[openvswitch] / vswitchd / vswitch.xml

<?xml version="1.0" encoding="utf-8"?>
<database title="Open vSwitch Configuration Database">
  <p>
    A database with this schema holds the configuration for one Open
    vSwitch daemon.  The top-level configuration for the daemon is the
    <ref table="Open_vSwitch"/> table, which must have exactly one
    record.  Records in other tables are significant only when they
    can be reached directly or indirectly from the <ref
    table="Open_vSwitch"/> table.  Records that are not reachable from
    the <ref table="Open_vSwitch"/> table are automatically deleted
    from the database, except for records in a few distinguished
    ``root set'' tables.
  </p>

  <h2>Common Columns</h2>

  <p>
    Most tables contain two special columns, named <code>other_config</code>
    and <code>external_ids</code>.  These columns have the same form and
    purpose each place that they appear, so we describe them here to save space
    later.
  </p>

  <dl>
    <dt><code>other_config</code>: map of string-string pairs</dt>
    <dd>
      <p>
        Key-value pairs for configuring rarely used features.  Supported keys,
        along with the forms taken by their values, are documented individually
        for each table.
      </p>
      <p>
        A few tables do not have <code>other_config</code> columns because no
        key-value pairs have yet been defined for them.
      </p>
    </dd>

    <dt><code>external_ids</code>: map of string-string pairs</dt>
    <dd>
      Key-value pairs for use by external frameworks that integrate with Open
      vSwitch, rather than by Open vSwitch itself.  System integrators should
      either use the Open vSwitch development mailing list to coordinate on
      common key-value definitions, or choose key names that are likely to be
      unique.  In some cases, where key-value pairs have been defined that are
      likely to be widely useful, they are documented individually for each
      table.
    </dd>
  </dl>

  <table name="Open_vSwitch" title="Open vSwitch configuration.">
    Configuration for an Open vSwitch daemon.  There must be exactly
    one record in the <ref table="Open_vSwitch"/> table.

    <group title="Configuration">
      <column name="bridges">
        Set of bridges managed by the daemon.
      </column>

      <column name="ssl">
        SSL used globally by the daemon.
      </column>

      <column name="external_ids" key="system-id">
        A unique identifier for the Open vSwitch's physical host.
        The form of the identifier depends on the type of the host.
        On a Citrix XenServer, this will likely be the same as
        <ref column="external_ids" key="xs-system-uuid"/>.
      </column>

      <column name="external_ids" key="xs-system-uuid">
        The Citrix XenServer universally unique identifier for the physical
        host as displayed by <code>xe host-list</code>.
      </column>
    </group>

    <group title="Status">
      <column name="next_cfg">
        Sequence number for client to increment.  When a client modifies
        any part of the database configuration and wishes to wait for
        Open vSwitch to finish applying the changes, it may increment
        this sequence number.
      </column>

      <column name="cur_cfg">
        Sequence number that Open vSwitch sets to the current value of
        <ref column="next_cfg"/> after it finishes applying a set of
        configuration changes.
      </column>

      <column name="capabilities">
        Describes functionality supported by the hardware and software platform
        on which this Open vSwitch is based.  Clients should not modify this
        column.  See the <ref table="Capability"/> description for defined
        capability categories and the meaning of associated
        <ref table="Capability"/> records.
      </column>

      <group title="Statistics">
        <p>
          The <code>statistics</code> column contains key-value pairs that
          report statistics about a system running an Open vSwitch.  These are
          updated periodically (currently, every 5 seconds).  Key-value pairs
          that cannot be determined or that do not apply to a platform are
          omitted.
        </p>

        <column name="other_config" key="enable-statistics"
                type='{"type": "boolean"}'>
          Statistics are disabled by default to avoid overhead in the common
          case when statistics gathering is not useful.  Set this value to
          <code>true</code> to enable populating the <ref column="statistics"/>
          column or to <code>false</code> to explicitly disable it.
        </column>

        <column name="statistics" key="cpu"
                type='{"type": "integer", "minInteger": 1}'>
          <p>
            Number of CPU processors, threads, or cores currently online and
            available to the operating system on which Open vSwitch is running,
            as an integer.  This may be less than the number installed, if some
            are not online or if they are not available to the operating
            system.
          </p>
          <p>
            Open vSwitch userspace processes are not multithreaded, but the
            Linux kernel-based datapath is.
          </p>
        </column>

        <column name="statistics" key="load_average">
          A comma-separated list of three floating-point numbers,
          representing the system load average over the last 1, 5, and 15
          minutes, respectively.
        </column>

        <column name="statistics" key="memory">
          <p>
            A comma-separated list of integers, each of which represents a
            quantity of memory in kilobytes that describes the operating
            system on which Open vSwitch is running.  In respective order,
            these values are:
          </p>

          <ol>
            <li>Total amount of RAM allocated to the OS.</li>
            <li>RAM allocated to the OS that is in use.</li>
            <li>RAM that can be flushed out to disk or otherwise discarded
            if that space is needed for another purpose.  This number is
            necessarily less than or equal to the previous value.</li>
            <li>Total disk space allocated for swap.</li>
            <li>Swap space currently in use.</li>
          </ol>

          <p>
            On Linux, all five values can be determined and are included.  On
            other operating systems, only the first two values can be
            determined, so the list will only have two values.
          </p>
        </column>

