ics-004-channel-and-packet-semantics

ics	title	stage	category	kind	requires	author	created	modified
4	Channel & Packet Semantics	draft	IBC/TAO	instantiation	2, 3, 5, 24	Christopher Goes <[email protected]>	2019-03-07	2019-08-25

Synopsis

The "channel" abstraction provides message delivery semantics to the interblockchain communication protocol, in three categories: ordering, exactly-once delivery, and module permissioning. A channel serves as a conduit for packets passing between a module on one chain and a module on another, ensuring that packets are executed only once, delivered in the order in which they were sent (if necessary), and delivered only to the corresponding module owning the other end of the channel on the destination chain. Each channel is associated with a particular connection, and a connection may have any number of associated channels, allowing the use of common identifiers and amortising the cost of header verification across all the channels utilising a connection & light client.

Channels are payload-agnostic. The modules which send and receive IBC packets decide how to construct packet data and how to act upon the incoming packet data, and must utilise their own application logic to determine which state transactions to apply according to what data the packet contains.

Motivation

The interblockchain communication protocol uses a cross-chain message passing model. IBC packets are relayed from one blockchain to the other by external relayer processes. Chain A and chain B confirm new blocks independently, and packets from one chain to the other may be delayed, censored, or re-ordered arbitrarily. Packets are visible to relayers and can be read from a blockchain by any relayer process and submitted to any other blockchain.

The IBC protocol must provide ordering (for ordered channels) and exactly-once delivery guarantees to allow applications to reason about the combined state of connected modules on two chains. For example, an application may wish to allow a single tokenized asset to be transferred between and held on multiple blockchains while preserving fungibility and conservation of supply. The application can mint asset vouchers on chain B when a particular IBC packet is committed to chain B, and require outgoing sends of that packet on chain A to escrow an equal amount of the asset on chain A until the vouchers are later redeemed back to chain A with an IBC packet in the reverse direction. This ordering guarantee along with correct application logic can ensure that total supply is preserved across both chains and that any vouchers minted on chain B can later be redeemed back to chain A.

In order to provide the desired ordering, exactly-once delivery, and module permissioning semantics to the application layer, the interblockchain communication protocol must implement an abstraction to enforce these semantics — channels are this abstraction.

Definitions

ConsensusState is as defined in ICS 2.

Connection is as defined in ICS 3.

Port and authenticate are as defined in ICS 5.

hash is a generic collision-resistant hash function, the specifics of which must be agreed on by the modules utilising the channel.

Identifier, get, set, delete, getCurrentHeight, and module-system related primitives are as defined in ICS 24.

A channel is a pipeline for exactly-once packet delivery between specific modules on separate blockchains, which has at least one end capable of sending packets and one end capable of receiving packets.

A bidirectional channel is a channel where packets can flow in both directions: from A to B and from B to A.

A unidirectional channel is a channel where packets can only flow in one direction: from A to B (or from B to A, the order of naming is arbitrary).

An ordered channel is a channel where packets are delivered exactly in the order which they were sent.

An unordered channel is a channel where packets can be delivered in any order, which may differ from the order in which they were sent.

enum ChannelOrder {
  ORDERED,
  UNORDERED,
}

Directionality and ordering are independent, so one can speak of a bidirectional unordered channel, a unidirectional ordered channel, etc.

All channels provide exactly-once packet delivery, meaning that a packet sent on one end of a channel is delivered no more and no less than once, eventually, to the other end.

This specification only concerns itself with bidirectional channels. Unidirectional channels can use almost exactly the same protocol and will be outlined in a future ICS.

An end of a channel is a data structure on one chain storing channel metadata:

interface ChannelEnd {
  state: ChannelState
  ordering: ChannelOrder
  counterpartyPortIdentifier: Identifier
  counterpartyChannelIdentifier: Identifier
  connectionHops: [Identifier]
  version: string
}

• The state is the current state of the channel end.
• The ordering field indicates whether the channel is ordered or unordered.
• The counterpartyPortIdentifier identifies the port on the counterparty chain which owns the other end of the channel.
• The counterpartyChannelIdentifier identifies the channel end on the counterparty chain.
• The nextSequenceSend, stored separately, tracks the sequence number for the next packet to be sent.
• The nextSequenceRecv, stored separately, tracks the sequence number for the next packet to be received.
• The connectionHops stores the list of connection identifiers, in order, along which packets sent on this channel will travel. At the moment this list must be of length 1. In the future multi-hop channels may be supported.
• The version string stores an opaque channel version, which is agreed upon during the handshake. This can determine module-level configuration such as which packet encoding is used for the channel. This version is not used by the core IBC protocol.

