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Multi-Player Game --- Motivation

It is interesting.

It is hard and challenging. Some difficulties are,

lack technology to achieve global synchronization

long network latency

high packet loss rate

inconsistency of state view among participants

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Multi-Player Game --- Term

Entity

Participant

Player

Entity Server

Entities Entities

Participant A Participant B

NetworkPlayer 1

control

Player 2

control

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Why do we care about synchronization and consistency?

Multi-Player Game --- Motivation

Synchronization preserves temporal and causal

relationships among events happening at geographically

distributed sites.

Participant C

Participant B

Entity1

Entity2

Entity3

Entity 1 shoots at entity 2

at time 100ms. The

message travels 50ms to

reach entity 2 and 100ms

to reach entity 3.

Entity 2 shoots at entity 3at time 90ms. The message

travels 70ms to reach

entity1 and 40ms to reach

entity 1.

Who is dead?

Wild Wild West

Participant A

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Multi-Player Game --- Related Work

Distributed Computing and Simulation

MiMaze (a simple multi-player game based onIP/UDP/RTP protocol stack on the Mbone)

DIS Standard (IEEE distributed interactive simulation)

HAL

Games

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Multi-Player Game --- Related Work

Client-server Architecture

Advantage

Disadvantage

It is easy to achieve consistency. Each

participant can have the same state

view by receiving the same data

from the server.

Server may become bottleneck.Nobody can play if server crashes.

Participants are treated as dumb

terminals.

Participant Participant Participant

Participants sendactions to the server

and the server sends updated entity

states back for display.

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Multi-Player Game --- Related Work

Fully Distributed Architecture

Advantage

Disadvantage

It is easy to scale. No server

bottleneck.

Synchronization and consistency.

Participant

Participant

Participant

Participant

Participants talk with each other

directly (not through the server). Entity

states are updated in distributed places.

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Multi-Player Game --- Proposed Work

Three Observations

Require global reference to resolve conflict of stateupdate.

Continuous entity states are transmitted repeatedly. Only the most

recent ones matter. No retransmission of lost entity states (similarto real time audio/video).

High reliability and strong consistency are not required forcontinuous entity states.

However, global reference does not have to reside in a

centralized place.

Continuous game play should not be disruptedby player joining and leaving.

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Multi-Player Game --- Proposed Work

High speed LAN, multicast

Master serverConnection Servers

Entity server running

entities automatically

by using AI engine

TCPTCP

TCP TCPTCP

UDP

UDPUDP

UDP

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A new player joins in a session by contacting the master server. The master server

finds a less busy server for the new participant to connect.

The new participant connects with the server. The server transfer game state and

entity control to the new participant.

Participant sends interacting events to the involved participant via unreliable

unicast.

Participant sends state updates of local entities to other participants via unreliable

unicast or multicast.

Each entity has a owner. Only the

owner of an entity can change its

state. Every other participant

updates the entity state by

receiving information from its

owner (subscribe and publish).

Only participants (entity server is

a special kind of participant) own

entities.

Master server performs session

management.

Connection server assigns entity

control to a participant. It is also

responsible for reassigning

entities after their owner leaves a

session.

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Multi-Player Game --- Proposed Work

Dead reckoning

Dead reckoning reduces traffic and deals with loss of continuous

entity state messages. The technique computes an entitys state based

on its most recently received state.

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Multi-Player Game --- Proposed Work

Latency tolerance

Latency tolerance is used to hide network delay. A fixed amount

of delay is added to participants action.

Issue stop

command

at 200ms

Issue stop

command

at 230ms

300ms 330ms

transmission

A fixed delay 100ms is added to every action. Assume that

max one way transmission time + process overhead < 100ms

Participant

Participant

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Multi-Player Game --- Proposed Work

Dynamic entity subscribe and publish

P1

P2P3 P4

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P3 subscribes to some entity controlled by P1.

P2 sends an interaction event involving entity controlled by P1.

P1 updates entity state based on the received interaction event.

P1 sends new entity state to other subscribers.

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