[Bloat] First draft of complete "Bufferbloat And You" enclosed.
esr at thyrsus.com
Sat Feb 5 05:23:06 PST 2011
I consider this draft coverage-complete for the basic introduction I was
aiming at. Suggestions from dtaht5 and jg have been incorporated where
appropriate. Critique and correct, but try not to make it longer. I'm a
bit unhappy about the length and may actually try to cut it.
You will note that the description of network failure modes is
somewhat broader than in jg's talk. So is the section on why QoS
fails to address the problem. This is me putting on my
system-architect head and doing original analysis; if you think I have
misunderstood the premises or reasoned about them incorrectly, tell
Please fix typos and outright grammatical errors. If you think you have spotted
a higher-level usage problem or awkwardness, check with me before changing it.
What you think is technically erroneous may be expressive voice.
Explanation: Style is the contrast between expectation and surprise.
Poets writing metric poetry learn to introduce small breaks in
scansion in order to induce tension-and-release cycles at a higher
level that will hold the reader's interest. The corresponding prose
trick is to bend usage rules or change the register of the writing
slightly away from what the reader unconsciously expects. If you try
to "fix" these you will probably be stepping on an intended effect.
So check first.
(I will also observe that unless you are already an unusually skilled
writer, you should *not* try to replicate this technique; the risk of
sounding affected or just teeth-jarringly bad is high. As Penn &
Teller puts it, "These stunts are being performed by trained,
Future directions: unless somebody stops me, I'm going to reorganize
what wiki docs there are around this thing. The basic idea is to make this
the page new visitors naturally land on *first*, with embedded
hotlinks to the more specialized stuff.
Explanation: Outlines and bulleted lists of stuff are deadly. They're
great for reference, but they scream "too much; don't read" to people
first trying to wrap their heads around a topic. Narrative
introductions with hotlinks are both less threatening and more
effective. The main reason they're not used more is that most people
find them quite hard to write. I don't.
If I decide I need to cut the length, I'll push some subsections down
to linked subpages.
I haven't learned Textile yet. I'll probably get to that this weekend.
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
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Bufferbloat is a huge drag on Internet performance created,
ironically, by previous attempts to make it work better.
The bad news is that bufferbloat is everywhere, in more devices and
programs than you can shake a stick at. The good news is, bufferbloat
is relatively easy to fix. The even better news is that fixing it may
solve a lot of the service problems now addressed by bandwidth caps
and metering, making the Internet faster and less expensive for both
consumers and providers.
== Packets on the Highway ==
To fix bufferbloat, you first have to understand it. Start by
imagining cars traveling down an imaginary road. They're trying to get
from one end to the other as fast as possible, so they travel nearly
bumper to bumper at the road's highest safe speed.
Our "cars" are standing in for Internet packets, of course, and our
road is a network link. The 'bandwidth' of the link is like the total
amount of stuff the cars can carry from one end to the other per
second; the 'latency' is like the amount of time it takes any given
car to get from one end to the other.
One of the problems road networks have to cope with is traffic
congestion. If too many cars try to use the road at once, bad things
happen. One of those bad things is cars running off the road and
crashing. The Internet analog of this is called 'packet loss'. We
want to hold it to a minimum.
There's an easy way to attack a road congestion problem that's not
actually used much because human drivers hate it. That's to interrupt
the road with a parking lot. A car drives in, waits to be told when it
can leave, and then drives out. By controlling the timing and rate at
which you tell cars they can leave, you can hold the number of cars on
the road downstream of the lot to a safe level.
For this technique to work, cars must enter the parking lot without
slowing down, otherwise you'd cause a backup on the upstream side of the
lot. Real cars can't do that, but Internet packets can, so please
think of this as a minor bug in the analogy and then ignore it.
The other thing that has to be true is that the lot doesn't exceed its
maximum capacity. That is, cars leave often enough relative to the
speed at which they come in that there's always space in the lot for
In the real world, this is a serious problem. On the Internet,
extremely large parking lots are so cheap to build that it's difficult
to fill them to capacity. So we can (mostly) ignore this problem with the
analogy as well. We'll explain later what happens when we can't.
The Internet analog of our parking lot is a packet buffer. People who
build network hardware and software have been raised up to hate losing
packets the same way highway engineers hate auto crashes. So they put
lots of huge buffers everywhere on the network.
In network jargon, this optimizes for bandwidth. That is, it
maximizes the amount of stuff you can bulk-ship through the network
without loss. The problem is that it does horrible things to latency.
To see why, let's go back to our cars on the road.
Suppose your rule for when a car gets to leave the parking lot is the
simplest possible: it fills up until it overflows, then cars are let
out the downstream side as fast as they can go. This is not a very
smart rule, and human beings wouldn't use it, but many Internet
devices actually do and it's a good place to start in understanding
bufferbloat. (We'll call this rule Simple Overflow Queuing, or SOQU
for short. Pronounce it "sock-you" or "soak-you"; you'll see why in a
Now, here's how the flow of cars will look if the lot starts empty and
the road is in light use. Cars will arrive at the parking lot, fill it
up, and then proceed out the other side and nobody will go off the
road. But - each car will be delayed by the time required to initially
fill up the parking lot.
There's another effect, too. The parking lot turns smooth traffic
into clumpy traffic. A constantly spaced string of cars coming in
tends to turn into a series of clumps coming out, with size of each
clump controlled by the width of the exit from the the parking lot.
This is a problem, because car clumps tend to cause car crashes.
