Technical Issues: Networking

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Technical IssuesNetworking
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Technical IssuesNetworking

• More and more gaming environments are supporting
  networked multiplayer play.
• Computer based games have supported network games
  for many years.
• Consoles are now also supporting network play.
• Not to mention the recent developments in gaming
  on mobile and wireless devices (cell phones,
  etc.).
• Most new games released these days make use of
  networking in some way.
• Support and user communities.
• Delivery of patches, updates, and upgrades.
• Multiplayer gaming experiences.

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Networking Requirementsfor Video Games

• Consistency and synchronization
• All game players should see the game world in the
  same state at the same time.
• Low latency
• Delays between players should be low.
• This helps promote consistency.
• Scalability
• The game should be able to support a large number
  of players without difficulties.
• The game should also be able to scale in real
  time as players enter and leave the game.

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Networking Requirementsfor Video Games

• Performance
• Sufficient network resources (e.g. bandwidth)
  should be available to ensure the game runs
  smoothly.
• Reliability
• If part of the game goes down, the rest should
  continue to function without major service
  disruptions.
• Security
• It should be easy to keep track of players and
  prevent them from breaking game rules, cheating,
  or abusing other players.

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Networked Game StructureThe Peer-to-Peer Model
Peer
Peer
. . .
Peer
Peer
Peer
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Networked Game StructureThe Peer-to-Peer Model

• Each player plays the game with the same program
  as every other player.
• Each such program is a peer.
• Each peer accepts player input, carries out game
  processing, and provides feedback.
• Each peer must communicate directly with all
  other peers involved in the game to maintain
  synchronization and game consistency.
• All peers are equivalent in functionality and
  standing in the networked game.

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Networked Game StructureThe Peer-to-Peer Model

• Advantages
• Conceptually very simple.
• All players have equal standing in the game,
  providing some element of fairness.
• Disadvantages
• Each players system must have sufficient
  resources to support the game or everyone can
  suffer.
• Each player must know about all other players in
  the game, with network resources consumed for
  each pair of players in the game. (Using
  multicasting or broadcasting will reduce this
  problem, but will increase complexity in the
  game.)
• Consistency and synchronization may be difficult
  to maintain, especially with many players.

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Networked Game StructureThe Peer-to-Peer Model
Screen shot from Doom. One of the first
multiplayer games, Doom used the peer-to-peer
model.
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Networked Game StructureThe Integrated Server
Model
Client/Server
Client
Client
Client
Client
. . .
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Networked Game StructureThe Integrated Server
Model

• The networked game is composed of two different
  kinds of programs.
• Multiple clients.
• A single integrated client/server.
• Clients are the player interfaces to the game,
  taking their input and providing audio, visual,
  and tactile feedback where appropriate.
• The integrated client/server acts as a client to
  one player, but has additional responsibilities.
• Coordinates and synchronizes all player
  activities.
• Hosts any non player characters.
• Carries out the majority of processing involved
  in serving the game out to players.

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Networked Game StructureThe Integrated Server
Model

• Advantages
• Players no longer need to know about every other
  player in the game.
• Total communication volume decreases.
• Consistency and synchronization is easier, as the
  server is in sole control over the game.
• Disadvantages
• Will the integrated server have the resources to
  adequately handle all the players?
• Will the player playing at the integrated server
  have an unfair advantage in terms of reduced
  latency and preferential server access?
• How well will this scale?
• What if the server goes down?

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Networked Game StructureThe Integrated Server
Model
Screen shot from Starcraft. Starcraft supported
many network play options, including an
integrated client/server mode, as well as
Battle.net.
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Networked Game StructureThe Dedicated Server
Model
Client
Client
. . .
Server
Client
Client
Client
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Networked Game StructureThe Dedicated Server
Model

• In this model, the game is again split into two
  different kinds of programs
• Multiple clients.
• A single dedicated server.
• Clients serve the same purpose as in the previous
  model, interfacing players to the game they are
  playing.
• The server in this case, however, is a separate
  program without any client functionality.
• Its sole purpose is to coordinate and synchronize
  the game, and to serve it out to the players
  clients.

