DDoS Defense by Offense
| View Paper |  | Public Review (by Adrian Perrig) | |
Michael Walfish, MIT
Mythili Vutukuru, MIT
Hari Balakrishnan, MIT
David Karger, MIT
Scott Shenker, University of California, Berkeley and ICSI
Abstract
This paper presents the design, implementation, analysis, and experimental evaluation of speak-up, a defense against applicationlevel distributed denial-of-service (DDoS), in which attackers cripple a server by sending legitimate-looking requests that consume computational resources (e.g., CPU cycles, disk). With speak-up, a victimized server encourages all clients, resources permitting, to automatically send higher volumes of traffic. We suppose that attackers are already using most of their upload bandwidth so cannot react to the encouragement. Good clients, however, have spare upload bandwidth and will react to the encouragement with drastically higher volumes of traffic. The intended outcome of this traffic inflation is that the good clients crowd out the bad ones, thereby capturing a much larger fraction of the server's resources than before. We experiment under various conditions and find that speak-up causes the server to spend resources on a group of clients in rough proportion to their aggregate upload bandwidth. This result makes the defense viable and effective for a class of real attacks.
The article is very interesting and proposes a novel technique in combating Application level DOS attacks.
I have some doubts about the method’s validity in this particular case.
Consider following:
1. The ‘g’ represents the aggregate demand from good clients, the useful “production” traffic.
2. The “G” denote the sum of the useful “g” traffic volume and the solicited artificial traffic, lets call it “A”
(1) G = g + A; where A >> g
3. Both g and A traffic is ‘good’ in a sense that both are created by the good servers, but there is one big difference. Loss of packets belonging to the A stream has no impact on the overall service provided by the server, while loss of a packets belonging to the g stream means service degradation, and this is what one wants to avoid and prevent.
4. The article assesses server’s resources fraction available under attack to the good traffic as:
(2) g/(g+B) , B>>g
While same fraction with activated defense mechanism is expressed as:
(3) G/(G + B),
(4) Where hopefully G > B, or close to.
5. The formula (3) is in my opinion incorrect because it does not take into account difference in meaning of lost g and A streams’ packets. Since the objective is to protect the g stream, one has to focus on g’s resources share which is :
(5) g/(g+A+B)
(6) where A+B >> B
6. Comparing expression (5) with (2) one can conclude that value of (2) >> value of (5) and thus the true production traffic gets much less traffic share with the defense mechanism active. Or in summary, the “speak-up” mechanism amplifies the attack.
Wlodek Stankiewicz
wstankiewicz@acm.org
Nice, France
Wlodek,
Thanks for your comment. I think there's a minor misunderstanding here: the "A" stream is dropped by the front-end and isn't sent to the server at all. The assumption (stated several times and justified in the paper) is that the network link feeding the front-end isn't the bottleneck.
Hence, the formulas stated in the paper and validated by our experiments are correct.
Do you agree?
Hari
Hari,
If you look up section 3.2, first sentence, says that thinner “drops the requests at random to reduce the rate to c”.
This is the base of my reasoning; I assume that ‘the requests’ mean all requests, that is G+B or g+A+B.
If all A are dropped by thinner then server gets g+B requests.
If g+B > c and the ‘speak up ‘does not make difference.
If g+B > c then some request got dropped at random and we are back at the initial situation, although thinner protected the server by not sending more then c requests.
Best regards
Wlodek
Hi Wlodek,
Let me try to clarify things here. I think that there are multiple misunderstandings. My summary would be that you are wrongly mixing ideas from the simple illustration, from section 3.2, and from section 3.3.
First, the "A stream" doesn't exist in the realization of speak-up based on request-and-retry. That realization is what is discussed in 3.2 (and illustrated at the beginning of 3). I'm going to focus on that realization. If you have questions about 3.3, we can answer them next.
In the request-retry realization (3.2), there is not a logically separate "A stream". There are simply requests issued by clients. Some of these requests may be repeats. These repeats arise because the thinner hears the previous request, decides to drop it, and issues a "please retry" signal. The thinner may decide to serve a "repeat" (though in the simple description, the thinner doesn't know which requests are repeats).
Thus, I think your view is not quite correct that none of the packets in the A stream can result in the client getting service.
Now, a point similar to the one that Hari made is that in order for the description I just gave to be true, all of the requests _must arrive at the thinner_. That is, the requests cannot be dropped _before_ they get to the thinner (if they were dropped before the thinner, the thinner could not issue the "please retry", and the good clients would not be able to amplify).
Please let us know if you'd like to discuss 3.3, in which there truly is a separate "payment stream".
Many thanks for your interest,
Mike
I hope that Mohit Lad from UCLA posts his slides from his OO talk - it contained a very humerous idea, but which also is an alternative approach to defense by offense - essentially, when ypo uare attacked, respond (like Ghandi) with an offer of lots of other sites the attacker might like to try - at the humerous level, this might cause an attacker to experience guilt, confusion or fear/uncertainty and doubt ("what if this guy is setting a trap) - at another level its a bit like the ideas of using p2p and entanglement to dilute the attackers' resources over lots more places - worth a look...
what is nice is that that shows it was right to accept this paper for sigcomm as it made people think about alterantives even when they might not have liked this particular proposal itself:
so mark handley and adam greenhalgh at UCL have done a bunch of work on ddos, and we discussed the idea that one might detect an incoming attacks scanning algorithm and infect it to cause it to hit a specific (large) set of targets next - as well as deflecting/diluting the attack, one might even simply direct an attacker at a botnet, which woud be a sweet irony:-)
Please post or send me the slides by mail.
Sorry for late answer, sometimes real security issue ‘blackhole’ my time.
As far as 3.2 is concerned, I agree, there is no issue as long as thinner removes duplicates, it really has to do that, or one will encounter the side effects I’ve talked about.
The 3.3 is really interesting. I wonder if botnet robots would answer to any solicitations at all, these are (in my limited experience) simple minded pieces of code.
So the solicitation my work as a ‘real user’/’robot code’ verification and the following ‘virtual auction’ will give advantage to real systems.
Best regards
Wlodek
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