P2P Media Streaming - Amir H. Payberah · Apr. 19, 2012 P2P Media Streaming Challenges •Data...

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1 Apr. 19, 2012  Amir H. Payberah - P2P Media Streaming P2P Media Streaming Amir H. Payberah ([email protected])

Transcript of P2P Media Streaming - Amir H. Payberah · Apr. 19, 2012 P2P Media Streaming Challenges •Data...

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1Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

P2P Media Streaming

Amir H. Payberah ([email protected])

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Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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3 Amir H. Payberah ­ P2P Media Streaming

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Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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4Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

Introduction

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Media Streaming

•Media streaming is multimedia that is sent over a network and played as it is being received by end users.

•Users do not need to wait to download all the media.

•They can play it while the media is being delivered by the provider.

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Media Streaming

•Live Media Streaming The streams are only available at a particular instant

in time.

•Video on Demand (VoD) The streams are stored on a server and are available to be transmitted 

at a user's request.

It provides a large subset of VCR functionality, e.g., pause, fast forward, fast rewind and ...

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Media Streaming Trend

Cisco's global consumer Internet traffic forecast

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 Solutions for Media Streaming

•Client­Server solution

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Client – Server

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Client – Server

•What is the problem with the Client-Server model for media streaming? [d]

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11 Amir H. Payberah ­ P2P Media Streaming

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Client – Server

•What is the problem with the Client-Server model for media streaming?

•Scalability•Single point of failure

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Client – Server

•What is the problem with the Client-Server model for media streaming?

•Scalability•Single point of failure

•Providing a scalable service, which is resistant to failure is very expensive.

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13 Amir H. Payberah ­ P2P Media Streaming

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Client – Server

Distributed serversContent Delivery Network (CDN)

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 Solutions for Media Streaming

•Client­Server solution

•Peer­to­Peer solution

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Peer­to­Peer

•The peers can help each other.

•The peers who have parts of the data can forward it to other requesting peers.

•The capacity increases with the number of peers.

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16 Amir H. Payberah ­ P2P Media Streaming

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Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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17Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

P2P Media Streaming

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P2P Media Streaming Challenges

• Data should be received with respect to certain timing constraints. A negligible startup delay Smooth playback A negligible playback latency (only for Live Streaming)

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P2P Media Streaming Challenges

• Data should be received with respect to certain timing constraints. A negligible startup delay Smooth playback A negligible playback latency (only for Live Streaming)

• Nodes join, leave and fail continuously. Called churn

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20 Amir H. Payberah ­ P2P Media Streaming

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P2P Media Streaming Challenges

• Data should be received with respect to certain timing constraints. A negligible startup delay Smooth playback A negligible playback latency (only for Live Streaming)

• Nodes join, leave and fail continuously. Called churn

• Network capacity changes.

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P2P Media Streaming Challenges

• Data should be received with respect to certain timing constraints. A negligible startup delay Smooth playback A negligible playback latency (only for Live Streaming)

• Nodes join, leave and fail continuously. Called churn

• Network capacity changes.

• Free­riding problem.

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P2P Media Streaming Challenges

• Data should be received with respect to certain timing constraints. A negligible startup delay Smooth playback A negligible playback latency (only for Live Streaming)

• Nodes join, leave and fail continuously. Called churn

• Network capacity changes.

• Free­riding problem.

• Connectivity Problem. NAT problem.

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Main Questions

•What type of overlay topology is useful for data dissemination?

•What algorithm is used for data dissemination?

•How do we construct and maintain this target overlay topology?

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Main Questions

•What type of overlay topology is useful for data dissemination?

•What algorithm is used for data dissemination?

•How do we construct and maintain this target overlay topology?

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Data Dissemination Overlay

•What overlay topology do we build to distribute data messages?

•Some possibilities include: Single tree Multiple tree Mesh

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Single Tree Structure

• Build a single multicast tree, in which the root is the media source and the interior nodes and leaves are peers.

Media Source

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Single Tree Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Single Tree Advantage/Disadvantage?

•Advantage The short latency of data delivery Easy to implement

•Disadvantage The fragility of the tree structure upon the failure of nodes close to the 

root All the traffic is only forwarded by the interior nodes

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Multiple­Tree Structure

• The media source splits the stream into a set of sub­streams.

