mss_clamp: TCP MSS clamping plugin

Type: feature

Configure TCP MSS clamping on an interface as follows:
set interface tcp-mss-clamp [rx|tx] <interface-name>
  ip4 [enable|disable|rx|tx] ip4-mss <size>
  ip6 [enable|disable|rx|tx] ip6-mss <size>

Change-Id: I45b04e50a0b70a33e14a9066f981c651292ebffb
Signed-off-by: Neale Ranns <neale.ranns@cisco.com>
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
Signed-off-by: Miklos Tirpak <miklos.tirpak@gmail.com>
Signed-off-by: Matthew Smith <mgsmith@netgate.com>

7 files changed, 1331 insertions, 0 deletions

diff --git a/src/plugins/mss_clamp/CMakeLists.txt b/src/plugins/mss_clamp/CMakeLists.txt
new file mode 100644
index 00000000000..d650d7fc528
--- /dev/null
+++ b/src/plugins/mss_clamp/CMakeLists.txt
@@ -0,0 +1,30 @@
+
+# Copyright (c) <current-year> <your-organization>
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+add_vpp_plugin(mss_clamp
+  SOURCES
+  mss_clamp.c
+  mss_clamp_api.c
+  mss_clamp_node.c
+
+
+  MULTIARCH_SOURCES
+  mss_clamp_node.c
+
+  API_FILES
+  mss_clamp.api
+
+  INSTALL_HEADERS
+  mss_clamp.h
+)
diff --git a/src/plugins/mss_clamp/mss_clamp.api b/src/plugins/mss_clamp/mss_clamp.api
new file mode 100644
index 00000000000..e1b9fdea6cb
--- /dev/null
+++ b/src/plugins/mss_clamp/mss_clamp.api
@@ -0,0 +1,112 @@
+/* Hey Emacs use -*- mode: C -*- */
+/*
+ * Copyright (c) 2016 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+option version = "1.0.0";
+import "vnet/interface_types.api";
+
+/** \brief TCP MSS Clamping direction flag
+ */
+enumflag mss_clamp_dir : u8 {
+    MSS_CLAMP_DIR_NONE = 0x0,
+    MSS_CLAMP_DIR_RX = 0x1,
+    MSS_CLAMP_DIR_TX = 0x2,
+};
+
+/** \brief Enable/Disable TCP MSS Clamping feature on an interface
+    @param client_index - opaque cookie to identify the sender
+    @param context - sender context, to match reply w/ request
+    @param sw_if_index - interface index on which clamping will be applied
+    @param ipv4_mss - Maximum Segment Size for IPv4/TCP
+    @param ipv6_mss - Maximum Segment Size for IPv6/TCP
+    @param ipv4_direction - Direction clamping is enabled on (IPv4/TCP)
+    @param ipv6_direction - Direction clamping is enabled on (IPv6/TCP)
+ */
+autoreply define mss_clamp_enable_disable {
+    u32 client_index;
+    u32 context;
+    vl_api_interface_index_t sw_if_index;
+    u16 ipv4_mss;
+    u16 ipv6_mss;
+    vl_api_mss_clamp_dir_t ipv4_direction;
+    vl_api_mss_clamp_dir_t ipv6_direction;
+};
+
+
+/** \brief Get the list of configured mss values
+    @param client_index - opaque cookie to identify the sender
+*/
+service {
+  rpc mss_clamp_get returns mss_clamp_get_reply
+    stream mss_clamp_details;
+};
+
+/** \brief Get the TCP MSS Clamping feature settings
+    @param client_index - opaque cookie to identify the sender
+    @param context - sender context, to match reply w/ request
+    @param cursor - cursor to continue when there is more to read
+    @param sw_if_index - interface index to filter the result,
+			 ~0 means no filter
+ */
+define mss_clamp_get {
+    u32 client_index;
+    u32 context;
+    u32 cursor;
+    vl_api_interface_index_t sw_if_index;
+};
+
+/** \brief Reply for get TCP MSS Clamping feature settings request
+    @param context - returned sender context, to match reply w/ request
+    @param retval - return code
+    @param cursor - cursor to continue when there is more to read
+ */
+define mss_clamp_get_reply {
+    u32 context;
+    i32 retval;
+    u32 cursor;
+};
+
+/** \brief Configured MSS values on an interface
+    @param context - returned sender context, to match reply w/ request
+    @param sw_if_index - interface index on which clamping is applied
+    @param ipv4_mss - Maximum Segment Size for IPv4/TCP
+    @param ipv6_mss - Maximum Segment Size for IPv6/TCP
+    @param ipv4_direction - Direction clamping is enabled on (IPv4/TCP)
+    @param ipv6_direction - Direction clamping is enabled on (IPv6/TCP)
+ */
+define mss_clamp_details {
+    u32 context;
+    vl_api_interface_index_t sw_if_index;
+    u16 ipv4_mss;
+    u16 ipv6_mss;
+    vl_api_mss_clamp_dir_t ipv4_direction;
+    vl_api_mss_clamp_dir_t ipv6_direction;
+};
+
+counters mss_clamp {
+  clamped {
+    severity info;
+    type counter64;
+    units "packets";
+    description "packets clamped";
+  };
+};
+paths {
+  "/err/tcp-mss-clamping-ip4-in" "mss-clamp";
+  "/err/tcp-mss-clamping-ip4-out" "mss-clamp";
+  "/err/tcp-mss-clamping-ip6-in" "mss-clamp";
+  "/err/tcp-mss-clamping-ip6-out" "mss-clamp";
