Cooperative Data Backup for Mobile Devices Ludovic Courts Advisors - PowerPoint PPT Presentation

Cooperative Data Backup for Mobile Devices Ludovic Courtès Advisors : David Powell, Marc-Olivier Killijian 23 November 2007

2 Context Thesis at LAAS-CNRS, Dependable & Fault-Tolerant Computing Team The MoSAIC Project • 3-year project started in Sept. 2004 • French national program: IRISA, Eurecom and LAAS-CNRS The Hidenets Project • 3-year EU IST project , FP6, started in Jan. 2006 • resilience for mobility-aware services ⇒ Improve Mobile Device Data Availability

3 • Introduction • Redundancy Management • Storage Mechanisms • Secure Cooperation • Implementation • Conclusions

4 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Problem Statement Mobile Devices are Subject to Damage, Loss, etc. Typical Data Backup Techniques… • “synchronization” betweenmobile device and desktop machine • requires access to desktop machine … Are Constraining or Costly • only intermittent access to one’s desktop machine • potentially costly communications (e.g., GPRS, UMTS) ⇒ Backup opportunities are rare, data is at risk

5 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Cooperative Approach to Data Backup Key Ideas • leverage computing device ubiquity • opportunistic replication to neighboring devices • … using wireless ad hoc networking (Wi-Fi, Bluetooth) Salient Points • adapted to intermittent connectivity scenarios • continuous backup & replication • protected against common-mode failures

6 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes

7 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes

8 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes

9 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes Internet

10 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes Internet

11 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes Internet Store Contributors Data Owner Internet

12 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes Internet

13 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Cooperative Backup and Recovery Processes Internet

14 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Storage Challenges Unpredictable Connection Encounters & Lifetime ⇒ limited transfer size ⇒ data must be fragmented ⇒ data blocks are scattered Limited Resources • minimize storage cost • optimize energy consumption

15 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Security Challenges Trustworthy Data Storage • ensure data confidentiality, integrity, authenticity • provide appropriate backup redundancy Secure Cooperation • participants have no a priori trust relationship • participants are mutually suspicious • protect against Denial-of-Service attacks

16 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Related Work Peer-to-Peer Backup • Pastiche , Samsara , PeerStore , etc. on the Internet ⇒ different connectivity assumptions • Personal Area Network Cooperative Backup • Flashback • devices are mutually trusted Persistent Stores for Sensor Networks (e.g., tinyPEDS ) Delay-Tolerant Networks • different evaluation criteria (e.g., delay) • usually assumes that devices are well-behaved

17 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Major Contributions of the Thesis • Definition of Dependability Goals & Backup Framework • Identification of Distributed Storage Requirements • Design of Core Security Mechanisms • Prototype Implementation of a Cooperative Backup Service

18 • Introduction • Redundancy Management • Storage Mechanisms • Secure Cooperation • Implementation • Conclusions

19 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Distributed Storage & Redundancy Management

20 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Improving Data Availability Problem Statement • contributors may fail • contributors are not trusted ⇒ Need for Data Replication Data Replication Goals • maximize storage efficiency … • … and data availability Methodology • devise replication strategies • evaluate the efficiency/availability tradeoff

21 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Simple Replication Strategy Algorithm send a total of n copies of each data item 1. 2. send 1 copy per contributor recover from any 1 contributor out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost f + 1 times the input data size • n = 3, f = 2

22 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Simple Replication Strategy Algorithm send a total of n copies of each data item 1. 2. send 1 copy per contributor recover from any 1 contributor out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost f + 1 times the input data size • n = 3, f = 2

23 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Simple Replication Strategy Algorithm send a total of n copies of each data item 1. 2. send 1 copy per contributor recover from any 1 contributor out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost f + 1 times the input data size • n = 3, f = 2

24 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Using Erasure Codes for Replication k = 4 source blocks Erasure Codes at a Glance k -block input → n coded blocks , n > k • m blocks suffice to recover input data k < m < n • storage cost : S = n / k • k = 1 ⇔ simple replication • Optimal Codes when m = k • • notation: ( n , k ) code n and k are user-defined parameters • n = 6 coded blocks

25 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Replication Strategy Using Erasure Codes Algorithm ( n , k ) erasure coding → n coded blocks 1. 2. send 1 coded block per contributor recover from any k contributors out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost k + f • times the input data size! k n = 5, k = 3 f = n − k = 2

26 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Replication Strategy Using Erasure Codes Algorithm ( n , k ) erasure coding → n coded blocks 1. 2. send 1 coded block per contributor recover from any k contributors out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost k + f • times the input data size! k n = 5, k = 3 f = n − k = 2

27 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions A Replication Strategy Using Erasure Codes Algorithm ( n , k ) erasure coding → n coded blocks 1. 2. send 1 coded block per contributor recover from any k contributors out of n 3. Dependability & Storage Cost Analysis tolerate f contributor faults ⇒ storage cost k + f • times the input data size! k n = 5, k = 3 f = n − k = 2

28 Introduction > Redundancy Management > Storage Mechanisms > Secure Cooperation > Implementation > Conclusions Example Erasure Code-Based Strategies Examples (3,1) code: f = 2 failures tolerated; storage cost: S = 3 • (5,3) code: f = 2 failures tolerated; storage cost: S = 1 . 67 • 2 failures tolerated: n = k + 2 3 stretch factor S S = k + 2 2 k 1 0 1 2 4 6 8 10 parameter k