diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index 94408f8..3690145 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -339,7 +339,67 @@
         sock.close()
 
 
-class _SelectorTransport(transports.Transport):
+class _FlowControlMixin(transports.Transport):
+    """All the logic for (write) flow control in a mix-in base class.
+
+    The subclass must implement get_write_buffer_size().  It must call
+    _maybe_pause_protocol() whenever the write buffer size increases,
+    and _maybe_resume_protocol() whenever it decreases.  It may also
+    override set_write_buffer_limits() (e.g. to specify different
+    defaults).
+
+    The subclass constructor must call super().__init__(extra).  This
+    will call set_write_buffer_limits().
+
+    The user may call set_write_buffer_limits() and
+    get_write_buffer_size(), and their protocol's pause_writing() and
+    resume_writing() may be called.
+    """
+
+    def __init__(self, extra=None):
+        super().__init__(extra)
+        self._protocol_paused = False
+        self.set_write_buffer_limits()
+
+    def _maybe_pause_protocol(self):
+        size = self.get_write_buffer_size()
+        if size <= self._high_water:
+            return
+        if not self._protocol_paused:
+            self._protocol_paused = True
+            try:
+                self._protocol.pause_writing()
+            except Exception:
+                logger.exception('pause_writing() failed')
+
+    def _maybe_resume_protocol(self):
+        if (self._protocol_paused and
+            self.get_write_buffer_size() <= self._low_water):
+            self._protocol_paused = False
+            try:
+                self._protocol.resume_writing()
+            except Exception:
+                logger.exception('resume_writing() failed')
+
+    def set_write_buffer_limits(self, high=None, low=None):
+        if high is None:
+            if low is None:
+                high = 64*1024
+            else:
+                high = 4*low
+        if low is None:
+            low = high // 4
+        if not high >= low >= 0:
+            raise ValueError('high (%r) must be >= low (%r) must be >= 0' %
+                             (high, low))
+        self._high_water = high
+        self._low_water = low
+
+    def get_write_buffer_size(self):
+        raise NotImplementedError
+
+
+class _SelectorTransport(_FlowControlMixin, transports.Transport):
 
     max_size = 256 * 1024  # Buffer size passed to recv().
 
@@ -362,8 +422,6 @@
         self._buffer = self._buffer_factory()
         self._conn_lost = 0  # Set when call to connection_lost scheduled.
         self._closing = False  # Set when close() called.
-        self._protocol_paused = False
-        self.set_write_buffer_limits()
         if self._server is not None:
             self._server.attach(self)
 
@@ -410,40 +468,6 @@
                 server.detach(self)
                 self._server = None
 
-    def _maybe_pause_protocol(self):
-        size = self.get_write_buffer_size()
-        if size <= self._high_water:
-            return
-        if not self._protocol_paused:
-            self._protocol_paused = True
-            try:
-                self._protocol.pause_writing()
-            except Exception:
-                logger.exception('pause_writing() failed')
-
-    def _maybe_resume_protocol(self):
-        if (self._protocol_paused and
-            self.get_write_buffer_size() <= self._low_water):
-            self._protocol_paused = False
-            try:
-                self._protocol.resume_writing()
-            except Exception:
-                logger.exception('resume_writing() failed')
-
-    def set_write_buffer_limits(self, high=None, low=None):
-        if high is None:
-            if low is None:
-                high = 64*1024
-            else:
-                high = 4*low
-        if low is None:
-            low = high // 4
-        if not high >= low >= 0:
-            raise ValueError('high (%r) must be >= low (%r) must be >= 0' %
-                             (high, low))
-        self._high_water = high
-        self._low_water = low
-
     def get_write_buffer_size(self):
         return len(self._buffer)