Virtual Reality Operations, User Views and Networ Considerations

For a distributed multi-party application, there are a number of constraints which we might try to satisfy.

• Convergence, or does the interface show a system that reaches eventual consistency.
• Determinism, or does the system behave according to users' expectations of the underlying model.
• Multiple Modifiers - does the system allow more than a single user to modify an object (write/update state associated with an object).
• Interactivity - does the system let users lock an object so that they then have exclusive write/update access.

A key point in the design of the system is that when operating over a network with loss, reordering, and different levels of throughput/latency between different senders and recipients, it is clear that it is impossible to satisfy all four of these at any one time.

It should be sufficient to relax one of the constraints at one time, but it will dependent on a particular application and user requirements which, constraint is the appropriate one to relax.

Network conditions that pertain to each of these constraints are:

• Capacity - bits per second - may differ between a sender and a set of recipients - a distribution tree rooted at each sender will have paths to recipients, and different parts of the sub tree may have different capacity.
• Errors/Loss - packet loss may be due to noise on the line, transient outage, or temporary congestion. Again, in a multi-party application, this may be different for different receivers.
• Availability - networks can partition. Some senders or recipients may crash, or reboot at different times during a multi-party session.
• Delay - delays over long-haul networks^8.4, even without queues, can be significant. Again, they will usually differ from one recipient to the next.