Zuse-Institute Berlin (ZIB) Computer Science Research Artur Andrzejak Zuse-Institute Berlin (ZIB) Overview: Challenges in P2P Systems

ZuseZuse-Institute -Institute BerlinBerlin (ZIB) (ZIB) Computer Science Computer Science ResearchResearch

Artur AndrzejakArtur AndrzejakZuse-Institute Berlin (ZIB)Zuse-Institute Berlin (ZIB)

Overview:Overview:

Challenges in P2P SystemsChallenges in P2P Systems


What is a Peer-To-Peer System?What is a Peer-To-Peer System?

Participants are autonomous (different owners) Participants are autonomous (different owners)

Resources are distributedResources are distributed

Sites have equal functionalitySites have equal functionality

clientsclients, when accessing information, when accessing information

serversservers, when serving information to other peers, when serving information to other peers

routersrouters, when forwarding information, when forwarding information

... and so are called „... and so are called „peerspeers““

Lange number of participantsLange number of participants


P2P – a Bad Idea?P2P – a Bad Idea?

„„Distribution is expensive, specialized functionality is Distribution is expensive, specialized functionality is

good!“ (Garcia-Molina)good!“ (Garcia-Molina)

If distribution is necessary (e.g. due to reliability):If distribution is necessary (e.g. due to reliability):

build centralized directory and use backupsbuild centralized directory and use backups

computational efficiency suffers in P2P-scenario!computational efficiency suffers in P2P-scenario!


So Why P2P-Systems Exist at All?So Why P2P-Systems Exist at All?

User‘s view:User‘s view:

exploiting existing inexpensive resourcesexploiting existing inexpensive resources

sharing costs among manysharing costs among many

legal protectionlegal protection

autonomyautonomy

anonymityanonymity

Researcher‘s view:Researcher‘s view:

ScalabilityScalability

Self-organization and low management costSelf-organization and low management cost

High availability and fault-toleranceHigh availability and fault-tolerance


Main ChallengesMain Challenges

SearchSearch

Reliability and securityReliability and security

(Resource Management)(Resource Management)



SearchSearch



Search Mechanism CharacteristicsSearch Mechanism Characteristics

Comprehensiveness and guaranteesComprehensiveness and guarantees

Many of today‘s systems do not guarantee that existing items Many of today‘s systems do not guarantee that existing items will be found at all, or they do not find all itemswill be found at all, or they do not find all items

Query expressivenessQuery expressiveness

Today: only key/keyword searches; range queries, aggregates Today: only key/keyword searches; range queries, aggregates and SQL-like queries desirableand SQL-like queries desirable

EfficiencyEfficiency

A major problem: too many messages for searching, some A major problem: too many messages for searching, some systems even use floodingsystems even use flooding

RobustnessRobustness

AutonomyAutonomy


Search Mechanism Determines..Search Mechanism Determines..

TopologyTopology

From arbitrary (Gnutella) to rigid (Napster)From arbitrary (Gnutella) to rigid (Napster)

Rigid topology increases efficiency but decreases autonomyRigid topology increases efficiency but decreases autonomy

Placement of Data/MetadataPlacement of Data/Metadata

Gnutella – only own data; Chord – data/metadata is carefully Gnutella – only own data; Chord – data/metadata is carefully distributed in whole network; superpeers – metadata for distributed in whole network; superpeers – metadata for superpeers is centralizedsuperpeers is centralized

Message RoutingMessage Routing

Each query message is sent to a group of peersEach query message is sent to a group of peers

From unstructured flooding (Gnutella) to sofisticated protocols From unstructured flooding (Gnutella) to sofisticated protocols (Chord, CAN etc.)(Chord, CAN etc.)


