CMPSEM027 Online Gaming

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CMPSEM027Online Gaming

• Abdennour El Rhalibi

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Massively Multiplayer Games Design and Issues and
Solutions
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Multiplayer Online Game?

• A Multiplayer Online Game (MOG) is a software
  system in which multiple users interact with each
  other in real-time, even though those users may
  be physically located around the world.
• Typically, each user accesses his/her own
  computer workstation or console, using it to
  provide a user interface to the game environment.
• These environments usually aim to provide users
  with a sense of realism by incorporating
  realistic 3D graphics, spatial sound other
  modalities to create an immersive experience.

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A MOG is distinguished by five common features

• A shared sense of space
• A shared sense of presence
• A shared sense of time
• A way to communicate
• A way to share

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Gameplay Issues

• Designing a game to have multiplayer options
  cannot in good practice be an add-on after the
  game has been developed.
• It has to be considered early in the design
  phase.
• Because multiplayer can increase the of
  polygons you have to be able to draw how much
  of the screen you have to split up.
• Vertical vs horizontal split-screen- depends
  where your enemys are most likely to appear- in
  nature horizontal is more important than
  vertical.
• Co-operative vs Melee
• Co-op
• Can co-op be played through the regular storyline
  of the game? (like Halo)
• Or only in special multiplayer arenas? (like
  Quake)
• What happens if one of you dies? E.g. Halo
  Resurrect when no one is shooting your partner.
• What happens if both of you die? Halo Restart
  from the level?

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Gameplay Decisions

• Can I bring in customized characters? Quake
  allows mods.
• How do you find other players to play with on a
  networked game? Match making service.
• How do you find each other during a networked
  game? Radars.
• Is the game persistent? e.g. Networked Quake vs
  Ultima Online / Everquest
• How many players can you support simultaneously?
• Will the game include NPCs during multiplayer
  sessions? Are NPCs your friend or enemy?

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E.g. Gauntlet

• 3rd person view- everyone on the same screen
• Play is co-operative
• What happens when players walk in different
  directions?
• Camera zooms out up to a limit.

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E.g. Quake / Jedi Outcast / Academy

• Entire screen dedicated to 1 player
• Co-op melee play possible over network.
• Multiple types of network games- free for all
  capture the flag
• Challenge network latency handling many many
  players and NPCs handling mods for customizable
  game characters.

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E.g. Halo

• Split screen co-op/melee and networked play
• Screen resolution limits polygon counts network
  latency handling many players and NPCs

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E.g. Ultima / Everquest / Star Wars Galaxies

• No single user game mode.
• Entire screen dedicated to 1 player.
• All players are on the network.
• Large scale persistent RPG
• Open-ended play
• Richness and replayability of the game is
  enhanced by users providing the content thru
  social interactions.
• Must handle 100s/1000s of players over periods
  of years.
• Large reliable databases in a constant running
  simulation environment.
• Database migration problems as of players
  grows.
• How to incorporate new game play capabilities
  over time as players ask for them without
  disrupting service.

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MOG are difficult to implement

• MOG are difficult to implement correctly or
  effectively. MOG are complex because they are
  multiple traditional types of software rolled
  into a single application.
• MOG are
• Distributed systems They must contend with all
  of the challenges of managing network resources,
  data loss, network failure, and concurrency.
• Graphical applications They must maintain
  smooth, real-time display frame rates and
  carefully allocate the CPU among rendering and
  other tasks.
• Interactive applications They must process
  real-time data input from users. Users should see
  the virtual environment as if it exists locally,
  even though its participants are distributed at
  multiple remote hosts.

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Additional complexity

• MOG design is made more complex because these
  systems must work with a number of existing
  application services.
• MOG typically must integrate with database
  systems that store persistent information about
  the game environment. These databases include,
  for example, detailed information about the
  environments terrain elevation, the location of
  buildings and other static items in the
  environment, and the initial MOG configuration.
• MOG need to support user authentication and may
  interact with commerce and other transaction
  systems.
• To support reproducible game play, MOG must be
  able to log events in real-time to a persistent
  storage this task is complicated by the fact
  that the complete state of the MOG may not
  actually be known at any single host in the
  system.

