Guardnode functionality

Proposed design for Guardnode functionality in the Ocean client.

Requests

Requests in the guardnode system must be permissioned (i.e. only authorised partners - client chains - can issue requests). Permission tokens are required to issue requests. This works as follows: Permission tokens (permissionAsset) are (optionally) issued to a given address in the genesis block, e.g.:

‘permissionassetcoins=5000000’ ‘permissioncoinsdestination=g4e638a7cc43….’’ (the scriptPubKey of the permissionasset output).

This scriptPubKey (multisig or P2PKH) is owned by the ‘controller’. The controller sends a quantity of the permissionAsset to a client chain so they can submit requests.

Requests are generated by (or on behalf of) a client (leaf) chain. The request transactions are identified by having a time (blockheight) locked (OP_CLTV) permissionAsset output paying to a 1-of-3 multisig output encoded with metadata. The 3 multisig pubkeys are as follows:

key1 -> target pubkey permission asset is locked at

key2 -> <02 pubkey prefix> <32 bytes client chain genesis block hash>

key3 -> <03 pubkey prefix>
        <4 bytes service period start time (block height)>
        <4 bytes target number of tickets>
        <4 bytes auction price decay constant>
        <4 bytes fee percentage>
        <15 bytes empty>

The request transaction is generated with a new RPC: createrawrequesttx. The RPC will take the arguments: 1. txid of input 2. vout of input. 3. output address 4. permission asset ID type, 5. client chain genesis block hash 6. start time (startBlockHeight) 7. end time (endBlockHeight) 8. target no. of tickets 9. auction decay constant 10. fee percentage.

The RPC will then create a new transaction. This tx will have one input (txid and vout) and two outputs. The first output will pay to the output address and have a time-lock set to the end time (block height). The first output asset ID tag will be set to RPC argument 4 (permission asset ID type). The first output script can be constructed like: .. code-block:: json

script = CScript() ToByteVector(endBlockHeight) << OP_CHECKLOCKTIMEVERIFY << OP_DROP << OP_DUP << OP_1 << ToByteVector(key1) << ToByteVector(key2) << ToByteVector(key3) << OP_3 << OP_CHECKMULTISIG

The raw transaction generated by this RPC will then be signed with the private key of the (permissionAsset) input and braodcast.

Active requests RPC

To view all active requests (i.e. requests that have been confirmed but not expired: blockheight > endBlockHeight) the getrequests RPC is used. This RPC scans the current UTXO set for request transactions (using the permission asset type) and returns details as a JSON array.

This array contains an object for each active request. Each object contains the request metadata and the blockheight at which it was confirmed.

e.g.

[
    {
        "startBlockHeight": 40060,
        "endBlockHeight": 90000,
        "confirmedBlockHeight": 30000,
        "genesisBlock": "fa1c7d059dab70cddb8cb3cc7b8971d385eecea4d68bd86c5cb6d75949789ba1",
        "numTickets": 100,
        "startPrice": 100000,
        "auctionPrice": 80000,
        "decayConst": 1000000,
        "feePercentage": 100,
        "txid": "cc1c7d059dab70cddb8cb3cc7b8971d385eecea4d68bd86c5cb6d75949789ba1"
    }
]

This RPC will be called by the Guardnode operators/interface to get current requests. If blockheight < startBlockHeight then the auction is potentially stil active. (this RPC can be modelled on existing functions like gettxoutsetinfo)

Decay function

The decay function will return the current ticket bid price (in CBT sats) for given parameters, as follows:

CAmount CRequest::GetAuctionPrice(uint32_t height)
{
     uint32_t t = height - nConfirmedBlockHeight;
     if(t < 0) return 0; // auction not started yet
     return nStartPrice*(1 + t)/(1 + t + pow(t,3)/nDecayConst);
}

Given the parameters in the object above, the ticket price is shown in the figure as function of t over 4000 blocks (~ 3 days at 1min per block).

Fig. 1.: Ticket price decay function with startPrice = 100000 CBT and decayConst = 1000000.

Request/bid table

An in-memory table (rtable) will list all current requests (if the node is configured with a -requestlist=1 flag). The table will be updated at each new block: new requests will be added as a block is recieved (in the ConnectBlock function) and removed when blockheight > endBlockHeight) e.g. with a function UpdateRequestList. In the event of a node re-start, the rtable will be regenerated by scanning the UTXO set with e.g. a function LoadRequestList. (This can be based on the UpdateFreezeList and LoadFreezeList functions). Each entry in the table will have all the request transaction parameters and the request transaction txid.