Channel ends have a state:

enum ChannelState {
  INIT,
  TRYOPEN,
  OPEN,
  CLOSED,
}

• A channel end in INIT state has just started the opening handshake.
• A channel end in TRYOPEN state has acknowledged the handshake step on the counterparty chain.
• A channel end in OPEN state has completed the handshake and is ready to send and receive packets.
• A channel end in CLOSED state has been closed and can no longer be used to send or receive packets.

A Packet, in the interblockchain communication protocol, is a particular interface defined as follows:

interface Packet {
  sequence: uint64
  timeoutHeight: uint64
  sourcePort: Identifier
  sourceChannel: Identifier
  destPort: Identifier
  destChannel: Identifier
  data: bytes
}

• The sequence number corresponds to the order of sends and receives, where a packet with an earlier sequence number must be sent and received before a packet with a later sequence number.
• The timeoutHeight indicates a consensus height on the destination chain after which the packet will no longer be processed, and will instead count as having timed-out.
• The sourcePort identifies the port on the sending chain.
• The sourceChannel identifies the channel end on the sending chain.
• The destPort identifies the port on the receiving chain.
• The destChannel identifies the channel end on the receiving chain.
• The data is an opaque value which can be defined by the application logic of the associated modules.

Note that a Packet is never directly serialised. Rather it is an intermediary structure used in certain function calls that may need to be created or processed by modules calling the IBC handler.

An OpaquePacket is a packet, but cloaked in an obscuring data type by the host state machine, such that a module cannot act upon it other than to pass it to the IBC handler. The IBC handler can cast a Packet to an OpaquePacket and vice versa.

type OpaquePacket = object

Desired Properties

Efficiency

• The speed of packet transmission and confirmation should be limited only by the speed of the underlying chains. Proofs should be batchable where possible.

Exactly-once delivery

• IBC packets sent on one end of a channel should be delivered exactly once to the other end.
• No network synchrony assumptions should be required for exactly-once safety. If one or both of the chains halt, packets may be delivered no more than once, and once the chains resume packets should be able to flow again.

Ordering

• On ordered channels, packets should be sent and received in the same order: if packet x is sent before packet y by a channel end on chain A, packet x must be received before packet y by the corresponding channel end on chain B.
• On unordered channels, packets may be sent and received in any order. Unordered packets, like ordered packets, have individual timeouts specified in terms of the destination chain's height.

Permissioning

• Channels should be permissioned to one module on each end, determined during the handshake and immutable afterwards (higher-level logic could tokenize channel ownership by tokenising ownership of the port). Only the module associated with a channel end should be able to send or receive on it.

Technical Specification

Dataflow visualisation

The architecture of clients, connections, channels and packets:

Preliminaries

Store paths

Channel structures are stored under a store path prefix unique to a combination of a port identifier and channel identifier:

function channelPath(portIdentifier: Identifier, channelIdentifier: Identifier): Path {
    return "ports/{portIdentifier}/channels/{channelIdentifier}"
}

The capability key associated with a channel is stored under the channelCapabilityPath:

function channelCapabilityPath(portIdentifier: Identifier, channelIdentifier: Identifier): Path {
  return "{channelPath(portIdentifier, channelIdentifier)}/key"
}

The nextSequenceSend and nextSequenceRecv unsigned integer counters are stored separately so they can be proved individually:

function nextSequenceSendPath(portIdentifier: Identifier, channelIdentifier: Identifier): Path {
    return "{channelPath(portIdentifier, channelIdentifier)}/nextSequenceSend"
}

function nextSequenceRecvPath(portIdentifier: Identifier, channelIdentifier: Identifier): Path {
    return "{channelPath(portIdentifier, channelIdentifier)}/nextSequenceRecv"
}

Constant-size commitments to packet data fields are stored under the packet sequence number:

function packetCommitmentPath(portIdentifier: Identifier, channelIdentifier: Identifier, sequence: uint64): Path {
    return "{channelPath(portIdentifier, channelIdentifier)}/packets/" + sequence
}

Absence of the path in the store is equivalent to a zero-bit.

