RFC: rent deposit spam protection (#439)

* RFC: rent deposit spam protection

* Update 0002-rent-deposit.md

* Update 0002-rent-deposit.md

* Update 0002-rent-deposit.md

* Update 0002-rent-deposit.md

* Update 0002-rent-deposit.md

rfcs/0002-rent-deposit.md

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+# Overview
+
+Spam protection of CPU resources is well documented and automated process in Substrate.
+
+Protecing storage is less documented process which seems essential to keep ntwork robust.
+
+This document descrbides various aspect of storage to be protected, consistent means to do so, user and developer experience of protected storage. 
+
+It overview some existing storage spam protection mechanics in different p2p computation models too.
+
+## Existing spam protection models
+
+### Solana
+
+Solana network forces to pay block based rent or rent except amount for each data account. 
+Some conracts allow only wokring with rent excempt accounts to protect users contracts from so fail.
+Rent amount is voted by validators. Rent depends on size of data.
+
+### P2P Storage with only small part of nodes stores replicas
+
+Overall, if node is interested in data it stores it.
+Interest to store data can be payed with some desposit.
+Proves of storage are obtained from nodes which where payed to store data. 
+Payed node must pin (copy and share on demand) specific data.
+Payment to store depends on size of data at least.
+
+Examples are Arweave, Filecoin, Sia.
+
+### Substrate 
+
+In substrate user accounts to pay rent and has some safety from being deleted from chain.
+
+### Solana protocols
+
+Some specific contracts take it rent on top of Solana too. 
+These server as gates to public lists. 
+Examples would be Order Book DEX Serum, which takes entry to create pool of 3 SOL (600) used.
+It prevent contract to be slow and polluting public list from garbage pairs
+
+## Storage spam consequences
+
+There are 2 consequnces.
+
+Many data items on chain will raise requirements for each node regarding size, so will not be able to choose speed only variants for storage.
+So overall limiting nodes can run parachain.
+Can if it will be econimical to attack pulling whole storage, will have hard time to clean storage from attack and from valid data later.
+So protecting storage is more important than CPU, as CPU attacks are only transient and easy fixed buy very hight multipliers.
+
+Another one is pefromance and polluting shared search lists. 
+Example, are long list of token pairs to choose DEX pool.
+
+## Forms paying - Rent/Deposit/Stake
+
+Slashable stake. Until slashed and unlike rent, returned fully to staker during deletion.
+
+Rent deposit is not return fully as it is used to pay rent.
+
+Liqudity provider, in some form adds liqudity to market and locks it for some period.
+
+## When can avoid paying for storage?
+
+When count of storage items is finite. 
+Example, set of well known currency pairs to form pools.
+Rule does not work if set is extended with custom pool configurations or with custom currencies added via registry.
+
+When storage item is added by root or councel.
+So storage is just premission based.
+
+## Proposal
+
+Each permissionless storage item of very large potential combinatorial number of items must be accompanited with deposit of native token.
+
+### Considerations
+
+We can consider next set of guidlines applicable when producing rent/deposit based models of storage consumption spam protection:
+
+- Use `rent/deposit` symbols in code base to allow easy to find relevant places
+- Motivate user to delete dead storage items by paing back part of rent. Can make burn of storage item block depenendat, so of chain observers can get prize for burn calls.
+- Increase rent depending on how influential storage item is. Higher size or longer some user interface search list because, the higher price of rent is.
+- Rent should be captured on storage item creation, so can gracefully handel reconfiguration of rent after runtme upgrade.
+- All rent must be documented as part of pallets docuemntation. Alternative would be to have RPC API prefixed with `function-name/rent/` to return rent size in native currency for all extrinsics taking rent.
+- If rent can be really big, add additional paramter into extrinsic indicating maximal rent user eager to pay for getting storage place holder (similar to slippage). This also would be good indicator to discover rent based dispatchables.
+- Each pallet for each storage item has its own rent requirmenets, so if one pallet calls other pallet, 2 can take rent. So can so simplify can allow rent free call from one pallet to other.
+- Can implement taking, so returning back can be imlemented after some time.