Skip to main content

​
Synopsis

This standards document describes the abstraction of an IBC connection: two stateful objects (connection ends) on two separate chains, each associated with a light client of the other chain, which together facilitate cross-chain sub-state verification and packet association (through channels). A protocol for safely establishing a connection between two chains is described.

​
Motivation

The core IBC protocol provides authorisation and ordering semantics for packets: guarantees, respectively, that packets have been committed on the sending blockchain (and according state transitions executed, such as escrowing tokens), and that they have been committed exactly once in a particular order and can be delivered exactly once in that same order. The connection abstraction specified in this standard, in conjunction with the client abstraction specified in ICS 2, defines the authorisation semantics of IBC. Ordering semantics are described in ICS 4).

​
Definitions

Client-related types & functions are as defined in ICS 2. Channel and packet-related functions are as defined in ICS 4. Commitment proof related types & functions are defined in ICS 23 Identifier and other host state machine requirements are as defined in ICS 24. The identifier is not necessarily intended to be a human-readable name (and likely should not be, to discourage squatting or racing for identifiers). The opening handshake protocol allows each chain to verify the identifier used to reference the connection on the other chain, enabling modules on each chain to reason about the reference on the other chain. An actor, as referred to in this specification, is an entity capable of executing datagrams who is paying for computation / storage (via gas or a similar mechanism) but is otherwise untrusted. Possible actors include:
  • End users signing with an account key
  • On-chain smart contracts acting autonomously or in response to another transaction
  • On-chain modules acting in response to another transaction or in a scheduled manner

​
Desired Properties

  • Implementing blockchains should be able to safely allow untrusted actors to open and update connections.

​
Pre-Establishment

Prior to connection establishment:
  • No further IBC sub-protocols should operate, since cross-chain sub-states cannot be verified.
  • The initiating actor (who creates the connection) must be able to specify an initial consensus state for the chain to connect to and an initial consensus state for the connecting chain (implicitly, e.g. by sending the transaction).

​
During Handshake

Once a negotiation handshake has begun:
  • Only the appropriate handshake datagrams can be executed in order.
  • No third chain can masquerade as one of the two handshaking chains

​
Post-Establishment

Once a negotiation handshake has completed:
  • The created connection objects on both chains contain the consensus states specified by the initiating actor.
  • No other connection objects can be maliciously created on other chains by replaying datagrams.

​
Technical Specification

​
Data Structures

This ICS defines the ConnectionState and ConnectionEnd types: 
enum ConnectionState {
  INIT,
  TRYOPEN,
  OPEN,
}

interface ConnectionEnd {
  state: ConnectionState
  counterpartyConnectionIdentifier: Identifier
  counterpartyPrefix: CommitmentPrefix
  clientIdentifier: Identifier
  counterpartyClientIdentifier: Identifier
  version: string | []string
  delayPeriodTime: uint64
  delayPeriodBlocks: uint64
}
  • The state field describes the current state of the connection end.
  • The counterpartyConnectionIdentifier field identifies the connection end on the counterparty chain associated with this connection.
  • The counterpartyPrefix field contains the prefix used for state verification on the counterparty chain associated with this connection. Chains should expose an endpoint to allow relayers to query the connection prefix. If not specified, a default counterpartyPrefix of "ibc" should be used.
  • The clientIdentifier field identifies the client associated with this connection.
  • The counterpartyClientIdentifier field identifies the client on the counterparty chain associated with this connection.
  • The version field is an opaque string which can be utilised to determine encodings or protocols for channels or packets utilising this connection. If not specified, a default version of "" should be used.
  • The delayPeriodTime indicates a period in time that must elapse after validation of a header before a packet, acknowledgement, proof of receipt, or timeout can be processed.
  • The delayPeriodBlocks indicates a period in blocks that must elapse after validation of a header before a packet, acknowledgement, proof of receipt, or timeout can be processed.