+};
+
diff --git a/src/plugins/mss_clamp/mss_clamp.c b/src/plugins/mss_clamp/mss_clamp.c
new file mode 100644
index 00000000000..cdac5456641
--- /dev/null
+++ b/src/plugins/mss_clamp/mss_clamp.c
@@ -0,0 +1,292 @@
+/*
+ * mss_clamp.c - TCP MSS clamping plug-in
+ *
+ * Copyright (c) 2018 Cisco and/or its affiliates
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <vnet/vnet.h>
+#include <vnet/plugin/plugin.h>
+#include <mss_clamp/mss_clamp.h>
+#include <mss_clamp/mss_clamp.api_types.h>
+
+mssc_main_t mssc_main;
+
+/* Action function shared between message handler and debug CLI */
+
+static void
+mssc_enable_disable_feat (u32 sw_if_index, u8 dir4, u8 dir6, int enable)
+{
+  if (dir4 == MSS_CLAMP_DIR_NONE && dir6 == MSS_CLAMP_DIR_NONE)
+    return;
+
+  // ip4
+  if ((dir4 & MSS_CLAMP_DIR_RX) != MSS_CLAMP_DIR_NONE)
+    vnet_feature_enable_disable ("ip4-unicast", "tcp-mss-clamping-ip4-in",
+				 sw_if_index, enable, 0, 0);
+  if ((dir4 & MSS_CLAMP_DIR_TX) != MSS_CLAMP_DIR_NONE)
+    vnet_feature_enable_disable ("ip4-output", "tcp-mss-clamping-ip4-out",
+				 sw_if_index, enable, 0, 0);
+  // ip6
+  if ((dir6 & MSS_CLAMP_DIR_RX) != MSS_CLAMP_DIR_NONE)
+    vnet_feature_enable_disable ("ip6-unicast", "tcp-mss-clamping-ip6-in",
+				 sw_if_index, enable, 0, 0);
+  if ((dir6 & MSS_CLAMP_DIR_TX) != MSS_CLAMP_DIR_NONE)
+    vnet_feature_enable_disable ("ip6-output", "tcp-mss-clamping-ip6-out",
+				 sw_if_index, enable, 0, 0);
+}
+
+int
+mssc_enable_disable (u32 sw_if_index, u8 dir4, u8 dir6, u16 mss4, u16 mss6)
+{
+  mssc_main_t *cm = &mssc_main;
+  u8 *dir_enabled4, *dir_enabled6;
+  int rv = 0;
+
+  if (dir4 == MSS_CLAMP_DIR_NONE)
+    mss4 = MSS_CLAMP_UNSET;
+  if (dir6 == MSS_CLAMP_DIR_NONE)
+    mss6 = MSS_CLAMP_UNSET;
+
+  vec_validate_init_empty (cm->dir_enabled4, sw_if_index, MSS_CLAMP_DIR_NONE);
+  vec_validate_init_empty (cm->dir_enabled6, sw_if_index, MSS_CLAMP_DIR_NONE);
+  vec_validate_init_empty (cm->max_mss4, sw_if_index, MSS_CLAMP_UNSET);
+  vec_validate_init_empty (cm->max_mss6, sw_if_index, MSS_CLAMP_UNSET);
+
+  cm->max_mss4[sw_if_index] = mss4;
+  cm->max_mss6[sw_if_index] = mss6;
+  dir_enabled4 = &cm->dir_enabled4[sw_if_index];
+  dir_enabled6 = &cm->dir_enabled6[sw_if_index];
+
+  // Disable the directions that are no longer needed
+  mssc_enable_disable_feat (sw_if_index, (*dir_enabled4) & ~dir4,
+			    (*dir_enabled6) & ~dir6, 0);
+  // Enable the new directions
+  mssc_enable_disable_feat (sw_if_index, ~(*dir_enabled4) & dir4,
+			    ~(*dir_enabled6) & dir6, 1);
+
+  *dir_enabled4 = dir4;
+  *dir_enabled6 = dir6;
+
+  return rv;
+}
+
+int
+mssc_get_mss (u32 sw_if_index, u8 *dir4, u8 *dir6, u16 *mss4, u16 *mss6)
+{
+  mssc_main_t *cm = &mssc_main;
+  int rv = VNET_API_ERROR_FEATURE_DISABLED;
+
+  if (vec_len (cm->dir_enabled4) > sw_if_index &&
+      MSS_CLAMP_DIR_NONE != cm->dir_enabled4[sw_if_index])
+    {
+      *mss4 = cm->max_mss4[sw_if_index];
+      *dir4 = cm->dir_enabled4[sw_if_index];
+      rv = 0;
+    }
+  else
+    {
+      *mss4 = MSS_CLAMP_DIR_NONE;
+      *dir4 = 0;
+    }
+
+  if (vec_len (cm->dir_enabled6) > sw_if_index &&
+      MSS_CLAMP_DIR_NONE != cm->dir_enabled6[sw_if_index])
+    {
+      *mss6 = cm->max_mss6[sw_if_index];
+      *dir6 = cm->dir_enabled6[sw_if_index];
+      rv = 0;
+    }
+  else
+    {
+      *mss6 = MSS_CLAMP_DIR_NONE;
+      *dir6 = 0;
+    }
+  return rv;
+}
+
+static uword
+unformat_mssc_dir (unformat_input_t *input, va_list *args)
+{
+  u8 *result = va_arg (*args, u8 *);
+  u8 dir = MSS_CLAMP_DIR_RX | MSS_CLAMP_DIR_TX;
+
+  if (unformat (input, "disable"))
+    dir = MSS_CLAMP_DIR_NONE;
+  else if (unformat (input, "enable"))
+    dir = MSS_CLAMP_DIR_RX | MSS_CLAMP_DIR_TX;
+  else if (unformat (input, "rx"))
+    dir = MSS_CLAMP_DIR_RX;
+  else if (unformat (input, "tx"))
+    dir = MSS_CLAMP_DIR_TX;
+  else
+    return 0;
+
+  *result = dir;
+  return 1;
+}
+
+static clib_error_t *
+mssc_enable_command_fn (vlib_main_t *vm, unformat_input_t *input,
+			vlib_cli_command_t *cmd)
+{
+  u32 sw_if_index = ~0;
+  u8 dir4 = ~0, dir6 = ~0;
+  u32 mss4 = ~0, mss6 = ~0;
+  int rv;
+
+  while (unformat_check_input (input) != UNFORMAT_END_OF_INPUT)
+    {
+      if (unformat (input, "ip4 %U", unformat_mssc_dir, &dir4))
+	;
+      else if (unformat (input, "ip6 %U", unformat_mssc_dir, &dir6))
+	;
+      else if (unformat (input, "ip4-mss %d", &mss4))
+	;
+      else if (unformat (input, "ip6-mss %d", &mss6))
+	;
