Belka
Now that you've got some metrics to know when you have a successful
design, start examining the trades.
Most systems have a turning point in their performance as load is
increased, you need manage the traffic so that the offered load does
not push the system over that point (or if it does, not for too long -
it doesn't have to be perfect all the time, just suitably good - this
is where stochastic come it, another story).
What you are doing here is trading efficient use of the infrastructure
(e.g. you are not going to try and run the servers close to 100%) for
responsive behaviour.
This is one of the reasons why people create multi-levelled web manage
architectures - you could think of the design as having an initial
stage that can analyse and identify the traffic an characterise it -
ideally it should be able to cope with line rate or at least a known
upper bound (which the load balancer in front knows and can be
configured for), it can then pass the real traffic on for processing.
This all comes down to creating back pressure: typically by mediating
between producer and consumer with a finite queue - then configuring
that queue to have the right length so that the back pressure is felt
at an appropriate point, too long and the experienced delay is too
large, too short and you are changing modes of behaviour too quickly -
again it that stochastics/queueing theory stuff.
Avoiding starvation is easy, FIFO service does that - but you'll find
that's not enough you also need to bound the service time for certain
requests people often think that 'priority' is the right answer but
usually that is too naive. Resources are finite, giving one source of
demand 'priority' means that the other sources loose out and that
trade is highly non-linear and creates its own denial-of-service.
Neil
On 2 May 2009, at 06:27, Belka wrote:
Thanks, Niel. :)
You actually motivated me to determine/specify defense requirements
<- that
I should have done long before writing here.
Now I'm not experienced in DDoSs defending, so my reasoning here
might be a
bit voulnerable. Few basic requirements:
1. Server has services that shouldn't be endangered by computational
resource starvation. That is why I use load balancing for SAR
(Services
under Attack Risk). I even use 2 types of load controls: one per
each SAR,
and the second - above all ARSes.
2. Even when under attack SAR should be able to serve. Of course, it's
effective input capability becomes much lower, but requirement here
is to
provide possible maximum of effectiveness. That is why
2.1. identification of bad request should be fast, and
2.2. request processing should be fair (without starvation on
acceptance
time).
After projecting this /\ specification on architecture plan, the
need in
*good* tryReadChan is now less sharp. However, it still would be
very useful
- I also have other applications for it.
The *good* tryReadChan would be atomic, immediate, and with
determinate
result (of type Maybe)...
----------
By the way, for
Actually, am I wrong thinking, that it can't be helped - and the
degradation
from cute concurency synchronization model of Chan is unavoidable?
I have an idea of such solution (without getting down to lower level
programming), - called it "fishing": one should complicate the flow
unit
(FlowUnit), that is being passed in the Channel. The FlowUnit
diversifies to
real bizness data, and service data. That way I now may gain control
over
blocking....
But this solution is not simple and lightweight. If anybody is
interested,
I could describe the concept in more details.
Belka
Neil Davies-2 wrote:
Belka
You've described what you don't want - what do you want?
Given that the fundamental premise of a DDoS attack is to saturate
resources
so that legitimate activity is curtailed - ultimately the only
response has to be to
discard load, preferably not the legitimate load (and therein lies
the
nub of the problem).
What are you trying to achieve here - a guarantee of progress for the
system?
a guarantee of a fairness property? (e.g. some legitimate traffic
will
get
processed) or, given that the DDoS load can be identified given some
initial
computation, guarantee to progress legitimate load up to some level
of
DDoS
attack?
Neil
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