Use empty TLV flag

Similar to what is done for `require_confirmed_inputs`.
We add a requirement for the receiver of `accept_channel` and
`accept_channel2`.

Author: t-bast

blip-0030.md

@@ -32,16 +32,12 @@ Additional TLV fields for the `open_channel2` message:
 1. `tlv_stream`: `open_channel2_tlvs`
 2. types:
     1. type: 32768 (`zero_reserve`)
-    2. data:
-        * [`byte`:`use_zero_reserve`]
 
 Additional TLV fields for the `accept_channel2` message:
 
 1. `tlv_stream`: `accept_channel2_tlvs`
 2. types:
     1. type: 32768 (`zero_reserve`)
-    2. data:
-        * [`byte`:`use_zero_reserve`]
 
 ### Requirements
 
@@ -51,7 +47,7 @@ A node that wants to support zero-reserve channels:
 
 When sending `open_channel`:
 
-* If `zero_reserve` was negotiated:
+* If the `zero_reserve` feature was negotiated:
   * MAY set `channel_reserve_satoshis` to `0`
 
 When receiving `open_channel`:
@@ -62,19 +58,31 @@ When receiving `open_channel`:
   * Otherwise:
     * MUST send an `error` and forget the channel
 
+When receiving `accept_channel`:
+
+* If `channel_reserve_satoshis` was set to `0` in `open_channel`, and it is
+  not set to `0` in `accept_channel`:
+  * MUST send an `error` and forget the channel
+
 When sending `open_channel2`:
 
-* If `zero_reserve` was negotiated:
-  * MAY set the `zero_reserve` TLV field to `1`
+* If the `zero_reserve` feature was negotiated:
+  * MAY set the `zero_reserve` TLV field
 
 When receiving `open_channel2`:
 
-* If `zero_reserve` is set to `1`:
+* If the `zero_reserve` TLV field is set:
   * If it wants to use `zero_reserve`:
-    * MUST set the `zero_reserve` TLV field to `1` in `accept_channel2`
+    * MUST set the `zero_reserve` TLV field in `accept_channel2`
   * Otherwise:
     * MUST send an `error` and forget the channel
 
+When receiving `accept_channel2`:
+
+* If `zero_reserve` was set in `open_channel2`, and it is not set in
+  `accept_channel2`:
+  * MUST send an `error` and forget the channel
+
 When sending or receiving `update_add_htlc`:
 
 * If `zero_reserve` has been negotiated: