# Asynchronous User-level ETH transfers over netted balance approach for EE-level ETH transfers

Here is a natural way for staging user-level ETH transfers on top of netted balance approach for EE-level transfers. Adopts ideas from non-sequential receipts (meta-ee) approach.

As is typical, the user-level transfers are split into a debit transaction on the sender shard, and a credit transaction on the recipient shard in a subsequent slot. However, utilising the netted balance approach, the EE-level transfers happen atomically and synchronously. Can we call this approach quasi-synchronous!

## Preliminaries

• s_1, s_2 shards.
• E_1, E_2 execution environments.
• a_i users on (s_1,E_1), b_j be users in (s_2, E_2) for some index sets that i and j range on.

## Transaction Types

• Cross-shard debit transfer
• a_i \stackrel{x_i}{\Longrightarrow} b_i, cross-shard transfer of x_i ETH from the user a_i on (s_1,E_1) to the user b_i on (s_2,E_2).
• Submitted on sender shard.
• Emits a Credit System event on success.
• Cross-shard credit transfer
• a_i \stackrel{x_i}{\longrightarrow} b_i, credit transfer of x_i ETH to b_i on (s_2,E_2), which is from a_i on (s_1, E_1).
• Submitted on recipient shard.
• Includes the Credit System Event and the Merkle Proof for it.
• Bitfields are used for replay protection.
• Emits a Revert System Event on failure.
• Cross-shard revert transfer
• a_i \stackrel{x_i}{\longleftarrow} b_i, a revert transfer of x_i ETH to a_i when the credit transfer of the form a_i \stackrel{x_i}{\longrightarrow} b_i fails.
• Includes the Revert System Event and the Merkle Proof for it.
• Bitfields are used for replay protection.

## System Event messages

System Events are similar to application event messages in contract code, but are unforgeable by the application. We have two System Events: Credit and Revert. All of them include sender details (shard-id, EE-id, user address), recipient details, transfer amount. They also include the block number of the shard block where this event is emitted, and an index number starting from 0 (for every block).

## Algorithm for the Block Proposer

Suppose a block proposer selects some transactions t_1, ..., t_n, where every t_i, 1 \le i \le n, is either a cross-shard debit transfer from a user in E_1 to a user in E_2, or a credit transfer from a user in E_2 to a user in E_1.

• net = 0; // tracks the net amount for EE-level transfers
• for i : 1 … n
• if t_i: b_i \stackrel{x_i}{\longrightarrow} a_i AND BitFieldCheck(t_i) passes AND Merkle Proof check of Credit System Event passes AND realBal(s_1,E_1) > net + x_i
• include t_i to the block
• if t_i executes successfully
• bal(a_i) += x_i; (is implied in a successful execution)
• SetBit(t_i);
• if failure
• emit Revert(b_i, x_i, a_i) System Event
• net += x_i
• if t_i : a_i \stackrel{x_i}{\Longrightarrow} b_i AND realBal(s_1,E_1) > net + x_i
- include t_i to the block
- if t_i executes successfully
- bal(a_i) -= x_i (implied)
- net += x_i
- emit Credit(a_i, x_i, b_i) System Event
• end for
• s_1.bal[s_1,E_1] -= net
• s_1.bal[s_2,E_2] += net

Note that when processing a pending cross-shard credit transfer transaction b_i \stackrel{x_i}{\longrightarrow} a_i, the EE-level transfer is already complete.

## Discussion

### Replay protection

Bitfields are used for replay protection. Recipient EE keeps a map from (\mathit{senderShard, senderEE, senderBlockNumber}) \mapsto b_1...b_n where b_i corresponds to $i$th credit transfer transaction created in the shard block numbered \mathit{senderBlockNumber} of \mathit{senderShard} .

• BitFieldCheck(t_i) checks to see if the given credit transfer t_i has already been processed. This is done by checking the bit corresponding to the index specified in the SystemEvent in the above map. If zero, returns true. If no entry is present, then a new entry is created, and the bits are initialised to zeros, and returns true. If the corresponding bit is set to 1, returns false.
• SetBit(t_i) sets the bit for this credit transfer to 1.

Similar mechanism is used for revert transfers protecting replays.

We need a sliding-window time-bound for processing credit transfer transactions. If this time-bound is two epochs, then all the bitfields related to block numbers prior to two epochs are discarded, and revert transactions are placed instead, after affecting the EE-level revert.

Problem 1 Note that the bit is first initialised to zero, when BP first tries to include the credit transfer in a block. If the client never creates and submits a credit-transfer, then the recipient (user) loses the amount, however, there is no ETH loss at the EE-level. The time-out applies to credit transfers that are submitted to the transaction pool. So, a submission of credit transfers need to be incentivised.

### Revert Transfers

Note that the revert transactions don’t appear in the above algorithm. They are processed as regular transactions, except that we check the Bitfields similar to Credit bitfields (explained above) to ensure double spending.

