bridge: Break bonding implementation out into library.

This removes over 1000 lines of code from bridge.c and will make it
easier to moving the bonding implementation into ofproto as part of
future development.

1 files changed, 1493 insertions, 0 deletions

diff --git a/lib/bond.c b/lib/bond.c
new file mode 100644
index 00000000..e27d5f40
--- /dev/null
+++ b/lib/bond.c
@@ -0,0 +1,1493 @@
+/*
+ * 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.
+ * 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 <config.h>
+
+#include "bond.h"
+
+#include <limits.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "coverage.h"
+#include "dynamic-string.h"
+#include "flow.h"
+#include "hmap.h"
+#include "lacp.h"
+#include "list.h"
+#include "netdev.h"
+#include "odp-util.h"
+#include "ofpbuf.h"
+#include "packets.h"
+#include "poll-loop.h"
+#include "tag.h"
+#include "timeval.h"
+#include "unixctl.h"
+#include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(bond);
+
+COVERAGE_DEFINE(bond_process_lacp);
+
+/* Bit-mask for hashing a flow down to a bucket.
+ * There are (BOND_MASK + 1) buckets. */
+#define BOND_MASK 0xff
+
+/* A hash bucket for mapping a flow to a slave.
+ * "struct bond" has an array of (BOND_MASK + 1) of these. */
+struct bond_entry {
+    struct bond_slave *slave;   /* Assigned slave, NULL if unassigned. */
+    uint64_t tx_bytes;          /* Count of bytes recently transmitted. */
+    tag_type tag;               /* Tag for entry<->slave association. */
+    struct list list_node;      /* In bond_slave's 'entries' list. */
+};
+
+/* A bond slave, that is, one of the links comprising a bond. */
+struct bond_slave {
+    struct hmap_node hmap_node; /* In struct bond's slaves hmap. */
+    struct bond *bond;          /* The bond that contains this slave. */
+    void *aux;                  /* Client-provided handle for this slave. */
+
+    struct netdev *netdev;      /* Network device, owned by the client. */
+    char *name;                 /* Name (a copy of netdev_get_name(netdev)). */
+
+    /* Link status. */
+    long long delay_expires;    /* Time after which 'enabled' may change. */
+    bool up;                    /* Last link status read from netdev. */
+    bool enabled;               /* May be chosen for flows? */
+    tag_type tag;               /* Tag associated with this slave. */
+
+    /* Rebalancing info.  Used only by bond_rebalance(). */
+    struct list bal_node;       /* In bond_rebalance()'s 'bals' list. */
+    struct list entries;        /* 'struct bond_entry's assigned here. */
+    uint64_t tx_bytes;          /* Sum across 'tx_bytes' of entries. */
+};
+
+/* A bond, that is, a set of network devices grouped to improve performance or
+ * robustness.  */
+struct bond {
+    struct hmap_node hmap_node; /* In 'all_bonds' hmap. */
+    char *name;                 /* Name provided by client. */
+
+    /* Slaves. */
+    struct hmap slaves;
+
+    /* Bonding info. */
+    enum bond_mode balance;     /* Balancing mode, one of BM_*. */
+    struct bond_slave *active_slave;
+    tag_type no_slaves_tag;     /* Tag for flows when all slaves disabled. */
+    int updelay, downdelay;     /* Delay before slave goes up/down, in ms. */
+
+    /* SLB specific bonding info. */
+    struct bond_entry *hash;     /* An array of (BOND_MASK + 1) elements. */
+    int rebalance_interval;      /* Interval between rebalances, in ms. */
+    long long int next_rebalance; /* Next rebalancing time. */
+    bool send_learning_packets;
+
+    /* LACP. */
+    struct lacp *lacp;          /* LACP object. NULL if LACP is disabled. */
+
+    /* Monitoring. */
+    enum bond_detect_mode detect;     /* Link status mode, one of BLSM_*. */
+    struct netdev_monitor *monitor;   /* detect == BLSM_CARRIER only. */
+    long long int miimon_interval;    /* Miimon status refresh interval. */
+    long long int miimon_next_update; /* Time of next miimon update. */
+
+    /* Legacy compatibility. */
+    long long int next_fake_iface_update; /* LLONG_MAX if disabled. */
+
+    /* Tag set saved for next bond_run().  This tag set is a kluge for cases
+     * where we can't otherwise provide revalidation feedback to the client.
+     * That's only unixctl commands now; I hope no other cases will arise. */
+    struct tag_set unixctl_tags;
+};
+
+static struct hmap all_bonds = HMAP_INITIALIZER(&all_bonds);
+
+static struct bond_slave *bond_slave_lookup(struct bond *, const void *slave_);
+static bool bond_is_link_up(struct bond *, struct netdev *);
+static void bond_enable_slave(struct bond_slave *, bool enable,
+                              struct tag_set *);
+static void bond_link_status_update(struct bond_slave *, struct tag_set *);
+static void bond_choose_active_slave(struct bond *, struct tag_set *);
+static bool bond_is_tcp_hash(const struct bond *);
+static unsigned int bond_hash_src(const uint8_t mac[ETH_ADDR_LEN],
+                                  uint16_t vlan);
+static unsigned int bond_hash_tcp(const struct flow *, uint16_t vlan);
+static struct bond_entry *lookup_bond_entry(const struct bond *,
+                                            const struct flow *,
+                                            uint16_t vlan);
+static tag_type bond_get_active_slave_tag(const struct bond *);
+static struct bond_slave *choose_output_slave(const struct bond *,
+                                              const struct flow *,
+                                              uint16_t vlan);
+static void bond_update_fake_slave_stats(struct bond *);
+
+/* Attempts to parse 's' as the name of a bond balancing mode.  If successful,
+ * stores the mode in '*balance' and returns true.  Otherwise returns false
+ * without modifying '*balance'. */
+bool
+bond_mode_from_string(enum bond_mode *balance, const char *s)
+{
+    if (!strcmp(s, bond_mode_to_string(BM_TCP))) {
+        *balance = BM_TCP;
+    } else if (!strcmp(s, bond_mode_to_string(BM_SLB))) {
+        *balance = BM_SLB;
+    } else if (!strcmp(s, bond_mode_to_string(BM_AB))) {
+        *balance = BM_AB;
+    } else {
+        return false;
+    }
+    return true;
+}
+
+/* Returns a string representing 'balance'. */
+const char *
+bond_mode_to_string(enum bond_mode balance) {
+    switch (balance) {
+    case BM_TCP:
+        return "balance-tcp";
+    case BM_SLB:
+        return "balance-slb";
+    case BM_AB:
+        return "active-backup";
+    }
+    NOT_REACHED();
+}
+
+/* Attempts to parse 's' as the name of a bond link status detection mode.  If
+ * successful, stores the mode in '*detect' and returns true.  Otherwise
+ * returns false without modifying '*detect'. */
+bool
+bond_detect_mode_from_string(enum bond_detect_mode *detect, const char *s)
+{
+    if (!strcmp(s, bond_detect_mode_to_string(BLSM_CARRIER))) {
+        *detect = BLSM_CARRIER;
+    } else if (!strcmp(s, bond_detect_mode_to_string(BLSM_MIIMON))) {
+        *detect = BLSM_MIIMON;
+    } else {
+        return false;
+    }
+    return true;
+}
+
+/* Returns a string representing 'detect'. */
+const char *
+bond_detect_mode_to_string(enum bond_detect_mode detect)
+{
+    switch (detect) {
+    case BLSM_CARRIER:
+        return "carrier";
+    case BLSM_MIIMON:
+        return "miimon";
+    }
+    NOT_REACHED();
+}
+
+/* Creates and returns a new bond whose configuration is initially taken from
+ * 's'.
