ICS28: Distinguish between unbonding periods on provider and consumers

[andrey-kuprianov]

Surfaced from @informalsystems continuous audit of ICS28 Specification, at commit f1f79b8

In the ICS 28 specifications, the files CCV: Overview and Basic Concepts and CCV: System Model and Properties talk about UnbondingPeriod in general terms, while in reality there is an unbonding period on the provider chain, and a (different) unbonding period on each consumer chain. E.g. the very first occurrence of UnbondingPeriod in CCV: Overview and Basic Concepts reads like this:

Some of the validator updates can decrease the voting power granted to validators. These decreases may be a consequence of unbonding operations (e.g., unbonding delegations) on the provider chain. which MUST NOT complete before reaching maturity on both the provider and all the consumer chains, i.e., the unbonding period (denoted as UnbondingPeriod) has elapsed on both the provider and all the consumer chains.

Recommendation

• In every mention of UnbondingPeriod in the specification make sure to quantify whether this is the unbonding period on the provider, or on one particular consumer, or some function of the set of unbonding periods of the provider and all consumers (e.g. maximum, minimum);
• Clarify in the system model whether the relationship between different unbonding periods has any consequences for system functioning and/or security, and what that relationship should be in order for the system to function correctly.