P2VoD: Providing Fault Tolerant Video-on-Demand Streaming in Peer-to-Peer Environment

Tai T.Do, Kien A. Hua, Mounir A. TantaouiProc. of the IEEE Int. Conf. on Communications (ICC 2004)

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Outline

Introduction Related Work Architecture of P2VoD Performance Evaluation Conclusion

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Introduction

P2P approach can potentially solve many serious problems posed in existing streaming systems Infeasibility of IP Multicast Network bottleneck at the video server

Several projects on “live streaming” Not trivial to apply these techniques into VoD syst

ems

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Introduction (cont.)

Live streaming Shorter end-to-end delay, more lively the stream Short tree rooted at the video server User simply joins an on-going live streaming

session User may not quit if QoS degrades

VoD streaming Liveness is irrelevant, video is pre-recorded Whole video must be delivered User will stop watching if QoS degrades

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Introduction (cont.)

Proposed techniques not based on any existing live streaming systems

Problems to be solved for a P2P VoD streaming system Fast and localized failure recovery without jitter Quick join Effective handling of clients’ asynchronous request

s Small control overhead

P2VoD (Peer-to-Peer approach for VoD streaming)

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Related Work: P2Cast

Architecture uses a P2P approach to stream video using patching

Build application level multicast tree Server streams the entire video over base tree

P2Cast clients provide two functions Base stream forwarding Patch serving

Base tree construction & patch server selection algorithm Best Fit (BF): find the “fattest pipes” to requesting clients BF-delay: also use network delay information BF-delay-approx: not using actual available bandwidth, (1 or

0 represents enough bandwidth or not)

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Architecture of P2VoD

A streaming connection is assumed to constant bit-rate equals to the playback rate of the video

Retrieval block (R-block) as a data unit of the video equivalent to one unit of playback time

Each client has a buffer, whose maximum size is worth of MB units of playback time (i.e. MB R-block)

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Architecture of P2VoD (cont.)

abX : actual amount of buffer storage used by a client X 1 ≤ abX ≤ MB

By using the cache, early arriving client can serve late coming clients by forwarding the stream tjX : the joining time of a client X X can serve clients whose joining time

[tjX , tjX + abX ]

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P2VoD system

abC1 = MB = 5, abC4

= 3 < MB

When C6 arrives to the system at time 3, the first R-block of the video is still in the buffer of C1

C1 can serve the video stream to C6

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Architecture of P2VoD (cont.)

“Generation” Group of clients having the same smallest number

ed R-block in their caches

Generations are also numbered G1 as the oldest generation to Gn as the youngest

generation

Clients in these generations, excluding the server, form a video session A video session is closed

none of the clients has the first R-block of the video

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P2VoD system

C1, C2 , C3 , C4 , and C5 all have the same smallest number R-block

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Data Caching and Generation (cont.)

Rules to build generations Generations are indexed by numbers starting from 1 Members of a lower indexed generation arrive to the system

no latter than any member of other higher indexed generations

A peer in generation i-th (i > 1) receives the video stream from a peer in generation (i – 1)-th

Peers at the first generation receive the video stream directly from the server

When joining a video session of the system, a new peer belongs to the current highest numbered generation, OR be the first member of a newly created generation of that video

session

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Data Caching and Generation (cont.)

Caching scheme with the generation concept X and Y join the same generation at time tjX and tjY

Caching Rule: The difference of actual cache sizes used by two peers in the same generation must offset the difference in their arrival times

abX – abY = tjY – tjX

E.g. abC2 = abC1 – (tjC2 – tjC1) = 5 – (1 - 0) = 4

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Data Caching and Generation (cont.)

Assume that at time 36, peer C3 fails.

C1, C2, C4, C5 available to C8

allow a quick and localized recovery for C8

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Control Protocol

Control Protocol is required to maintain the system connectivity

X sends control information < addX, G(X), > to server when joining the system : expiration time when the first R-block of the vi

deo is no longer in the cache of X

Xtexp

Xtexp

Neighbors of peer X Siblings (peer in same G): need to keep a list of their IP address

es Periodically update the list Update the list on-demand (joining/leaving)

Child: agree to forward the video stream, X needs to send the IP addresses of X and its siblings to its child

Parent: first join the system, X sends the IP address to its parent

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Join Algorithm

Server S has the list of peers at the youngest generation for each video session, denoted as Gy

Expiration time is the same for every member of the youngest generation

Initially, when the system is empty is set to minus infinity Set of youngest generation peers contains only S

Gytexp

Gytexp

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Join Algorithm (cont.)