+ *
+ * The caller should register each slave on the new bond by calling
+ * bond_slave_register().  */
+struct bond *
+bond_create(const struct bond_settings *s)
+{
+    struct bond *bond;
+
+    bond = xzalloc(sizeof *bond);
+    hmap_init(&bond->slaves);
+    bond->no_slaves_tag = tag_create_random();
+    bond->miimon_next_update = LLONG_MAX;
+    bond->next_fake_iface_update = LLONG_MAX;
+
+    bond_reconfigure(bond, s);
+
+    tag_set_init(&bond->unixctl_tags);
+
+    return bond;
+}
+
+/* Frees 'bond'. */
+void
+bond_destroy(struct bond *bond)
+{
+    struct bond_slave *slave, *next_slave;
+
+    if (!bond) {
+        return;
+    }
+
+    hmap_remove(&all_bonds, &bond->hmap_node);
+
+    HMAP_FOR_EACH_SAFE (slave, next_slave, hmap_node, &bond->slaves) {
+        hmap_remove(&bond->slaves, &slave->hmap_node);
+        /* Client owns 'slave->netdev'. */
+        free(slave->name);
+        free(slave);
+    }
+    hmap_destroy(&bond->slaves);
+
+    free(bond->hash);
+
+    lacp_destroy(bond->lacp);
+
+    netdev_monitor_destroy(bond->monitor);
+
+    free(bond->name);
+    free(bond);
+}
+
+/* Updates 'bond''s overall configuration to 's'.
+ *
+ * The caller should register each slave on 'bond' by calling
+ * bond_slave_register().  This is optional if none of the slaves'
+ * configuration has changed, except that it is mandatory if 's' enables LACP
+ * and 'bond' previously didn't have LACP enabled.  In any case it can't
+ * hurt. */
+void
+bond_reconfigure(struct bond *bond, const struct bond_settings *s)
+{
+    if (!bond->name || strcmp(bond->name, s->name)) {
+        if (bond->name) {
+            hmap_remove(&all_bonds, &bond->hmap_node);
+            free(bond->name);
+        }
+        bond->name = xstrdup(s->name);
+        hmap_insert(&all_bonds, &bond->hmap_node, hash_string(bond->name, 0));
+    }
+
+    bond->balance = s->balance;
+    bond->detect = s->detect;
+    bond->miimon_interval = s->miimon_interval;
+    bond->updelay = s->up_delay;
+    bond->downdelay = s->down_delay;
+    bond->rebalance_interval = s->rebalance_interval;
+
+    if (bond->balance != BM_AB) {
+        if (!bond->hash) {
+            bond->hash = xcalloc(BOND_MASK + 1, sizeof *bond->hash);
+            bond->next_rebalance = time_msec() + bond->rebalance_interval;
+        }
+    } else {
+        if (bond->hash) {
+            free(bond->hash);
+            bond->hash = NULL;
+        }
+    }
+
+    if (bond->detect == BLSM_CARRIER) {
+        struct bond_slave *slave;
+
+        if (!bond->monitor) {
+            bond->monitor = netdev_monitor_create();
+        }
+
+        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+            netdev_monitor_add(bond->monitor, slave->netdev);
+        }
+    } else {
+        netdev_monitor_destroy(bond->monitor);
+        bond->monitor = NULL;
+
+        if (bond->miimon_next_update == LLONG_MAX) {
+            bond->miimon_next_update = time_msec() + bond->miimon_interval;
+        }
+    }
+
+    if (s->lacp) {
+        if (!bond->lacp) {
+            bond->lacp = lacp_create();
+        }
+        lacp_configure(bond->lacp, s->lacp);
+    } else {
+        lacp_destroy(bond->lacp);
+        bond->lacp = NULL;
+    }
+
+    if (s->fake_iface) {
+        if (bond->next_fake_iface_update == LLONG_MAX) {
+            bond->next_fake_iface_update = time_msec();
+        }
+    } else {
+        bond->next_fake_iface_update = LLONG_MAX;
+    }
+}
+
+/* Registers 'slave_' as a slave of 'bond'.  The 'slave_' pointer is an
+ * arbitrary client-provided pointer that uniquely identifies a slave within a
+ * bond.  If 'slave_' already exists within 'bond' then this function
+ * reconfigures the existing slave.
+ *
+ * 'netdev' must be the network device that 'slave_' represents.  It is owned
+ * by the client, so the client must not close it before either unregistering
+ * 'slave_' or destroying 'bond'.
+ *
+ * If 'bond' has a LACP configuration then 'lacp_settings' must point to LACP
+ * settings for 'slave_'; otherwise 'lacp_settings' is ignored.
+ */
+void
+bond_slave_register(struct bond *bond, void *slave_, struct netdev *netdev,
+                    const struct lacp_slave_settings *lacp_settings)
+{
+    struct bond_slave *slave = bond_slave_lookup(bond, slave_);
+
+    if (!slave) {
+        slave = xzalloc(sizeof *slave);
+
+        hmap_insert(&bond->slaves, &slave->hmap_node, hash_pointer(slave_, 0));
+        slave->bond = bond;
+        slave->aux = slave_;
+        slave->delay_expires = LLONG_MAX;
+        slave->up = bond_is_link_up(bond, netdev);
+        slave->enabled = slave->up;
+    }
+
+    slave->netdev = netdev;
+    free(slave->name);
+    slave->name = xstrdup(netdev_get_name(netdev));
+
+    if (bond->lacp) {
+        assert(lacp_settings != NULL);
+        lacp_slave_register(bond->lacp, slave, lacp_settings);
+    }
+}
+
+/* Unregisters 'slave_' from 'bond'.  If 'bond' does not contain such a slave
+ * then this function has no effect.
