+#if LUSTRE_VERSION_CODE < OBD_OCD_VERSION(3, 0, 53, 0)
+ /* Don't allow clients to connect that are using old 1.8 format
+ * protocol conventions (LUSTRE_MSG_MAGIC_v1, !MSGHDR_CKSUM_INCOMPAT18,
+ * ldlm_flock_policy_wire format, MDT_ATTR_xTIME_SET, etc). The
+ * FULL20 flag should be set on all connections since 2.0, but no
+ * longer affects behaviour.
+ *
+ * Later this check will be disabled and the flag can be retired
+ * completely once interop with 3.0 is no longer needed.
+ */
+ if (!(data->ocd_connect_flags & OBD_CONNECT_FULL20))
+ GOTO(out, rc = -EPROTO);
+#endif
+