        <column name="statistics" key="process_NAME">
          <p>
            One such key-value pair, with <code>NAME</code> replaced by
            a process name, will exist for each running Open vSwitch
            daemon process, with <var>name</var> replaced by the
            daemon's name (e.g. <code>process_ovs-vswitchd</code>).  The
            value is a comma-separated list of integers.  The integers
            represent the following, with memory measured in kilobytes
            and durations in milliseconds:
          </p>

          <ol>
            <li>The process's virtual memory size.</li>
            <li>The process's resident set size.</li>
            <li>The amount of user and system CPU time consumed by the
            process.</li>
            <li>The number of times that the process has crashed and been
            automatically restarted by the monitor.</li>
            <li>The duration since the process was started.</li>
            <li>The duration for which the process has been running.</li>
          </ol>

          <p>
            The interpretation of some of these values depends on whether the
            process was started with the <option>--monitor</option>.  If it
            was not, then the crash count will always be 0 and the two
            durations will always be the same.  If <option>--monitor</option>
            was given, then the crash count may be positive; if it is, the
            latter duration is the amount of time since the most recent crash
            and restart.
          </p>

          <p>
            There will be one key-value pair for each file in Open vSwitch's
            ``run directory'' (usually <code>/var/run/openvswitch</code>)
            whose name ends in <code>.pid</code>, whose contents are a
            process ID, and which is locked by a running process.  The
            <var>name</var> is taken from the pidfile's name.
          </p>

          <p>
            Currently Open vSwitch is only able to obtain all of the above
            detail on Linux systems.  On other systems, the same key-value
            pairs will be present but the values will always be the empty
            string.
          </p>
        </column>

        <column name="statistics" key="file_systems">
          <p>
            A space-separated list of information on local, writable file
            systems.  Each item in the list describes one file system and
            consists in turn of a comma-separated list of the following:
          </p>

          <ol>
            <li>Mount point, e.g. <code>/</code> or <code>/var/log</code>.
            Any spaces or commas in the mount point are replaced by
            underscores.</li>
            <li>Total size, in kilobytes, as an integer.</li>
            <li>Amount of storage in use, in kilobytes, as an integer.</li>
          </ol>

          <p>
            This key-value pair is omitted if there are no local, writable
            file systems or if Open vSwitch cannot obtain the needed
            information.
          </p>
        </column>
      </group>
    </group>

    <group title="Version Reporting">
      <p>
        These columns report the types and versions of the hardware and
        software running Open vSwitch.  We recommend in general that software
        should test whether specific features are supported instead of relying
        on version number checks.  These values are primarily intended for
        reporting to human administrators.
      </p>

      <column name="ovs_version">
        The Open vSwitch version number, e.g. <code>1.1.0</code>.
        If Open vSwitch was configured with a build number, then it is
        also included, e.g. <code>1.1.0+build6579</code>.
      </column>

      <column name="db_version">
        <p>
          The database schema version number in the form
          <code><var>major</var>.<var>minor</var>.<var>tweak</var></code>,
          e.g. <code>1.2.3</code>.  Whenever the database schema is changed in
          a non-backward compatible way (e.g. deleting a column or a table),
          <var>major</var> is incremented.  When the database schema is changed
          in a backward compatible way (e.g. adding a new column),
          <var>minor</var> is incremented.  When the database schema is changed
          cosmetically (e.g. reindenting its syntax), <var>tweak</var> is
          incremented.
        </p>

        <p>
          The schema version is part of the database schema, so it can also be
          retrieved by fetching the schema using the Open vSwitch database
          protocol.
        </p>
      </column>

      <column name="system_type">
        <p>
          An identifier for the type of system on top of which Open vSwitch
          runs, e.g. <code>XenServer</code> or <code>KVM</code>.
        </p>
        <p>
          System integrators are responsible for choosing and setting an
          appropriate value for this column.
        </p>
      </column>

      <column name="system_version">
        <p>
          The version of the system identified by <ref column="system_type"/>,
          e.g. <code>5.6.100-39265p</code> on XenServer 5.6.100 build 39265.
        </p>
        <p>
          System integrators are responsible for choosing and setting an
          appropriate value for this column.
        </p>
      </column>

    </group>

    <group title="Database Configuration">
      <p>
        These columns primarily configure the Open vSwitch database
        (<code>ovsdb-server</code>), not the Open vSwitch switch
        (<code>ovs-vswitchd</code>).  The OVSDB database also uses the <ref
        column="ssl"/> settings.
      </p>

      <p>
        The Open vSwitch switch does read the database configuration to
        determine remote IP addresses to which in-band control should apply.
      </p>

      <column name="manager_options">
        Database clients to which the Open vSwitch database server should
        connect or to which it should listen, along with options for how these
        connection should be configured.  See the <ref table="Manager"/> table
        for more information.
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Bridge">
    <p>
      Configuration for a bridge within an
      <ref table="Open_vSwitch"/>.
    </p>
    <p>
      A <ref table="Bridge"/> record represents an Ethernet switch with one or
      more ``ports,'' which are the <ref table="Port"/> records pointed to by
      the <ref table="Bridge"/>'s <ref column="ports"/> column.
    </p>