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A unified system

• The components of a MOG system interact in
  complex ways, so the designer must regard the
  application as a unified system.
• Invariably, trying to optimize one element of a
  MOG can adversely impact the behavior of other
  components.
• In effect, MOG development is a difficult
  balancing act of engineering tradeoffs.

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Architecture
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Single ThreadNetGames
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Processing Systems
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Fundamental Networking Issues

• Protocols (TCP/IP Transfer Control Protocol /
  Internet Protocol)
• TCP Completely reliable data arrives as a
  continuous streams connection-oriented
• Because data arrives as a stream there is no
  guarantee of when each byte of data will arrive.
• UDP Unreliable data arrives as discrete
  packets/messages each packet is correct but
  no guarantee of the order of the packet -
  non-connection oriented
• Bandwidth measured in bits per second.
• Latency
• This is your biggest enemy!
• Usually due to distance router and end-system
  overflows packet loss.
• Roundtrip times from New York TransUS 50ms
  TransAtlantic 120ms TransPacific 200ms
• Measured in milliseconds
• Latency in TCP is higher than UDP because TCP
  needs to acknowledge that data stream is correct.
• 200ms roundtrip latency is considered minimum
  acceptable latency for tightly coupled
  interactive applications.

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Fundamental Networking Issues

• Unicast, broadcast, multicast
• Broadcast sends message to multiple receivers one
  at a time.
• Multicast sends 1 message and allows router to
  replicate and send message (based on UDP).
• Problem is, multicast is not enabled on most
  routers since it can flood the Internet.
• Firewalls
• Usually networked applications operate over known
  port numbers.
• Firewalls prevent unintentional network traffic
  from entering computer.
• But it can prevent network traffic from entering
  you game application.
• Game should provide info about port used so that
  firewall can be opened.
• Unfortunately this creates a vulnerability for
  hackers.

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Networking Issues that can Ruin aGame Experience
Routers and end-systems have network buffers
(queues) which fill up during congestion. Congesti
on is usually caused by an aggregate of network
traffic entering the routers which far exceeds
the rate at which the router is able to deliver
it..
Assume Bottleneck here
Receive buffer
Send buffer
Client
Client
Router
Also if sender sends faster than the network
link, send buffers will fill up packets will
get dropped at the sender. Same can happen for
receiver.
Router
Client
Bottleneck can cause queues to fill up
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Networking Issues that can Ruin aGame Experience

• When buffers are full, packets will get dropped.
• Effect on TCP
• Latency will increase because TCP will retransmit
  in order to guarantee reliable delivery.
• Latency will increase because TCP will reduce
  transmission rate to attempt to reduce congestion
• Effect on UDP
• UDP will simply lose data.
• Is there an easy way to tell or determine what
  the best sending rate is?
• Unfortunately NO- there is no Quality of Service
  over the Internet best you can do is to assume
  the core of the network will never slow down and
  you have to dynamically adjust your sending rate
  to accommodate the slowest player- down to a
  minimum acceptable threshold.
• Rate adjustment is commonly used in audio/video
  streaming.
• For all clients, as a rule, you should process
  any incoming packets as fast as possible to avoid
  congestion at the end points.
• But if your client spends all its time processing
  packets, it may take time away from making the
  game go! So a balance must be found.

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Distributed Interaction

• Distributed interaction is one of the defining
  qualities of a MOG system.
• Users see each others real-time activity and
  react to this information in real-time.
• To be effective, the MOG system must present each
  user with the illusion that the entire
  environment is located on the local machine and
  that his or her actions are having a direct and
  immediate impact on the environment.
• The system needs to mask any artifacts that might
  arise because of the applications distributed
  nature.

