presented by Anwitaman Datta

EPFL-I&C-LSIR [P-Grid.org] Workshop on Distributed Data and Structures’04

NCCR-MICS [IP5]

presented byAnwitaman Datta

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

Route maintenance

overheads in DHT overlays

EPFL-I&C-LSIR [P-Grid.org] Workshop on Distributed Data and Structures’04

NCCR-MICS [IP5]

Overheads in overlays © 2004, A. Datta, K. Aberer, M. Hauswirth @ WDAS’04 2

P2P

Need XYZ

ContentXYZ

Network

Central index

Unstructured

Flooding

Super-peer

Power-law networks …

P2P is more than justFile sharing or Pirate to Pirate!


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, …

A Case for P2P


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)

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 infrastructuree.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

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

1 0,1

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

7 0,1

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

4 2,3

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

9 2,3

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

5 4,5

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

14 4,5

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

10 6,7

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

3 6,7

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

11 8,9

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

6 8,9

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

13 10,11

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

8 10,11

0 : 5,710 : 6,13

12 12,13,14

0 : 1,1410 : 11,13

2 12,13,14

000 001 010 011

0 1

00 01 10

100 101

11

ID peer identifier Prefix Routing in P-Grid

1 : 12, 13 routing table entry

query(101) @ 7

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

Replicas


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

directory(logical ID <-> IP address)

lookup IP address

P-Grid

routing based on logical address

Self-referential directory implemented by P-Grid

routing based on logical address

lookup IP addressin case of failure

Self-healing, self-referential directory

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

1 1

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

7 1

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

4 2,3

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

9 2,3

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

5 4,5

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

14 4,5

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

10 6,7

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

3 6,7

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

11 8,9

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

6 8,9

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

13 10,11

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

8 10,11

0 : 5,710 : 6,13

12 12,13,14

0 : 1,1410 : 11,13

2 12,13,14

0 1

000 001

01

010 011

10

100 101

11

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 )

001 : 2 ,12

Stale cache

Up-to-date cache

ID Presently online

ID Presently offnline

ID Stores mappings of peers

Example

offline

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

Routingentries

repaired


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

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’

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

Dynamic equilibrium equation


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

0 refstale

1 refstale

2 refstale

r refstale…

repairs

IDchange

IDchange

IDchange

IDchange

N.B. We distinguish stale entries from offline peers


Analysis vs. Simulations (Lazy recursion)


Overhead with varying pon


Contour: Zone of operation for a maximum cost (Lazy)


CoU (eager) vs. CoF (lazy)

Taxonomy of route maintenance mechanisms

Our approaches Reactivestrategies


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

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

Efficient, self-contained handling of identity in Peer-to-Peer systems,

Karl Aberer, Anwitaman Datta, Manfred Hauswirth; IEEE Transactions on Knowledge and Data Engineering 16(7), July 2004

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

Questions?

presented by Anwitaman Datta

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