introduction to peer-to-peer systems

ALMA MATER STUDIORUM – UNIVERSITA’ DI BOLOGNA

Introduction toPeer-to-Peer Systems

Ozalp Babaoglu

© Babaoglu Complex Systems

Introduction

■ Peer-to-peer (P2P) systems have become extremely popular and contribute to vast amounts of Internet traffic

■ P2P basic definition:■ A P2P system is a distributed collection of peer nodes■ Each node is both a server and a client:

▴ May provide services to other peers▴ May consume services from other peers

■ Very different from the client-server model

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P2P History: 1969 - 1990

■ The origins:■ In the beginning, all nodes in Arpanet/Internet were peers■ Each node was capable of:

▴ Performing routing (locate machines)▴ Accepting ftp connections (file sharing)▴ Accepting telnet connections (distributed computation)

3 © Babaoglu Complex Systems

P2P History: 1999 - today

■ The advent of Napster:■ Jan 1999: the first version of Napster was released by Shawn

Fanning, student at Northeastern University■ July 1999: Napster Inc. founded■ Feb 2001: Napster closed down

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Napster

Napster Central Index Server

Napster Client 4

Napster Client 3

Napster Client 2

Napster Client 1

Your ComputerQuery: song.mp3

Client 2

Copy: song.mp3

- Client/Server search- P2P download- Napster is not “pure P2P”


Client/Server vs. Peer-to-Peer

• Servers well connected to the “core” of the Internet

• Servers carry out critical tasks• Clients only talk to servers

• Nodes located at the “periphery of the Internet”

• Tasks distributed across all nodes• Clients talk to other clients


Example – Video sharing

Client-Server: YouTubeAdvantages

• Client can disconnect after upload

• Uploader needs little bandwidth

• Other users can find the file easily(just use search on server webpage)

Disadvantages

• Server may not accept file or remove it later (according to content policy)

• Whole system depends on the server(what if shut down like Napster?)

• Server storage and bandwidthare expensive!

client-server

uploader

downloader downloader

downloader

downloaderdownloader


Example – Video sharing

Peer-to-peer: BitTorrentAdvantages

• Does not depend on a central server

• Bandwidth shared across nodes(downloaders also act as uploaders)

• High scalability, low cost

Disadvantages

• Seeder must remain on-line to guarantee file availability

• Content is more difficult to find(downloaders must find .torrent file)

• Freeloaders cheat in order to download without uploading

peer-to-peer

seeder

downloader downloader

downloader

downloaderdownloader


P2P vs. client-server

Client-server

Asymmetric: client and servers carry out different tasks

Global knowledge: servers have a global view of the network

Centralization: communications and management are centralized

Single point of failure: a server failure brings down the system

Limited scalability: servers easily overloaded

Expensive: server storage and bandwidth capacity is not cheap

Peer-to-peer

Symmetric: No distinction between node; they are peers

Local knowledge: nodes only know a small set of other nodes

Decentralization: no global knowledge, only local interactions

Robustness: several nodes may fail with little or no impact

High scalability: high aggregate capacity, load distribution

Low-cost: storage and bandwidth are contributed by users


P2P and Overlay Networks

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■ Peer-to-Peer systems are usually structured as “overlays”■ Logical structures built on top of a physical routed

communication infrastructure (IP) that creates the allusion of a completely-connected graph

■ Links based on logical “knows” relationships rather than physical connectivity

© Babaoglu Complex Systems11

Overlay networks

Physical network: “who has a communication link to whom”© Babaoglu Complex Systems

12

Overlay networks

Logical network: “who can communicate with whom”


Overlay networks

Overlay network (ring): “who knows whom”


Overlay networks

Overlay network (binary tree): “who knows whom”


P2P Environment

■ Completely decentralized control with limited local states■ High latency, low bandwidth communication■ Churn

■ Nodes may disconnect temporarily■ New nodes are continuously joining the system, while others

leave permanently■ Security

■ P2P clients runs on machines under the total control of their owners

■ Malicious users may try to bring down the system■ Selfishness

■ Users may run hacked clients in order to avoid contributing resources


Why P2P?

■ Decentralization enables deployment of applications that are:■ Highly available■ Fault-tolerant■ Self organizing■ Scalable■ Difficult or impossible to shut down

■ This results in a “grassroots” approach and “democratization” of the Internet

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© Babaoglu Complex Systems 17

P2P Problems

■ Overlay construction and maintenance■ e.g., random, two-level, ring, etc.

