Temporal-DHT and its Application in P2P-VoD

Systems

Abhishek Bhattacharya, Zhenyu Yang & Shiyun Zhang

Roadmap

•Introduction•Indexing•Buffer Management•Content Distribution•Results•Summary

Introduction• Peer-to-Peer (P2P) applications gained

prominence due to decentralized/self-organizing behavior, scalability, tolerance dynamics (churn/flash crowd).

• P2P-based multimedia streaming services poses challenges that are different from file-sharing applications.

• Live Streaming applications already popular with Internet-scale deployments (PPLive, CoolStreaming, Uusee, etc.)

• Video-on-Demand (VoD) systems present unique challenges with asynchronous/dynamic user interactivity.

Introduction

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• Content Discovery: Tracking Server Decentralized Indexing

Structures

• Content Distribution: Overlay Tree/Multi-Tree/Mesh

Introduction• Current P2P-VoD systems are classified into 2

categories:

Cache-and-Relay: indexing based on playing position.

Content Dissemination based on playing position proximity

Smooth and easy content availability from parent

Not resilient to asynchronous jumps and peer dynamics (leave/failure)

High streaming efficiency due to seamless in-order playback continuity

Dynamic/In-Order Caching

Introduction Static-Cache: indexing based on a static

distributed storage contributed by all the peers and are independent of playing position.

Content dissemination is independent of playing position.

Frequent parent change: after each segment playback.

Increased resiliency to peer dynamics and efficient support for random access patterns.

Avoids exploiting playing position proximity thereby reduces streaming efficiency.

Out-of-Order/Static Caching.

Introduction• Distributed Hash Tables (DHT) are stable

substrates for P2P based applications with decentralized operations.

• DHT s are efficient for indexing static data like file-sharing applications where the content remains same throughout peer lifetime.

• DHTs are efficiently used by Static-Cache based systems since the cached segments remain constant.

• DHTs are unable to efficiently handle data with temporal dynamics (Cache-and-Relay based systems) thereby invoking high update overheads.

Indexing• Temporal-DHT(t-DHT) for dynamic indexing with

respect to playing position.

• t-DHT augments the generic DHT interface for indexing dynamic content by modifying query resolution and indexing record structure.

• t-DHT reduce update overhead by using lazy updation with a coarser granularity of periodicity.

• t-DHT estimates the playing position by exploiting predictive temporal dynamics of the content.

Indexing: t-DHT semantics

Generic DHT indexing record: < pi , cj >

where pi is the peer currently hosting the content cj

t-DHT indexing record: < pi , cj , TTL>

where TTL is the time-to-live for the particular indexing record.

t-DHT also utilizes a publish interval (T) which signifies the periodicity of the lazy updates. T is a design choice and can be defined as a multiple of playback data rate.

Indexing: Query Resolution …Ci+1Ci Ci+2 Ci+z… … …Ci Ci

Ci

T

Range Query Reformulation in t-DHT

Given the size of playback buffer k, the position of representative segment r, the publish interval z, a peer that searches for dynamic segment ci needs to perform a range query for segments in [ci-k+r-z , ci+r-1] ∩ [c1 , cM]Detailed proof in the paper as Theorem IV.1

Given the size of playback buffer k, the position of representative segment r, the publish interval z, and the target segment ci , an index record of < p , c , TTL> can be filtered out of if id(c)+k-r+z-TTL < i or id(c)-r+1+z > i. Proof in the paper as Theorem IV.3

Indexing: Query Resolution

Indexing A semantic overlay is layered on top of DHT network for allowing easy and efficient in-order access.

In-order consecutive segments are highly correlated with high access probabilities.

Each t-DHT peer maintains content predecessor/successor pointers which link to the next and previous in-ordered segments respectively in the stream.

The range query is accelerated with the help of these links and also lowers messaging cost instead of invoking multiple exact-match DHT queries.

Indexing Dynamic Indexing is implemented by t-DHT augmented semantics with each record as < pi , cj , TTL> .

Indexing based on playing position and thereby updating the t-DHT after every publish interval. Query resolution is performed by range query reformulation.

Static Indexing is also implemented by t-DHT only but mostly retains generic DHT operations with each record as < pi , cj , >.

Indexing is independent of playing position and the records are constant with no updations required. Query resolution is performed similar to exact-match generic DHT routing.

Buffer Management Continuous playback achieved by dynamic caching and random access by static caching are conflicting features.

t-DHT organizes an integrated approach by utilizing static and dynamic caching in a single framework.

Buffer is divided into 2 parts: static and dynamic. Static buffer contents remain same but dynamic buffer contents keep changing.

Temporal-DHT based Mesh (TDM) is constructed on top of static-dynamic buffers by forming the content linkage pointers and parent-child pointers.

Buffer Management The static buffer is filled up by randomly downloading b segments from other peers or server.

The static buffer contents are published to the t-DHT by a one-time post operation with the format for static indexing records.

The dynamic buffer is filled up by caching k segments around the current playing position.

The dynamic buffer contents are published to the t-DHT by a chosen representative segment and performs continuous update operations with a certain interval.

Content Distribution VoD users frequently perform random seeks during the initial period after joining and then either leaves the system or stabilizes to continuous in-order playback mode [15].

TDM is motivated from the above observation by dividing the users duration into 2 modes: random seek and continuous playback.

TDM follows adaptive content distribution by associating randomized and synchronous dissemination.

Continuous playback mode is handled by the construction of an overlay tree after the peer enters this mode and the distribution follows along the synchronous parent-child pointers.

Content Distribution After joining the system, each VoD peer is placed into random access mode where static indexing/querying is used for content location/distribution.

Decision on transition from random access mode to continuous playback mode!

TDM relies on user workload profiling to perform seamless transition from random to continuous mode.

After entering the continuous mode, VoD peer joins an overlay tree and streams content always from tree parent. This process avoids unnecessary parent search-switch.

18

Results

Summary We propose Temporal-DHT which is a novel augmentation to generic DHT for indexing contents with temporal dynamics.

Temporal-DHT applies lazy updates to reduce update overhead and query reformulation/TTL filtering techniques for query resolution.

An integrated static/dynamic caching and buffer management mechanism is presented to efficiently harness the advantages of both the approaches.

Adaptive Content Distribution is utilized by exploiting user request pattern to efficiently support random and continuous content access within a single framework.

Please send all your questions to: abhat002@cis.fiu.edu