Computer networks consist of machines, that are separated in space. This separation in space means that events will be observed by different machines at different times. It also means that as the propagation of observation increases, the origin time of the event becomes less certain. In computer networks, origin time is made more certain, by the addition of information, for example a timestamp. The interpretation / accuracy of a timestamp may depend on other mechanisms, such as time synchronization technology. In this treatment, network nodes are assumed to have no velocity relative to each other, so neither special of general relativity are considered.
As a simplification, we assume light travels in a straight line.
- R0 is observes that a port has failed.
- R0 constructs a protocol message and sends it to R1.
- The protocol message says “Port X, down, now”
- R1 receives the protocol message at T1 (T0 + (D1/V))
- T1 is different than T0, so “now” has a different meaning in time for R1, than R0
- R1 forwards the message to R2, without changing “now”.
- Now R2, R1, and R0 all believe the Port down event occurred at different times.
The basic problem in information networks is that information processing nodes are separated in space, and therefore they observe events at different times.
A common mitigation is to include timestamps in information headers. The accuracy of this approach depends on how well different nodes in the network are synchronized to a common time, for example UTC.
Synchronization accuracy can be improved through various time synchronization approaches:
- Network Time Protocol
If an approach, for example NTP, is only accurate to within a few milliseconds, then that may be good enough for some applications, but sill less than what would be required to change the operation of routing protocols / the IP control plane, because a great deal could happen in a few milliseconds.
Quantum technology may provide enhanced synchronization, in the future.
Appendix A – Special and General Relativity
As computer networks generally consist of machines that are at rest with respect to a similar reference frame, special and general relativity are not considered here. In other words, the case where, for example, one IP router has a velocity relative to another IP router, is not considered.
In the context of special relativity, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends on the object’s velocity relative to the observer.Spacetime, Wikipedia
Information has velocity in computer networks, but for the sake of this writing, the case where information understands its own time, is not considered.