+ *
+ * Unregistering a slave invalidates all flows. */
+void
+bond_slave_unregister(struct bond *bond, const void *slave_)
+{
+    struct bond_slave *slave = bond_slave_lookup(bond, slave_);
+    bool del_active;
+
+    if (!slave) {
+        return;
+    }
+
+    del_active = bond->active_slave == slave;
+    if (bond->hash) {
+        struct bond_entry *e;
+        for (e = bond->hash; e <= &bond->hash[BOND_MASK]; e++) {
+            if (e->slave == slave) {
+                e->slave = NULL;
+            }
+        }
+    }
+
+    free(slave->name);
+
+    hmap_remove(&bond->slaves, &slave->hmap_node);
+    /* Client owns 'slave->netdev'. */
+    free(slave);
+
+    if (del_active) {
+        struct tag_set tags;
+
+        tag_set_init(&tags);
+        bond_choose_active_slave(bond, &tags);
+        bond->send_learning_packets = true;
+    }
+}
+
+/* Callback for lacp_run(). */
+static void
+bond_send_pdu_cb(void *slave_, const struct lacp_pdu *pdu)
+{
+    static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 10);
+    struct bond_slave *slave = slave_;
+    uint8_t ea[ETH_ADDR_LEN];
+    int error;
+
+    error = netdev_get_etheraddr(slave->netdev, ea);
+    if (!error) {
+        struct lacp_pdu *packet_pdu;
+        struct ofpbuf packet;
+
+        ofpbuf_init(&packet, 0);
+        packet_pdu = eth_compose(&packet, eth_addr_lacp, ea, ETH_TYPE_LACP,
+                                 sizeof *packet_pdu);
+        *packet_pdu = *pdu;
+        error = netdev_send(slave->netdev, &packet);
+        if (error) {
+            VLOG_WARN_RL(&rl, "bond %s: sending LACP PDU on slave %s failed "
+                         "(%s)",
+                         slave->bond->name, slave->name, strerror(error));
+        }
+        ofpbuf_uninit(&packet);
+    } else {
+        VLOG_ERR_RL(&rl, "bond %s: cannot obtain Ethernet address of slave "
+                    "%s (%s)",
+                    slave->bond->name, slave->name, strerror(error));
+    }
+}
+
+/* Performs periodic maintenance on 'bond'.  The caller must provide 'tags' to
+ * allow tagged flows to be invalidated.
+ *
+ * The caller should check bond_should_send_learning_packets() afterward. */
+void
+bond_run(struct bond *bond, struct tag_set *tags)
+{
+    struct bond_slave *slave;
+
+    /* Update link status. */
+    if (bond->detect == BLSM_CARRIER
+        || time_msec() >= bond->miimon_next_update)
+    {
+        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+            slave->up = bond_is_link_up(bond, slave->netdev);
+        }
+        bond->miimon_next_update = time_msec() + bond->miimon_interval;
+    }
+
+    /* Update LACP. */
+    if (bond->lacp) {
+        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+            lacp_slave_enable(bond->lacp, slave, slave->enabled);
+        }
+
+        lacp_run(bond->lacp, bond_send_pdu_cb);
+    }
+
+    /* Enable slaves based on link status and LACP feedback. */
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        bond_link_status_update(slave, tags);
+    }
+    if (!bond->active_slave || !bond->active_slave->enabled) {
+        bond_choose_active_slave(bond, tags);
+    }
+
+    /* Update fake bond interface stats. */
+    if (time_msec() >= bond->next_fake_iface_update) {
+        bond_update_fake_slave_stats(bond);
+        bond->next_fake_iface_update = time_msec() + 1000;
+    }
+
+    /* Invalidate any tags required by  */
+    tag_set_union(tags, &bond->unixctl_tags);
+    tag_set_init(&bond->unixctl_tags);
+}
+
+/* Causes poll_block() to wake up when 'bond' needs something to be done. */
+void
+bond_wait(struct bond *bond)
+{
+    struct bond_slave *slave;
+
+    if (bond->detect == BLSM_CARRIER) {
+        netdev_monitor_poll_wait(bond->monitor);
+    } else {
+        poll_timer_wait_until(bond->miimon_next_update);
+    }
+
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        if (slave->delay_expires != LLONG_MAX) {
+            poll_timer_wait_until(slave->delay_expires);
+        }
+    }
+
+    if (bond->next_fake_iface_update != LLONG_MAX) {
+        poll_timer_wait_until(bond->next_fake_iface_update);
+    }
+
+    /* Ensure that any saved tags get revalidated right away. */
+    if (!tag_set_is_empty(&bond->unixctl_tags)) {
+        poll_immediate_wake();
+    }
+
+    /* We don't wait for bond->next_rebalance because rebalancing can only run
+     * at a flow account checkpoint.  ofproto does checkpointing on its own
+     * schedule and bond_rebalance() gets called afterward, so we'd just be
+     * waking up for no purpose. */
+}
+
+/* MAC learning table interaction. */
+
+static bool
+may_send_learning_packets(const struct bond *bond)
+{
+    return !lacp_negotiated(bond->lacp) && bond->balance != BM_AB;
+}
+
+/* Returns true if 'bond' needs the client to send out packets to assist with
+ * MAC learning on 'bond'.  If this function returns true, then the client
+ * should iterate through its MAC learning table for the bridge on which 'bond'
+ * is located.  For each MAC that has been learned on a port other than 'bond',
+ * it should call bond_send_learning_packet().
+ *
+ * This function will only return true if 'bond' is in SLB mode and LACP is not
+ * negotiated.  Otherwise sending learning packets isn't necessary.
+ *
+ * Calling this function resets the state that it checks. */
+bool
+bond_should_send_learning_packets(struct bond *bond)
+{
+    bool send = bond->send_learning_packets && may_send_learning_packets(bond);
+    bond->send_learning_packets = false;
+    return send;
+}
+
+/* Sends a gratuitous learning packet on 'bond' from 'eth_src' on 'vlan'.
