diff --git a/Makefile.am b/Makefile.am
index 102e7c4..3a7c3d4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,7 +19,8 @@
 include vpnor/Makefile.am.include
 else
 mboxd_SOURCES += flash.c \
-	lpc_reset.c
+	lpc_reset.c \
+	protocol_negotiate_version.c
 endif
 
 mboxctl_SOURCES = mboxctl.c
diff --git a/protocol.c b/protocol.c
index 293934f..2bd91ab 100644
--- a/protocol.c
+++ b/protocol.c
@@ -448,55 +448,9 @@
 	return 0;
 }
 
-static const struct protocol_ops protocol_ops_v1 = {
-	.reset = protocol_v1_reset,
-	.get_info = protocol_v1_get_info,
-	.get_flash_info = protocol_v1_get_flash_info,
-	.create_window = protocol_v1_create_window,
-	.mark_dirty = protocol_v1_mark_dirty,
-	.erase = NULL,
-	.flush = protocol_v1_flush,
-	.close = protocol_v1_close,
-	.ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops protocol_ops_v2 = {
-	.reset = protocol_v1_reset,
-	.get_info = protocol_v2_get_info,
-	.get_flash_info = protocol_v2_get_flash_info,
-	.create_window = protocol_v2_create_window,
-	.mark_dirty = protocol_v2_mark_dirty,
-	.erase = protocol_v2_erase,
-	.flush = protocol_v2_flush,
-	.close = protocol_v2_close,
-	.ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops *protocol_ops_map[] = {
-	[0] = NULL,
-	[1] = &protocol_ops_v1,
-	[2] = &protocol_ops_v2,
-};
-
-int protocol_negotiate_version(struct mbox_context *context,
-				   uint8_t requested)
-{
-	/* Check we support the version requested */
-	if (requested < API_MIN_VERSION)
-		return -EINVAL;
-
-	context->version = (requested > API_MAX_VERSION) ?
-				API_MAX_VERSION : requested;
-
-	context->protocol = protocol_ops_map[context->version];
-
-	return context->version;
-}
-
 int protocol_init(struct mbox_context *context)
 {
-	context->version = API_MAX_VERSION;
-	context->protocol = protocol_ops_map[context->version];
+	protocol_negotiate_version(context, API_MAX_VERSION);
 
 	return 0;
 }
diff --git a/protocol_negotiate_version.c b/protocol_negotiate_version.c
new file mode 100644
index 0000000..849363f
--- /dev/null
+++ b/protocol_negotiate_version.c
@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2018 IBM Corp.
+#include "config.h"
+
+#include <errno.h>
+
+#include "mbox.h"
+#include "protocol.h"
+
+static const struct protocol_ops protocol_ops_v1 = {
+	.reset = protocol_v1_reset,
+	.get_info = protocol_v1_get_info,
+	.get_flash_info = protocol_v1_get_flash_info,
+	.create_window = protocol_v1_create_window,
+	.mark_dirty = protocol_v1_mark_dirty,
+	.erase = NULL,
+	.flush = protocol_v1_flush,
+	.close = protocol_v1_close,
+	.ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops protocol_ops_v2 = {
+	.reset = protocol_v1_reset,
+	.get_info = protocol_v2_get_info,
+	.get_flash_info = protocol_v2_get_flash_info,
+	.create_window = protocol_v2_create_window,
+	.mark_dirty = protocol_v2_mark_dirty,
+	.erase = protocol_v2_erase,
+	.flush = protocol_v2_flush,
+	.close = protocol_v2_close,
+	.ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops *protocol_ops_map[] = {
+	[0] = NULL,
+	[1] = &protocol_ops_v1,
+	[2] = &protocol_ops_v2,
+};
+
+int protocol_negotiate_version(struct mbox_context *context,
+				   uint8_t requested)
+{
+	/* Check we support the version requested */
+	if (requested < API_MIN_VERSION)
+		return -EINVAL;
+
+	context->version = (requested > API_MAX_VERSION) ?
+				API_MAX_VERSION : requested;
+
+	context->protocol = protocol_ops_map[context->version];
+
+	return context->version;
+}
diff --git a/test/Makefile.am.include b/test/Makefile.am.include
index 71fa851..fab9e14 100644
--- a/test/Makefile.am.include
+++ b/test/Makefile.am.include
@@ -25,7 +25,8 @@
 	lpc_reset.c \
 	common.c \
 	flash.c \
-	protocol.c
+	protocol.c \
+	protocol_negotiate_version.c
 
