datapath: Consider tunnels to have no MTU, fixing jumbo frame support.

Until now, tunnel vports have had a specific MTU, in the same way that
ordinary network devices have an MTU, but treating them this way does not
always make sense.  For example, consider a datapath that has three ports:
the local port, a GRE tunnel to another host, and a physical port.  If
the physical port is configured with a jumbo MTU, it should be possible to
send jumbo packets across the tunnel: the tunnel can do fragmentation or
the physical port traversed by the tunnel might have a jumbo MTU.

However, until now, tunnels always had a 1500-byte MTU by default.  It
could be adjusted using ODP_VPORT_MTU_SET, but nothing actually did this.
One alternative would be to make ovs-vswitchd able to set the vport's MTU.
This commit, however, takes a different approach, of dropping the concept
of MTU entirely for tunnel vports.  This also solves the problem described
above, without making any additional work for anyone.

I tested that, without this change, I could not send 1600-byte "pings"
between two machines whose NICs had 2000-byte MTUs that were connected to
vswitches that were in turn connected over GRE tunnels with the default
1500-byte MTU.  With this change, it worked OK, regardless of the MTU of
the network traversed by the GRE tunnel.

This patch also makes "patch" ports MTU-less.

It might make sense to remove vport_set_mtu() and the associated callback
now, since ordinary network devices are the only vports that support it
now.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Suggested-by: Jesse Gross <jesse@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Bug #3728.

diff --git a/datapath/datapath.c b/datapath/datapath.c
index 74f276b556c269e4c2a98a4dd24282bc3f3d4ba7..ba32e37fae9dbb91797e7de4af6266b0724f5d12 100644 (file)
--- a/datapath/datapath.c
+++ b/datapath/datapath.c
@@ -773,6 +773,8 @@ int dp_min_mtu(const struct datapath *dp)
                        continue;
 
                dev_mtu = vport_get_mtu(p);
+               if (!dev_mtu)
+                       continue;
                if (!mtu || dev_mtu < mtu)
                        mtu = dev_mtu;
        }
@@ -1582,6 +1584,7 @@ static int odp_vport_cmd_fill_info(struct vport *vport, struct sk_buff *skb,
        struct odp_header *odp_header;
        struct nlattr *nla;
        int ifindex, iflink;
+       int mtu;
        int err;
 
        odp_header = genlmsg_put(skb, pid, seq, &dp_vport_genl_family,
@@ -1603,7 +1606,9 @@ static int odp_vport_cmd_fill_info(struct vport *vport, struct sk_buff *skb,
 
        NLA_PUT(skb, ODP_VPORT_ATTR_ADDRESS, ETH_ALEN, vport_get_addr(vport));
 
-       NLA_PUT_U32(skb, ODP_VPORT_ATTR_MTU, vport_get_mtu(vport));
+       mtu = vport_get_mtu(vport);
+       if (mtu)
+               NLA_PUT_U32(skb, ODP_VPORT_ATTR_MTU, mtu);
 
        err = vport_get_options(vport, skb);
        if (err == -EMSGSIZE)

diff --git a/datapath/tunnel.c b/datapath/tunnel.c
index bd3a9e0a8b06258deab8d72166ba539a074a3fe9..6d0e7b9c603b35e0e63753ee31a223cf7de3a049 100644 (file)
--- a/datapath/tunnel.c
+++ b/datapath/tunnel.c
@@ -663,7 +663,6 @@ bool tnl_frag_needed(struct vport *vport, const struct tnl_mutable_config *mutab
        }
 #endif
 
-       total_length = min(total_length, mutable->mtu);
        payload_length = total_length - header_length;
 
        nskb = dev_alloc_skb(NET_IP_ALIGN + eth_hdr_len + header_length +
@@ -1428,7 +1427,6 @@ struct vport *tnl_create(const struct vport_parms *parms,
        }
 
        vport_gen_rand_ether_addr(mutable->eth_addr);
-       mutable->mtu = ETH_DATA_LEN;
 
        get_random_bytes(&initial_frag_id, sizeof(int));
        atomic_set(&tnl_vport->frag_id, initial_frag_id);
@@ -1477,7 +1475,6 @@ int tnl_set_options(struct vport *vport, struct nlattr *options)
        old_mutable = rtnl_dereference(tnl_vport->mutable);
        mutable->seq = old_mutable->seq + 1;
        memcpy(mutable->eth_addr, old_mutable->eth_addr, ETH_ALEN);
-       mutable->mtu = old_mutable->mtu;
 