Packet acknowledgement data are stored under the packetAcknowledgementPath:

function packetAcknowledgementPath(portIdentifier: Identifier, channelIdentifier: Identifier, sequence: uint64): Path {
    return "{channelPath(portIdentifier, channelIdentifier)}/acknowledgements/" + sequence
}

Unordered channels MUST always write a acknowledgement (even an empty one) to this path so that the absence of such can be used as proof-of-timeout. Ordered channels MAY write an acknowledgement, but are not required to.

Versioning

During the handshake process, two ends of a channel come to agreement on a version bytestring associated with that channel. The contents of this version bytestring are and will remain opaque to the IBC core protocol. Host state machines MAY utilise the version data to indicate supported IBC/APP protocols, agree on packet encoding formats, or negotiate other channel-related metadata related to custom logic on top of IBC.

Host state machines MAY also safely ignore the version data or specify an empty string.

Sub-protocols

Note: If the host state machine is utilising object capability authentication (see ICS 005), all functions utilising ports take an additional capability parameter.

Identifier validation

Channels are stored under a unique (portIdentifier, channelIdentifier) prefix. The validation function validatePortIdentifier MAY be provided.

type validateChannelIdentifier = (portIdentifier: Identifier, channelIdentifier: Identifier) => boolean

If not provided, the default validateChannelIdentifier function will always return true.

Channel lifecycle management

Initiator	Datagram	Chain acted upon	Prior state (A, B)	Posterior state (A, B)
Actor	ChanOpenInit	A	(none, none)	(INIT, none)
Relayer	ChanOpenTry	B	(INIT, none)	(INIT, TRYOPEN)
Relayer	ChanOpenAck	A	(INIT, TRYOPEN)	(OPEN, TRYOPEN)
Relayer	ChanOpenConfirm	B	(OPEN, TRYOPEN)	(OPEN, OPEN)

Initiator	Datagram	Chain acted upon	Prior state (A, B)	Posterior state (A, B)
Actor	ChanCloseInit	A	(OPEN, OPEN)	(CLOSED, OPEN)
Relayer	ChanCloseConfirm	B	(CLOSED, OPEN)	(CLOSED, CLOSED)

Opening handshake

The chanOpenInit function is called by a module to initiate a channel opening handshake with a module on another chain.

The opening channel must provide the identifiers of the local channel identifier, local port, remote port, and remote channel identifier.

When the opening handshake is complete, the module which initiates the handshake will own the end of the created channel on the host ledger, and the counterparty module which it specifies will own the other end of the created channel on the counterparty chain. Once a channel is created, ownership cannot be changed (although higher-level abstractions could be implemented to provide this).

function chanOpenInit(
  order: ChannelOrder,
  connectionHops: [Identifier],
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  counterpartyPortIdentifier: Identifier,
  counterpartyChannelIdentifier: Identifier,
  version: string): CapabilityKey {
    abortTransactionUnless(validateChannelIdentifier(portIdentifier, channelIdentifier))

    abortTransactionUnless(connectionHops.length === 1) // for v1 of the IBC protocol

    abortTransactionUnless(provableStore.get(channelPath(portIdentifier, channelIdentifier)) === null)
    connection = provableStore.get(connectionPath(connectionHops[0]))

    // optimistic channel handshakes are allowed
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state !== CLOSED)
    abortTransactionUnless(authenticate(privateStore.get(portPath(portIdentifier))))
    channel = ChannelEnd{INIT, order, counterpartyPortIdentifier,
                         counterpartyChannelIdentifier, connectionHops, version}
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
    key = generate()
    provableStore.set(channelCapabilityPath(portIdentifier, channelIdentifier), key)
    provableStore.set(nextSequenceSendPath(portIdentifier, channelIdentifier), 1)
    provableStore.set(nextSequenceRecvPath(portIdentifier, channelIdentifier), 1)
    return key
}

The chanOpenTry function is called by a module to accept the first step of a channel opening handshake initiated by a module on another chain.