■ Data location■ locate a given data object among a large number of nodes

■ Data dissemination■ propagate data in an efficient and robust manner

■ Global reasoning with local information■ maintain local views with small per node state

■ Tolerance to churn■ maintain system invariants (e.g., topology, data location, data

availability) despite node arrivals and departures


P2P Applications

■ Sharing of content:■ File sharing, content delivery networks■ Gnutella, eMule, Akamai

■ Sharing of storage:■ Distributed file systems

■ Sharing of CPU time:■ Parallel computing, Grid■ Seti@home, Folding@home, FightAids@home

(typically not pure P2P)

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P2P Topologies

■ Unstructured■ Structured

■ Centralized■ Hierarchical

■ Hybrid


Evaluating topologies

■ Manageability■ How hard is it to keep working?

■ Information coherence■ How authoritative is info?

■ Extensibility■ How easy is it to grow?

■ Fault tolerance■ How well can it handle failures?

■ Censorship■ How hard is it to shut down?

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Unstructured

■ Manageable■ Difficult, many owners

■ Coherent■ Difficult, unreliable peers

■ Extensible■ Anyone can join in

■ Fault tolerance■ redundancy

■ Censorship■ Difficult to shut down


Structured: Centralized

■ Manageable■ System is all in one place

■ Coherent■ Information is centralized

■ Extensible■ No

■ Fault tolerance■ Single point of failure

■ Censorship■ Easy to shut down


Structured: Hierarchical

■ Manageable■ Chain of authority

■ Coherent■ Cache consistency

■ Extensible■ Add more leaves, rebalance

■ Fault tolerance■ Root is vulnerable

■ Censorship■ Just shut down the root


Hybrid: Superpeers

■ Manageable■ Same as decentralized

■ Coherent■ Better than decentralized

■ Extensible■ Anyone can join in

■ Fault tolerance■ redundancy

■ Censorship■ Difficult to shut down


Some Common Topologies

■ Flat unstructured: a node can connect to any other node■ only constraint: maximum degree dmax

■ fast join procedure■ good for data dissemination, bad for location

■ Two-level unstructured: nodes connect to a superpeer■ superpeers form a small overlay■ used for indexing and forwarding■ high load on superpeer

■ Flat structured: constraints based on node ids■ allows for efficient data location■ constraints require long join and leave procedures


Data location in unstructured networks:Flooding

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■ Problem: find the set of nodes S that store a copy of object O

■ Flooding: forward the search message to all neighbors (first Gnutella protocol)■ A search message contains either keywords or an object id■ Advantages:

▴ simplicity▴ no topology constraints

■ Disadvantages:▴ high network overhead (huge traffic generated by each search request)▴ flooding stopped by TTL (which produces search horizon)▴ only applicable to small number of nodes


Data location in unstructured networks:Flooding

■ Flooding (cont.)■ Flooding in a flat unstructured network:

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search horizon forTTL = 2

obj

Objects that lie outside of the horizon are not found


Data location in unstructured networks:Superpeers

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■ Two-level overlay: use superpeers to track the locations of an object [Gnutella 2, BitTorrent]■ Each node connects to a superpeer and advertises the list of

objects it stores■ Search requests are sent to the supernode, which forwards them

to other supernodes■ Advantages:

▴ highly scalable■ Disadvantages:

▴ superpeers must be reliable, powerful and well connected to the Internet (expensive)

▴ superpeers must maintain large state▴ the system relies on a small number of superpeers


Data location in unstructured networks:Superpeers

■ Two-Level Overlay (cont.)

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A two-level overlay is a partially centralized system

In some systems, superpeers may be disconnected (e.g., BitTorrent)

responserequestobj


Data location in structured networks:Key-Based Routing

■ Structured networks: use a routing algorithm that implements Key-Based Routing (KBR) [Chord, Pastry, Overnet, Kad, eMule]

■ KBR (also known as Distributed Hash Tables) works as follows:■ nodes are (randomly) assigned unique node identifiers (nodeId)■ given a key k, the node whose nodeId is numerically closest to k

among all nodes in the network is known as the root of key k■ given a key k, a KBR algorithm can route a message to the root

of k in a small number of hops, usually O(log n)■ the location of an object with id objectId is tracked by the root of

k = objectId■ thus, one can find the location of an object by routing a message

to the root of k = objectId and querying the root for the location of the object

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Structured overlay network: Chord

Basics

Each peer is assigned a unique m-bit identifier id .Every peer is assumed to store data contained in a file.Each file has a unique m-bit key k .Peer with smallest identifier id � k is responsible forstoring file with key k .succ(k): The peer (i.e., node) with the smallest identifierp � k .

NoteAll arithmetic is done modulo M = 2m. In other words, ifx = k ·M +y , then x mod M = y .

22 / 53

Structured overlay network: Chord

Basics

Each peer is assigned a unique m-bit identifier id .Every peer is assumed to store data contained in a file.Each file has a unique m-bit key k .Peer with smallest identifier id � k is responsible forstoring file with key k .succ(k): The peer (i.e., node) with the smallest identifierp � k .