Gnutella – How it WorksGnutella – How it Works

query hitquery hit

downloaddownload


Gnutella – CharacteristicsGnutella – Characteristics

Characteristics Gnutella

Comprehensivness ++

Expressivness ++++

Efficiency +

Autonomy ++++

Robustness +++

Architectural Properties

Gnutella

Topology power law

Data Placement arbitrary

Message Routing flooding


Chord – How it WorksChord – How it Works

A key is stored at its successor:node with next higher ID

N32

N90

N105

K80

K20

K5

Circular 160-bitID space

Key 5

Node 105

N80

½¼

1/8

1/161/321/641/128

112

N120

Finger i points to successor of n+2i


Chord - CharacteristicsChord - Characteristics

Characteristics Gnutella Chord

Comprehensivness ++ ++++

Expressivness ++++ +

Efficiency + ++++

Autonomy ++++ ++

Robustness +++ ++

Architectural Properties

Gnutella Chord

Topology power law ring

Data Placement arbitrary hashing

Message Routing flooding directed


Decouple Efficiency, Autonomy, RobustnessDecouple Efficiency, Autonomy, Robustness

autonomy

robustness efficiency

+

+

+

gnutellachord

(From „Open Problems in Data Sharing Peer-To-Peer Systems“ by Hector (From „Open Problems in Data Sharing Peer-To-Peer Systems“ by Hector Garcia-Molina)Garcia-Molina)


Novelty: Location-Independent RoutingNovelty: Location-Independent Routing

Each unique document or endpoint has a globally unique Each unique document or endpoint has a globally unique identifier (GUID)identifier (GUID)

Locating data can be seen as a routing problem:Locating data can be seen as a routing problem:

clients construct messages addressed with GUIDs and let clients construct messages addressed with GUIDs and let peers pass these messages until object is locatedpeers pass these messages until object is located

Known as Known as Decentralized Object Location and Routing (DOLR) Decentralized Object Location and Routing (DOLR) paradigm or paradigm or Distributed Hash Table (DHT)Distributed Hash Table (DHT)

Advantages: Advantages:

allows for routing messages to objects without knowing their allows for routing messages to objects without knowing their locationlocation

data can be stored anywhere, amidst millions of peers data can be stored anywhere, amidst millions of peers scalabilityscalability

provides locality: use of local resources instead of distant, if provides locality: use of local resources instead of distant, if possiblepossible

Implemented in Chord, CAN, Pastry, Tapestry Implemented in Chord, CAN, Pastry, Tapestry



SearchSearch



Essence: Untrusted/Unreliable Components Essence: Untrusted/Unreliable Components Centralized systems have componentsCentralized systems have components which are professionally which are professionally

maintained andmaintained and trusted to behave well trusted to behave well

Components of a P2P-system may crash or fail at any time Components of a P2P-system may crash or fail at any time

((unreliable componentsunreliable components))

Also, the participants might be adversarial, attempting to damage Also, the participants might be adversarial, attempting to damage

the system (the system (untrusted componentsuntrusted components))

Failure rate ~ system size Failure rate ~ system size larger P2P-systems are guaranteed larger P2P-systems are guaranteed

to have malfunctioning components to have malfunctioning components

P2P-system builders must invoke P2P-system builders must invoke new design principles to new design principles to achieve guaranteesachieve guarantees

„„only the aggregate behaviour of many peers can be trusted“only the aggregate behaviour of many peers can be trusted“

Techniques for untrusted components solve issues for unreliable Techniques for untrusted components solve issues for unreliable ones (converse is not true)ones (converse is not true)


Achieving Reliability and Security Achieving Reliability and Security

ReplicationReplication

CryptographyCryptography

Byzantine AgreementByzantine Agreement

Exploiting differencesExploiting differences

„„Thermodynamic“ Systems DesignThermodynamic“ Systems Design


ReplicationReplication Redundancy helps to achieve fault tolerance by providing Redundancy helps to achieve fault tolerance by providing

online replacements for faulty resourcesonline replacements for faulty resources

Advanced P2P Systems (Intermemory, OceanStore, Advanced P2P Systems (Intermemory, OceanStore, FreeHaven) use so called FreeHaven) use so called erasure codingerasure coding

Each chunk of data is transformed into many fragmentsEach chunk of data is transformed into many fragments

Very low Fraction of Blocks Lost Per Year (FBLPY)Very low Fraction of Blocks Lost Per Year (FBLPY)

