ADR 017: Chain Versions

TODO

clarify how to handle slashing when ChainID changes

Changelog

28-07-2018: Updates from review
- split into two ADRs - one for protocol, one for chains
16-07-2018: Initial draft - was originally joint ADR for protocol and chain versions

Context

Software and Protocol versions are covered in a separate ADR.

Here we focus on chain versions.

Requirements

We need to version blockchains across protocols, networks, forks, etc. We need chain identifiers and descriptions so we can talk about a multitude of chains, and especially the differences between them, in a meaningful way.

Networks

We need to support many independent networks running the same version of the software, even possibly starting from the same initial state. They must have distinct identifiers so that peers know which one they are joining and so validators and users can prevent replay attacks.

Call this the NetworkName (note we currently call this ChainID in the software. In this ADR, ChainID has a different meaning). It represents both the application being run and the community or intention of running it.

Peers only connect to other peers with the same NetworkName.

Forks

We need to support existing networks upgrading and forking, wherein they may do any of:

- revert back to some height, continue with the same versions but new blocks
- arbitrarily mutate state at some height, continue with the same versions (eg. Dao Fork)
- change the AppVersion at some height

Note because of Tendermint's voting power threshold rules, a chain can only be extended under the "original" rules and under the new rules if 1/3 or more is double signing, which is expressly prohibited, and is supposed to result in their punishment on both chains. Since they can censor the punishment, the chain is expected to be hardforked to remove the validators. Thus, if both branches are to continue after a fork, they will each require a new identifier, and the old chain identifier will be retired (ie. only useful for syncing history, not for new blocks)..

TODO: explain how to handle slashing when chain id changed!

We need a consistent way to describe forks.

Proposal

ChainDescription

ChainDescription is a complete immutable description of a blockchain. It takes the following form:

ChainDescription = <NetworkName>/<BlockVersion>/<AppVersion>/<StateHash>/<ValHash>/<ConsensusParamsHash>

Here, StateHash is the merkle root of the initial state, ValHash is the merkle root of the initial Tendermint validator set, and ConsensusParamsHash is the merkle root of the initial Tendermint consensus parameters.

The genesis.json file must contain enough information to compute this value. It need not contain the StateHash or ValHash itself, but contain the state from which they can be computed with the given protocol versions.

NOTE: consider splitting NetworkName into NetworkName and AppName - this allows folks to independently use the same application for different networks (ie we could imagine multiple communities of validators wanting to put up a Hub using the same app but having a distinct network name. Arguably not needed if differences will come via different initial state / validators).

ChainID

Define ChainID = TMHASH(ChainDescriptor). It's the unique ID of a blockchain.

It should be Bech32 encoded when handled by users, eg. with cosmoschain prefix.

Forks and Uprades

When a chain forks or upgrades but continues the same history, it takes a new ChainDescription as follows:

ChainDescription = <ChainID>/x/<Height>/<ForkDescription>

Where

ChainID is the ChainID from the previous ChainDescription (ie. its hash)
x denotes that a change occured
Height is the height the change occured
ForkDescription has the same form as ChainDescription but for the fork
this allows forks to specify new versions for tendermint or the app, as well as arbitrary changes to the state or validator set

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