text: /*
   * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2016 Nicira, Inc.
   *
   * 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.
   */
  
  /* Based on sample implementation in 802.1D-1998.  Above copyright and license
   * applies to all modifications. */
  
  #include <config.h>
  
  #include "stp.h"
  #include <sys/types.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <inttypes.h>
  #include <stdlib.h>
  #include "byte-order.h"
  #include "connectivity.h"
  #include "ofpbuf.h"
  #include "ovs-atomic.h"
  #include "dp-packet.h"
  #include "packets.h"
  #include "seq.h"
  #include "unixctl.h"
  #include "util.h"
  #include "openvswitch/vlog.h"
  
  VLOG_DEFINE_THIS_MODULE(stp);
  
  static struct vlog_rate_limit stp_rl = VLOG_RATE_LIMIT_INIT(60, 60);
  
  #define STP_PROTOCOL_ID 0x0000
  #define STP_PROTOCOL_VERSION 0x00
  #define STP_TYPE_CONFIG 0x00
  #define STP_TYPE_TCN 0x80
  
  OVS_PACKED(
  struct stp_bpdu_header {
      ovs_be16 protocol_id;       /* STP_PROTOCOL_ID. */
      uint8_t protocol_version;   /* STP_PROTOCOL_VERSION. */
      uint8_t bpdu_type;          /* One of STP_TYPE_*. */
  });
  BUILD_ASSERT_DECL(sizeof(struct stp_bpdu_header) == 4);
  
  enum stp_config_bpdu_flags {
      STP_CONFIG_TOPOLOGY_CHANGE_ACK = 0x80,
      STP_CONFIG_TOPOLOGY_CHANGE = 0x01
  };
  
  OVS_PACKED(
  struct stp_config_bpdu {
      struct stp_bpdu_header header; /* Type STP_TYPE_CONFIG. */
      uint8_t flags;                 /* STP_CONFIG_* flags. */
      ovs_be64 root_id;              /* 8.5.1.1: Bridge believed to be root. */
      ovs_be32 root_path_cost;       /* 8.5.1.2: Cost of path to root. */
      ovs_be64 bridge_id;            /* 8.5.1.3: ID of transmitting bridge. */
      ovs_be16 port_id;              /* 8.5.1.4: Port transmitting the BPDU. */
      ovs_be16 message_age;          /* 8.5.1.5: Age of BPDU at tx time. */
      ovs_be16 max_age;              /* 8.5.1.6: Timeout for received data. */
      ovs_be16 hello_time;           /* 8.5.1.7: Time between BPDU generation. */
      ovs_be16 forward_delay;        /* 8.5.1.8: State progression delay. */
  });
  BUILD_ASSERT_DECL(sizeof(struct stp_config_bpdu) == 35);
  
  OVS_PACKED(
  struct stp_tcn_bpdu {
      struct stp_bpdu_header header; /* Type STP_TYPE_TCN. */
  });
  BUILD_ASSERT_DECL(sizeof(struct stp_tcn_bpdu) == 4);
  
  struct stp_timer {
      bool active;                 /* Timer in use? */
      int value;                   /* Current value of timer, counting up. */
  };
  
  struct stp_port {
      struct stp *stp;
      char *port_name;                /* Human-readable name for log messages. */
      void *aux;                      /* Auxiliary data the user may retrieve. */
      int port_id;                    /* 8.5.5.1: Unique port identifier. */
      enum stp_state state;           /* 8.5.5.2: Current state. */
      int path_cost;                  /* 8.5.5.3: Cost of tx/rx on this port. */
      stp_identifier designated_root; /* 8.5.5.4. */
      int designated_cost;            /* 8.5.5.5: Path cost to root on port. */
      stp_identifier designated_bridge; /* 8.5.5.6. */
      int designated_port;            /* 8.5.5.7: Port to send config msgs on. */
      bool topology_change_ack;       /* 8.5.5.8: Flag for next config BPDU. */
      bool config_pending;            /* 8.5.5.9: Send BPDU when hold expires? */
      bool change_detection_enabled;  /* 8.5.5.10: Detect topology changes? */
  
      struct stp_timer message_age_timer; /* 8.5.6.1: Age of received info. */
      struct stp_timer forward_delay_timer; /* 8.5.6.2: State change timer. */
      struct stp_timer hold_timer;        /* 8.5.6.3: BPDU rate limit timer. */
  
      int tx_count;                   /* Number of BPDUs transmitted. */
      int rx_count;                   /* Number of valid BPDUs received. */
      int error_count;                /* Number of bad BPDUs received. */
  
      bool state_changed;
  };
  
  struct stp {
      struct ovs_list node;           /* Node in all_stps list. */
  
      /* Static bridge data. */
      char *name;                     /* Human-readable name for log messages. */
      stp_identifier bridge_id;       /* 8.5.3.7: This bridge. */
      int max_age;                    /* 8.5.3.4: Time to drop received data. */
      int hello_time;                 /* 8.5.3.5: Time between sending BPDUs. */
      int forward_delay;              /* 8.5.3.6: Delay between state changes. */
      int bridge_max_age;             /* 8.5.3.8: max_age when we're root. */
      int bridge_hello_time;          /* 8.5.3.9: hello_time as root. */
      int bridge_forward_delay;       /* 8.5.3.10: forward_delay as root. */
      int rq_max_age;                 /* User-requested max age, in ms. */
      int rq_hello_time;              /* User-requested hello time, in ms. */
      int rq_forward_delay;           /* User-requested forward delay, in ms. */
      int elapsed_remainder;          /* Left-over msecs from last stp_tick(). */
  
      /* Dynamic bridge data. */
      stp_identifier designated_root; /* 8.5.3.1: Bridge believed to be root. */
      unsigned int root_path_cost;    /* 8.5.3.2: Cost of path to root. */
      struct stp_port *root_port;     /* 8.5.3.3: Lowest cost port to root. */
      bool topology_change_detected;  /* 8.5.3.11: Detected a topology change? */
      bool topology_change;           /* 8.5.3.12: Received topology change? */
  
      /* Bridge timers. */
      struct stp_timer hello_timer;   /* 8.5.4.1: Hello timer. */
      struct stp_timer tcn_timer;     /* 8.5.4.2: Topology change timer. */
      struct stp_timer topology_change_timer; /* 8.5.4.3. */
  
      /* Ports. */
      struct stp_port ports[STP_MAX_PORTS];
  
      /* Interface to client. */
      bool fdb_needs_flush;          /* MAC learning tables needs flushing. */
      struct stp_port *first_changed_port;
      void (*send_bpdu)(struct dp_packet *bpdu, int port_no, void *aux);
      void *aux;
  
      struct ovs_refcount ref_cnt;
  };
  
  static struct ovs_mutex mutex;
  static struct ovs_list all_stps__ = OVS_LIST_INITIALIZER(&all_stps__);
  static struct ovs_list *const all_stps OVS_GUARDED_BY(mutex) = &all_stps__;
  