Losses per year for Losses per year for 6 months repair 6 months repair interval:interval:

Std: Std: 0.03 blocks0.03 blocksErasure: Erasure: 1010-35-35 blocks blocks


Byzantine Agreement Byzantine Agreement

Immutable (read-only) data can be easily signed Immutable (read-only) data can be easily signed

(„sealed“) by cryptographic means to detect and discard („sealed“) by cryptographic means to detect and discard

faulty informationfaulty information

Also repairs are possible by these techniquesAlso repairs are possible by these techniques

However, However, some decisions are activesome decisions are active: e.g. changing, : e.g. changing,

replacing or deleting informationreplacing or deleting information

These decisions must be taken collectively to eliminate These decisions must be taken collectively to eliminate

corrupted nodescorrupted nodes

Here Here Byzantine AgreementByzantine Agreement can be used: only if a correct can be used: only if a correct

number of nodes agree, a unified decision is takennumber of nodes agree, a unified decision is taken

Works if no more than 1/3 of the nodes are compromizedWorks if no more than 1/3 of the nodes are compromized

Applied in OceanStore and FarsiteApplied in OceanStore and Farsite


Exploiting Differences Exploiting Differences

Some peers are „more equal“ than others:Some peers are „more equal“ than others:

Different CPUs, memory, storage cap., network connectivityDifferent CPUs, memory, storage cap., network connectivity

Some are professionally managed, others notSome are professionally managed, others not

Physically, some are locked in secure rooms, others are Physically, some are locked in secure rooms, others are

publicpublic

We can exploit these differences to tune performance, We can exploit these differences to tune performance,

availability, reliability, securityavailability, reliability, security

Examples:Examples:

Computers with higher connectivity as supernodesComputers with higher connectivity as supernodes

Actively managed nodes for Byzantine AgreementActively managed nodes for Byzantine Agreement

Placing archival data on servers deep in mountainsPlacing archival data on servers deep in mountains


„„Thermodynamic“ Systems Design Thermodynamic“ Systems Design A new concept of John Kubiatiowicz – A new concept of John Kubiatiowicz – „Stability through „Stability through

Statistics“Statistics“

We can give We can give guarantees on collective behaviourguarantees on collective behaviour while individual while individual nodes are not predictablenodes are not predictable

Over time, the latent order of a system is destroyed – this Over time, the latent order of a system is destroyed – this resembles the 2nd law of thermodynamics: „resembles the 2nd law of thermodynamics: „entropy of closed entropy of closed systems increasessystems increases““

Therefore, Therefore, self-organizing behaviour is necessaryself-organizing behaviour is necessary::

Servers must continuously collect, regenerate and Servers must continuously collect, regenerate and redistribute fragments in a data storage systemredistribute fragments in a data storage system

They must adjust routing links in the DOLR to correct They must adjust routing links in the DOLR to correct changeschanges

They must recognize faults without global communicationThey must recognize faults without global communication

Entropy reduction can be also achieved by Entropy reduction can be also achieved by introspectionintrospection

System observes itself, applies analyses, then adapts System observes itself, applies analyses, then adapts accordinglyaccordingly

Research in the area of IBM‘s Autonomic ComputingResearch in the area of IBM‘s Autonomic Computing


P2P-Research at ZIB: CSR-DMS P2P-Research at ZIB: CSR-DMS

Management of large scientific data-sets (up to 400 Management of large scientific data-sets (up to 400

Mio. files)Mio. files)

Should improve existing approaches in the area of Should improve existing approaches in the area of

GRID technologiesGRID technologies

Also as a framework for researchAlso as a framework for research

Architecture is P2P-basedArchitecture is P2P-based

Should exhibit self-management abilitiesShould exhibit self-management abilities

Candidates for Diplomarbeiten are very welcome! Candidates for Diplomarbeiten are very welcome!

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Zuse-Institute Berlin (ZIB) Computer Science Research Artur Andrzejak Zuse-Institute Berlin (ZIB) Overview: Challenges in P2P Systems