Countering DoS Attacks with Stateless Multipath Overlays Presented by Yan Zhang

Overview Background Problem formulation Architecture Implementation Evaluation

DDoS Attack Distributed Denial of Service An attacker is able to recruit a number of hosts (zombies) throughout the Internet to simultaneously or in a coordinated fashion launch an attack upon the target. Typical DDoS: SYN flood attack, ICMP attack

DDoS Attack-Direct

DDoS Attack-Indirect

Overlay Network Overlay network :A computer network which is built on top of another network. Node: in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network

IP network as an overlay network

Overlay network & Benefits Purpose: To implement a network service that is not available in the existing network --Routing, Addressing, Security, Multicast, Mobility Benefits: 1. Do not have to deploy new equipment, or modify existing software/protocols 2. Do not have to deploy at every node

Overview Background Problem formulation Architecture Implementation Evaluation

Traditional ION Traditional Indirection-based overlay network methods (like SOS,MayDay) make two assumptions: Attack on fixed and bounded set of overlay nodes can only affect a small fraction of users Attacker could not eavesdrop on link inside the network

Problem Traditional ION has weakness: Target attack: Attacker can follow the client s connection and bring down the nodes which client tries to connect to. Sweep attack: Degrade the connection by bringing down a portion of the overlay nodes at a time

Related work SOS (Keromytis et al) --Suggested using an overlay network to route traffic from legitimate users to a secret node Stateless flow filter (Xuan et al) --By adding capabilities to packets Ticket mechanism (Gligor ) -- Clients must obtain tickets before they are allowed to access protected service

Overview Background Problem formulation Architecture Implementation Evaluation

Spread-spectrum Electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. CDMA is a typical spread spectrum communication

Intuitive To prevent following attack: By adopting spread spectrum approach, the client spreads its packets randomly across all access points. To verify the authenticity: Using a token, at the expense of bandwidth

Attack models Sweep attack: Without internal knowledge of system, blindly sweep all nodes ---TCP SYN, ICMP flooding etc ----Like radio jamming in all channels Targeted attack: Know which overlay node a client is using. More sophisticated ----Like eavesdrop and jam target frequency

Traffic spreading issues Spread the packets from clients across all overlay nodes in a pesudo-random manner Randomly attack will only cause a fraction of packets loss Duplicate the packets or using forward error correction to recover the loss

Traffic Spreading

Key and ticket establishment Protocol Randomly redirect the authentication The client sends packet to a random overlay The receiving node forward the request to another random overlay node The attacker cold not determine which nodes to target

Key and ticket establishment Protocol One round-trip only use first and last connection (from A to D) Two round-trip guarantees the liveness

Client-Overlay communication protocol

Key and ticket establishment Protocol To avoid reuse of the same ticket by multiple DDoS zombies, the range of valid sequence numbers for the ticket is kept relatively small (e.g., 500 packets) The ticket is bound to the client s IP,

Overview Background Problem formulation Architecture Implementation Evaluation

Implementation Connection Establishment Phase -- As described in the protocol part -- Establish session key and ticket -- Usually two round-trip

Implementation Packet Transmission Phase the client computes the index in the sorted list of IPs as: index = UMAC(Ku XOR sequence number) mod(n) Ticket Renewal Phase When valid tickets are about to expire, the overlay node issues a new ticket with the same session key but larger max sequence number.

Overview Background Problem formulation Architecture Implementation Evaluation

Evaluation Impact of Sweeping attack with a modest amount of packet replication and striping at the client, the proposed method can handle even massive DoS attacks against the overlay General ION attack resistance

Performance evaluation Throughput under attack Only 33% in the worst case scenario Increase the replication rate, the throughput get closer to the direct connection

Performance evaluation As the replication factor is increased, and for larger networks, we get better average latency results. In the worst-case scenario, we get a 2.5 increase in latency,

Performance evaluation The attack happens on a random fraction of the overlay nodes. Packet replication helps us achieve higher network resilience.

Performance evaluation Latency V.S. Node failures

Summary Proposed the first non-trivial attack model: both the simple types of flooding attacks, as well as more sophisticated attackers that can eavesdrop the victim s communication link Proposed the use of a spread-spectrum-like paradigm to create per-packet path diversity.