Hybrid delayPeriod

[milosevic]

In the current specification (https://github.com/cosmos/ibc/tree/master/spec/core/ics-003-connection-semantics) delayPeriod parameter is time based, i.e., the protocol enforces that at minimum delayPeriod must pass between the point header (consensusState) is installed and the corresponding packet data is verified. This provides security window during which correct relayer can submit a proof of misbehaviour and freeze client in case consensusState is invalid.

However, if the destination chain (where invalid consensusState is installed) does not create blocks or is censoring misbehaviour processing during delayPeriod, then security protection is of no use. Potential solution that would increase security guarantees would be having hybrid delayPeriod where it is expressed both in terms of time and number of blocks. In this case verification would be possible only when both some time and number of blocks has passed.

Although it is very hard completely preventing censorship issue (with the existing system), this proposal should still increase security guarantees as it decreases a chance for censorship of misbehaviour by increasing packetDelay.