function chanOpenTry(
  order: ChannelOrder,
  connectionHops: [Identifier],
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  counterpartyPortIdentifier: Identifier,
  counterpartyChannelIdentifier: Identifier,
  version: string,
  counterpartyVersion: string,
  proofInit: CommitmentProof,
  proofHeight: uint64): CapabilityKey {
    abortTransactionUnless(validateChannelIdentifier(portIdentifier, channelIdentifier))
    abortTransactionUnless(connectionHops.length === 1) // for v1 of the IBC protocol
    previous = provableStore.get(channelPath(portIdentifier, channelIdentifier))
    abortTransactionUnless(
      (previous === null) ||
      (previous.state === INIT &&
       previous.order === order &&
       previous.counterpartyPortIdentifier === counterpartyPortIdentifier &&
       previous.counterpartyChannelIdentifier === counterpartyChannelIdentifier &&
       previous.connectionHops === connectionHops &&
       previous.version === version)
      )
    abortTransactionUnless(authenticate(privateStore.get(portPath(portIdentifier))))
    connection = provableStore.get(connectionPath(connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    expected = ChannelEnd{INIT, order, portIdentifier,
                          channelIdentifier, connectionHops.reverse(), counterpartyVersion}
    abortTransactionUnless(connection.verifyChannelState(
      proofHeight,
      proofInit,
      counterpartyPortIdentifier,
      counterpartyChannelIdentifier,
      expected
    ))
    channel = ChannelEnd{TRYOPEN, order, counterpartyPortIdentifier,
                         counterpartyChannelIdentifier, connectionHops, version}
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
    key = generate()
    provableStore.set(channelCapabilityPath(portIdentifier, channelIdentifier), key)
    provableStore.set(nextSequenceSendPath(portIdentifier, channelIdentifier), 1)
    provableStore.set(nextSequenceRecvPath(portIdentifier, channelIdentifier), 1)
    return key
}

The chanOpenAck is called by the handshake-originating module to acknowledge the acceptance of the initial request by the counterparty module on the other chain.

function chanOpenAck(
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  counterpartyVersion: string,
  proofTry: CommitmentProof,
  proofHeight: uint64) {
    channel = provableStore.get(channelPath(portIdentifier, channelIdentifier))
    abortTransactionUnless(channel.state === INIT || channel.state === TRYOPEN)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(portIdentifier, channelIdentifier))))
    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    expected = ChannelEnd{TRYOPEN, channel.order, portIdentifier,
                          channelIdentifier, channel.connectionHops.reverse(), counterpartyVersion}
    abortTransactionUnless(connection.verifyChannelState(
      proofHeight,
      proofTry,
      channel.counterpartyPortIdentifier,
      channel.counterpartyChannelIdentifier,
      expected
    ))
    channel.state = OPEN
    channel.version = counterpartyVersion
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
}

The chanOpenConfirm function is called by the handshake-accepting module to acknowledge the acknowledgement of the handshake-originating module on the other chain and finish the channel opening handshake.

function chanOpenConfirm(
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  proofAck: CommitmentProof,
  proofHeight: uint64) {
    channel = provableStore.get(channelPath(portIdentifier, channelIdentifier))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state === TRYOPEN)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(portIdentifier, channelIdentifier))))
    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    expected = ChannelEnd{OPEN, channel.order, portIdentifier,
                          channelIdentifier, channel.connectionHops.reverse(), channel.version}
    abortTransactionUnless(connection.verifyChannelState(
      proofHeight,
      proofAck,
      channel.counterpartyPortIdentifier,
      channel.counterpartyChannelIdentifier,
      expected
    ))
    channel.state = OPEN
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
}

Closing handshake

The chanCloseInit function is called by either module to close their end of the channel. Once closed, channels cannot be reopened.

Calling modules MAY atomically execute appropriate application logic in conjunction with calling chanCloseInit.

Any in-flight packets can be timed-out as soon as a channel is closed.

function chanCloseInit(
  portIdentifier: Identifier,
  channelIdentifier: Identifier) {
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(portIdentifier, channelIdentifier))))
    channel = provableStore.get(channelPath(portIdentifier, channelIdentifier))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state !== CLOSED)
    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    channel.state = CLOSED
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
}

The chanCloseConfirm function is called by the counterparty module to close their end of the channel, since the other end has been closed.

Calling modules MAY atomically execute appropriate application logic in conjunction with calling chanCloseConfirm.

Once closed, channels cannot be reopened.

function chanCloseConfirm(
  portIdentifier: Identifier,
  channelIdentifier: Identifier,
  proofInit: CommitmentProof,
  proofHeight: uint64) {
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(portIdentifier, channelIdentifier))))
    channel = provableStore.get(channelPath(portIdentifier, channelIdentifier))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state !== CLOSED)
    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    expected = ChannelEnd{CLOSED, channel.order, portIdentifier,
                          channelIdentifier, channel.connectionHops.reverse(), channel.version}
    abortTransactionUnless(connection.verifyChannelState(
      proofHeight,
      proofInit,
      channel.counterpartyPortIdentifier,
      channel.counterpartyChannelIdentifier,
      expected
    ))
    channel.state = CLOSED
    provableStore.set(channelPath(portIdentifier, channelIdentifier), channel)
}

Packet flow & handling

A day in the life of a packet

The following sequence of steps must occur for a packet to be sent from module 1 on machine A to module 2 on machine B, starting from scratch.