  #define FOR_EACH_ENABLED_PORT(PORT, STP)                        \
      for ((PORT) = stp_next_enabled_port((STP), (STP)->ports);   \
           (PORT);                                                \
           (PORT) = stp_next_enabled_port((STP), (PORT) + 1))
  static struct stp_port *
  stp_next_enabled_port(const struct stp *stp, const struct stp_port *port)
      OVS_REQUIRES(mutex)
  {
      for (; port < &stp->ports[ARRAY_SIZE(stp->ports)]; port++) {
          if (port->state != STP_DISABLED) {
              return CONST_CAST(struct stp_port *, port);
          }
      }
      return NULL;
  }
  
  #define MESSAGE_AGE_INCREMENT 1
  
  static void stp_transmit_config(struct stp_port *) OVS_REQUIRES(mutex);
  static bool stp_supersedes_port_info(const struct stp_port *,
                                       const struct stp_config_bpdu *)
      OVS_REQUIRES(mutex);
  static void stp_record_config_information(struct stp_port *,
                                            const struct stp_config_bpdu *)
      OVS_REQUIRES(mutex);
  static void stp_record_config_timeout_values(struct stp *,
                                               const struct stp_config_bpdu  *)
      OVS_REQUIRES(mutex);
  static bool stp_is_designated_port(const struct stp_port *)
      OVS_REQUIRES(mutex);
  static void stp_config_bpdu_generation(struct stp *) OVS_REQUIRES(mutex);
  static void stp_transmit_tcn(struct stp *) OVS_REQUIRES(mutex);
  static void stp_configuration_update(struct stp *) OVS_REQUIRES(mutex);
  static bool stp_supersedes_root(const struct stp_port *root,
                                  const struct stp_port *) OVS_REQUIRES(mutex);
  static void stp_root_selection(struct stp *) OVS_REQUIRES(mutex);
  static void stp_designated_port_selection(struct stp *) OVS_REQUIRES(mutex);
  static void stp_become_designated_port(struct stp_port *)
      OVS_REQUIRES(mutex);
  static void stp_port_state_selection(struct stp *) OVS_REQUIRES(mutex);
  static void stp_make_forwarding(struct stp_port *) OVS_REQUIRES(mutex);
  static void stp_make_blocking(struct stp_port *) OVS_REQUIRES(mutex);
  static void stp_set_port_state(struct stp_port *, enum stp_state)
      OVS_REQUIRES(mutex);
  static void stp_topology_change_detection(struct stp *) OVS_REQUIRES(mutex);
  static void stp_topology_change_acknowledged(struct stp *)
      OVS_REQUIRES(mutex);
  static void stp_acknowledge_topology_change(struct stp_port *)
      OVS_REQUIRES(mutex);
  static void stp_received_config_bpdu(struct stp *, struct stp_port *,
                                       const struct stp_config_bpdu *)
      OVS_REQUIRES(mutex);
  static void stp_received_tcn_bpdu(struct stp *, struct stp_port *)
      OVS_REQUIRES(mutex);
  static void stp_hello_timer_expiry(struct stp *) OVS_REQUIRES(mutex);
  static void stp_message_age_timer_expiry(struct stp_port *)
      OVS_REQUIRES(mutex);
  static bool stp_is_designated_for_some_port(const struct stp *)
      OVS_REQUIRES(mutex);
  static void stp_forward_delay_timer_expiry(struct stp_port *)
      OVS_REQUIRES(mutex);
  static void stp_tcn_timer_expiry(struct stp *) OVS_REQUIRES(mutex);
  static void stp_topology_change_timer_expiry(struct stp *)
      OVS_REQUIRES(mutex);
  static void stp_hold_timer_expiry(struct stp_port *) OVS_REQUIRES(mutex);
  static void stp_initialize_port(struct stp_port *, enum stp_state)
      OVS_REQUIRES(mutex);
  static void stp_become_root_bridge(struct stp *) OVS_REQUIRES(mutex);
  static void stp_update_bridge_timers(struct stp *) OVS_REQUIRES(mutex);
  
  static int clamp(int x, int min, int max);
  static int ms_to_timer(int ms);
  static int timer_to_ms(int timer);
  static void stp_start_timer(struct stp_timer *, int value);
  static void stp_stop_timer(struct stp_timer *);
  static bool stp_timer_expired(struct stp_timer *, int elapsed, int timeout);
  
  static void stp_send_bpdu(struct stp_port *, const void *, size_t)
      OVS_REQUIRES(mutex);
  static void stp_unixctl_tcn(struct unixctl_conn *, int argc,
                              const char *argv[], void *aux);
  
  void
  stp_init(void)
  {
      static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER;
  
      if (ovsthread_once_start(&once)) {
          /* We need a recursive mutex because stp_send_bpdu() could loop back
           * into the stp module through a patch port.  This happens
           * intentionally as part of the unit tests.  Ideally we'd ditch
           * the call back function, but for now this is what we have. */
          ovs_mutex_init_recursive(&mutex);
  
          unixctl_command_register("stp/tcn", "[bridge]", 0, 1, stp_unixctl_tcn,
                                   NULL);
          ovsthread_once_done(&once);
      }
  }
  
  /* Creates and returns a new STP instance that initially has no ports enabled.
   *
   * 'bridge_id' should be a 48-bit MAC address as returned by
   * eth_addr_to_uint64().  'bridge_id' may also have a priority value in its top
   * 16 bits; if those bits are set to 0, STP_DEFAULT_BRIDGE_PRIORITY is used.
   * (This priority may be changed with stp_set_bridge_priority().)
   *
   * When the bridge needs to send out a BPDU, it calls 'send_bpdu'.  This
   * callback may be called from stp_tick() or stp_received_bpdu().  The
   * arguments to 'send_bpdu' are an STP BPDU encapsulated in 'bpdu',
   * the spanning tree port number 'port_no' that should transmit the
   * packet, and auxiliary data to be passed to the callback in 'aux'.
   */
  struct stp *
  stp_create(const char *name, stp_identifier bridge_id,
             void (*send_bpdu)(struct dp_packet *bpdu, int port_no, void *aux),
             void *aux)
  {
      struct stp *stp;
      struct stp_port *p;
  
      stp_init();
  
      ovs_mutex_lock(&mutex);
      stp = xzalloc(sizeof *stp);
      stp->name = xstrdup(name);
      stp->bridge_id = bridge_id;
      if (!(stp->bridge_id >> 48)) {
          stp->bridge_id |= (uint64_t) STP_DEFAULT_BRIDGE_PRIORITY << 48;
      }
  
      stp->rq_max_age = STP_DEFAULT_MAX_AGE;
      stp->rq_hello_time = STP_DEFAULT_HELLO_TIME;
      stp->rq_forward_delay = STP_DEFAULT_FWD_DELAY;
      stp_update_bridge_timers(stp);
      stp->max_age = stp->bridge_max_age;
      stp->hello_time = stp->bridge_hello_time;
      stp->forward_delay = stp->bridge_forward_delay;
  
      stp->designated_root = stp->bridge_id;
      stp->root_path_cost = 0;
      stp->root_port = NULL;
      stp->topology_change_detected = false;
      stp->topology_change = false;
  
      stp_stop_timer(&stp->tcn_timer);
      stp_stop_timer(&stp->topology_change_timer);
      stp_start_timer(&stp->hello_timer, 0);
  
      stp->send_bpdu = send_bpdu;
      stp->aux = aux;
  
      stp->first_changed_port = &stp->ports[ARRAY_SIZE(stp->ports)];
      for (p = stp->ports; p < &stp->ports[ARRAY_SIZE(stp->ports)]; p++) {
          p->stp = stp;
          p->port_id = (stp_port_no(p) + 1) | (STP_DEFAULT_PORT_PRIORITY << 8);
          p->path_cost = 19;      /* Recommended default for 100 Mb/s link. */
          stp_initialize_port(p, STP_DISABLED);
      }
      ovs_refcount_init(&stp->ref_cnt);
  
      list_push_back(all_stps, &stp->node);
      ovs_mutex_unlock(&mutex);
      return stp;
  }
  
  struct stp *
  stp_ref(const struct stp *stp_)
  {
      struct stp *stp = CONST_CAST(struct stp *, stp_);
      if (stp) {
          ovs_refcount_ref(&stp->ref_cnt);
      }
      return stp;
  }
  
