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Concepts

NOTE: if you are not familiar with the IBC terminology and concepts, please read this document as prerequisite reading.

Client Creation, Updates, and Upgrades

IBC clients are on chain light clients. The light client is responsible for verifying counterparty state. A light client can be created by any user submitting a valid initial ClientState and ConsensusState. The client identifier is auto generated using the client type and the global client counter appended in the format: {client-type}-{N}. Clients are given a client identifier prefixed store to store their associated client state and consensus states. Consensus states are stored using their associated height.

Clients can be updated by any user submitting a valid Header. The client state callback to CheckHeaderAndUpdateState is responsible for verifying the header against previously stored state. The function should also return the updated client state and consensus state if the header is considered a valid update. A light client, such as Tendermint, may have client specific parameters like TrustLevel which must be considered valid in relation to the Header. The update height is not necessarily the lastest height of the light client. Updates may fill in missing consensus state heights.

Clients may be upgraded. The upgrade should be verified using VerifyUpgrade. It is not a requirement to allow for light client upgrades. For example, the solo machine client will simply return an error on VerifyUpgrade. Clients which implement upgrades are expected to account for, but not necessarily support, planned and unplanned upgrades.

Client Misbehaviour

IBC clients must freeze when the counterparty chain becomes byzantine and takes actions that could fool the light client into accepting invalid state transitions. Thus, relayers are able to submit Misbehaviour proofs that prove that a counterparty chain has signed two Headers for the same height. This constitutes misbehaviour as the IBC client could have accepted either header as valid. Upon verifying the misbehaviour the IBC client must freeze at that height so that any proof verifications for the frozen height or later fail.

Note, there is a difference between the chain-level Misbehaviour that IBC is concerned with and the validator-level Evidence that Tendermint is concerned with. Tendermint must be able to detect, submit, and punish any evidence of individual validators breaking the Tendermint consensus protocol and attempting to mount an attack. IBC clients must only act when an attack is successful and the chain has successfully forked. In this case, valid Headers submitted to the IBC client can no longer be trusted and the client must freeze.

Governance may then choose to override a frozen client and provide the correct, canonical Header so that the client can continue operating after the Misbehaviour submission.

ClientUpdateProposal

A governance proposal may be passed to update a specified client using another client known as the "substitute client". This is useful in unfreezing clients or updating expired clients, thereby making the effected channels active again. Each client is expected to implement this functionality. A client may choose to disallow an update by a governance proposal by returning an error in the client state function 'CheckSubstituteAndUpdateState'.

The localhost client cannot be updated by a governance proposal.

The solo machine client requires the boolean flag 'AllowUpdateAfterProposal' to be set to true in order to be updated by a proposal. This is set upon client creation and cannot be updated later.

The tendermint client has two flags update flags, 'AllowUpdateAfterExpiry' and 'AllowUpdateAfterMisbehaviour'. The former flag can only be used to unexpire clients. The latter flag can be used to unfreeze a client and if necessary it will also unexpire the client. It is best practice to initialize a new substitute client instead of using an existing one This avoids potential issues of the substitute becoming frozen due to misbehaviour or the subject client becoming refrozen due to misbehaviour not being expired at the time the proposal passes. These boolean flags are set upon client creation and cannot be updated later.

The CheckSubstituteAndUpdateState function provides the light client with its own client store, the client store of the substitute, the substitute client state, and the intitial height that should be used when referring to the substitute client. Most light client implementations should copy consensus states from the substitute to the subject, but are not required to do so. Light clients may copy informationa as they deem necessary.

It is not recommended to use a substitute client in normal operations since the subject light client will be given unrestricted access to the substitute client store. Governance should not pass votes which enable byzantine light client modules from modifying the state of the substitute.

IBC Client Heights

IBC Client Heights are represented by the struct:

type Height struct {
   RevisionNumber uint64
   RevisionHeight  uint64
}

The RevisionNumber represents the revision of the chain that the height is representing. An revision typically represents a continuous, monotonically increasing range of block-heights. The RevisionHeight represents the height of the chain within the given revision.

On any reset of the RevisionHeight, for example, when hard-forking a Tendermint chain, the RevisionNumber will get incremented. This allows IBC clients to distinguish between a block-height n of a previous revision of the chain (at revision p) and block-height n of the current revision of the chain (at revision e).