+      else if (unformat (input, "%U", unformat_vnet_sw_interface,
+			 vnet_get_main (), &sw_if_index))
+	;
+      else
+	break;
+    }
+
+  if (sw_if_index == ~0)
+    return clib_error_return (0, "Please specify an interface");
+
+  if (dir4 == (u8) ~0 || dir6 == (u8) ~0)
+    return clib_error_return (
+      0, "Please specify the MSS clamping direction for ip4 and ip6");
+
+  if (dir4 != MSS_CLAMP_DIR_NONE)
+    {
+      if (mss4 == ~0)
+	return clib_error_return (
+	  0, "Please specify the Max Segment Size for ip4");
+      if (mss4 >= MSS_CLAMP_UNSET)
+	return clib_error_return (0, "Invalid Max Segment Size");
+    }
+  if (dir6 != MSS_CLAMP_DIR_NONE)
+    {
+      if (mss6 == ~0)
+	return clib_error_return (
+	  0, "Please specify the Max Segment Size for ip6");
+      if (mss6 >= MSS_CLAMP_UNSET)
+	return clib_error_return (0, "Invalid Max Segment Size");
+    }
+
+  rv = mssc_enable_disable (sw_if_index, dir4, dir6, mss4, mss6);
+
+  if (rv)
+    return clib_error_return (0, "Failed: %d = %U", rv, format_vnet_api_errno,
+			      rv);
+
+  return (NULL);
+}
+
+VLIB_CLI_COMMAND (mssc_enable_disable_command, static) = {
+  .path = "set interface tcp-mss-clamp",
+  .short_help = "set interface tcp-mss-clamp <interface-name> "
+		"ip4 [enable|disable|rx|tx] ip4-mss <size> "
+		"ip6 [enable|disable|rx|tx] ip6-mss <size>",
+  .function = mssc_enable_command_fn,
+};
+
+static u8 *
+format_mssc_clamping (u8 *s, va_list *args)
+{
+  u8 dir = va_arg (*args, u32);
+  u16 mss = va_arg (*args, u32);
+#define DIR2S(d)                                                              \
+  (((d) == (MSS_CLAMP_DIR_RX | MSS_CLAMP_DIR_TX)) ?                           \
+     "" :                                                                     \
+     (((d) == MSS_CLAMP_DIR_RX) ? " [RX]" : " [TX]"))
+
+  if (MSS_CLAMP_DIR_NONE == dir)
+    {
+      return format (s, "disabled");
+    }
+  u32 mss_u32 = mss;
+  return format (s, "%d%s", mss_u32, DIR2S (dir));
+}
+
+static clib_error_t *
+mssc_show_command_fn (vlib_main_t *vm, unformat_input_t *input,
+		      vlib_cli_command_t *cmd)
+{
+  mssc_main_t *cm = &mssc_main;
+  u32 sw_if_index = ~0;
+  u32 ii;
+
+  while (unformat_check_input (input) != UNFORMAT_END_OF_INPUT)
+    {
+      if (unformat (input, "%U", unformat_vnet_sw_interface, vnet_get_main (),
+		    &sw_if_index))
+	;
+      else
+	break;
+    }
+
+  if (sw_if_index == ~0)
+    {
+      vec_foreach_index (ii, cm->dir_enabled4)
+	{
+	  u8 dir4 = cm->dir_enabled4[ii];
+	  u8 dir6 = cm->dir_enabled6[ii];
+	  if (MSS_CLAMP_DIR_NONE != dir4 || MSS_CLAMP_DIR_NONE != dir6)
+	    {
+	      u16 mss4 = cm->max_mss4[ii];
+	      u16 mss6 = cm->max_mss6[ii];
+	      vlib_cli_output (vm, "%U: ip4: %U ip6: %U",
+			       format_vnet_sw_if_index_name, vnet_get_main (),
+			       ii, format_mssc_clamping, dir4, mss4,
+			       format_mssc_clamping, dir6, mss6);
+	    }
+	}
+    }
+  else
+    {
+      u16 mss4, mss6;
+      u8 dir4, dir6;
+      mssc_get_mss (sw_if_index, &dir4, &dir6, &mss4, &mss6);
+      vlib_cli_output (vm, "%U: ip4: %U ip6: %U", format_vnet_sw_if_index_name,
+		       vnet_get_main (), sw_if_index, format_mssc_clamping,
+		       dir4, mss4, format_mssc_clamping, dir6, mss6);
+    }
+
+  return (NULL);
+}
+
+VLIB_CLI_COMMAND (mssc_show_command, static) = {
+  .path = "show interface tcp-mss-clamp",
+  .short_help = "show interface tcp-mss-clamp [interface-name]",
+  .long_help = "show TCP MSS clamping configurations",
+  .function = mssc_show_command_fn,
+};
+
+static clib_error_t *
+mssc_init (vlib_main_t *vm)
+{
+  return NULL;
+}
+
+VLIB_INIT_FUNCTION (mssc_init);
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
diff --git a/src/plugins/mss_clamp/mss_clamp.h b/src/plugins/mss_clamp/mss_clamp.h
new file mode 100644
index 00000000000..46aa12aa81f
--- /dev/null
+++ b/src/plugins/mss_clamp/mss_clamp.h
@@ -0,0 +1,55 @@
+/*
+ * mss_clamp.h - TCP MSS clamping plug-in header file
+ *
+ * Copyright (c) 2018 Cisco and/or its affiliates
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef __included_mss_clamp_h__
+#define __included_mss_clamp_h__
+
+#include <stdbool.h> /* for bool in .api */
+#include <vnet/vnet.h>
+
+extern int mssc_enable_disable (u32 sw_if_index, u8 dir4, u8 dir6, u16 mss4,
+				u16 mss6);
+extern int mssc_get_mss (u32 sw_if_index, u8 *dir4, u8 *dir6, u16 *mss4,
+			 u16 *mss6);
+
+typedef struct
+{
+  /* Maximum segment size per interface for IPv4/IPv6 */
+  u16 *max_mss4;
+  u16 *max_mss6;
+
