Submitted by WatchPug.
Per the document: https://github.com/code-423n4/2022-01-livepeer#l2---l1-lpt-withdrawal
The method (a) described above wont work in the current implementation due to the missing interface on L2LPTGateway.
When initiate a withdraw from the Arbitrum Gateway Router, L2GatewayRouter will call outboundTransfer(address,address,uint256,uint256,uint256,bytes) on ITokenGateway(gateway):
https://github.com/OffchainLabs/arbitrum/blob/b8366005a697000dda1f57a78a7bdb2313db8fe2/packages/arb-bridge-peripherals/contracts/tokenbridge/arbitrum/gateway/L2GatewayRouter.sol#L57-L64
https://github.com/OffchainLabs/arbitrum/blob/b8366005a697000dda1f57a78a7bdb2313db8fe2/packages/arb-bridge-peripherals/contracts/tokenbridge/libraries/gateway/GatewayRouter.sol#L78-L102
However, L2LPTGateway dose not implement outboundTransfer(address,address,uint256,uint256,uint256,bytes) but only outboundTransfer(address,address,uint256,bytes):
https://github.com/livepeer/arbitrum-lpt-bridge/blob/ebf68d11879c2798c5ec0735411b08d0bea4f287/contracts/L2/gateway/L2LPTGateway.sol#L65-L89
Therefore, the desired feature to withdraw LPT from L2 to L1 via Arbitrum Router will not be working properly.
Consider implementing the method used by  Arbitrum Router.
See also the implementation of L2DaiGateway by arbitrum-dai-bridge: https://github.com/makerdao/arbitrum-dai-bridge/blob/master/contracts/l2/L2DaiGateway.sol#L88-L95
yondonfu (Livepeer) confirmed and resolved:
