Preamble
MIP#: 45
Title: Liquidations 2.0 - Liquidation System Redesign
Author(s): Smart Contracts Domain Team
Type: Technical
Status: Formal Submission
Date Proposed: 2021-02-03
Dependencies:
Replaces: Liquidation System 1.2 (LIQ-1.2)
Sentence Summary
This MIP defines a new Liquidation System for the Maker Protocol.
Paragraph Summary
If accepted, this MIP would resolve some inefficiencies in the current liquidation system by replacing the current English style auction system, where bids start low, with a new Dutch style auction system, where auction prices generally start high and drop over time. The key advantage to the new liquidation system is that the entire Vaultâs collateral is available at a given price. As time passes, the DAI the protocol is willing to take for the collateral falls. This allows keepers to leverage all liquidity available on-chain using single block composability. The design is intended to reduce the risk that auctions clear at lower-than-market prices, while removing most liquidity constraints from keepers. Liquidations 2.0 is also intended to reduce price volatility risk from keepers.
This document supersedes the Liquidations 2.0: Technical Summary.
Specification
MIP45c1 Governance Parameters
Abacus/LinearDecrease â tau [seconds]
Seconds after auction start when the price reaches zero.
Abacus/StairstepExponentialDecrease â cut [ray]
Per-step multiplicative factor.
cut = 0.99 * RAY is 1% drop on each step.
Abacus/StairstepExponentialDecrease â step [seconds]
Length of time between price drops.
Clipper â buf [ray]
The multiplicative factor to increase the starting price of an auction.
E.g. if the current OSM price of an asset is 1,000 and buf = 1.2 * RAY (20% above), then the initial price of that auction will be 1,200.
Clipper â calc [address]
The contract address of the price calculator function. Adheres to Abacus interface. Some examples of price functions are found in abaci.sol file.
Clipper â chip [wad]
Percentage of tab to suck from vow to incentivize keepers when liquidating a vault or resetting an already existing auction.
chip = 0.02 * WAD is 2%.
Clipper â cusp [ray]
Percentage drop before auction reset. Together with the tail, they are the two variables that determine that an auction needs to be reset (whichever triggers first).
E.g. if the initial price of an auction (top) is set to 1,200 and cusp = 0.6 * RAY (60% of the starting price), then the auction will need to be reset when reaching just below the price of 720.
Clipper â dog [address]
The address of the liquidation module contract.
Clipper â spotter [address]
The Collateral price module contract address.
Clipper â tail [seconds]
Time elapsed before auction reset. Together with the cusp, they are the two variables that determine that an auction needs to be reset (whichever triggers first).
Clipper â tip [rad]
Flat fee to suck from vow to incentivize keepers when liquidating a vault or resetting an already existing auction.
tip = 100 * RAD is 100 DAI.
Clipper â vow [address]
The address of the accounting system contract. The recipient of DAI raised in auctions.
Dog â Hole [rad]
Max DAI needed to cover debt + liquidation penalty of active auctions.
Hole = 10,000,000 * RAD is 10M DAI.
Dog â ilk.chop [wad]
Liquidation Penalty per collateral (ilk).
E.g. if there is a vault ready to be liquidated that has a debt of 1,000 DAI and chop = 1.13 * WAD (13% above), then the max amount to be raised by the auction will be 1,130 DAI.
Dog â ilk.clip [address]
The address of the auction module contract. One clip per collateral (ilk).
Dog â ilk.hole [rad]
Max DAI needed to cover debt + liquidation penalty of active auctions per collateral (ilk).
hole = 10,000,000 * RAD is 10M DAI.
Dog â vow [address]
The address of the accounting system contract. The recipient of the bad debt coming from a vault when itâs liquidated.
MIP45c2 Vault Liquidation
https://github.com/makerdao/dss/blob/liq-2.0/src/dog.sol
The Vault liquidation function (Dog.bark) takes three caller supplied arguments:
-
ilk: the collateral ilk -
urn: the Vault to be liquidated -
kpr: the address where DAI incentives will be sent
Dog.bark performs several actions:
- confiscates the given Vault
urnif itâs undercollateralized and- sends the collateral to the
ilk'sClipper - increments the
vow's bad debt accumulator
- sends the collateral to the
- pushes the bad debt onto the debt queue
- adds the bad debt to the
Holewith theDirtaccumulator - adds the bad debt to the
ilk.holewith theilk.dirtaccumulator - initiates the auction by calling
Clipper.kick() - fires the
Bark()event
In the context of the Maker protocol, a âliquidationâ is the automatic transfer of collateral from an insufficiently collateralized Vault, along with the transfer of that Vaultâs debt to the protocol. In both the current liquidation contract (the Cat) and the 2.0 version (the Dog), an auction is started promptly to sell the transferred collateral for DAI in an attempt to cancel out the debt now assigned to the protocol. This makes the behavior of the new contract very similar to that of the current one, but there are some important differences, explained below.