        /* Parse the others configured by userspace. */
        err = tnl_set_config(options, tnl_vport->tnl_ops, vport, mutable);
@@ -1548,22 +1545,6 @@ int tnl_destroy(struct vport *vport)
        return 0;
 }
 
-int tnl_set_mtu(struct vport *vport, int mtu)
-{
-       struct tnl_vport *tnl_vport = tnl_vport_priv(vport);
-       struct tnl_mutable_config *mutable;
-
-       mutable = kmemdup(rtnl_dereference(tnl_vport->mutable),
-                         sizeof(struct tnl_mutable_config), GFP_KERNEL);
-       if (!mutable)
-               return -ENOMEM;
-
-       mutable->mtu = mtu;
-       assign_config_rcu(vport, mutable);
-
-       return 0;
-}
-
 int tnl_set_addr(struct vport *vport, const unsigned char *addr)
 {
        struct tnl_vport *tnl_vport = tnl_vport_priv(vport);
@@ -1592,12 +1573,6 @@ const unsigned char *tnl_get_addr(const struct vport *vport)
        return rcu_dereference_rtnl(tnl_vport->mutable)->eth_addr;
 }
 
-int tnl_get_mtu(const struct vport *vport)
-{
-       const struct tnl_vport *tnl_vport = tnl_vport_priv(vport);
-       return rcu_dereference_rtnl(tnl_vport->mutable)->mtu;
-}
-
 void tnl_free_linked_skbs(struct sk_buff *skb)
 {
        if (unlikely(!skb))

diff --git a/datapath/tunnel.h b/datapath/tunnel.h
index c3bc8ddc12cabb33cca3cd54d6c89c18561fa251..6ce9c468aefc94530e9f5d2bcb6bb04177206c88 100644 (file)
--- a/datapath/tunnel.h
+++ b/datapath/tunnel.h
@@ -52,7 +52,6 @@
  * @tunnel_hlen: Tunnel header length.
  * @eth_addr: Source address for packets generated by tunnel itself
  * (e.g. ICMP fragmentation needed messages).
- * @mtu: MTU of tunnel.
  * @in_key: Key to match on input, 0 for wildcard.
  * @out_key: Key to use on output, 0 if this tunnel has no fixed output key.
  * @flags: TNL_F_* flags.
@@ -70,7 +69,6 @@ struct tnl_mutable_config {
        unsigned tunnel_hlen;
 
        unsigned char eth_addr[ETH_ALEN];
-       unsigned mtu;
 
        /* Configured via ODP_TUNNEL_ATTR_* attributes. */
        __be64  in_key;
@@ -223,11 +221,9 @@ int tnl_destroy(struct vport *);
 int tnl_set_options(struct vport *, struct nlattr *);
 int tnl_get_options(const struct vport *, struct sk_buff *);
 
-int tnl_set_mtu(struct vport *vport, int mtu);
 int tnl_set_addr(struct vport *vport, const unsigned char *addr);
 const char *tnl_get_name(const struct vport *vport);
 const unsigned char *tnl_get_addr(const struct vport *vport);
-int tnl_get_mtu(const struct vport *vport);
 int tnl_send(struct vport *vport, struct sk_buff *skb);
 void tnl_rcv(struct vport *vport, struct sk_buff *skb);
 

