ip: Path MTU

Type: feature

Support setting the MTU for a peer on an interface. The minimum value of
the path and interface MTU is used at forwarding time.

the path MTU is specified for a given peer, by address and table-ID.
In the forwarding plane the MTU is enfored either:
 1 - if the peer is attached, then the MTU is set on the peer's
adjacency
 2 - if the peer is not attached, it is remote, then a DPO is added to
the peer's FIB entry to perform the necessary fragmentation.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I8b9ea6a07868b50e97e2561f18d9335407dea7ae

1 files changed, 883 insertions, 0 deletions

diff --git a/src/vnet/ip/ip_path_mtu.c b/src/vnet/ip/ip_path_mtu.c
new file mode 100644
index 00000000000..38adb44065b
--- /dev/null
+++ b/src/vnet/ip/ip_path_mtu.c
@@ -0,0 +1,883 @@
+/*
+ *------------------------------------------------------------------
+ * ip_path_mtu.c
+ *
+ * Copyright (c) 2021 Graphiant.
+ * 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/ip/ip_path_mtu.h>
+#include <vnet/ip/ip_frag.h>
+#include <vnet/adj/adj_delegate.h>
+#include <vnet/adj/adj_nbr.h>
+#include <vnet/fib/fib_table.h>
+#include <vnet/fib/fib_entry_track.h>
+
+#include <vnet/dpo/drop_dpo.h>
+
+/**
+ * Path MTU
+ *
+ * A path is a peer. A peer is known by an IP address (in a table).
+ * Insert a DPO in the forwarding chain for the peer to perform the
+ * fragmentation.
+ * For attached peers, all traffic will use the peer's adjacency, there
+ * is already an MTU chekc in the adjacency (for the link's MTU) so as an
+ * optimisation, instead of using a DPO, we add a delegate to the adjacency
+ * to set the adjacency's MTU to the path MTU.
+ */
+
+/**
+ * the logger
+ */
+static vlib_log_class_t ip_pmtu_logger;
+
+static adj_delegate_type_t ip_pmtu_adj_delegate_type;
+static fib_source_t ip_pmtu_source;
+
+/**
+ * DPO pool
+ */
+ip_pmtu_dpo_t *ip_pmtu_dpo_pool;
+
+/**
+ * DPO type registered for these GBP FWD
+ */
+static dpo_type_t ip_pmtu_dpo_type;
+
+/**
+ * Fib node type for the tracker
+ */
+static fib_node_type_t ip_pmtu_fib_type;
+
+/**
+ * Path MTU tracker pool
+ */
+ip_pmtu_t *ip_pmtu_pool;
+
+/**
+ * Delegate added to adjacencies to track path MTU
+ */
+typedef struct ip_path_mtu_adj_delegate_t_
+{
+  u16 pmtu;
+} ip_path_mtu_adj_delegate_t;
+
+static ip_path_mtu_adj_delegate_t *ip_path_mtu_adj_delegate_pool;
+
+/* DB of all FIB PMTU settings */
+typedef struct ip_pmtu_key_t_
+{
+  ip46_address_t nh;
+  u32 table_id;
+  fib_protocol_t fproto;
+} __clib_packed ip_pmtu_key_t;
+
+static uword *ip_pmtu_db;
+
+#define IP_PMTU_TRKR_DBG(_ipt, _fmt, _args...)                                \
+  {                                                                           \