+ *
+ * See bond_should_send_learning_packets() for description of usage. */
+int
+bond_send_learning_packet(struct bond *bond,
+                          const uint8_t eth_src[ETH_ADDR_LEN],
+                          uint16_t vlan)
+{
+    struct bond_slave *slave;
+    struct ofpbuf packet;
+    struct flow flow;
+    int error;
+
+    assert(may_send_learning_packets(bond));
+    if (!bond->active_slave) {
+        /* Nowhere to send the learning packet. */
+        return 0;
+    }
+
+    memset(&flow, 0, sizeof flow);
+    memcpy(flow.dl_src, eth_src, ETH_ADDR_LEN);
+    slave = choose_output_slave(bond, &flow, vlan);
+
+    ofpbuf_init(&packet, 0);
+    compose_benign_packet(&packet, "Open vSwitch Bond Failover", 0xf177,
+                          eth_src);
+    if (vlan) {
+        eth_set_vlan_tci(&packet, htons(vlan));
+    }
+    error = netdev_send(slave->netdev, &packet);
+    ofpbuf_uninit(&packet);
+
+    return error;
+}
+
+/* Checks whether a packet that arrived on 'slave_' within 'bond', with an
+ * Ethernet destination address of 'eth_dst', should be admitted.
+ *
+ * The return value is one of the following:
+ *
+ *    - BV_ACCEPT: Admit the packet.
+ *
+ *    - BV_DROP: Drop the packet.
+ *
+ *    - BV_DROP_IF_MOVED: Consult the MAC learning table for the packet's
+ *      Ethernet source address and VLAN.  If there is none, or if the packet
+ *      is on the learned port, then admit the packet.  If a different port has
+ *      been learned, however, drop the packet (and do not use it for MAC
+ *      learning).
+ */
+enum bond_verdict
+bond_check_admissibility(struct bond *bond, const void *slave_,
+                         const uint8_t eth_dst[ETH_ADDR_LEN], tag_type *tags)
+{
+    /* Admit all packets if LACP has been negotiated, because that means that
+     * the remote switch is aware of the bond and will "do the right thing". */
+    if (lacp_negotiated(bond->lacp)) {
+        return BV_ACCEPT;
+    }
+
+    /* Drop all multicast packets on inactive slaves. */
+    if (eth_addr_is_multicast(eth_dst)) {
+        *tags |= bond_get_active_slave_tag(bond);
+        if (bond->active_slave != bond_slave_lookup(bond, slave_)) {
+            return BV_DROP;
+        }
+    }
+
+    /* Drop all packets for which we have learned a different input port,
+     * because we probably sent the packet on one slave and got it back on the
+     * other.  Gratuitous ARP packets are an exception to this rule: the host
+     * has moved to another switch.  The exception to the exception is if we
+     * locked the learning table to avoid reflections on bond slaves. */
+    return BV_DROP_IF_MOVED;
+}
+
+/* Returns the slave (registered on 'bond' by bond_slave_register()) to which
+ * a packet with the given 'flow' and 'vlan' should be forwarded.  Returns
+ * NULL if the packet should be dropped because no slaves are enabled.
+ *
+ * 'vlan' is not necessarily the same as 'flow->vlan_tci'.  First, 'vlan'
+ * should be a VID only (i.e. excluding the PCP bits).  Second,
+ * 'flow->vlan_tci' is the VLAN TCI that appeared on the packet (so it will be
+ * nonzero only for trunk ports), whereas 'vlan' is the logical VLAN that the
+ * packet belongs to (so for an access port it will be the access port's VLAN).
+ *
+ * Adds a tag to '*tags' that associates the flow with the returned slave.
+ */
+void *
+bond_choose_output_slave(struct bond *bond, const struct flow *flow,
+                         uint16_t vlan, tag_type *tags)
+{
+    struct bond_slave *slave = choose_output_slave(bond, flow, vlan);
+    if (slave) {
+        *tags |= slave->tag;
+        return slave->aux;
+    } else {
+        *tags |= bond->no_slaves_tag;
+        return NULL;
+    }
+}
+
+/* Processes LACP packet 'packet', which was received on 'slave_' within
+ * 'bond'.
+ *
+ * The client should use this function to pass along LACP messages received on
+ * any of 'bond''s slaves. */
+void
+bond_process_lacp(struct bond *bond, void *slave_, const struct ofpbuf *packet)
+{
+    if (bond->lacp) {
+        struct bond_slave *slave = bond_slave_lookup(bond, slave_);
+        const struct lacp_pdu *pdu = parse_lacp_packet(packet);
+        if (slave && pdu) {
+            COVERAGE_INC(bond_process_lacp);
+            lacp_process_pdu(bond->lacp, slave, pdu);
+        }
+    }
+}
+
+/* Rebalancing. */
+
+/* Notifies 'bond' that 'n_bytes' bytes were sent in 'flow' within 'vlan'. */
+void
+bond_account(struct bond *bond, const struct flow *flow, uint16_t vlan,
+             uint64_t n_bytes)
+{
+    switch (bond->balance) {
+    case BM_AB:
+        /* Nothing to do. */
+        break;
+
+    case BM_SLB:
+    case BM_TCP:
+        lookup_bond_entry(bond, flow, vlan)->tx_bytes += n_bytes;
+        break;
+
+    default:
+        NOT_REACHED();
+    }
+}
+
+static struct bond_slave *
+bond_slave_from_bal_node(struct list *bal)
+{
+    return CONTAINER_OF(bal, struct bond_slave, bal_node);
+}
+
+static void
+log_bals(struct bond *bond, const struct list *bals)
+{
+    if (VLOG_IS_DBG_ENABLED()) {
+        struct ds ds = DS_EMPTY_INITIALIZER;
+        const struct bond_slave *slave;
+
+        LIST_FOR_EACH (slave, bal_node, bals) {
+            if (ds.length) {
+                ds_put_char(&ds, ',');
+            }
+            ds_put_format(&ds, " %s %"PRIu64"kB",
+                          slave->name, slave->tx_bytes / 1024);
+
+            if (!slave->enabled) {
+                ds_put_cstr(&ds, " (disabled)");
+            }
+            if (!list_is_empty(&slave->entries)) {
+                struct bond_entry *e;
+
+                ds_put_cstr(&ds, " (");
+                LIST_FOR_EACH (e, list_node, &slave->entries) {
+                    if (&e->list_node != list_front(&slave->entries)) {
+                        ds_put_cstr(&ds, " + ");
+                    }
+                    ds_put_format(&ds, "h%td: %"PRIu64"kB",
+                                  e - bond->hash, e->tx_bytes / 1024);
+                }
+                ds_put_cstr(&ds, ")");
+            }
+        }
+        VLOG_DBG("bond %s:%s", bond->name, ds_cstr(&ds));
+        ds_destroy(&ds);
+    }
+}
+
+/* Shifts 'hash' from its current slave to 'to'. */
+static void
+bond_shift_load(struct bond_entry *hash, struct bond_slave *to,
+                struct tag_set *set)
+{
+    struct bond_slave *from = hash->slave;
+    struct bond *bond = from->bond;
+    uint64_t delta = hash->tx_bytes;
+
+    VLOG_INFO("bond %s: shift %"PRIu64"kB of load (with hash %td) "
+              "from %s to %s (now carrying %"PRIu64"kB and "
+              "%"PRIu64"kB load, respectively)",
+              bond->name, delta / 1024, hash - bond->hash,
+              from->name, to->name,
+              (from->tx_bytes - delta) / 1024,
+              (to->tx_bytes + delta) / 1024);
+
+    /* Shift load away from 'from' to 'to'. */
+    from->tx_bytes -= delta;
+    to->tx_bytes += delta;
+
+    /* Arrange for flows to be revalidated. */
+    tag_set_add(set, hash->tag);
+    hash->slave = to;
+    hash->tag = tag_create_random();
+}
+
+/* Pick and returns a bond_entry to migrate to 'to' (the least-loaded slave),
+ * given that doing so must decrease the ratio of the load on the two slaves by
+ * at least 0.1.  Returns NULL if there is no appropriate entry.