Heights that share the same revision number can be compared by simply comparing their respective RevisionHeights. Heights that do not share the same revision number will only be compared using their respective RevisionNumbers. Thus a height h with revision number e+1 will always be greater than a height g with revision number e, REGARDLESS of the difference in revision heights.

Ex:

Height{RevisionNumber: 3, RevisionHeight: 0} > Height{RevisionNumber: 2, RevisionHeight: 100000000000}

When a Tendermint chain is running a particular revision, relayers can simply submit headers and proofs with the revision number given by the chain's chainID, and the revision height given by the Tendermint block height. When a chain updates using a hard-fork and resets its block-height, it is responsible for updating its chain-id to increment the revision number. IBC Tendermint clients then verifies the revision number against their ChainId and treat the RevisionHeight as the Tendermint block-height.

Tendermint chains wishing to use revisions to maintain persistent IBC connections even across height-resetting upgrades must format their chain-ids in the following manner: {chainID}-{revision_number}. On any height-resetting upgrade, the chainID MUST be updated with a higher revision number than the previous value.

Ex:

Before upgrade ChainID: gaiamainnet-3
After upgrade ChainID: gaiamainnet-4

Clients that do not require revisions, such as the solo-machine client, simply hardcode 0 into the revision number whenever they need to return an IBC height when implementing IBC interfaces and use the RevisionHeight exclusively.

Other client-types may implement their own logic to verify the IBC Heights that relayers provide in their Update, Misbehavior, and Verify functions respectively.

The IBC interfaces expect an ibcexported.Height interface, however all clients should use the concrete implementation provided in 02-client/types and reproduced above.

Connection Handshake

The connection handshake occurs in 4 steps as defined in ICS 03.

ConnOpenInit is the first attempt to initialize a connection on the executing chain. The handshake is expected to succeed if the version selected is supported. The connection identifier for the counterparty connection must be left empty indicating that the counterparty must select its own identifier. The connection identifier is auto derived in the format: connection{N} where N is the next sequence to be used. The counter begins at 0 and increments by 1. The connection is set and stored in the INIT state upon success.

ConnOpenTry is a response to a chain executing ConnOpenInit. The executing chain will validate the chain level parameters the counterparty has stored such as its chainID. The executing chain will also verify that if a previous connection exists for the specified connection identifier that all the parameters match and its previous state was in INIT. This may occur when both chains execute ConnOpenInit simultaneously. If the connection does not exist then a connection identifier is generated in the same format done in ConnOpenInit. The executing chain will verify that the counterparty created a connection in INIT state. The executing chain will also verify The ClientState and ConsensusState the counterparty stores for the executing chain. The executing chain will select a version from the intersection of its supported versions and the versions set by the counterparty. The connection is set and stored in the TRYOPEN state upon success.

ConnOpenAck may be called on a chain when the counterparty connection has entered TRYOPEN. A previous connection on the executing chain must exist in either INIT or TRYOPEN. The executing chain will verify the version the counterparty selected. If the counterparty selected its own connection identifier, it will be validated in the basic validation of a MsgConnOpenAck. The counterparty connection state is verified along with the ClientState and ConsensusState stored for the executing chain. The connection is set and stored in the OPEN state upon success.

ConnOpenConfirm is a response to a chain executing ConnOpenAck. The executing chain's connection must be in TRYOPEN. The counterparty connection state is verified to be in the OPEN state. The connection is set and stored in the OPEN state upon success.

Connection Version Negotiation

During the handshake procedure for connections a version is agreed upon between the two parties. This occurs during the first 3 steps of the handshake.

During ConnOpenInit, party A is expected to set all the versions they wish to support within their connection state. It is expected that this set of versions is from most preferred to least preferred. This is not a strict requirement for the SDK implementation of IBC because the party calling ConnOpenTry will greedily select the latest version it supports that the counterparty supports as well. A specific version can optionally be passed as Version to ensure that the handshake will either complete with that version or fail.

During ConnOpenTry, party B will select a version from the counterparty's supported versions. Priority will be placed on the latest supported version. If a matching version cannot be found an error is returned.