 TEST_MOCK_SRCS = %reldir%/tmpf.c %reldir%/mbox.c %reldir%/system.c
 
diff --git a/transport_mbox.c b/transport_mbox.c
index bc5e1e2..0d3d9b7 100644
--- a/transport_mbox.c
+++ b/transport_mbox.c
@@ -47,7 +47,7 @@
 
 static const struct errno_map errno_map_v2[] = {
 	{ 0, MBOX_R_SUCCESS },
-	{ EACCES, MBOX_R_PARAM_ERROR },
+	{ EACCES, MBOX_R_WINDOW_ERROR },
 	{ EBUSY, MBOX_R_BUSY },
 	{ EINVAL, MBOX_R_PARAM_ERROR },
 	{ EPERM, MBOX_R_WINDOW_ERROR },
diff --git a/vpnor/Makefile.am.include b/vpnor/Makefile.am.include
index 38fc631..75bd92e 100644
--- a/vpnor/Makefile.am.include
+++ b/vpnor/Makefile.am.include
@@ -3,7 +3,8 @@
 	%reldir%/flash.cpp \
 	%reldir%/pnor_partition.cpp \
 	%reldir%/lpc_reset.cpp \
-	%reldir%/transport_mbox.cpp
+	%reldir%/protocol.cpp \
+	%reldir%/protocol_negotiate_version.cpp
 
 mboxd_LDFLAGS += -lstdc++fs \
 	$(SDBUSPLUS_LIBS) \
diff --git a/vpnor/mboxd_pnor_partition_table.cpp b/vpnor/mboxd_pnor_partition_table.cpp
index cc0fccf..f787fc8 100644
--- a/vpnor/mboxd_pnor_partition_table.cpp
+++ b/vpnor/mboxd_pnor_partition_table.cpp
@@ -13,7 +13,6 @@
 #include "xyz/openbmc_project/Common/error.hpp"
 #include <phosphor-logging/elog-errors.hpp>
 #include <experimental/filesystem>
-#include "vpnor/transport_mbox.hpp"
 
 int init_vpnor(struct mbox_context *context)
 {
@@ -48,8 +47,6 @@
 
     if (context && !context->vpnor)
     {
-        context->handlers = vpnor_mbox_handlers;
-
         try
         {
             context->vpnor = new vpnor_partition_table;
diff --git a/vpnor/protocol.cpp b/vpnor/protocol.cpp
new file mode 100644
index 0000000..57b35a7
--- /dev/null
+++ b/vpnor/protocol.cpp
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2018 IBM Corp.
+#include "config.h"
+
+extern "C" {
+#include "mbox.h"
+#include "protocol.h"
+#include "vpnor/protocol.h"
+}
+
+#include "vpnor/pnor_partition_table.hpp"
+
+/* XXX: Maybe this should be a method on a class? */
+static bool vpnor_partition_is_readonly(const pnor_partition &part)
+{
+    return part.data.user.data[1] & PARTITION_READONLY;
+}
+
+typedef int (*create_window_fn)(struct mbox_context *context,
+                                struct protocol_create_window *io);
+
+static int generic_vpnor_create_window(struct mbox_context *context,
+                                       struct protocol_create_window *io,
+                                       create_window_fn create_window)
+{
+    if (io->req.ro)
+    {
+        return create_window(context, io);
+    }
+
+    /* Only allow write windows on regions mapped by the ToC as writeable */
+    size_t offset = io->req.offset;
+    offset <<= context->block_size_shift;
+    try
+    {
+        const pnor_partition &part = context->vpnor->table->partition(offset);
+        if (vpnor_partition_is_readonly(part))
+        {
+            return -EPERM;
+        }
+    }
+    catch (const openpower::virtual_pnor::UnmappedOffset &e)
+    {
+        /*
+         * Writes to unmapped areas are not meaningful, so deny the request.
+         * This removes the ability for a compromised host to abuse unused
+         * space if any data was to be persisted (which it isn't).
+         */
+        return -EACCES;
+    }
+
+    return create_window(context, io);
+}
+
+int protocol_v1_vpnor_create_window(struct mbox_context *context,
+                                    struct protocol_create_window *io)
+{
+    return generic_vpnor_create_window(context, io, protocol_v1_create_window);
+}
+
+int protocol_v2_vpnor_create_window(struct mbox_context *context,
+                                    struct protocol_create_window *io)
+{
+    return generic_vpnor_create_window(context, io, protocol_v2_create_window);
+}
diff --git a/vpnor/protocol.h b/vpnor/protocol.h
new file mode 100644
index 0000000..8cebda6
--- /dev/null
+++ b/vpnor/protocol.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+/* Copyright (C) 2018 IBM Corp. */
+#ifndef VPNOR_PROTOCOL_H
+#define VPNOR_PROTOCOL_H
+
+#include "protocol.h"
+
+/* Protocol v1 */
+int protocol_v1_vpnor_create_window(struct mbox_context *context,
+                                    struct protocol_create_window *io);
+
+/* Protocol v2 */
+int protocol_v2_vpnor_create_window(struct mbox_context *context,
+			            struct protocol_create_window *io);
+
+#endif /* VPNOR_PROTOCOL_H */
diff --git a/vpnor/protocol_negotiate_version.cpp b/vpnor/protocol_negotiate_version.cpp
new file mode 100644
index 0000000..4aff8db
--- /dev/null
+++ b/vpnor/protocol_negotiate_version.cpp
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2018 IBM Corp.
+#include "config.h"
+
+#include <errno.h>
+
+extern "C" {
+#include "mbox.h"
+#include "protocol.h"
+#include "vpnor/protocol.h"
+}
+
+/* We need to hijack create_window for vpnor */
+
+static const struct protocol_ops protocol_ops_v1 = {
+    .reset = protocol_v1_reset,
+    .get_info = protocol_v1_get_info,
+    .get_flash_info = protocol_v1_get_flash_info,
+    .create_window = protocol_v1_vpnor_create_window,
+    .mark_dirty = protocol_v1_mark_dirty,
+    .erase = NULL,
+    .flush = protocol_v1_flush,
+    .close = protocol_v1_close,
+    .ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops protocol_ops_v2 = {
+    .reset = protocol_v1_reset,
+    .get_info = protocol_v2_get_info,
+    .get_flash_info = protocol_v2_get_flash_info,
+    .create_window = protocol_v2_vpnor_create_window,
+    .mark_dirty = protocol_v2_mark_dirty,
+    .erase = protocol_v2_erase,
+    .flush = protocol_v2_flush,
+    .close = protocol_v2_close,
+    .ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops *protocol_ops_map[] = {
+    [0] = NULL,
+    [1] = &protocol_ops_v1,
+    [2] = &protocol_ops_v2,
+};
+
+int protocol_negotiate_version(struct mbox_context *context, uint8_t requested)
+{
+    /* Check we support the version requested */
+    if (requested < API_MIN_VERSION)
+        return -EINVAL;
+
+    uint8_t version =
+        (requested > API_MAX_VERSION) ? API_MAX_VERSION : requested;
+    context->version = static_cast<enum api_version>(version);
+
+    context->protocol = protocol_ops_map[context->version];
+
+    return context->version;
+}
diff --git a/vpnor/test/Makefile.am.include b/vpnor/test/Makefile.am.include
index 2444606..b029b37 100644
--- a/vpnor/test/Makefile.am.include
+++ b/vpnor/test/Makefile.am.include
@@ -12,9 +12,10 @@
 	vpnor/lpc_reset.cpp \
 	vpnor/mboxd_pnor_partition_table.cpp \
 	vpnor/flash.cpp \
-	vpnor/transport_mbox.cpp \
 	vpnor/pnor_partition.cpp \
 	vpnor/pnor_partition_table.cpp \
+	vpnor/protocol.cpp \
+	vpnor/protocol_negotiate_version.cpp \
 	%reldir%/tmpd.cpp
 
