Networked Games - consistency and real-time

1
Networked Games - consistency and real-time

• Objectives
• Understand the problems associated with networked
  games.
• Realize the importance of satisfying real-time
  and consistency requirements .

2
First networked games

• Allow multiple users to interact in a single
  gaming environment via text based communications.
• Initially made popular by Multi User Dungeons
  (MUDS).
• These are traditionally text based and reached
  their highest popularity in the 70s and 80s.
• Required users to build up their abilities (e.g.,
  physical strength, magical capability) over a
  number of sessions.
• Games last a long time. Users participate in a
  number of sessions over a period of days, months
  and even years.
• Replayability value is high. Difficult to stop
  playing when you have amassed so many
  points/skills over a prolonged period of time.

3
Real-time networked games

• Allow users to interact in real-time, usually
  involving graphical representation of the gaming
  arena.
• X-Pilot is an example of such a game.
• Very popular in the open source community.
• Fly a space ship around a 2D gaming arena,
  avoiding rocks while shooting other users or
  automated ships (known as robots simplified to
  bots in later years).
• Recent games tend to be centered around first
  person shooters.
• Unreal Tournament, Quake 3, etc.

4
Classification of networked games

• Networked games may be considered a sub-category
  of networked virtual environments (net-VEs).
• Net-VEs describe two active areas of research.
• Distributed Interactive Simulation (DIS)
  Emphasis is on modeling real world scenarios.
  Usually war type simulations.
• Collaborative Virtual Environments (CVE)
  Emphasis is on encouraging human interaction in
  collaborative working scenarios.
• Networked games tend to borrow from both DIS
  and CVE lines of research
• For example, simulate fighting scenarios while
  allowing user collaboration (simply via
  messaging).

5
DIS

• Research primarily funded by the military.
• War simulations for training troops, tank drivers
  etc.
• Attempted to link simulation units (such as tank
  simulators and flight simulators) that are
  geographically separated (e.g., different cities)
  so they can interact in battle scenes.
• Emphasis is on realism.
• Simulators are replicas of real hardware
  (interior).
• Battles are played out in real-time.
• Special stations allowing generals to monitor
  battles.
• Recent research looking at collaborative
  techniques.
• Allowing battle units to cooperate.
• Difficult to achieve as many units are dummy
  entities (actions are partly controlled by
  computer, may only be ordered by a general).
• Integrating AI controlled entities with human
  controlled entities in a collaborative manner is
  difficult.

6
CVE

• Allow geographically distributed users to
  cooperate.
• May mimic well known physical collaborative
  techniques such as lectures, tutorials, common
  rooms.
• Due to the virtual nature of the environment
  other collaborations are possible.
• Walk through a house yet to be built.
• Collaboration of engineers on 3D designs.
• Molecule modeling.
• Emphasis is on improving collaboration between
  users.
• Not as important as real world model. Real world
  physics is not required (most of the time).
• Representation of different individuals,
  different mechanisms of communication (e.g.,
  gesturing) are most important.
• Primarily funded by research grants related to
  work carried out in the Computer Supported
  Collaborative Work Community (CSCW).

7
Problems with networked games that have hard
real-time requirements

• Must satisfy real-time requirements while
  satisfying consistency requirements.
• Which is more important?
• If real-time requirements are not met then
  realism suffers.
• If consistency requirements are not met then user
  interaction becomes difficult to achieve.
• Real-time?
• If user U1 opens a door by time t then all users
  should see U1 open a door by time t.
• Consistency?
• If user U1 opens a door and then passes through
  the doorway then all users should see U1 open the
  same door then pass through the same doorway.

8
Example of real-time problem

• Three users (U1, U2, U3) participate in a first
  person shooter.
• U2 appears in 6 different positions in
  consecutive frames.
• At frame 3 U2 enters the line of fire of U1 and
  U3.

• Due to message latency, U1 views U2 at frame 3
  sufficiently late for U3 to gain an unfair
  advantage.

9
Example of consistency problem

• Three users (U1, U2, U3) participate in a first
  person shooter.
• As viewed from U1 U1 pushes a button that
  disarms all opponents.
• As viewed from U2 Just before U1 pushes the
  button U2 shoots U1.
• What does U3 see?
• Ordering of events (even if they appear to happen
  concurrently) is required.
• Ensuring every user views events in the same
  order is commonly termed identical ordering or
  total ordering.

10
Attempting to satisfy real-time requirements

• Two methods exist predictive modeling
  scoping/zoning.
• Predictive modeling is sometimes termed dead
  reckoning.
• Useful if we can quite accurately predict future
  events related to a received event.
• An example would be the firing of a rocket.
  Provided with velocity, acceleration and starting
  position we can predict its flight through the
  air.
• Scoping requires limiting the number of users
  that are expected to receive an event.
• Commonly achieved via the division of the virtual
  space.
• This can be via rooms, or simply users views.

11
Attempting to satisfy consistency requirements

• Some form of message ordering protocol is
  required.
• A trivial attempt at satisfying ordering is to
  use TCP to ensure FIFO and have a central server
  through which all messages must pass through.
• The central server, together with TCP, ensures
  all nodes receive the same messages in the same
  order (we are ignoring node failure and
  disallowing nodes to act on their own messages
  without traversal through the server).
• UDP is used rather than TCP as TCP may introduce
  excessive latency time for message transit. This
  prevents FIFO ordering (apart from over LAN where
  broadcast may be used).
• UDP introduces the problem of message loss (even
  though TCP does not overcome this totally it does
  provide some indication when messages are not
  received in some instances).