The module can interface with the IBC handler through ICS 25 or ICS 26.

1. Initial client & port setup, in any order
   1. Client created on A for B (see ICS 2)
   2. Client created on B for A (see ICS 2)
   3. Module 1 binds to a port (see ICS 5)
   4. Module 2 binds to a port (see ICS 5), which is communicated out-of-band to module 1
2. Establishment of a connection & channel, optimistic send, in order
   1. Connection opening handshake started from A to B by module 1 (see ICS 3)
   2. Channel opening handshake started from 1 to 2 using the newly created connection (this ICS)
   3. Packet sent over the newly created channel from 1 to 2 (this ICS)
3. Successful completion of handshakes (if either handshake fails, the connection/channel can be closed & the packet timed-out)
   1. Connection opening handshake completes successfully (see ICS 3) (this will require participation of a relayer process)
   2. Channel opening handshake completes successfully (this ICS) (this will require participation of a relayer process)
4. Packet confirmation on machine B, module 2 (or packet timeout if the timeout height has passed) (this will require participation of a relayer process)
5. Acknowledgement (possibly) relayed back from module 2 on machine B to module 1 on machine A

Represented spatially, packet transit between two machines can be rendered as follows:

Sending packets

The sendPacket function is called by a module in order to send an IBC packet on a channel end owned by the calling module to the corresponding module on the counterparty chain.

Calling modules MUST execute application logic atomically in conjunction with calling sendPacket.

The IBC handler performs the following steps in order:

• Checks that the channel & connection are open to send packets
• Checks that the calling module owns the sending port
• Checks that the packet metadata matches the channel & connection information
• Checks that the timeout height specified has not already passed on the destination chain
• Increments the send sequence counter associated with the channel
• Stores a constant-size commitment to the packet data & packet timeout

Note that the full packet is not stored in the state of the chain - merely a short hash-commitment to the data & timeout value. The packet data can be calculated from the transaction execution and possibly returned as log output which relayers can index.

function sendPacket(packet: Packet) {
    channel = provableStore.get(channelPath(packet.sourcePort, packet.sourceChannel))

    // optimistic sends are permitted once the handshake has started
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state !== CLOSED)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.sourcePort, packet.sourceChannel))))
    abortTransactionUnless(packet.destPort === channel.counterpartyPortIdentifier)
    abortTransactionUnless(packet.destChannel === channel.counterpartyChannelIdentifier)
    connection = provableStore.get(connectionPath(channel.connectionHops[0]))

    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state !== CLOSED)

    consensusState = provableStore.get(consensusStatePath(connection.clientIdentifier))
    abortTransactionUnless(consensusState.getHeight() < packet.timeoutHeight)

    nextSequenceSend = provableStore.get(nextSequenceSendPath(packet.sourcePort, packet.sourceChannel))
    abortTransactionUnless(packet.sequence === nextSequenceSend)

    // all assertions passed, we can alter state

    nextSequenceSend = nextSequenceSend + 1
    provableStore.set(nextSequenceSendPath(packet.sourcePort, packet.sourceChannel), nextSequenceSend)
    provableStore.set(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence), hash(packet.data, packet.timeout))

    // log that a packet has been sent
    emitLogEntry("sendPacket", {sequence: packet.sequence, data: packet.data, timeout: packet.timeout})
}

Receiving packets

The recvPacket function is called by a module in order to receive & process an IBC packet sent on the corresponding channel end on the counterparty chain.

Calling modules MUST execute application logic atomically in conjunction with calling recvPacket, likely beforehand to calculate the acknowledgement value.