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Networked Game StructureThe Dedicated Server
Model

• Advantages
• With separated functionality, there is no single
  client with a clear unfair advantage.
• If a dedicated machine is unavailable, the server
  can be run on the same machine as one of the
  clients, falling back to essentially an
  integrated model.
• Disadvantages
• Will clients closer to the server still have an
  advantage over clients farther away?
• Will the dedicated server have the resources to
  support all of the clients?
• How well will this scale?
• What if the server goes down?

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Networked Game StructureThe Dedicated Server
Model
Screen shot from Quake II. The Quake series of
games pushed the dedicated server model, and
provided dedicated servers for many different
platforms.
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Networked Game StructureThe Flat Server Model
Client
Client
Client
Client
Server
Server
Server
Client
Client
Client
Client
Server
Client
Client
Client
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Networked Game StructureThe Flat Server Model

• In this model, there are multiple dedicated
  servers for scalability to support a much larger
  number of players.
• Each server must know about every other server in
  use.
• The servers must communicate with one another to
  ensure consistency and synchronization between
  all game players.
• Each player uses a client to connect with a
  dedicated server to access the game.

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Networked Game StructureThe Flat Server Model

• Advantages
• More scalable than single server approaches.
• More reliable, as there are multiple servers.
• Conceptually easy to manage servers.
• Disadvantages
• As number of servers grow, so does communication
  overhead (unless multicasting or broadcasting is
  used).
• Consistency maintenance and synchronization are
  more difficult now than before, especially with
  more clients.

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Networked Game StructureThe Hierarchical Server
Model
Client
Client
Server
Client
Client
Server
Server
Client
Server
Client
Client
Client
Server
Client
Client
Client
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Networked Game StructureThe Hierarchical Server
Model

• In this model, additional servers are added
  solely for the purpose of managing and
  coordinating other servers.
• This creates a hierarchy of servers, in a
  tree-like configuration.
• The server at the root of the tree has ultimate
  responsibility over the entire game, but ensures
  scalability by delegating many of its tasks to
  lower level servers.

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Networked Game StructureThe Hierarchical Server
Model

• Advantages
• More scalable than flat server model.
• Reduced communications overhead.
• Disadvantages
• As the number of levels in the hierarchy grows,
  latency between the various parts of the game
  grows as well.
• Consistency maintenance and synchronization are
  even more difficult now than in the flat server
  model, due to this increased latency.
• What if the root server goes down?

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Networked Game StructureThe Grid Computing Model
Client
Client
Server
Server
Server
Client
Client
Server
Server
Client
Server
Client
Client
Client
Server
Server
Server
Client
Client
Client
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Networked Game StructureThe Grid Computing Model

• In this model, all of the servers act as a single
  powerful parallel computer.
• To the clients, the servers seem as if they are a
  single logical computer.
• Software on the servers automatically
  redistributes player load to make use of free
  resources and to reduce client latency.
• All servers in the grid may be in the same
  location, or they can be geographically
  distributed around the world.

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Networked Game StructureThe Grid Computing Model

• Advantages
• Can scale to support a large number of players
  adding servers to the grid is transparent.
• Very reliable if a server is lost, player load
  is shifted automatically to a new server.
• Consistency and synchronization is handled
  automatically by the grid software.
• Disadvantages
• Very complex grid software required, although
  standard software is emerging.
• Very costly to provide.

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Networked Game StructureThe Grid Computing Model
The Butterfly Grid, shown to the left, isbeing
developed byButterfly.net to provide a
massively multiplayer environment for
videogames. It will use IBM technologies based
onLinux to support a variety of games on
various platforms, including Sonys
Playstation 2, and others.
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Game World Structuring

• Once you decide how to structure your game, you
  must decide how to spread your game world over
  the servers used to support your game.
• Will each server be independent, or will they
  work together to support a single massive game
  world?
• This is an important decision, and can have a
  large impact on your game.