  /* Destroys 'stp'. */
  void
  stp_unref(struct stp *stp)
  {
      if (stp && ovs_refcount_unref_relaxed(&stp->ref_cnt) == 1) {
          size_t i;
  
          ovs_mutex_lock(&mutex);
          list_remove(&stp->node);
          ovs_mutex_unlock(&mutex);
          free(stp->name);
  
          for (i = 0; i < STP_MAX_PORTS; i++) {
              free(stp->ports[i].port_name);
          }
          free(stp);
      }
  }
  
  /* Runs 'stp' given that 'ms' milliseconds have passed. */
  void
  stp_tick(struct stp *stp, int ms)
  {
      struct stp_port *p;
      int elapsed;
  
      ovs_mutex_lock(&mutex);
      /* Convert 'ms' to STP timer ticks.  Preserve any leftover milliseconds
       * from previous stp_tick() calls so that we don't lose STP ticks when we
       * are called too frequently. */
      ms = clamp(ms, 0, INT_MAX - 1000) + stp->elapsed_remainder;
      elapsed = ms_to_timer(ms);
      stp->elapsed_remainder = ms - timer_to_ms(elapsed);
      if (!elapsed) {
          goto out;
      }
  
      if (stp_timer_expired(&stp->hello_timer, elapsed, stp->hello_time)) {
          stp_hello_timer_expiry(stp);
      }
      if (stp_timer_expired(&stp->tcn_timer, elapsed, stp->bridge_hello_time)) {
          stp_tcn_timer_expiry(stp);
      }
      if (stp_timer_expired(&stp->topology_change_timer, elapsed,
                            stp->max_age + stp->forward_delay)) {
          stp_topology_change_timer_expiry(stp);
      }
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (stp_timer_expired(&p->message_age_timer, elapsed, stp->max_age)) {
              stp_message_age_timer_expiry(p);
          }
      }
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (stp_timer_expired(&p->forward_delay_timer, elapsed,
                                stp->forward_delay)) {
              stp_forward_delay_timer_expiry(p);
          }
          if (stp_timer_expired(&p->hold_timer, elapsed, ms_to_timer(1000))) {
              stp_hold_timer_expiry(p);
          }
      }
  
  out:
      ovs_mutex_unlock(&mutex);
  }
  
  static void
  set_bridge_id(struct stp *stp, stp_identifier new_bridge_id)
      OVS_REQUIRES(mutex)
  {
      if (new_bridge_id != stp->bridge_id) {
          bool root;
          struct stp_port *p;
  
          root = stp_is_root_bridge(stp);
          FOR_EACH_ENABLED_PORT (p, stp) {
              if (stp_is_designated_port(p)) {
                  p->designated_bridge = new_bridge_id;
              }
          }
          stp->bridge_id = new_bridge_id;
          stp_configuration_update(stp);
          stp_port_state_selection(stp);
          if (stp_is_root_bridge(stp) && !root) {
              stp_become_root_bridge(stp);
          }
      }
  }
  
  void
  stp_set_bridge_id(struct stp *stp, stp_identifier bridge_id)
  {
      const uint64_t mac_bits = (UINT64_C(1) << 48) - 1;
      const uint64_t pri_bits = ~mac_bits;
      ovs_mutex_lock(&mutex);
      set_bridge_id(stp, (stp->bridge_id & pri_bits) | (bridge_id & mac_bits));
      ovs_mutex_unlock(&mutex);
  }
  
  void
  stp_set_bridge_priority(struct stp *stp, uint16_t new_priority)
  {
      const uint64_t mac_bits = (UINT64_C(1) << 48) - 1;
      ovs_mutex_lock(&mutex);
      set_bridge_id(stp, ((stp->bridge_id & mac_bits)
                          | ((uint64_t) new_priority << 48)));
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the desired hello time for 'stp' to 'ms', in milliseconds.  The actual
   * hello time is clamped to the range of 1 to 10 seconds and subject to the
   * relationship (bridge_max_age >= 2 * (bridge_hello_time + 1 s)).  The bridge
   * hello time is only used when 'stp' is the root bridge. */
  void
  stp_set_hello_time(struct stp *stp, int ms)
  {
      ovs_mutex_lock(&mutex);
      stp->rq_hello_time = ms;
      stp_update_bridge_timers(stp);
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the desired max age for 'stp' to 'ms', in milliseconds.  The actual max
   * age is clamped to the range of 6 to 40 seconds and subject to the
   * relationships (2 * (bridge_forward_delay - 1 s) >= bridge_max_age) and
   * (bridge_max_age >= 2 * (bridge_hello_time + 1 s)).  The bridge max age is
   * only used when 'stp' is the root bridge. */
  void
  stp_set_max_age(struct stp *stp, int ms)
  {
      ovs_mutex_lock(&mutex);
      stp->rq_max_age = ms;
      stp_update_bridge_timers(stp);
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the desired forward delay for 'stp' to 'ms', in milliseconds.  The
   * actual forward delay is clamped to the range of 4 to 30 seconds and subject
   * to the relationship (2 * (bridge_forward_delay - 1 s) >= bridge_max_age).
   * The bridge forward delay is only used when 'stp' is the root bridge. */
  void
  stp_set_forward_delay(struct stp *stp, int ms)
  {
      ovs_mutex_lock(&mutex);
      stp->rq_forward_delay = ms;
      stp_update_bridge_timers(stp);
      ovs_mutex_unlock(&mutex);
  }
  
  /* Returns the name given to 'stp' in the call to stp_create(). */
  const char *
  stp_get_name(const struct stp *stp)
  {
      char *name;
  
      ovs_mutex_lock(&mutex);
      name = stp->name;
      ovs_mutex_unlock(&mutex);
      return name;
  }
  
  /* Returns the bridge ID for 'stp'. */
  stp_identifier
  stp_get_bridge_id(const struct stp *stp)
  {
      stp_identifier bridge_id;
  
      ovs_mutex_lock(&mutex);
      bridge_id = stp->bridge_id;
      ovs_mutex_unlock(&mutex);
      return bridge_id;
  }
  