The IBC handler performs the following steps in order:

• Checks that the channel & connection are open to receive packets
• Checks that the calling module owns the receiving port
• Checks that the packet metadata matches the channel & connection information
• Checks that the packet sequence is the next sequence the channel end expects to receive (for ordered channels)
• Checks that the timeout height has not yet passed
• Checks the inclusion proof of packet data commitment in the outgoing chain's state
• Sets the opaque acknowledgement value at a store path unique to the packet (if the acknowledgement is non-empty or the channel is unordered)
• Increments the packet receive sequence associated with the channel end (ordered channels only)

function recvPacket(
  packet: OpaquePacket,
  proof: CommitmentProof,
  proofHeight: uint64,
  acknowledgement: bytes): Packet {

    channel = provableStore.get(channelPath(packet.destPort, packet.destChannel))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state === OPEN)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.destPort, packet.destChannel))))
    abortTransactionUnless(packet.sourcePort === channel.counterpartyPortIdentifier)
    abortTransactionUnless(packet.sourceChannel === channel.counterpartyChannelIdentifier)

    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)

    abortTransactionUnless(getConsensusHeight() < packet.timeoutHeight)

    abortTransactionUnless(connection.verifyPacketCommitment(
      proofHeight,
      proof,
      packet.sourcePort,
      packet.sourceChannel,
      packet.sequence,
      hash(packet.data, packet.timeout)
    ))

    // all assertions passed (except sequence check), we can alter state

    if (acknowledgement.length > 0 || channel.order === UNORDERED)
      provableStore.set(
        packetAcknowledgementPath(packet.destPort, packet.destChannel, packet.sequence),
        hash(acknowledgement)
      )

    if (channel.order === ORDERED) {
      nextSequenceRecv = provableStore.get(nextSequenceRecvPath(packet.destPort, packet.destChannel))
      abortTransactionUnless(packet.sequence === nextSequenceRecv)
      nextSequenceRecv = nextSequenceRecv + 1
      provableStore.set(nextSequenceRecvPath(packet.destPort, packet.destChannel), nextSequenceRecv)
    }

    // log that a packet has been received & acknowledged
    emitLogEntry("recvPacket", {sequence: packet.sequence, timeout: packet.timeout, data: packet.data, acknowledgement})

    // return transparent packet
    return packet
}

Acknowledgements

The acknowledgePacket function is called by a module to process the acknowledgement of a packet previously sent by the calling module on a channel to a counterparty module on the counterparty chain. acknowledgePacket also cleans up the packet commitment, which is no longer necessary since the packet has been received and acted upon.

Calling modules MAY atomically execute appropriate application acknowledgement-handling logic in conjunction with calling acknowledgePacket.

function acknowledgePacket(
  packet: OpaquePacket,
  acknowledgement: bytes,
  proof: CommitmentProof,
  proofHeight: uint64): Packet {

    // abort transaction unless that channel is open, calling module owns the associated port, and the packet fields match
    channel = provableStore.get(channelPath(packet.sourcePort, packet.sourceChannel))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state === OPEN)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.sourcePort, packet.sourceChannel))))
    abortTransactionUnless(packet.sourceChannel === channel.counterpartyChannelIdentifier)

    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    abortTransactionUnless(connection !== null)
    abortTransactionUnless(connection.state === OPEN)
    abortTransactionUnless(packet.sourcePort === channel.counterpartyPortIdentifier)

    // verify we sent the packet and haven't cleared it out yet
    abortTransactionUnless(provableStore.get(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))
           === hash(packet.data, packet.timeout))

    // abort transaction unless correct acknowledgement on counterparty chain
    abortTransactionUnless(connection.verifyPacketAcknowledgement(
      proofHeight,
      proof,
      packet.destPort,
      packet.destChannel,
      packet.sequence,
      hash(acknowledgement)
    ))

    // all assertions passed, we can alter state

    // delete our commitment so we can't "acknowledge" again
    provableStore.delete(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))

    // return transparent packet
    return packet
}

Timeouts

Application semantics may require some timeout: an upper limit to how long the chain will wait for a transaction to be processed before considering it an error. Since the two chains have different local clocks, this is an obvious attack vector for a double spend - an attacker may delay the relay of the receipt or wait to send the packet until right after the timeout - so applications cannot safely implement naive timeout logic themselves.

Note that in order to avoid any possible "double-spend" attacks, the timeout algorithm requires that the destination chain is running and reachable. One can prove nothing in a complete network partition, and must wait to connect; the timeout must be proven on the recipient chain, not simply the absence of a response on the sending chain.

Sending end

The timeoutPacket function is called by a module which originally attempted to send a packet to a counterparty module, where the timeout height has passed on the counterparty chain without the packet being committed, to prove that the packet can no longer be executed and to allow the calling module to safely perform appropriate state transitions.

Calling modules MAY atomically execute appropriate application timeout-handling logic in conjunction with calling timeoutPacket.

In the case of an ordered channel, timeoutPacket checks the recvSequence of the receiving channel end and closes the channel if a packet has timed out.

In the case of an unordered channel, timeoutPacket checks the absence of an acknowledgement (which will have been written if the packet was received). Unordered channels are expected to continue in the face of timed-out packets.

If relations are enforced between timeout heights of subsequent packets, safe bulk timeouts of all packets prior to a timed-out packet can be performed. This specification omits details for now.

function timeoutPacket(
  packet: OpaquePacket,
  proof: CommitmentProof,
  proofHeight: uint64,
  nextSequenceRecv: Maybe<uint64>): Packet {

    channel = provableStore.get(channelPath(packet.sourcePort, packet.sourceChannel))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state === OPEN)

    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.sourcePort, packet.sourceChannel))))
    abortTransactionUnless(packet.destChannel === channel.counterpartyChannelIdentifier)

    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    // note: the connection may have been closed
    abortTransactionUnless(packet.destPort === channel.counterpartyPortIdentifier)

    // check that timeout height has passed on the other end
    abortTransactionUnless(proofHeight >= packet.timeoutHeight)

    // check that packet has not been received
    abortTransactionUnless(nextSequenceRecv < packet.sequence)

    // verify we actually sent this packet, check the store
    abortTransactionUnless(provableStore.get(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))
           === hash(packet.data, packet.timeout))

    if channel.order === ORDERED
      // ordered channel: check that the recv sequence is as claimed
      abortTransactionUnless(connection.verifyNextSequenceRecv(
        proofHeight,
        proof,
        packet.destPort,
        packet.destChannel,
        nextSequenceRecv
      ))
    else
      // unordered channel: verify absence of acknowledgement at packet index
      abortTransactionUnless(connection.verifyPacketAcknowledgementAbsence(
        proofHeight,
        proof,
        packet.sourcePort,
        packet.sourceChannel,
        packet.sequence
      ))

    // all assertions passed, we can alter state

    // delete our commitment
    provableStore.delete(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))

    if channel.order === ORDERED {
      // ordered channel: close the channel
      channel.state = CLOSED
      provableStore.set(channelPath(packet.sourcePort, packet.sourceChannel), channel)
    }

    // return transparent packet
    return packet
}

Timing-out on close

The timeoutOnClose function is called by a module in order to prove that the channel to which an unreceived packet was addressed has been closed, so the packet will never be received (even if the timeoutHeight has not yet been reached).

function timeoutOnClose(
  packet: Packet,
  proof: CommitmentProof,
  proofClosed: CommitmentProof,
  proofHeight: uint64,
  nextSequenceRecv: Maybe<uint64>): Packet {

    channel = provableStore.get(channelPath(packet.sourcePort, packet.sourceChannel))
    // note: the channel may have been closed
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.sourcePort, packet.sourceChannel))))
    abortTransactionUnless(packet.destChannel === channel.counterpartyChannelIdentifier)

    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    // note: the connection may have been closed
    abortTransactionUnless(packet.destPort === channel.counterpartyPortIdentifier)

    // verify we actually sent this packet, check the store
    abortTransactionUnless(provableStore.get(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))
           === hash(packet.data, packet.timeout))

    // check that the opposing channel end has closed
    expected = ChannelEnd{CLOSED, channel.order, channel.portIdentifier,
                          channel.channelIdentifier, channel.connectionHops.reverse(), channel.version}
    abortTransactionUnless(connection.verifyChannelState(
      proofHeight,
      proofClosed,
      channel.counterpartyPortIdentifier,
      channel.counterpartyChannelIdentifier,
      expected
    ))

    if channel.order === ORDERED
      // ordered channel: check that the recv sequence is as claimed
      abortTransactionUnless(connection.verifyNextSequenceRecv(
        proofHeight,
        proof,
        packet.destPort,
        packet.destChannel,
        nextSequenceRecv
      ))
    else
      // unordered channel: verify absence of acknowledgement at packet index
      abortTransactionUnless(connection.verifyPacketAcknowledgementAbsence(
        proofHeight,
        proof,
        packet.sourcePort,
        packet.sourceChannel,
        packet.sequence
      ))

    // all assertions passed, we can alter state

    // delete our commitment
    provableStore.delete(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))

    // return transparent packet
    return packet
}

Cleaning up state

cleanupPacket is called by a module to remove a received packet commitment from storage. The receiving end must have already processed the packet (whether regularly or past timeout).

In the ordered channel case, cleanupPacket cleans-up a packet on an ordered channel by proving that the packet has been received on the other end.