When this happens on the Internet, the buffer adds latency to the
connection. Packets that arrive where they're supposed to go will
have large time delays. Smooth network traffic turns into a
herky-jerky stuttering thing; as a result, packet loss rises.
Performance is worse than if the buffer weren't there at all. And -
this is an important point - the larger the buffer is, the worse the
== From Highway to Network ==
Now imagine a whole network of highways, each with parking lots
scattered randomly along them and at their intersections. Cars trying
to get through it will experience multiple delays, and initially
smooth traffic will become clumpy and chaotic. Clumps from upstream
buffers will clog downstream buffers that might have handled the same
volume of traffic as a smooth flow, leading to serious and sometimrds
unrecoverable packet loss.
As the total traffic becomes heavier, network traffic patterns will
grow burstier and more chaotic. Usage of individual links will swing
rapidly and crazily between emptiness and overload. Latency, and total
packet times, will zig from instantaneous to
check-again-next-week-please and zag back again in no predictable
Packet losses - the problem all those buffers were put in to prevent -
will begin to increase once all the buffers are full, because the
occasional crash is the only thing that can currently tell Internet
routers to slow down their sending. It doesn't take too long before
you start getting the Internet equivalent of 60-car pileups.
Bad consequences of this are legion. One of the most obvious is what
latency spikes do to the service that converts things like website names
to actual network addresses - DNS lookups get painfully slow.
Voice-over-IP services like Skype and video streamers like YouTube
become stuttery, prone to dropouts, and painful to use. Gamers get
For the more technically-inclined reader, there are several other
important Internet service protocols that degrade badly in an
enviroment with serious latency spikes: NTP, ARP, DHCP, and various
routing protocols. Yes, things as basic as your system clock time can
get messed up!
And - this is the key point - the larger and more numerous the buffers
on the network are, the worse these problems get. This is the bufferbloat
problem in a nutshell.
One of the most insidious things about bufferbloat is that it easily
masquerades as something else: underprovisioning of the network. But buying
fatter pipes doesn't fix the bufferbloat cascades, and buying larger
buffers actually makes them worse!
Those of us who have been studying bufferbloat believe that many of
the problems now attributed to under-capacity and bandwidth hogging
are actually symptoms of bufferbloat. We think fixing the bufferbloat
problem may well make many contentious arguments about usage metering,
bandwidth caps, and tiered pricing unnecessary. At the very least, we
think networks should be systematically adited for bufferbloat before
more resources are plowed into fixing problems that may be completely
== Three Cures and a Blind Alley ==
Now that we understand it, what can we do about it?
We can start by understanding how we got into this mess; mainly, by
equating "The data must get through!" with zero packet loss.
Hating packet loss enough to want to stamp it out completely is
actually a bad mental habit. Unlike real cars on real highways, the
Internet is designed to respond to crashes by resending an identical
copy when a packet send is not acknowledged. In fact, the Internet's
normal mechanisms for avoiding congestion rely on the occasional
packet loss to trigger them. Thus, the perfect is the enemy of the
good; some packet loss is essential.
But, historically, the designers of network hardware and software have
tended to break in the other direction, bloating buffers in order to
drive packet losses to zero. Undoing this mistake will pay off hugely
in improved network oerformance.
There are three main tactics:
First, we can *pay attention*! Bufferbloat is easy to test for once
you know how to spot it. Watching networks for bufferbloat cascades
and fixing them needs to be part of the normal job duties of every
Second, we can decrease buffer sizes. This cuts the delay due to
latency and decreases the clumping effect on the traffic. It can
increase packet loss, but that problem is coped with pretty well by the
Internet's normal congestion-avoidance methods. As long as packet
losses remain unusual events (below the levels produced by bufferbloat
cascades), resends will happen as needed and the data will get through.
Third, we can use smarter rules than SOQU for when and by how much a
buffer should try to empty itself. That is, we need buffer-management
rules that we can expect to statistically smooth network traffic
rather than clumpifying it. The reasons smarter rules have not been
universally deployed already are mainly historical; now, this can and
should be fixed.
Next we need to point out one tactic that won't work.
Some people think the answer to Internet congestion is to turn each link
into a multi-lane highway, with fast lanes and slow lanes. The theory
of QoS ("Quality Of Service") is that you can put priority traffic in
fast lanes and bulk traffic in slow ones.
This approach has historical roots in things telephone companies used to
do. It works well for analog traffic that doesn't use buffering, only
switching. It doesn't work for Internet traffic, because all the lanes
have to use the same buffers.
If you try to implement QoS on a digital packet network, what you end
up with is extremely complicated buffer-management rules with so many
brittle assumptions baked into them that they harm performance when
the shape of network demand is even slightly different than the
Really smart buffer-management rules are simple enough not to have
strange corner cases where they break down and jam up the traffic.
Complicated ones break down and jam up the traffic. QOS rules
== Less Hard ==
We started by asserting that bufferbloat is easy to fix. Here
are the reasons for optimism:
First, it's easy to detect once you understand it - and verifying
that you've fixed it is easy, too.
Second, the fixes are cheap and give direct benefits as soon as
they're applied. You don't have to wait for other people to fix
bufferbloat in their devices to improve the performance of your own.
Third, you usually only have to fix it once per device; continual
tuning isn't necessary.
Fourth, it's basically all software fixes. No expensive hardware
upgrades are required.
Finally (and importantly!), trying to fix it won't significantly
increase your risk of a network failure. If you fumble the first time,
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