  /* Returns the bridge ID of the bridge currently believed to be the root. */
  stp_identifier
  stp_get_designated_root(const struct stp *stp)
  {
      stp_identifier designated_root;
  
      ovs_mutex_lock(&mutex);
      designated_root = stp->designated_root;
      ovs_mutex_unlock(&mutex);
      return designated_root;
  }
  
  /* Returns true if 'stp' believes itself to the be root of the spanning tree,
   * false otherwise. */
  bool
  stp_is_root_bridge(const struct stp *stp)
  {
      bool is_root;
  
      ovs_mutex_lock(&mutex);
      is_root = stp->bridge_id == stp->designated_root;
      ovs_mutex_unlock(&mutex);
      return is_root;
  }
  
  /* Returns the cost of the path from 'stp' to the root of the spanning tree. */
  int
  stp_get_root_path_cost(const struct stp *stp)
  {
      int cost;
  
      ovs_mutex_lock(&mutex);
      cost = stp->root_path_cost;
      ovs_mutex_unlock(&mutex);
      return cost;
  }
  
  /* Returns the bridge hello time, in ms.  The returned value is not necessarily
   * the value passed to stp_set_hello_time(): it is clamped to the valid range
   * and quantized to the STP timer resolution.  */
  int
  stp_get_hello_time(const struct stp *stp)
  {
      int time;
  
      ovs_mutex_lock(&mutex);
      time = timer_to_ms(stp->bridge_hello_time);
      ovs_mutex_unlock(&mutex);
      return time;
  }
  
  /* Returns the bridge max age, in ms.  The returned value is not necessarily
   * the value passed to stp_set_max_age(): it is clamped to the valid range,
   * quantized to the STP timer resolution, and adjusted to match the constraints
   * due to the hello time.  */
  int
  stp_get_max_age(const struct stp *stp)
  {
      int time;
  
      ovs_mutex_lock(&mutex);
      time = timer_to_ms(stp->bridge_max_age);
      ovs_mutex_unlock(&mutex);
      return time;
  }
  
  /* Returns the bridge forward delay, in ms.  The returned value is not
   * necessarily the value passed to stp_set_forward_delay(): it is clamped to
   * the valid range, quantized to the STP timer resolution, and adjusted to
   * match the constraints due to the forward delay.  */
  int
  stp_get_forward_delay(const struct stp *stp)
  {
      int time;
  
      ovs_mutex_lock(&mutex);
      time = timer_to_ms(stp->bridge_forward_delay);
      ovs_mutex_unlock(&mutex);
      return time;
  }
  
  /* Returns true if something has happened to 'stp' which necessitates flushing
   * the client's MAC learning table.  Calling this function resets 'stp' so that
   * future calls will return false until flushing is required again. */
  bool
  stp_check_and_reset_fdb_flush(struct stp *stp)
  {
      bool needs_flush;
  
      ovs_mutex_lock(&mutex);
      needs_flush = stp->fdb_needs_flush;
      stp->fdb_needs_flush = false;
      ovs_mutex_unlock(&mutex);
      return needs_flush;
  }
  
  /* Returns the port in 'stp' with index 'port_no', which must be between 0 and
   * STP_MAX_PORTS. */
  struct stp_port *
  stp_get_port(struct stp *stp, int port_no)
  {
      struct stp_port *port;
  
      ovs_mutex_lock(&mutex);
      ovs_assert(port_no >= 0 && port_no < ARRAY_SIZE(stp->ports));
      port = &stp->ports[port_no];
      ovs_mutex_unlock(&mutex);
      return port;
  }
  
  /* Returns the port connecting 'stp' to the root bridge, or a null pointer if
   * there is no such port. */
  struct stp_port *
  stp_get_root_port(struct stp *stp)
  {
      struct stp_port *port;
  
      ovs_mutex_lock(&mutex);
      port = stp->root_port;
      ovs_mutex_unlock(&mutex);
      return port;
  }
  
  /* Finds a port whose state has changed.  If successful, stores the port whose
   * state changed in '*portp' and returns true.  If no port has changed, stores
   * NULL in '*portp' and returns false. */
  bool
  stp_get_changed_port(struct stp *stp, struct stp_port **portp)
  {
      struct stp_port *end, *p;
      bool changed = false;
  
      ovs_mutex_lock(&mutex);
      end = &stp->ports[ARRAY_SIZE(stp->ports)];
      for (p = stp->first_changed_port; p < end; p++) {
          if (p->state_changed) {
              p->state_changed = false;
              stp->first_changed_port = p + 1;
              *portp = p;
              changed = true;
              goto out;
          }
      }
      stp->first_changed_port = end;
      *portp = NULL;
  
  out:
      ovs_mutex_unlock(&mutex);
      return changed;
  }
  
  /* Returns the name for the given 'state' (for use in debugging and log
   * messages). */
  const char *
  stp_state_name(enum stp_state state)
  {
      switch (state) {
      case STP_DISABLED:
          return "disabled";
      case STP_LISTENING:
          return "listening";
      case STP_LEARNING:
          return "learning";
      case STP_FORWARDING:
          return "forwarding";
      case STP_BLOCKING:
          return "blocking";
      default:
          OVS_NOT_REACHED();
      }
  }
  
  /* Returns true if 'state' is one in which packets received on a port should
   * be forwarded, false otherwise.
   */
  bool
  stp_forward_in_state(enum stp_state state)
  {
      return (state & STP_FORWARDING) != 0;
  }
  
  /* Returns true if 'state' is one in which MAC learning should be done on
   * packets received on a port, false otherwise.
   */
  bool
  stp_learn_in_state(enum stp_state state)
  {
      return (state & (STP_LEARNING | STP_FORWARDING)) != 0;
  }
  
  /* Returns true if 'state' is one in which bpdus should be forwarded on a
   * port, false otherwise.
   *
   * Returns true if 'state' is STP_DISABLED, since in that case the port does
   * not generate the bpdu and should just forward it (e.g. patch port on pif
   * bridge). */
  bool
  stp_should_forward_bpdu(enum stp_state state)
  {
      return (state &
              ( STP_DISABLED | STP_LISTENING | STP_LEARNING
                | STP_FORWARDING)) != 0;
  }
  
  /* Returns the name for the given 'role' (for use in debugging and log
   * messages). */
  const char *
  stp_role_name(enum stp_role role)
  {
      switch (role) {
      case STP_ROLE_ROOT:
          return "root";
      case STP_ROLE_DESIGNATED:
          return "designated";
      case STP_ROLE_ALTERNATE:
          return "alternate";
      case STP_ROLE_DISABLED:
          return "disabled";
      default:
          OVS_NOT_REACHED();
      }
  }
  
