| ics | title | stage | category | requires | kind | author | created | modified |
|---|---|---|---|---|---|---|---|---|
20 |
Fungible Token Transfer |
draft |
IBC/APP |
25, 26 |
instantiation |
Christopher Goes <[email protected]> |
2019-07-15 |
2019-08-25 |
This standard document specifies packet data structure, state machine handling logic, and encoding details for the transfer of fungible tokens over an IBC channel between two modules on separate chains. The state machine logic presented allows for safe multi-chain denomination handling with permissionless channel opening. This logic constitutes a "fungible token transfer bridge module", interfacing between the IBC routing module and an existing asset tracking module on the host state machine.
Users of a set of chains connected over the IBC protocol might wish to utilise an asset issued on one chain on another chain, perhaps to make use of additional features such as exchange or privacy protection, while retaining fungibility with the original asset on the issuing chain. This application-layer standard describes a protocol for transferring fungible tokens between chains connected with IBC which preserves asset fungibility, preserves asset ownership, limits the impact of Byzantine faults, and requires no additional permissioning.
The IBC handler interface & IBC routing module interface are as defined in ICS 25 and ICS 26, respectively.
- Preservation of fungibility (two-way peg).
- Preservation of total supply (constant or inflationary on a single source chain & module).
- Permissionless token transfers, no need to whitelist connections, modules, or denominations.
- Symmetric (all chains implement the same logic, no in-protocol differentiation of hubs & zones).
- Fault containment: prevents Byzantine-inflation of tokens originating on chain
A, as a result of chainB's Byzantine behaviour (though any users who sent tokens to chainBmay be at risk).
Only one packet data type, FungibleTokenPacketData, which specifies the denomination, amount, sending account, receiving account, and whether the sending chain is the source of the asset, is required.
interface FungibleTokenPacketData {
denomination: string
amount: uint256
sender: string
receiver: string
source: boolean
}The fungible token transfer bridge module tracks escrow addresses associated with particular channels in state. Fields of the ModuleState are assumed to be in scope.
interface ModuleState {
channelEscrowAddresses: Map<Identifier, string>
}The sub-protocols described herein should be implemented in a "fungible token transfer bridge" module with access to a bank module and to the IBC routing module.
The setup function must be called exactly once when the module is created (perhaps when the blockchain itself is initialised) to bind to the appropriate port and create an escrow address (owned by the module).
function setup() {
routingModule.bindPort("bank", ModuleCallbacks{
onChanOpenInit,
onChanOpenTry,
onChanOpenAck,
onChanOpenConfirm,
onChanCloseInit,
onChanCloseConfirm,
onRecvPacket,
onTimeoutPacket,
onAcknowledgePacket,
onTimeoutPacketClose
})
}Once the setup function has been called, channels can be created through the IBC routing module between instances of the fungible token transfer module on separate chains.
An administrator (with the permissions to create connections & channels on the host state machine) is responsible for setting up connections to other state machines & creating channels to other instances of this module (or another module supporting this interface) on other chains. This specification defines packet handling semantics only, and defines them in such a fashion that the module itself doesn't need to worry about what connections or channels might or might not exist at any point in time.
Both machines A and B accept new channels from any module on another machine, if and only if:
- The other module is bound to the "bank" port.
- The channel being created is unordered.
- The version string is empty.
function onChanOpenInit(
order: ChannelOrder,
connectionHops: [Identifier],
portIdentifier: Identifier,
channelIdentifier: Identifier,
counterpartyPortIdentifier: Identifier,
counterpartyChannelIdentifier: Identifier,
version: string) {
// only unordered channels allowed
abortTransactionUnless(order === UNORDERED)
// only allow channels to "bank" port on counterparty chain
abortTransactionUnless(counterpartyPortIdentifier === "bank")
// version not used at present
abortTransactionUnless(version === "")
// allocate an escrow address
channelEscrowAddresses[channelIdentifier] = newAddress()
}function onChanOpenTry(
order: ChannelOrder,
connectionHops: [Identifier],
portIdentifier: Identifier,
channelIdentifier: Identifier,
counterpartyPortIdentifier: Identifier,
counterpartyChannelIdentifier: Identifier,
version: string,
counterpartyVersion: string) {
// only unordered channels allowed
abortTransactionUnless(order === UNORDERED)
// version not used at present
abortTransactionUnless(version === "")
abortTransactionUnless(counterpartyVersion === "")
// only allow channels to "bank" port on counterparty chain
abortTransactionUnless(counterpartyPortIdentifier === "bank")
// allocate an escrow address
channelEscrowAddresses[channelIdentifier] = newAddress()
}function onChanOpenAck(
portIdentifier: Identifier,
channelIdentifier: Identifier,
version: string) {
// version not used at present
abortTransactionUnless(version === "")
// port has already been validated
}function onChanOpenConfirm(
portIdentifier: Identifier,
channelIdentifier: Identifier) {
// accept channel confirmations, port has already been validated
}function onChanCloseInit(
portIdentifier: Identifier,
channelIdentifier: Identifier) {
// no action necessary
}function onChanCloseConfirm(
portIdentifier: Identifier,
channelIdentifier: Identifier) {
// no action necessary
}In plain English, between chains A and B:
- When acting as the source zone, the bridge module escrows an existing local asset denomination on the sending chain and mints vouchers on the receiving chain.
