good person who is trying to do the right thing for everyone. Why would a *good* person have decided to do what you're angry about? Remember, it's a hypothetical honest person, doing what they honestly think is right. Why do they think that? Could someone reach such a high level of office and be *that* dumb? Imagine you were that dumb; what would you do to achieve that level of power? Or are you perhaps missing something? Can you figure out what you've missed?

Try to remember that Canada is not the United States. We have a vastly, vastly lower degree of corruption, for many reasons. To skip the idealism, one theory is simply that, at 1/10th the size, it's just not worth infiltrating us. Maybe the person making the decision really *is* a good person.

Note that at least one of the big political parties has policies that aim to make the rich richer. The funny thing is, virtually everyone reading this article *is* rich, by the literal definition average people use, or they will be rich before they retire. You might not agree with this party's policies, but if you're *surprised* when a lot of those policies end up directly benefitting you, then you are much less objective than you think you are. You hate them, but you've forgotten why. If you're honest with yourself, you'll find that you're in the uncomfortable position of disagreeing with policies that are really, really good for you personally. That can be a form of high virtue. Does it feel like hating them is virtuous, or reflexive? There's a big difference.

Most of all:

Do your own research. We have the Internet. We can just *look stuff up*. Government policies and decisions are published in the open, with tonnes of public review. Yet almost nobody ever actually goes to the primary sources before forming an opinion; they trust what their friends tell them. Ask your friends: did you actually read the primary sources? If they say no, they just heard it from their friends, STOP THEM BEFORE THEY HURT SOMEONE.

The only cure for mob mentality is thinking for yourself.

Syndicated 2011-02-03 09:58:07 from apenwarr - Business is Programming

Moore's law and iPad-sized "retina displays"

Normally I don't bother to engage in Apple product speculation, but in this case, I have some actual knowledge (from a brief foray into working at a semiconductor company, some time ago) that might be interesting to share, and Apple is a good excuse. Of course, that was a long time ago - before LCD monitors had any significant popularity - and I'm just a software guy, so I might totally misinterpret everything.

But if I were to take what I think I know and conjecture wildly without looking at any reference material, it would look like this:

LCD-style displays (at least in that family; that is, a two-dimensional array of transistors that light up or darken or whatever to form an image) are all built using something like semiconductor crystal growth processes.

Semiconductor processes more or less conform to Moore's Law, one statement of which is: the price per transistor approximately halves every 18 months. (It is often stated in terms of "performance," but we have already long passed the point where doubling the number of transistors automatically improves performance. And in a display, we care about the literal number of transistors anyway.)

Why does the price per transistor halve? I'm not really sure, but the short answer is "because we get better at growing perfect (imperfection-free) semiconductor crystals at a smaller size." So there are two variables: how small are the transistors? And how perfect are the transistors?

Roughly, if you can pack more transistors into a smaller space, the chance that you'll run into an imperfection in the crystal in the space you use will be less; the error rate in a crystal is about constant - and very low, but not quite zero.

Transistor sizes ("processes") come in steps. These are the sizes you hear about every time Intel builds a new fab: 0.04 micron, and so on. When a new process is introduced, it'll be buggy and produce a higher rate of imperfect transistors for a while; with practice, the manufacturing engineers get that error rate down, saving money because more of the chips on a particular wafer of silicon will be imperfection-free.

By the way, that's how it works: you buy these wafers of silicon, then you "print" circuits on it using one of many extremely crazy technologies, then you chop up the wafer and make chips out of it. Some of the chips will work (no imperfections), and some of them won't (at least one imperfection). Testing chips is a serious business, because it can sometimes be really hard to find the one transistor out of 25 million that is slightly wrong sometimes (at the wrong temperature) because of an imperfection. But they manage to do it. Very cool.

Also by the way, multicore processors are an interesting way to reduce the cost of imperfections. Normally, a single imperfection makes your chip garbage. If you print multiple cores on the same chip, it massively increases the area of the chip, making the probability of failure *much* higher. For example, a quad-core processor takes roughly 4x the area, so if p is the probability of failure for one core, the probability of overall success is (1-p)**4. So if one core has a 90% success rate (where I worked, that was considered very good :)), four cores have only a 65% success rate, and so on. Similarly, a single-core megacomplicated processor four times as big as the original would have only a 65% success rate. But if you're building a multi-core processor, when you find that *one* of the cores is damaged, you can disable *just* that one core, and the rest of the processor is still good! So as a manufacturer, you save *tons* of money versus just throwing it away.