+    vlib_log_debug (ip_pmtu_logger, "[%U]: " _fmt ": ", format_ip_pmtu,       \
+		    _ipt - ip_pmtu_pool, ##_args);                            \
+  }
+#define IP_PMTU_DBG(_fmt, _args...)                                           \
+  {                                                                           \
+    vlib_log_debug (ip_pmtu_logger, _fmt ": ", ##_args);                      \
+  }
+
+static u8 *
+format_ip_pmtu_flags (u8 *s, va_list *ap)
+{
+  ip_pmtu_flags_t f = va_arg (*ap, ip_pmtu_flags_t);
+
+  if (0)
+    ;
+#define _(a, b, c) else if (f & IP_PMTU_FLAG_##a) s = format (s, "%s ", c);
+  foreach_ip_pmtu_flag
+#undef _
+
+    return (s);
+}
+
+u32
+ip_pmtu_get_table_id (const ip_pmtu_t *ipt)
+{
+  const fib_prefix_t *pfx;
+  u32 fib_index;
+
+  pfx = fib_entry_get_prefix (ipt->ipt_fib_entry);
+  fib_index = fib_entry_get_fib_index (ipt->ipt_fib_entry);
+
+  return (fib_table_get_table_id (fib_index, pfx->fp_proto));
+}
+
+void
+ip_pmtu_get_ip (const ip_pmtu_t *ipt, ip_address_t *ip)
+{
+  const fib_prefix_t *pfx;
+
+  pfx = fib_entry_get_prefix (ipt->ipt_fib_entry);
+  ip_address_from_46 (&pfx->fp_addr, pfx->fp_proto, ip);
+}
+
+static u8 *
+format_ip_pmtu (u8 *s, va_list *ap)
+{
+  ip_pmtu_t *ipt;
+  index_t ipti = va_arg (*ap, index_t);
+  const fib_prefix_t *pfx;
+  u32 fib_index;
+
+  ipt = pool_elt_at_index (ip_pmtu_pool, ipti);
+  pfx = fib_entry_get_prefix (ipt->ipt_fib_entry);
+  fib_index = fib_entry_get_fib_index (ipt->ipt_fib_entry);
+
+  s =
+    format (s, "[%d] [tbl:[%d:%d]] %U pmtu:[cfg:%d, oper:%d, parent:%d] [%U]",
+	    ipti, ip_pmtu_get_table_id (ipt), fib_index, format_fib_prefix,
+	    pfx, ipt->ipt_cfg_pmtu, ipt->ipt_oper_pmtu, ipt->ipt_parent_pmtu,
+	    format_ip_pmtu_flags, ipt->ipt_flags);
+
+  return (s);
+}
+
+static u8 *
+format_ip_path_mtu_adj_delegate (const adj_delegate_t *aed, u8 *s)
+{
+  ip_path_mtu_adj_delegate_t *ip_adj;
+
+  ip_adj = pool_elt_at_index (ip_path_mtu_adj_delegate_pool, aed->ad_index);
+
+  s = format (s, "IP path-MTU: %d", ip_adj->pmtu);
+
+  return (s);
+}
+
+static void
+ip_pmtu_adj_delegate_adj_created (adj_index_t ai)
+{
+  ip_path_mtu_adj_delegate_t *ipp_ad;
+  const ip_pmtu_t *ipt;
+  ip_adjacency_t *adj;
+  u32 table_id;
+  uword *p;
+
+  adj = adj_get (ai);
+
+  switch (adj->lookup_next_index)
+    {
+    case IP_LOOKUP_NEXT_DROP:
+    case IP_LOOKUP_NEXT_PUNT:
+    case IP_LOOKUP_NEXT_LOCAL:
+    case IP_LOOKUP_NEXT_GLEAN:
+    case IP_LOOKUP_NEXT_MCAST:
+    case IP_LOOKUP_NEXT_BCAST:
+    case IP_LOOKUP_NEXT_MCAST_MIDCHAIN:
+    case IP_LOOKUP_NEXT_ICMP_ERROR:
+    case IP_LOOKUP_N_NEXT:
+      return;
+
+    case IP_LOOKUP_NEXT_ARP:
+    case IP_LOOKUP_NEXT_REWRITE:
+    case IP_LOOKUP_NEXT_MIDCHAIN:
+      break;
+    }
+
