Standard Transaction Rules
Some transactions will not be accepted by miners unless they appear in a block. This is equivalent to the 'IsStandard' function in Bitcoin. This file dictates the rules for standard Sia transactions.
Consensus rules limit the size of a block, but not the size of a transaction. Standard rules however limit the size of a single transaction to 16kb.
A chain of dependent transactions cannot exceed 500kb.
Double Spend Rules
When two conflicting transactions are seen, the first transaction is the only one that is kept. If the blockchain reorganizes, the transaction that is kept is the transaction that was most recently in the blockchain. This is to discourage double spending, and enforce that the first transaction seen is the one that should be kept by the network. Other conflicts are thrown out.
Transactions are currently included into blocks using a first-come first-serve algorithm. Eventually, transactions will be rejected if the fee does not meet a certain minimum. For the near future, there are no plans to prioritize transactions with substantially higher fees. Other mining software may take alternative approaches.
File Contract Rules
File Contracts that start in less than 10 blocks time are not accepted into the transaction pool. This is because a file contract becomes invalid if it is not accepted into the blockchain by the start block, and this might result in a cascade of invalidated unconfirmed transactions, which may make it easier to launch double spend attacks on zero confirmation outputs. 10 blocks is plenty of time on the other hand for a file contract to make it into the blockchain.
Miners will reject transactions that have public keys using algorithms that the miner does not understand.
Arbitrary Data Usage
Arbitrary data can be used to make verifiable announcements, or to have other protocols sit on top of Sia. The arbitrary data can also be used for soft forks, and for protocol relevant information. Any arbitrary data is allowed by consensus, but only certain arbitrary data is considered standard.
Arbitrary data that is prefixed by the string 'NonSia' is always allowed. This indicates that the remaining data has no relevance to Sia protocol rules, and never will.
Arbitrary data that is prefixed by the string 'HostAnnouncement' is allowed, but only if the data within accurately decodes to the HostAnnouncement struct found in modules/hostdb.go, and contains no extra information.