diff --git a/README.md b/README.md
index 1a5b51a..83e440c 100644
--- a/README.md
+++ b/README.md
@@ -87,7 +87,7 @@ for example:
 
 #### Listing
 
-It is also possible to "list" the available protocols. A list message is simply:
+Implementations MAY support "listing" of the available protocols. A list message is simply:
 
 ```
 ls\n
@@ -139,6 +139,8 @@ For example
 < ...
 ```
 
+Note that `ls` support is OPTIONAL, i.e. implementations MAY support sending and/or receiving `ls`. This in turn implies that implementations MUST NOT depend on a remote node supporting `ls`.
+
 #### Implementation Recommendations
 
 **Protocol Selection**