+  /* Direction the feature is enabled for IPv4/IPv6 (rx, tx, both) */
+  u8 *dir_enabled4;
+  u8 *dir_enabled6;
+
+  /* API message ID base */
+  u16 msg_id_base;
+} mssc_main_t;
+
+extern mssc_main_t mssc_main;
+
+#define MSS_CLAMP_UNSET 0xffff
+
+#endif /* __included_mss_clamp_h__ */
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
diff --git a/src/plugins/mss_clamp/mss_clamp_api.c b/src/plugins/mss_clamp/mss_clamp_api.c
new file mode 100644
index 00000000000..cb47cd6ce06
--- /dev/null
+++ b/src/plugins/mss_clamp/mss_clamp_api.c
@@ -0,0 +1,136 @@
+/*
+ * mss_clamp_api.c - TCP MSS clamping plug-in
+ *
+ * Copyright (c) <current-year> <your-organization>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <vnet/vnet.h>
+#include <vnet/plugin/plugin.h> /* VLIB_PLUGIN_REGISTER */
+#include <mss_clamp/mss_clamp.h>
+#include <mss_clamp/mss_clamp.api_enum.h>
+#include <mss_clamp/mss_clamp.api_types.h>
+#include <vlibapi/api.h>
+#include <vlibmemory/api.h>
+#include <vpp/app/version.h> /* VPP_BUILD_VER */
+
+#define REPLY_MSG_ID_BASE cm->msg_id_base
+#include <vlibapi/api_helper_macros.h>
+
+/* API message handler */
+static void
+vl_api_mss_clamp_enable_disable_t_handler (
+  vl_api_mss_clamp_enable_disable_t *mp)
+{
+  mssc_main_t *cm = &mssc_main;
+  vl_api_mss_clamp_enable_disable_reply_t *rmp;
+  int rv;
+
+  VALIDATE_SW_IF_INDEX (mp);
+
+  rv = mssc_enable_disable (ntohl (mp->sw_if_index), mp->ipv4_direction,
+			    mp->ipv6_direction, ntohs (mp->ipv4_mss),
+			    ntohs (mp->ipv6_mss));
+
+  BAD_SW_IF_INDEX_LABEL;
+  REPLY_MACRO (VL_API_MSS_CLAMP_ENABLE_DISABLE_REPLY);
+}
+
+static void
+send_mss_clamp_details (u32 sw_if_index, vl_api_registration_t *rp,
+			u32 context)
+{
+  mssc_main_t *cm = &mssc_main;
+  vl_api_mss_clamp_details_t *rmp;
+  u16 mss4, mss6;
+  u8 dir4, dir6;
+  int rv;
+
+  mss4 = mss6 = 0;
+  dir4 = dir6 = MSS_CLAMP_DIR_NONE;
+  rv = mssc_get_mss (sw_if_index, &dir4, &dir6, &mss4, &mss6);
+  if (rv == VNET_API_ERROR_FEATURE_DISABLED)
+    return;
+
+  REPLY_MACRO_DETAILS4 (VL_API_MSS_CLAMP_DETAILS, rp, context, ({
+			  rmp->sw_if_index = htonl (sw_if_index);
+			  rmp->ipv4_mss = htons (mss4);
+			  rmp->ipv6_mss = htons (mss6);
+			  rmp->ipv4_direction = dir4;
+			  rmp->ipv6_direction = dir6;
+			}));
+}
+
+static void
+vl_api_mss_clamp_get_t_handler (vl_api_mss_clamp_get_t *mp)
+{
+  mssc_main_t *cm = &mssc_main;
+  vl_api_mss_clamp_get_reply_t *rmp;
+  int rv = 0;
+  u32 sw_if_index = ntohl (mp->sw_if_index);
+  vl_api_registration_t *reg;
+
+  reg = vl_api_client_index_to_registration (mp->client_index);
+  if (!reg)
+    return;
+
+  if (sw_if_index == ~0)
+    {
+      if (vec_len (cm->dir_enabled4) == 0)
+	{
+	  REPLY_MACRO2 (VL_API_MSS_CLAMP_GET_REPLY, ({ rmp->cursor = ~0; }));
+	  return;
+	}
+
+      REPLY_AND_DETAILS_MACRO (
+	VL_API_MSS_CLAMP_GET_REPLY, cm->dir_enabled4,
+	({ send_mss_clamp_details (cursor, reg, mp->context); }));
+    }
+  else
+    {
+      VALIDATE_SW_IF_INDEX (mp);
+      send_mss_clamp_details (sw_if_index, reg, mp->context);
+
+      BAD_SW_IF_INDEX_LABEL;
+      REPLY_MACRO2 (VL_API_MSS_CLAMP_GET_REPLY, ({ rmp->cursor = ~0; }));
+    }
+}
+
+/* API definitions */
+#include <vnet/format_fns.h>
+#include <mss_clamp/mss_clamp.api.c>
+
+/* Set up the API message handling tables */
+static clib_error_t *
+mssc_api_hookup (vlib_main_t *vm)
+{
+  mssc_main_t *cm = &mssc_main;
+
+  cm->msg_id_base = setup_message_id_table ();
+  return 0;
+}
+
+VLIB_API_INIT_FUNCTION (mssc_api_hookup);
+
+VLIB_PLUGIN_REGISTER () = {
+  .version = VPP_BUILD_VER,
+  .description = "TCP MSS clamping plugin",
+};
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
diff --git a/src/plugins/mss_clamp/mss_clamp_node.c b/src/plugins/mss_clamp/mss_clamp_node.c
new file mode 100644
index 00000000000..4a40b2329e1
--- /dev/null
+++ b/src/plugins/mss_clamp/mss_clamp_node.c
@@ -0,0 +1,411 @@
+/*
+ * mss_clamp_node.c - Node implementing TCP MSS clamping
+ *
+ * Copyright (c) 2018 Cisco and/or its affiliates
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <vlib/vlib.h>
+#include <vnet/vnet.h>
+#include <vnet/pg/pg.h>
+#include <vppinfra/error.h>
+#include <mss_clamp/mss_clamp.h>
+#include <mss_clamp/mss_clamp.api_enum.h>
+#include <vnet/fib/fib_types.h>
+#include <vnet/feature/feature.h>
+#include <vnet/ip/ip4.h>
+#include <vnet/ip/ip6.h>
+
+extern vlib_node_registration_t mssc_ip4_in_node, mssc_ip4_out_node;
+extern vlib_node_registration_t mssc_ip6_in_node, mssc_ip6_out_node;
+
+typedef struct mssc_trace_t_
+{
+  u32 max_mss;
+  u32 clamped;
+} mssc_trace_t;
+
+/* packet trace format function */
+static u8 *
+format_mssc_trace (u8 *s, va_list *args)
+{
+  CLIB_UNUSED (vlib_main_t * vm) = va_arg (*args, vlib_main_t *);
+  CLIB_UNUSED (vlib_node_t * node) = va_arg (*args, vlib_node_t *);
+  mssc_trace_t *t = va_arg (*args, mssc_trace_t *);
+
+  s = format (s, "max mss: %d clamped: %d", t->max_mss, t->clamped);
+  return s;
+}
+
+typedef enum
+{
+  MSSC_NEXT_DROP,
+  MSSC_N_NEXT,
+} mssc_next_t;
+
+/*
+ * fixup the maximum segment size if it's a syn packet
+ * return 1 if the mss was changed otherwise 0
+ */
+always_inline u32
+mssc_mss_fixup (vlib_buffer_t *b0, tcp_header_t *tcp0, u16 max_mss0)
+{
+  ip_csum_t sum0;
+
+  if (PREDICT_FALSE (tcp_syn (tcp0)))
+    {
+      u8 opt_len, opts_len, kind;
+      const u8 *data;
+      u16 mss0, new_mss0;
+
+      opts_len = (tcp_doff (tcp0) << 2) - sizeof (tcp_header_t);
+      data = (const u8 *) (tcp0 + 1);
+
+      for (; opts_len > 0; opts_len -= opt_len, data += opt_len)
+	{
+	  kind = data[0];
+
+	  /* Get options length */
+	  if (kind == TCP_OPTION_EOL)
+	    break;
+	  else if (kind == TCP_OPTION_NOOP)
+	    {
+	      opt_len = 1;
+	      continue;
+	    }
+	  else
+	    {
+	      /* broken options */
+	      if (opts_len < 2)
+		return 0;
+	      opt_len = data[1];
+
+	      /* weird option length */
+	      if (opt_len < 2 || opt_len > opts_len)
+		return 0;
+	    }
+
+	  if (kind == TCP_OPTION_MSS)
+	    {
+	      mss0 = *(u16 *) (data + 2);
+	      if (clib_net_to_host_u16 (mss0) > max_mss0)
+		{
+		  new_mss0 = clib_host_to_net_u16 (max_mss0);
+		  *((u16 *) (data + 2)) = new_mss0;
+		  sum0 = tcp0->checksum;
+		  sum0 = ip_csum_update (sum0, mss0, new_mss0, tcp_header_t,
+					 checksum);
+		  tcp0->checksum = ip_csum_fold (sum0);
+		  return 1;
+		}
+	    }
+	}
+    }
+
+  return 0;
+}
+
+always_inline uword
+mssc_inline (vlib_main_t *vm, vlib_node_runtime_t *node, vlib_frame_t *frame,
+	     vlib_dir_t dir, fib_protocol_t fproto)
+{
+  mssc_main_t *cm = &mssc_main;
+  vlib_buffer_t *bufs[VLIB_FRAME_SIZE], **b;
+  u16 nexts[VLIB_FRAME_SIZE], *next;
+  u32 n_left, *from;
+  u32 pkts_clamped = 0;
+
+  from = vlib_frame_vector_args (frame);
+  n_left = frame->n_vectors;
+  b = bufs;
+  next = nexts;
+
+  vlib_get_buffers (vm, from, bufs, n_left);
+
+  while (n_left >= 4)
+    {
+      u32 sw_if_index0, sw_if_index1;
+      const u8 *h0, *h1;
+      u32 clamped0, clamped1;
+
+      /* Prefetch next iteration. */
+      {
+	vlib_prefetch_buffer_header (b[2], LOAD);
+	vlib_prefetch_buffer_header (b[3], LOAD);
+	vlib_prefetch_buffer_data (b[2], LOAD);
+	vlib_prefetch_buffer_data (b[3], LOAD);
+      }
+
+      sw_if_index0 = vnet_buffer (b[0])->sw_if_index[dir];
+      sw_if_index1 = vnet_buffer (b[1])->sw_if_index[dir];
+      clamped0 = clamped1 = 0;
+
+      /* speculatively enqueue b0 to the current next frame */
+      vnet_feature_next_u16 (&next[0], b[0]);
+      vnet_feature_next_u16 (&next[1], b[1]);
+
+      h0 = (u8 *) vlib_buffer_get_current (b[0]);
+      h1 = (u8 *) vlib_buffer_get_current (b[1]);
+      if (VLIB_TX == dir)
+	{
+	  h0 += vnet_buffer (b[0])->ip.save_rewrite_length;
+	  h1 += vnet_buffer (b[1])->ip.save_rewrite_length;
+	}
+
+      if (FIB_PROTOCOL_IP4 == fproto)
+	{
+	  ip4_header_t *ip0 = (ip4_header_t *) h0;
+	  ip4_header_t *ip1 = (ip4_header_t *) h1;
+
+	  if (IP_PROTOCOL_TCP == ip0->protocol)
+	    {
+	      clamped0 = mssc_mss_fixup (b[0], ip4_next_header (ip0),
+					 cm->max_mss4[sw_if_index0]);
+	    }
+	  if (IP_PROTOCOL_TCP == ip1->protocol)
+	    {
+	      clamped1 = mssc_mss_fixup (b[1], ip4_next_header (ip1),
+					 cm->max_mss4[sw_if_index1]);
+	    }
+	}
+      else if (FIB_PROTOCOL_IP6 == fproto)
+	{
+	  ip6_header_t *ip0 = (ip6_header_t *) h0;
+	  ip6_header_t *ip1 = (ip6_header_t *) h1;
+
+	  if (IP_PROTOCOL_TCP == ip0->protocol)
+	    {
+	      clamped0 = mssc_mss_fixup (b[0], ip6_next_header (ip0),
+					 cm->max_mss6[sw_if_index0]);
+	    }
+	  if (IP_PROTOCOL_TCP == ip1->protocol)
+	    {
+	      clamped1 = mssc_mss_fixup (b[1], ip6_next_header (ip1),
+					 cm->max_mss6[sw_if_index1]);
+	    }
+	}
+
+      pkts_clamped += clamped0 + clamped1;
+
+      if (PREDICT_FALSE (node->flags & VLIB_NODE_FLAG_TRACE))
+	{
+	  if (b[0]->flags & VLIB_BUFFER_IS_TRACED)
+	    {
+	      mssc_trace_t *t;
+
+	      t = vlib_add_trace (vm, node, b[0], sizeof (*t));
+	      t->max_mss = (FIB_PROTOCOL_IP4 == fproto) ?
