feature: define option_zero_fee_second_stage

Author: halseth

03-transactions.md

@@ -417,10 +417,14 @@ Note the reference to the "base fee" for a commitment transaction in the require
 #### Requirements
 
 The fee for an HTLC-timeout transaction:
-  - MUST BE calculated to match:
+  - If `option_zero_fee_second_stage` applies:
+    1. MUST BE 0.
+  - Otherwise, MUST BE calculated to match:
     1. Multiply `feerate_per_kw` by 663 (666 if `option_anchor_outputs` applies) and divide by 1000 (rounding down).
 
 The fee for an HTLC-success transaction:
+  - If `option_zero_fee_second_stage` applies:
+    1. MUST BE 0.
   - MUST BE calculated to match:
     1. Multiply `feerate_per_kw` by 703 (706 if `option_anchor_outputs` applies) and divide by 1000 (rounding down).
 

09-features.md

@@ -39,6 +39,7 @@ The Context column decodes as follows:
 | 16/17 | `basic_mpp`                      | Node can receive basic multi-part payments                | IN9      | `payment_secret`  | [BOLT #4][bolt04-mpp]                 |
 | 18/19 | `option_support_large_channel`   | Can create large channels                                 | IN       |                   | [BOLT #2](02-peer-protocol.md#the-open_channel-message) |
 | 20/21 | `option_anchor_outputs`          | Anchor outputs                                            | IN       | `option_static_remotekey` | [BOLT #3](03-transactions.md)         |
+| 22/23 | `option_zero_fee_second_stage`   | Anchor commitment type with zero fee second-stage HTLC txs| IN       | `option_anchor_outputs` | [BOLT #3](03-transactions.md)         |
 
 ## Requirements