    <group title="Core Features">
      <column name="name">
        Bridge identifier.  Should be alphanumeric and no more than about 8
        bytes long.  Must be unique among the names of ports, interfaces, and
        bridges on a host.
      </column>

      <column name="ports">
        Ports included in the bridge.
      </column>

      <column name="mirrors">
        Port mirroring configuration.
      </column>

      <column name="netflow">
        NetFlow configuration.
      </column>

      <column name="sflow">
        sFlow configuration.
      </column>

      <column name="flood_vlans">
        <p>
          VLAN IDs of VLANs on which MAC address learning should be disabled,
          so that packets are flooded instead of being sent to specific ports
          that are believed to contain packets' destination MACs.  This should
          ordinarily be used to disable MAC learning on VLANs used for
          mirroring (RSPAN VLANs).  It may also be useful for debugging.
        </p>
        <p>
          SLB bonding (see the <ref table="Port" column="bond_mode"/> column in
          the <ref table="Port"/> table) is incompatible with
          <code>flood_vlans</code>.  Consider using another bonding mode or
          a different type of mirror instead.
        </p>
      </column>
    </group>

    <group title="OpenFlow Configuration">
      <column name="controller">
        <p>
          OpenFlow controller set.  If unset, then no OpenFlow controllers
          will be used.
        </p>

        <p>
          If there are primary controllers, removing all of them clears the
          flow table.  If there are no primary controllers, adding one also
          clears the flow table.  Other changes to the set of controllers, such
          as adding or removing a service controller, adding another primary
          controller to supplement an existing primary controller, or removing
          only one of two primary controllers, have no effect on the flow
          table.
        </p>
      </column>

      <column name="flow_tables">
        Configuration for OpenFlow tables.  Each pair maps from an OpenFlow
        table ID to configuration for that table.
      </column>

      <column name="fail_mode">
        <p>When a controller is configured, it is, ordinarily, responsible
        for setting up all flows on the switch.  Thus, if the connection to
        the controller fails, no new network connections can be set up.
        If the connection to the controller stays down long enough,
        no packets can pass through the switch at all.  This setting
        determines the switch's response to such a situation.  It may be set
        to one of the following:
        <dl>
          <dt><code>standalone</code></dt>
          <dd>If no message is received from the controller for three
          times the inactivity probe interval
          (see <ref column="inactivity_probe"/>), then Open vSwitch
          will take over responsibility for setting up flows.  In
          this mode, Open vSwitch causes the bridge to act like an
          ordinary MAC-learning switch.  Open vSwitch will continue
          to retry connecting to the controller in the background
          and, when the connection succeeds, it will discontinue its
          standalone behavior.</dd>
          <dt><code>secure</code></dt>
          <dd>Open vSwitch will not set up flows on its own when the
          controller connection fails or when no controllers are
          defined.  The bridge will continue to retry connecting to
          any defined controllers forever.</dd>
        </dl>
        </p>
        <p>If this value is unset, the default is implementation-specific.</p>
        <p>When more than one controller is configured,
        <ref column="fail_mode"/> is considered only when none of the
        configured controllers can be contacted.</p>
        <p>
          Changing <ref column="fail_mode"/> when no primary controllers are
          configured clears the flow table.
        </p>
      </column>

      <column name="datapath_id">
        Reports the OpenFlow datapath ID in use.  Exactly 16 hex digits.
        (Setting this column has no useful effect.  Set <ref
        column="other-config" key="datapath-id"/> instead.)
      </column>

      <column name="other_config" key="datapath-id">
        Exactly 16 hex digits to set the OpenFlow datapath ID to a specific
        value.  May not be all-zero.
      </column>

      <column name="other_config" key="disable-in-band"
              type='{"type": "boolean"}'>
        If set to <code>true</code>, disable in-band control on the bridge
        regardless of controller and manager settings.
      </column>

      <column name="other_config" key="in-band-queue"
              type='{"type": "integer", "minInteger": 0, "maxInteger": 4294967295}'>
        A queue ID as a nonnegative integer.  This sets the OpenFlow queue ID
        that will be used by flows set up by in-band control on this bridge.
        If unset, or if the port used by an in-band control flow does not have
        QoS configured, or if the port does not have a queue with the specified
        ID, the default queue is used instead.
      </column>
    </group>

    <group title="Spanning Tree Configuration">
      The IEEE 802.1D Spanning Tree Protocol (STP) is a network protocol
      that ensures loop-free topologies.  It allows redundant links to
      be included in the network to provide automatic backup paths if
      the active links fails.

      <column name="stp_enable">
        Enable spanning tree on the bridge.  By default, STP is disabled
        on bridges.  Bond, internal, and mirror ports are not supported
        and will not participate in the spanning tree.
      </column>
 
      <column name="other_config" key="stp-system-id">
        The bridge's STP identifier (the lower 48 bits of the bridge-id)
        in the form
        <var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>.
        By default, the identifier is the MAC address of the bridge.
      </column>

      <column name="other_config" key="stp-priority"
              type='{"type": "integer", "minInteger": 0, "maxInteger": 65535}'>
        The bridge's relative priority value for determining the root
        bridge (the upper 16 bits of the bridge-id).  A bridge with the
        lowest bridge-id is elected the root.  By default, the priority
        is 0x8000.
      </column>