In addition, each request in the table will have a vector of valid bid transactions that have been recieved against the request. As valid bids are recieved, the transaction IDs are added to this vector (along with the bif block height) up to a max of numTickets. A valid bid is decribed below, and are added to the vector by the UpdateRequestList function.

So the table will look like this:

[
    {
        "requestTxID": "0a22fe0103a2f583f37d3feb94df941a6c90d8d0c3113548e0776f3413f33346",
        "confirmedBlockHeight": 30000,
        "startBlockHeight": 40060,
        "endBlockHeight": 90000,
        "genesisBlock": "fa1c7d059dab70cddb8cb3cc7b8971d385eecea4d68bd86c5cb6d75949789ba1",
        "numTickets": 100,
        "startPrice": 100000,
        "auctionPrice": 80000,
        "decayConst": 1000000,
        "feePercentage": 100,

        "bids": [
            { "txid": "65eacf082247aaf0b1624539a0d7e3bb667b73211269907b0504a3b8f8ab0a22",
             "feePubKey": "0300adf7a8f55f92f8be6a5ed7619d1821c5bc9901f5592badea04677043b83656" },
            { "txid": "af3d49ff538a9a2bcd78b924aa27f102fb391811c387e7b5b06fc034d56cd4d8",
             "feePubKey": "0311adf7a8f55f92f8be6a5ed7619d1821c5bc9901f5592badea04677043b83656" },
            { "txid": "64c787adf54983f90be8d6a72ba9c3e2523117804b2087f8b6324ccb4b29ac0d",
             "feePubKey": "0322adf7a8f55f92f8be6a5ed7619d1821c5bc9901f5592badea04677043b83656"},
            { "txid": "9a5afcbd6892a2b7c8b6926f764f947df2ef22bc25be4fdb743079b7a03df56f",
             "feePubKey": "0333adf7a8f55f92f8be6a5ed7619d1821c5bc9901f5592badea04677043b83656"}
        ]
    }
]

A new RPC getrequestbids will output this vector of bids (with txids and block heights) for a given request transaction ID (by querying the in memory table).

Bid transactions

Bid transactions will be created with a new RPC createrawbidtx. This will take as arguments: 1. input txid 2. input vout 3. lock height (i.e. the endBlockHeight of the request) 4. The txid of the request. 5. The bid amount (CBT). 6. Stake address (the address to which the stake will be paid back at the end of the service period) 7. Fee address (base-58 address for fee payment on the client chain). This RPC will then output a hex encoded raw unsigned bid transaction with three outputs:

  1. The first output will be a CLTV locked 1-of-3 multisig (of CBT asset type)
  2. The second output will be a P2PKH output paying any change from the input
  3. Transaction fee.

The first output should be locked for the same duration as the ending blockheight of the request.

The 3 multisig pubkeys are as follows:

key1 -> target pubkey CBT asset is locked at key2 -> <02 pubkey prefix> <32 bytes request transaction hash> key3 -> pubkey to receive fees on client chain

Any excess amount will have to be returned to an address owned by the user, using “change” and “change” fields in the output object. These are optional and should only be included when the input amount exceeds the bid amount.

Bid transaction validity

When a bid transaction is received into a block, the UpdateRequestBidList function will determine its validity, and if it is valid, the TxID and other bid information will be added to the relevant request bid set in the request list. The validity will be determined as follows:

  1. Check if transaction is encoded as a bid transaction.
  2. Read request TxID from the second pubkey in the CLTV locked multisig
  3. Get the decayConst, startPrice, blockheight (when the request transaction was confirmed), startBlockHeight, endBlockHeight and numTickets from the request list.
  4. Check that endBlockHeight in the bid transaction time-lock CLTV is greater than or equal to the request endBlockHeight.
  5. Calculate the current bid price based on the request parameters and the current blockheight with ticket_auction_price.
  6. Check that the value of CBT in output 1 is greater than or equal to the current bid price.
  7. Check that the auction has not ended and that the request number of tickets has not been reached.

If valid the, bid transaction TxID and bid information is added to the request bid set in the request list.

Bid output policy

The request bid set is used for two purposes: 1. to enable the coordinator to pay client chain fees to the winning bidders, and 2. to lock the winning bid outputs for the duration of the service period. The locking is performed via the CLTV locked multisig output and the bid is added to the bid set only if it matches all the above prerequisites.

This bid set will also allow winning bids to collect the change. At the end of the auction the final request bid will be calculated and guardnodes will be able to get the overbid - see the guardnode tecdoc.