  /* Notifies the STP entity that bridge protocol data unit 'bpdu', which is
   * 'bpdu_size' bytes in length, was received on port 'p'.
   *
   * This function may call the 'send_bpdu' function provided to stp_create(). */
  void
  stp_received_bpdu(struct stp_port *p, const void *bpdu, size_t bpdu_size)
  {
      struct stp *stp = p->stp;
      const struct stp_bpdu_header *header;
  
      ovs_mutex_lock(&mutex);
      if (p->state == STP_DISABLED) {
          goto out;
      }
  
      if (bpdu_size < sizeof(struct stp_bpdu_header)) {
          VLOG_WARN("%s: received runt %"PRIuSIZE"-byte BPDU", stp->name, bpdu_size);
          p->error_count++;
          goto out;
      }
  
      header = bpdu;
      if (header->protocol_id != htons(STP_PROTOCOL_ID)) {
          VLOG_WARN("%s: received BPDU with unexpected protocol ID %"PRIu16,
                    stp->name, ntohs(header->protocol_id));
          p->error_count++;
          goto out;
      }
      if (header->protocol_version != STP_PROTOCOL_VERSION) {
          VLOG_DBG("%s: received BPDU with unexpected protocol version %"PRIu8,
                   stp->name, header->protocol_version);
      }
  
      switch (header->bpdu_type) {
      case STP_TYPE_CONFIG:
          if (bpdu_size < sizeof(struct stp_config_bpdu)) {
              VLOG_WARN("%s: received config BPDU with invalid size %"PRIuSIZE,
                        stp->name, bpdu_size);
              p->error_count++;
              goto out;
          }
          stp_received_config_bpdu(stp, p, bpdu);
          break;
  
      case STP_TYPE_TCN:
          if (bpdu_size != sizeof(struct stp_tcn_bpdu)) {
              VLOG_WARN("%s: received TCN BPDU with invalid size %"PRIuSIZE,
                        stp->name, bpdu_size);
              p->error_count++;
              goto out;
          }
          stp_received_tcn_bpdu(stp, p);
          break;
  
      default:
          VLOG_WARN("%s: received BPDU of unexpected type %"PRIu8,
                    stp->name, header->bpdu_type);
          p->error_count++;
          goto out;
      }
      p->rx_count++;
  
  out:
      ovs_mutex_unlock(&mutex);
  }
  
  /* Returns the STP entity in which 'p' is nested. */
  struct stp *
  stp_port_get_stp(struct stp_port *p)
  {
      struct stp *stp;
  
      ovs_mutex_lock(&mutex);
      stp = p->stp;
      ovs_mutex_unlock(&mutex);
      return stp;
  }
  
  void
  stp_port_set_name(struct stp_port *p, const char *name)
  {
      char *old;
  
      ovs_mutex_lock(&mutex);
      old = p->port_name;
      p->port_name = xstrdup(name);
      free(old);
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the 'aux' member of 'p'.
   *
   * The 'aux' member will be reset to NULL when stp_port_disable() is
   * called or stp_port_enable() is called when the port is in a Disabled
   * state. */
  void
  stp_port_set_aux(struct stp_port *p, void *aux)
  {
      ovs_mutex_lock(&mutex);
      p->aux = aux;
      ovs_mutex_unlock(&mutex);
  }
  
  /* Returns the 'aux' member of 'p'. */
  void *
  stp_port_get_aux(struct stp_port *p)
  {
      void *aux;
  
      ovs_mutex_lock(&mutex);
      aux = p->aux;
      ovs_mutex_unlock(&mutex);
      return aux;
  }
  
  /* Returns the index of port 'p' within its bridge. */
  int
  stp_port_no(const struct stp_port *p)
  {
      struct stp *stp;
      int index;
  
      ovs_mutex_lock(&mutex);
      stp = p->stp;
      ovs_assert(p >= stp->ports && p < &stp->ports[ARRAY_SIZE(stp->ports)]);
      index = p - p->stp->ports;
      ovs_mutex_unlock(&mutex);
      return index;
  }
  
  /* Returns the port ID for 'p'. */
  int
  stp_port_get_id(const struct stp_port *p)
  {
      int port_id;
  
      ovs_mutex_lock(&mutex);
      port_id = p->port_id;
      ovs_mutex_unlock(&mutex);
      return port_id;
  }
  
  /* Returns the state of port 'p'. */
  enum stp_state
  stp_port_get_state(const struct stp_port *p)
  {
      enum stp_state state;
  
      ovs_mutex_lock(&mutex);
      state = p->state;
      ovs_mutex_unlock(&mutex);
      return state;
  }
  
  /* Returns the role of port 'p'. */
  enum stp_role
  stp_port_get_role(const struct stp_port *p)
  {
      struct stp_port *root_port;
      enum stp_role role;
  
      ovs_mutex_lock(&mutex);
      root_port = p->stp->root_port;
      if (root_port && root_port->port_id == p->port_id) {
          role = STP_ROLE_ROOT;
      } else if (stp_is_designated_port(p)) {
          role = STP_ROLE_DESIGNATED;
      } else if (p->state == STP_DISABLED) {
          role = STP_ROLE_DISABLED;
      } else {
          role = STP_ROLE_ALTERNATE;
      }
      ovs_mutex_unlock(&mutex);
      return role;
  }
  
  /* Retrieves BPDU transmit and receive counts for 'p'. */
  void
  stp_port_get_counts(const struct stp_port *p,
                      int *tx_count, int *rx_count, int *error_count)
  {
      ovs_mutex_lock(&mutex);
      *tx_count = p->tx_count;
      *rx_count = p->rx_count;
      *error_count = p->error_count;
      ovs_mutex_unlock(&mutex);
  }
  
  /* Disables STP on port 'p'. */
  void
  stp_port_disable(struct stp_port *p)
  {
      struct stp *stp;
  
      ovs_mutex_lock(&mutex);
      stp = p->stp;
      if (p->state != STP_DISABLED) {
          bool root = stp_is_root_bridge(stp);
          stp_become_designated_port(p);
          stp_set_port_state(p, STP_DISABLED);
          p->topology_change_ack = false;
          p->config_pending = false;
          stp_stop_timer(&p->message_age_timer);
          stp_stop_timer(&p->forward_delay_timer);
          stp_configuration_update(stp);
          stp_port_state_selection(stp);
          if (stp_is_root_bridge(stp) && !root) {
              stp_become_root_bridge(stp);
          }
          p->aux = NULL;
      }
      ovs_mutex_unlock(&mutex);
  }
  
  /* Enables STP on port 'p'.  The port will initially be in "blocking" state. */
  void
  stp_port_enable(struct stp_port *p)
  {
      ovs_mutex_lock(&mutex);
      if (p->state == STP_DISABLED) {
          stp_initialize_port(p, STP_BLOCKING);
          stp_port_state_selection(p->stp);
      }
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the priority of port 'p' to 'new_priority'.  Lower numerical values
   * are interpreted as higher priorities. */
  void
  stp_port_set_priority(struct stp_port *p, uint8_t new_priority)
  {
      uint16_t new_port_id;
  
      ovs_mutex_lock(&mutex);
      new_port_id  = (p->port_id & 0xff) | (new_priority << 8);
      if (p->port_id != new_port_id) {
          struct stp *stp = p->stp;
          if (stp_is_designated_port(p)) {
              p->designated_port = new_port_id;
          }
          p->port_id = new_port_id;
          if (stp->bridge_id == p->designated_bridge
              && p->port_id < p->designated_port) {
              stp_become_designated_port(p);
              stp_port_state_selection(stp);
          }
      }
      ovs_mutex_unlock(&mutex);
  }
  