diff --git a/datapath/vport-capwap.c b/datapath/vport-capwap.c
index 4fb43933021ad4e93c0b6fa354fdfb3d909fcfd9..65f1f1bdda515564e8e34ffd814e49a7bfa98306 100644 (file)
--- a/datapath/vport-capwap.c
+++ b/datapath/vport-capwap.c
@@ -654,7 +654,6 @@ const struct vport_ops capwap_vport_ops = {
        .exit           = capwap_exit,
        .create         = capwap_create,
        .destroy        = tnl_destroy,
-       .set_mtu        = tnl_set_mtu,
        .set_addr       = tnl_set_addr,
        .get_name       = tnl_get_name,
        .get_addr       = tnl_get_addr,
@@ -663,7 +662,6 @@ const struct vport_ops capwap_vport_ops = {
        .get_dev_flags  = vport_gen_get_dev_flags,
        .is_running     = vport_gen_is_running,
        .get_operstate  = vport_gen_get_operstate,
-       .get_mtu        = tnl_get_mtu,
        .send           = tnl_send,
 };
 

diff --git a/datapath/vport-gre.c b/datapath/vport-gre.c
index fc489e9252b632eecd144aed981afad4255399b2..0f45f8f705027bf2239045df62eb900c7391c99c 100644 (file)
--- a/datapath/vport-gre.c
+++ b/datapath/vport-gre.c
@@ -391,7 +391,6 @@ const struct vport_ops gre_vport_ops = {
        .exit           = gre_exit,
        .create         = gre_create,
        .destroy        = tnl_destroy,
-       .set_mtu        = tnl_set_mtu,
        .set_addr       = tnl_set_addr,
        .get_name       = tnl_get_name,
        .get_addr       = tnl_get_addr,
@@ -400,6 +399,5 @@ const struct vport_ops gre_vport_ops = {
        .get_dev_flags  = vport_gen_get_dev_flags,
        .is_running     = vport_gen_is_running,
        .get_operstate  = vport_gen_get_operstate,
-       .get_mtu        = tnl_get_mtu,
        .send           = tnl_send,
 };

diff --git a/datapath/vport-patch.c b/datapath/vport-patch.c
index 7dfea6ba68d7ed0757b2d99c99df60d5b4977849..1c4d2c5e2d3b90942d27ebdc2cf457f2e651390c 100644 (file)
--- a/datapath/vport-patch.c
+++ b/datapath/vport-patch.c
@@ -20,7 +20,6 @@ struct patch_config {
 
        char peer_name[IFNAMSIZ];
        unsigned char eth_addr[ETH_ALEN];
-       unsigned int mtu;
 };
 
 struct patch_vport {
@@ -148,10 +147,6 @@ static struct vport *patch_create(const struct vport_parms *parms)
 
        vport_gen_rand_ether_addr(patchconf->eth_addr);
 
-       /* Make the default MTU fairly large so that it doesn't become the
-        * bottleneck on systems using jumbo frames. */
-       patchconf->mtu = 65535;
-
        rcu_assign_pointer(patch_vport->patchconf, patchconf);
 
        peer_name = patchconf->peer_name;
@@ -236,22 +231,6 @@ static void update_peers(const char *name, struct vport *vport)
        }
 }
 
-static int patch_set_mtu(struct vport *vport, int mtu)
-{
-       struct patch_vport *patch_vport = patch_vport_priv(vport);
-       struct patch_config *patchconf;
-
-       patchconf = kmemdup(rtnl_dereference(patch_vport->patchconf),
-                         sizeof(struct patch_config), GFP_KERNEL);
-       if (!patchconf)
-               return -ENOMEM;
-
-       patchconf->mtu = mtu;
-       assign_config_rcu(vport, patchconf);
-
-       return 0;
-}
-
 static int patch_set_addr(struct vport *vport, const unsigned char *addr)
 {
        struct patch_vport *patch_vport = patch_vport_priv(vport);
@@ -289,12 +268,6 @@ static int patch_get_options(const struct vport *vport, struct sk_buff *skb)
        return nla_put_string(skb, ODP_PATCH_ATTR_PEER, patchconf->peer_name);
 }
 