+			     cm->max_mss4[sw_if_index0] :
+			     cm->max_mss6[sw_if_index0];
+	      t->clamped = clamped0;
+	    }
+	  if (b[1]->flags & VLIB_BUFFER_IS_TRACED)
+	    {
+	      mssc_trace_t *t;
+
+	      t = vlib_add_trace (vm, node, b[1], sizeof (*t));
+	      t->max_mss = (FIB_PROTOCOL_IP4 == fproto) ?
+			     cm->max_mss4[sw_if_index1] :
+			     cm->max_mss6[sw_if_index1];
+	      t->clamped = clamped1;
+	    }
+	}
+
+      b += 2;
+      next += 2;
+      n_left -= 2;
+    }
+
+  while (n_left > 0)
+    {
+      u32 sw_if_index0;
+      const u8 *h0;
+      u32 clamped0;
+
+      sw_if_index0 = vnet_buffer (b[0])->sw_if_index[dir];
+      clamped0 = 0;
+
+      /* speculatively enqueue b0 to the current next frame */
+      vnet_feature_next_u16 (&next[0], b[0]);
+
+      h0 = (u8 *) vlib_buffer_get_current (b[0]);
+      if (VLIB_TX == dir)
+	h0 += vnet_buffer (b[0])->ip.save_rewrite_length;
+
+      if (FIB_PROTOCOL_IP4 == fproto)
+	{
+	  ip4_header_t *ip0 = (ip4_header_t *) h0;
+
+	  if (IP_PROTOCOL_TCP == ip0->protocol)
+	    {
+	      clamped0 = mssc_mss_fixup (b[0], ip4_next_header (ip0),
+					 cm->max_mss4[sw_if_index0]);
+	    }
+	}
+      else if (FIB_PROTOCOL_IP6 == fproto)
+	{
+	  ip6_header_t *ip0 = (ip6_header_t *) h0;
+
+	  if (IP_PROTOCOL_TCP == ip0->protocol)
+	    {
+	      clamped0 = mssc_mss_fixup (b[0], ip6_next_header (ip0),
+					 cm->max_mss6[sw_if_index0]);
+	    }
+	}
+
+      pkts_clamped += clamped0;
+
+      if (PREDICT_FALSE ((node->flags & VLIB_NODE_FLAG_TRACE) &&
+			 (b[0]->flags & VLIB_BUFFER_IS_TRACED)))
+	{
+	  mssc_trace_t *t;
+
+	  t = vlib_add_trace (vm, node, b[0], sizeof (*t));
+	  t->max_mss = (FIB_PROTOCOL_IP4 == fproto) ?
+			 cm->max_mss4[sw_if_index0] :
+			 cm->max_mss6[sw_if_index0];
+	  t->clamped = clamped0;
+	}
+
+      b++;
+      next++;
+      n_left--;
+    }
+
+  vlib_buffer_enqueue_to_next (vm, node, from, nexts, frame->n_vectors);
+  vlib_node_increment_counter (vm, node->node_index, MSS_CLAMP_ERROR_CLAMPED,
+			       pkts_clamped);
+
+  return frame->n_vectors;
+}
+
+static uword
+mssc_ip4_in (vlib_main_t *vm, vlib_node_runtime_t *node, vlib_frame_t *frame)
+{
+  return (mssc_inline (vm, node, frame, VLIB_RX, FIB_PROTOCOL_IP4));
+}
+
+static uword
+mssc_ip4_out (vlib_main_t *vm, vlib_node_runtime_t *node, vlib_frame_t *frame)
+{
+  return (mssc_inline (vm, node, frame, VLIB_TX, FIB_PROTOCOL_IP4));
+}
+
+static uword
+mssc_ip6_in (vlib_main_t *vm, vlib_node_runtime_t *node, vlib_frame_t *frame)
+{
+  return (mssc_inline (vm, node, frame, VLIB_RX, FIB_PROTOCOL_IP6));
+}
+
+static uword
+mssc_ip6_out (vlib_main_t *vm, vlib_node_runtime_t *node, vlib_frame_t *frame)
+{
+  return (mssc_inline (vm, node, frame, VLIB_TX, FIB_PROTOCOL_IP6));
+}
+
+VLIB_REGISTER_NODE (mssc_ip4_in_node) =
+{
+  .function = mssc_ip4_in,
+  .name = "tcp-mss-clamping-ip4-in",
+  .vector_size = sizeof (u32),
+  .format_trace = format_mssc_trace,
+  .type = VLIB_NODE_TYPE_INTERNAL,
+
+  .n_errors = MSS_CLAMP_N_ERROR,
+  .error_counters = mss_clamp_error_counters,
+
+  .n_next_nodes = MSSC_N_NEXT,
+  .next_nodes = {
+        [MSSC_NEXT_DROP] = "error-drop",
+  },
+};
+
+VLIB_REGISTER_NODE (mssc_ip4_out_node) =
+{
+  .function = mssc_ip4_out,
+  .name = "tcp-mss-clamping-ip4-out",
+  .vector_size = sizeof (u32),
+  .format_trace = format_mssc_trace,
+  .type = VLIB_NODE_TYPE_INTERNAL,
+
+  .n_errors = MSS_CLAMP_N_ERROR,
+  .error_counters = mss_clamp_error_counters,
+
+  .n_next_nodes = MSSC_N_NEXT,
+  .next_nodes = {
+        [MSSC_NEXT_DROP] = "error-drop",
+  },
+};
+
+VLIB_REGISTER_NODE (mssc_ip6_in_node) =
+{
+  .function = mssc_ip6_in,
+  .name = "tcp-mss-clamping-ip6-in",
+  .vector_size = sizeof (u32),
+  .format_trace = format_mssc_trace,
+  .type = VLIB_NODE_TYPE_INTERNAL,
+
+  .n_errors = MSS_CLAMP_N_ERROR,
+  .error_counters = mss_clamp_error_counters,
+
+  .n_next_nodes = MSSC_N_NEXT,
+  .next_nodes = {
+        [MSSC_NEXT_DROP] = "error-drop",
+  },
+};
+
+VLIB_REGISTER_NODE (mssc_ip6_out_node) =
+{
+  .function = mssc_ip6_out,
+  .name = "tcp-mss-clamping-ip6-out",
+  .vector_size = sizeof (u32),
+  .format_trace = format_mssc_trace,
+  .type = VLIB_NODE_TYPE_INTERNAL,
+
+  .n_errors = MSS_CLAMP_N_ERROR,
+  .error_counters = mss_clamp_error_counters,
+