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Distributed Interaction

• Maintaining the illusion of a single system is
  difficult because of the messaging required to
  exchange information within the MOG.
• For example, networks impose a noticeable delay
  from the time that a message is transmitted to
  the time that a message is actually received at
  its destination.
• Moreover, different messages may incur different
  delays depending on the type of network and on
  the locations of the source and destination
  hosts.
• Each host must therefore attempt to present a
  consistent real-time view of the MOG and contend
  with the fact that all of the incoming
  information about remote users is already
  out-of-date when it arrives.

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Network Game Management

• If your multiplayer game needs to retrieve large
  data files (like 3D models), cache it locally in
  the same way a web browser caches images.
• Visual representation of networked entities
• avatar
• Body position orientation
• Head orientation
• Pre-recorded gestures
• Body parts list to load _at_ each client
• Send absolute state data not key or button
  presses.
• Use UDP to send data that is normally sent
  repeatedly- e.g. a players position.
• With UDP try to keep packets no larger than
  1Kbytes otherwise there is no guarantee of
  arrival.
• Pack several variables together- dont send one
  at a time.
• UDP does not guarantee the order of packets so
  include a packet in the message so that old
  messages can be filtered out.

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What is dynamic shared state?

• The dynamic information that multiple hosts must
  maintain about the MOG
• Accurate dynamic shared state is fundamental to
  creating realistic virtual environments. It is
  what makes a game multi-user.
• Management is one of the most difficult
  challenges facing the MOG designer. The trade off
  is between resources and realism.

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Consistency-Throughput Tradeoff

• It is impossible to allow dynamic shared state
  to change frequently and guarantee that all hosts
  simultaneously access identical versions of that
  state.
• We can have either a dynamic world or a
  consistent world, but not both.

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Design Implications

• For a highly dynamic shared state, hosts must
  transmit more frequent data updates.
• To guarantee consistent views of the shared
  state, hosts must employ reliable data delivery.
• Available network bandwidth must be split between
  these two constraints.

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Managing Shared States
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Massive Multiplayer

• Very large client server networks (thousands of
  clients)
• Uses advanced networking concepts such as
• Data Extrapolation
• Hierarchical Messaging
• Spatial Subdivision.
• WoW, Planetside,

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Data Extrapolation

• Uses timestamps and polynomials to interpolate
  player positions in between position updates.
• The prediction polynomial is recalculated after
  each position update.
• As lag increases, the difference between
  predicted player positions and actual position
  increases. This creates a "jump" in perceived
  player position.

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Data Extrapolation
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Hierarchical Messaging

• Ranks types of messages in order of importance.
• Enables players with less bandwidth to play.
• Based on bandwidth tests, a player receives the
  most important messages while less important
  messages are dropped in order to stay within
  bandwidth limitations.

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Hierarchical Messaging
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Spatial Subdivision

• Method of sending updates only to players who are
  close to each other.
• Stores players in a spatial index which enables
  the server to determine which clients to update
  with a particular player's new position.

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Connectivity models

• Shared Centralized clients connected to central
  server simplest to implement maintain
  consistency across all clients but load at
  server can restrict of participants that can be
  supported simultaneously.
• Replicated Homogeneous clients with no
  centralized control (multicast) most scalable
  solution for large of clients, but multicast is
  not deployed across the whole Internet.
• Shared distributed with peer-to-peer updates
  clients with full connectivity to each other
  often a central mediator initiates the peer
  connections.
• Shared distributed using client-server
  subgrouping subgroups for areas of interest
  management most scalable for very large worlds
  (like online persistent worlds- Ultima Online,
  etc..)

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New Trends

• Problem
• Trend is toward Massively Multiplayer Games
• Traditional client-server architectures not
  scalable
• Server clusters expensive to deploy, maintain
• Current Solutions
• TeraZona - middleware
• Butterfly.net -
• Issues
• Tailored for MMORPGs
• Still very costly and centralized