------ An Overview
Zhang Fu
zhafu@chalmers.se
Outline
 What is DDoS ? How it can be done?
 Different types of DDoS attacks.
 Reactive VS Proactive Defence
 Some noticeable solutions
 Crux issues
 Discussion
 The “battle” is going on.
DDoS Attacks
 A Denial of Service (DoS) attack is an attempt by the attacker to
prevent the legitimate users of a service from using that service.
 If the attack is launched from multiple compromised machines, then it
is a Distributed Denial of Service (DDoS) attack.
 Basic Types of DDoS attacks:
 Sending malformed packets to confuse systems (protocol or application).
Can be also called semantic attack.

Example: SYN-flooding, Teardrop Attacks
 Flooding packets to the victim to depleting key resources (bandwidth). Can
be also called brute-force attack.

Example: DNS request flooding, Smurf attack.
DDoS Attacks (cont.)
 What makes DDoS possible?
 End-to-End paradigm : intermediate network provides
best-effort packets delivery service.
 Different networks do not have effective cooperation.
Victim’s security relies on the rest of the network
 End hosts can not control the bandwidth allocation or
queuing mechanism of the network
Zhang Fu
 Control is distributed.zhafu@chalmers.se

Steps of launching DDoS attacks
 Recruiting and Exploit.
 Propagation.
 Launching attacks.
order
attack
Victim Types
 Application
 Target on a given application. If the resource is not completely consumed,
other application maybe still available.
 The attack traffic volume is usually small, and seems to be normal packets.
E.g. signature attack.
 Host
 Overwhelming the host’s communication mechanism. Or make the host
crash/reboot.
 The attack traffic is usually big. The host can not solve the problem alone.
 Resource Attack
 Attack some critical entities in the victim’s network, such as DNS server.
 Congest some critical links of the network.
 The attack traffic is big and easy to detect. But need cooperation to defense.
Victim Types (cont.)
 Infrastructure
 Aim to disable the critical service of the whole Internet, such as root DNS
server, core network, certificate server.
 The attack can aggregate a huge volume of traffic with in a very short time
period.
 Need cooperation to defend against this attack.
Impact of the attack
 Disruptive: completely disable the victim’s service.
 The victim can recover automatically after the attack. Some
may need human to be involved. And some may be not
recoverable.
 Degrading: consume some portion of the victim’s
resource.
 Success depends on the service. QoS plays an important role.
 Not easy to detect.
 Tradeoff between deploying a defense mechanism and losing
market caused by the degradation.
Summary of DDoS attacks
 What is a DoS / DDoS attack?
 Why DDoS attacks can be launched successfully?
 DDoS attacks target both in application layer and
network layer.
 Some DDoS attacks aims to completely deplete the
resource of the victim, while others aims to degrade
the quality of the victim’s service.
Challenges for defense mechanisms
 DDoS is a problem in distributed manner. It needs to
be solved in a distributed way. However, assumption of
global deployment would be rather strong.
 Some attacks can be hardly defined. Many factors may
be involved, such as number of compromised
machines, attack rate, attack duration, impact of the
attack.
 Lack of universal benchmark.
 Lack of test platforms for large scale network.
Principles for counter measures
 Security
 The attacker can hardly break the secrets used in the system. Or
find a semantic flaw to attack the system.
 Accuracy
 The system should filter out the malicious traffic as much as
possible and affect the legitimate traffic as little as possible.
 Efficiency
 Keep the overhead within a acceptable threshold
 Safe Failure
 When the system is fail, the situation can not be worse than that
before the deployment.
Which way to go? Proactive VS Reactive
 Proactive solutions aim at prevent the DDoS attacks
from beginning. Or the victim’s service is not denied
during the attacks.


How to prevent DDoS attacks? Secure the hosts, Build
DDoS-resilient protocol. We need both police and doctor!
How to make system tolerate DDoS attacks? Resource
accounting, provide more resource.
 Examples of proactive solutions: puzzle based solutions,
network capability, secure overlay.
Proactive VS Reactive (cont.)
 Reactive solutions aims at mitigate DDoS attacks when the
victim suffers those attacks, or some DDoS attacks are
detected.
 Need some detection mechanisms. Less overhead in the
normal situation.
 The problem is how to identify DDoS attacks, what are
the proper responses for different kinds of attacks?
•
•
Use models of attacks to detect. We can also define abnormal
behaviors for detection, But have to be careful with false
positive.
Block identified zombies, or rate limiting /filtering.
Network Layer Defense
 Network Capability
 Choose a path from source to the destination
 Capability Establishment (Sending Request and getting Capability)
 Sending Packets with Capability
 Capability Refreshing
Internet
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Packet Marking (Trace back)
• Packets will be marked by the routers along the path.
• When DDoS attacks occur, the victim will identify the attacking sources.
• Victim will also send control command to the router which is near to the
sources to limit the malicious traffic.
What’re the
advantages and
disadvantages?
SOS: Secure Overlay Service
Application Layer Defense
 How can network-based applications
defend by themselves？
 Solutions inspired by Frequency Hopping.
 ACK-based port hopping (Badishi et al. 2005)
 Port hopping with bounded clock offset (Lee and Ting 2004)
 Hopping authentication code (Srivatsa et al 2006)
 Port hopping in the presence of clock drifts. (Zhang et al.2008)
Crux Issues
 IP spoofing.
 Network topology dependency.
 Refreshing secrets.
 Feedback mechanisms.
 Space efficiency.
We can hardly solve DDoS problem completely. The ideal solution could be
very complicated. We might need an integrated solution. However, it’s
unclear about the optimal integration.
Summary
 What is DDoS
 Why it is possible
 What is the main category of defence mechanisms
 We want secure, robust, efficient solutions for the
problem.
Zhang Fu
zhafu@chalmers.se
The End
Thank You
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