• A single tree is created for each sub­stream.

• A peer to receive the whole media should join all trees.

1 2 3 4 5 6

1 3

Media Server

5

2 4 6

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Multiple­Tree Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Multiple­Tree Advantage/Disadvantage?

•Advantage Resilient to node failure Good load balancing

•Disadvantage Difficult to implement If a node fails, the sub­tree rooted at that node does not receive data, 

while they rejoin the system again

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Mesh­based Structure

• The media source into small blocks.

• Nodes are connected in a mesh­network.

• Nodes periodically exchange their buffer maps.

1 2 3 4 5 6

1

3

5

2

46

Media Server

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Mesh Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Mesh Advantage/Disadvantage?

•Advantage Resilient to node failure Good load balancing Easy to implement

•Disadvantage Unpredictable latencies due to the frequent exchange of notifications 

and requests

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Main Questions

•What type of overlay topology is useful for data dissemination?

•What algorithm is used for data dissemination?

•How do we construct and maintain this target overlay topology?

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Data Dissemination Algorithms

•How to distribute data messages.

• It could be: Push­based Pull­base Push­Pull­based

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Push­based Data Dissemination

•A node actively pushes a received block to its neighbours.

•Mostly used in tree­based overlays.

•What about mesh­based overlays? [d]

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Push­based Data Dissemination

•A node actively pushes a received block to its neighbours.

•Mostly used in tree­based overlays.

•What about mesh­based overlays? Redundant messages: a node might blindly push a block to a node 

already has that block.

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Pull­based Data Dissemination

•Nodes periodically exchange data availability (buffer maps).

•After receiving a buffer map, a node can decide and schedule to pull which block from which node.

•Mostly used in mesh­based overlays.

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Pull­based Data Dissemination

•Nodes periodically exchange data availability (buffer maps).

•After receiving a buffer map, a node can decide and schedule to pull which block from which node.

•Mostly used in mesh­based overlays.In order

Rarest firstHybrid

Will be back to this

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Push­Pull­based Data Dissemination

• Usually blocks are pushed through a tree and missed blocks are pulled from the mesh neighbours.

Media Source

Page 42: P2P Media Streaming - Amir H. Payberah · Apr. 19, 2012 P2P Media Streaming Challenges •Data should be received with respect to certain timing constraints. A negligible startup

42 Amir H. Payberah ­ P2P Media Streaming

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Main Questions

•What type of overlay topology is useful for data dissemination?

•What algorithm is used for data dissemination?

•How do we construct and maintain this target overlay topology?

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43 Amir H. Payberah ­ P2P Media Streaming

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The Overlay Construction and Maintenance

•How to build and maintain the data distribution overlay.

•Using the control messages for this purpose.

• It could be: Centralized Hierarchical DHT­based Control flooding Gossip­based

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Apr. 19, 2012

Centralized Method

Media Server Central Server

1

2

3

4

New Node

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Centralized Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Centralized Advantage/Disadvantage?

•Advantage Fast Easy to apply optimization methods Easy to implement

•Disadvantage Not scalable Single point of failure

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Hierarchical Method

Layer 3

Layer 2

Layer 1

1

2

4

New Node

3

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Hierarchical Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Hierarchical Advantage/Disadvantage?

•Advantage Scalable No single point of failure

•Disadvantage Slow convergence Difficult to implement

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DHT­based Method

Media Server

New Node

1

2

3

4

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DHT­based Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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DHT­based Advantage/Disadvantage?

•Advantage Scalable No single point of failure

•Disadvantage Difficult to implement

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Controlled Flooding Method

New node

ttl=2

ttl=1

ttl=1

OK

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Flooding Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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Flooding Advantage/Disadvantage?

•Advantage Scalable No single point of failure

•Disadvantage No guarantee to find supplier node Slow convergence

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Gossip­based Method

OK

• Peers periodically send their data availability to their neighbours.

Request / Reply

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Apr. 19, 2012

Gossip­based Advantage/Disadvantage?

•Advantage/Disadvantage [d]

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58 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Gossip­based Advantage/Disadvantage?