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Game World StructuringMultiple Worlds

• Each game server supports its own independent
  game world.
• The world, levels, characters, and game rules can
  differ from server to server.
• This is the easiest approach, as there is little
  consistency management needed.
• Players also like this approach, as they can find
  a server providing a game world more to their
  needs and style of game play.
• Ultimately, however, this approach limits the
  number of players in a single game at a time.

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Game World StructuringMultiple Worlds
Screen shot from Counter Strike. There are many
Counter Strike servers around the world, each
providing their own independent game.
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Game World StructuringThe Divided Single World

• A single world is supported across all servers,
  but each server provides its own independent part
  of the world.
• Geographic region (planet, country, island, part
  of a dungeon, etc.)
• Level of game play.
• Chapter of an unfolding story.
• Players move between servers as they play.
• Each server still supports a limited number of
  players in its region at a time.
• By allowing players to move from server to
  server, they have the illusion of a much larger
  game world with many more players.

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Game World StructuringThe Divided Single World
Screen shot from Everquest. Everquest features
many different regions and zones across multiple
servers to support a very large world.
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Game World StructuringThe Single World

• In this approach, a single massive game world in
  its entirety is supported by many servers at the
  same time.
• There are no independent servers they all
  interact to support the same game world.
• All players exist in the same world at the same
  time, allowing for vastly different multiplayer
  experiences.
• Providing scalability to support this and
  maintain consistency at the same time is quite
  difficult.

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Game World StructuringThe Single World
Concept art for VibeForce. VibeForce is a MMORPG
for the PC, Playstation 2, and wireless devices
that will make use of the Butterfly Grid to
support a single massive game world.
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Locating Networked Games

• The first step to playing a networked game is to
  locate one to participate in.
• There are a variety of different techniques used.
• The choice of technique is very important to the
  success of a game.
• If it is too hard, users will find it difficult
  to play.
• Mechanisms that simplify the procedure are more
  expensive, and can incur continuing costs
  throughout the life time of a game.

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Locating Networked GamesDirect Connections

• The oldest and simplest approach of locating a
  networked game requires a direct physical
  connection between the two gaming systems.
• PC systems required a serial connection.
• Consoles use a variety of techniques the Xbox
  uses a crossover Ethernet connection.
• With a direct connection, location is quite
  simple the other game system can be found at the
  other end of the connection.
• While simple, it ultimately limits the number of
  players possible, and restricts the location of
  players to within a few metres of each other.

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Locating Networked GamesDirect Connections
Screen shot from Mech Assault. It supports
direct connection play to another Xbox with the
system link cable.
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Locating Networked GamesFixed Addresses

• In this approach, a game peer or server to
  contact is listening on a fixed address for new
  game players.
• A specific phone number for a dial-up connection
  using a modem.
• A network address and port number for Internet
  based games.
• It is up to the player to know this address in
  advance to connect into the game.
• No location is actually required, as the game
  simply contacts the fixed address directly to
  join.

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Locating Networked GamesFixed Addresses
Screen shot of Quake II. The address book in
Quake II allowed players to record fixed
addresses to contact their favourite servers.
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Locating Networked GamesBroadcasting

• In this approach, game clients or peers broadcast
  data onto the network to search for servers or
  other peers.
• When a server or peer receives a broadcast
  message, it responds to notify the sender of
  their address to join the game.
• This approach is quite simple to implement (using
  UDP, for example), but is typically restricted to
  finding networked games on the same network.
• Broadcast messages are usually not propagated
  across routers and other network gear.