+  table_id = fib_table_get_table_id_for_sw_if_index (
+    adj->ia_nh_proto, adj->rewrite_header.sw_if_index);
+
+  ip_pmtu_key_t key = {
+    .nh = adj->sub_type.nbr.next_hop,
+    .table_id = table_id,
+    .fproto = adj->ia_nh_proto,
+  };
+
+  p = hash_get_mem (ip_pmtu_db, &key);
+
+  if (NULL == p)
+    return;
+
+  ipt = pool_elt_at_index (ip_pmtu_pool, p[0]);
+
+  pool_get (ip_path_mtu_adj_delegate_pool, ipp_ad);
+  ipp_ad->pmtu = ipt->ipt_cfg_pmtu;
+
+  adj_delegate_add (adj, ip_pmtu_adj_delegate_type,
+		    ipp_ad - ip_path_mtu_adj_delegate_pool);
+
+  adj_nbr_set_mtu (ai, ipp_ad->pmtu);
+
+  IP_PMTU_TRKR_DBG (ipt, "adj-added:", ai);
+}
+
+static void
+ip_pmtu_adj_delegate_adj_deleted (adj_delegate_t *ad)
+{
+  pool_put_index (ip_path_mtu_adj_delegate_pool, ad->ad_index);
+}
+
+static void
+ip_pmtu_adj_delegate_adj_modified (adj_delegate_t *ad)
+{
+  ip_path_mtu_adj_delegate_t *ipp_ad;
+
+  ipp_ad = pool_elt_at_index (ip_path_mtu_adj_delegate_pool, ad->ad_index);
+
+  adj_nbr_set_mtu (ad->ad_adj_index, ipp_ad->pmtu);
+}
+
+const adj_delegate_vft_t ip_path_adj_delegate_vft = {
+  .adv_format = format_ip_path_mtu_adj_delegate,
+  .adv_adj_deleted = ip_pmtu_adj_delegate_adj_deleted,
+  .adv_adj_modified = ip_pmtu_adj_delegate_adj_modified,
+  .adv_adj_created = ip_pmtu_adj_delegate_adj_created,
+};
+
+static bool
+ip_path_mtu_value_invalid (u16 pmtu)
+{
+  return (pmtu == 0 || pmtu == 0xffff);
+}
+
+static adj_walk_rc_t
+ip_ptmu_adj_walk_remove (adj_index_t ai, void *ctx)
+{
+  adj_delegate_t *ad;
+
+  ad = adj_delegate_get (adj_get (ai), ip_pmtu_adj_delegate_type);
+
+  if (ad)
+    {
+      adj_nbr_set_mtu (ai, 0);
+
+      pool_put_index (ip_path_mtu_adj_delegate_pool, ad->ad_index);
+      adj_delegate_remove (ai, ip_pmtu_adj_delegate_type);
+    }
+  return (ADJ_WALK_RC_CONTINUE);
+}
+
+static adj_walk_rc_t
+ip_ptmu_adj_walk_update (adj_index_t ai, void *ctx)
+{
+  ip_path_mtu_adj_delegate_t *ipp_ad;
+  adj_delegate_t *ad;
+  u16 *pmtup;
+
+  pmtup = ctx;
+  ad = adj_delegate_get (adj_get (ai), ip_pmtu_adj_delegate_type);
+
+  if (ad)
+    ipp_ad = pool_elt_at_index (ip_path_mtu_adj_delegate_pool, ad->ad_index);
+  else
+    {
+      pool_get (ip_path_mtu_adj_delegate_pool, ipp_ad);
+
+      adj_delegate_add (adj_get (ai), ip_pmtu_adj_delegate_type,
+			ipp_ad - ip_path_mtu_adj_delegate_pool);
+    }
+
+  ipp_ad->pmtu = *pmtup;
+
+  adj_nbr_set_mtu (ai, ipp_ad->pmtu);
+
+  return (ADJ_WALK_RC_CONTINUE);
+}
+
+static ip_pmtu_dpo_t *
+ip_pmtu_dpo_alloc (void)
+{
+  ip_pmtu_dpo_t *ipm;
+
+  pool_get_aligned_zero (ip_pmtu_dpo_pool, ipm, sizeof (ip_pmtu_dpo_t));
+
+  return (ipm);
+}
+
+static ip_pmtu_dpo_t *
+ip_pmtu_dpo_get_from_dpo (const dpo_id_t *dpo)
+{
+  ASSERT (ip_pmtu_dpo_type == dpo->dpoi_type);