-static int patch_get_mtu(const struct vport *vport)
-{
-       const struct patch_vport *patch_vport = patch_vport_priv(vport);
-       return rcu_dereference_rtnl(patch_vport->patchconf)->mtu;
-}
-
 static int patch_send(struct vport *vport, struct sk_buff *skb)
 {
        struct patch_vport *patch_vport = patch_vport_priv(vport);
@@ -319,7 +292,6 @@ const struct vport_ops patch_vport_ops = {
        .exit           = patch_exit,
        .create         = patch_create,
        .destroy        = patch_destroy,
-       .set_mtu        = patch_set_mtu,
        .set_addr       = patch_set_addr,
        .get_name       = patch_get_name,
        .get_addr       = patch_get_addr,
@@ -328,6 +300,5 @@ const struct vport_ops patch_vport_ops = {
        .get_dev_flags  = vport_gen_get_dev_flags,
        .is_running     = vport_gen_is_running,
        .get_operstate  = vport_gen_get_operstate,
-       .get_mtu        = patch_get_mtu,
        .send           = patch_send,
 };

diff --git a/datapath/vport.c b/datapath/vport.c
index 852a3c69f5f00dbb15cfcdfecdec3e81333503f9..8ef96f73e1fc6ea7e77cb1881d5617e0be93ab85 100644 (file)
--- a/datapath/vport.c
+++ b/datapath/vport.c
@@ -605,12 +605,14 @@ int vport_get_iflink(const struct vport *vport)
  *
  * @vport: vport from which to retrieve MTU
  *
- * Retrieves the MTU of the given device.
- *
- * Must be called with RTNL lock or rcu_read_lock.
+ * Retrieves the MTU of the given device.  Returns 0 if @vport does not have an
+ * MTU (as e.g. some tunnels do not).  Either RTNL lock or rcu_read_lock must
+ * be held.
  */
 int vport_get_mtu(const struct vport *vport)
 {
+       if (!vport->ops->get_mtu)
+               return 0;
        return vport->ops->get_mtu(vport);
 }
 
@@ -710,7 +712,7 @@ int vport_send(struct vport *vport, struct sk_buff *skb)
        int sent;
 
        mtu = vport_get_mtu(vport);
-       if (unlikely(packet_length(skb) > mtu && !skb_is_gso(skb))) {
+       if (mtu && unlikely(packet_length(skb) > mtu && !skb_is_gso(skb))) {
                if (net_ratelimit())
                        pr_warn("%s: dropped over-mtu packet: %d > %d\n",
                                dp_name(vport->dp), packet_length(skb), mtu);

diff --git a/datapath/vport.h b/datapath/vport.h
index ee3b127d4f0b46f323c8aae40a820f5459529c8f..8e70762853f4ce0217a6ee32b9d58a36ff3db0f3 100644 (file)
--- a/datapath/vport.h
+++ b/datapath/vport.h
@@ -175,7 +175,8 @@ struct vport_parms {
  * @get_iflink: Get the system interface index associated with the device that
  * will be used to send packets (may be different than ifindex for tunnels).
  * May be null if the device does not have an iflink.
- * @get_mtu: Get the device's MTU.
+ * @get_mtu: Get the device's MTU.  May be %NULL if the device does not have an
+ * MTU (as e.g. some tunnels do not).
  * @send: Send a packet on the device.  Returns the length of the packet sent.
  */
 struct vport_ops {

diff --git a/include/openvswitch/datapath-protocol.h b/include/openvswitch/datapath-protocol.h
index 8645096ab816e6014ede682ea0286e7fc3f7a08e..eafc80032b59717d14468a0b3a747147ab49cedf 100644 (file)
--- a/include/openvswitch/datapath-protocol.h
+++ b/include/openvswitch/datapath-protocol.h
@@ -237,7 +237,8 @@ enum odp_vport_cmd {
  * @ODP_VPORT_ATTR_STATS: A &struct rtnl_link_stats64 giving statistics for
  * packets sent or received through the vport.
  * @ODP_VPORT_ATTR_ADDRESS: A 6-byte Ethernet address for the vport.
- * @ODP_VPORT_ATTR_MTU: MTU for the vport.
+ * @ODP_VPORT_ATTR_MTU: MTU for the vport.  Omitted if the vport does not have
+ * an MTU as, e.g., some tunnels do not.
  * @ODP_VPORT_ATTR_IFINDEX: ifindex of the underlying network device, if any.
  * @ODP_VPORT_ATTR_IFLINK: ifindex of the device on which packets are sent (for
  * tunnels), if any.