NoteAll arithmetic is done modulo M = 2m. In other words, ifx = k ·M +y , then x mod M = y .

22 / 53

Example

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Peer�9�storesfiles�with�keys5,�6,�7,�8,�9

Peer�1�storesfiles�with�keys29,�30,�31,�0,�1

Peer�20�storesfile�with�key�21

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Efficient lookups

Partial view = finger table

Each node p maintains a finger table FTp[] with at most mentries:

FTp[i] = succ(p +2i�1)

Note: FTp[i] points to the first node succeeding p by atleast 2i�1.To look up a key k , node p forwards the request to nodewith index j satisfying

q = FTp[j] k < FTp[j +1]

If p < k < FTp[1], the request is also forwarded to FTp[1]

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Example finger tables

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Example lookup: 15@4

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4 p = 14 < 18 < FTp[1]) 14! 18

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4 p = 14 < 18 < FTp[1]) 14! 18

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4 p = 14 < 18 < FTp[1]) 14! 18

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The Chord graph

EssenceEach peer represented by a vertex; if FTp[i] = j , add arc h

�!i , ji,

but keep directed graph strict.

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Chord: path lengths

ObservationWith dn

2 (i , j) = min{|i� j |,n� |i� j |}, we can see that every peeris joined with another peer at distance 1

2n, 14n, 1

8n, . . . ,1.

7.5

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ath

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Network�size�(x�1000)

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Chord: degree distribution

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Chord: clustering coefficient

0.12

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NoteCC is computed over undirected Chord graph; x-axis showsnumber of 1000 nodes.

32 / 53 © Babaoglu Complex Systems

Data location in structured networks:Key-Based Routing

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■ Structured networks■ Advantages:

▴ completely decentralized (no need for superpeers)▴ routing algorithm achieves low hop count for large network sizes

■ Disadvantages:▴ each object must be tracked by a different node▴ objects are tracked by unreliable nodes (which may disconnect)▴ keyword-based searches are more difficult to implement than with

superpeers (because objects are located by their objectid)▴ the overlay must be structured according to a given topology in order to

achieve a low hop count▴ routing tables must be updated every time a node joins or leaves the

overlay


Effects of Churn

■ Churn can have several effects on a P2P system:■ data objects may be become unavailable if all replicas disconnect■ routing tables may become inconsistent

(e.g., entries may point to disconnected nodes)■ the overlay may become partitioned if many nodes suddenly

disconnect:


Churn - Preventing Partitions

■ A naïve approach to preventing partitions is to increase the average node degree

■ Ring partitions can be avoided by keep a list of successor nodes

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Churn Tolerance

■ Node arrivals and departures must not disrupt the normal behavior of the system■ system invariants must be maintained

▴ connected overlay (i.e., no partitions), low average path length▴ data objects accessible from anywhere in the network

■ two types of churn tolerance:▴ dynamic recovery: ability to react to changes in the overlay to maintain

system invariants (e.g., heal partitions)▴ static resilience: ability to continue operating correctly before adaptation

occurs (e.g., route messages through alternate paths)


Security

■ Security in P2P systems is hard to enforce:■ Users have full control of their computers■ Modified clients may not follow the standard protocol■ Data may be corrupted■ Private data stored on remote computers may disclosed

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Security - Weak identities

■ The user may leave the system and rejoin it with a new identity (different user id)

■ If an attack is detected, the attacker can re-enter the system with a new id

■ An attacker may create a large number of false identities (Sybil attack)

A

S1 S2

S3

S4S5

S6

Example of Sybil attack:

- Nodes S1 to S6 are actually 6 instances of the P2P client running on the same machine

- The attacker can intercept all traffic coming from or going to node A


Security - Strong identities

■ The user cannot change its identity■ Solution: use a centralized, trusted Certification Authority

(CA)■ Each new user must obtain an identity certificate■ The certificate is digitally signed by the CA, whose public key is

known by all users■ A certificate cannot be forged (require the CA’s private key)■ To prove his identity, a user signs a message with his private key,

and attaches the corresponding certificate signed by the CA■ Strong identities prevent Sybil Attacks■ If an attacker is caught, it cannot easily rejoin the system

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Security - Weak vs. Strong identities

■ Strong identities require a centralized CA■ New nodes must contact the CA before joining the network:

▴ The CA response may be slow▴ If the CA is unavailable, new nodes cannot join

■ The security of the system depends on CA:▴ The CA must correctly verify the identity of the requester▴ The CA’s private key must be secret

■ Many P2P systems use weak identities■ IP addresses already gives some identity information■ Some systems ensure anonymity

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introduction to peer-to-peer systems

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