+
+  return (ip_pmtu_dpo_get (dpo->dpoi_index));
+}
+
+static index_t
+ip_pmtu_dpo_get_index (ip_pmtu_dpo_t *ipm)
+{
+  return (ipm - ip_pmtu_dpo_pool);
+}
+
+static void
+ip_pmtu_dpo_lock (dpo_id_t *dpo)
+{
+  ip_pmtu_dpo_t *ipm;
+
+  ipm = ip_pmtu_dpo_get_from_dpo (dpo);
+  ipm->ipm_locks++;
+}
+
+static void
+ip_pmtu_dpo_unlock (dpo_id_t *dpo)
+{
+  ip_pmtu_dpo_t *ipm;
+
+  ipm = ip_pmtu_dpo_get_from_dpo (dpo);
+  ipm->ipm_locks--;
+
+  if (0 == ipm->ipm_locks)
+    {
+      dpo_reset (&ipm->ipm_dpo);
+      pool_put (ip_pmtu_dpo_pool, ipm);
+    }
+}
+
+static u32
+ip_pmtu_dpo_get_urpf (const dpo_id_t *dpo)
+{
+  ip_pmtu_dpo_t *ipm;
+
+  ipm = ip_pmtu_dpo_get_from_dpo (dpo);
+
+  return (dpo_get_urpf (&ipm->ipm_dpo));
+}
+
+void
+ip_pmtu_dpo_add_or_lock (fib_protocol_t fproto, u16 pmtu, dpo_id_t *dpo)
+{
+  ip_pmtu_dpo_t *ipm;
+  dpo_id_t parent = DPO_INVALID;
+
+  ipm = ip_pmtu_dpo_alloc ();
+
+  ipm->ipm_proto = fib_proto_to_dpo (fproto);
+  ipm->ipm_pmtu = pmtu;
+
+  dpo_copy (&parent, drop_dpo_get (ipm->ipm_proto));
+  dpo_stack (ip_pmtu_dpo_type, ipm->ipm_proto, &ipm->ipm_dpo, &parent);
+  dpo_set (dpo, ip_pmtu_dpo_type, ipm->ipm_proto, ip_pmtu_dpo_get_index (ipm));
+}
+
+u8 *
+format_ip_pmtu_dpo (u8 *s, va_list *ap)
+{
+  index_t index = va_arg (*ap, index_t);
+  u32 indent = va_arg (*ap, u32);
+  ip_pmtu_dpo_t *ipm = ip_pmtu_dpo_get (index);
+
+  s = format (s, "ip-pmtu-dpo: %U, mtu:%d", format_dpo_proto, ipm->ipm_proto,
+	      ipm->ipm_pmtu);
+  s = format (s, "\n%U", format_white_space, indent + 2);
+  s = format (s, "%U", format_dpo_id, &ipm->ipm_dpo, indent + 4);
+
+  return (s);
+}
+
+/**
+ * Interpose a path MTU DPO
+ */
+static void
+ip_pmtu_dpo_interpose (const dpo_id_t *original, const dpo_id_t *parent,
+		       dpo_id_t *clone)
+{
+  ip_pmtu_dpo_t *ipm, *ipm_clone;
+
+  ipm_clone = ip_pmtu_dpo_alloc ();
+  ipm = ip_pmtu_dpo_get (original->dpoi_index);
+
+  ipm_clone->ipm_proto = ipm->ipm_proto;
+  ipm_clone->ipm_pmtu = ipm->ipm_pmtu;
+
+  dpo_stack (ip_pmtu_dpo_type, ipm_clone->ipm_proto, &ipm_clone->ipm_dpo,
+	     parent);
+  dpo_set (clone, ip_pmtu_dpo_type, ipm_clone->ipm_proto,
+	   ip_pmtu_dpo_get_index (ipm_clone));
+}
+
+static u16
+ip_pmtu_dpo_get_mtu (const dpo_id_t *dpo)
+{
+  ip_pmtu_dpo_t *ipd;
+
+  ipd = pool_elt_at_index (ip_pmtu_dpo_pool, dpo->dpoi_index);
+
+  return (ipd->ipm_pmtu);
+}
+
+const static dpo_vft_t ip_pmtu_dpo_vft = {
+  .dv_lock = ip_pmtu_dpo_lock,
+  .dv_unlock = ip_pmtu_dpo_unlock,
+  .dv_format = format_ip_pmtu_dpo,
+  .dv_get_urpf = ip_pmtu_dpo_get_urpf,
+  .dv_mk_interpose = ip_pmtu_dpo_interpose,
+  .dv_get_mtu = ip_pmtu_dpo_get_mtu,
+};
+
+/**
+ * @brief The per-protocol VLIB graph nodes that are assigned to a glean
+ *        object.