+  .n_next_nodes = MSSC_N_NEXT,
+  .next_nodes = {
+        [MSSC_NEXT_DROP] = "error-drop",
+  },
+};
+
+VNET_FEATURE_INIT (mssc_ip4_in_feat, static) = {
+  .arc_name = "ip4-unicast",
+  .node_name = "tcp-mss-clamping-ip4-in",
+  .runs_after = VNET_FEATURES ("ip4-policer-classify"),
+};
+
+VNET_FEATURE_INIT (mssc_ip4_out_feat, static) = {
+  .arc_name = "ip4-output",
+  .node_name = "tcp-mss-clamping-ip4-out",
+};
+
+VNET_FEATURE_INIT (mssc_ip6_in_feat, static) = {
+  .arc_name = "ip6-unicast",
+  .node_name = "tcp-mss-clamping-ip6-in",
+  .runs_after = VNET_FEATURES ("ip6-policer-classify"),
+};
+
+VNET_FEATURE_INIT (mssc_ip6_out_feat, static) = {
+  .arc_name = "ip6-output",
+  .node_name = "tcp-mss-clamping-ip6-out",
+};
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
diff --git a/src/plugins/mss_clamp/test/test_mss_clamp.py b/src/plugins/mss_clamp/test/test_mss_clamp.py
new file mode 100644
index 00000000000..23495b6050b
--- /dev/null
+++ b/src/plugins/mss_clamp/test/test_mss_clamp.py
@@ -0,0 +1,295 @@
+#!/usr/bin/env python3
+
+import unittest
+
+from framework import VppTestCase, VppTestRunner
+
+from scapy.layers.inet import IP, TCP
+from scapy.layers.inet6 import IPv6
+from scapy.layers.l2 import Ether
+from scapy.packet import Raw
+
+
+class TestMSSClamp(VppTestCase):
+    """ TCP MSS Clamping Test Case """
+
+    def setUp(self):
+        super(TestMSSClamp, self).setUp()
+
+        # create 2 pg interfaces
+        self.create_pg_interfaces(range(2))
+
+        for i in self.pg_interfaces:
+            i.admin_up()
+            i.config_ip4()
+            i.resolve_arp()
+            i.config_ip6()
+            i.resolve_ndp()
+
+    def tearDown(self):
+        for i in self.pg_interfaces:
+            i.unconfig_ip4()
+            i.unconfig_ip6()
+            i.admin_down()
+        super(TestMSSClamp, self).tearDown()
+
+    def verify_pkt(self, rx, expected_mss):
+        # check that the MSS size equals the expected value
+        # and the IP and TCP checksums are correct
+        tcp = rx[TCP]
+        tcp_csum = tcp.chksum
+        del tcp.chksum
+        ip_csum = 0
+        if (rx.haslayer(IP)):
+            ip_csum = rx[IP].chksum
+            del rx[IP].chksum
+
+        opt = tcp.options
+        self.assertEqual(opt[0][0], 'MSS')
+        self.assertEqual(opt[0][1], expected_mss)
+        # recalculate checksums
+        rx = rx.__class__(bytes(rx))
+        tcp = rx[TCP]
+        self.assertEqual(tcp_csum, tcp.chksum)
+        if (rx.haslayer(IP)):
+            self.assertEqual(ip_csum, rx[IP].chksum)
+
+    def send_and_verify_ip4(self, src_pg, dst_pg, mss, expected_mss):
+        # IPv4 TCP packet with the requested MSS option.
+        # from a host on src_pg to a host on dst_pg.
+        p = (Ether(dst=src_pg.local_mac,
+                   src=src_pg.remote_mac) /
+             IP(src=src_pg.remote_ip4,
+                dst=dst_pg.remote_ip4) /
+             TCP(sport=1234, dport=1234,
+                 flags="S",
+                 options=[('MSS', (mss)), ('EOL', None)]) /
+             Raw('\xa5' * 100))
+
+        rxs = self.send_and_expect(src_pg, p * 65, dst_pg)
+
+        for rx in rxs:
+            self.verify_pkt(rx, expected_mss)
+
+    def send_and_verify_ip6(self, src_pg, dst_pg, mss, expected_mss):
+        #
+        # IPv6 TCP packet with the requested MSS option.
+        # from a host on src_pg to a host on dst_pg.
+        #
+        p = (Ether(dst=src_pg.local_mac,
+                   src=src_pg.remote_mac) /
+             IPv6(src=src_pg.remote_ip6,
+                  dst=dst_pg.remote_ip6) /
+             TCP(sport=1234, dport=1234,
+                 flags="S",
+                 options=[('MSS', (mss)), ('EOL', None)]) /
+             Raw('\xa5' * 100))
+
+        rxs = self.send_and_expect(src_pg, p * 65, dst_pg)
+
+        for rx in rxs:
+            self.verify_pkt(rx, expected_mss)
+
+    def test_tcp_mss_clamping_ip4_tx(self):
+        """ IP4 TCP MSS Clamping TX """
+
+        # enable the TCP MSS clamping feature to lower the MSS to 1424.
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1424, ipv6_mss=0,
+                                           ipv4_direction=3, ipv6_direction=0)
+
+        # Verify that the feature is enabled.