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Locating Networked GamesBroadcasting
Screen shot from Unreal Tournament 2003. It
allows you to browse through servers on the local
network using a broadcast mechanism.
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Locating Networked GamesLocation Services

• Location services run at fixed network addresses
  known to the game itself.
• When a new game server is started, it registers
  its address and game information with the
  location service.
• When the player wishes to join a multiplayer
  game, the game contacts the location service to
  help track down a game meeting the players needs
  and requirements.
• When the player has selected a game to join, the
  appropriate server is contacted according to its
  registration information and play can begin as
  usual.

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Locating Networked GamesLocation Services

• Location services can be hosted by the game
  producer or by a third party.
• Location services can provide many features to
  game players
• They can choose games based on server location,
  game rules, levels, characters, and a variety of
  other criteria.
• They do not necessarily need to know any of the
  other players to join a game.
• Other features include the ability to run private
  games, as well as other community services.
• Because of their ease of use, these services are
  becoming more common, despite their costs.

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Locating Networked GamesLocation Services
Screen shot from Xbox Live. Many features allow
easy multiplayer experiences on this console.
You can play with friends or complete
strangers. You can even have the system perform
an optimatch to link you up with players of
similar skill and ability automatically.
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Networked Communication

• Once a networked game is located and joined, the
  game can proceed.
• There are still, however, many communications
  issues that must be resolved.
• What to communicate.
• When to communicate.
• Where to communicate.
• How to communicate.
• These issues can have a great impact on the
  success of a networked game.

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Networked CommunicationRules to Follow

• The following general guidelines should apply to
  almost any networked game
• Only send what is needed.
• Only send when it is needed.
• Only send to where it is needed.
• Following these rules is a good step towards
  meeting the requirements of networked games
  discussed earlier.

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Networked CommunicationWhat is Needed?

• The state of a game is essentially the complete
  set of attributes describing all elements of a
  game at a given point in time.
• Remember that we want all players to see the game
  world in the same state at the same time.
• To do this, all changes in state must be sent to
  all parts of the networked game.
• Keep in mind that we want to minimize the amount
  of communication, so we need to determine what to
  send and what not to send.

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Networked CommunicationWhat is Needed?

• All changes in state that affect game play and
  the outcome of the game must be sent.
• Character movement and actions, object placement,
  level condition, and so on.
• Changes in backgrounds, ambience, atmosphere, and
  so on should not be sent.
• This can be a large amount of information!
• These can be handled in the individual peers and
  clients independently if they do not affect game
  play.
• If desired, periodically synchronize random
  number keys controlling these events for some
  consistency.

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Networked CommunicationWhen is it Needed?

• Only communicate to indicate changes in the state
  of the game.
• When there is essentially nothing happening in a
  game, there should be no network traffic being
  generated!
• Instead of communicating every change as it
  happens individually, it might be more efficient
  to collect them for a short period of time and
  send several state changes at once.
• Do not wait too long, however, or the games
  state will become inconsistent across players.
• You will need to experiment and tune this.

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Networked CommunicationWhere is it Needed?

• Not every change in game state needs to be sent
  to every player in the game.
• Only send changes in state to players that will
  be affected by them!
• For example, a player does not need to be
  notified of character movement across the level
  if the player cannot see or hear it once they
  are in range, it becomes important.
• Making these decisions can increase game
  complexity, but it can provide large benefits to
  network efficiency.

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Networked CommunicationHow to Communicate?

• Assuming that network play involves communication
  over the Internet, the choice of transports to
  use is limited.
• There are two main choices
• TCP Transmission Control Protocol, a
  connection-oriented Internet protocol.
• UDP User Datagram Protocol, a connectionless
  Internet protocol.
• There are other protocols to use, but they can be
  more complex and lack widespread support.
• DiffServ, RSVP, and so on.

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Networked CommunicationHow to Communicate?

