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- # Fast Sync Subprotocol Specification
-
- This directory contains English and TLA+ specifications for the FastSync
- protocol as it is currently implemented in the Tendermint Core codebase.
-
- ## English Specification
-
- The [English Specification](fastsync.md) provides a detailed description of the
- fast sync problem and the properties a correct protocol must satisfy. It also
- includes a detailed description of the protocol as currently implemented in Go,
- and an anlaysis of the implementation with respect to the properties.
-
- It was found that the current implementation does not satisfy certain
- properties, and is therefore not a correct solution to the fast sync problem.
- The issue discovered holds for all previous implementations of the protocol. A
- fix is described which is straight forward to implement.
-
- ## TLA+ Specification
-
- Two TLA+ specifications are provided: a high level [specification
- of the protocol](fastsync.tla) and a low level [specification of the scheduler
- component of the implementation](scheduler.tla). Both specifications contain
- properties that may be checked by the TLC model checker, though only for small
- values of the relevant parameters.
-
- We will continue to refine these specifications in our research work,
- to deduplicate
- the redundancies between them, improve their utility to researchers and
- engineers, and to improve their verifiability. For now, they provide a complete
- description of the fast sync protocol in TLA+; especially the
- [scheduler.tla](scheduler.tla), which maps very closely to the current
- implementation of the [scheduler in Go](https://github.com/tendermint/tendermint/blob/master/blockchain/v2/scheduler.go).
-
- The [scheduler.tla](scheduler.tla) can be model checked in TLC with the following
- parameters:
-
- - Constants:
- - numRequests <- 2
- - PeerIDs <- 0..2
- - ultimateHeight <- 3
- - Invariants:
- - TypeOK
- - Properties:
- - TerminationWhenNoAdvance
- - TerminationGoodPeers
- - TerminationAllCases
- - Proofs that properties are not vacuously true:
- - TerminationGoodPeersPre
- - TerminationAllCases
- - SchedulerIncreasePre
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