+ *
+ * The list of entries isn't sorted.  I don't know of a reason to prefer to
+ * shift away small hashes or large hashes. */
+static struct bond_entry *
+choose_entry_to_migrate(const struct bond_slave *from, uint64_t to_tx_bytes)
+{
+    struct bond_entry *e;
+
+    if (list_is_short(&from->entries)) {
+        /* 'from' carries no more than one MAC hash, so shifting load away from
+         * it would be pointless. */
+        return NULL;
+    }
+
+    LIST_FOR_EACH (e, list_node, &from->entries) {
+        double old_ratio, new_ratio;
+        uint64_t delta;
+
+        if (to_tx_bytes == 0) {
+            /* Nothing on the new slave, move it. */
+            return e;
+        }
+
+        delta = e->tx_bytes;
+        old_ratio = (double)from->tx_bytes / to_tx_bytes;
+        new_ratio = (double)(from->tx_bytes - delta) / (to_tx_bytes + delta);
+        if (old_ratio - new_ratio > 0.1) {
+            /* Would decrease the ratio, move it. */
+            return e;
+        }
+    }
+
+    return NULL;
+}
+
+/* Inserts 'slave' into 'bals' so that descending order of 'tx_bytes' is
+ * maintained. */
+static void
+insert_bal(struct list *bals, struct bond_slave *slave)
+{
+    struct bond_slave *pos;
+
+    LIST_FOR_EACH (pos, bal_node, bals) {
+        if (slave->tx_bytes > pos->tx_bytes) {
+            break;
+        }
+    }
+    list_insert(&pos->bal_node, &slave->bal_node);
+}
+
+/* Removes 'slave' from its current list and then inserts it into 'bals' so
+ * that descending order of 'tx_bytes' is maintained. */
+static void
+reinsert_bal(struct list *bals, struct bond_slave *slave)
+{
+    list_remove(&slave->bal_node);
+    insert_bal(bals, slave);
+}
+
+/* If 'bond' needs rebalancing, does so.
+ *
+ * The caller should have called bond_account() for each active flow, to ensure
+ * that flow data is consistently accounted at this point. */
+void
+bond_rebalance(struct bond *bond, struct tag_set *tags)
+{
+    struct bond_slave *slave;
+    struct bond_entry *e;
+    struct list bals;
+
+    if (bond->balance == BM_AB || time_msec() < bond->next_rebalance) {
+        return;
+    }
+    bond->next_rebalance = time_msec() + bond->rebalance_interval;
+
+    /* Add each bond_entry to its slave's 'entries' list.
+     * Compute each slave's tx_bytes as the sum of its entries' tx_bytes. */
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        slave->tx_bytes = 0;
+        list_init(&slave->entries);
+    }
+    for (e = &bond->hash[0]; e <= &bond->hash[BOND_MASK]; e++) {
+        if (e->slave && e->tx_bytes) {
+            e->slave->tx_bytes += e->tx_bytes;
+            list_push_back(&e->slave->entries, &e->list_node);
+        }
+    }
+
+    /* Add enabled slaves to 'bals' in descending order of tx_bytes.
+     *
+     * XXX This is O(n**2) in the number of slaves but it could be O(n lg n)
+     * with a proper list sort algorithm. */
+    list_init(&bals);
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        if (slave->enabled) {
+            insert_bal(&bals, slave);
+        }
+    }
+    log_bals(bond, &bals);
+
+    /* Shift load from the most-loaded slaves to the least-loaded slaves. */
+    while (!list_is_short(&bals)) {
+        struct bond_slave *from = bond_slave_from_bal_node(list_front(&bals));
+        struct bond_slave *to = bond_slave_from_bal_node(list_back(&bals));
+        uint64_t overload;
+
+        overload = from->tx_bytes - to->tx_bytes;
+        if (overload < to->tx_bytes >> 5 || overload < 100000) {
+            /* The extra load on 'from' (and all less-loaded slaves), compared
+             * to that of 'to' (the least-loaded slave), is less than ~3%, or
+             * it is less than ~1Mbps.  No point in rebalancing. */
+            break;
+        }
+
+        /* 'from' is carrying significantly more load than 'to', and that load
+         * is split across at least two different hashes. */
+        e = choose_entry_to_migrate(from, to->tx_bytes);
+        if (e) {
+            bond_shift_load(e, to, tags);
+
+            /* Delete element from from->entries.
+             *
+             * We don't add the element to to->hashes.  That would only allow
+             * 'e' to be migrated to another slave in this rebalancing run, and
+             * there is no point in doing that. */
+            list_remove(&e->list_node);
+
+            /* Re-sort 'bals'. */
+            reinsert_bal(&bals, from);
+            reinsert_bal(&bals, to);
+        } else {
+            /* Can't usefully migrate anything away from 'from'.