We do not want revert transfers to fail. So, there are two design choices:
1. Zero-gas By making the revert transfers not consume any gas, we can expect it to succeed always.
2. ETH loss at user-level If the revert transfers run out of gas and fail, then we can emit a RevertFail System Event. The sender (user) loses ETH, however, the sender EE does not lose ETH. One solution is to invoke a EE host function periodically (say, at every checkpoint), and process the accumulated RevertFail System Events and push the ETH back to the sender (user).

There is still one remote case where the sender account itself disappears before a revert is processed. Then the revert transfer fails. We end up in a state where the EE balance does not match up with its user-level balances.

Problem 2: @liochon pointed out that if revert transfers are cheap or zero-gas, then a malicious BP can deliberately (maliciously) fail credit transfers and load up the system with many cheap revert transfers. Because a successful credit transfer does not depend on anything but the presence of the recipient account, a malicious BP cannot fail the credit transfer on demand. However, he has the choice not to include it in the block. This defers the transaction inclusion in the block, and increases the probability of a credit time-out.

### Gas billing EE-level transfers

It is easy to bill the EE-level transfers to the transactions that contribute to it, by a small modification in the above algorithm, which book-keeps the participating transactions.

### Failing EE-level balance checks

Because we have deferred transfers at user-level, we need to check real balances at EE-level (as shown in the above algorithm). The flexibility of this approach is that depending on the transfer amounts and the EE balance some transactions get included and some not.

## Benefits

• No locking / blocking.
• No constraint on the block proposer to pick specific transactions or to order them.
• EE-level transfers can be billed uniformly to the contributing transactions. Client’s responsibility to put enough gas limit on debit and credit transfers to accomodate EE-level transfers as well.

• Problem 1
• Problem 2

## Examples

### Optimistic case

Block on shard s_1

a_1 \stackrel{10}{\Longrightarrow} b_1\\ a_2 \stackrel{20}{\Longrightarrow} b_2\\ a_3 \stackrel{30}{\Longrightarrow} b_3\\ E_1 \stackrel{60}{\Longrightarrow} E_2

Subsequent block on shard s_2

b_1 \stackrel{5}{\Longrightarrow} a_2\\ a_1 \stackrel{10}{\longrightarrow} b_1\\ a_2 \stackrel{20}{\longrightarrow} b_2\\ a_3 \stackrel{30}{\longrightarrow} b_3\\ E_2 \stackrel{5}{\Longrightarrow} E_1

A credit transfer of the form b_1 \stackrel{5}{\longrightarrow} a_2 gets included in a subsequent block of shard s_1.

### When debit fails

Block on shard s_1

a_1 \stackrel{10}{\Longrightarrow} b_1\\ a_2 \stackrel{20}{\Longrightarrow} b_2\\ \mathbf{FAIL}~~~a_3 \stackrel{30}{\Longrightarrow} b_3 \\ E_1 \stackrel{30}{\Longrightarrow} E_2

Subsequent block on shard s_2

b_1 \stackrel{5}{\Longrightarrow} a_2\\ a_1 \stackrel{10}{\longrightarrow} b_1\\ a_2 \stackrel{20}{\longrightarrow} b_2\\ E_2 \stackrel{5}{\Longrightarrow} E_1

### When credit fails

Block on shard s_1

a_1 \stackrel{10}{\Longrightarrow} b_1\\ a_2 \stackrel{20}{\Longrightarrow} b_2\\ a_3 \stackrel{30}{\Longrightarrow} b_3\\ E_1 \stackrel{60}{\Longrightarrow} E_2

Subsequent block on shard s_2

b_1 \stackrel{5}{\Longrightarrow} a_2\\ a_1 \stackrel{10}{\longrightarrow} b_1\\ a_2 \stackrel{20}{\longrightarrow} b_2\\ \mathbf{FAIL}~~~a_3 \stackrel{30}{\longrightarrow} b_3\\ E_2 \stackrel{35}{\Longrightarrow} E_1

Subsequent blocks on shard s_1 include a revert transfer: a_3 \stackrel{30}{\longleftarrow} b_3, and a credit transfer: b_1 \stackrel{5}{\longrightarrow} a_2. Note that we should not allow revert to fail.

Thanks @drinkcoffee, @hmijail and @SamWilsn for your inputs.

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What is to prevent/discourage a Byzantine node from issuing a debit transfer if no Merkle proof is necessary for debit transactions?

Thanks for the question.

I was modelling or rather extending the transactions from its Ethereum1 kind. So, the cross-shard debit transfers (\Longrightarrow) are signed by the sender. The v, r, and s fields would hold the sender signature. Similar to Ethereum1, a BP can derive the sender address from v, r, and s and check if it matches the sender account details specified for the cross-shard transfer. This means, only the sender user has the right to submit a cross-shard debit transfer.

Because credit transfers and revert transfers contain Merkle Proofs of respective events, we can allow anyone to submit these transfers. The fields v, r and s are of no use here.

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