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accounting: round to nearest whole satoshi for balances, not down #958

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accounting: round to nearest whole satoshi for balances, not down #958

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1908634
accounting: round to nearest whole satoshi for balances, not down
niftynei Feb 10, 2022
a296aba
fixup: account for 500/500 even split case
niftynei Feb 14, 2022
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15 changes: 13 additions & 2 deletions 03-transactions.md
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Original file line number Diff line number Diff line change
Expand Up @@ -99,7 +99,12 @@ To allow an opportunity for penalty transactions, in case of a revoked commitmen
The reason for the separate transaction stage for HTLC outputs is so that HTLCs can timeout or be fulfilled even though they are within the `to_self_delay` delay.
Otherwise, the required minimum timeout on HTLCs is lengthened by this delay, causing longer timeouts for HTLCs traversing the network.

The amounts for each output MUST be rounded down to whole satoshis. If this amount, minus the fees for the HTLC transaction, is less than the `dust_limit_satoshis` set by the owner of the commitment transaction, the output MUST NOT be produced (thus the funds add to fees).
The amounts for the `to_local` and `to_remote` outputs MUST be rounded
to the nearest satoshi. Any amount less than 500msat is rounded down;
any amount greater than 499msat is rounded up. This minimizes loss to fees
over the lifetime of a node.

The amounts for each HTLC output MUST be rounded *down* to whole satoshis. If this amount, minus the fees for the HTLC transaction, is less than the `dust_limit_satoshis` set by the owner of the commitment transaction, the output MUST NOT be produced (thus the funds add to fees).

#### `to_local` Output

Expand Down Expand Up @@ -363,7 +368,7 @@ Note that there are two possible variants for each node.
### Requirements

Each node offering a signature:
- MUST round each output down to whole satoshis.
- MUST round each output to the nearest whole satoshi.
- MUST subtract the fee given by `fee_satoshis` from the output to the funder.
- MUST remove any output below its own `dust_limit_satoshis`.
- MAY eliminate its own output.
Expand All @@ -386,6 +391,12 @@ has been used.
There will be at least one output, if the funding amount is greater
than twice `dust_limit_satoshis`.

Output amounts are rounded to the nearest whole satoshi. Any amount less
than 500msat is rounded down; any amount greater than 499msat is rounded up.

This ensures that over the lifetime of a node, the number of satoshis
a node earns/spends will be approximately accurate.

## Fees

### Fee Calculation
Expand Down