Add context for option_anchors_zero_fee_htlc_tx

Author: ariard

02-peer-protocol.md

@@ -841,7 +841,9 @@ dust exposure. The exact value used is a matter of node policy.
 
 For channels that don't use `option_anchors_zero_fee_htlc_tx`, an increase of
 the `feerate_per_kw` may trim multiple htlcs from commitment transactions,
-which could create a large increase in dust exposure.
+which could create a large increase in dust exposure. If `option_anchors_zero_fee_htlc_tx`
+has been negotiated, the HTLC feerate is implied to be 0, the checks should
+still be applied as a node can be exposed due to the sum of the trimmed HTLCs.
 
 ### Adding an HTLC: `update_add_htlc`
 
@@ -1165,13 +1167,14 @@ The node _not responsible_ for paying the Bitcoin fee:
   - MUST NOT send `update_fee`.
 
 A sending node:
-  - if the `update_fee` increases `feerate_per_kw`:
-    - if the dust balance of the remote transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
-      - MAY NOT send `update_fee`
-      - MAY fail the channel
-    - if the dust balance of the local transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
-      - MAY NOT send `update_fee`
-      - MAY fail the channel
+  - if `option_anchors_zero_fee_htlc_tx` was not negotiated:
+    - if the `update_fee` increases `feerate_per_kw`:
+      - if the dust balance of the remote transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
+        - MAY NOT send `update_fee`
+        - MAY fail the channel
+      - if the dust balance of the local transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
+        - MAY NOT send `update_fee`
+        - MAY fail the channel
 
 A receiving node:
   - if the `update_fee` is too low for timely processing, OR is unreasonably large:
@@ -1182,11 +1185,12 @@ A receiving node:
   current commitment transaction:
     - SHOULD fail the channel,
       - but MAY delay this check until the `update_fee` is committed.
-  - if the `update_fee` increases `feerate_per_kw`:
-    - if the dust balance of the remote transaction at the updated `feerate_per_kw` is greater then `max_dust_htlc_exposure_msat`:
-      - MAY fail the channel
-    - if the dust balance of the local transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
-      - MAY fail the channel
+    - if `option_anchors_zero_fee_htlc_tx` was not negotiated:
+      - if the `update_fee` increases `feerate_per_kw`:
+        - if the dust balance of the remote transaction at the updated `feerate_per_kw` is greater then `max_dust_htlc_exposure_msat`:
+          - MAY fail the channel
+      - if the dust balance of the local transaction at the updated `feerate_per_kw` is greater than `max_dust_htlc_exposure_msat`:
+          - MAY fail the channel
 
 #### Rationale