1 von: a scalable peer-to-peer network for virtual environments shun-yun hu ( 胡舜元 )...
TRANSCRIPT
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VON:A Scalable Peer-to-Peer Network
for Virtual Environments
Shun-Yun Hu (胡舜元 ) ([email protected])
CSIE, National Central University, Taiwan
2005/10/19
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Outline
Introduction Voronoi-based Overlay Network (VON) Simulation Results Analysis Conclusion
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What is Networked Virtual Environment (NVE)?
Virtual Reality + Internet
3D environment with people (avatar), objects, terrain, agents
Military simulations (’80) Massively Multiplayer Online Games (mid-‘90)
Trends: larger scale, more realistic simulation
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NVE: A Shared Space
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Issues for Creating NVE
Consistency (events/states)
Responsiveness multiplayer Security
Scalability Persistency massively multiplayer Reliability (Fault-tolerance)
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The Scalability Problem
Many nodes on a 2D plane ( > 1,000) Message exchange with those within Area of Interest (AOI) How does each node receive the relevant messages?
Area of Interest
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A simple solution (point-to-point)
N * (N-1) connections ≈ O(N2) Not scalable!
Source: [Funkhouser95]
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A better solution (client-server)
Message filtering at server to reduce trafficN connections = O(N) server is bottleneck
Source: [Funkhouser95]
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Current solution (server-cluster)
Still limited by servers. Expansive to deploy & maintain.
Source: [Funkhouser95]
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Scalability Analysis
Scalability constrains Computing resource (CPU) Network resource (Bandwidth)
Non-scalable system vs. Scalable system
x: number of entities
y: resource consumption at the limiting system component
Resource limit
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Server-cluster issues
Insufficient total resourceHardware provisioning over-provision!
High user density (crowding)User limits limits scale & realism!
Excessive inter-server communicationsLess load balancing difficult balance!
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What Next?
Strategies Increase resource More servers Decrease consumption Message filtering
Architectures Scale Point-to-point (LAN) tens 10^1 Client-server hundreds 10^2 Server-cluster thousands 10^3 ? millions 10^6 …
Peer-to-Peer
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What is Peer-to-Peer (P2P)?
[Stoica et al. 2003] Distributed systems without any centralized control
or hierarchical organization Runs software with equivalent functionality
Examples File-sharing: Napster, Gnutella, eDonkey Distributed Search: Chord, CAN, Tapestry, Pastry VoIP: Skype
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Peer-to-Peer Overlay
A P2P overlay network source: [Keller & Simon 2003]
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Promise & Challenge of P2P
Promises Growing resource, decentralized
Scalable Commodity hardware Affordable
Challenges Topology maintenance dynamic join/leave Efficient content retrieval no global knowledge
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Issues for Creating P2P NVE
Consistency (events/states)
Responsiveness multiplayer Security
Scalability Persistency massively multiplayer Reliability (Fault-tolerance)
Consistency (topology) P2P NVE
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Related Work (1):
DHT-based: SimMUD
[Knutsson et al. 2004] (UPenn)
Pastry + Scribe Regions Coordinators
(super-nodes)
Fixed-size region Relay overhead
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Related Work (2):
Neighbor-list Exchange
[Kawahara et al. 2004] (Univ. of Tokyo)
Fully-distributed Nearest-neighbors List exchange
High transmission Overlay partition
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Related Work (3):
Mutual Notification: Solipsis
[Keller & Simon 2003] (France Telecomm R&D)
Links with AOI neighbor Mutual cooperation Inside convex hull
Potentially slow discovery Inconsistent topology
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Outline
Introduction Voronoi-based Overlay Network (VON) Simulation Results Analysis Conclusion
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Design Goals
Observation: for virtual environment applications, the contents we want
are messages from AOI neighbors Content discovery is a neighbor discovery problem
Solve the Neighbor Discovery Problem in a fully-distributed, message-efficient manner.
Specific goals: Scalable Limit & minimize message traffics Responsive Direct connection with AOI neighbors
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Voronoi Diagram
2D Plane partitioned into regions by sites, each region contains all the points closest to its site
Can be used to find k-nearest neighbor easily
Neighbors
Site
Region
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Design Concepts
Identify enclosing and boundary neighbors Each node constructs a Voronoi of its neighbors Enclosing neighbors are minimally maintained Mutual collaboration in neighbor discovery
Circle Area of Interest (AOI)
White self
Yellow enclosing neighbor (E.N.)
L. Blue boundary neighbor (B.N.)
Pink E.N. & B.N.
Green AOI neighbor
L. Green unknown neighbor
Use Voronoi to solve the neighbor discovery problem
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Procedure (JOIN)
1) Joining node sends coordinates to any existing node
Join request is forwarded to acceptor
2) Acceptor sends back its own neighbor list
joining node connects with other nodes on the list
Acceptor’s region Joining node
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Procedure (MOVE)
1) Positions sent to all neighbors, mark messages to B.N.
B.N. checks for overlaps between mover’s AOI and its E.N.
2) Connect to new nodes upon notification by B.N.
Disconnect any non-overlapped neighbor
Boundary neighbors
New neighbors
Non-overlapped neighbors
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Procedure (LEAVE)
1) Simply disconnect
2) Others then update their Voronoi
new B.N. is discovered via existing B.N.
