diff --git a/ipywidgets/_version.py b/ipywidgets/_version.py
index 761d079d7f2..3f06ee21f23 100644
--- a/ipywidgets/_version.py
+++ b/ipywidgets/_version.py
@@ -8,7 +8,7 @@
 __version__ = '%s.%s.%s%s'%(version_info[0], version_info[1], version_info[2],
   '' if version_info[3]=='final' else _specifier_[version_info[3]]+str(version_info[4]))
 
-__protocol_version__ = '2.0.0'
+__protocol_version__ = '2.1.0'
 
 # These are *protocol* versions for each package, *not* npm versions. To check, look at each package's src/version.ts file for the protocol version the package implements.
 __jupyter_widgets_base_version__ = '1.2.0'
diff --git a/ipywidgets/widgets/widget.py b/ipywidgets/widgets/widget.py
index 94e466dd12a..ee3198cdaaa 100644
--- a/ipywidgets/widgets/widget.py
+++ b/ipywidgets/widgets/widget.py
@@ -319,6 +319,10 @@ def _call_widget_constructed(widget):
 
     @classmethod
     def handle_comm_opened_control(cls, comm, msg):
+        version = msg.get('metadata', {}).get('version', '')
+        if version.split('.')[0] != PROTOCOL_VERSION_MAJOR:
+            raise ValueError("Incompatible widget protocol versions: received version %r, expected version %r"%(version, __protocol_version__))
+
         cls.get_manager_state()
         widgets = Widget.widgets.values()
         # build a single dict with the full widget state
@@ -332,10 +336,7 @@ def handle_comm_opened_control(cls, comm, msg):
                 'state': widget.get_state(drop_defaults=drop_defaults),
             }
         full_state, buffer_paths, buffers = _remove_buffers(full_state)
-        # the message is also send as buffer, so it does not get handled by jupyter_server
-        msg = jsondumps([full_state, buffer_paths]).encode('utf8')
-        buffers.insert(0, msg)
-        comm.send(buffers=buffers)
+        comm.send([full_state, buffer_paths], buffers=buffers)
 
     @staticmethod
     def handle_comm_opened(comm, msg):
diff --git a/packages/base/src/version.ts b/packages/base/src/version.ts
index 475502b53b1..8fbd0e7e5b9 100644
--- a/packages/base/src/version.ts
+++ b/packages/base/src/version.ts
@@ -5,5 +5,4 @@ export
 const JUPYTER_WIDGETS_VERSION = '1.2.0';
 
 export
-const PROTOCOL_VERSION = '2.0.0';
-
+const PROTOCOL_VERSION = '2.1.0';
diff --git a/packages/schema/messages.md b/packages/schema/messages.md
index c2576e38bc3..96afb475489 100644
--- a/packages/schema/messages.md
+++ b/packages/schema/messages.md
@@ -338,3 +338,16 @@ To display a widget, the kernel sends a Jupyter [iopub `display_data` message](h
   }
 }
 ```
+
+
+
+
+# Widget messaging protocol, version 2.1
+
+This is implemented in ipywidgets 7.7.
+
+## Changes from version 2
+
+### The `jupyter.widget.control` comm target
+
+A kernel-side Jupyter widgets library registers a `jupyter.widget.control` comm target that is used for fetching all widgets states through a "one shot" comm message (one for all widget instances). The kernel-side widgets library must answer to the "comm-open" message with a comm message containing the full state of all widget instances.