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# System prepended metadata

title: Notes on batch auction-style FCFS

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# Notes on batch auction-style FCFS
Notes on [batch auction-style FCFS](https://research.arbitrum.io/t/transaction-ordering-policy/127/2)

This protocol builds ontop of Themis, "a scheme for introducing fair ordering of transactions into (permissioned) Byzantine consensus protocols with at most f faulty nodes among n≥4f+1. Themis is the first such scheme to achieve (optimistic) linear communication complexity. At the same time, it enforces the strongest notion of fair ordering proposed to date. Themis also achieves standard liveness, rather than the weaker notion of previous work." - [Source](https://eprint.iacr.org/2021/1465)



Themis definition of ***Fairness granularity***: For granlularity g, we consider that timestamps are bucketed into slots of interval g time each (e.g. [0,,g), [g,2g) and so on). Events within a time buket are assumed to happen at the same time.


#### FBA-FCFS implementation notes
- multiple (or one central) nodes with encrypted mempools only visible to validators and every txn is broadcasted to all validators
- Each node collects txns for a period g and reports a partial ordering to the chosen consensus leader at the time.
- After aggregating each individual nodes preferences, the leader gets a weak ordering (by FCFS) with unordered batches in it. The leader then tries to resolve the order of the batches using auctions (i.e. `conbase.transfer()`)
- FBA-FCFS adovactes for a fairness granularity of 500ms based on analysis of burst periods in gas for NFT sales


#### general notes
- Author argues FBA-FCFS is better than vanilla FCFS in two ways:
	- makes the fairness of the ordering more robust, in the sense of network latency tolerance
	- more fair by removing the negative externality caused by inefficient resource allocation and increases the welfare upperbound of users (helps to attract future users)
- FBA-FCFS targets eradiciting sandwiching and generalized front running by requiring validators to violate privacy and receive order honesty. Liquidations,Abitrage,NFT sniping, and statistical arb/market making do not invalidate honest validator behavior under this design
- 


##### robustness
- with vanila FCFS, validators cant perform sandwiching or generalized front even by breaking the privacy assumption, while in FBA-FCFS by violating privay they can perform those extraction methods. 
	- Mitigations for this trade-off:
		- 


#### game view


#### others concerns


#### my concerns