During ConnOpenAck, party A will verify that they can support the version party B selected. If they do not support the selected version an error is returned. After this step, the connection version is considered agreed upon.

A Version is defined as follows:

type Version struct {
	// unique version identifier
	Identifier string 
	// list of features compatible with the specified identifier
	Features []string 
}

A version must contain a non empty identifier. Empty feature sets are allowed, but each feature must be a non empty string.

::: warning A set of versions should not contain two versions with the same identifier, but differing feature sets. This will result in undefined behavior with regards to version selection in ConnOpenTry. Each version in a set of versions should have a unique version identifier. :::

Channel Handshake

The channel handshake occurs in 4 steps as defined in ICS 04.

ChanOpenInit is the first attempt to initialize a channel on top of an existing connection. The handshake is expected to succeed if the version selected for the existing connection is a supported IBC version. The portID must correspond to a port already binded upon InitChain. The channel identifier for the counterparty channel must be left empty indicating that the counterparty must select its own identifier. The channel identifier is auto derived in the format: channel{N} where N is the next sequence to be used. The channel is set and stored in the INIT state upon success. The channel parameters NextSequenceSend, NextSequenceRecv, and NextSequenceAck are all set to 1 and a channel capability is created for the given portID and channelID path.

ChanOpenTry is a response to a chain executing ChanOpenInit. If the executing chain is calling ChanOpenTry after previously executing ChanOpenInit then the provided channel parameters must match the previously selected parameters. If the previous channel does not exist then a channel identifier is generated in the same format as done in ChanOpenInit. The connection the channel is created on top of must be an OPEN state and its IBC version must support the desired channel type being created (ORDERED, UNORDERED, etc). The executing chain will verify that the channel state of the counterparty is in INIT. The executing chain will set and store the channel state in TRYOPEN. The channel parameters NextSequenceSend, NextSequenceRecv, and NextSequenceAck are all set to 1 and a channel capability is created for the given portID and channelID path only if the channel did not previously exist.

ChanOpenAck may be called on a chain when the counterparty channel has entered TRYOPEN. A previous channel on the executing chain must exist be in either INIT or TRYOPEN state. If the counterparty selected its own channel identifier, it will be validated in the basic validation of MsgChanOpenAck. The executing chain verifies that the counterparty channel state is in TRYOPEN. The channel is set and stored in the OPEN state upon success.

ChanOpenConfirm is a response to a chain executing ChanOpenAck. The executing chain's previous channel state must be in TRYOPEN. The executing chain verifies that the counterparty channel state is OPEN. The channel is set and stored in the OPEN state upon success.

Channel Version Negotiation

During the channel handshake procedure a version must be agreed upon between the two parties. The selection process is largely left to the callers and the verification of valid versioning must be handled by application developers in the channel handshake callbacks.

During ChanOpenInit, a version string is passed in and set in party A's channel state.

During ChanOpenTry, a version string for party A and for party B are passed in. The party A version string must match the version string used in ChanOpenInit otherwise channel state verification will fail. The party B version string could be anything (even different than the proposed one by party A). However, the proposed version by party B is expected to be fully supported by party A.

During the ChanOpenAck callback, the application module is expected to verify the version proposed by party B using the MsgChanOpenAck CounterpartyVersion field. The application module should throw an error if the version string is not valid.

In general empty version strings are to be considered valid options for an application module.

Application modules may implement their own versioning system, such as semantic versioning, or they may lean upon the versioning system used for in connection version negotiation. To use the connection version semantics the application would simply pass the proto encoded version into each of the handshake calls and decode the version string into a Version instance to do version verification in the handshake callbacks.

Implementations which do not feel they would benefit from versioning can do basic string matching using a single compatible version.

Sending, Receiving, Acknowledging Packets

Terminology: Packet Commitment A hash of the packet stored on the sending chain. Packet Receipt A single bit indicating that a packet has been received. Used for timeouts. Acknowledgement Data written to indicate the result of receiving a packet. Typically conveying either success or failure of the receive.