MIP45c3 Partial vs. Total Liquidations
In the current system, in each call to the liquidation function (Cat.bite) transfers a fixed amount of debt (the dunk) from the affected Vault, along with a proportional amount of the Vaultâs collateral to the protocol. For example, if 50% of a Vaultâs debt is taken by the protocol, then half of its collateral is taken as well. If the Vaultâs debt is less than the dunk, then all debt and collateral is taken. In 2.0, all debt is taken when the liquidation function (Dog.bark) is called, and no analogue of the dunk parameter exists. The reasoning behind this change is that because the new auctions allow partial purchases of collateral, the liquidity available to a participant no longer limits their ability to participate in auctions, so instead the total number of auctions should be minimized. Just to emphasize, there is no longer a minimum DAI liquidity requirement for the sale of collateral on a per-participant basis.
MIP45c4 Limits on DAI Needed to Cover Debt and Fees of Active Auctions
In situations involving large amounts of collateral at auction, the current and new designs modify the behavior described in the previous section. Both liquidations 1.2 and 2.0 implement a limit on the total amount of DAI needed to cover the summed debt and liquidation penalty associated with all active auctions. In LIQ-1.2 this is called the box, and in LIQ-2.0 we call this the Hole. Whenever the maximum possible addition to this value is less than the amount of debt+fees that would otherwise be sent to auction, a partial liquidation is performed so as not to exceed this amount. In the current system there is only a global limit; in 2.0, in addition to the global limit, there is also a per-collateral limit as well. Similar to how there is an ilk.line for a collateralâs debt ceiling and a Line for the overall system debt ceiling, there is now an ilk.hole to correspond with the Hole. This ensures that typical market depth against DAI can be taken into account on a per-collateral basis by those determining risk parameters.
MIP45c5 Auction Initiation
https://github.com/makerdao/dss/blob/liq-2.0/src/clip.sol
The auction initiation function (Clipper.kick) takes four caller supplied arguments:
-
tab: the target DAI to raise from the auction (debt+stability fees+liquidation penalty) -
lot: the amount of collateral available for purchase -
usr: the Vault under liquidation -
kpr: the address where DAI incentives will be sent
Clipper.kick performs several checks and actions:
- checks that the caller is authorized (only the
Dogor governance may callClipper.kick) - checks that liquidations are enabled in the three-stage circuit breaker
- increments a counter and assigns a unique numerical
idto the new auction - inserts the
idinto a list tracking active auctions - creates a structure to record the parameters of the auction; this includes:
- itâs position in the active auctions list
- the target amount of DAI to raise from bidders (
tab) - the amount of collateral available for purchase (
lot) - the Vault that was liquidated to create this auction
- allows return of collateral if not all of it is sold
- allows the return of collateral and
tabwhenClipper.yankis called byEnd.snip
- the timestamp of the auctions creation (as a Unix epoch)
- the initial price of collateral in the auction (
top)
- sends an incentive denominated in DAI to the
kpr - fires the
Kick()event
The initial price is set by reading the current price in the corresponding Oracle Security Module (OSM) and multiplying by a configurable percentage (the buf parameter). Note that the current OSM price is between one and two hours delayed relative to the actual market price. A keeper doesnât make a call to Clipper.kick directly, but rather makes a call to Dog.bark which in turn calls Clipper.kick.
MIP45c6 Liquidation Incentive Mechanism
In this design, there is less incentive to quickly liquidate Vaults than in the current system, because there is no inherent advantage obtained by doing so. In contrast, the current auction system grants the account triggering a liquidation the privilege of making the first bid in the resulting auction. It is unclear whether this matters significantly in practice, or whether some stronger incentive should be added (for example, a small DAI reward paid to liquidators).
To ensure there was a remedy for this potential issue, an incentive mechanism was added for liquidators. The form of the incentive is, on a per-collateral type basis, a constant amount of DAI plus an amount of DAI that scales linearly with the amount of debt associated with the liquidation. Either contribution can be set to zero. Such a structure is justified by the following:
- The reward is set per-collateral to give maximum flexibility to include not just per-collateral risk parameters like
mat(collateralization ratio) andchop(liquidation penalty) in its setting, but also to allow for unique market conditions that might only apply to one or a few collateral types. - The component of the reward that increases linearly with the total Vault debt is intended to be used to reward liquidators for reducing risk to the system, as risk itself scales with the size of undercollateralized Vaultsâa Vault that is twice as big as another represents twice the risk of bad debt accrual. Or viewed another way, liquidating two vaults of size X represents the same risk reduction as liquidating one Vault of size 2Xâthus the reward to a liquidator ought to be similar. Further, the system can afford to pay more for larger liquidations, because the liquidation penalty is also proportional to the amount of debt outstanding for a given Vault.
- The constant component of the reward can be used to cover gas costs (which are per-Vault for liquidators) or to allow MKR holders to effectively pay Keepers to clear small Vaults that would otherwise not be attractive for liquidation.
These parameters must be set extremely carefully, lest it be possible to exploit the system by âfarmingâ liquidation rewards (e.g. creating Vaults with the intention of liquidating them and profiting from the too-high rewards). Generally, the liquidation reward should remain less than the liquidation penalty by some margin of safety, at least to ensure the system is not accruing a deficit from liquidations.
MIP45c7 Three-Stage Liquidation Circuit Breaker
In this section, weâll cover all the functions of the three-stage liquidation circuit breaker. In contrast to LIQ-1.2, Liquidations 2.0 comes with a three-stage circuit breaker built into the Clipper contract. The stages are:
-
liquidations enabled(
0): This means the breaker is not tripped and the protocol can liquidate new Vaults as well as service old liquidations. -
new liquidations disabled(
1): This means no new liquidations (Clipper.kick) -
all liquidations disabled(
2): This means no new liquidations (Clipper.kick), no takes (Clipper.take), and no redos (Clipper.redo)
Just like in LIQ-1.2, the circuit breaker will be available through a ClipperMom contract giving governance the ability to bypass the GSM delay for circuit breaker actions.
MIP45c8 Bidding (Purchasing)
https://github.com/makerdao/dss/blob/liq-2.0/src/clip.sol
The purchasing function (Clipper.take) takes five caller supplied arguments:
-
id: the numerical id of the auction to bid on -
amt: the maximum amount of collateral to buy (amt) â a purchase behaves like a limit order -
max: the maximum acceptable price in DAI per unit collateral (max) -
who: address that will receive the collateral (who) -
data: an arbitrary bytestring (if provided, the addresswhois assumed to be a contract with an interface as described below, to which this data is passed)
Clipper.take performs several initial checks:
- a reentrancy check to ensure the function is not being recursively invoked
- that the three-stage circuit breaker is not tripped
- that the auction id corresponds to a valid auction
- that the auction does not need to be reset, either due to having experienced too large a percentage decrease in price, or having existed for too long of a time duration
- that the callerâs specified maximum price is at least the current auction price
Then, the amount of collateral to attempt to purchase is computed as the minimum of the collateral left in the auction (lot) and the callerâs specified quantity (amt)âthe resulting value is the slice. This value is then multiplied by the current price of the auction to compute the DAI owed in exchange (owe). If owe exceeds the DAI collection target of the auction (tab), then owe is adjusted to be equal to tab, and slice is set to tab / price (i.e. the auction will not sell more collateral than is needed to cover debt+fees from the liquidated Vault).
Once the collateral to buy and corresponding DAI owed are determined, a check is done to ensure that the remaining DAI collection target either exceeds a minimum amount or is zero; if not, the function reverts. Callers should be aware of this possibility and account for it when possible.
Next, collateral is transferred (within the protocolâs accounting module) to the who address provided by the caller. If the caller provided a bytestring with greater than zero length, an external call is made to the who address, assuming it exposes a function, follow Solidity conventions, with the following signature:
clipperCall(
address, // recipient of DAI
uint256, // owe [rad]
uint256, // slice [wad]
bytes calldata
)
The first argument is the recipient of DAI. That is, this is the address the clipperCallee must return DAI to. The second argument is DAI owed (as a 45 decimal digit fixed-precision integer, or rad), the third argument is the collateral being purchased (as an 18 decimal digit fixed-precision integer, or wad), regardless of the precision of the external token contract, and the last argument is identical to what bytestring the caller originally supplied to the purchase function. As mentioned earlier, a locking mechanism prevents reentry into the purchase function during this external call, and the Clipper.redo call, for security reasons.
After the external call completes (or immediately following the transfer of collateral, if no external call was executed), DAI is transferred (internally, within the core accounting module) from msg.sender to the protocol.