Case 1 If all of the existing video sessions are closed, X

will be admitted if server S still has enough outbound-bandwidth

Otherwise, X will be rejected

Case 2 For the case where there is at least one existing

video session still open, X will try to join that video session

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Join Algorithm (cont.)

Case 2 proceeds as follows. Step1

X contacts a random member of the Gy set

acquires the list of peers at the super generation of Gy.

Step2: If texp at the super generation > tjX, then go to Step3 Otherwise go to Step4

Step3: (generation not expired) X selects a parent from the super generation If such a candidate exists, X becomes a member of Gy

X starts receiving the video stream from its selected parent X sets its actual cache size abX to conform to the caching rule

Otherwise go to Step4

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Join Algorithm (cont.)

Case 2 proceeds as follows. Step4: (generation expired)

A new generation is formed below Gy

X becomes the first member of that generation Actual cache size abX is set to equal to MB

X selects a peer from Gy as its parent If no such peer exists, X tries other open video sessions

or contacts the server

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Parent Selection Criteria

Round Robin Selection promote the fairness among peers the requesting peer should select a peer, who hasn’t served

any client for the longest period of time Smallest Delay Selection

minimize the joining time of the requesting peer the requesting peer picks the first peer in the candidate

group, who has enough out-bound bandwidth Smallest Distance Selection

reduce the number of links involved in the underlying network

the requesting peer chooses a peer, who has enough out-bound bandwidth and closest to it

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Failure Recovery

P2VoD uses a two-phase failure recovery protocol

1) Detecting failure Grateful failures: When a peer leaves the system,

it forwards the LEAVE_MESSAGE to its children Unexpected failures: it happens unexpectedly

without any explicit warning, peers in P2VoD are required to monitor their incoming traffic

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Failure Recovery (cont.)

2) Recovering from failures Failure at a peer X, the whole sub-tree under X is

affected The rest of the sub-tree are informed to wait throu

gh the WAIT msg A disrupted peer X finds a new parent

X contacts the siblings of its parent If no such parent exists, X contacts the server Otherwise, X is rejected

X succeeds, the sub-tree is recovered X fails, the immediate children of X will invoke the

recovery process

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Performance Evaluation

Examine the effects of two parameters in P2VoD:

max number of clients allowed in the first generation of each video session, K

max buffer size each client can use, MB

Compare P2VoD with P2Cast Network topology using GT-ITM Clients arriving to system follow

the Poisson distribution one Transit network (with 4 nodes) 12 stub domains (with 96 nodes) backbone link support 25 streams edge link support 7 concurrent streams

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Performance Evaluation (cont.)

Rejection probability probability that a client tries to join the system but

can not get the service

Server stress amount of bandwidth used at the server, or the

number of streams established at the server

Number of contacts during the recovery number of nodes a client has to contact during the

recovery process

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Rejection probability for various maximum buffer sizes (K = 3) MB is varied from 0.1 to 0.4 of the length of the video When doubling size of MB, the rejection probability is reduced by half

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Server stress with various values of K (MB = 0.1)

In light workload, almost every clients get the video stream directly from the server

In heavy workload, most clients get the video stream from some other client, not the server

With small K, a failure at the first generation will likely cause disruption the whole video session

With large K, such failure only cause a portion of the

video session to be in

recovery mode

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P2VoD vs. P2Cast: Client rejection probability P2VoD use K =6 & MB = 0.2 The threshold for P2Cast is set at 0.2. P2VoD outperforms BF because P2Cast requires each client to obtain

two streams

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P2VoD vs. P2Cast: Server stress

In P2VoD, a video session is open as long as clients in the Gy still have the first block of the video buffer

In P2Cast, a video session is open for a short period of time starting when the first client join the video session and last for the length of the threshold

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P2VoD vs. P2Cast: Failure overhead

Clients arrive to the system at a rate of 0.4 per second during a period of 2 hours (3516 clients)

When the failure probability increases, the number of clients contacted during a recovery decreases

more clients leave the system, network bandwidth is also released easier for an affected client to find a new parent

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Conclusion

P2VoD, a new system for Video-On-Demand streaming in a Peer-to-Peer environment

the concept of generation and caching scheme to deal effectively with failures

An efficient control protocol help P2VoD to allow clients join the system fast, as well as to recover failures in a quick and localized manner

Simulation-based performance study shows that P2VoD is superior than P2Cast