  /* Convert 'speed' (measured in Mb/s) into the path cost. */
  uint16_t
  stp_convert_speed_to_cost(unsigned int speed)
  {
      uint16_t ret;
  
      ovs_mutex_lock(&mutex);
      ret = speed >= 10000 ? 2  /* 10 Gb/s. */
          : speed >= 1000 ? 4 /* 1 Gb/s. */
          : speed >= 100 ? 19 /* 100 Mb/s. */
          : speed >= 16 ? 62  /* 16 Mb/s. */
          : speed >= 10 ? 100 /* 10 Mb/s. */
          : speed >= 4 ? 250  /* 4 Mb/s. */
          : 19;             /* 100 Mb/s (guess). */
      ovs_mutex_unlock(&mutex);
      return ret;
  }
  
  /* Sets the path cost of port 'p' to 'path_cost'.  Lower values are generally
   * used to indicate faster links.  Use stp_port_set_speed() to automatically
   * generate a default path cost from a link speed. */
  void
  stp_port_set_path_cost(struct stp_port *p, uint16_t path_cost)
  {
      ovs_mutex_lock(&mutex);
      if (p->path_cost != path_cost) {
          struct stp *stp = p->stp;
          p->path_cost = path_cost;
          stp_configuration_update(stp);
          stp_port_state_selection(stp);
      }
      ovs_mutex_unlock(&mutex);
  }
  
  /* Sets the path cost of port 'p' based on 'speed' (measured in Mb/s). */
  void
  stp_port_set_speed(struct stp_port *p, unsigned int speed)
  {
      stp_port_set_path_cost(p, stp_convert_speed_to_cost(speed));
  }
  
  /* Enables topology change detection on port 'p'. */
  void
  stp_port_enable_change_detection(struct stp_port *p)
  {
      p->change_detection_enabled = true;
  }
  
  /* Disables topology change detection on port 'p'. */
  void
  stp_port_disable_change_detection(struct stp_port *p)
  {
      p->change_detection_enabled = false;
  }
  
  static void
  stp_transmit_config(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      struct stp *stp = p->stp;
      bool root = stp_is_root_bridge(stp);
      if (!root && !stp->root_port) {
          return;
      }
      if (p->hold_timer.active) {
          VLOG_DBG_RL(&stp_rl, "bridge: %s, port: %s, transmit config bpdu pending",
                      stp->name, p->port_name);
          p->config_pending = true;
      } else {
          struct stp_config_bpdu config;
          memset(&config, 0, sizeof config);
          config.header.protocol_id = htons(STP_PROTOCOL_ID);
          config.header.protocol_version = STP_PROTOCOL_VERSION;
          config.header.bpdu_type = STP_TYPE_CONFIG;
          config.flags = 0;
          if (p->topology_change_ack) {
              config.flags |= STP_CONFIG_TOPOLOGY_CHANGE_ACK;
          }
          if (stp->topology_change) {
              config.flags |= STP_CONFIG_TOPOLOGY_CHANGE;
          }
          config.root_id = htonll(stp->designated_root);
          config.root_path_cost = htonl(stp->root_path_cost);
          config.bridge_id = htonll(stp->bridge_id);
          config.port_id = htons(p->port_id);
          if (root) {
              config.message_age = htons(0);
          } else {
              config.message_age = htons(stp->root_port->message_age_timer.value
                                         + MESSAGE_AGE_INCREMENT);
          }
          config.max_age = htons(stp->max_age);
          config.hello_time = htons(stp->hello_time);
          config.forward_delay = htons(stp->forward_delay);
          if (ntohs(config.message_age) < stp->max_age) {
              p->topology_change_ack = false;
              p->config_pending = false;
              VLOG_DBG_RL(&stp_rl, "bridge: %s, port: %s, transmit config bpdu",
                          stp->name, p->port_name);
              stp_send_bpdu(p, &config, sizeof config);
              stp_start_timer(&p->hold_timer, 0);
          }
      }
  }
  
  static bool
  stp_supersedes_port_info(const struct stp_port *p,
                           const struct stp_config_bpdu *config)
       OVS_REQUIRES(mutex)
  {
      if (ntohll(config->root_id) != p->designated_root) {
          return ntohll(config->root_id) < p->designated_root;
      } else if (ntohl(config->root_path_cost) != p->designated_cost) {
          return ntohl(config->root_path_cost) < p->designated_cost;
      } else if (ntohll(config->bridge_id) != p->designated_bridge) {
          return ntohll(config->bridge_id) < p->designated_bridge;
      } else {
          return (ntohll(config->bridge_id) != p->stp->bridge_id
                  || ntohs(config->port_id) <= p->designated_port);
      }
  }
  
  static void
  stp_record_config_information(struct stp_port *p,
                                const struct stp_config_bpdu *config)
       OVS_REQUIRES(mutex)
  {
      p->designated_root = ntohll(config->root_id);
      p->designated_cost = ntohl(config->root_path_cost);
      p->designated_bridge = ntohll(config->bridge_id);
      p->designated_port = ntohs(config->port_id);
      stp_start_timer(&p->message_age_timer, ntohs(config->message_age));
  }
  
  static void
  stp_record_config_timeout_values(struct stp *stp,
                                   const struct stp_config_bpdu  *config)
       OVS_REQUIRES(mutex)
  {
      stp->max_age = ntohs(config->max_age);
      stp->hello_time = ntohs(config->hello_time);
      stp->forward_delay = ntohs(config->forward_delay);
      stp->topology_change = config->flags & STP_CONFIG_TOPOLOGY_CHANGE;
  }
  
  static bool
  stp_is_designated_port(const struct stp_port *p) OVS_REQUIRES(mutex)
  {
      return (p->designated_bridge == p->stp->bridge_id
              && p->designated_port == p->port_id);
  }
  
  static void
  stp_config_bpdu_generation(struct stp *stp) OVS_REQUIRES(mutex)
  {
      struct stp_port *p;
  
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (stp_is_designated_port(p)) {
              stp_transmit_config(p);
          }
      }
  }
  
  static void
  stp_transmit_tcn(struct stp *stp) OVS_REQUIRES(mutex)
  {
      struct stp_port *p = stp->root_port;
      struct stp_tcn_bpdu tcn_bpdu;
  
      if (!p) {
          return;
      }
      VLOG_DBG_RL(&stp_rl, "bridge: %s, root port: %s, transmit tcn", stp->name,
                  p->port_name);
      tcn_bpdu.header.protocol_id = htons(STP_PROTOCOL_ID);
      tcn_bpdu.header.protocol_version = STP_PROTOCOL_VERSION;
      tcn_bpdu.header.bpdu_type = STP_TYPE_TCN;
      stp_send_bpdu(p, &tcn_bpdu, sizeof tcn_bpdu);
  }
  
  static void
  stp_configuration_update(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp_root_selection(stp);
      stp_designated_port_selection(stp);
      seq_change(connectivity_seq_get());
  }
  
  static bool
  stp_supersedes_root(const struct stp_port *root, const struct stp_port *p)
      OVS_REQUIRES(mutex)
  {
      int p_cost = p->designated_cost + p->path_cost;
      int root_cost = root->designated_cost + root->path_cost;
  
