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NCCR - MICS [IP5]. Route maintenance overheads in DHT overlays. presented by Anwitaman Datta Joint work with Karl Aberer and Manfred Hauswirth {Karl.Aberer, Anwitaman.Datta, Manfred.Hauswirth}@epfl.ch. EPFL-I&C-LSIR [P-Grid.org]. Workshop on Distributed Data and Structures ’04. P2P.

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slide1

NCCR-MICS [IP5]

Route maintenanceoverheads in DHT overlays

presented byAnwitaman Datta

Joint work with Karl Aberer and Manfred Hauswirth{Karl.Aberer, Anwitaman.Datta, Manfred.Hauswirth}@epfl.ch

EPFL-I&C-LSIR [P-Grid.org]

Workshop on Distributed Data and Structures’04

slide2
P2P
  • Central index
  • Unstructured
    • Flooding
  • Super-peer
  • Power-law networks …

ContentXYZ

Need XYZ

Network

P2P is more than just

File sharing or Pirate to Pirate!

slide3

A Case for P2P

  • Goals:
    • Efficient, scalable and reliable resource discovery/name resolution in a decentralized internet scale system
    • Content addressable network - disentangle the underlying network from the applications and services
    • A reliable substrate for other distributed applications
      • Semantic Web, Grid computing, Web services, large scale event notification, P2P web-search, …
challenges
Challenges
  • Unreliability of autonomous participants (peers)
  • Unreliability of the communication network
  • Lack of global knowledge and coordination
  • Scalability and robustness (fault tolerance)
  • Performance
approach issues
Approach (& issues)
  • Distributed indexing and routing in an overlay (disentangled from the underlying network)
    • Just like the world wide web, we can realize an overlay (but more structured) on top of the internet infrastructure

e.g., Distributed Hash Table/Distributed Indexing

    • P-Grid, Chord, DKS, CAN, Pastry, Kademlia, …
  • Peers (re-)joining/leaving the overlay => Maintenance of the overlay routes is required
    • Self-healing while routing
  • Flux in overlay => system operates in dynamic equlibrium
    • Two forces:
      • Changes in the network making routing information unusable
      • Maintenance operations repairing routing information
slide6

Prefix Routing in P-Grid

0

1

00

01

10

2

0 : 1,1410 : 11,13

12,13,14

14

1 : 2,1200 : 9,4011: 3,10

4,5

7

1 : 12, 1301 : 5,14001: 9,4

0,1

1 : 8, 1300 : 7,9011: 3,10

5

4,5

10

1 : 6,800 : 1,7010: 5,14

6,7

6

0 : 1,311 : 2,12101: 8,13

8,9

1 : 6,1301 :10,14000: 1,7

4

2,3

peer identifier

ID

Peer 9 holds keys with prefix 001, so we call, its path is 001

1 : 12, 13 routing table entry

query(101) @ 7

11

000

001

010

011

100

101

Replicas

12

0 : 5,710 : 6,13

12,13,14

3

1 : 11,1200 : 1,9010: 5,14

6,7

11

0 : 4,711 : 2,12101: 8,13

8,9

0 : 5,911 : 2,12100: 6,11

13

10,11

1

1 : 12, 1301 : 5, 10001: 9,4

0,1

9

1 : 8,201 : 3, 10000: 1,7

2,3

8

10,11

0 : 4,911 : 2,12100: 6,11

slide7

P-Grid

Self-referential directoryimplemented by P-Grid

lookup IP address

in case of failure

routing based on

logical address

routing based on

logical address

lookup IP address

directory

(logical ID <-> IP address)

Self-healing, self-referential directory

  • Any distributed access structure (such as P-Grid) requires mapping of a logical ID (associated data key) to physical ID (IP address)
  • This mapping cannot by static in the presence of dynamic IP addresses (hence requires a directory service)
  • A very important problem for the implementation of any P2P system
slide8

Example

Presently online

ID

ID

Presently offnline

Routingentriesrepaired

Up-to-date cache

1 :2 ,12

Stale cache

2

0 : 1,1410 : 11,13

12,13,14

Key as 4 bits for ID (2=0010 etc.)

