diff --git a/protocol.h b/protocol.h
new file mode 100644
index 0000000..c147139
--- /dev/null
+++ b/protocol.h
@@ -0,0 +1,50 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+/* Copyright (C) 2018 IBM Corp. */
+
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+struct mbox_context;
+
+/*
+ * The GET_MBOX_INFO command is special as it can change the interface based on
+ * negotiation. As such we need to accommodate all response types
+ */
+struct protocol_get_info {
+	struct {
+		uint8_t api_version;
+	} req;
+	struct {
+		uint8_t api_version;
+		union {
+			struct {
+				uint16_t read_window_size;
+				uint16_t write_window_size;
+			} v1;
+			struct {
+				uint8_t block_size_shift;
+				uint16_t timeout;
+			} v2;
+		};
+	} resp;
+};
+
+struct protocol_ops {
+	int (*get_info)(struct mbox_context *context,
+			struct protocol_get_info *io);
+};
+
+int protocol_init(struct mbox_context *context);
+void protocol_free(struct mbox_context *context);
+
+int protocol_negotiate_version(struct mbox_context *context, uint8_t requested);
+
+/* Protocol v1 */
+int protocol_v1_get_info(struct mbox_context *context,
+			 struct protocol_get_info *io);
+
+/* Protocol v2 */
+int protocol_v2_get_info(struct mbox_context *context,
+			 struct protocol_get_info *io);
+
+#endif /* PROTOCOL_H */