v2 opens: proposal to get rid of the minimum estimated fee

Prior versions of the v2 dual-funding protocol assumed a 'minimum fee'
payment for any witness stack of any input, as a way to simplify fee
checks.

The suggested min feerate didn't make sense for taproot spend paths etc;
instead we remove this check entirely.

02-peer-protocol.md

@@ -211,6 +211,7 @@ The sending node:
     - MUST send even `serial_id`s
   - if is the *non-initiator*:
     - MUST send odd `serial_id`s
+  - MUST provide a best estimate for the weight of the witness for this input
 
 The receiving node:
   - MUST add all received inputs to the transaction
@@ -411,7 +412,7 @@ prefixed with its length (since it is already specified in the `len` field).
 While the `minimum fee` is calculated and verified at `tx_complete` conclusion,
 it is possible for the fee for the exchanged witness data to be underpaid.
 It is the responsibility of the sending peer to correctly account for the
-required fee, e.g. a multisig witness stack whose weight exceeds 107.
+required fee.
 
 ### The `tx_init_rbf` Message
 

03-transactions.md

@@ -873,64 +873,6 @@ Multiplying non-witness data by 4 results in a weight of:
 
       overall_weight = funding_transaction_weight + witness_weight
 
-### Calculating Fees for Collaborative Transaction Construction
-
-Every participant in a collaborative transaction covers the fees for
-their own inputs and outputs. The initiator also provides funds to
-cover for the common transaction fields.
-
-Upon successful exchange of `tx_complete` messages, each peer must at
-least cover a minimum estimated fee. Here is how to calculate the minimum
-fee for each participant.
-
-The minimum witness weight for an input is 107.
-
-In the following, the initiator has provided one input (P2WPKH), one change output
-(P2WPKH), and the funding output. The contributor has provided two
-inputs (P2WPKH) and two change outputs (P2WPKH).
-
-Assuming a `feerate` of 253 per kiloweight, the initiator's minimum fee is
-calculated as follows.  Note that the feerate is rounded up to the nearest satoshi.
-
-    initiator_weight = transaction_fields * 4
-                     + segwit_fields
-                     + p2wpkh_input * 4
-                     + funding_output * 4
-                     + p2wpkh_output * 4
-                     + input_count * 107 (minimum witness weight)
-
-    initiator_weight = 10 * 4
-                     + 2
-                     + 41 * 4
-                     + 43 * 4
-                     + 31 * 4
-                     + 107
-
-    initiator_weight = 609
-
-    initiator_fees = initiator_weight * feerate
-    initiator_fees = 609 * 253 / 1000
-    initiator_fees = 155 sats
-
-The contributor's minimum fee is calculated as follows.
-
-    contributor_weight = 2 * p2wpkh_input * 4
-                       + 2 * p2wpkh_output * 4
-                       + input_count * 107 (minimum witness weight)
-
-    contributor_weight = 2 * 41 * 4
-                       + 2 * 31 * 4
-                       + 2 * 107
-
-    contributor_weight = 790
-
-    contributor_fees = contributor_weight * feerate
-    contributor_fees = 790 * 253 / 1000
-    contributor_fees = 200 sats
-
-This is an estimated fee. The peer MUST at least contribute the estimated fee,
-and MUST exceed the minimum fee in the case that their witness weight is greater
-than the estimated weight of 107 per input.
 
 ## Expected Weight of the Commitment Transaction