      <column name="other_config" key="stp-hello-time"
              type='{"type": "integer", "minInteger": 1, "maxInteger": 10}'>
        The interval between transmissions of hello messages by
        designated ports, in seconds.  By default the hello interval is
        2 seconds.
      </column>

      <column name="other_config" key="stp-max-age"
              type='{"type": "integer", "minInteger": 6, "maxInteger": 40}'>
        The maximum age of the information transmitted by the bridge
        when it is the root bridge, in seconds.  By default, the maximum
        age is 20 seconds.
      </column>

      <column name="other_config" key="stp-forward-delay"
              type='{"type": "integer", "minInteger": 4, "maxInteger": 30}'>
        The delay to wait between transitioning root and designated
        ports to <code>forwarding</code>, in seconds.  By default, the
        forwarding delay is 15 seconds.
      </column>
    </group>

    <group title="Other Features">
      <column name="datapath_type">
        Name of datapath provider.  The kernel datapath has
        type <code>system</code>.  The userspace datapath has
        type <code>netdev</code>.
      </column>

      <column name="external_ids" key="bridge-id">
        A unique identifier of the bridge.  On Citrix XenServer this will
        commonly be the same as
        <ref column="external_ids" key="xs-network-uuids"/>.
      </column>

      <column name="external_ids" key="xs-network-uuids">
        Semicolon-delimited set of universally unique identifier(s) for the
        network with which this bridge is associated on a Citrix XenServer
        host.  The network identifiers are RFC 4122 UUIDs as displayed by,
        e.g., <code>xe network-list</code>.
      </column>

      <column name="other_config" key="hwaddr">
        An Ethernet address in the form
        <var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>
        to set the hardware address of the local port and influence the
        datapath ID.
      </column>

      <column name="other_config" key="flow-eviction-threshold"
              type='{"type": "integer", "minInteger": 0}'>
        <p>
          A number of flows as a nonnegative integer.  This sets number of
          flows at which eviction from the kernel flow table will be triggered.
          If there are a large number of flows then increasing this value to
          around the number of flows present can result in reduced CPU usage
          and packet loss.
        </p>
        <p>
          The default is 1000.  Values below 100 will be rounded up to 100.
        </p>
      </column>

      <column name="other_config" key="forward-bpdu"
              type='{"type": "boolean"}'>
        Option to allow forwarding of BPDU frames when NORMAL action is
        invoked.  Frames with reserved Ethernet addresses (e.g. STP
        BPDU) will be forwarded when this option is enabled and the
        switch is not providing that functionality.  If STP is enabled
        on the port, STP BPDUs will never be forwarded.  If the Open
        vSwitch bridge is used to connect different Ethernet networks,
        and if Open vSwitch node does not run STP, then this option
        should be enabled.  Default is disabled, set to
        <code>true</code> to enable.
      </column>
    </group>

    <group title="Bridge Status">
      <p>
        Status information about bridges.
      </p>
      <column name="status">
        Key-value pairs that report bridge status.
      </column>
      <column name="status" key="stp_bridge_id">
        <p>
          The bridge-id (in hex) used in spanning tree advertisements.
          Configuring the bridge-id is described in the
          <code>stp-system-id</code> and <code>stp-priority</code> keys
          of the <code>other_config</code> section earlier.
        </p>
      </column>
      <column name="status" key="stp_designated_root">
        <p>
          The designated root (in hex) for this spanning tree.
        </p>
      </column>
      <column name="status" key="stp_root_path_cost">
        <p>
          The path cost of reaching the designated bridge.  A lower
          number is better.
        </p>
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Port" table="Port or bond configuration.">
    <p>A port within a <ref table="Bridge"/>.</p>
    <p>Most commonly, a port has exactly one ``interface,'' pointed to by its
    <ref column="interfaces"/> column.  Such a port logically
    corresponds to a port on a physical Ethernet switch.  A port
    with more than one interface is a ``bonded port'' (see
    <ref group="Bonding Configuration"/>).</p>
    <p>Some properties that one might think as belonging to a port are actually
    part of the port's <ref table="Interface"/> members.</p>

    <column name="name">
      Port name.  Should be alphanumeric and no more than about 8
      bytes long.  May be the same as the interface name, for
      non-bonded ports.  Must otherwise be unique among the names of
      ports, interfaces, and bridges on a host.
    </column>

    <column name="interfaces">
      The port's interfaces.  If there is more than one, this is a
      bonded Port.
    </column>

    <group title="VLAN Configuration">
      <p>Bridge ports support the following types of VLAN configuration:</p>
      <dl>
        <dt>trunk</dt>
        <dd>
          <p>
            A trunk port carries packets on one or more specified VLANs
            specified in the <ref column="trunks"/> column (often, on every
            VLAN).  A packet that ingresses on a trunk port is in the VLAN
            specified in its 802.1Q header, or VLAN 0 if the packet has no
            802.1Q header.  A packet that egresses through a trunk port will
            have an 802.1Q header if it has a nonzero VLAN ID.
          </p>

          <p>
            Any packet that ingresses on a trunk port tagged with a VLAN that
            the port does not trunk is dropped.
          </p>
        </dd>

        <dt>access</dt>
        <dd>
          <p>
            An access port carries packets on exactly one VLAN specified in the
            <ref column="tag"/> column.  Packets egressing on an access port
            have no 802.1Q header.
          </p>