+        rv, reply = self.vapi.mss_clamp_get(sw_if_index=self.pg1.sw_if_index)
+        self.assertEqual(reply[0].ipv4_mss, 1424)
+        self.assertEqual(reply[0].ipv4_direction, 3)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip4(self.pg0, self.pg1, 1460, 1424)
+
+        # check the stats
+        stats = self.statistics.get_counter(
+            '/err/tcp-mss-clamping-ip4-out/clamped')
+        self.assertEqual(sum(stats), 65)
+
+        # Send syn packets with small enough MSS values and verify they are
+        # unchanged.
+        self.send_and_verify_ip4(self.pg0, self.pg1, 1400, 1400)
+
+        # enable the the feature only in TX direction
+        # and change the max MSS value
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1420, ipv6_mss=0,
+                                           ipv4_direction=2, ipv6_direction=0)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip4(self.pg0, self.pg1, 1460, 1420)
+
+        # enable the the feature only in RX direction
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1424, ipv6_mss=0,
+                                           ipv4_direction=1, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip4(self.pg0, self.pg1, 1460, 1460)
+
+        # disable the feature
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=0,
+                                           ipv4_direction=0, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip4(self.pg0, self.pg1, 1460, 1460)
+
+    def test_tcp_mss_clamping_ip4_rx(self):
+        """ IP4 TCP MSS Clamping RX """
+
+        # enable the TCP MSS clamping feature to lower the MSS to 1424.
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1424, ipv6_mss=0,
+                                           ipv4_direction=3, ipv6_direction=0)
+
+        # Verify that the feature is enabled.
+        rv, reply = self.vapi.mss_clamp_get(sw_if_index=self.pg1.sw_if_index)
+        self.assertEqual(reply[0].ipv4_mss, 1424)
+        self.assertEqual(reply[0].ipv4_direction, 3)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip4(self.pg1, self.pg0, 1460, 1424)
+
+        # check the stats
+        stats = self.statistics.get_counter(
+            '/err/tcp-mss-clamping-ip4-in/clamped')
+        self.assertEqual(sum(stats), 65)
+
+        # Send syn packets with small enough MSS values and verify they are
+        # unchanged.
+        self.send_and_verify_ip4(self.pg1, self.pg0, 1400, 1400)
+
+        # enable the the feature only in RX direction
+        # and change the max MSS value
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1420, ipv6_mss=0,
+                                           ipv4_direction=1, ipv6_direction=0)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip4(self.pg1, self.pg0, 1460, 1420)
+
+        # enable the the feature only in TX direction
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=1424, ipv6_mss=0,
+                                           ipv4_direction=2, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip4(self.pg1, self.pg0, 1460, 1460)
+
+        # disable the feature
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=0,
+                                           ipv4_direction=0, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip4(self.pg1, self.pg0, 1460, 1460)
+
+    def test_tcp_mss_clamping_ip6_tx(self):
+        """ IP6 TCP MSS Clamping TX """
+
+        # enable the TCP MSS clamping feature to lower the MSS to 1424.
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1424,
+                                           ipv4_direction=0, ipv6_direction=3)
+
+        # Verify that the feature is enabled.
+        rv, reply = self.vapi.mss_clamp_get(sw_if_index=self.pg1.sw_if_index)
+        self.assertEqual(reply[0].ipv6_mss, 1424)
+        self.assertEqual(reply[0].ipv6_direction, 3)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip6(self.pg0, self.pg1, 1460, 1424)
+
+        # check the stats
+        stats = self.statistics.get_counter(
+            '/err/tcp-mss-clamping-ip6-out/clamped')
+        self.assertEqual(sum(stats), 65)
+
+        # Send syn packets with small enough MSS values and verify they are
+        # unchanged.
+        self.send_and_verify_ip6(self.pg0, self.pg1, 1400, 1400)
+
+        # enable the the feature only in TX direction
+        # and change the max MSS value
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1420,
+                                           ipv4_direction=0, ipv6_direction=2)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip6(self.pg0, self.pg1, 1460, 1420)
+
+        # enable the the feature only in RX direction
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1424,
+                                           ipv4_direction=0, ipv6_direction=1)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip6(self.pg0, self.pg1, 1460, 1460)
+
+        # disable the feature
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=0,
+                                           ipv4_direction=0, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip6(self.pg0, self.pg1, 1460, 1460)
+
+    def test_tcp_mss_clamping_ip6_rx(self):
+        """ IP6 TCP MSS Clamping RX """
+
+        # enable the TCP MSS clamping feature to lower the MSS to 1424.
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1424,
+                                           ipv4_direction=0, ipv6_direction=3)
+
+        # Verify that the feature is enabled.
+        rv, reply = self.vapi.mss_clamp_get(sw_if_index=self.pg1.sw_if_index)
+        self.assertEqual(reply[0].ipv6_mss, 1424)
+        self.assertEqual(reply[0].ipv6_direction, 3)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip6(self.pg1, self.pg0, 1460, 1424)
+
+        # check the stats
+        stats = self.statistics.get_counter(
+            '/err/tcp-mss-clamping-ip6-in/clamped')
+        self.assertEqual(sum(stats), 65)
+
+        # Send syn packets with small enough MSS values and verify they are
+        # unchanged.
+        self.send_and_verify_ip6(self.pg1, self.pg0, 1400, 1400)
+
+        # enable the the feature only in RX direction
+        # and change the max MSS value
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1420,
+                                           ipv4_direction=0, ipv6_direction=1)
+
+        # Send syn packets and verify that the MSS value is lowered.
+        self.send_and_verify_ip6(self.pg1, self.pg0, 1460, 1420)
+
+        # enable the the feature only in TX direction
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=1424,
+                                           ipv4_direction=0, ipv6_direction=2)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip6(self.pg1, self.pg0, 1460, 1460)
+
+        # disable the feature
+        self.vapi.mss_clamp_enable_disable(self.pg1.sw_if_index,
+                                           ipv4_mss=0, ipv6_mss=0,
+                                           ipv4_direction=0, ipv6_direction=0)
+
+        # Send the packets again and ensure they are unchanged.
+        self.send_and_verify_ip6(self.pg1, self.pg0, 1460, 1460)
+
+
+if __name__ == '__main__':
+    unittest.main(testRunner=VppTestRunner)