diff --git a/lib/dhcp-client.c b/lib/dhcp-client.c
index b9ebefe09b7aa9600a3636228e4b44f5bacc1fad..e59dc34c0ad460157d1e95e9ddffa4354709aefe 100644 (file)
--- a/lib/dhcp-client.c
+++ b/lib/dhcp-client.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010, 2011 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -934,10 +934,8 @@ do_receive_msg(struct dhclient *cli, struct dhcp_msg *msg)
     const char *cli_name = dhclient_get_name(cli);
     uint8_t cli_mac[ETH_ADDR_LEN];
     struct ofpbuf b;
-    int mtu;
 
-    netdev_get_mtu(cli->netdev, &mtu);
-    ofpbuf_init(&b, mtu + VLAN_ETH_HEADER_LEN);
+    ofpbuf_init(&b, ETH_TOTAL_MAX + VLAN_ETH_HEADER_LEN);
     netdev_get_etheraddr(cli->netdev, cli_mac);
     for (; cli->received < 50; cli->received++) {
         const struct ip_header *ip;

diff --git a/lib/dpif-linux.c b/lib/dpif-linux.c
index f1d42dc230275351bba8cbf5a73d4ca58ad1b155..509174c869d1ce18b5ad9fd759374450742677f1 100644 (file)
--- a/lib/dpif-linux.c
+++ b/lib/dpif-linux.c
@@ -1108,6 +1108,8 @@ dpif_linux_vport_from_ofpbuf(struct dpif_linux_vport *vport,
     }
     if (a[ODP_VPORT_ATTR_MTU]) {
         vport->mtu = nl_attr_get_u32(a[ODP_VPORT_ATTR_MTU]);
+    } else {
+        vport->mtu = INT_MAX;
     }
     if (a[ODP_VPORT_ATTR_OPTIONS]) {
         vport->options = nl_attr_get(a[ODP_VPORT_ATTR_OPTIONS]);
@@ -1158,7 +1160,7 @@ dpif_linux_vport_to_ofpbuf(const struct dpif_linux_vport *vport,
                           vport->address, ETH_ADDR_LEN);
     }
 
-    if (vport->mtu) {
+    if (vport->mtu && vport->mtu != INT_MAX) {
         nl_msg_put_u32(buf, ODP_VPORT_ATTR_MTU, vport->mtu);
     }
 

diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index acc14a8b251b0c8244596ef92007c5449ca0bfbc..93dd1cde99a89d1e36865524dde42d9aced45ec2 100644 (file)
--- a/lib/dpif-netdev.c
+++ b/lib/dpif-netdev.c
@@ -375,7 +375,7 @@ do_add_port(struct dp_netdev *dp, const char *devname, const char *type,
     port->internal = internal;
 
     netdev_get_mtu(netdev, &mtu);
-    if (mtu > max_mtu) {
+    if (mtu != INT_MAX && mtu > max_mtu) {
         max_mtu = mtu;
     }
 

diff --git a/lib/netdev-linux.c b/lib/netdev-linux.c
index 9deea57da6408e1f9734d8cbdaf7e8d94fb53849..1428ce6617fc76647ae60d4b105e1a3aa1966a56 100644 (file)
--- a/lib/netdev-linux.c
+++ b/lib/netdev-linux.c
@@ -2365,6 +2365,11 @@ htb_setup_class__(struct netdev *netdev, unsigned int handle,
     int mtu;
 
     netdev_get_mtu(netdev, &mtu);
+    if (mtu == INT_MAX) {
+        VLOG_WARN_RL(&rl, "cannot set up HTB on device %s that lacks MTU",
+                     netdev_get_name(netdev));
+        return EINVAL;
+    }
 
     memset(&opt, 0, sizeof opt);
     tc_fill_rate(&opt.rate, class->min_rate, mtu);
@@ -2484,6 +2489,13 @@ htb_parse_class_details__(struct netdev *netdev,
     const char *priority_s = shash_find_data(details, "priority");
     int mtu;
 
