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-Design Doc 003: Fee Structure Metrics
-.. note::
- This design document is currently a draft, it
- does not reflect any implementation decisions yet.
-We discuss criterea for evaluating an exchange's denomination and fee structure.
-Currently, users of a Taler wallet can't easily judge the fee structure of an
-exchange and what it will mean for them. Thus we want to define some metrics
-that allow a user to make more informed decisions.
-Similarly, the fee structure metrics might be used by exchange operators
-as a senity check.
-An auditor might also enforce ranges on these metrics as a condition for
-auditing a denomination structure.
-* Privacy: A denomination structure that has too many denominations
- will result in reduced anonymity.
-* Efficiency: A denomination structure that has too few denominations
- is inefficient.
-* Predictability:
- * The amount of fees for an operation should not come
- as a total surprise for the user.
- * The user should know beforehand when they have to be online
- to refresh their balance, and they should know how much
- this will cost them.
-The metrics for the exchange should be advisory, i.e. an informed user should
-be able to accept the withdrawal anyway.
-The exchange's business interests may be in conflict with (1) a transparency of
-cost for the user and and (2) restrictions on denomination/fee structures.
-Thus the metrics should still allow some degree of variability between
-We make the assumption that wallet always prefer operations with better
-privacy. For example, a single coin should only be used for at most one
-spend operation and at most one refresh operation.
-Proposed Solution
-The following yes/no criteria are applied to determine if a denomination/fee structure
-is *reasonable*:
-* For a denomination of value ``v``, either
- (a) ``v`` must be the the smallest offered denomination, or
- (b) there must be another denomination with a value of at least ``v / 10``.
-Is there a relationship between the smallest denomination and the size of fees?
-Idea: When when only doing spends on a coin that are a multiple of the smallest spending amount,
-we constrain the number of coins that are refreshed into.
-When evaluating the e2e cost of a denomiation, look at:
- * the cost of withdrawing the coin by itself and spending it fully, directly
- * the cost of withdrawing the coin, directly refreshing it into the next smallest
- fully fitting currency (or use greedy fit!) and add up the withdraw, refresh and re-withdraw fees.
- * percentage is always computed with sum against the full coin's value
- * smallest coins are an exception
-Actually, smallest spendable amount should be a multiple of the smallest coins, since
-we still need to deal with fees even at that level
-Refund fees should not be higher than if the user got a refund through
-a different channel than Taler and then purchased the product again.
-The following metrics are also defined:
-* Shortest time from withdrawal expiration to deposit expiration.
-* Upkeep, i.e. how much does it cost the customer to keep coins in their wallet
- per time period, on average over a long period.
-* Assurance period, i.e. the time span for which the exchange has already announced
- its fees.
-* Spending amount range.
-* End-to-end fee range. This is the minimum/maximum cost to withdraw electronic cash
- (within the spending range), spend some of it and then obtain change for the transaction.
-* Merchant contribution range. This is the percentage of the end-to-end fees that the merchant
- can cover if they choose to do so.
-* Users can manually study the fee structure.
-* The auditor could impose a fee structure and not
- allow any variability between exchanges
-* The approach does not work well in some special-purpose deployments,
- where the coin structure is tailored to the products of merchants,
- and refreshing would never even happen.
-* The approach also does not consider more "creative" fee structures,
- such as ones where coins that are valid for a longer time have higher
- fees.
-Discussion / Q&A