          <p>
            Any packet with an 802.1Q header with a nonzero VLAN ID that
            ingresses on an access port is dropped, regardless of whether the
            VLAN ID in the header is the access port's VLAN ID.
          </p>
        </dd>

        <dt>native-tagged</dt>
        <dd>
          A native-tagged port resembles a trunk port, with the exception that
          a packet without an 802.1Q header that ingresses on a native-tagged
          port is in the ``native VLAN'' (specified in the <ref column="tag"/>
          column).
        </dd>

        <dt>native-untagged</dt>
        <dd>
          A native-untagged port resembles a native-tagged port, with the
          exception that a packet that egresses on a native-untagged port in
          the native VLAN will not have an 802.1Q header.
        </dd>
      </dl>
      <p>
        A packet will only egress through bridge ports that carry the VLAN of
        the packet, as described by the rules above.
      </p>

      <column name="vlan_mode">
        <p>
          The VLAN mode of the port, as described above.  When this column is
          empty, a default mode is selected as follows:
        </p>
        <ul>
          <li>
            If <ref column="tag"/> contains a value, the port is an access
            port.  The <ref column="trunks"/> column should be empty.
          </li>
          <li>
            Otherwise, the port is a trunk port.  The <ref column="trunks"/>
            column value is honored if it is present.
          </li>
        </ul>
      </column>

      <column name="tag">
        <p>
          For an access port, the port's implicitly tagged VLAN.  For a
          native-tagged or native-untagged port, the port's native VLAN.  Must
          be empty if this is a trunk port.
        </p>
      </column>

      <column name="trunks">
        <p>
          For a trunk, native-tagged, or native-untagged port, the 802.1Q VLAN
          or VLANs that this port trunks; if it is empty, then the port trunks
          all VLANs.  Must be empty if this is an access port.
        </p>
        <p>
          A native-tagged or native-untagged port always trunks its native
          VLAN, regardless of whether <ref column="trunks"/> includes that
          VLAN.
        </p>
      </column>

      <column name="other_config" key="priority-tags"
              type='{"type": "boolean"}'>
        <p>
          An 802.1Q header contains two important pieces of information: a VLAN
          ID and a priority.  A frame with a zero VLAN ID, called a
          ``priority-tagged'' frame, is supposed to be treated the same way as
          a frame without an 802.1Q header at all (except for the priority).
        </p>

        <p>
          However, some network elements ignore any frame that has 802.1Q
          header at all, even when the VLAN ID is zero.  Therefore, by default
          Open vSwitch does not output priority-tagged frames, instead omitting
          the 802.1Q header entirely if the VLAN ID is zero.  Set this key to
          <code>true</code> to enable priority-tagged frames on a port.
        </p>

        <p>
          Regardless of this setting, Open vSwitch omits the 802.1Q header on
          output if both the VLAN ID and priority would be zero.
        </p>

        <p>
          All frames output to native-tagged ports have a nonzero VLAN ID, so
          this setting is not meaningful on native-tagged ports.
        </p>
      </column>
    </group>

    <group title="Bonding Configuration">
      <p>A port that has more than one interface is a ``bonded port.'' Bonding
      allows for load balancing and fail-over.  Some kinds of bonding will
      work with any kind of upstream switch:</p>

      <dl>
        <dt><code>balance-slb</code></dt>
        <dd>
          Balances flows among slaves based on source MAC address and output
          VLAN, with periodic rebalancing as traffic patterns change.
        </dd>

        <dt><code>active-backup</code></dt>
        <dd>
          Assigns all flows to one slave, failing over to a backup slave when
          the active slave is disabled.
        </dd>
      </dl>

      <p>
        The following modes require the upstream switch to support 802.3ad with
        successful LACP negotiation:
      </p>

      <dl>
        <dt><code>balance-tcp</code></dt>
        <dd>
          Balances flows among slaves based on L2, L3, and L4 protocol
          information such as destination MAC address, IP address, and TCP
          port.
        </dd>

        <dt><code>stable</code></dt>
        <dd>
          <p>Attempts to always assign a given flow to the same slave
          consistently.  In an effort to maintain stability, no load
          balancing is done.  Uses a similar hashing strategy to
          <code>balance-tcp</code>, always taking into account L3 and L4
          fields even if LACP negotiations are unsuccessful. </p>
          <p>Slave selection decisions are made based on <ref table="Interface"
          column="other_config" key="bond-stable-id"/> if set.  Otherwise,
          OpenFlow port number is used.  Decisions are consistent across all
          <code>ovs-vswitchd</code> instances with equivalent
          <ref table="Interface" column="other_config" key="bond-stable-id"/>
          values.</p>
        </dd>
      </dl>

      <p>These columns apply only to bonded ports.  Their values are
      otherwise ignored.</p>

      <column name="bond_mode">
        <p>The type of bonding used for a bonded port.  Defaults to
        <code>active-backup</code> if unset.
        </p>
      </column>

      <column name="other_config" key="bond-hash-basis"
              type='{"type": "integer"}'>
        An integer hashed along with flows when choosing output slaves in load
        balanced bonds.  When changed, all flows will be assigned different
        hash values possibly causing slave selection decisions to change.  Does
        not affect bonding modes which do not employ load balancing such as
        <code>active-backup</code>.
      </column>