+    netdev_get_mtu(netdev, &mtu);
+    if (mtu == INT_MAX) {
+        VLOG_WARN_RL(&rl, "cannot parse HTB class on device %s that lacks MTU",
+                     netdev_get_name(netdev));
+        return EINVAL;
+    }
+
     /* min-rate.  Don't allow a min-rate below 1500 bytes/s. */
     if (!min_rate_s) {
         /* min-rate is required. */
@@ -2509,7 +2521,6 @@ htb_parse_class_details__(struct netdev *netdev,
      * doesn't include the Ethernet header, we need to add at least 14 (18?) to
      * the MTU.  We actually add 64, instead of 14, as a guard against
      * additional headers get tacked on somewhere that we're not aware of. */
-    netdev_get_mtu(netdev, &mtu);
     hc->burst = burst_s ? strtoull(burst_s, NULL, 10) / 8 : 0;
     hc->burst = MAX(hc->burst, mtu + 64);
 

diff --git a/lib/netdev-provider.h b/lib/netdev-provider.h
index 4181758fa762a2c9c2e3837ed5f5a15a37a95d9b..be51c48a541f8d94d3847edc7dbf79c8a159efa3 100644 (file)
--- a/lib/netdev-provider.h
+++ b/lib/netdev-provider.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009, 2010 Nicira Networks.
+ * Copyright (c) 2009, 2010, 2011 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -237,7 +237,10 @@ struct netdev_class {
      *
      * The MTU is the maximum size of transmitted (and received) packets, in
      * bytes, not including the hardware header; thus, this is typically 1500
-     * bytes for Ethernet devices.*/
+     * bytes for Ethernet devices.
+     *
+     * If 'netdev' does not have an MTU (e.g. as some tunnels do not), then
+     * this function should set '*mtup' to INT_MAX. */
     int (*get_mtu)(const struct netdev *netdev, int *mtup);
 
     /* Returns the ifindex of 'netdev', if successful, as a positive number.

diff --git a/lib/netdev.c b/lib/netdev.c
index 24b616a3df76092768a5e33574320ab7921523bb..f06742aecb441c67c4f052fc1b9376839cf0f8ca 100644 (file)
--- a/lib/netdev.c
+++ b/lib/netdev.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010, 2011 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -521,8 +521,9 @@ netdev_get_name(const struct netdev *netdev)
  * (and received) packets, in bytes, not including the hardware header; thus,
  * this is typically 1500 bytes for Ethernet devices.
  *
- * If successful, returns 0 and stores the MTU size in '*mtup'.  On failure,
- * returns a positive errno value and stores ETH_PAYLOAD_MAX (1500) in
+ * If successful, returns 0 and stores the MTU size in '*mtup'.  Stores INT_MAX
+ * in '*mtup' if 'netdev' does not have an MTU (as e.g. some tunnels do not).On
+ * failure, returns a positive errno value and stores ETH_PAYLOAD_MAX (1500) in
  * '*mtup'. */
 int
 netdev_get_mtu(const struct netdev *netdev, int *mtup)

diff --git a/vswitchd/bridge.c b/vswitchd/bridge.c
index 54830dcd9d6a8b2b128dd72166df7ecb748fc1c1..bb67e17fc91d041800e6d97d358e63bc9b89365a 100644 (file)
--- a/vswitchd/bridge.c
+++ b/vswitchd/bridge.c
@@ -1207,7 +1207,7 @@ iface_refresh_status(struct iface *iface)
                                     ? "up" : "down");
 
     error = netdev_get_mtu(iface->netdev, &mtu);
-    if (!error) {
+    if (!error && mtu != INT_MAX) {
         mtu_64 = mtu;
         ovsrec_interface_set_mtu(iface->cfg, &mtu_64, 1);
     }

diff --git a/vswitchd/vswitch.xml b/vswitchd/vswitch.xml
index bb7afe8861ef13977a0b9b65f55b6830b86938bb..f4515f4fe175f3f5b4b7f80a01eebb561305768a 100644 (file)
--- a/vswitchd/vswitch.xml
+++ b/vswitchd/vswitch.xml
@@ -1065,6 +1065,10 @@
           and many kinds of virtual interfaces can be configured with
           higher MTUs.
         </p>
+        <p>
+          This column will be empty for an interface that does not
+          have an MTU as, for example, some kinds of tunnels do not.
+        </p>
       </column>
 
       <column name="status">