Add reference about “protocol message types” and “protocol message formats”.
index 4899bacda7..5793936b42 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -2069,8 +2069,9 @@ The commands accepted in replication mode are:
</para>
<para>
- WAL data is sent as a series of CopyData messages. (This allows
- other information to be intermixed; in particular the server can send
+ WAL data is sent as a series of CopyData messages
+ (See <xref linkend="protocol-message-types"/> and <xref linkend="protocol-message-formats"/>).
+ (This allows other information to be intermixed; in particular the server can send
an ErrorResponse message if it encounters a failure after beginning
to stream.) The payload of each CopyData message from server to the
client contains a message of one of the following formats: