3. This is not an impossible architecture – Incremental Deployment Compatible Unlike any previous papers, this paper addresses a lot of issues connected to DoS attacks This paper comes from the author of Active Networks! Remember the 835 citations? This paper has 83 citations and was regarded as a powerful technique by many authors. These techniques are powerful because they seek to block or explicitly limit unauthorized users…

4. This presentation was inspired from D. Witherall’s original work Presentation by: Rahul Potharaju

5. Ingress filtering Traceback Fair Queuing SOS, MayDay SIFF Discards packets with widely spoofed addresses at the edge of the network Uses routers to create state so that receivers can reconstruct the path of unwanted traffic Each flow gets its fair share of Bottleneck bandwidth Rely on an offline authenticator Refines the previously proposed capability approach Attack packets are dropped before they arrive at the destination! k hosts attacking a destination would lower the bandwidth by 1/k Requires prior-arrangement Doesn’t concentrate much on security of proposed method

6. The base already existed and improving – TVA is a robust approach to the earlier proposed methods using capabilities It allows the destination to control what it receives – No more false bills! Overcomes the shortcomings of current packet filtering techniques Automated validation of senders without prior arrangement Ensures that any two nodes can communicate regardless of the ‘k’ malicious nodes

7. Capabilities must be granted by the destination to the sender, so that they can be stamped on packets. Capabilities must be unforgeable and not readily transferable across senders and destinations Routers must be able to verify capabilities without trusting hosts. Capabilities must expire so that a destination can cut off a sender from whom it no longer wants to receive packets. Capabilities must add little overhead in the common case.

8. The network must discard unwanted packets before they reach the destination  Each packet must carry information that tells a router whether it is wanted by the destination in the first place. And that’s it… couldn’t be much simpler than this! The explicit identification information is known as Capability

9. SenderRouter Destination Request (Part of TCP SYN packet) Response (Part of TCP SYN/ACK packet) Routers add pre-capabilities Adds capabilities

10. Initial request channel should not be open to DoS attacks either by: Flooding the destination Rate limit requests at all network locations so that they cannot consume all bandwidth Request packets comprise only 5% of the total link capacity Blocking the requests of legitimate senders Use per-source fair queuing to ensure that no source can overwhelm others Spoofing? No problem because PSFQ requires authenticated source identifier

11. Path Identifier – Each router at the trust boundary tags (16—bit values) the packet with a small value derived from its incoming interface – Identifies the upstream party Fair queue requests using the tag to identify a queue Two Advantages: Tag space is bounded  Number of queues is bounded  Router resources are bounded Uses Trust domains

12. Rate-limited Preferential Remaining Bandwidth

13. This is a matter of policy, and it depends on the role, the destination plays in the network. Client -> Server possible but other way not possible (firewalls, NAT etc.) Public server -> Clients: Grant all requests with a default number of bytes and timeout, using the path identifier to fairly serve different sources when the load is high. If ( senders_action == misbehave ) Add sender to blacklist As a result: Sender’s capability will expire

14. Hashing Mechanism = f ( source IP, destination IP, timestamp, router secret) Each router that forwards a request packet generates its own pre-capability and attaches it to the packet. Router secret is changed at twice the rate of the timestamp rollover, and uses the previous secret to validate capability.  Pre-Capability expires within at most the timestamp rollover period and each is valid for about the same time period

15. Receiver: Oops… I authorized some unwanted traffic Attacker: Good… I can flood until my capability expires! Fatal problem: Evan a small number can raise a DoS attack Solution: Grant the right to send up to N bytes along a path within the next T seconds. Routers verify their portion of the capabilities by re-computing the hashes much as before, expect that now two hashes are required instead of one. The router now perform two checks, one for N and one for T.

16. Bounding Algorithm: If ( flow rate for a valid capability > N/T ) Keep router state If ( pkt_rcvd && no_state_for_pkt) Track byte count and associate a minimal ttl with state = L*T/N If ( ttl == 0) Reclaim the state for use with a new capability Max bytes used Creates statettl hits 0  Reclaim state Worst caseTotal of 2N bytes before capability expires

17. Efficient Capabilities: – Security benefits from long capabilities ( a long key length). Route Changes and Failures: – Demote packets to be the same priority as legacy traffic by changing a bit in the capability header. Balancing Authorized Traffic: – Floods of authorized traffic: A pair of colluding attackers can authorize high-rate transfers between themselves and disrupt other authorized traffic that shares the bottleneck – Use Fair Queuing Short, Slow or Asymmetric Flows: – TVA is designed to run with low overhead for long, fast flows that have a reverse channel. Short or slow connections will experience a higher relative overhead. But wait: Most bytes belong to long flows  Effect on aggregate efficiency would be small Design does not introduce any latency on handshaking  Remember piggybacking? Short flows are less likely because flows are defined from a sender to a destination basis

18. Packets carrying capability information Hosts that act as senders and destinations TVA Protocol Routers that process capability information Two types of capability packets: Request Packets: Carry a list of blank capabilities and path identifiers that are filled in by routers as requests travel towards destination Regular Packets: Two types, packets that carry both a flow nonce and a list of valid capabilities, and packets that carry only a flow nonce If a regular packet does not pass the capability check, it may be demoted to low priority traffic that is treated as legacy traffic. There are no separate capability packets! All packets carry a header that extends the behavior of IP. SenderRouter Destinatio n Request (Part of TCP SYN packet) Response (Part of TCP SYN/ACK packet) Routers add pre-capabilities Adds capabilities