Leaving node (also a B.N.) New boundary neighbor
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Dynamic AOI
Crowding within AOI can overload a particular node
It’s better if AOI-radius can be adjusted in real time
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Adjustment Conditions
AOI-radius decrease Number of connections > connection limits
AOI-radius increase Maximum connections not exceeded Current AOI-radius < preferred AOI-radius
Mutual awareness rule Do not disconnect a neighbor who sees me
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Demonstration
Simulation video General movements (40 nodes, 800x600 world) Local vs. global view Dynamic AOI adjustment
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Outline
Introduction Voronoi-based Overlay Network (VON) Simulation Results Analysis Conclusion
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Simulation Method C++ implementation of VON (open source VAST library)
World size: 1200 x 1200, AOI: 100 Trials from 100 – 1000 nodes Connection limit per node: 20 1000 time-steps
(~ 100 simulated seconds, assuming 10 updates/seconds)
Behavior model Random movement: random destination Constant velocity: 5 units/step Movement duration: random (until destination is reached)
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Consistency Metrics
Topology Consistency [Kawahara, 2004]
observed AOI neighbors
actual AOI neighbors
Drift Distance [Diot, 1999]Distance between observed position and actual position
(average over all nodes)
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Scalability: Avg. Transmission / sec
0
2
4
6
8
10
12
14
16
0 200 400 600 800 1000Number of Nodes
Siz
e (
kb
)
basic
dAOI
basic (fixed density after 500 nodes)dAOI (fixed density after 500 nodes)
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Scalability: Max. Transmission / sec
0
10
20
30
40
50
60
0 200 400 600 800 1000Number of Nodes
Siz
e (
kb
)
basic
dAOI
basic (fixed density after 500 nodes)dAOI (fixed density after 500 nodes)
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Scalability: Avg. Neighbor Size
0
5
10
15
20
25
30
35
40
45
50
0 200 400 600 800 1000Number of Nodes
Ne
igh
bo
r S
ize
connected neighbors (basic)
AOI neighbors (basic)
connected neighbors (dAOI)
AOI neighbors (dAOI)
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Consistency: Topology Consistency
99.90
99.91
99.92
99.93
99.94
99.95
99.96
99.97
99.98
99.99
100.00
100 200 300 400 500 600 700 800 900 1000Number of Nodes
To
po
log
y C
on
sis
ten
cy
(%
)
basicdAOI
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Consistency: Drift Distance
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
100 200 300 400 500 600 700 800 900 1000
Number of Nodes
Ave
rag
e D
rift
Dis
tan
ce
basic
dAOI
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Reliability: Effects of Packet Loss
0
10
20
30
40
50
60
70
80
90
100
0% 20% 40% 60% 80% 100%Loss Rate
Un
its
Topology Consistency (%)
Recovery Steps
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Outline
Introduction Voronoi-based Overlay Network (VON) Simulation Results Analysis Conclusion
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Analysis of Design
Scalability Bounded resource consumption dynamic AOI
Consistency (Topology) Topology is fully connected enclosing neighbors
Reliability Self-organizing distributed neighbor discovery
Responsiveness Lowest latency direct connection, no relay
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P2P NVE Comparisons
DHT-based
Neighbor-list exchange
Solipsis VON
Consistency (topology)
DHT & Supernode
(consistent)
Neighbor list-exchange
(partitioning)
Neighbor notify&query
(undiscovery)
Neighbor notify
(consistent)
Responsive-ness
two to many
One hop One hop One hop
Scalability O(n) on supernode
Constant in crowding
Constant if fixed density
Constant in crowding
Con Latency too high
Overlay partitioning
Occasional undiscovery
Circular node line-up
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Problems of Voronoi Approach
Message traffic Circular round-up of nodes Redundant message sending
(inherent to fully-distributed design)
Incomplete neighbor discovery Can happen with inconsistent / incorrect neighbor list Fast moving node
Limited AOI Direct connections
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Outline
Introduction Voronoi-based Overlay Network (VON) Simulation Results Analysis Conclusion
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Summary
NVE scalability is achievable with P2P architecture and is a neighbor discovery problem
A promising solution: Voronoi-based P2P Overlay Leverage knowledge of each peer to maintain topology
Properties Scalable: fully-distributed, dynamic AOI Efficient: low irrelevant messages, zero relay Simple: simple protocol and procedure
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Potential Applications
Online gamesPosition updates in current MMOGs, Voice-chats
MilitaryEnable large-scale, affordable military training simulation
3D WebProvide multi-user interactivity to static 3D world
Scientific simulationsDistribute spatial simulation requiring frequent synchronization
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Future Perspectives
Short-term Distributed event/state consistency Customizable AOI (Heterogeneity in P2P) Recovery from overlay partition and fast-moving nodes
Long-term Persistency issue (P2P-based database) Security issue (protection from malicious nodes) 3D content distribution (3D streaming on P2P)
Massive, persistent 3D environment sharable by all!
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Acknowledgements Dr. Jui-Fa Chen (陳瑞發老師 ) Tsu-Han Chen (鄭子涵 ) Members of the Alpha Lab, TKU CS
Dr. Chin-Kun Hu (胡進錕老師 ) Guan-Ming Liao (廖冠名 ) LSCP, Institute of Physics, Academia Sinica
Joaquin Keller (France Tele. R&D, Solipsis) Jon Watte (there.com) Kuan-Ta Chen (陳寬達 , NTU)
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Inconsistency caused by dAOI
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Topology Consistency in NLE
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Slow Discovery in Solipsis