+ *
+ * this means that these graph nodes are ones from which a glean is the
+ * parent object in the DPO-graph.
+ */
+const static char *const ip_pmtu_dpo_ip4_nodes[] = {
+  "ip4-pmtu-dpo",
+  NULL,
+};
+
+const static char *const ip_pmtu_dpo_ip6_nodes[] = {
+  "ip6-pmtu-dpo",
+  NULL,
+};
+
+const static char *const *const ip_pmtu_dpo_nodes[DPO_PROTO_NUM] = {
+  [DPO_PROTO_IP4] = ip_pmtu_dpo_ip4_nodes,
+  [DPO_PROTO_IP6] = ip_pmtu_dpo_ip6_nodes,
+};
+
+static bool
+ip_mtu_fib_entry_is_attached (fib_node_index_t fib_entry)
+{
+  const fib_prefix_t *pfx;
+  u32 cover, fib_index;
+
+  fib_index = fib_entry_get_fib_index (fib_entry);
+  pfx = fib_entry_get_prefix (fib_entry);
+
+  /*
+   * If the tracked prefix's cover is attached, then all packets that
+   * are forwarded to this neighbour will use the adjacency, this is a
+   * more efficient place to perform the MTU check and fragging
+   */
+  cover = fib_table_get_less_specific (fib_index, pfx);
+
+  return (FIB_ENTRY_FLAG_ATTACHED & fib_entry_get_flags (cover) ||
+	  FIB_ENTRY_FLAG_ATTACHED & fib_entry_get_flags (fib_entry));
+}
+
+static index_t
+ip_pmtu_alloc (u32 fib_index, const fib_prefix_t *pfx,
+	       const ip_pmtu_key_t *key, u16 pmtu)
+{
+  dpo_id_t ip_dpo = DPO_INVALID;
+  ip_pmtu_t *ipt;
+  fib_node_index_t cover;
+  const dpo_id_t *lb_dpo;
+  index_t ipti;
+
+  pool_get (ip_pmtu_pool, ipt);
+  fib_node_init (&(ipt->ipt_node), ip_pmtu_fib_type);
+
+  ipti = ipt - ip_pmtu_pool;
+  hash_set_mem_alloc (&ip_pmtu_db, key, ipti);
+
+  ipt->ipt_cfg_pmtu = pmtu;
+  ipt->ipt_fib_entry = fib_entry_track (fib_index, pfx, ip_pmtu_fib_type, ipti,
+					&ipt->ipt_sibling);
+
+  /*
+   * If the tracked prefix's cover is attached, then all packets that
+   * are forwarded to this neighbour will use the adjacency, this is a
+   * more efficient place to perform the MTU check and fragging
+   */
+  cover = fib_table_get_less_specific (fib_index, pfx);
+
+  if (ip_mtu_fib_entry_is_attached (ipt->ipt_fib_entry))
+    {
+      u32 sw_if_index;
+
+      ipt->ipt_flags |= IP_PMTU_FLAG_ATTACHED;
+      ipt->ipt_oper_pmtu = ipt->ipt_cfg_pmtu;
+
+      sw_if_index = fib_entry_get_resolving_interface (cover);
+
+      /* walk all adjs to add/update delegate */
+      adj_nbr_walk_nh (sw_if_index, pfx->fp_proto, &pfx->fp_addr,
+		       ip_ptmu_adj_walk_update, &ipt->ipt_oper_pmtu);
+    }
+  else
+    {
+      ipt->ipt_flags |= IP_PMTU_FLAG_REMOTE;
+
+      lb_dpo = fib_entry_contribute_ip_forwarding (ipt->ipt_fib_entry);
+
+      ipt->ipt_oper_pmtu = clib_min (dpo_get_mtu (lb_dpo), ipt->ipt_cfg_pmtu);
+
+      /*
+       * interpose a policy DPO from the nh so that MTU is applied
+       */
+      ip_pmtu_dpo_add_or_lock (pfx->fp_proto, ipt->ipt_oper_pmtu, &ip_dpo);
+
+      fib_table_entry_special_dpo_add (fib_index, pfx, ip_pmtu_source,
+				       FIB_ENTRY_FLAG_INTERPOSE, &ip_dpo);
+      dpo_reset (&ip_dpo);
+    }
+
+  IP_PMTU_TRKR_DBG (ipt, "create");
+
+  return (ipti);
+}
+
+static void
+ip_pmtu_stack (ip_pmtu_t *ipt)
+{
+  bool was_attached, is_attached;