14

1 : 2,1200 : 9,4011: 3,10

4,5

1 : 12, 1301 : 5,14001: 9,4

7

1

5

1 : 8, 1300 : 7,9011: 3,10

4,5

1 : 6,800 : 1,7010: 5,14

10

6,7

0 : 1,311 : 2,12101: 8,13

6

8,9

1 : 6,1301 :10,14000: 1,7

4

2,3

offline

query(01*) @ 7

…query(0101) @ 7 (for stale entry 5, cycle -> abort)

…query(1110) @ 7 (for stale entry 14, forward to 12 or 13)

…query(1110) @ 12 (is offline)

…query(1110) @ 13 (for stale entry 2)

……query(0010) @ 13 (forward to 5)

……query(0010) @ 5 (forward to 7)

……query(0010) @ 7 (forward to 9)

……query(0010) @ 9 (new entry for 2 found !)

…query(1110) @ 2 (new entry for 14 found !)

query(01*) @ 14 (finally )

0

1

Stores mappings

of peers

ID

00

01

10

11

000

001

010

011

100

101

12

0 : 5,710 :6,13

12,13,14

3

1 : 11,1200 : 1,9010: 5,14

6,7

11

0 : 4,711 : 2,12101: 8,13

8,9

13

0 : 5,911 :2,12100: 6,11

10,11

1 : 12, 1301 : 5, 10001: 9,4

1

1

9

1 : 8,201 : 3, 10000: 1,7

2,3

8

10,11

0 : 4,911 : 2,12100: 6,11

possible strategies
Possible strategies
  • Eager - Correction on Use (CoU)
    • While using a routing table, try correcting stale entries even if the present query can be routed using alternate routes (available locally).
      • Some entries of a particular level of routing table are unusable, but other entries of the same level are still usable.
  • Lazy - Correction on Failure (CoF)
    • While using a routing table, try correcting stale entries only if no alternate routes for the present query is available locally.
      • All entries of a particular level of routing table are unusable, but other levels may still be usable.
performance of overlays in flux
Performance of overlays in flux
  • Static resilience
    • Given a state of the network, and no more changes, how does the network perform?
      • P-Grid, Chord, various topologies …
  • Dealing with network churn
    • Given flux in the network, what maintenance cost is required to maintain a certain state.
      • e.g., Lower bound (MIT/Chord)
      • Simulations … (many groups)
  • Dynamic equilibrium
    • Given any flux in the network, and any maintenance strategy, what equilibrium state will the network operate in, and what will the maintenance cost and performance in the equilibrium state be?
eager recursion a k a correction on use cou
Eager recursion a.k.a. Correction on Use ‘CoU’

Dynamic equilibrium equation

  • LHS
    • Rate at which repair of stale routing entries occur
      • rup changes per 1-rup queries
    • Nrec – 1 additional recursive queries
    • Repair makes sense only if the routing entry to be repaired corresponds to an online peer
    • A repair is possible only if recursive query succeeds
  • RHS
    • Rate of entries turning stale
    • rup changes
    • 1-pdyn probability of non-stale references (only these can turn stale)
    • r references at each peer for each of log2n levels
lazy repair strategy correction on failure cof
Try to rectify stale references only when none of the references in a given level are usable

Not all routing entries are treated uniformly (unlike in CoU).

The number of stale entries for each routing level at each peer defines the state of that level.

Markovian model.

Dynamic equilibrium equation determined by equating inflow and outflow for each state

At dynamic equilibrium, the number of routing levels with given number of stale entries over the whole system should not change

Lazy Repair Strategy (Correction on Failure ‘CoF’ )

ID

change

ID

change

ID

change

ID

change

0 ref

stale

1 ref

stale

2 ref

stale

r ref

stale

repairs

N.B. We distinguish stale entries from offline peers

slide17

Reactivestrategies

Our approaches

Taxonomy of route maintenance mechanisms

summary
Summary
  • Self-referential decentralized directory with self-healing routing
  • Dynamic equilibrium of overlay network in flux (model & analysis)
  • Route maintenance mechanisms
    • Correction on Use
    • Correction on Failure
  • Taxonomy of maintenance mechanisms
other open issues
Other/open issues
  • Security/DDoS/…
    • Identity/Authentication
    • Authorization/Privacy
    • Reputation/Trust
    • Quorums/Web-of-trust
  • Garbage collection of references
  • Generic analysis (for various DHTs)
  • Sensor networks or MANETs and overlays
references
References
  • Efficient, self-contained handling of identity in Peer-to-Peer systems,

Karl Aberer, Anwitaman Datta, Manfred Hauswirth; IEEE Transactions on Knowledge and Data Engineering16(7), July 2004

& other papers @ http://www.p-grid.org

Questions?