Transfer Limits
Token transfer limits are the way to secure the bridge operations
The transfer limits are applicable for those bridge modes where ERC20 or native tokens are involved in operations: native-to-erc20, erc20-to-native and erc20-to-erc20.
Transfer limits were introduced for the following reasons:
avoid drying out the validators accounts by relaying requests with low value
secure the bridge for the case when the bridge contract on one side was compromised by rejecting transactions with very high value
avoid malicious actions from the validators by rejecting their confirmations for requests with very high value
Transfer limits for deposits
There are two cases how the limits are applied when tokens are deposited to another chain through the bridge.
Actively controlled deposit
This case is applicable for the native-to-erc20 mode and for the mode erc20-to-erc20 when tokens supporting ERC677 (transferAndCall) are used on both sides of the bridge.
The first set of checks whether the value of the transaction is within the limits happens on step 1 for thenative-to-erc20 mode or on step 2 for the mode erc20-to-erc20 after invocation of theonTokenTransfer method:
the transferred value must be greater or equal than minimum allowable value per transaction
the transferred value must be less or equal than maximum allowable value per transaction
the accumulated value of all transfers sent in this day in this direction must be less or equal than maximum allowable accumulated values per day
The second set of checks occurs on step 3 for native-to-erc20 and on step 4 for erc20-to-erc20:
the value of the transfer confirmed by the validator must be less or equal than minimum allowable value per transaction
the accumulated value of all confirmed transfers sent in this day in this direction must be less or equal than maximum allowable accumulated values per day
The checks on the Home side of the bridge are intended to limit bridge operations in case of malicious actions of validators.
Although so far the limits are not synchronized automatically on both side of the bridge, the bridge owners must make sure that maximum allowable deposit value per transaction must be the same on both sides of the bridge. Similar rule is for the maximum allowable accumulated deposits value per day.
Passive perception of deposit
The checks described here happen during tokens transfers in the erc20-to-native mode and in the mode erc20-to-erc20 when generic ERC20 tokens are used on the Foreign side of the bridge.
Since the bridge contract on the Foreign side is not involved in the transfer operation no checks occur on this side. That's why in these bridge modes it is not possible to check minimal allowable value per transaction.
The checks are only applied on step 3:
the value of the transfer confirmed by the validator must be less or equal than minimum allowable value per transaction
the accumulated value of all confirmed transfers sent in this day in this direction must be less or equal than maximum allowable values per day
Transfer limits for withdrawals
It is assumed that the tokens participating in withdrawals are controlled by the bridge (or bridge owners) that is why the transfer limits behave similarly in all bridge modes.
The first set of checks for detection whether the withdrawal is within the limits happens on step 1 for the erc20-to-native mode and on step 2 for the modes native-to-erc20 and erc20-to-erc20. It makes sure that:
the transferred value is greater or equal than minimum allowable value per transaction
the transferred value is less or equal than maximum allowable value per transaction
the accumulated value of all transfers sent in this day in this direction is less or equal than maximum allowable accumulated values per day
The checks are applied in the second time when one oracle sends all collected confirmations to the bridge contract on the Foreign side of the bridge (on step 5 in the case of the erc20-to-native mode and on step 6 in other modes):
the value of the transfer confirmed by the validator must be less or equal than minimum allowable value per transaction
the accumulated value of all confirmed transfers sent in this day in this direction must be less or equal than maximum allowable values per day
Again, the bridge owners are responsible for synchronization of the maximum allowable withdrawal per transaction value and the maximum allowable accumulated withdrawals per day value between two sides of the bridge.
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