 VPNOR_LDADD = -lstdc++fs \
diff --git a/vpnor/transport_mbox.cpp b/vpnor/transport_mbox.cpp
deleted file mode 100644
index 620006e..0000000
--- a/vpnor/transport_mbox.cpp
+++ /dev/null
@@ -1,58 +0,0 @@
-#include "config.h"
-
-extern "C" {
-#include "mbox.h"
-#include "transport_mbox.h"
-};
-
-#include "vpnor/transport_mbox.hpp"
-#include "vpnor/pnor_partition_table.hpp"
-
-// clang-format off
-const mboxd_mbox_handler vpnor_mbox_handlers[NUM_MBOX_CMDS] =
-{
-	mbox_handle_reset,
-	mbox_handle_mbox_info,
-	mbox_handle_flash_info,
-	mbox_handle_read_window,
-	mbox_handle_close_window,
-	vpnor_handle_write_window,
-	mbox_handle_dirty_window,
-	mbox_handle_flush_window,
-	mbox_handle_ack,
-	mbox_handle_erase_window
-};
-// clang-format on
-
-/* XXX: Maybe this should be a method on a class? */
-static bool vpnor_partition_is_readonly(const pnor_partition& part)
-{
-    return part.data.user.data[1] & PARTITION_READONLY;
-}
-
-int vpnor_handle_write_window(struct mbox_context* context,
-                              union mbox_regs* req, struct mbox_msg* resp)
-{
-    size_t offset = get_u16(&req->msg.args[0]);
-    offset <<= context->block_size_shift;
-    try
-    {
-        const pnor_partition& part = context->vpnor->table->partition(offset);
-        if (vpnor_partition_is_readonly(part))
-        {
-            return -MBOX_R_WINDOW_ERROR;
-        }
-    }
-    catch (const openpower::virtual_pnor::UnmappedOffset& e)
-    {
-        /*
-         * Writes to unmapped areas are not meaningful, so deny the request.
-         * This removes the ability for a compromised host to abuse unused
-         * space if any data was to be persisted (which it isn't).
-         */
-        return -MBOX_R_WINDOW_ERROR;
-    }
-
-    /* Defer to the default handler */
-    return mbox_handle_write_window(context, req, resp);
-}
diff --git a/vpnor/transport_mbox.hpp b/vpnor/transport_mbox.hpp
deleted file mode 100644
index d9a05bb..0000000
--- a/vpnor/transport_mbox.hpp
+++ /dev/null
@@ -1,8 +0,0 @@
-extern "C" {
-#include "mbox.h"
-
-extern const mboxd_mbox_handler vpnor_mbox_handlers[NUM_MBOX_CMDS];
-
-int vpnor_handle_write_window(struct mbox_context *context,
-                              union mbox_regs *req, struct mbox_msg *resp);
-};