Lecture 8Hybrid Architecture

for Games

15 March 2010

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singlecentralized

server

completelydecentralized

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P2P with Central Arbiter

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A trusted central arbiter receives all game traffic and resolves conflicts if necessary.

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Benefits ofP2P + Arbiter

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low latency among players

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have trusted authority now

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Drawbacks ofP2P + Arbiter

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limited scalability

arbiter becomes bottleneck

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Mirrored ServersArchitecture

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Multiple servers, each replicates the complete game states and serves clients from different geographical regions. Each client connects to one server.

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Servers may be connected with high speed, provisioned networked, reducing synchronization latency among the servers.

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Updates from a client are sent to its server, which then forwards it to other servers. The servers then forward the updates to all relevant clients.

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P2P-style state synchronization among servers is easier, since 1. servers can be trusted2. clocks can be synchronized3. IP multicast may be used4. higher availability5. shorter delay

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Benefits ofMirrored Servers

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no single point of failures

A client can use another replicated server if their nearest server fails.

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increase total network capacity

good if networking bandwidth is the bottleneck

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lower latency between client and server

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Drawbacks ofMirrored Servers

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latency between players increases

due to additional forwarding

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cannot scale to large world

each server still keeps the complete states

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Q: How to assign players to servers?

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Assigning players to closest servers may lead to overloaded servers.

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Assigning players to remote servers may increase end-to-end latency.

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But we only care about latency between players that are relevant to each other.

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Zoned ServersArchitecture

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Divide the game world into independent regions. One server is in-charged of one region.

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A client connects to the server serving its current region, and is handed-off to another server when it moves to another region.

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Servers seldom communicate with each other.

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Updates from a client are sent to its server, which then forwards it to all relevant clients in the same region.

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Advantages of Zoned Servers

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Can scale to large world

Just add more servers

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Drawbacks of Zoned Servers

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Single point of failures

(But can deploy mirrors for each region)

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Constrained Game Designs

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Supporting Seamless Game Worlds

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Divide the game world into regions. One server is in-charged of one region.

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Player may move around the world seamlessly

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A client may see events and objects from neighboring regions.

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Events occurring in one region may affectobjects in another region.

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Advantage: Partitioning is transparent to the players. No artificial constraints in game design.

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Advantage: We can now resize the regions to load-balance among the servers, improving server utilization.

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A client connects to the server serving its current region, and is handed-off to another server when it moves to another region.

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Server communicates with each other to transfer states, update states etc.

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Updates from a client are sent to its server, which then forwards it to all relevant clients in the same region or neighboring regions.

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or

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A client connects to the servers serving its current region and regions it subscribes to. Updates from neighboring server are sent directly.

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Q: How to partition the game world?

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Ideally: each server should handle the same

number of players

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Ideally: minimize communications between servers

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Ideally: minimize the number of hand offs

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Generic Gaming Proxies

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Proxy: trusted host providing specialized services for games

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Proxy: typically close to the players

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Examples of Proxy Services:

Time-stampingMessage Ordering

Interest Management

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Example 1:Time-stamping Services

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Timestamp Cheat inP2P Games

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Player generates fake, earlier timestamp to gain advantage after knowing the opponent's move.

(m1,t1)

(m2,t0)

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(m1,t1)

(m2,t2)

But such cheat is impossible if a trusted proxytimestamp the messages instead of the players.

m2

(m2,t2)(m2,t2)

m1

Proxy

Proxy

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(m1,t1,s)

(m2,t2,s)

The proxy can digitally signed the message to proof that he is the one that added the timestamp.

m2

(m2,t2)(m2,t2)

m1

Proxy

Proxy

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Timestamp Servers is generic and is game

independent

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Example 2:Message Ordering

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Client/Server Architecture

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Game is divided into rounds based on updatesfrom the server.

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Messages (action) from players are grouped into rounds. Reaction time is the time between receiving

message from server and sending an action.

Reaction Time

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We want to deliver messages to the server in the following order:

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1. for a player, messages are delivered in the order they are generated.

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2. messages from the same round from different players are delivered in increasing order of “reaction time”.

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Messages are not delivered according to reaction time.

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3. messages in a round is delivered to the server before messages from the subsequence rounds.

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The second green message from the top playerviolates this condition.

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Server

Server Proxy

Player Proxies

Players

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Ui

(Ui, i)(Ui, i)

The server proxy tags each message (“updates”) from the server with the round number.

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Ri

(Ui, i)(Ui, i)

The player proxy remembers Ri, the time message Ui is received, and forwards it to players.

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Aij

Action messages from the players are tagged by the player proxies with (i) round number i (ii) reaction time tij and (iii) order of action messages from that player.

Aij, i, tij, k

Pj

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Example of tags inserted by the proxy.

ProxyP2

Proxy

t12 t22 t32

k=1 k=2 k=1 k=1

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The server proxy now determines two things:(i) in what order to deliver the messages, and (ii) when to delivery the messages

Aij, i, tij, k

Pj

Aij’, i’, tij’, k’

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Delivery order: sort the queue according to round number, then by reaction time. Deliver messages in the order inside the queue.

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Delivery time: can deliver the head of the queue as long as no other messages from the same round is still in transit.

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ServerProxy

PlayerProxy j

PlayerProxy

Receive Time = RTTj + tij + Si

Si

tij

RTTj + tij + Si

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RTTj + tij + Si

Message from k within the same round i should be sent first if tik < tij

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RTTj + tij + Si < RTTk + tik + Si

If message from k is still in transit when message from j is received, then

< RTTk + tij + Si

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RTTj + tij + Si < RTTk + tik + Si

If message from k is still in transit when message from j is received, then

< RTTk + tij + Si

If message from j is delivered at this time, then message from k should have arrived

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max {RTTk} + tij + Si

It is safe to set the delivery time of a message from j to be

k in the set of hostswhose messages have not arrived

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Example: Consider a given round i. Si = 0ti1 = 1 ti2 = 2 ti3 = 3RTT1 = 10 RTT2 = 20 RTT3 = 10

At time 11, message from Player 1 is received.Delivery time for message 1 = 21

1

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1

Example: Consider a given round i. Si = 0ti1 = 1 ti2 = 2 ti3 = 3RTT1 = 10 RTT2 = 20 RTT3 = 10

At time 13, message from Player 3 is received.Delivery time for message 3 = 23

3

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Example: Consider a given round i. Si = 0ti1 = 1 ti2 = 2 ti3 = 3RTT1 = 10 RTT2 = 20 RTT3 = 10

At time 21, message from Player 1 is delivered

3

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Example: Consider a given round i. Si = 0ti1 = 1 ti2 = 2 ti3 = 3RTT1 = 10 RTT2 = 20 RTT3 = 10

At time 22, message from Player 2 is received.Delivery time for message 2 = 12. The messageis sent right away. At time 23, message from Player 3 is delivered.

(We can actually delivery message 3 earlier as an optimization)

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When a message is inserted into the queue, the delivery time of messages ahead in the queue may shorten, while the delivery time messages behind remains unchanged.

max {RTTk} + tij + Sik in set of hostswhose messages have not arrived

max {RTTk} + tij + Sik in set of hostswhose messages have not arrived

now

>

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max {Ti-1, max {RTTk} + tij + Si}

Consider the case where messages from previous round is still in transit,

the delivery time should be set as

k = set of hostswhose messages have not arrived

where Ti-1 is the time where maximum delivery time from previous round

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What if a message is late?

Then it is delivered immediately to the server

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Advantages: No need to synchronize clock or estimate one way delay.

Estimating RTT is easier.

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Server

Server Proxy

Player Proxies

Players

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Examples of Proxy Services:

Time-stampingMessage Ordering

Interest Management

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Example 3:Interest Management

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Servers

Proxies

Players

Regions 1 2 3 4

Regions 1 3 41 2 32

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Servers

Proxies

Players

Regions 1 2 3 4

Regions 1 3 41 2 32

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Servers

Proxies

Players

Regions 1 2 3 4

Regions 1 3 41 23 32

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Advantages:

Reduced network cost at server

No connect/disconnect at clients when player moves/region migrates

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P2P + arbitermirrored servers

zoned serversservers + proxies

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