      if (p->designated_root != root->designated_root) {
          return p->designated_root < root->designated_root;
      } else if (p_cost != root_cost) {
          return p_cost < root_cost;
      } else if (p->designated_bridge != root->designated_bridge) {
          return p->designated_bridge < root->designated_bridge;
      } else if (p->designated_port != root->designated_port) {
          return p->designated_port < root->designated_port;
      } else {
          return p->port_id < root->port_id;
      }
  }
  
  static void
  stp_root_selection(struct stp *stp) OVS_REQUIRES(mutex)
  {
      struct stp_port *p, *root;
  
      root = NULL;
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (stp_is_designated_port(p)
              || p->designated_root >= stp->bridge_id) {
              continue;
          }
          if (root && !stp_supersedes_root(root, p)) {
              continue;
          }
          root = p;
      }
      stp->root_port = root;
      if (!root) {
          stp->designated_root = stp->bridge_id;
          stp->root_path_cost = 0;
      } else {
          stp->designated_root = root->designated_root;
          stp->root_path_cost = root->designated_cost + root->path_cost;
      }
  }
  
  static void
  stp_designated_port_selection(struct stp *stp) OVS_REQUIRES(mutex)
  {
      struct stp_port *p;
  
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (stp_is_designated_port(p)
              || p->designated_root != stp->designated_root
              || stp->root_path_cost < p->designated_cost
              || (stp->root_path_cost == p->designated_cost
                  && (stp->bridge_id < p->designated_bridge
                      || (stp->bridge_id == p->designated_bridge
                          && p->port_id <= p->designated_port))))
          {
              stp_become_designated_port(p);
          }
      }
  }
  
  static void
  stp_become_designated_port(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      struct stp *stp = p->stp;
      p->designated_root = stp->designated_root;
      p->designated_cost = stp->root_path_cost;
      p->designated_bridge = stp->bridge_id;
      p->designated_port = p->port_id;
  }
  
  static void
  stp_port_state_selection(struct stp *stp) OVS_REQUIRES(mutex)
  {
      struct stp_port *p;
  
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (p == stp->root_port) {
              p->config_pending = false;
              p->topology_change_ack = false;
              stp_make_forwarding(p);
          } else if (stp_is_designated_port(p)) {
              stp_stop_timer(&p->message_age_timer);
              stp_make_forwarding(p);
          } else {
              p->config_pending = false;
              p->topology_change_ack = false;
              stp_make_blocking(p);
          }
      }
  }
  
  static void
  stp_make_forwarding(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      if (p->state == STP_BLOCKING) {
          stp_set_port_state(p, STP_LISTENING);
          stp_start_timer(&p->forward_delay_timer, 0);
      }
  }
  
  static void
  stp_make_blocking(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      if (!(p->state & (STP_DISABLED | STP_BLOCKING))) {
          if (p->state & (STP_FORWARDING | STP_LEARNING)) {
              if (p->change_detection_enabled) {
                  stp_topology_change_detection(p->stp);
              }
          }
          stp_set_port_state(p, STP_BLOCKING);
          stp_stop_timer(&p->forward_delay_timer);
      }
  }
  
  static void
  stp_set_port_state(struct stp_port *p, enum stp_state state)
      OVS_REQUIRES(mutex)
  {
      if (state != p->state && !p->state_changed) {
          p->state_changed = true;
          if (p < p->stp->first_changed_port) {
              p->stp->first_changed_port = p;
          }
          seq_change(connectivity_seq_get());
      }
      p->state = state;
  }
  
  static void
  stp_topology_change_detection(struct stp *stp) OVS_REQUIRES(mutex)
  {
      static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 5);
  
      if (stp_is_root_bridge(stp)) {
          stp->topology_change = true;
          stp_start_timer(&stp->topology_change_timer, 0);
      } else if (!stp->topology_change_detected) {
          stp_transmit_tcn(stp);
          stp_start_timer(&stp->tcn_timer, 0);
      }
      stp->fdb_needs_flush = true;
      stp->topology_change_detected = true;
      seq_change(connectivity_seq_get());
      VLOG_INFO_RL(&rl, "%s: detected topology change.", stp->name);
  }
  
  static void
  stp_topology_change_acknowledged(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp->topology_change_detected = false;
      stp_stop_timer(&stp->tcn_timer);
  }
  
  static void
  stp_acknowledge_topology_change(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      p->topology_change_ack = true;
      stp_transmit_config(p);
  }
  
  static void
  stp_received_config_bpdu(struct stp *stp, struct stp_port *p,
                           const struct stp_config_bpdu *config)
      OVS_REQUIRES(mutex)
  {
      if (ntohs(config->message_age) >= ntohs(config->max_age)) {
          VLOG_WARN("%s: received config BPDU with message age (%u) greater "
                    "than max age (%u)",
                    stp->name,
                    ntohs(config->message_age), ntohs(config->max_age));
          return;
      }
      if (p->state != STP_DISABLED) {
          bool root = stp_is_root_bridge(stp);
          if (stp_supersedes_port_info(p, config)) {
              stp_record_config_information(p, config);
              stp_configuration_update(stp);
              stp_port_state_selection(stp);
              if (!stp_is_root_bridge(stp) && root) {
                  stp_stop_timer(&stp->hello_timer);
                  if (stp->topology_change_detected) {
                      stp_stop_timer(&stp->topology_change_timer);
                      stp_transmit_tcn(stp);
                      stp_start_timer(&stp->tcn_timer, 0);
                  }
              }
              if (p == stp->root_port) {
                  stp_record_config_timeout_values(stp, config);
                  stp_config_bpdu_generation(stp);
                  if (config->flags & STP_CONFIG_TOPOLOGY_CHANGE_ACK) {
                      stp_topology_change_acknowledged(stp);
                  }
                  if (config->flags & STP_CONFIG_TOPOLOGY_CHANGE) {
                      stp->fdb_needs_flush = true;
                  }
              }
          } else if (stp_is_designated_port(p)) {
              stp_transmit_config(p);
          }
      }
  }
  
  static void
  stp_received_tcn_bpdu(struct stp *stp, struct stp_port *p)
      OVS_REQUIRES(mutex)
  {
      if (p->state != STP_DISABLED) {
          if (stp_is_designated_port(p)) {
              stp_topology_change_detection(stp);
              stp_acknowledge_topology_change(p);
          }
      }
  }
  
  static void
  stp_hello_timer_expiry(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp_config_bpdu_generation(stp);
      stp_start_timer(&stp->hello_timer, 0);
  }
  
  static void
  stp_message_age_timer_expiry(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      struct stp *stp = p->stp;
      bool root = stp_is_root_bridge(stp);
  
      VLOG_DBG_RL(&stp_rl, "bridge: %s, port: %s, message age timer expired",
                  stp->name, p->port_name);
      stp_become_designated_port(p);
      stp_configuration_update(stp);
      stp_port_state_selection(stp);
      if (stp_is_root_bridge(stp) && !root) {
          stp->max_age = stp->bridge_max_age;
          stp->hello_time = stp->bridge_hello_time;
          stp->forward_delay = stp->bridge_forward_delay;
          stp_topology_change_detection(stp);
          stp_stop_timer(&stp->tcn_timer);
          stp_config_bpdu_generation(stp);
          stp_start_timer(&stp->hello_timer, 0);
      }
  }
  
  static bool
  stp_is_designated_for_some_port(const struct stp *stp) OVS_REQUIRES(mutex)
  {
      const struct stp_port *p;
  