Lastly, various values are updated to record the changed state of the auction: the DAI needed to cover debt and fees for outstanding auctions, and outstanding auctions of the given collateral type, are reduced (via a callback to the liquidator contract) is reduced by owe, and the tab (DAI collection target) and lot (collateral for sale) of the auction are adjusted as well. If all collateral has been drained from an auction, all its data is cleared and it is removed from the active auctions list. If collateral remains, but the DAI collection target has been reached, the same is done and excess collateral is returned to the liquidated Vault.
MIP45c9 Example Liquidation
Figure MIP45c9.1: NOTE: in the above figure,
tau is tail.
In this example we can see a linear decrease function (calc), with an ETH-A OSM price of 200 DAI, a buf of 20%, a tail (tau) of 21600 seconds, a tab of *60,000 DAI with a lot of 347.32 ETH. There are two bidders, Alice and Bob. Alice calls take first and is willing to give *50,000 DAI in return for 256.41 ETH collateral, a price of 195 DAI per ETH. The price of ETH continues to fall over time, and Bob picks up the remaining 90.91 ETH for 10,000 DAI, a price of 110 DAI per ETH. If the auction reached the tail value, or fell to less than cusp percent of top, then Clipper.take would revert if called, and the auction would need to be reset with a Clipper.redo call.
MIP45c10 Features
As discussed previously, collateral auctions are being changed from English to Dutch style. The current auction functionality is described in the official Maker documentation; here, we will focus on the new Dutch design, and only reference some contrasting points relative to the current design.
MIP45c11 Instant Settlement
Unlike the current English auctions, in which DAI bids are placed, with a participantâs capital remaining locked until they are outbid or until the auction terminates, Dutch auctions settle instantly. They do so according to a price calculated from the initial price and the time elapsed since the auction began. Price versus time curves are discussed more later. The lack of a lock-up period mitigates much of the price volatility risk for auction participants and allows for faster capital recycling.
MIP45c12 Flash Lending of Collateral
This feature, enabled by instant settlement, eliminates any capital requirement for bidders (excepting gas)âin the sense that even a participant with zero DAI (and nothing to trade for DAI) could still purchase from an auction by directing the sale of the auctionâs collateral into other protocols in exchange for DAI. Thus, all DAI liquidity available across DeFi can be used by any participant to purchase collateral, subject only to gas requirements. The exact mechanics are discussed above, but essentially a participant needs to specify a contract who (which conforms to a particular interface), and calldata to supply to it, and the auction contract will automatically invoke whatever logic is in the external contract.
MIP45c13 Price as a Function of Time
https://github.com/makerdao/dss/blob/liq-2.0/src/abaci.sol
Price-versus-time curves are specified through an interface that treats price at the current time as a function of the initial price of an auction and the time at which it was initiated. How to determine the most effective price curve for a given collateral is still an active area of research; some initial options (linear and stair-step exponential) have been implemented for research purposes. Other candidates besides these include a piecewise linear curve and a piecewise exponential curve. This module is configurable and can be replaced in the course of innovation. Feedback or suggestions are particularly welcome in this area.
MIP45c14 Resetting an Auction
As mentioned above, auctions can reach a defunct state that requires resetting for two reasons:
- too much time has elapsed since the auction started (controlled by the
tailgovernance parameter) - the ratio of the current price to the initial price has fallen below a certain level (specified by the
cuspgovernance parameter).
The reset function, when called, first ensures that at least one of these conditions holds. Then it adjusts the starting time of the auction to the current time, and sets the starting price in exactly the same way as is done in the initialization function (i.e. the current OSM price increased percentage-wise by the buf parameter). This process will repeat until all collateral has been sold or the whole debt has been collected; contrast this behavior with the current auctions, which reset until at least one bid is received.
MIP45c15 Improved Keeper Wallet Security
If keepers decide to use the clipperCallee pattern, then they need not store DAI or collateral on that account. This means a keeper need only hold enough ETH to execute transactions that can orchestrate the Clipper.take call, sending collateral to a contract that returns DAI to the msg.sender to pay for the collateral all in one transaction. The Clipper.take call can send any remaining collateral or DAI beyond owe to a cold wallet address inaccessible to the keeper. NOTE: If the keeper chooses to behave as an EOA address, then the DAI and collateral would be exposed just as in LIQ-1.2 unless special care is taken to create a proxy contract.