- When acting as the sink zone, the bridge module burns local vouchers on the sending chains and unescrows the local asset denomination on the receiving chain.
- When a packet times-out, local assets are unescrowed back to the sender or vouchers minted back to the sender appropriately.
- No acknowledgement data is necessary.
createOutgoingPacket must be called by a transaction handler in the module which performs appropriate signature checks, specific to the account owner on the host state machine.
function createOutgoingPacket(
denomination: string,
amount: uint256,
sender: string,
receiver: string,
source: boolean) {
if source {
// sender is source chain: escrow tokens
// determine escrow account
escrowAccount = channelEscrowAddresses[packet.sourceChannel]
// construct receiving denomination, check correctness
prefix = "{packet/destPort}/{packet.destChannel}"
abortTransactionUnless(denomination.slice(0, len(prefix)) === prefix)
// escrow source tokens (assumed to fail if balance insufficient)
bank.TransferCoins(sender, escrowAccount, denomination.slice(len(prefix)), amount)
} else {
// receiver is source chain, burn vouchers
// construct receiving denomination, check correctness
prefix = "{packet/sourcePort}/{packet.sourceChannel}"
abortTransactionUnless(denomination.slice(0, len(prefix)) === prefix)
// burn vouchers (assumed to fail if balance insufficient)
bank.BurnCoins(sender, denomination, amount)
}
FungibleTokenPacketData data = FungibleTokenPacketData{denomination, amount, sender, receiver, source}
handler.sendPacket(packet)
}onRecvPacket is called by the routing module when a packet addressed to this module has been received.
function onRecvPacket(packet: Packet): bytes {
FungibleTokenPacketData data = packet.data
if data.source {
// sender was source chain: mint vouchers
// construct receiving denomination, check correctness
prefix = "{packet/destPort}/{packet.destChannel}"
abortTransactionUnless(data.denomination.slice(0, len(prefix)) === prefix)
// mint vouchers to receiver (assumed to fail if balance insufficient)
bank.MintCoins(data.receiver, data.denomination, data.amount)
} else {
// receiver is source chain: unescrow tokens
// determine escrow account
escrowAccount = channelEscrowAddresses[packet.destChannel]
// construct receiving denomination, check correctness
prefix = "{packet/sourcePort}/{packet.sourceChannel}"
abortTransactionUnless(data.denomination.slice(0, len(prefix)) === prefix)
// unescrow tokens to receiver (assumed to fail if balance insufficient)
bank.TransferCoins(escrowAccount, data.receiver, data.denomination.slice(len(prefix)), data.amount)
}
return 0x
}onAcknowledgePacket is called by the routing module when a packet sent by this module has been acknowledged.
function onAcknowledgePacket(
packet: Packet,
acknowledgement: bytes) {
// nothing is necessary, likely this will never be called since it's a no-op
}onTimeoutPacket is called by the routing module when a packet sent by this module has timed-out (such that it will not be received on the destination chain).
function onTimeoutPacket(packet: Packet) {
FungibleTokenPacketData data = packet.data
if data.source {
// sender was source chain, unescrow tokens
// determine escrow account
escrowAccount = channelEscrowAddresses[packet.destChannel]
// construct receiving denomination, check correctness
prefix = "{packet/sourcePort}/{packet.sourceChannel}"
abortTransactionUnless(data.denomination.slice(0, len(prefix)) === prefix)
// unescrow tokens back to sender
bank.TransferCoins(escrowAccount, data.sender, data.denomination.slice(len(prefix)), data.amount)
} else {
// receiver was source chain, mint vouchers
// construct receiving denomination, check correctness
prefix = "{packet/sourcePort}/{packet.sourceChannel}"
abortTransactionUnless(data.denomination.slice(0, len(prefix)) === prefix)
// mint vouchers back to sender
bank.MintCoins(data.sender, data.denomination, data.amount)
}
}function onTimeoutPacketClose(packet: Packet) {
// can't happen, only unordered channels allowed
}This implementation preserves both fungibility & supply.
Fungibility: If tokens have been sent to the counterparty chain, they can be redeemed back in the same denomination & amount on the source chain.
Supply: Redefine supply as unlocked tokens. All send-recv pairs sum to net zero. Source chain can change supply.
This does not yet handle the "diamond problem", where a user sends a token originating on chain A to chain B, then to chain D, and wants to return it through D -> C -> A — since the supply is tracked as owned by chain B, chain C cannot serve as the intermediary. It is not yet clear whether that case should be dealt with in-protocol or not — it may be fine to just require the original path of redemption (and if there is frequent liquidity and some surplus on both paths the diamond path will work most of the time). Complexities arising from long redemption paths may lead to the emergence of central chains in the network topology.
- Each chain, locally, could elect to keep a lookup table to use short, user-friendly local denominations in state which are translated to and from the longer denominations when sending and receiving packets.
- Additional restrictions may be imposed on which other machines may be connected to & which channels may be established.
Not applicable.
A future version of this standard could use a different version in the channel handshake.
Coming soon.
Coming soon.
Jul 15, 2019 - Draft written
Jul 29, 2019 - Major revisions; cleanup
Aug 25, 2019 - Major revisions, more cleanup
All content herein is licensed under Apache 2.0.