• About TCP
• More overhead, in establishing a connection, and
  in maintaining connection resources.
• Provides reliable, in order delivery of data in a
  stream of bytes.
• Also provides flow and congestion control to
  regulate transmission rate of data.
• About UDP
• No connection, so there is less overhead.
• Data can be lost or delivered out of order.
• Data is delivered with explicit boundaries
  between messages.
• UDP may have problems crossing through firewalls,
  however, so this must be kept in mind.

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Networked CommunicationHow to Communicate?

• Ultimately, you must assess the needs and
  requirements of your game and make your decisions
  accordingly.
• If reliability is important and no loss of data
  can be tolerated, TCP may be better. (You can
  still build reliability on top of UDP, however,
  if you need to.)
• If loss of data can be tolerated, UDP might be
  more efficient in the end.
• If you want to support massive numbers of
  players, UDP might be necessary to avoid
  connection overhead and connection restrictions.

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Other Networking IssuesPersistence

• Will the game world continue to exist and
  function once a player leaves it?
• If so, the game world is persistent.
• Persistence introduces several interesting
  issues
• What happens to player characters, their items,
  and so on when the player leaves?
• How does time pass in the persistent world?
• How does the game story unfold when certain
  players are not always around?
• How can the game action be split into reasonable
  pieces that coexists with this persistence?

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Other Networking IssuesPersistence

• Keep in mind that persistence works better for
  some games and game genres than others.
• It is well suited to role-playing games.
• It can work well with action and adventure games,
  if they are designed properly.
• It can be difficult to work well with strategy
  and sports games. (Who will take the players
  place when they are gone, or will their side or
  team be defeated again and again in their
  absence?)
• Persistence must be worked into a game early as
  early in the design process as possible.

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Other Networking IssuesPersistence
Screen shot from Everquest. Games such as
Everquest provide a persistent world, even when
you are not around to participate in it.
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Other Networking IssuesSecurity in Games

• Traditional network security measures are
  starting to appear in networked games for a
  variety of purposes.
• Authenticating game peers and clients to prevent
  cheating.
• Authenticating players for access control and
  accounting purposes.
• Authenticating and authorizing players to access
  special game content.
• Authenticating and authorizing administrators to
  adjust game settings.
• Preventing people from tampering with or
  disrupting networked games.
• Preventing players from abusing each other.

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Other Networking IssuesSecurity in Games
Screen shot from Shadowbane. Hackers once used
various exploits to wipe out new players
gathering in safe-holds, gain God-like powers,
and banish anentire city of people to under
water.
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Other Networking IssuesSecurity in Games
Screen shots from Dark Age of Camelot. In an
August 2003 security breach,hackers spawned
multiple high level creatures, shown above, that
wreakedhavoc and caused a lot of trouble on game
servers.
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Other Networking IssuesSecurity in Games

• How do we accomplish this?
• Password systems.
• Cryptographic mechanisms, for public key and
  private key encryption.
• Secure communication protocols such as SSL
  (Secure Sockets Layer), TLS (Transport Layer
  Security), and IPSec.
• What are the costs?
• Increased complexity in game code.
• Increased processing requirements.
• Increased network requirements, as more data
  must be exchanged to support security.

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Other Networking IssuesGoing Mobile

• Many companies are investigating mobile devices
  as gaming platforms.
• The traditional game developers.
• Manufacturers of cell phones and PDAs, as well as
  service providers.
• This would allow gamers to play at any time, with
  anyone, anywhere.
• This raises several interesting possibilities,
  particularly for persistent games.

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Other Networking IssuesGoing Mobile
Nokia hopes that its N-Gage mobile gaming device
will lead to a whole newgeneration of networked
games. (That and a decent market share
whencompeting against Nintendos hand helds!)
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Other Networking IssuesGoing Mobile

• This kind of platform raises several interesting
  and challenging networking issues.
• Network and device capabilities.
• Packet and data loss.
• Interference.
• Disconnection of players.
• Relocation of players, both between network
  segments and possibly devices as well.