MIP45c16 Incentive to call redo()
https://github.com/makerdao/dss/blob/liq-2.0/src/clip.sol
The auction initiation function (Clipper.redo) takes two caller supplied arguments:
-
id: the current auction id -
kpr: the address where DAI incentives will be sent
Clipper.redo performs several checks and actions:
- a mutex check to ensure the
Clipper.takefunction is not already being invoked fromclipperCallee - that the three-stage circuit breaker is not tripped
- that the auction id corresponds to a valid auction
- that the auction needs to be reset, either due to having experienced too large a percentage decrease in price, or having existed for too long of a time duration
- updates several fields of the existing auction
-
ticto reset the auction start time -
topwith the current OSM price andbufpercent
-
-
vat.sucks DAI to thekpras an incentive if eligible - fires the
Redo()event
An auction that has expired or which is currently offered at a value higher than the oracle threshold will likely not complete at favorable values. The system therefore provides a direct incentive to Clipper.redo the auction, resetting itâs expiration and setting the starting price to match the current oracle price + buf. The redo includes the same Dai incentive to the keeper as the Clipper.kick, which is based on the flat fee plus the governance-defined percentage of collateral. There is one exception to this incentive. If the tab or lot * price remaining is under the ilk.dust limit, then there will be no incentive paid to redo the auction. This is to help prevent incentive farming attacks where no keepers bid on dusty lots, and Clipper.redo is called repeatedly. If auctions in this state build up, governance may choose to pay a keeper to clean them up.
MIP45c17 Shutdown (Global Settlement, Emergency Shutdown)
https://github.com/makerdao/dss/blob/liq-2.0/src/end.sol
A started auction can be reverted via the auth function called yank in Clipper contract.
This function requires that the auction exists and executes the following actions:
- calls
dog.digsin order to increment itsHoleandilk.holevalues by the remaining auctiontab - sends the remaining collateral to its
msg.sender - removes the auction struct data
- fires the
Yank()event
This function might be thought of as a general purpose operation to migrate existing auctions to a new contract; however, the only use-case now is in the End module, which takes care of system shutdown.
The End module will be upgraded together with the auction contracts as a new function End.snip is required.
This function End.snip is responsible for calling Clipper.yank for any running auction when shutdown is triggered. It will receive the collateral from Clipper.yank and will send it back to the vault owner together with the remaining debt to recover. One consideration to note is that the debt that the vault owner will receive includes the liquidation penalty part already charged.
MIP45c18 Interfaces
Dog Interface
function wards(address) external view returns (uint256);
function rely(address) external;
function deny(address) external;
Standard MakerDAO authorization structure.
function vat() external view returns (address);
function vow() external view returns (address);
DSS core address introspection.
function ilks(bytes32 ilk) external view returns (
address clip, uint256 chop, uint256 hole, uint256 dirt);
Returns values configured for a given ilk (ex. ETH-A)
function live() external view returns (uint256);
Returns 1 if the system is active.
function Hole() external view returns (uint256);
function Dirt() external view returns (uint256);
Getters for the global Hole and Dirt configuration.
function file(bytes32 what, address data) external;
function file(bytes32 what, uint256 data) external;
function file(bytes32 ilk, bytes32 what, address data) external;
function file(bytes32 ilk, bytes32 what, uint256 data) external;
(Authenticated) Parameter modification functions, available to governance.
function chop(bytes32 ilk) external view returns (uint256);
Getter for the chop value of a given ilk.
function bark(bytes32 ilk, address urn, address kpr)
external returns (uint256 id);
The main liquidation function. Initiates an auction. A keeper calls this function to begin the auction of urn for a particular ilk. kpr is the address where the keeper incentive is sent, allowing keepers to have liquidation rewards sent to a different address than the caller.
function digs(bytes32 ilk, uint256 rad) external;
(Authenticated) Removes collateral from the accumulator. Called by the Clipper.
function cage() external;
(Authenticated) Deactivates the Dog and sets live to 0.
Clipper Interface
function wards(address) external view returns (uint256);
function rely(address) external;
function deny(address) external;
Standard MakerDAO authorization structure.
function ilk() external view returns (bytes32);
The ilk that this Clipper is associated with.
function dog() external view returns (address);
function vow() external view returns (address);
function spotter() external view returns (address);
DSS core address introspection.
function calc() external view returns (address);
The address of the pricing function used by this Clipper.
function buf() external view returns (uint256);
function tail() external view returns (uint256);
function cusp() external view returns (uint256);
function chip() external view returns (uint256);
function tip() external view returns (uint256);
Getters for governance-configured auction parameters.
function kicks() external view returns (uint256);
Auction counter. Increments each time an auction is initiated.
function active() external view returns (uint256[]);
Returns an array containing the active auctions. To save gas, this array is not sorted.