A packet may be associated with one of the following states:

the packet does not exist (ie it has not been sent)
the packet has been sent but not received (the packet commitment exists on the sending chain, but no receipt exists on the receiving chain)
the packet has been received but not acknowledged (packet commitment exists on the sending chain, a receipt exists on the receiving chain, but no acknowledgement exists on the receiving chain)
the packet has been acknowledgement but the acknowledgement has not been relayed (the packet commitment exists on the sending chain, the receipt and acknowledgement exist on the receiving chain)
the packet has completed its life cycle (the packet commitment does not exist on the sending chain, but a receipt and acknowledgement exist on the receiving chain)

Sending of a packet is initiated by a call to the ChannelKeeper.SendPacket function by an application module. Packets being sent will be verified for correctness (core logic only). If the packet is valid, a hash of the packet will be stored as a packet commitment using the packet sequence in the key. Packet commitments are stored on the sending chain.

A message should be sent to the receving chain indicating that the packet has been committed on the sending chain and should be received on the receiving chain. The light client on the receiving chain, which verifies the sending chain's state, should be updated to the lastest sending chain state if possible. The verification will fail if the latest state of the light client does not include the packet commitment. The receiving chain is responsible for verifying that the counterparty set the hash of the packet. If verification of the packet to be received is successful, the receiving chain should store a receipt of the packet and call application logic if necessary. An acknowledgement may be processed and stored at this time (synchronously) or at another point in the future (asynchronously).

Acknowledgements written on the receiving chain may be verified on the sending chain. If the sending chain successfully verifies the acknowledgement then it may delete the packet commitment stored at that sequence. There is no requirement for acknowledgements to be written. Only the hash of the acknowledgement is stored on the chain. Application logic may be executed in conjunction with verifying an acknowledgement. For example, in fungible cross-chain token transfer, a failed acknowledgement results in locked or burned funds being refunded.

Relayers are responsible for reconstructing packets between the sending, receiving, and acknowledging of packets.

IBC applications sending and receiving packets are expected to appropriately handle data contained within a packet. For example, cross-chain token transfers will unmarshal the data into proto definitions representing a token transfer.

Future optimizations may allow for storage cleanup. Stored packet commitments could be removed from channels which do not write packet acknowledgements and acknowledgements could be removed when a packet has completed its life cycle.

Timing out Packets

A packet may be timed out on the receiving chain if the packet timeout height or timestamp has been surpassed on the receving chain or the channel has closed. A timed out packet can only occur if the packet has never been received on the receiving chain. ORDERED channels will verify that the packet sequence is greater than the NextSequenceRecv on the receiving chain. UNORDERED channels will verify that the packet receipt has not been written on the receiving chain. A timeout on channel closure will additionally verify that the counterparty channel has been closed. A successful timeout may execute application logic as appropriate.

Both the packet's timeout timestamp and the timeout height must have been surpassed on the receiving chain for a timeout to be valid. A timeout timestamp or timeout height with a 0 value indicates the timeout field may be ignored. Each packet is required to have at least one valid timeout field.

Closing Channels

Closing a channel occurs in occurs in 2 handshake steps as defined in ICS 04.

ChanCloseInit will close a channel on the executing chain if the channel exists, it is not already closed and the connection it exists upon is OPEN. Channels can only be closed by a calling module or in the case of a packet timeout on an ORDERED channel.

ChanCloseConfirm is a response to a counterparty channel executing ChanCloseInit. The channel on the executing chain will be closed if the channel exists, the channel is not already closed, the connection the channel exists upon is OPEN and the executing chain successfully verifies that the counterparty channel has been closed.

Port and Channel Capabilities

Hostname Validation

Hostname validation is implemented as defined in ICS 24.

The 24-host sub-module parses and validates identifiers. It also builds the key paths used to store IBC related information.

A valid identifier must conatin only alphanumeric characters or the following list of allowed characters: ".", "_", "+", "-", "#", "[", "]", "<", ">"

Client identifiers must contain between 9 and 64 characters.
Connection identifiers must contain between 10 and 64 characters.
Channel identifiers must contain between 10 and 64 characters.
Port identifiers must contain between 2 and 64 characters.

Proofs

Proofs for counterparty state validation are provided as bytes. These bytes can be unmarshaled into proto definitions as necessary by light clients. For example, the Tendermint light client will use the bytes as a merkle proof where as the solo machine client will unmarshal the proof into several layers proto definitions used for signature verficiation.

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