IBC Client functionality

[adizere]

Context

IBC Clients are responsible for verifying the state of foreign chains in IBC. They are stored on-chain, subject to client specific updates. The IBC relayer is in charge of driving client updates. There are two primary interfaces that compose IBC clients:

1. client handlers: functions exposed by the IBC module to permit creating, updating, and upgrading an IBC client;
2. verification methods: functions exposed by clients to provide verification of incoming messages.

Further reading

IBC Client English spec: ics-002-client-semantics
IBC Client handlers TLA+ model: ICS02ClientHandlers.tla part of IBC-core TLA+ spec

Dependencies

For basic implementation:

• ICS23 vector-commitments

For complete implementation & acceptance:

• ICS26: ics-026-routing-module
• ICS18: ics-018-relayer-algorithms
• model-based testing (MBT) harness

Acceptance criteria

Component	Functionality	Criteria	Status
Module	Create, Update Handlers	Impl & unit tests	[X]
Module	Upgrade Handler	Impl & unit tests	[ ]
Module	Create, Update, Upgrade Handlers	MBT tests	[ ]
Module	Create, Update Handlers	TLA+ specs	[X]
Module	Verification methods	Impl & mock tests	[ ]
CLI	Create, Update	Impl & end-to-end tests	[X]
CLI	Upgrade	Impl & end-to-end tests	under review
CLI	Periodic client updates	Impl & mock tests	[ ]

Related issues

The following ongoing issues provide a more fine-grained perspective on the different parts of IBC client functionality:

• Incomplete ICS 02 verification functions: https://github.com/informalsystems/ibc-rs/issues/726
• Upgrade Client:
  • Handler: Handler for upgrade client #722
  • CLI support: Relayer CLI for client upgrade message #357
• MBT: Model-based testing for ICS02 via ICS26 #324
• CLI support for periodic client updates: MVP Client Relayer: Periodic Client Updates #216

[adizere]

Closing: this admin issue served its purpose of helping clarify the missing pieces in our roadmap.