• Contract Name: cat.sol

• Type/Category: DSS

• ​Associated MCD System Diagram​

• ​Contract Source​

• ​Etherscan​

1. Introduction (Summary)

The Cat is the system's liquidation agent, it enables keepers to mark positions as unsafe and sends them to auction.

2. Contract Details

Glossary (Cat)

Math

• mul(uint, uint)/rmul(uint, uint) - will revert on overflow or underflow

• bite(bytes32, address) will revert if lot or art are larger than or equal to 2^255.

• bite will not leave a Vault with debt and no collateral.

Auth

• wards are allowed to call protected functions (Administration and cage())

Storage

• ilks stores Ilk structs

  • Ilk is the struct with the address of the collateral auction contract (flip), the penalty for that collateral to be liquidated (chop) and the maximum size of collateral that can be auctioned at once (dunk).

• live must be 1 for the Cat to bite. (see cage in mechanisms)

• box the limit on the debt and penalty fees available for auction. [RAD]

• dunk ("debt chunk") amount of debt plus penalty fee per auction, in Dai. [RAD]

• vat address that conforms to a VatLike interface (see vat documentation for more info). It is set during the constructor and cannot be changed.

• vow address that conforms to a VowLike interface (see vow documentation for more info).

The values of all parameters here (except vat) are changeable by an address that is relyed on. For instance, the End module should be authed to allow for it to call cage() and update live from 1 to 0. Governance (through an authed address) should be able to add collateral types to Cat, update their parameters, and change the vow.

Unsafe

bite can be called at anytime but will only succeed if the Vault is unsafe. A Vault is unsafe when its locked collateral (ink) times its collateral's liquidation price (spot) is less than its debt (art times the fee for the collateral rate). Liquidation price is the oracle-reported price scaled by the collateral's liquidation ratio.

Events

• Bite: emitted when a bite(bytes32, address) is successfully executed. Contains:

  • ilk: Collateral

  • urn: Vault address

  • ink: see lot in bite

  • art: see art in bite

  • tab: see tab in bite

  • flip: address of the auction contract

  • id: ID of the auction in the Flipper

3. Key Mechanisms & Concepts

cage()

• auth

• sets live to 0 (prevents bite). See End documentation for further description.

• Once live=0 it cannot be set back to 1.

bite(bytes32 ilk, address urn)

• bytes32 ilk parameter represents the collateral type that is being bitten.

• address urn the address that identifies the Vault being bitten.

• returns uint id which is the ID of the new auction in the Flipper.

• bite checks if the Vault is in an unsafe position and if it is, it starts a Flip auction for a piece of the collateral to cover a share of the debt.

The following is a line-by-line explanation of what bite does.

function bite(bytes32 ilk, address urn) public returns (uint id) {
  // Get the rate, spot, and dust from the ilk in the vat.
  (,uint256 rate,uint256 spot,,uint256 dust) = vat.ilks(ilk);
  // get the ink and art from the urn from the Vat.
  (uint256 ink, uint256 art) = vat.urns(ilk, urn);
​
  // ensure End has not happened
  require(live == 1);
  // require the Vault to be unsafe (see definition above).
  require(spot > 0 && mul(ink, spot) < mul(art, rate), "Cat/not-unsafe");
​
	// Loads the `ilk` data into memory as an optimization.
  Ilk memory milk = ilks[ilk];
  // Declares a variable that will be assigned in the following scope.
  uint256 dart;
​
	// Defines a new scope, this prevents a stack too deep error in solidity
  {
		// Calculate the available space in the box
    uint256 room = sub(box, litter);
​
    // test whether the remaining space in the litterbox is dusty
    require(litter < box && room >= dust, "Cat/liquidation-limit-hit");
​
		// Sets the amount of debt to be covered by the auction.
    // (smaller of either the amount of normalized debt, maximum debt chunk size, or space in the box)
    // divided by the rate, divided by the penalty fee.
    dart = min(art, mul(min(milk.dunk, room), WAD) / rate / milk.chop);
  }
​
	// Takes the minimum of the collateral balance or the
  // amount of collateral represented by the debt to be covered
  uint256 dink = min(ink, mul(ink, dart) / art);
​
	// Prevents no-collateral auctions
  require(dart >  0      && dink >  0     , "Cat/null-auction");
  // Protects against int overflow when converting from uint to int
  require(dart <= 2**255 && dink <= 2**255, "Cat/overflow"    );
​
	// Called in this way, vat.grab will:
  // - Remove the dink and the dart from the bitten Vault (urn)
  // - Adds the collateral (dink) to the Cat's gem
  // - Adds the debt (dart) to the Vow's debt (vat.sin[vow])
  // - Increases the total unbacked dai (vice) in the system
  // This may leave the CDP in a dusty state
  vat.grab(
    ilk, urn, address(this), address(vow), -int256(dink), -int256(dart)
  );
  // Adds the debt to the debt-queue in Vow (Vow.Sin and Vow.sin[now])
  vow.fess(mul(dart, rate));
​
  { // Avoid stack too deep
    // This calcuation will overflow if dart*rate exceeds ~10^14,
    // i.e. the maximum dunk is roughly 100 trillion DAI.
    // Multiplies the accumulated rate by the normalized debt to be covered 
    // to get the total debt tab (debt + stability fee + liquidation penalty) for the auction.
    uint256 tab = mul(mul(dart, rate), milk.chop) / WAD;
    // Updates the amount of litter in the box
    litter = add(litter, tab);
​
		// Calls kick on the collateral's Flipper contract.
    // tab is the total debt to be sent to auction
    // gal: address(vow) sets up the Vow as the recipient of the Dai income for this auction
    // bid: 0 indicates that this is the opening bid
    // This moves the collateral from the Cat's gem to the Flipper's gem in the Vat
    id = Kicker(milk.flip).kick({
        urn: urn,
        gal: address(vow),
        tab: tab,
        lot: dink,
        bid: 0
    });
  }
​
  // Emits an event about the bite to notify actors (for instance keepers) about the new auction
  emit Bite(ilk, urn, dink, dart, mul(dart, rate), milk.flip, id);
}

