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In uncommon situations, a highly valued client may become frozen due to uncontrollable circumstances. A highly valued client might have hundreds of channels being actively used. Some of those channels might have a significant amount of locked tokens used for ICS 20. If the one third of the validator set of the chain the client represents decides to collude, they can sign off on two valid but conflicting headers each signed by the other one third of the honest validator set. The light client can now be updated with two valid, but conflicting headers at the same height. The light client cannot know which header is trustworthy and therefore evidence of such misbehaviour is likely to be submitted resulting in a frozen light client. Frozen light clients cannot be updated under any circumstance except via a governance proposal. Since a quorum of validators can sign arbitrary state roots which may not be valid executions of the state machine, a governance proposal has been added to ease the complexity of unfreezing or updating clients which have become “stuck”. Without this mechanism, validator sets would need to construct a state root to unfreeze the client. Unfreezing clients, re-enables all of the channels built upon that client. This may result in recovery of otherwise lost funds. Tendermint light clients may become expired if the trusting period has passed since their last update. This may occur if relayers stop submitting headers to update the clients. An unplanned upgrade by the counterparty chain may also result in expired clients. If the counterparty chain undergoes an unplanned upgrade, there may be no commitment to that upgrade signed by the validator set before the chain-id changes. In this situation, the validator set of the last valid update for the light client is never expected to produce another valid header since the chain-id has changed, which will ultimately lead the on-chain light client to become expired. In the case that a highly valued light client is frozen, expired, or rendered non-updateable, a governance proposal may be submitted to update this client, known as the subject client. The proposal includes the client identifier for the subject and the client identifier for a substitute client. Light client implementations may implement custom updating logic, but in most cases, the subject will be updated to the latest consensus state of the substitute client, if the proposal passes. The substitute client is used as a “stand in” while the subject is on trial. It is best practice to create a substitute client after the subject has become frozen to avoid the substitute from also becoming frozen. An active substitute client allows headers to be submitted during the voting period to prevent accidental expiry once the proposal passes.

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How to recover an expired client with a governance proposal

See also the relevant documentation: ADR-026, IBC client recovery mechanisms

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Preconditions

  • The chain is updated with ibc-go >= v1.1.0.
  • The client identifier of an active client for the same counterparty chain.
  • The governance deposit.

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Steps

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Step 1

Check if the client is attached to the expected chain-id. For example, for an expired Tendermint client representing the Akash chain the client state looks like this on querying the client state: 
{
  client_id: 07-tendermint-146
  client_state:
  '@type': /ibc.lightclients.tendermint.v1.ClientState
  allow_update_after_expiry: true
  allow_update_after_misbehaviour: true
  chain_id: akashnet-2
}
The client is attached to the expected Akash chain-id. Note that although the parameters (allow_update_after_expiry and allow_update_after_misbehaviour) exist to signal intent, these parameters have been deprecated and will not enforce any checks on the revival of client. See ADR-026 for more context on this deprecation.

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Step 2

If the chain has been updated to ibc-go >= v1.1.0, anyone can submit the governance proposal to recover the client by executing this via cli: 
<binary> tx gov submit-proposal update-client <expired-client-id> <active-client-id>
The <active-client-id> should be a client identifier on the same chain as the expired or frozen client. This client identifier should connect to the same chain as the expired or frozen client. This means: use the active client that is currently being used to relay packets between the two chains as the replacement client. After this, it is just a question of who funds the governance deposit and if the chain in question votes yes.

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Important considerations

Please note that from v1.0.0 of ibc-go it will not be allowed for transactions to go to expired clients anymore, so please update to at least this version to prevent similar issues in the future. Please also note that if the client on the other end of the transaction is also expired, that client will also need to update. This process updates only one client.