+             * Don't reconsider it. */
+            list_remove(&from->bal_node);
+        }
+    }
+
+    /* Implement exponentially weighted moving average.  A weight of 1/2 causes
+     * historical data to decay to <1% in 7 rebalancing runs.  1,000,000 bytes
+     * take 20 rebalancing runs to decay to 0 and get deleted entirely. */
+    for (e = &bond->hash[0]; e <= &bond->hash[BOND_MASK]; e++) {
+        e->tx_bytes /= 2;
+        if (!e->tx_bytes) {
+            e->slave = NULL;
+        }
+    }
+}
+
+/* Bonding unixctl user interface functions. */
+
+static struct bond *
+bond_find(const char *name)
+{
+    struct bond *bond;
+
+    HMAP_FOR_EACH_WITH_HASH (bond, hmap_node, hash_string(name, 0),
+                             &all_bonds) {
+        if (!strcmp(bond->name, name)) {
+            return bond;
+        }
+    }
+    return NULL;
+}
+
+static struct bond_slave *
+bond_lookup_slave(struct bond *bond, const char *slave_name)
+{
+    struct bond_slave *slave;
+
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        if (!strcmp(slave->name, slave_name)) {
+            return slave;
+        }
+    }
+    return NULL;
+}
+
+static void
+bond_unixctl_list(struct unixctl_conn *conn,
+                  const char *args OVS_UNUSED, void *aux OVS_UNUSED)
+{
+    struct ds ds = DS_EMPTY_INITIALIZER;
+    const struct bond *bond;
+
+    ds_put_cstr(&ds, "bond\ttype\tslaves\n");
+
+    HMAP_FOR_EACH (bond, hmap_node, &all_bonds) {
+        const struct bond_slave *slave;
+        size_t i;
+
+        ds_put_format(&ds, "%s\t%s\t",
+                      bond->name, bond_mode_to_string(bond->balance));
+
+        i = 0;
+        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+            if (i++ > 0) {
+                ds_put_cstr(&ds, ", ");
+            }
+            ds_put_cstr(&ds, slave->name);
+        }
+        ds_put_char(&ds, '\n');
+    }
+    unixctl_command_reply(conn, 200, ds_cstr(&ds));
+    ds_destroy(&ds);
+}
+
+static void
+bond_unixctl_show(struct unixctl_conn *conn,
+                  const char *args, void *aux OVS_UNUSED)
+{
+    struct ds ds = DS_EMPTY_INITIALIZER;
+    const struct bond_slave *slave;
+    const struct bond *bond;
+
+    bond = bond_find(args);
+    if (!bond) {
+        unixctl_command_reply(conn, 501, "no such bond");
+        return;
+    }
+
+    ds_put_format(&ds, "bond_mode: %s\n",
+                  bond_mode_to_string(bond->balance));
+
+    if (bond->lacp) {
+        ds_put_format(&ds, "lacp: %s\n",
+                      lacp_is_active(bond->lacp) ? "active" : "passive");
+    } else {
+        ds_put_cstr(&ds, "lacp: off\n");
+    }
+
+    if (bond->balance != BM_AB) {
+        ds_put_format(&ds, "bond-hash-algorithm: %s\n",
+                      bond_is_tcp_hash(bond) ? "balance-tcp" : "balance-slb");
+    }
+
+    ds_put_format(&ds, "bond-detect-mode: %s\n",
+                  bond->monitor ? "carrier" : "miimon");
+
+    if (!bond->monitor) {
+        ds_put_format(&ds, "bond-miimon-interval: %lld\n",
+                      bond->miimon_interval);
+    }
+
+    ds_put_format(&ds, "updelay: %d ms\n", bond->updelay);
+    ds_put_format(&ds, "downdelay: %d ms\n", bond->downdelay);
+
+    if (bond->balance != BM_AB) {
+        ds_put_format(&ds, "next rebalance: %lld ms\n",
+                      bond->next_rebalance - time_msec());
+    }
+
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        struct bond_entry *be;
+        struct flow flow;
+
+        /* Basic info. */
+        ds_put_format(&ds, "\nslave %s: %s\n",
+                      slave->name, slave->enabled ? "enabled" : "disabled");
+        if (slave == bond->active_slave) {
+            ds_put_cstr(&ds, "\tactive slave\n");
+        }
+        if (slave->delay_expires != LLONG_MAX) {
+            ds_put_format(&ds, "\t%s expires in %lld ms\n",
+                          slave->enabled ? "downdelay" : "updelay",
+                          slave->delay_expires - time_msec());
+        }
+
+        if (bond->balance == BM_AB) {
+            continue;
+        }
+
+        /* Hashes. */
+        memset(&flow, 0, sizeof flow);
+        for (be = bond->hash; be <= &bond->hash[BOND_MASK]; be++) {
+            int hash = be - bond->hash;
+
+            if (be->slave != slave) {
+                continue;
+            }
+
+            ds_put_format(&ds, "\thash %d: %"PRIu64" kB load\n",
+                          hash, be->tx_bytes / 1024);
+
+            if (bond->balance != BM_SLB) {
+                continue;
+            }
+
+            /* XXX How can we list the MACs assigned to hashes? */
+        }
+    }
+    unixctl_command_reply(conn, 200, ds_cstr(&ds));
+    ds_destroy(&ds);
+}
+
+static void
+bond_unixctl_migrate(struct unixctl_conn *conn, const char *args_,
+                     void *aux OVS_UNUSED)
+{
+    char *args = (char *) args_;
+    char *save_ptr = NULL;
+    char *bond_s, *hash_s, *slave_s;
+    struct bond *bond;
+    struct bond_slave *slave;
+    struct bond_entry *entry;
+    int hash;
+
+    bond_s = strtok_r(args, " ", &save_ptr);
+    hash_s = strtok_r(NULL, " ", &save_ptr);
+    slave_s = strtok_r(NULL, " ", &save_ptr);
+    if (!slave_s) {
+        unixctl_command_reply(conn, 501,
+                              "usage: bond/migrate BOND HASH SLAVE");
+        return;
+    }
+
+    bond = bond_find(bond_s);
+    if (!bond) {
+        unixctl_command_reply(conn, 501, "no such bond");
+        return;
+    }
+
+    if (bond->balance != BM_SLB) {
+        unixctl_command_reply(conn, 501, "not an SLB bond");
+        return;
+    }
+
+    if (strspn(hash_s, "0123456789") == strlen(hash_s)) {
+        hash = atoi(hash_s) & BOND_MASK;
+    } else {
+        unixctl_command_reply(conn, 501, "bad hash");
+        return;
+    }
+
+    slave = bond_lookup_slave(bond, slave_s);
+    if (!slave) {
+        unixctl_command_reply(conn, 501, "no such slave");
+        return;
+    }
+
+    if (!slave->enabled) {
+        unixctl_command_reply(conn, 501, "cannot migrate to disabled slave");
+        return;
+    }
+
+    entry = &bond->hash[hash];
+    tag_set_add(&bond->unixctl_tags, entry->tag);
+    entry->slave = slave;
+    entry->tag = tag_create_random();