Remember those three-core AMD processors that people found out they could hack into being four-core AMD processors? I wouldn't, if I were you.

Anyway, where was I? Oh, right.

So every now and then, Intel will build a new fab with a newer, smaller process, and start making their old chips in the new process, which results in a vastly lower imperfection rate, which means far fewer of the chips will be thrown away, which means they make much more money per chip, which means it's time to cut the price. Along with improved manufacturing processes etc, this happens at approximately a 2x ratio every 18 months, if Mr. Moore was correct. And he seems to have been, more or less, although some of us might wonder whether it's a self-fulfilling prophecy at this point.

Now here's the interesting part: LCD-like displays, being based on similar crystal-growing methods, follow similar trends!

A "higher DPI" - more dots in less space - corresponds to a new crystal growth process. Initially, there will be a certain number of imperfections per unit area; over time, those wonderful manufacturing engineers will get the error rate down lower.

Remember when LCD monitors used to come with "dead pixels?" The small print in your monitor's warranty - at the time, maybe not now - said that up to 1, or 2, or 5 dead pixels was "unavoidable" and not grounds for returning the product to the manufacturer. Why? Because it was just way too expensive to make it perfect. Maybe the probability of one dead pixel in a 17" screen was, say, 80%, but the probability of two dead pixels was only 50%. If you allow zero dead pixels, you have to throw away 80% of your units; if you allow one, you only throw away 50%; and so on. Accepting those dead pixels made a *huge* difference to profitability. LCD-like displays might never have been cheap enough if we didn't put up with the stupid dead pixels.

But those were the old days. Nowadays, thanks to greatly improved processes and lots of competition, we've gotten used to zero dead pixels. Yay capitalism. So let's assume that from now on, all displays will always have zero dead pixels.

The question now is: how long until I can get an iPad with double the resolution?

Well, now we apply Moore's Law.

If you watched the Android vs. iPhone debate over the last couple of years, you saw a series of Android phones with successively higher DPI in the same physical area, followed by the iPhone 4, which had a just slightly higher DPI than the most recent Android phone at the time, but which happened to be double the resolution of the original iPhone, because the iPhone hadn't been trying to keep up. (Now that the pixels are so tiny that they're invisible, are Android phones still coming out with even higher resolutions? I haven't heard.)

The so-called "retina display" wasn't a revolution, of course - it was just the next step in displays. (Holding off until you could get an integer 2x multiplier, however, was frickin' genius. Sometimes the best ideas sound obvious in retrospect.)

Each of those successively higher resolution screens was a "new process" in semiconductor speak. With each new process came a higher density of imperfections per unit area, which is why smaller screens were the first to hit these crazy-high densities. We can assume that the biggest screens at "retina" density as of the iPhone 4's release - 960x640 in June 2010 - was the state of the art at the time. We can also assume that the iPad was the biggest you could go with the best process for that size at the time, 1024x768 in March 2010.

Let's assume that the iPad wants to achieve a similar DPI before it hops to the next resolution. I think it's safe to assume they'll wait until they can do a 2x hop like they did with the iPhone. However, the exact DPI isn't such a good assumption; people tolerate lower DPI on a bigger screen. But since I don't have real numbers, let's just assume the same DPI as the iPhone 4, ie. the same "process."

We want to achieve 1024x768 times two = 2048x1536 = (2.13*960)x(2.40*640) = about 5x the physical area of the iPhone 4.

Moore's law says we get 2x every 18 months. So 5x is 2**2.32, ie. 2.32 doubling periods, or 3.48 years.

By that calculation, we can expect to see a double-resolution iPad 3.48 years from its original release date in March 2010, ie. Christmas 2013. (We can expect that Apple will have secret test units long before that, as they would have with the iPhone 4, but that doesn't change anything since they do it consistently. We can also assume that if you're willing to pay zillions of dollars, you could have a large display like that - produced by lucky fluke in an error-prone process - much sooner. And of course you'll get almost-as-good-but-not-retina very-high-res Android tablets sooner than that.)