19. 19 Indicate the type and format of the packet Each capability has a 64-bit value, broken down into 8 bits of router timestamp in seconds ( a modulo 256 clock) and 56 bits of a keyed hash

20. Packets carrying capability information Hosts that act as senders and destinations TVA Protocol Routers that process capability information SenderRouter Destination Request (Part of TCP SYN packet) Response (Part of TCP SYN/ACK packet) Routers add pre-capabilities Adds capabilities Sender Side: If ( no valid capability ) Piggyback request onto TCP SYN Destination Side If ( willing to authorize ) Piggyback capabilities onto TCP SYN/ACK Send request for reverse directions Else Return empty capability list piggybacked onto TCP RST

21. Packets carrying capability information Hosts that act as senders and destinations TVA Protocol Routers that process capability information To process a request packet if( at trust boundary ) add pre-capability to the end of the list and add a new path identifier To process a regular packet If ( pkt_authorized ) update cached information & schedule for forwarding Locate an entry for the flow using source & destination IPs if ( cached entry ) update byte count else check capability and allocate entry if ( valid_capability & renewal pkt) mint pre-capability Routers route and forward packets as required by IP and additionally process packets according to the capability information that they carry.. SenderRouter Destinatio n Request (Part of TCP SYN packet) Response (Part of TCP SYN/ACK packet) Routers add pre-capabilities Adds capabilities

22. Uses ns to simulate TVA, SIFF, pushback and legacy Internet Rate-limit capability requests to 1% of link capacity (down from default 5% to stress the design)! Setup fixed length transfer between legitimate users and destination under various attacks and measure: i) the average fraction of completed transfers ii) the average time of the transfers that complete Reason: DoS attacks cause heavy loss  Slow transfers and eventual application abortion Dumbbell Topology SIFF - treats capacity requests as legacy traffic, does not limit the number of times a capability is used to forward traffic, and does not balance authorized traffic sent to different destinations Pushback - recursively pushes destination-based network filters backwards across the incoming link that contributes most of the flood.

23. Simulation Results Attacker floods the destination with legacy traffic at 1 Mb/s TVA: Legacy traffic has a lower priority compared to request traffic SIFF: Treats both legacy and request packets as equally low priority traffic Pushback: Performs well but as the number of attackers increase, it cannot isolate attack & legitimate traffic

24. Attacker floods the destination with request packets at 1 Mb/s TVA: Request packets are rate-limited SIFF: Treats both legacy and request packets as equally low priority traffic Pushback: Treats requests as regular data

25. Attacker is supported by a colluder TVA: Transfer time slightly increases but no user will be starved SIFF: Treats both legacy and request packets as equally low priority traffic  Users are completely starved Pushback: Treats requests as regular data

26. Prototyped TVA using the Linux netfilter framework Host Portion – Implemented as a user-space proxy :: Allows legacy applications to run without modifications Router Portion Kernel module using the AES-hash for pre-capabilities and SHA1 for capabilities Dual-processor 3.2 GHz Pentium Xeon machine running Linux 2.6.8 Kernel Kernel packet generator to generate different types of packets Each run: One million packets of each type. Five Runs: Recorded the average number of instruction cycles for the router to process each type of packet

27. Regular with a cache entry is the most common so not a big deal! Regular without a cached entry – Two has functions Renewal without cached entry – Three hash functions Output rate α Input rate Reaches a peak depending on the type of input packet Problem of spoofed short renewal packets can be solved using Lazy Receiver Processing Implementation can handle 100 Mbps interfaces and intend to demonstrate gigabit handling in the near future

28. TVA uses standard cryptographic functions with sufficient amount of keys material and changes keys every 128 seconds! Breaking the hash scheme TVA uses a packet format that does not expose pre- capabilities in the first 8 bytes that are visible in ICMP error messages Observe pre-capabilities (using ICMP error messages) Attacker will not generally be able to send packets along the same path as the authorized sender because capabilities are bound to a specific source, destination and router. Steal Capabilities The compromised router is another attacker! As long as there are capability routers in the middle, DoS attacks are still limited. Eavesdropping (includes a compromised router) This attack has little effect on a sender’s traffic if per- destination queuing is used, which is TVA’s default. Spoof authorized traffic

29. No Flag Day: The design requires both routers and host to be upgraded, but does not require a flag day. Incremental Deployment Compatible : Routers can be upgraded incrementally, at trust boundaries and locations of congestion, i.e., the ingress and egress of edge ISP’s Host must be upgraded. This can be occurred with proxies at the edges of customer networks in the manner of a NAT box or Firewall. Expect to use DNS to signal which hosts can handle capabilities in the same manner as upgrades

30. There are three components in a network: Sender, Receiver and Destination. If one considers DoS as a serious problem, then we have to alters one of the three components! Main contribution is to flesh out the design of a comprehensive and practical capability system for the first time Practical design with an implementation to support it Constrained design to be easy to transition into practice – Incremental deployment

31. We have a website! http://www.ics.uci.edu/~xwy/doslimit.html Agreed that this is a new architecture but wait… it has its advantages and is incrementally deployable This architecture can be implemented and tested either internally or globally We considered attackers to be like-minded and are working on getting results for dynamic behavior patterns The attacker number we used will be increased by the next paper! Hash functions affect the performance but then there is a law of equivalence: One cannot gain anything without first giving something in return! – If there’s a better way, let us know and we’ll evaluate it… We don’t claim this to be the best design, its just a better design…