•Advantage Scalable No single point of failure Easy to implement

•Disadvantage No guarantee to find supplier node in time

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59 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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60Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

Classification of P2P Streaming Solutions

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61 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Related Work

• SplitStream• DONet/Coolsteraming• CoopNet• Orchard• Bullet• Prime• Pulsar• NICE• Zigzag• DirectStream• MeshCast

• mtreeBone• PULSE• GnuStream• SAAR• ChainSaw• ChunkySpread• BulkTree• ForestCast• AnySee• DagStream• Climber

• CollectCast• HyMoNet• GridMedia• Promise• Yoid• Zebra• Tribler• CliqueStream• GradienTv• Sepidar• GLive

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62 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Data Dissemination Overlay

•Data dissemination: Push – Single­tree Push – Multiple­tree Pull – Mesh Push­Pull

•Overlay maintenance: Centralized Hierarchical DHT­based Control flooding Gossip­based

20 different combinations

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63 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Data Dissemination Overlay 

Push (Single Tree) Push (Multiple-tree) Pull (Mesh) Push-Pull

CoopnetForestCast

ZebraSpliStream

SAAROrchard

ChunkySpread

DirectedStreamHyMoNet

YoidNice

ZigZagClimberSAAR

SepidarGradienTv

PrimePulsar

CliqueStreamBullet

NewCoolStreamingMtreeboneGridMedia

GLiveBulkTree

CollectCastPromiseSAAR

GnuStreamCoolStreaming

PulseChainsawMeshCast

TriblerDagstream

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64 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Overlay Construction and Maintenance Methods

Centralized

Hierarchical

DHT-based

Flooding

Gossip-based

DirectedStream, HyMoNet, Yoid, CoopNetForestCast, Zebra, Prime

GLive, Sepidar, GradienTv, Orchard, ChunkySpread, CoolStreaming, Pulse, ChainsawMeshCast, Tribler, DagStream, Bullet, mTreebone, GridMedia

NICE, ZigZag, Climber, BulkTree, Prime

SAAR, SplitStream, CollectCast, Promise, CliqueStream, Pulsar

GnuStream

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65 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

All Together

Push (Single tree) Push (Multiple-tree) Pull (Mesh) Push-Pull

Centralized

Hierarchical

DHT-based

Flooding

Gossip-based

Glive - CoolStreaming – Pulse - Chainsaw – MeshCast - Tribler -

DagStream

Sepidar - GradienTvBullet - mTreebone

GridMedia

CoopnetForestCast

Zebra

DirectedStreamHyMoNet

Yoid

Prime

Prime

PulsarCliqueStream

OrchardChunkySpread

NICEZigZagClimber

SAAR SAARSplitStream

SAARCollectCast

Promise

GnuStream

BulkTree

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66 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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67Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

Security in P2P Streaming Systems

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68 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Free­riding Problem

• Free­riders are the nodes that uses the resources in the system, without contributing in data distribution.

• Incentivzing mechanism Tit­for­tat Transitive auditing

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69 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Collusion

• The attacks can exacerbate by collusion.

• A collection of nodes conduct correlated attack.

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70 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Forgery and Repudiation attacks Forgery: fabricating or tampering data stream. Repudiation: denying the received data stream or to acknowledge with 

false information.

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71 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Forgery and Repudiation attacks Forgery: fabricating or tampering data stream. Repudiation: denying the received data stream or to acknowledge with 

false information.

• Pollution attacks Mixing or substituting junk data into the stream.

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72 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Forgery and Repudiation attacks Forgery: fabricating or tampering data stream. Repudiation: denying the received data stream or to acknowledge with 

false information.

• Pollution attacks Mixing or substituting junk data into the stream.

•Membership and Eclipse attacks Compromising the underlying overlay or membership protocol, e.g., the 

routing mechanism.

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73 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Neighbour selection attacks Controlling the neighbour selection mechanism of some nodes.

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74 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Neighbour selection attacks Controlling the neighbour selection mechanism of some nodes.

• Sybil attacks Used when the reputation mechanism established in a P2P system. Creating a large number of entities, which bear the same disguised 

identifier.

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75 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Neighbour selection attacks Controlling the neighbour selection mechanism of some nodes.