      FOR_EACH_ENABLED_PORT (p, stp) {
          if (p->designated_bridge == stp->bridge_id) {
              return true;
          }
      }
      return false;
  }
  
  static void
  stp_forward_delay_timer_expiry(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      if (p->state == STP_LISTENING) {
          stp_set_port_state(p, STP_LEARNING);
          stp_start_timer(&p->forward_delay_timer, 0);
      } else if (p->state == STP_LEARNING) {
          stp_set_port_state(p, STP_FORWARDING);
          if (stp_is_designated_for_some_port(p->stp)) {
              if (p->change_detection_enabled) {
                  stp_topology_change_detection(p->stp);
              }
          }
      }
  }
  
  static void
  stp_tcn_timer_expiry(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp_transmit_tcn(stp);
      stp_start_timer(&stp->tcn_timer, 0);
  }
  
  static void
  stp_topology_change_timer_expiry(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp->topology_change_detected = false;
      stp->topology_change = false;
  }
  
  static void
  stp_hold_timer_expiry(struct stp_port *p) OVS_REQUIRES(mutex)
  {
      if (p->config_pending) {
          stp_transmit_config(p);
      }
  }
  
  static void
  stp_initialize_port(struct stp_port *p, enum stp_state state)
      OVS_REQUIRES(mutex)
  {
      ovs_assert(state & (STP_DISABLED | STP_BLOCKING));
      stp_become_designated_port(p);
  
      if (!p->state && state == STP_DISABLED) {
          p->state = state; /* Do not trigger state change when initializing. */
      } else {
          stp_set_port_state(p, state);
      }
      p->topology_change_ack = false;
      p->config_pending = false;
      p->change_detection_enabled = true;
      p->aux = NULL;
      stp_stop_timer(&p->message_age_timer);
      stp_stop_timer(&p->forward_delay_timer);
      stp_stop_timer(&p->hold_timer);
      p->tx_count = p->rx_count = p->error_count = 0;
  }
  
  static void
  stp_become_root_bridge(struct stp *stp) OVS_REQUIRES(mutex)
  {
      stp->max_age = stp->bridge_max_age;
      stp->hello_time = stp->bridge_hello_time;
      stp->forward_delay = stp->bridge_forward_delay;
      stp_topology_change_detection(stp);
      stp_stop_timer(&stp->tcn_timer);
      stp_config_bpdu_generation(stp);
      stp_start_timer(&stp->hello_timer, 0);
  }
  
  static void
  stp_start_timer(struct stp_timer *timer, int value) OVS_REQUIRES(mutex)
  {
      timer->value = value;
      timer->active = true;
  }
  
  static void
  stp_stop_timer(struct stp_timer *timer) OVS_REQUIRES(mutex)
  {
      timer->active = false;
  }
  
  static bool
  stp_timer_expired(struct stp_timer *timer, int elapsed, int timeout)
      OVS_REQUIRES(mutex)
  {
      if (timer->active) {
          timer->value += elapsed;
          if (timer->value >= timeout) {
              timer->active = false;
              return true;
          }
      }
      return false;
  }
  
  /* Returns the number of whole STP timer ticks in 'ms' milliseconds.  There
   * are 256 STP timer ticks per second. */
  static int
  ms_to_timer(int ms)
  {
      return ms * 0x100 / 1000;
  }
  
  /* Returns the number of whole milliseconds in 'timer' STP timer ticks.  There
   * are 256 STP timer ticks per second. */
  static int
  timer_to_ms(int timer)
  {
      return timer * 1000 / 0x100;
  }
  
  static int
  clamp(int x, int min, int max)
  {
      return x < min ? min : x > max ? max : x;
  }
  
  static void
  stp_update_bridge_timers(struct stp *stp) OVS_REQUIRES(mutex)
  {
      int ht, ma, fd;
  
      ht = clamp(stp->rq_hello_time, 1000, 10000);
      ma = clamp(stp->rq_max_age, MAX(2 * (ht + 1000), 6000), 40000);
      fd = clamp(stp->rq_forward_delay, ma / 2 + 1000, 30000);
  
      stp->bridge_hello_time = ms_to_timer(ht);
      stp->bridge_max_age = ms_to_timer(ma);
      stp->bridge_forward_delay = ms_to_timer(fd);
  
      if (stp_is_root_bridge(stp)) {
          stp->max_age = stp->bridge_max_age;
          stp->hello_time = stp->bridge_hello_time;
          stp->forward_delay = stp->bridge_forward_delay;
      }
  }
  
  static void
  stp_send_bpdu(struct stp_port *p, const void *bpdu, size_t bpdu_size)
      OVS_REQUIRES(mutex)
  {
      struct eth_header *eth;
      struct llc_header *llc;
      struct dp_packet *pkt;
  
      /* Skeleton. */
      pkt = dp_packet_new(ETH_HEADER_LEN + LLC_HEADER_LEN + bpdu_size);
      eth = dp_packet_put_zeros(pkt, sizeof *eth);
      llc = dp_packet_put_zeros(pkt, sizeof *llc);
      dp_packet_reset_offsets(pkt);
      dp_packet_set_l3(pkt, dp_packet_put(pkt, bpdu, bpdu_size));
  
      /* 802.2 header. */
      eth->eth_dst = eth_addr_stp;
      /* p->stp->send_bpdu() must fill in source address. */
      eth->eth_type = htons(dp_packet_size(pkt) - ETH_HEADER_LEN);
  
      /* LLC header. */
      llc->llc_dsap = STP_LLC_DSAP;
      llc->llc_ssap = STP_LLC_SSAP;
      llc->llc_cntl = STP_LLC_CNTL;
  
      p->stp->send_bpdu(pkt, stp_port_no(p), p->stp->aux);
      p->tx_count++;
  }
  
  /* Unixctl. */
  
  static struct stp *
  stp_find(const char *name) OVS_REQUIRES(mutex)
  {
      struct stp *stp;
  
      LIST_FOR_EACH (stp, node, all_stps) {
          if (!strcmp(stp->name, name)) {
              return stp;
          }
      }
      return NULL;
  }
  
  static void
  stp_unixctl_tcn(struct unixctl_conn *conn, int argc,
                  const char *argv[], void *aux OVS_UNUSED)
  {
      ovs_mutex_lock(&mutex);
      if (argc > 1) {
          struct stp *stp = stp_find(argv[1]);
  
          if (!stp) {
              unixctl_command_reply_error(conn, "no such stp object");
              goto out;
          }
          stp_topology_change_detection(stp);
      } else {
          struct stp *stp;
  
          LIST_FOR_EACH (stp, node, all_stps) {
              stp_topology_change_detection(stp);
          }
      }
  
      unixctl_command_reply(conn, "OK");
  
  out:
      ovs_mutex_unlock(&mutex);
  }
  