Apply suggestions by @rustyrussell

02-peer-protocol.md

@@ -542,6 +542,11 @@ the other node then replies similarly, using a fee it thinks is fair.  This
 exchange continues until both agree on the same fee or when one side fails
 the channel.
 
+In the modern method, the funder sends its permissable fee range, and the
+non-funder has to pick a fee in this range. If it chooses the same value,
+negotiation is complete after two messages, otherwise the funder will reply
+with the same value (completing after three messages).
+
 1. type: 39 (`closing_signed`)
 2. data:
    * [`channel_id`:`channel_id`]
@@ -589,10 +594,20 @@ The receiving node:
   and the channel does not use `option_anchor_outputs`:
     - MUST fail the connection.
   - if the message contains a `fee_range`:
-    - if it disagrees with that `fee_range`:
+    - if there is no overlap between that and its own `fee_range`:
       - SHOULD fail the connection
     - otherwise:
-      - MUST propose a `fee_satoshis` in that range
+      - if it is the funder:
+        - if `fee_satoshis` is not in the overlap between the sent and received `fee_range`:
+          - SHOULD fail the connection
+        - otherwise:
+          - MUST reply with the same `fee_satoshis`.
+      - otherwise (it is not the funder):
+        - if it has already sent a `closing_signed`:
+          - if `fee_satoshis` is not the same as the value it sent:
+            - SHOULD fail the connection.
+        - otherwise:
+          - MUST propose a `fee_satoshis` in the overlap between received and (about-to-be) sent `fee_range`.
   - otherwise, if `fee_satoshis` is not strictly between its last-sent `fee_satoshis`
   and its previously-received `fee_satoshis`, UNLESS it has since reconnected:
     - SHOULD fail the connection.