      <group title="Link Failure Detection">
        <p>
          An important part of link bonding is detecting that links are down so
          that they may be disabled.  These settings determine how Open vSwitch
          detects link failure.
        </p>

        <column name="other_config" key="bond-detect-mode"
                type='{"type": "string", "enum": ["set", ["carrier", "miimon"]]}'>
          The means used to detect link failures.  Defaults to
          <code>carrier</code> which uses each interface's carrier to detect
          failures.  When set to <code>miimon</code>, will check for failures
          by polling each interface's MII.
        </column>

        <column name="other_config" key="bond-miimon-interval"
                type='{"type": "integer"}'>
          The interval, in milliseconds, between successive attempts to poll
          each interface's MII.  Relevant only when <ref column="other_config"
          key="bond-detect-mode"/> is <code>miimon</code>.
        </column>

        <column name="bond_updelay">
          <p>
            The number of milliseconds for which carrier must stay up on an
            interface before the interface is considered to be up.  Specify
            <code>0</code> to enable the interface immediately.
          </p>

          <p>
            This setting is honored only when at least one bonded interface is
            already enabled.  When no interfaces are enabled, then the first
            bond interface to come up is enabled immediately.
          </p>
        </column>

        <column name="bond_downdelay">
          The number of milliseconds for which carrier must stay down on an
          interface before the interface is considered to be down.  Specify
          <code>0</code> to disable the interface immediately.
        </column>
      </group>

      <group title="LACP Configuration">
        <p>
          LACP, the Link Aggregation Control Protocol, is an IEEE standard that
          allows switches to automatically detect that they are connected by
          multiple links and aggregate across those links.  These settings
          control LACP behavior.
        </p>

        <column name="lacp">
          Configures LACP on this port.  LACP allows directly connected
          switches to negotiate which links may be bonded.  LACP may be enabled
          on non-bonded ports for the benefit of any switches they may be
          connected to.  <code>active</code> ports are allowed to initiate LACP
          negotiations.  <code>passive</code> ports are allowed to participate
          in LACP negotiations initiated by a remote switch, but not allowed to
          initiate such negotiations themselves.  If LACP is enabled on a port
          whose partner switch does not support LACP, the bond will be
          disabled.  Defaults to <code>off</code> if unset.
        </column>

        <column name="other_config" key="lacp-system-id">
          The LACP system ID of this <ref table="Port"/>.  The system ID of a
          LACP bond is used to identify itself to its partners.  Must be a
          nonzero MAC address. Defaults to the bridge Ethernet address if
          unset.
        </column>

        <column name="other_config" key="lacp-system-priority"
                type='{"type": "integer", "minInteger": 1, "maxInteger": 65535}'>
          The LACP system priority of this <ref table="Port"/>.  In LACP
          negotiations, link status decisions are made by the system with the
          numerically lower priority.
        </column>

        <column name="other_config" key="lacp-time">
          <p>
            The LACP timing which should be used on this <ref table="Port"/>.
            Possible values are <code>fast</code>, <code>slow</code> and a
            positive number of milliseconds.  By default <code>slow</code> is
            used.  When configured to be <code>fast</code> LACP heartbeats are
            requested at a rate of once per second causing connectivity
            problems to be detected more quickly.  In <code>slow</code> mode,
            heartbeats are requested at a rate of once every 30 seconds.
          </p>

          <p>
            Users may manually set a heartbeat transmission rate to increase
            the fault detection speed further.  When manually set, OVS expects
            the partner switch to be configured with the same transmission
            rate.  Manually setting <code>lacp-time</code> to something other
            than <code>fast</code> or <code>slow</code> is not supported by the
            LACP specification.
          </p>
        </column>

        <column name="other_config" key="lacp-heartbeat"
                type='{"type": "boolean"}'>
          Treat LACP like a simple heartbeat protocol for link state
          monitoring.  Most features of the LACP protocol are disabled
          when this mode is in use.  The default if not specified is
          <code>false</code>.
        </column>
      </group>

      <group title="SLB Configuration">
        <p>
          These settings control behavior when a bond is in
          <code>balance-slb</code> mode, regardless of whether the bond was
          intentionally configured in SLB mode or it fell back to SLB mode
          because LACP negotiation failed.
        </p>

        <column name="other_config" key="bond-rebalance-interval"
                type='{"type": "integer", "minInteger": 0, "maxInteger": 10000}'>
          For a load balanced bonded port, the number of milliseconds between
          successive attempts to rebalance the bond, that is, to move flows
          from one interface on the bond to another in an attempt to keep usage
          of each interface roughly equal.  If zero, load balancing is disabled
          on the bond (carrier status changes still cause flows to move).  If
          less than 1000ms, the rebalance interval will be 1000ms.
        </column>
      </group>

      <column name="bond_fake_iface">
        For a bonded port, whether to create a fake internal interface with the
        name of the port.  Use only for compatibility with legacy software that
        requires this.
      </column>
    </group>

    <group title="Spanning Tree Configuration">
      <column name="other_config" key="stp-enable"
              type='{"type": "boolean"}'>
        If spanning tree is enabled on the bridge, member ports are
        enabled by default (with the exception of bond, internal, and
        mirror ports which do not work with STP).  If this column's
        value is <code>false</code> spanning tree is disabled on the
        port.
      </column>