+  const fib_prefix_t *pfx;
+  u32 fib_index;
+
+  pfx = fib_entry_get_prefix (ipt->ipt_fib_entry);
+  fib_index = fib_entry_get_fib_index (ipt->ipt_fib_entry);
+
+  was_attached = !!(ipt->ipt_flags & IP_PMTU_FLAG_ATTACHED);
+  is_attached = ip_mtu_fib_entry_is_attached (ipt->ipt_fib_entry);
+
+  if (was_attached && !is_attached)
+    {
+      /* transition from attached to remote - walk all adjs to remove delegate
+       */
+      adj_nbr_walk_nh (fib_entry_get_resolving_interface (ipt->ipt_fib_entry),
+		       pfx->fp_proto, &pfx->fp_addr, ip_ptmu_adj_walk_remove,
+		       &ipt->ipt_oper_pmtu);
+      ipt->ipt_flags &= ~IP_PMTU_FLAG_ATTACHED;
+    }
+  if (!was_attached && is_attached)
+    {
+      /* transition from remote to attached - remove the DPO */
+      fib_table_entry_special_remove (fib_index, pfx, ip_pmtu_source);
+      ipt->ipt_flags &= ~IP_PMTU_FLAG_REMOTE;
+    }
+
+  if (is_attached)
+    {
+      /* walk all adjs to add/update delegate */
+      ipt->ipt_oper_pmtu = ipt->ipt_cfg_pmtu;
+      adj_nbr_walk_nh (fib_entry_get_resolving_interface (ipt->ipt_fib_entry),
+		       pfx->fp_proto, &pfx->fp_addr, ip_ptmu_adj_walk_update,
+		       &ipt->ipt_oper_pmtu);
+      ipt->ipt_flags |= IP_PMTU_FLAG_ATTACHED;
+    }
+  else
+    {
+      const dpo_id_t *lb_dpo;
+      u16 dpo_mtu;
+
+      fib_table_entry_special_remove (fib_index, pfx, ip_pmtu_source);
+
+      ipt->ipt_flags |= IP_PMTU_FLAG_REMOTE;
+      lb_dpo = fib_entry_contribute_ip_forwarding (ipt->ipt_fib_entry);
+      dpo_mtu = dpo_get_mtu (lb_dpo);
+
+      ipt->ipt_oper_pmtu = clib_min (dpo_mtu, ipt->ipt_cfg_pmtu);
+
+      /*
+       * if the configured path-MTU is less that the egress/interface then
+       * interpose a policy DPO from the nh so that MTU is applied
+       */
+      if (ipt->ipt_oper_pmtu < dpo_mtu)
+	{
+	  dpo_id_t ip_dpo = DPO_INVALID;
+
+	  ip_pmtu_dpo_add_or_lock (pfx->fp_proto, ipt->ipt_oper_pmtu, &ip_dpo);
+
+	  fib_table_entry_special_dpo_update (
+	    fib_index, pfx, ip_pmtu_source, FIB_ENTRY_FLAG_INTERPOSE, &ip_dpo);
+	  dpo_reset (&ip_dpo);
+	}
+    }
+  IP_PMTU_TRKR_DBG (ipt, "stack");
+}
+
+static void
+ip_pmtu_update (index_t ipti, u16 pmtu)
+{
+  ip_pmtu_t *ipt;
+
+  ipt = pool_elt_at_index (ip_pmtu_pool, ipti);
+  ipt->ipt_flags &= ~IP_PMTU_FLAG_STALE;
+  ipt->ipt_cfg_pmtu = pmtu;
+
+  ip_pmtu_stack (ipt);
+}
+
+static index_t
+ip_pmtu_destroy (index_t ipti, const ip_pmtu_key_t *key)
+{
+  ip_pmtu_t *ipt;
+  const fib_prefix_t *pfx;
+
+  ipt = pool_elt_at_index (ip_pmtu_pool, ipti);
+  pfx = fib_entry_get_prefix (ipt->ipt_fib_entry);
+
+  IP_PMTU_TRKR_DBG (ipt, "destroy");
+
+  if (ipt->ipt_flags & IP_PMTU_FLAG_REMOTE)
+    fib_table_entry_special_remove (
+      fib_entry_get_fib_index (ipt->ipt_fib_entry), pfx, ip_pmtu_source);
+  else
+    /* remove the delegate from all the adjacencies */
+    adj_nbr_walk_nh (fib_entry_get_resolving_interface (ipt->ipt_fib_entry),
+		     pfx->fp_proto, &pfx->fp_addr, ip_ptmu_adj_walk_remove,
+		     NULL);
+
+  /*