In the unordered channel case, cleanupPacket cleans-up a packet on an unordered channel by proving that the associated acknowledgement has been written.

function cleanupPacket(
  packet: OpaquePacket,
  proof: CommitmentProof,
  proofHeight: uint64,
  nextSequenceRecvOrAcknowledgement: Either<uint64, bytes>): Packet {

    channel = provableStore.get(channelPath(packet.sourcePort, packet.sourceChannel))
    abortTransactionUnless(channel !== null)
    abortTransactionUnless(channel.state === OPEN)
    abortTransactionUnless(authenticate(privateStore.get(channelCapabilityPath(packet.sourcePort, packet.sourceChannel))))
    abortTransactionUnless(packet.destChannel === channel.counterpartyChannelIdentifier)

    connection = provableStore.get(connectionPath(channel.connectionHops[0]))
    // note: the connection may have been closed
    abortTransactionUnless(packet.destPort === channel.counterpartyPortIdentifier)

    // abortTransactionUnless packet has been received on the other end
    abortTransactionUnless(nextSequenceRecv > packet.sequence)

    // verify we actually sent the packet, check the store
    abortTransactionUnless(provableStore.get(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))
               === hash(packet.data, packet.timeout))

    if channel.order === ORDERED
      // check that the recv sequence is as claimed
      abortTransactionUnless(connection.verifyNextSequenceRecv(
        proofHeight,
        proof,
        packet.destPort,
        packet.destChannel,
        nextSequenceRecvOrAcknowledgement
      ))
    else
      // abort transaction unless acknowledgement on the other end
      abortTransactionUnless(connection.verifyPacketAcknowledgement(
        proofHeight,
        proof,
        packet.destPort,
        packet.destChannel,
        packet.sequence,
        nextSequenceRecvOrAcknowledgement
      ))

    // all assertions passed, we can alter state

    // clear the store
    provableStore.delete(packetCommitmentPath(packet.sourcePort, packet.sourceChannel, packet.sequence))

    // return transparent packet
    return packet
}

Reasoning about race conditions

Simultaneous handshake attempts

If two machines simultaneously initiate channel opening handshakes with each other, attempting to use the same identifiers, both will fail and new identifiers must be used.

Identifier allocation

There is an unavoidable race condition on identifier allocation on the destination chain. Modules would be well-advised to utilise pseudo-random, non-valuable identifiers. Managing to claim the identifier that another module wishes to use, however, while annoying, cannot man-in-the-middle a handshake since the receiving module must already own the port to which the handshake was targeted.

Timeouts / packet confirmation

There is no race condition between a packet timeout and packet confirmation, as the packet will either have passed the timeout height prior to receipt or not.

Man-in-the-middle attacks during handshakes

Verification of cross-chain state prevents man-in-the-middle attacks for both connection handshakes & channel handshakes since all information (source, destination client, channel, etc.) is known by the module which starts the handshake and confirmed prior to handshake completion.

Connection / channel closure with in-flight packets

If a connection or channel is closed while packets are in-flight, the packets can no longer be received on the destination chain and can be timed-out on the source chain.

Querying channels

Channels can be queried with queryChannel:

function queryChannel(connId: Identifier, chanId: Identifier): ChannelEnd | void {
    return provableStore.get(channelPath(connId, chanId))
}

Properties & Invariants

• The unique combinations of channel & port identifiers are first-come-first-serve: once a pair has been allocated, only the modules owning the ports in question can send or receive on that channel.
• Packets are delivered exactly once, assuming that the chains are live within the timeout window, and in case of timeout can be timed-out exactly once on the sending chain.
• The channel handshake cannot be man-in-the-middle attacked by another module on either blockchain or another blockchain's IBC handler.

Backwards Compatibility

Not applicable.

Forwards Compatibility

Data structures & encoding can be versioned at the connection or channel level. Channel logic is completely agnostic to packet data formats, which can be changed by the modules any way they like at any time.

Example Implementation

Coming soon.

Other Implementations

Coming soon.

History

Jun 5, 2019 - Draft submitted

Jul 4, 2019 - Modifications for unordered channels & acknowledgements

Jul 16, 2019 - Alterations for multi-hop routing future compatibility

Jul 29, 2019 - Revisions to handle timeouts after connection closure

Aug 13, 2019 - Various edits

Aug 25, 2019 - Cleanup

Copyright

All content herein is licensed under Apache 2.0.