​
Store paths

Connection paths are stored under a unique identifier. 
function connectionPath(id: Identifier): Path {
  return "connections/{id}"
}
A reverse mapping from clients to a set of connections (utilised to look up all connections using a client) is stored under a unique prefix per-client: 
function clientConnectionsPath(clientIdentifier: Identifier): Path {
  return "clients/{clientIdentifier}/connections"
}

​
Helper functions

addConnectionToClient is used to add a connection identifier to the set of connections associated with a client. 
function addConnectionToClient(
  clientIdentifier: Identifier,
  connectionIdentifier: Identifier) {
    conns = privateStore.get(clientConnectionsPath(clientIdentifier))
    conns.add(connectionIdentifier)
    privateStore.set(clientConnectionsPath(clientIdentifier), conns)
}
Helper functions are defined by the connection to pass the CommitmentPrefix associated with the connection to the verification function provided by the client. In the other parts of the specifications, these functions MUST be used for introspecting other chains’ state, instead of directly calling the verification functions on the client. 
function verifyClientConsensusState(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  clientIdentifier: Identifier,
  consensusStateHeight: Height,
  consensusState: ConsensusState
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, consensusStatePath(clientIdentifier, consensusStateHeight))
  return verifyMembership(clientState, height, 0, 0, proof, path, consensusState)
}

function verifyClientState(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  clientState: ClientState
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, clientStatePath(clientIdentifier)
  return verifyMembership(clientState, height, 0, 0, proof, path, clientState)
}

function verifyConnectionState(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  connectionIdentifier: Identifier,
  connectionEnd: ConnectionEnd
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, connectionPath(connectionIdentifier))
  return verifyMembership(clientState, height, 0, 0, proof, path, connectionEnd)
}

function verifyChannelState(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  channelEnd: ChannelEnd
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, channelPath(portIdentifier, channelIdentifier))
  return verifyMembership(clientState, height, 0, 0, proof, path, channelEnd)
}

function verifyPacketCommitment(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  sequence: uint64,
  commitmentBytes: bytes
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, packetCommitmentPath(portIdentifier, channelIdentifier, sequence))
  return verifyMembership(clientState, height, connection.delayPeriodTime, connection.delayPeriodBlocks, proof, path, commitmentBytes)
}

function verifyPacketAcknowledgement(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  sequence: uint64,
  acknowledgement: bytes
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, packetAcknowledgementPath(portIdentifier, channelIdentifier, sequence))
  return verifyMembership(clientState, height, connection.delayPeriodTime, connection.delayPeriodBlocks, proof, path, acknowledgement)
}

function verifyPacketReceiptAbsence(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  sequence: uint64
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, packetReceiptPath(portIdentifier, channelIdentifier, sequence))
  return verifyNonMembership(clientState, height, connection.delayPeriodTime, connection.delayPeriodBlocks, proof, path)
}

// OPTIONAL: verifyPacketReceipt is only required to support new channel types beyond ORDERED and UNORDERED.
function verifyPacketReceipt(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  sequence: uint64,
  receipt: bytes
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, packetReceiptPath(portIdentifier, channelIdentifier, sequence))
  return verifyMembership(clientState, height, connection.delayPeriodTime, connection.delayPeriodBlocks, connection.counterpartyPrefix, proof, portIdentifier, channelIdentifier, sequence, receipt)
}

function verifyNextSequenceRecv(
  connection: ConnectionEnd,
  height: Height,
  proof: CommitmentProof,
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  sequence: uint64,
  nextSequenceRecv: uint64
) {
  clientState = queryClientState(connection.clientIdentifier)
  path = applyPrefix(connection.counterpartyPrefix, nextSequenceRecvPath(portIdentifier, channelIdentifier, sequence))
  return verifyMembership(clientState, height, connection.delayPeriodTime, connection.delayPeriodBlocks, proof, path, nextSequenceRecv)
}

function verifyMultihopMembership(
  connection: ConnectionEnd, // the connection end corresponding to the receiving chain.
  height: Height,
  proof: MultihopProof,
  connectionHops: []Identifier,
  key: CommitmentPath,
  value: bytes
) {
  // the connectionEnd corresponding to the end of the multi-hop channel path (sending/counterparty chain).
  multihopConnectionEnd = abortTransactionUnless(getMultihopConnectionEnd(proof))
  prefix = multihopConnectionEnd.GetCounterparty().GetPrefix()
  client = queryClient(connection.clientIdentifier)
  consensusState = queryConsensusState(connection.clientIdentifier, height)

  abortTransactionUnless(client.Status() === "active")
  abortTransactionUnless(client.GetLatestHeight() >= height)