• Sybil attacks Used when the reputation mechanism established in a P2P system. Creating a large number of entities, which bear the same disguised 

identifier

• DoS attacks Sending excessive amount of requests and ...

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76 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

• Neighbour selection attacks Controlling the neighbour selection mechanism of some nodes.

• Sybil attacks Used when the reputation mechanism established in a P2P system. Creating a large number of entities, which bear the same disguised 

identifier

• DoS attacks Sending excessive amount of requests and ...

• Omission attacks Not sending the data according to the protocol. Other extreme than DoS attack.

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77 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Common Attacks in P2P Streaming Systems

Attack Target

Forgery Data

Pollution Data

Eclipse Overlay, Protocol

Neighbor Protocol

Sybil Protocol

DoS Peers

Omission Peers, Data

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78 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Outline

• Introduction

•P2P media streaming

•Classification of P2P streaming systems

•Security in P2P streaming systems

•Sepidar/GLive – two P2P streaming systems

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79Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

Sepidar

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80 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Problem Description (1/5)

• Building and optimizing a P2P overlay for live media streaming minimize playback latency improve timely delivery of the stream

• The media stream is split into a number of sub­streams or stripes.

• A node can create a bounded number of download connections, and accept a bounded number of upload connections.

Download slot

Upload slot

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81 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Problem Description (2/5)

• In order to provide the full media to all the nodes every download­slot needs to be assigned to an upload­slot. download­slots at a node must download different stripes.

• This problem can be defined as an assignment problem.

Child

Parent

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82 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Problem Description (3/5)

• A connection between a download­slot i and an upload­slot j for a stripe k is associated with a cost cijk, which is the number of hops from the owner of the upload­slots j, to the media source for the stripe k.

• A complete assignment, A,  is an assignment that each download­slot is assigned to an upload­slot.

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83 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Problem Description (4/5)

• Formulating as an optimization problem:

• Objective function We want to find a complete assignment over all the complete 

assignments that minimizes the total cost:

• Subject to Every download­slot is assigned to exactly one upload­slot. Each upload­slot is assigned to at most one download­slot. The download­slots owned by the same node download distinct stripes.

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84 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

• Centralized solution: Needs global knowledge. Possible for small system sizes.

• Our distributed market­based approach: Inspired by auction algorithms. Each node knows only a small number of nodes in the system (partial 

view).

Problem Description (5/5)

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85 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

•What overlay topology is built for data dissemination? Tree Multiple­tree Mesh

•What algorithm is used for data dissemination? Push Pull Push­Pull

•How to construct and maintain this overlay? Centralized DHT Gossip­based ...

Designing a P2P Media Streaming System

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86 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Multiple­Tree Structure

• The media source splits the stream into a set of sub­streams.

• A single tree is created for each sub­stream.

• A peer to receive the whole media should join all trees.

1 2 3 4 5 6

1 3

Media Server

5

2 4 6

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87 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model ­ Node Properties

• Currency: The the number of upload slots at a node.

• Price: The price of a node that has an unused upload slot is zero, otherwise the node’s price equals the lowest currency of its already connected children.

• Cost: The length of its path to the root. 

Currency = 3Price = 1Cost = 3

currency = 2 currency = 1 currency = 2

Source

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88 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model ­ Streaming Overlay Construction

• Our market model is based on minimizing costs through nodes iteratively bidding for upload slots.

• The depth of a node in each tree is inversely proportional to its currency.

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89 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model – Child Side

Currency: 3Cost: 5

Q:  Price: 1      Cost: 4

N:  Price: 2      Cost: 6

M:  Price: 5      Cost: 4

X:  Price: 3      Cost: 5

P

P

Q

N

MX

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90 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model – Child Side

Currency: 3Cost: 5

Q:  Price: 1      Cost: 4

N:  Price: 2      Cost: 6

M:  Price: 5      Cost: 4

X:  Price: 3      Cost: 5

P

P

Q

N

MX

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91 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model – Parent Side

Q

Request

P

Price: 1

Currency: 2 Currency: 3 Currency: 3 Currency: 1

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92 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Market Model – Parent Side

Q

P

Price: 1

Currency: 2 Currency: 3 Currency: 3 Currency: 3

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93 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Freerider Detector

• Freeriders are nodes that supply less upload bandwidth than claimed.