+    unixctl_command_reply(conn, 200, "migrated");
+}
+
+static void
+bond_unixctl_set_active_slave(struct unixctl_conn *conn, const char *args_,
+                              void *aux OVS_UNUSED)
+{
+    char *args = (char *) args_;
+    char *save_ptr = NULL;
+    char *bond_s, *slave_s;
+    struct bond *bond;
+    struct bond_slave *slave;
+
+    bond_s = strtok_r(args, " ", &save_ptr);
+    slave_s = strtok_r(NULL, " ", &save_ptr);
+    if (!slave_s) {
+        unixctl_command_reply(conn, 501,
+                              "usage: bond/set-active-slave BOND SLAVE");
+        return;
+    }
+
+    bond = bond_find(bond_s);
+    if (!bond) {
+        unixctl_command_reply(conn, 501, "no such bond");
+        return;
+    }
+
+    slave = bond_lookup_slave(bond, slave_s);
+    if (!slave) {
+        unixctl_command_reply(conn, 501, "no such slave");
+        return;
+    }
+
+    if (!slave->enabled) {
+        unixctl_command_reply(conn, 501, "cannot make disabled slave active");
+        return;
+    }
+
+    if (bond->active_slave != slave) {
+        tag_set_add(&bond->unixctl_tags, bond_get_active_slave_tag(bond));
+        bond->active_slave = slave;
+        bond->active_slave->tag = tag_create_random();
+        VLOG_INFO("bond %s: active interface is now %s",
+                  bond->name, slave->name);
+        bond->send_learning_packets = true;
+        unixctl_command_reply(conn, 200, "done");
+    } else {
+        unixctl_command_reply(conn, 200, "no change");
+    }
+}
+
+static void
+enable_slave(struct unixctl_conn *conn, const char *args_, bool enable)
+{
+    char *args = (char *) args_;
+    char *save_ptr = NULL;
+    char *bond_s, *slave_s;
+    struct bond *bond;
+    struct bond_slave *slave;
+
+    bond_s = strtok_r(args, " ", &save_ptr);
+    slave_s = strtok_r(NULL, " ", &save_ptr);
+    if (!slave_s) {
+        char *usage = xasprintf("usage: bond/%s-slave BOND SLAVE",
+                                enable ? "enable" : "disable");
+        unixctl_command_reply(conn, 501, usage);
+        free(usage);
+        return;
+    }
+
+    bond = bond_find(bond_s);
+    if (!bond) {
+        unixctl_command_reply(conn, 501, "no such bond");
+        return;
+    }
+
+    slave = bond_lookup_slave(bond, slave_s);
+    if (!slave) {
+        unixctl_command_reply(conn, 501, "no such slave");
+        return;
+    }
+
+    bond_enable_slave(slave, enable, &bond->unixctl_tags);
+    unixctl_command_reply(conn, 501, enable ? "enabled" : "disabled");
+}
+
+static void
+bond_unixctl_enable_slave(struct unixctl_conn *conn, const char *args,
+                          void *aux OVS_UNUSED)
+{
+    enable_slave(conn, args, true);
+}
+
+static void
+bond_unixctl_disable_slave(struct unixctl_conn *conn, const char *args,
+                           void *aux OVS_UNUSED)
+{
+    enable_slave(conn, args, false);
+}
+
+static void
+bond_unixctl_hash(struct unixctl_conn *conn, const char *args_,
+                  void *aux OVS_UNUSED)
+{
+    char *args = (char *) args_;
+    uint8_t mac[ETH_ADDR_LEN];
+    uint8_t hash;
+    char *hash_cstr;
+    unsigned int vlan;
+    char *mac_s, *vlan_s;
+    char *save_ptr = NULL;
+
+    mac_s  = strtok_r(args, " ", &save_ptr);
+    vlan_s = strtok_r(NULL, " ", &save_ptr);
+
+    if (vlan_s) {
+        if (sscanf(vlan_s, "%u", &vlan) != 1) {
+            unixctl_command_reply(conn, 501, "invalid vlan");
+            return;
+        }
+    } else {
+        vlan = OFP_VLAN_NONE;
+    }
+
+    if (sscanf(mac_s, ETH_ADDR_SCAN_FMT, ETH_ADDR_SCAN_ARGS(mac))
+        == ETH_ADDR_SCAN_COUNT) {
+        hash = bond_hash_src(mac, vlan) & BOND_MASK;
+
+        hash_cstr = xasprintf("%u", hash);
+        unixctl_command_reply(conn, 200, hash_cstr);
+        free(hash_cstr);
+    } else {
+        unixctl_command_reply(conn, 501, "invalid mac");
+    }
+}
+
+void
+bond_init(void)
+{
+    lacp_init();
+
+    unixctl_command_register("bond/list", bond_unixctl_list, NULL);
+    unixctl_command_register("bond/show", bond_unixctl_show, NULL);
+    unixctl_command_register("bond/migrate", bond_unixctl_migrate, NULL);
+    unixctl_command_register("bond/set-active-slave",
+                             bond_unixctl_set_active_slave, NULL);
+    unixctl_command_register("bond/enable-slave", bond_unixctl_enable_slave,
+                             NULL);
+    unixctl_command_register("bond/disable-slave", bond_unixctl_disable_slave,
+                             NULL);
+    unixctl_command_register("bond/hash", bond_unixctl_hash, NULL);
+}
+
+static struct bond_slave *
+bond_slave_lookup(struct bond *bond, const void *slave_)
+{
+    struct bond_slave *slave;
+
+    HMAP_FOR_EACH_IN_BUCKET (slave, hmap_node, hash_pointer(slave_, 0),
+                             &bond->slaves) {
+        if (slave->aux == slave_) {
+            return slave;
+        }
+    }
+
+    return NULL;
+}
+
+static bool
+bond_is_link_up(struct bond *bond, struct netdev *netdev)
+{
+    return (bond->detect == BLSM_CARRIER
+            ? netdev_get_carrier(netdev)
+            : netdev_get_miimon(netdev));
+}
+
+static void
+bond_enable_slave(struct bond_slave *slave, bool enable, struct tag_set *tags)
+{
+    slave->delay_expires = LLONG_MAX;
+    if (enable != slave->enabled) {
+        slave->enabled = enable;
+        if (!slave->enabled) {
+            VLOG_WARN("interface %s: disabled", slave->name);
+            tag_set_add(tags, slave->tag);
+        } else {
+            VLOG_WARN("interface %s: enabled", slave->name);
+            slave->tag = tag_create_random();
+        }
+    }
+}
+
+static void
+bond_link_status_update(struct bond_slave *slave, struct tag_set *tags)
+{
+    struct bond *bond = slave->bond;
+    bool up;
+
+    up = slave->up && lacp_slave_may_enable(bond->lacp, slave);
+    if ((up == slave->enabled) != (slave->delay_expires == LLONG_MAX)) {
+        static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
+        VLOG_INFO_RL(&rl, "interface %s: link state %s",
+                     slave->name, up ? "up" : "down");
+        if (up == slave->enabled) {
+            slave->delay_expires = LLONG_MAX;
+            VLOG_INFO_RL(&rl, "interface %s: will not be %s",
