A Charging and Rewarding Scheme for Packet Forwarding in Multi-hop Cellular Networks

1. NCCR/MICS A Charging and Rewarding Scheme for Packet Forwarding in Multi-hop Cellular Networks N. Ben Salem*, L. Buttyán**, J.-P. Hubaux* and M. Jakobsson*** *Laboratory of Computer Communications and Applications (LCA) Swiss Federal Institute of Technology – Lausanne (EPFL), Switzerland **Department of Telecommunications, Budapest University of Technology and Economics, Hungary ***RSA Laboratories, Hoboken, NJ, USA

2. Outline • Multi-hop Cellular Networks • 2. Model • System and trust model • Adversarial model • 3. The protocol • Session setup • Packet sending • Payment redemption • Security analysis • 5. Overhead of the solution • Communication Overhead • Computation Overhead 6. Conclusions and future work

3. Set of base stations connected to a backbone • Cell = The geographical area under the control of a base station • A node beyond the reach of the base station coverage can • use other mobile stations as relays Backbone B Backbone A Multi-hop cellular networks • Combine the characteristics of cellular and ad hoc networks • Advantages: • Increase the coverage of the network • Small number of base stations (fixed antennas) • Reduce the energy consumption of the sending mobile station • Reduced interference

4. Why would the intermediate nodes use their battery to relay packets for other nodes? Charge the initiator A of the communication Reward the cooperative forwarding nodes (and the operator) Initiator Correspondent BSB BSA j B A 1 i 1 Problem statement • Multi-hop cellular networks represent a new and promising paradigm, but … • No cooperation = the network does not work • We exclusively consider the packet forwarding service

5. The initiator A wants to communicate with the correspondent B • A has to establish an end-to-end session with B (a session is a • secure route on which all the nodes are authenticated) • An initiator session between A and BSA • A correspondent session between BSB and B • Then, A and B exchange packets BSB BSA j B A 1 i 1 End-to-end session Initiator session Correspondent session Packet exchange Model System model: • This is done by establishing: • The operator charges A for the traffic (in both directions)

6. Model Trust model and assumptions: • Node i shares a symmetric key Kiwith theoperator • The nodes trust the operator for: • not revealing secret keys • correctly transmitting packets • correctly performing billing and auditing • The nodes do not trust each other • The underlying routing protocol is secure • All the communications go through a base station • Nodes are mobile but we have a certain level of route stability Adversarial Model: • The nodes are rational: • they are potential attackers if cheating is beneficial • they will cooperate if they expect a gain • Collusions are possible • We consider the pessimistic case where all the attackers are under the control of a single entity

7. AReq0 AReqID oldASID ARoute TrafficInfo MACA AReqi-1 AReqID oldASID ARoute TrafficInfo MACi-1 AReqi AReqID oldASID ARoute TrafficInfo MACi AReqa AReqID oldASID ARoute TrafficInfo MACa A layered MAC that BSA can verify Session Setup BSB BSA B A i j

8. Req BReq0 PADi,1 PADi,2 PADi, SID seedi BReqID oldBSID BRoute TrafficInfo Ki MaxLength MaxLength MaxLength AConf Layered MAC authentication BConf MACa AReqID ASID MACA … MAC1 BReqID BSID MAC1 … MACb MACB Session Setup BSB BSA B A i j Stream Cipher Generation

9. SPkt0,l SSID l Payloadl MACS SPkti-1,l Bodyi-1,l SSID  PADi,l = SSID l Bodyi,l SPkti,l SPkts,l Bodys,l SSID Encrypted data that BSS can decrypt Packet Sending BSD BSS D S i j Body0,l

10. l Payloadl MACD DSID  PAD1,l Acknowledgement for the packet  PAD1,l = DSID Body0,l DPkti,l Iterative XOR DPktd,l Bodyd,l DSID The Body is decrypted MACD l Payloadl Packet Sending BSD BSS D S i j Body’0,l

11. BSB BSA j B=D A=S 1 i 1 Payment Redemption • Charging and rewarding mechanism: • - When the packet SPkt of length L reaches BSS • A is charged n (L) • The forwarders in the up-stream are rewarded  (L)each • The operator is rewarded • - When the packet DPkt is injected in the down-stream • D is charged a small amount  • - When the packet DAck is received by a base station • The forwarders in the down-stream are rewarded  (L)each • D is refunded for each packet it acknowledges

12. DPktd,l DSID Acknowledgement for the packet • D maintains:Batch= MACKD(DSID |  | Payload )  LastPkt;  LostPkts MACD l Payloadl Payment Redemption • Destination Acknowledgment: • One acknowledgement per session: • DAck = [ DSID | Batch | lastPkt | LostPkts | • MACKD(DSID | Batch | lastPkt | LostPkts ) ] • DAck is sent offline after the session is closed

13. Security Analysis • Incentive to cooperate: • The up-stream nodes get rewarded only if SPkt reaches BSS • The down-stream nodes get rewarded only if D acknowledges DPkt • D is refunded only if it acknowledges DPkt • Disincentive against cheating: • Refusal to pay: • The MAC in the packet uniquely identifies S • Incorrect reward claims: • A node i is credited if it is part of both the session setup and the packet sending phases • A node i is the only node that is able to correctly compute the layered MAC in the session setup and the PAD in the packet sending • Free-riding: • The packets are encrypted at each hop • The nodes are not rewarded and the transmitted data is garbled • Emulated nodes: • A node is in several physical locations simultaneously • Some nodes seem to be always neighbors • Capture a rogue device

14. Communication Overhead • Sizes of the fields: • Session Setup Phase: 144+NbFwdrs*64 bytes • Packet Sending Phase: 20 bytes per packet • Sending the Acknowledgement: 38+2*NbLostPkts bytes per session •  Numerical values? • Simulations: • 100 nodes in a 500x500 m2 cell with one base station in the center • Fixed power range of 100 m • RWP: uniform speed  [0,20] m/s; pause time  {0,60,120,300,600} s • We discard the first 1000 s of simulation time • 100 simulations for each value of the pause time • Figures of interest: • Average lifetime of a route (AverageLifeTime) • Average number of forwarding nodes (NbFwdrs) • Average percentage of disconnected nodes (NotConnected).

15. Communication Overhead • Simulation Results: • Numerical example: • Mobility = 0s Pause time • Application = Voice over IP • Codec G.711 • frame size = 200 bytes • Values of the overhead: • During the 8.2s, it is possible to transmit 410 packets (= 65.6 kbytes) • Session setup : 0.3% of the total payload of the session • Packet sending : 11% of the packet size • Payment redemption : 0.3% of the total payload of the session for the pessimistic case where NbLostPkts=100

16. Computation Overhead • Session Setup Phase (per session): • 2 MAC operationsfor each node • Packet Sending Phase (per packet): • 1 stream cipher encryption for each node (except D) • 1 MAC operation for S and D • Acknowledgment computation (for D): • 1 XOR operation per packet • 1MAC computation per session • What is the cost of a stream cipher encryption?

17. Conclusions and future work • Conclusions: • We have addressed the problem of cooperation for packet forwarding in multi-hop cellular networks • We have proposed a solution based of a charging and rewarding mechanism • We have shown that the protocol encourages cooperation and that it resists to number of rational attacks • We have quantified the life time of the sessions and shown that the usage of our scheme leads to a very moderate overhead • Future work: • Malicious attacks • Several operators • Charge the correspondent