Note: all of the above assumes that Apple will choose to define "retina display" as the same DPI for the iPad as for the iPhone. They probably won't. Being masters of reality distortion, I would bet on them being able to deliver their newly-redefined "retina display" iPads at least a year sooner at a lower DPI. So let's say sometime in 2012 instead. But not 2011.

By the way, if we grew the screen further, to what's now a 1600x1200 display, that's another 2.44x the area beyond the iPad, so 1.29 more doublings, which is about 2 more years. So we can have retina quality laptop or desktop monitors in, say, Christmas 2015. Since I've been wanting 200dpi monitors for most of my life, I'll be looking forward to it.

Bonus Prediction

I bet "2x" mode for viewing iPhone apps on the iPad 2 will look a lot prettier than on the iPad 1, though. After all, iPhone 4 screens already *have* twice the pixels, so "scaling it up" by 2x isn't actually scaling it up, it's just displaying the same pixels at a larger size. The iPad 2 will probably pretend to be an iPhone 4 to iPhone apps.

Syndicated 2011-01-19 06:58:40 from apenwarr - Business is Programming

18 Jan 2011 (updated 18 Jan 2011 at 21:09 UTC) »

redo surpasses sshuttle in github popularity

Today my redo project (based on djb's redo design) passed my sshuttle project in terms of number of github followers, despite being significantly newer. bup still has about twice as many followers as either of them, though.

...and yes, that's right. I'm competing with myself. I like it better than competing with other people, because it keeps me on my toes: I always lose, but it's never depressing.

Syndicated 2011-01-18 20:02:27 (Updated 2011-01-18 21:09:50) from apenwarr - Business is Programming

14 Jan 2011 (updated 10 Feb 2011 at 03:09 UTC) »

bufferbloat vs. wireless networks, and other stories

It seems my previous article on bufferbloat struck a chord with some people. Most people, quite rightly, didn't know what to do with it and thought it was super confusing. (It was. It was rambly and poorly edited, and reading the responses mostly reminded me how disorganized all my thoughts on the topic were. Oops. It turns out disappearing into a hole and studying something for a month straight can quickly make you lose perspective on what the average person does/doesn't know about TCP/IP and traffic shaping. Sorry about that.)

On the other hand, some people actually think there's useful information hidden in there, which there is. Other people thought I was just wrong, which I'm not; that article, confusing as it was, isn't *conjecture*, it's a disjointed disorganized overly-summarized report of my actual careful experiments and observations.

Now, vague inexact reports of my careful observations are just as useful to you as totally wrong conjectures, so I apologize for that. Perhaps I'll try again later.

Meanwhile, I do want to correct a couple of misconceptions in particular, just in case people *did* believe me, because there are some things I *wasn't* claiming because they aren't true and/or I don't know if they're true.

Wireless networks have problems, but they are not these problems

My article was specifically about a very simple network topology that looks like this:

  computers -> wires -> router -> DSL/cable/etc -> internet.

I didn't talk about wireless anywhere in the article, somewhat on purpose, because wireless has its own problems and those new problems confound your ability to do straightforward experiments to see the original router problems I was talking about.

Now, it's safe to say that if your wireless speed is much much more than your router speed, and all your wireless traffic is internet traffic (ie. not talking to other wireless devices on your LAN), then everything in my article remains true. Any wireless problems are dwarfed by problems caused by your router's crappy queue, and can be solved by doing traffic shaping correctly.

TCP is smart, so it knows not to send data any faster than your router can handle. Since that's much slower than your local wireless network, the only queue that ever fills up is the one at the bottleneck: at your router or the ISP's router on the other end. All the boxes on your WLAN will have essentially empty queues, so it doesn't matter how their maximum queue length is configured.

However, let's imagine that this isn't the case on your network; nowadays you might have 20 megabit cable, or 50 megabit FIOS, or whatever, which means your Internet connection is actually faster than your wireless network. Or maybe your Internet connection *is* a long range shared wireless network, in which case your wireless speed is obviously not much much greater than your router's speed, as we had been assuming. Or maybe you're just connecting two computers together on your LAN, with no router involved at all. What happens then?

What happens then is, well, a whole different story than I had been talking about. And so of course the solution will have to be totally different.

Now, as it happens, I've been thinking about creating a new startup to build the ultimate wireless router - one with built in traffic shaping *that works out of the box*. So while I don't have as much concrete test data for wireless as I do for wired routers, I do have quite a bit of information that comes from studying the situation in theory and a bit of practice.