+                         slave->name, up ? "disabled" : "enabled");
+        } else {
+            int delay = (lacp_negotiated(bond->lacp) ? 0
+                         : up ? bond->updelay : bond->downdelay);
+            slave->delay_expires = time_msec() + delay;
+            if (delay) {
+                VLOG_INFO_RL(&rl, "interface %s: will be %s if it stays %s "
+                             "for %d ms",
+                             slave->name,
+                             up ? "enabled" : "disabled",
+                             up ? "up" : "down",
+                             delay);
+            }
+        }
+    }
+
+    if (time_msec() >= slave->delay_expires) {
+        bond_enable_slave(slave, up, tags);
+    }
+}
+
+static bool
+bond_is_tcp_hash(const struct bond *bond)
+{
+    return bond->balance == BM_TCP && lacp_negotiated(bond->lacp);
+}
+
+static unsigned int
+bond_hash_src(const uint8_t mac[ETH_ADDR_LEN], uint16_t vlan)
+{
+    return hash_bytes(mac, ETH_ADDR_LEN, vlan);
+}
+
+static unsigned int
+bond_hash_tcp(const struct flow *flow, uint16_t vlan)
+{
+    struct flow hash_flow = *flow;
+    hash_flow.vlan_tci = vlan;
+
+    /* The symmetric quality of this hash function is not required, but
+     * flow_hash_symmetric_l4 already exists, and is sufficient for our
+     * purposes, so we use it out of convenience. */
+    return flow_hash_symmetric_l4(&hash_flow, 0);
+}
+
+static struct bond_entry *
+lookup_bond_entry(const struct bond *bond, const struct flow *flow,
+                  uint16_t vlan)
+{
+    assert(bond->balance != BM_AB);
+    return &bond->hash[(bond_is_tcp_hash(bond)
+                        ? bond_hash_tcp(flow, vlan)
+                        : bond_hash_src(flow->dl_src, vlan)) & BOND_MASK];
+}
+
+static struct bond_slave *
+choose_output_slave(const struct bond *bond, const struct flow *flow,
+                    uint16_t vlan)
+{
+    struct bond_entry *e;
+
+    switch (bond->balance) {
+    case BM_AB:
+        return bond->active_slave;
+
+    case BM_SLB:
+    case BM_TCP:
+        e = lookup_bond_entry(bond, flow, vlan);
+        if (!e->slave || !e->slave->enabled) {
+            /* XXX select interface properly.  The current interface selection
+             * is only good for testing the rebalancing code. */
+            e->slave = bond->active_slave;
+            e->tag = tag_create_random();
+        }
+        return e->slave;
+
+    default:
+        NOT_REACHED();
+    }
+}
+
+static struct bond_slave *
+bond_choose_slave(const struct bond *bond)
+{
+    struct bond_slave *slave, *best;
+
+    /* Find an enabled slave. */
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        if (slave->enabled) {
+            return slave;
+        }
+    }
+
+    /* All interfaces are disabled.  Find an interface that will be enabled
+     * after its updelay expires.  */
+    best = NULL;
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        if (slave->delay_expires != LLONG_MAX
+            && lacp_slave_may_enable(bond->lacp, slave)
+            && (!best || slave->delay_expires < best->delay_expires)) {
+            best = slave;
+        }
+    }
+    return best;
+}
+
+static void
+bond_choose_active_slave(struct bond *bond, struct tag_set *tags)
+{
+    static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
+    struct bond_slave *old_active_slave = bond->active_slave;
+
+    bond->active_slave = bond_choose_slave(bond);
+    if (bond->active_slave) {
+        if (bond->active_slave->enabled) {
+            VLOG_INFO_RL(&rl, "bond %s: active interface is now %s",
+                         bond->name, bond->active_slave->name);
+        } else {
+            VLOG_INFO_RL(&rl, "bond %s: active interface is now %s, skipping "
+                         "remaining %lld ms updelay (since no interface was "
+                         "enabled)", bond->name, bond->active_slave->name,
+                         bond->active_slave->delay_expires - time_msec());
+            bond_enable_slave(bond->active_slave, true, tags);
+        }
+
+        if (!old_active_slave) {
+            tag_set_add(tags, bond->no_slaves_tag);
+        }
+
+        bond->send_learning_packets = true;
+    } else if (old_active_slave) {
+        VLOG_WARN_RL(&rl, "bond %s: all interfaces disabled", bond->name);
+    }
+}
+
+/* Returns the tag for 'bond''s active slave, or 'bond''s no_slaves_tag if
+ * there is no active slave. */
+static tag_type
+bond_get_active_slave_tag(const struct bond *bond)
+{
+    return (bond->active_slave
+            ? bond->active_slave->tag
+            : bond->no_slaves_tag);
+}
+
+/* Attempts to make the sum of the bond slaves' statistics appear on the fake
+ * bond interface. */
+static void
+bond_update_fake_slave_stats(struct bond *bond)
+{
+    struct netdev_stats bond_stats;
+    struct bond_slave *slave;
+    struct netdev *bond_dev;
+
+    memset(&bond_stats, 0, sizeof bond_stats);
+
+    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
+        struct netdev_stats slave_stats;
+
+        if (!netdev_get_stats(slave->netdev, &slave_stats)) {
+            /* XXX: We swap the stats here because they are swapped back when
+             * reported by the internal device.  The reason for this is
+             * internal devices normally represent packets going into the
+             * system but when used as fake bond device they represent packets
+             * leaving the system.  We really should do this in the internal
+             * device itself because changing it here reverses the counts from
+             * the perspective of the switch.  However, the internal device
+             * doesn't know what type of device it represents so we have to do
+             * it here for now. */
+            bond_stats.tx_packets += slave_stats.rx_packets;
+            bond_stats.tx_bytes += slave_stats.rx_bytes;
+            bond_stats.rx_packets += slave_stats.tx_packets;
+            bond_stats.rx_bytes += slave_stats.tx_bytes;
+        }
+    }
+
+    if (!netdev_open_default(bond->name, &bond_dev)) {
+        netdev_set_stats(bond_dev, &bond_stats);
+        netdev_close(bond_dev);
+    }
+}