Administration

Various file function signatures for administering Cat:

• Setting new vow (file(bytes32, address))

• Setting new collateral (file(bytes32, bytes32, address))

• Setting penalty or dunk size for collateral (file(bytes32, bytes32, uint))

Usage

The primary usage will be for keepers to call bite on a Vault they believe to be unsafe in order to start the auction process.

4. Gotchas (Potential source of user error)

• When the Cat is upgraded, there are multiple references to it that must be updated at the same time (End, Vat.rely, Vow.rely). It must also rely on the End, the system's pause.proxy()

• A Vat upgrade will require a new Cat

• The Cat stores each Ilk's liquidation penalty and maximum auction size.

• Each ilk will be initiated with the file for their Flipper; however, auctions cannot start until file is also called to set the chop and the dunk. Without these auctions for either 0 gems or 0 dai would be created by calling bite on an unsafe Vault.

• bite needs to be called n times where n = urn.ink / ilks[ilk].dunk if n > 1. This allows for the possibility that the Vault becomes safe between bite calls either through increased collateral (in value or quantity), or decreased debt.

• Calling bite returns the auction id and emits and event with the id. This is required to bid in the Flipper on the auction.

5. Failure Modes (Bounds on Operating Conditions & External Risk Factors)

Coding Error

A bug in the Cat could lead to loss (or locking) of Dai and Collateral by assigning it to an address that cannot recover it (i.e. a bad Vow address or an incorrectly programmed Flipper). The main coding failure mode of Cat is if it has a bug that causes auctions to cease to function. This would require upgrading the system to a corrected Cat contract. If there is a bug in Cat that reverts on cage then it would cause Shutdown could fail (until a correct Cat is launched).

Feeds

The Cat relies indirectly on the price Feeds as it looks to the Vat's tracking of the collateral prices (spot) to determine Vault safety. If this system breaks down, it could lead to theft of collateral (too low spot) or unbacked Dai (incorrectly high spot).

Governance

Governance can authorize and configure new collaterals for Cat. This could lead to misconfiguration or inefficiencies in the system. Misconfiguration could cause Cat not to operate properly or at all. For instance, if an Ilk.dunk is set to be greater than 2**255 could allow for very, very large Vaults to be un-bite-able.

Inefficiencies in the dunk or chop could affect auctions. For instance, a dunk that is too large or too small could lead to disincentives for keepers to participate in auctions. A chop that is too small would not sufficiently dis-incentivize risky Vaults and too large could lead to it being converted to bad debt. Further discussion of how the parameters could lead to system attacks is described in this Auction Grinding paper.

Flipper

The Cat relies on an external Flipper contract to run the auction and moves the collateral from the Cat to the Flipper contracts in the Vat. A faulty collateral auction contract could result in the loss of collateral or dai or non-functioning auctions.