function sales(uint256) external view returns (
uint256 pos,
uint256 tab,
uint256 lot,
address usr,
uint96 tic,
uint256 top);
Returns information on a particular auction. Completed auctions are removed from the mapping.
function stopped() external view returns (uint256);
Returns the current circuit breaker status. 0 for no breaker, 1 if new kicks cannot be called, 2 if no new kick, redo or take can be called.
function file(bytes32 what, uint256 data) external;
function file(bytes32 what, uint256 data) external;
(Authenticated) Parameter modification functions, available to governance.
function kick(uint256 tab, uint256 lot, address usr, address kpr)
external returns (uint256 id);
(Authenticated) Initiates the auction. Called by the Dog.
function redo(uint256 id, address kpr) external;
Called to reset an auction due to expiry or price deviation.
function take(uint256 id, uint256 amt,
uint256 max, address who, bytes calldata data) external;
Called to purchase collateral.
function list() external view returns (uint256[] memory);
function count() external view returns (uint256);
function getId(uint256 pos) external view returns (uint256 id);
Helpers for iterating a list of active auctions. Get list() to get the array of auctions or get the count and iterate by pos using getId
function getStatus(uint256 id) external view returns (bool needsRedo, uint256 price);
Returns a bool if the auction is eligible for redo and the current price.
function setBreaker(uint256 level) external;
(Authenticated) Sets the circuit-breaker level for disabling kicks or redoâs.
function yank(uint256 id) external;
(Authenticated) Allows an auction to be removed during Emergency Shutdown or via a goveranance action.
Known Risks
This section covers some of the known risks with Liquidations 2.0
MIP45c19 Incentive Farming
Periodically, governance may increase the ilk.dust amount. When this happens, itâs usually because gas has become so expensive it impacts the efficiency of liquidations. That is, the cost of calling Dog.bark(), Clipper.take(), or Clipper.redo() may exceed the collateral offered. Incentives may be used as a remedy to this potential issue and possibly a way for the protocol to keep ilk.dust lower; however, governance must take care when increasing the ilks dust, tip, or chip not to incentivize the creation of many Vaults to farm this incentive. An example of an exploit is as follows:
- governance decides to increment
dustby 1500 DAI at the same time they scaleilk.tipto subsidize auctions. - an attacker realizes it would be profitable between gas and the
chopto shard (fork) their existing Vault into many Vaults or create many new Vaults. - the spell is voted on and passed
- using one transaction the attacker puts their Vaults at the edge of unsafe,
poke()s the OSM when next price is going down, then callsDog.bark()on all their Vaults to collect the incentive. - Using the gains, they can also slightly overbid for their Vaults auctions.
In order to thwart this attack governance must be careful when setting ilk.tip and ilk.chip so as not to create this perverse incentive.
MIP45c20 Price Decrease Function Too Fast
If the price function decreases too fast it can have the following consequences:
- the auction ends without any bid, then it needs to be reset and possibly this will keep happening
- bidders end up having reverts due the auction ended before tx confirmation
- bidders end up paying much less than what they were willing to pay (possibly generating permanent bad debt)
MIP45c21 Price Decrease Function Too Slow
If the price function decreases too slow it can have the following consequences:
- auction price never catches up with the market price, eventually being reset
- After the reset the price catches up, but is less than an optimal market price
- After the reset the price catches up, but still leaves bad debt
- After the reset the auction price still might not catch up, causing more resets and very likely leaving bad debt
MIP45c22 Front-Running
In LIQ-1.2 there is limited front-running risk as it requires capital to participate in auctions; however, in liquidations 2.0 if a keeper chooses to participate with no capital, there is substantial front-running risk from generalized front-running bots. The easier it is to replace the from address of the transaction with oneâs own, the greater the risk. To mitigate this risk keepers are encouraged to used authorized proxy contracts to interact with liquidations 2.0 and provide some amount of their own capital when bidding. More aggressive gas prices may also work. Unfortunately, we found no great way to prevent generalized front-running that preserves single-block composability.
MIP45c23 OSM Risk for Start Price
Because Clipper.kick and Clipper.redo consult the OSM for the collateral price, we are vulnerable to an oracle attack that can only be mitigated by the oracle delay, Dog.Hole, and ilk.hole. We must rely on the number of guards in place to prevent price manipulation and oracle attacks.
MIP45c24 Setting Hole or ilk.hole Too High
While Dog.Hole and ilk.hole can be set much higher in liquidations 2.0, there are still risks to setting this too high. A value for Dog.Hole thatâs set too high could result in far too much DAI demand, breaking the peg high. This is somewhat mitigated by the PSM and stablecoin collateral types, but should still factor in to how this parameter is set. An ilk.hole that is set too high, may have the additional result of causing a downward spiral as the liquidations push the asset price lower. In addition, if there is an oracle attack, this parameter can be thought of as our maximum exposure.
MIP45c25 Setting Hole or ilk.hole Too Low
If we set either Dog.Hole or ilk.hole too low, we run the risk of not being able to liquidate enough collateral at once. This could lead to a buildup of undercollateralized positions in the system, eventually causing the accrual of bad debt.
Audits
There are a number of auditors engaged on Liquidations 2.0. As audit reports come in this MIP will be updated.
Formulas
MIP45c26 Dog.bark formulas
The following formulas are still a work in progress, they might contain errors.
FV considerations
This function computes tab, the target amount of DAI to be raised in an auction and lot, the collateral to put for sale.
They are defined as:
tab = dart * ilk.rate * ilk.chop
lot = dink = urn.ink * dart / urn.art
room is defined as the remaining space of available DAI to be offered in auctions:
room = min(Hole - Dirt, ilk.hole - ilk.dust)
There is a precondition about room that needs to be satisfied in order to create an auction, otherwise the transaction fails:
room > 0 && room >= ilk.dust
Then dart is defined as:
if urn.art * ilk.rate * ilk.chop ||
urn.art * ilk.rate * ilk.chop > room && urn.art * ilk.rate * ilk.chop < room + dust
dart = urn.art
otherwise
dart = room / ilk.rate / ilk.chop
Leaving the final formulas as:
if urn.art * ilk.rate * ilk.chop ||
urn.art * ilk.rate * ilk.chop > room && urn.art * ilk.rate * ilk.chop < room + dust:
tab = urn.art * ilk.rate * ilk.chop
lot = urn.ink
otherwise
tab = room
lot = urn.ink * (room / ilk.rate / ilk.chop) / urn.art
Invariants:
dink <= urn.ink
dart <= urn.art
dart / urn.art == dink / urn.ink
urn.art' / urn.art == urn.ink' / urn.ink
tab <= min(ilk.hole + ilk.dust - ilk.dirt, Hole + max(ilk[*].dust) - Dirt)
considering ilk[*].dust can't change after set up
Implementation breakdown
This function computes tab, the target amount of DAI to be raised in an auction, and lot, the collateral to be auctioned off. They are defined as:
tab = dart * ilk.rate * ilk.chop (A)
lot = dink = urn.ink * dart / urn.art (B)
Liquidations and normalized debt
The debt of a vault is expressed in normalized terms:
vault.debt = vault.art * ilk.rate (C)
This normalized debt is decreased after a liquidation is triggered:
vault.art' = vault.art - dart (D)
This dart value stands for delta art and it is equal to or less than the normalized debt:
dart <= vault.art (E)
The computation of dart depends on the hole and dust restrictions as explained below.
hole restrictions
When a liquidation is successfully started, its tab value is added to the collateralâs dirt value:
ilk.dirt' = ilk.dirt + tab (F)
To prevent too many liquidations from being triggered at once, the tab value of new liquidations added to the collateralâs dirt shouldnât surpass the collateralâs hole value:
tab + ilk.dirt <= ilk.hole
Solving for tab,
tab <= ilk.hole - ilk.dirt (G)
There is also a general Dirt value that accumulates tabs from every collateral, and a general Hole value for which the same condition applies:
tab <= Hole - Dirt (H)
Joining equations G and H,
tab <= min(ilk.hole - ilk.dirt, Hole - Dirt) (H)
Solving for dart using equation A,
dart * ilk.rate * ilk.chop <= min(ilk.hole - ilk.dirt, Hole - Dirt)
dart <= min(ilk.hole - ilk.dirt, Hole - Dirt) / (ilk.rate * ilk.chop) (I)
Definition of dart in terms of room
room is defined as the remaining space of available DAI to be offered in auctions:
room = min(Hole - Dirt, ilk.hole - ilk.dust)
There is a precondition about room that needs to be satisfied in order to create an auction:
room > 0 && room >= ilk.dust
Otherwise, the transaction fails.
Then dart is defined as:
dart = vault.art
, for vault.art * ilk.rate <= room
|| (vault.art * ilk.rate > room
&& vault.art * ilk.rate < room + dust)
dart = room / ilk.rate / ilk.chop , otherwise
dust restrictions
When a vault is partially liquidated because of hole restrictions, its new debt value cannot be dusty. This means that it has to be either zero, or at least equal to the collateralâs dust value:
vault.art' * ilk.rate >= ilk.dust , or
vault.art' = 0 (J)
Solving for dart in both cases using equation D,
vault.art' >= ilk.dust / ilk.rate
vault.art - dart >= ilk.dust / ilk.rate
-dart >= ilk.dust / ilk.rate - vault.art
dart <= vault.art - ilk.dust / ilk.rate , or
vault.art - dart = 0
dart = vault.art (K)
When the value of dart is one such that vault.art' will be dusty, the whole vault is liquidated instead, even if it surpasses the hole restrictions. In order to obtain the range in which this is the case, we negate equation J and solve for dart:
vault.art' * ilk.rate < ilk.dust && vault.art' > 0
vault.art' < ilk.dust / ilk.rate && vault.art' > 0
0 < vault.art' < ilk.dust / ilk.rate
0 < vault.art - dart < ilk.dust / ilk.rate
-vault.art < -dart < ilk.dust / ilk.rate - vault.art
vault.art > dart > vault.art - ilk.dust / ilk.rate
vault.art - ilk.dust / ilk.rate < dart < vault.art (L)
General equation for tab in terms of dart
tab = min(
dart * ilk.rate * ilk.chop,
ilk.hole - ilk.dirt,
Hole - Dirt
) , for dart <= vault.art - ilk.dust / ilk.rate
tab = vault.art * ilk.rate * ilk.chop
, for vault.art - ilk.dust / ilk.rate < dart <= vault.art
General equations for tab and lot
tab = vault.art * ilk.rate * ilk.chop
lot = vault.ink , for vault.art * ilk.rate <= room
|| (vault.art * ilk.rate> room
&& vault.art < room + dust)
tab = room
lot = vault.ink * (room / ilk.rate / ilk.chop) / vault.art , otherwise
MIP45c27 Clipper.take formulas
The following formulas are still a work in progress, they might contain errors.
FV considerations
This function computes owe as the amount of DAI to pay in the auction
and slice as the amount of collateral being bought for that DAI paid
amt = desired max amount of collateral being purchased
tab = current amount of DAI to be recovered in the auction
lot = maximum amount of collateral available for selling
abacus.price = current price of collateral
The values of owe and slice will be
defined according to these different cases:
if min(lot, amt) * abacus.price >= tab
owe = tab
slice = tab / abacus.price
otherwise
if amt < lot && tab - amt * abacus.price < ilk.dust
if tab > ilk.dust
owe = tab - ilk.dust
slice = owe / abacus.price
otherwise
tx fails
otherwise
owe = min(lot, amt) * abacus.price
slice = min(lot, amt)
Implementation breakdown
This function computes owe, i.e. the amount of DAI needed to purchase
amt of collateral at the current price, as determined by the auctionâs
abacus:
owe = amt * abacus.price
If amt is equal or higher than the auctionâs lot, it means that all
the available collateral is being bought.
owe = auction.lot * abacus.price , for amt >= auction.lot
owe = amt * abacus.price , otherwise
Note that the first case means that the auction is over.
Additionally, owe needs to be less than or equal to the auctionâs
tab:
owe = min(auction.lot * abacus.price, auction.tab)
, for amt >= auction.lot
owe = min(amt * abacus.price, auction.tab) , otherwise
After each successful call to this function, the auctionâs tab value
decreases:
auction.tab' = auction.tab - owe
However, there is a restriction regarding the resulting tab value: it
cannot be less than the dust value of its collateral type:
auction.tab' >= collateral.dust
Solving for owe in the two equations above:
auction.tab - owe >= collateral.dust
-owe >= collateral.dust - auction.tab
owe <= auction.tab - collateral.dust
Thus, if the auction is to continue, a valid owe amount has to be less
than or equal to the difference between the auctionâs tab and the
collateralâs dust value:
owe = min(auction.lot * abacus.price, auction.tab)
, for amt >= auction.lot
owe = min(amt * abacus.price, auction.tab - collateral.dust)
, otherwise
Note that, since the first case means that the auction is over, the dust
restriction does not apply to it.
Another case in which the auction is over is when
owe = auction.tab
In this case, the dust value is also irrelevant. On the other hand, we
know that
owe = amt * abacus.price
Solving for amt in the two equations above:
auction.tab = amt * abacus.price
amt = auction.tab / abacus.price
Thus, the general computation of owe can be expressed as:
owe = min(auction.lot * abacus.price, auction.tab)
, for amt >= auction.lot
owe = auction.tab , for amt = auction.tab / abacus.price
owe = min(amt * abacus.price, auction.tab - collateral.dust)
, otherwise
What can be better than that!