• The freerider detector component.

• Nodes identify freeriders through transitive auditing using their children’s children.

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94 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Detecting Freeriders

• F is the sum of the number of audit responses not received before a timeout. the number of negative audit responses. the free upload slots.

• If F is more than M% of claimed upload slots, Q is suspected as a freerider. 

• If Q becomes suspected in N consecutive iterations, it is detected as a freerider. 

• The higher the value of N, the more accurate but slower the detection is.

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95 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Freerider – Punishment (1/4)

Q

Price: 2

Currency: 2 Currency: 3 Currency: 2

Currency = 3

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96 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Freerider – Punishment (2/4)

Q

Currency: 2 Currency: 3 Currency: 2

Currency = 3

Price: 2 Price = 0

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97 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Freerider – Punishment (3/4)

Q

Currency: 2 Currency: 3 Currency: 2

Currency = 3

Price: 2 Price = 0

P

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98 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Freerider – Punishment (4/4)

Q

P

Price: 1

Currency: 2 Currency: 3 Currency: 1 Currency: 2

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99Apr. 19, 2012 Amir H. Payberah ­ P2P Media Streaming

Optimization

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100 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Node Discovery

• Naïve solution: nodes in partial views are selected randomly from all the nodes.

• Optimization: nodes use the Gradient overlay to construct and maintain their partial view of the system.

Random view 

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101 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

The Gradient Overlay

• The Gradient overlay is a class of P2P overlays that arranges nodes using a local utility function at each node, such that nodes are ordered in descending utility values away from a core of the highest utility nodes.

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102 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

A Peer Partners

Similar­view 

Finger 

• Rather than have nodes explore the whole system for better parents, the Gradient enables nodes to limit exploration to the set of nodes with asimilar number of upload slots.

Random view 

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Distributed Optimization of P2P Media Delivert – Amir H. Payberah 103Apr. 19, 2012

GLive

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104 Amir H. Payberah ­ P2P Media Streaming

Apr. 19, 2012

Shortcoming of Sepidar

• Tree structure

•Fragile in massive failures

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•What overlay topology is built for data dissemination? Tree Multiple­tree Mesh

•What algorithm is used for data dissemination? Push Pull Push­Pull

•How to construct and maintain this overlay? Centralized DHT Gossip­based ...

Designing a P2P Media Streaming System

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Mesh Overlay

• Divide he main stream into a small blocks.• Nodes are connected in a mesh­network.

1 2 3 4 5 6

1

3

5

2

46

Media Server

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The Market Model ­ Node Properties

• Currency: The total number of blocks uploaded to children during the last 10 seconds.

• Price: The price of a node that has an unused upload slot is zero, otherwise the node’s price equals the lowest currency of its already connected children.

• Cost: The length of its path to the root via its shortest path.

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The Market Model – Parent/Child Selection

•The same as Sepidar.

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•Each parent node periodically sends its buffer map and its load to all its assigned children.

•A child node, pull the required blocks using the received information.

Data Dissemination (1/2)

In orderRarest first

Hybrid

I'm back

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•Sliding window

Data Dissemination (2/2)

In­order set Rare set

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•Each child assigns a score to each of its parents, for a time window covering the last 10 seconds. 

•When a child requests and receives a non­duplicate block from a parent within the last 10 seconds, it increments the score of that parent.

•A node periodically sends a score request to its grandchildren.

Freerider Detection (1/2)

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•Threshold s to detect freeriders.

•When a node with no free upload connection receives a connection request, it sorts its children based on their latest scores.  If there exist children with score less than s, the lowest score child is 

abandoned. Otherwise, accepts if the new node offers more money than the lowest 

money of its existing children.

Freerider Detection (2/2)

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DONE!

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Summary

• Media Streaming Live VoD

• Client-Server model Expensive

• P2P model The peers can help each other and the capacity increases with the number

of peers.

• Challenges Time constraint Churn Connectivity Security

• Main questions What overlay topology? What algorithm for data dissemination? How to construct the topology

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Any Questions?