Here are some things you need to know:

First, various operating systems and drivers have oversized network buffers on their wireless card drivers (and on their wired ethernet drivers too, in some cases). Since your internet router is no longer the bottleneck, suddenly the *client machine's* buffers are what matter, and those buffers are oversized. So yeah, if you start a big transfer, your interactive performance will suck. Solution: reduce the buffer size. Or install traffic shaping on your client machine, but that's a bit silly since it's your own LAN, so you should just fix the buffer size.

If you're downloading from a really dumb local server that has oversized transmit queues, same problem. You should get the server admin to lower the queue size. I guess your admin might be mean and refuse to do that, in which case you can improve your personal interactive performance when communicating with that server by installing traffic *policing* (downstream) on your client, but that's gross and fiddly.

Incidentally, traffic shaping/policing on a wireless network is totally disgusting because the available bandwidth is *variable*. How do you set the rates to 99%/80% of the bandwidth when the bandwidth keeps changing? You can't, at least not without having a direct line to the network driver to find out the current bitrate. And anyway, it's all stupid, because the right thing to do is simply lower the driver's tx queue size. Do that.

Next, rumour has it that if one machine starts blasting data all over your wireless network at top speed, this will hurt performance of other machines on the same WLAN. I personally have never experienced this, so warning: I'm now talking theory, not experimental evidence. But if it were to happen, I claim that this can't *possibly* have anything to do with the queue size of any of your devices. Why? Because no matter how big its queue, the wireless device on any computer can't send faster than its maximum transmit rate. A shorter or longer queue affects latency *on the machine with the queue*, and *on any machine forwarding through that machine*, but to everyone else, it just looks like the same packets coming out at the same speed.

(If your problem is that one machine on your wireless network starts blasting tons of data and overwhelms your router, well, yeah. Your router queue is too big. But if it overwhelms your router, we're in the case we talked about last time, not the current case where your router is too fast to ever be overwhelmed.)

Now, just because the problem isn't caused by your queue length doesn't mean I'm claiming it doesn't exist. There are lots of reasons the problem might exist, *especially* on a wireless network.

First of all, 802.11 wireless networks retransmit packets when they get dropped. This sounds obvious - doesn't everybody just retransmit stuff? - but no, it's *very strange*. In fact, ethernet *doesn't* retransmit stuff. If it sends a packet and the packet is lost due to noise or because the receiver has no buffer space, then tough. The packet is lost. It's TCP's job to retransmit it.

802.11 wireless networks don't work the same way. The reason is this: TCP *depends* on information about how many packets are lost and when. TCP assumes it's on a wired network, which is generally reliable, so when a packet is dropped, it's almost always because the receiver (or actually, any router along the path) has no more buffers. If that's true, it means the router is overwhelmed, so TCP needs to slow down... which it does. And that's how the whole internet works! That's why you don't need to know the speed of *my* DSL modem in order to send data to *my* DSL modem at exactly the speed my DSL modem can tolerate; no more, and no less. It's very cool.

Anyway, wireless networks are way less reliable than wired ones, because wireless networks are much more susceptible to random noise. So packets get dropped all the time, and it's not because you're sending too fast, it's because you're unlucky. Thus, the 802.11 standard calls for devices to retransmit packets that were lost at the physical layer. They can still be dropped by the receiving device, but only *after* they've been acknowledged by 802.11, which means the transmitter knows you got them. So in 802.11, there are two kinds of packet loss: the kind you're supposed to fix (noise), and the kind you aren't (buffer full).

Generally it's the driver's job to do the 802.11 retransmits. This calls into question: how many times do you retry transmitting? How long between retransmits? What if the noise was caused by someone else's retransmits, and we get into lockstep, constantly ruining each other's packets? On the receiving end, how and when do we report which kind of receive failure is which?

Answer: it's complicated. But it's obviously easy to do wrong. I heard a rumour that some Linux drivers will retry the same packet up to 256 times in a row if it doesn't work; that seems rather desperate to me. I can also imagine bugs where a driver, finding the kernel's receive queue is full, reports the 802.11 packet as corrupted ("please retransmit") rather than as delivered-but-then-dropped. If it did that, it would utterly destroy TCP's behaviour and potentially clog the network really badly. Maybe this bug exists; I have no idea, I've only heard rumours. If it does, then it would surely be a problem. It would also have nothing to do with queue length, and could not be solved by any form of traffic shaping.

