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BOLT 2: correct sat/Kb to sat/Kw conversion to avoid over paying (#176)

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This commit fixes an advisory error in the current spec draft. We
currently use `fee-per-kw` where `kw = 1000` weight to determine the
proper fee to pay for commitment transactions. Currently, the spec
advises implementer to take the typical sat/Kb at _multiply_ by 4. This
will result in implementations overpaying for commitment transactions
as the scaling should actually be in the _opposite_ direction. As the
weight is scaled up by 4, for fee-per-kw should be scaled down by 4.
So: sat/Kb * 1/4, instead of sat/Kb * 4.

[Minor fixup: "1/4th" to "1/4", better english, and doesn't trip spellcheck. -- RR]
This commit is contained in:
Olaoluwa Osuntokun 2017-05-17 17:18:29 -07:00 • committed by Rusty Russell
parent 1b4195c842
commit 3a359c8e1e
1 changed files with 1 additions and 1 deletions
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02-peer-protocol.md
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@ -111,7 +111,7 @@ The `temporary_channel_id` is used to identify this channel until the funding tr
`max_htlc_value_in_flight_msat` is a cap on total value of outstanding HTLCs, which allows a node to limit its exposure to HTLCs; similarly `max_accepted_htlcs` limits the number of outstanding HTLCs the other node can offer. `channel_reserve_satoshis` is the minimum amount that the other node is to keep as a direct payment. `htlc_minimum_msat` indicates the smallest value HTLC this node will accept.
`feerate_per_kw` indicates the initial fee rate by 1000-weight (ie. 4 times by the more normally-used 'feerate per kilobyte') which this side will pay for commitment and HTLC transactions as described in [BOLT #3](03-transactions.md#fee-calculation) (this can be adjusted later with an `update_fee` message). `to_self_delay` is the number of blocks that the other nodes to-self outputs must be delayed, using `OP_CHECKSEQUENCEVERIFY` delays; this is how long it will have to wait in case of breakdown before redeeming its own funds.
`feerate_per_kw` indicates the initial fee rate by 1000-weight (ie. 1/4 the more normally-used 'feerate per kilobyte') which this side will pay for commitment and HTLC transactions as described in [BOLT #3](03-transactions.md#fee-calculation) (this can be adjusted later with an `update_fee` message). `to_self_delay` is the number of blocks that the other nodes to-self outputs must be delayed, using `OP_CHECKSEQUENCEVERIFY` delays; this is how long it will have to wait in case of breakdown before redeeming its own funds.
The `funding_pubkey` is the public key in the 2-of-2 multisig script of the funding transaction output. The `revocation_basepoint` is combined with the revocation preimage for this commitment transaction to generate a unique revocation key for this commitment transaction. The `payment_basepoint` and `delayed_payment_basepoint` are similarly used to generate a series of keys for any payments to this node: `delayed_payment_basepoint` is used to for payments encumbered by a delay. Varying these keys ensures that the transaction ID of each commitment transaction is unpredictable by an external observer, even if one commitment transaction is seen: this property is very useful for preserving privacy when outsourcing penalty transactions to third parties.