diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 494c5ddba6..ce14c6f2c4 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -35,6 +35,7 @@ from synapse.replication.tcp.commands import (
     UserIpCommand,
     UserSyncCommand,
 )
+from synapse.replication.tcp.protocol import AbstractConnection
 from synapse.replication.tcp.streams import STREAMS_MAP, Stream
 from synapse.util.async_helpers import Linearizer
 
@@ -82,7 +83,7 @@ class ReplicationCommandHandler:
         self._factory = None  # type: Optional[ReplicationClientFactory]
 
         # The currently connected connections.
-        self._connections = []  # type: List[Any]
+        self._connections = []  # type: List[AbstractConnection]
 
         LaterGauge(
             "synapse_replication_tcp_resource_total_connections",
@@ -278,7 +279,7 @@ class ReplicationCommandHandler:
         """
         return self._presence_handler.get_currently_syncing_users()
 
-    def new_connection(self, connection):
+    def new_connection(self, connection: AbstractConnection):
         """Called when we have a new connection.
         """
         self._connections.append(connection)
@@ -295,7 +296,7 @@ class ReplicationCommandHandler:
         if self._factory:
             self._factory.resetDelay()
 
-    def lost_connection(self, connection):
+    def lost_connection(self, connection: AbstractConnection):
         """Called when a connection is closed/lost.
         """
         try:
diff --git a/synapse/replication/tcp/protocol.py b/synapse/replication/tcp/protocol.py
index d74fde7e0b..bb12d6a14b 100644
--- a/synapse/replication/tcp/protocol.py
+++ b/synapse/replication/tcp/protocol.py
@@ -46,6 +46,7 @@ indicate which side is sending, these are *not* included on the wire::
     > ERROR server stopping
     * connection closed by server *
 """
+import abc
 import fcntl
 import logging
 import struct
@@ -485,6 +486,22 @@ class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
         self.send_command(ReplicateCommand())
 
 
+class AbstractConnection(abc.ABC):
+    """An interface for replication connections.
+    """
+
+    @abc.abstractmethod
+    def send_command(self, cmd: Command):
+        """Send the command down the connection
+        """
+        pass
+
+
+# This tells python that `BaseReplicationStreamProtocol` implements the
+# interface.
+AbstractConnection.register(BaseReplicationStreamProtocol)
+
+
 # The following simply registers metrics for the replication connections
 
 pending_commands = LaterGauge(