Which brings up another really important point: the whole Internet actually only works at all because of a truly amazing level of voluntary cooperation. Anybody anywhere could deliberately mis-tune their TCP layer and get faster downloads and uploads than anybody else, at the cost of messing up things for everybody else. Similarly, a WLAN driver that mis-reported dropped packets could get much higher thruput than anybody else on the WLAN, at the cost of screwing over everybody else.

TCP is kind of like the "prisoner's dilemma" - as long as we all cooperate, we all win, but if any of us defects, we might as well all defect, because the party is over. And as with experimental tests of the "iterated prisoner's dilemma" in real life (where you play the game with your partners over and over, rather than just once), we have mostly converged on not cheating. Because if we didn't, someone else would cheat too, and the party would be over.

Overly long transmit queues are not defection, by the way; they're just stupid. You don't get better performance at other people's expense by using overly long queues. You just give yourself worse performance at your own expense. Nice work, genius.

So that's not this. And to repeat myself, queue length is also not what would cause one wireless device to steal bandwidth from another wireless device. 802.11 retransmit problems? Yeah, that could cause problems all right.

Now, even if we assume our 802.11 drivers don't have bugs (other than queue length, which doesn't screw anybody but the perpetrator), which might even be the case, there are *still* some obscure reasons that one station's WLAN traffic might stomp on another's. Here are a couple that I know about; there may be more.

1) Ethernet-style collision detection (which is used, sort of, sometimes, in 802.11) is prone to timing problems; the more people you have on your network, and the faster the network, the more collisions you'll have. Background: in original 10MBit ethernet, wire lengths were limited so that the first byte of a packet A would reach all nodes by the time the last byte had been sent out. It was still possible that a second node would start transmitting its own packet B between the first node starting to send A and the second node start to receive A; however, the timing was arranged so that both A and B would know by the end that they had stomped on each other, so they could wait for a random delay and then retransmit.

Unfortunately, with 100MBit and faster networks, this collision detection got a lot worse, because the same packet was 1/10th the "length" (if you think of a packet travelling through a wire as a wave moving at the speed of light, the distance from start to end is the "length" of the packet), and that length was so short that it wasn't really reasonable to require nodes to be physically so close together. That's why hubs were sensible in the 10MBit days, but rapidly fell into disuse in the 100MBit days in favour of switches, which don't need collision detection because they just give every node its own wire.

With wireless, you have no choice: everybody is on the same "wire", ie. the airwaves. You can sort of help by using multiple channels, which 802.11 does, half-heartedly. (There are only 3 non-overlapping channels in 802.11's North American standard, at least, which means three packets at a time in an optimistically configured topology.) There are interesting mathematical improvements on this (eg. frequency hopping and ultrawideband transmission), but 802.11 doesn't use them; cell phone networks do, nowadays, because they have so darned many people using them at once that nothing else would suffice.

*Anyway*, with 802.11, you're back to old-fashioned collision detection, only now you have outside noise, uncontrolled distances, random signal reflection, *and* fast transmit rates. It's awful, and frankly, I'm amazed it works at all.

The collision detection algorithms have improved somewhat since the ethernet CSMACD (carrier-sense multiple access collision detection) days, but not much. And the various 802.11 standards - I haven't read them all yet - seem to define multiple different ways around the problem. One of them is actually, ha ha, a token-ring like scheme: the wireless access point assigns timeslots to each node, and the node is only allowed to transmit during its timeslot, thus avoiding the possibility for collisions (while also bringing back all the bad stuff about token ring that killed it in the first place). I'm a bit fuzzy on when this mode is enabled and when it's not; I think it's mostly not, so this is more academic than useful.

So anyway, collision detection is hard. And the more you transmit, the more collisions you'll have. The more different nodes you have transmitting, the more collisions you have. The more your driver's auto-retransmit algorithm is broken, the *vastly* more collisions you'll have. And so on. One busy station with a wrong retransmit algorithm could really ruin it for everyone. Of course, like I said, I have no evidence that such a thing exists; I've just heard rumours.

2) This one is even more insidious. It's called the "hidden node problem." It works like this: station A can talk to the access point, and station B can talk to the access point, but A and B are on opposite ends of the building, so they're too far away to see *each other*. So let's say they both decide to start blasting data at the access point at the same time at full speed.

No matter what collision detection method they try to use, they'll never know that every single one of their packets is colliding with the other guy! As far as the access point is concerned, it's seeing a nonsense combination of signals from A and B. As far as A and B are concerned, all is well,1 except for some reason the access point isn't acknowledging any of their packets, so they have to retransmit.

If you're running a long-range wireless network, your access point probably has a really great well-positioned antenna, and the other stations probably don't. That means you probably have the hidden node problem in a *big* way. That means tons of horrible unexplained packet loss, which means tons of retransmits. And, rumour has it, those retransmits might be compounded by buggy retransmission algorithms. As the operator of such a network, your life, in short, sucks big time.

I don't have any experience running long-range multi-user wireless networks, although I have fantasized about doing so, in what now appears to be a rather obsessive-compulsive level of detail. And the hidden node problem is where my fantasies turn into nightmares.

I don't know the solution to this one. I do know three things that *might* be potential solutions, but they'd need to be thoroughly tested:

a) That 802.11 timeslot mechanism I mentioned above, if it actually was turned on and supported. (I read about it in the spec, and got the distinct impression that it was a great idea that wouldn't do me any good because I couldn't use it. But I now don't remember why. I think it's part of the 802.11 QoS spec, and even if routers support that, none of the standard workstation drivers do.)

b) Some projects (frottle, WiCCP) that actually do timeslicing at the user level. At least frottle has some user reports that say it dramatically improved results. However, it requires *everyone* on your network to run special software; if anybody isn't, they'll probably stomp on all the packets from everybody else.

c) An slightly insane specialized application of traffic policing at the access point. This requires that everybody talks only to the access point directly, never to other nodes on the WLAN, which is a pretty safe assumption since the hidden node problem means they can't see each other anyhow. Basically, if you do things exactly right, you should be able to slow down everyone's traffic so they all get 1/n of the total bandwidth, and you should be able to reduce traffic burstiness so those packets mostly statistically end up not colliding. If it worked, it would be a totally amazing hack, because you wouldn't have to fix the software on any of the stations, and you wouldn't have to use explicit timeslots; the timeslots would be implicit based on your traffic policing algorithm.

And no, the transmit queue length has no bearing on this problem. In fact, somewhat entertainingly, activating policing on the access point's WLAN link could actually force the transmit queues to be shorter (since the TCP window size would be shorter since policing is dropping so many packets), thus solving that problem too.

Assorted other clarifications about my last article

While we're here, note that:

  • the claims in my previous article don't depend on bandwidth; they are true at fast and slow speeds, you just use different constants when doing the math.
  • 256 kbits (with excessive buffers) is a fun example because it makes all the problems worse; you can't half-ass your traffic shaping or it just won't work. That means experiments are easier on that link than on a *good* link, which is why I took advantage of it during my sublet back in September.
  • My claim of 250ms of latency under high load needs explanation: that 250ms is configurable and asymmetric. Basically, all the latency was *downstream*, because incoming traffic (policing) is much harder to control than outgoing traffic (shaping). And you can lower the latency by lowering your downstream bandwidth limit, or raise the latency by raising the downstream bandwidth limit; it's a tradeoff. I talked about 250ms and 80% downstream because that was the tradeoff that worked for *me* with 10 sessions and Skype on a 256k link.
  • I "dialed in" 250ms because that's about the limit of bearability for interactive ssh traffic. No other reason. I could have picked basically any number.
I doubt all that is very helpful. But that's the best I can do for now.


1 You can simulate the hidden node problem at home while doing the dishes. Go stand by the kitchen sink and turn on the tap. Now talk at a normal volume to someone across the room, and have them talk back to you at a normal volume. To them, you'll be audible; what they hear is about a 1:1 ratio of tap noise to voice, which is good enough to understand. But to you, they'll be inaudible; by the time their voice reaches you, it has decayed a lot (ie. by the square of the distance between you and them), but the water noise is full blast, totally overwhelming their voice.

Syndicated 2011-01-14 04:33:37 (Updated 2011-02-10 03:09:25) from apenwarr - Business is Programming

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