+   * Drop the fib entry we're tracking
+   */
+  fib_entry_untrack (ipt->ipt_fib_entry, ipt->ipt_sibling);
+
+  /*
+   * remove from DB and return to pool
+   */
+  hash_unset_mem_free (&ip_pmtu_db, key);
+  pool_put (ip_pmtu_pool, ipt);
+
+  return (ipti);
+}
+
+int
+ip_path_mtu_update (const ip_address_t *nh, u32 table_id, u16 pmtu)
+{
+  fib_prefix_t pfx;
+  u32 fib_index;
+  uword *p;
+
+  ip_address_to_fib_prefix (nh, &pfx);
+  fib_index = fib_table_find (pfx.fp_proto, table_id);
+
+  if (~0 == fib_index)
+    return (VNET_API_ERROR_NO_SUCH_TABLE);
+
+  ip_pmtu_key_t key = {
+    .fproto = pfx.fp_proto,
+    .table_id = table_id,
+    .nh = pfx.fp_addr,
+  };
+
+  p = hash_get_mem (ip_pmtu_db, &key);
+
+  if (!ip_path_mtu_value_invalid (pmtu))
+    {
+      /* Add or update of path MTU */
+      if (NULL == p)
+	ip_pmtu_alloc (fib_index, &pfx, &key, pmtu);
+      else
+	ip_pmtu_update (p[0], pmtu);
+    }
+  else
+    {
+      if (NULL != p)
+	ip_pmtu_destroy (p[0], &key);
+    }
+
+  return (0);
+}
+
+static walk_rc_t
+ip_path_mtu_walk_mark (index_t ipti, void *ctx)
+{
+  ip_pmtu_t *ipt;
+
+  ipt = ip_path_mtu_get (ipti);
+
+  ipt->ipt_flags |= IP_PMTU_FLAG_STALE;
+
+  return (WALK_CONTINUE);
+}
+
+typedef struct ip_path_mtu_walk_sweep_ctx_t_
+{
+  index_t *indicies;
+} ip_path_mtu_walk_sweep_ctx_t;
+
+static walk_rc_t
+ip_path_mtu_walk_sweep (index_t ipti, void *arg)
+{
+  ip_path_mtu_walk_sweep_ctx_t *ctx = arg;
+  ip_pmtu_t *ipt;
+
+  ipt = ip_path_mtu_get (ipti);
+
+  if (ipt->ipt_flags & IP_PMTU_FLAG_STALE)
+    {
+      vec_add1 (ctx->indicies, ipti);
+    }
+
+  return (WALK_CONTINUE);
+}
+
+int
+ip_path_mtu_replace_begin (void)
+{
+  IP_PMTU_DBG ("replace-begin");
+
+  ip_path_mtu_walk (ip_path_mtu_walk_mark, NULL);
+
+  return (0);
+}
+
+int
+ip_path_mtu_replace_end (void)
+{
+  index_t *ipti;
+
+  IP_PMTU_DBG ("replace-end");
+
+  /*
+   * not safe to walk the pool whilst deleting, so create
+   * temporary storage of stale entries
+   */
+  ip_path_mtu_walk_sweep_ctx_t ctx = {
+    .indicies = NULL,
+  };
+
+  ip_path_mtu_walk (ip_path_mtu_walk_sweep, &ctx);
+
+  vec_foreach (ipti, ctx.indicies)
+    {
+      ip_pmtu_t *ipt;
+      ip_address_t ip;
+
+      ipt = ip_path_mtu_get (*ipti);
+      ip_pmtu_get_ip (ipt, &ip);
+      ip_path_mtu_update (&ip, ip_pmtu_get_table_id (ipt), 0);
+    }
+
+  vec_free (ctx.indicies);
+
+  return (0);
+}
+
+void
+ip_path_mtu_walk (ip_path_mtu_walk_t fn, void *ctx)
+{
+  index_t ipmi;
+
+  pool_foreach_index (ipmi, ip_pmtu_pool)
+    {
+      if (WALK_STOP == fn (ipmi, ctx))
+	break;
+    }
+}
+
+static fib_node_t *
+ip_pmtu_get_node (fib_node_index_t index)
+{
+  ip_pmtu_t *ipt;
+
+  ipt = pool_elt_at_index (ip_pmtu_pool, index);
+
+  return (&(ipt->ipt_node));
+}
+
+static ip_pmtu_t *
+ip_pmtu_get_from_node (fib_node_t *node)
+{
+  return (
+    (ip_pmtu_t *) (((char *) node) - STRUCT_OFFSET_OF (ip_pmtu_t, ipt_node)));
+}
+
+static void
+ip_pmtu_last_lock_gone (fib_node_t *node)
+{
+  /*
+   * the lifetime of the entry is managed by the API.