  // verify maximum delay period has passed
  expectedTimePerBlock = queryMaxExpectedTimePerBlock()
  delayPeriodTime = abortTransactionUnless(getMaximumDelayPeriod(proof, connection))
  delayPeriodBlocks = getBlockDelay(delayPeriodTime, expectedTimePerBlock)
  abortTransactionUnless(tendermint.VerifyDelayPeriodPassed(height, delayPeriodTime, delayPeriodBlocks))

  return multihop.VerifyMultihopMembership(consensusState, connectionHops, proof, prefix, key, value) // see ics-033
}

function verifyMultihopNonMembership(
  connection: ConnectionEnd, // the connection end corresponding to the receiving chain.
  height: Height,
  proof: MultihopProof,
  connectionHops: Identifier[],
  key: CommitmentPath
) {
  // the connectionEnd corresponding to the end of the multi-hop channel path (sending/counterparty chain).
  multihopConnectionEnd = abortTransactionUnless(getMultihopConnectionEnd(proof))
  prefix = multihopConnectionEnd.GetCounterparty().GetPrefix()
  client = queryClient(connection.clientIdentifier)
  consensusState = queryConsensusState(connection.clientIdentifier, height)

  abortTransactionUnless(client.Status() === "active")
  abortTransactionUnless(client.GetLatestHeight() >= height)

  // verify maximum delay period has passed
  expectedTimePerBlock = queryMaxExpectedTimePerBlock()
  delayPeriodTime = abortTransactionUnless(getMaximumDelayPeriod(proof, connection))
  delayPeriodBlocks = getBlockDelay(delayPeriodTime, expectedTimePerBlock)
  abortTransactionUnless(tendermint.VerifyDelayPeriodPassed(height, delayPeriodTime, delayPeriodBlocks))

  return multihop.VerifyMultihopNonMembership(consensusState, connectionHops, proof, prefix, key) // see ics-033
}

// Return the maximum expected time per block from the paramstore.
// See 03-connection - GetMaxExpectedTimePerBlock.
function queryMaxExpectedTimePerBlock(): uint64

function getTimestampAtHeight(
  connection: ConnectionEnd,
  height: Height
) {
  return queryConsensusState(connection.clientIdentifier, height).getTimestamp()
}

// Return the connectionEnd corresponding to the source chain.
function getMultihopConnectionEnd(proof: MultihopProof): ConnectionEnd {
  return abortTransactionUnless(Unmarshal(proof.ConnectionProofs[proof.ConnectionProofs.length - 1].Value))
}

// Return the maximum delay period in seconds across all connections in the channel path.
function getMaximumDelayPeriod(proof: MultihopProof, lastConnection: ConnectionEnd): number {
  delayPeriodTime = lastConnection.GetDelayPeriod()
  for connData in range proofs.ConnectionProofs {
    connectionEnd = abortTransactionUnless(Unmarshal(connData.Value))
    if (connectionEnd.DelayPeriod > delayPeriodTime) {
      delayPeriodTime = connectionEnd.DelayPeriod
    }
  }
  return delayPeriodTime
}

​
Sub-protocols

This ICS defines the opening handshake subprotocol. Once opened, connections cannot be closed and identifiers cannot be reallocated (this prevents packet replay or authorisation confusion). Header tracking and misbehaviour detection are defined in ICS 2. State Machine Diagram

​
Identifier validation

Connections are stored under a unique Identifier prefix. The validation function validateConnectionIdentifier MAY be provided. 
type validateConnectionIdentifier = (id: Identifier) => boolean
If not provided, the default validateConnectionIdentifier function will always return true.

​
Versioning

During the handshake process, two ends of a connection come to agreement on a version associated with that connection. This Version datatype is defined as: 
interface Version {
  identifier: string
  features: [string]
}
The identifier field specifies a unique version identifier. A value of "1" specifies IBC 1.0.0. The features field specifies a list of features compatible with the specified identifier. The values "ORDER_UNORDERED" and "ORDER_ORDERED" specify unordered and ordered channels, respectively. Host state machine MUST utilise the version data to negotiate encodings, priorities, or connection-specific metadata related to custom logic on top of IBC. It is assumed that the two chains running the opening handshake have at least one compatible version in common (i.e., the compatible versions of the two chains must have a non-empty intersection). If the two chains do not have any mutually acceptable versions, the handshake will fail. An implementation MUST define a function getCompatibleVersions which returns the list of versions it supports, ranked by descending preference order. 
type getCompatibleVersions = () => [Version]
An implementation MUST define a function pickVersion to choose a version from a list of versions. 
type pickVersion = ([Version]) => Version

​
Opening Handshake

The opening handshake sub-protocol serves to initialise consensus states for two chains on each other. The opening handshake defines four datagrams: ConnOpenInit, ConnOpenTry, ConnOpenAck, and ConnOpenConfirm. A correct protocol execution flows as follows (note that all calls are made through modules per ICS 25):
InitiatorDatagramChain acted uponPrior state (A, B)Posterior state (A, B)
ActorConnOpenInitA(none, none)(INIT, none)
RelayerConnOpenTryB(INIT, none)(INIT, TRYOPEN)
RelayerConnOpenAckA(INIT, TRYOPEN)(OPEN, TRYOPEN)
RelayerConnOpenConfirmB(OPEN, TRYOPEN)(OPEN, OPEN)
At the end of an opening handshake between two chains implementing the sub-protocol, the following properties hold:
  • Each chain has each other’s correct consensus state as originally specified by the initiating actor.
  • Each chain has knowledge of and has agreed to its identifier on the other chain.
This sub-protocol need not be permissioned, modulo anti-spam measures. Chains MUST implement a function generateIdentifier which chooses an identifier, e.g. by incrementing a counter: 
type generateIdentifier = () -> Identifier
A specific version can optionally be passed as version to ensure that the handshake will either complete with that version or fail. ConnOpenInit initialises a connection attempt on chain A. 
function connOpenInit(
  counterpartyPrefix: CommitmentPrefix,
  clientIdentifier: Identifier,
  counterpartyClientIdentifier: Identifier,
  version: string,
  delayPeriodTime: uint64,
  delayPeriodBlocks: uint64) {
    // generate a new identifier
    identifier = generateIdentifier()

    abortTransactionUnless(queryClientState(clientIdentifier) !== null)
    abortTransactionUnless(provableStore.get(connectionPath(identifier)) == null)

    state = INIT
    if version != "" {
      // manually selected version must be one we can support
      abortTransactionUnless(getCompatibleVersions().indexOf(version) > -1)
      versions = [version]
    } else {
      versions = getCompatibleVersions()
    }
    connection = ConnectionEnd{state, "", counterpartyPrefix,
      clientIdentifier, counterpartyClientIdentifier, versions, delayPeriodTime, delayPeriodBlocks}
    provableStore.set(connectionPath(identifier), connection)
    addConnectionToClient(clientIdentifier, identifier)
}
ConnOpenTry relays notice of a connection attempt on chain A to chain B (this code is executed on chain B). 
function connOpenTry(
  counterpartyConnectionIdentifier: Identifier,
  counterpartyPrefix: CommitmentPrefix,
  counterpartyClientIdentifier: Identifier,
  clientIdentifier: Identifier,
  clientState: ClientState, // DEPRECATED
  counterpartyVersions: string[],
  delayPeriodTime: uint64,
  delayPeriodBlocks: uint64,
  proofInit: CommitmentProof,
  proofClient: CommitmentProof, // DEPRECATED
  proofConsensus: CommitmentProof, // DEPRECATED
  proofHeight: Height,
  consensusHeight: Height,
  hostConsensusStateProof?: bytes, // DEPRECATED
) {
    // generate a new identifier
    identifier = generateIdentifier()

    abortTransactionUnless(queryClientState(clientIdentifier) !== null)
    expectedConnectionEnd = ConnectionEnd{INIT, "", getCommitmentPrefix(), counterpartyClientIdentifier,
                             clientIdentifier, counterpartyVersions, delayPeriodTime, delayPeriodBlocks}

    versionsIntersection = intersection(counterpartyVersions, getCompatibleVersions())
    version = pickVersion(versionsIntersection) // aborts transaction if there is no intersection

    connection = ConnectionEnd{TRYOPEN, counterpartyConnectionIdentifier, counterpartyPrefix,
                               clientIdentifier, counterpartyClientIdentifier, [version], delayPeriodTime, delayPeriodBlocks}
    abortTransactionUnless(connection.verifyConnectionState(proofHeight, proofInit, counterpartyConnectionIdentifier, expectedConnectionEnd))
    
    provableStore.set(connectionPath(identifier), connection)
    addConnectionToClient(clientIdentifier, identifier)
}
ConnOpenAck relays acceptance of a connection open attempt from chain B back to chain A (this code is executed on chain A). 
function connOpenAck(
  identifier: Identifier,
  clientState: ClientState, // DEPRECATED
  version: string,
  counterpartyIdentifier: Identifier,
  proofTry: CommitmentProof,
  proofClient: CommitmentProof, // DEPRECATED
  proofConsensus: CommitmentProof, // DEPRECATED
  proofHeight: Height,
  consensusHeight: Height,
  hostConsensusStateProof?: bytes, // DEPRECATED
) {
    connection = provableStore.get(connectionPath(identifier))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === INIT && connection.versions.indexOf(version) !== -1)
    expectedConnectionEnd = ConnectionEnd{
      TRYOPEN,
      identifier,
      getCommitmentPrefix(),
      connection.counterpartyClientIdentifier,
      connection.clientIdentifier,
      [version],
      connection.delayPeriodTime,
      connection.delayPeriodBlocks
    }
    abortTransactionUnless(connection.verifyConnectionState(proofHeight, proofTry, counterpartyIdentifier, expectedConnectionEnd))
    connection.state = OPEN
    connection.versions = [version]
    connection.counterpartyConnectionIdentifier = counterpartyIdentifier
    provableStore.set(connectionPath(identifier), connection)
}
ConnOpenConfirm confirms opening of a connection on chain A to chain B, after which the connection is open on both chains (this code is executed on chain B). 
function connOpenConfirm(
  identifier: Identifier,
  proofAck: CommitmentProof,
  proofHeight: Height) {
    connection = provableStore.get(connectionPath(identifier))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === TRYOPEN)
    expected = ConnectionEnd{OPEN, identifier, getCommitmentPrefix(), connection.counterpartyClientIdentifier,
                             connection.clientIdentifier, connection.versions, connection.delayPeriodTime, connection.delayPeriodBlocks}
    abortTransactionUnless(connection.verifyConnectionState(proofHeight, proofAck, connection.counterpartyConnectionIdentifier, expected))
    connection.state = OPEN
    provableStore.set(connectionPath(identifier), connection)
}

​
Querying

Connections can be queried by identifier with queryConnection. 
function queryConnection(id: Identifier): ConnectionEnd | void {
    return provableStore.get(connectionPath(id))
}
Connections associated with a particular client can be queried by client identifier with queryClientConnections. 
function queryClientConnections(id: Identifier): Set<Identifier> {
    return privateStore.get(clientConnectionsPath(id))
}

​
Properties & Invariants

  • Connection identifiers are first-come-first-serve: once a connection has been negotiated, a unique identifier pair exists between two chains.
  • The connection handshake cannot be man-in-the-middled by another blockchain’s IBC handler.

​
Backwards Compatibility

In the latest specification of the connection handshake, connOpenTry and connOpenAck will no longer validate that the counterparty’s clien state and consensus state is a valid client of the executing chain’s consensus protocol. Thus, clientState, proofClient, proofConsensus and consensusHeight fields in the ConnOpenTry and ConnOpenACk datagrams are deprecated and will eventually be removed.

​
Forwards Compatibility

A future version of this ICS will include version negotiation in the opening handshake. Once a connection has been established and a version negotiated, future version updates can be negotiated per ICS 6. The consensus state can only be updated as allowed by the updateConsensusState function defined by the consensus protocol chosen when the connection is established.

​
Example Implementations

​
History

Parts of this document were inspired by the previous IBC specification. Mar 29, 2019 - Initial draft version submitted May 17, 2019 - Draft finalised Jul 29, 2019 - Revisions to track connection set associated with client Jul 27, 2022 - Addition of ClientState validation in connOpenTry and connOpenAck Jul 23, 2024 - Removal of ClientState and ConsensusState validation in connOpenTry and connOpenAck. For information on the consequences of these changes see the attached diagram and consequences document All content herein is licensed under Apache 2.0.