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Handling ClientMessages: updates and misbehaviour

As mentioned before in the documentation about implementing the ConsensusState interface, ClientMessage is an interface used to update an IBC client. This update may be performed by:

  • a single header
  • a batch of headers
  • evidence of misbehaviour,
  • or any type which when verified produces a change to the consensus state of the IBC client.

This interface has been purposefully kept generic in order to give the maximum amount of flexibility to the light client implementer.

Implementing the ClientMessage interface​

Find the ClientMessageinterface in modules/core/exported:

type ClientMessage interface {
  proto.Message

  ClientType() string
  ValidateBasic() error
}

The ClientMessage will be passed to the client to be used in UpdateClient, which retrieves the ClientState by client ID (available in MsgUpdateClient). This ClientState implements the ClientState interface for its specific consenus type (e.g. Tendermint).

UpdateClient will then handle a number of cases including misbehaviour and/or updating the consensus state, utilizing the specific methods defined in the relevant ClientState.

VerifyClientMessage(ctx sdk.Context, cdc codec.BinaryCodec, clientStore sdk.KVStore, clientMsg ClientMessage) error
CheckForMisbehaviour(ctx sdk.Context, cdc codec.BinaryCodec, clientStore sdk.KVStore, clientMsg ClientMessage) bool
UpdateStateOnMisbehaviour(ctx sdk.Context, cdc codec.BinaryCodec, clientStore sdk.KVStore, clientMsg ClientMessage)
UpdateState(ctx sdk.Context, cdc codec.BinaryCodec, clientStore sdk.KVStore, clientMsg ClientMessage) []Height

Handling updates and misbehaviour​

The functions for handling updates to a light client and evidence of misbehaviour are all found in the ClientState interface, and will be discussed below.

It is important to note that Misbehaviour in this particular context is referring to misbehaviour on the chain level intended to fool the light client. This will be defined by each light client.

VerifyClientMessage​

VerifyClientMessage must verify a ClientMessage. A ClientMessage could be a Header, Misbehaviour, or batch update. To understand how to implement a ClientMessage, please refer to the Implementing the ClientMessage interface section.

It must handle each type of ClientMessage appropriately. Calls to CheckForMisbehaviour, UpdateState, and UpdateStateOnMisbehaviour will assume that the content of the ClientMessage has been verified and can be trusted. An error should be returned if the ClientMessage fails to verify.

For an example of a VerifyClientMessage implementation, please check the Tendermint light client.

CheckForMisbehaviour​

Checks for evidence of a misbehaviour in Header or Misbehaviour type. It assumes the ClientMessage has already been verified.

For an example of a CheckForMisbehaviour implementation, please check the Tendermint light client.

The Tendermint light client defines Misbehaviour as two different types of situations: a situation where two conflicting Headers with the same height have been submitted to update a client's ConsensusState within the same trusting period, or that the two conflicting Headers have been submitted at different heights but the consensus states are not in the correct monotonic time ordering (BFT time violation). More explicitly, updating to a new height must have a timestamp greater than the previous consensus state, or, if inserting a consensus at a past height, then time must be less than those heights which come after and greater than heights which come before.

UpdateStateOnMisbehaviour​

UpdateStateOnMisbehaviour should perform appropriate state changes on a client state given that misbehaviour has been detected and verified. This method should only be called when misbehaviour is detected, as it does not perform any misbehaviour checks. Notably, it should freeze the client so that calling the Status function on the associated client state no longer returns Active.

For an example of a UpdateStateOnMisbehaviour implementation, please check the Tendermint light client.

UpdateState​

UpdateState updates and stores as necessary any associated information for an IBC client, such as the ClientState and corresponding ConsensusState. It should perform a no-op on duplicate updates.

It assumes the ClientMessage has already been verified.

For an example of a UpdateState implementation, please check the Tendermint light client.

Putting it all together​

The 02-client Keeper module in ibc-go offers a reference as to how these functions will be used to update the client.

if err := clientState.VerifyClientMessage(clientMessage); err != nil {
  return err
}

foundMisbehaviour := clientState.CheckForMisbehaviour(clientMessage)
if foundMisbehaviour {
  clientState.UpdateStateOnMisbehaviour(clientMessage)
  // emit misbehaviour event
  return 
}

clientState.UpdateState(clientMessage) // expects no-op on duplicate header
// emit update event
return