+   */
+  ASSERT (0);
+}
+
+/*
+ * A back walk has reached this BIER entry
+ */
+static fib_node_back_walk_rc_t
+ip_pmtu_back_walk_notify (fib_node_t *node, fib_node_back_walk_ctx_t *ctx)
+{
+  /*
+   * re-populate the ECMP tables with new choices
+   */
+  ip_pmtu_t *ipr = ip_pmtu_get_from_node (node);
+
+  ip_pmtu_stack (ipr);
+
+  /*
+   * no need to propagate further up the graph, since there's nothing there
+   */
+  return (FIB_NODE_BACK_WALK_CONTINUE);
+}
+
+static const fib_node_vft_t ip_ptmu_fib_node_vft = {
+  .fnv_get = ip_pmtu_get_node,
+  .fnv_last_lock = ip_pmtu_last_lock_gone,
+  .fnv_back_walk = ip_pmtu_back_walk_notify,
+};
+
+static clib_error_t *
+ip_path_module_init (vlib_main_t *vm)
+{
+  ip_pmtu_adj_delegate_type =
+    adj_delegate_register_new_type (&ip_path_adj_delegate_vft);
+  ip_pmtu_source = fib_source_allocate ("path-mtu", FIB_SOURCE_PRIORITY_HI,
+					FIB_SOURCE_BH_SIMPLE);
+  ip_pmtu_fib_type = fib_node_register_new_type (&ip_ptmu_fib_node_vft);
+
+  ip_pmtu_db = hash_create_mem (0, sizeof (ip_pmtu_key_t), sizeof (index_t));
+  ip_pmtu_logger = vlib_log_register_class ("ip", "pmtu");
+  ip_pmtu_dpo_type =
+    dpo_register_new_type (&ip_pmtu_dpo_vft, ip_pmtu_dpo_nodes);
+
+  return (NULL);
+}
+
+VLIB_INIT_FUNCTION (ip_path_module_init);
+
+static clib_error_t *
+show_ip_pmtu_command (vlib_main_t *vm, unformat_input_t *input,
+		      vlib_cli_command_t *cmd)
+{
+  index_t ipti;
+
+  if (unformat (input, "%d", &ipti))
+    {
+      /*
+       * show one in detail
+       */
+      if (!pool_is_free_index (ip_pmtu_pool, ipti))
+	vlib_cli_output (vm, "%U", format_ip_pmtu, ipti);
+      else
+	vlib_cli_output (vm, "entry %d invalid", ipti);
+    }
+  else
+    {
+      /*
+       * show all
+       */
+      pool_foreach_index (ipti, ip_pmtu_pool)
+	{
+	  vlib_cli_output (vm, "%U", format_ip_pmtu, ipti);
+	}
+    }
+
+  return (NULL);
+}
+
+VLIB_CLI_COMMAND (show_fib_entry, static) = {
+  .path = "show ip pmtu",
+  .function = show_ip_pmtu_command,
+  .short_help = "show ip path MTU",
+};
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */