diff --git a/src/lib/python/isc/cc/session.py b/src/lib/python/isc/cc/session.py
index f6b62653be..33a47bdfcb 100644
--- a/src/lib/python/isc/cc/session.py
+++ b/src/lib/python/isc/cc/session.py
@@ -72,7 +72,7 @@ class Session:
         self._lname = None
         self._closed = True
 
-    def sendmsg(self, env, msg = None):
+    def sendmsg(self, env, msg=None):
         with self._lock:
             if self._closed:
                 raise SessionError("Session has been closed.")
@@ -82,15 +82,24 @@ class Session:
                 raise ProtocolError("Envelope too large")
             if type(msg) == dict:
                 msg = isc.cc.message.to_wire(msg)
-            self._socket.setblocking(1)
             length = 2 + len(env);
-            if msg:
+            if msg is not None:
                 length += len(msg)
-            self._socket.send(struct.pack("!I", length))
-            self._socket.send(struct.pack("!H", len(env)))
-            self._socket.send(env)
-            if msg:
-                self._socket.send(msg)
+
+            # Build entire message.
+            data = struct.pack("!I", length)
+            data += struct.pack("!H", len(env))
+            data += env
+            if msg is not None:
+                data += msg
+
+            # Send it in the blocking mode.  On some systems send() may
+            # actually send only part of the data, so we need to repeat it
+            # until all data have been sent out.
+            self._socket.setblocking(1)
+            while len(data) > 0:
+                cc = self._socket.send(data)
+                data = data[cc:]
 
     def recvmsg(self, nonblock = True, seq = None):
         """Reads a message. If nonblock is true, and there is no
diff --git a/src/lib/python/isc/cc/tests/session_test.py b/src/lib/python/isc/cc/tests/session_test.py
index 772ed0c961..e589085189 100644
--- a/src/lib/python/isc/cc/tests/session_test.py
+++ b/src/lib/python/isc/cc/tests/session_test.py
@@ -29,6 +29,7 @@ class MySocket():
         self.recvqueue = bytearray()
         self.sendqueue = bytearray()
         self._blocking = True
+        self.send_limit = None
 
     def connect(self, to):
         pass
@@ -40,7 +41,14 @@ class MySocket():
         self._blocking = val
 
     def send(self, data):
-        self.sendqueue.extend(data);
+        # If the upper limit is specified, only "send" up to the specified
+        # limit
+        if self.send_limit is not None and len(data) > self.send_limit:
+            self.sendqueue.extend(data[0:self.send_limit])
+            return self.send_limit
+        else:
+            self.sendqueue.extend(data)
+            return len(data)
 
     def readsent(self, length):
         if length > len(self.sendqueue):
@@ -101,6 +109,17 @@ class MySocket():
     def gettimeout(self):
         return 0
 
+    def set_send_limit(self, limit):
+        '''Specify the upper limit of the transmittable data at once.
+
+        By default, the send() method of this class "sends" all given data.
+        If this method is called and the its parameter is not None,
+        subsequent calls to send() will only transmit the specified amount
+        of data.  This can be used to emulate the situation where send()
+        on a real socket object results in partial write.
+        '''
+        self.send_limit = limit
+
 #
 # We subclass the Session class we're testing here, only
 # to override the __init__() method, which wants a socket,
@@ -157,6 +176,16 @@ class testSession(unittest.TestCase):
         #print(sent)
         #self.assertRaises(SessionError, sess.sendmsg, {}, {"hello": "a"})
 
+    def test_session_sendmsg_shortwrite(self):
+        sess = MySession()
+        # Specify the upper limit of the size that can be transmitted at
+        # a single send() call on the faked socket (10 is an arbitrary choice,
+        # just reasonably small).
+        sess._socket.set_send_limit(10)
+        sess.sendmsg({'to': 'someone', 'reply': 1}, {"hello": "a"})
+        # The complete message should still have been transmitted in the end.
+        sent = sess._socket.readsentmsg();
+
     def recv_and_compare(self, session, bytes, env, msg):
         """Adds bytes to the recvqueue (which will be read by the
            session object, and compare the resultinv env and msg to