       <column name="other_config" key="stp-port-num"
               type='{"type": "integer", "minInteger": 1, "maxInteger": 255}'>
        The port number used for the lower 8 bits of the port-id.  By
        default, the numbers will be assigned automatically.  If any
        port's number is manually configured on a bridge, then they
        must all be.
      </column>

       <column name="other_config" key="stp-port-priority"
               type='{"type": "integer", "minInteger": 0, "maxInteger": 255}'>
        The port's relative priority value for determining the root
        port (the upper 8 bits of the port-id).  A port with a lower
        port-id will be chosen as the root port.  By default, the
        priority is 0x80.
      </column>

       <column name="other_config" key="stp-path-cost"
               type='{"type": "integer", "minInteger": 0, "maxInteger": 65535}'>
        Spanning tree path cost for the port.  A lower number indicates
        a faster link.  By default, the cost is based on the maximum
        speed of the link.
      </column>
    </group>

    <group title="Other Features">
      <column name="qos">
        Quality of Service configuration for this port.
      </column>

      <column name="mac">
        The MAC address to use for this port for the purpose of choosing the
        bridge's MAC address.  This column does not necessarily reflect the
        port's actual MAC address, nor will setting it change the port's actual
        MAC address.
      </column>

      <column name="fake_bridge">
        Does this port represent a sub-bridge for its tagged VLAN within the
        Bridge?  See ovs-vsctl(8) for more information.
      </column>

      <column name="external_ids" key="fake-bridge-id-*">
        External IDs for a fake bridge (see the <ref column="fake_bridge"/>
        column) are defined by prefixing a <ref table="Bridge"/> <ref
        table="Bridge" column="external_ids"/> key with
        <code>fake-bridge-</code>,
        e.g. <code>fake-bridge-xs-network-uuids</code>.
      </column>
    </group>

    <group title="Port Status">
      <p>
        Status information about ports attached to bridges.
      </p>
      <column name="status">
        Key-value pairs that report port status.
      </column>
      <column name="status" key="stp_port_id">
        <p>
          The port-id (in hex) used in spanning tree advertisements for
          this port.  Configuring the port-id is described in the
          <code>stp-port-num</code> and <code>stp-port-priority</code>
          keys of the <code>other_config</code> section earlier.
        </p>
      </column>
      <column name="status" key="stp_state"
              type='{"type": "string", "enum": ["set",
                            ["disabled", "listening", "learning",
                             "forwarding", "blocking"]]}'>
        <p>
          STP state of the port.
        </p>
      </column>
      <column name="status" key="stp_sec_in_state"
              type='{"type": "integer", "minInteger": 0}'>
        <p>
          The amount of time (in seconds) port has been in the current
          STP state.
        </p>
      </column>
      <column name="status" key="stp_role"
              type='{"type": "string", "enum": ["set",
                            ["root", "designated", "alternate"]]}'>
        <p>
          STP role of the port.
        </p>
      </column>
    </group>

    <group title="Port Statistics">
      <p>
        Key-value pairs that report port statistics.
      </p>
      <group title="Statistics: STP transmit and receive counters">
        <column name="statistics" key="stp_tx_count">
          Number of STP BPDUs sent on this port by the spanning
          tree library.
        </column>
        <column name="statistics" key="stp_rx_count">
          Number of STP BPDUs received on this port and accepted by the
          spanning tree library.
        </column>
        <column name="statistics" key="stp_error_count">
          Number of bad STP BPDUs received on this port.  Bad BPDUs
          include runt packets and those with an unexpected protocol ID.
        </column>
      </group>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Interface" title="One physical network device in a Port.">
    An interface within a <ref table="Port"/>.

    <group title="Core Features">
      <column name="name">
        Interface name.  Should be alphanumeric and no more than about 8 bytes
        long.  May be the same as the port name, for non-bonded ports.  Must
        otherwise be unique among the names of ports, interfaces, and bridges
        on a host.
      </column>

      <column name="mac">
        <p>Ethernet address to set for this interface.  If unset then the
        default MAC address is used:</p>
        <ul>
          <li>For the local interface, the default is the lowest-numbered MAC
          address among the other bridge ports, either the value of the
          <ref table="Port" column="mac"/> in its <ref table="Port"/> record,
          if set, or its actual MAC (for bonded ports, the MAC of its slave
          whose name is first in alphabetical order).  Internal ports and
          bridge ports that are used as port mirroring destinations (see the
          <ref table="Mirror"/> table) are ignored.</li>
          <li>For other internal interfaces, the default MAC is randomly
          generated.</li>
          <li>External interfaces typically have a MAC address associated with
          their hardware.</li>
        </ul>
        <p>Some interfaces may not have a software-controllable MAC
        address.</p>
      </column>

      <column name="ofport">
        <p>OpenFlow port number for this interface.  Unlike most columns, this
        column's value should be set only by Open vSwitch itself.  Other
        clients should set this column to an empty set (the default) when
        creating an <ref table="Interface"/>.</p>
        <p>Open vSwitch populates this column when the port number becomes
        known.  If the interface is successfully added,
        <ref column="ofport"/> will be set to a number between 1 and 65535
        (generally either in the range 1 to 65279, inclusive, or 65534, the
        port number for the OpenFlow ``local port'').  If the interface
        cannot be added then Open vSwitch sets this column
        to -1.</p>
      </column>
    </group>

    <group title="System-Specific Details">
      <column name="type">
        <p>
          The interface type, one of:
        </p>

        <dl>
          <dt><code>system</code></dt>
          <dd>An ordinary network device, e.g. <code>eth0</code> on Linux.
          Sometimes referred to as ``external interfaces'' since they are
          generally connected to hardware external to that on which the Open
          vSwitch is running.  The empty string is a synonym for
          <code>system</code>.</dd>

          <dt><code>internal</code></dt>
          <dd>A simulated network device that sends and receives traffic.  An
          internal interface whose <ref column="name"/> is the same as its
          bridge's <ref table="Open_vSwitch" column="name"/> is called the
          ``local interface.''  It does not make sense to bond an internal
          interface, so the terms ``port'' and ``interface'' are often used
          imprecisely for internal interfaces.</dd>

          <dt><code>tap</code></dt>
          <dd>A TUN/TAP device managed by Open vSwitch.</dd>

          <dt><code>gre</code></dt>
          <dd>
            An Ethernet over RFC 2890 Generic Routing Encapsulation over IPv4
            tunnel.  See <ref group="Tunnel Options"/> for information on
            configuring GRE tunnels.
          </dd>

          <dt><code>ipsec_gre</code></dt>
          <dd>
            An Ethernet over RFC 2890 Generic Routing Encapsulation over IPv4
            IPsec tunnel.  
          </dd>

          <dt><code>capwap</code></dt>
          <dd>
            An Ethernet tunnel over the UDP transport portion of CAPWAP (RFC
            5415).  This allows interoperability with certain switches that do
            not support GRE.  Only the tunneling component of the protocol is
            implemented.  UDP ports 58881 and 58882 are used as the source and
            destination ports respectively.  CAPWAP is currently supported only
            with the Linux kernel datapath with kernel version 2.6.26 or later.
          </dd>

          <dt><code>patch</code></dt>
          <dd>
            A pair of virtual devices that act as a patch cable.
          </dd>

          <dt><code>null</code></dt>
          <dd>An ignored interface.</dd>
        </dl>
      </column>
    </group>

    <group title="Tunnel Options">
      <p>
        These options apply to interfaces with <ref column="type"/> of
        <code>gre</code>, <code>ipsec_gre</code>, and <code>capwap</code>.
      </p>

      <p>
        Each tunnel must be uniquely identified by the combination of <ref
        column="type"/>, <ref column="options" key="remote_ip"/>, <ref
        column="options" key="local_ip"/>, and <ref column="options"
        key="in_key"/>.  If two ports are defined that are the same except one
        has an optional identifier and the other does not, the more specific
        one is matched first.  <ref column="options" key="in_key"/> is
        considered more specific than <ref column="options" key="local_ip"/> if
        a port defines one and another port defines the other.
      </p>

      <column name="options" key="remote_ip">
        <p>
          Required.  The tunnel endpoint.  Unicast and multicast endpoints are
          both supported.
        </p>

        <p>
          When a multicast endpoint is specified, a routing table lookup occurs
          only when the tunnel is created.  Following a routing change, delete
          and then re-create the tunnel to force a new routing table lookup.
        </p>
      </column>

      <column name="options" key="local_ip">
        Optional.  The destination IP that received packets must match.
        Default is to match all addresses.  Must be omitted when <ref
        column="options" key="remote_ip"/> is a multicast address.
      </column>

      <column name="options" key="in_key">
        <p>Optional.  The key that received packets must contain, one of:</p>

        <ul>
          <li>
            <code>0</code>.  The tunnel receives packets with no key or with a
            key of 0.  This is equivalent to specifying no <ref column="options"
            key="in_key"/> at all.
          </li>
          <li>
            A positive 32-bit (for GRE) or 64-bit (for CAPWAP) number.  The
            tunnel receives only packets with the specified key.
          </li>
          <li>
            The word <code>flow</code>.  The tunnel accepts packets with any
            key.  The key will be placed in the <code>tun_id</code> field for
            matching in the flow table.  The <code>ovs-ofctl</code> manual page
            contains additional information about matching fields in OpenFlow
            flows.
          </li>
        </ul>

        <p>
        </p>
      </column>

      <column name="options" key="out_key">
        <p>Optional.  The key to be set on outgoing packets, one of:</p>

        <ul>
          <li>
            <code>0</code>.  Packets sent through the tunnel will have no key.
            This is equivalent to specifying no <ref column="options"
            key="out_key"/> at all.
          </li>
          <li>
            A positive 32-bit (for GRE) or 64-bit (for CAPWAP) number.  Packets
            sent through the tunnel will have the specified key.
          </li>
          <li>
            The word <code>flow</code>.  Packets sent through the tunnel will
            have the key set using the <code>set_tunnel</code> Nicira OpenFlow
            vendor extension (0 is used in the absence of an action).  The
            <code>ovs-ofctl</code> manual page contains additional information
            about the Nicira OpenFlow vendor extensions.
          </li>
        </ul>
      </column>

      <column name="options" key="key">
        Optional.  Shorthand to set <code>in_key</code> and
        <code>out_key</code> at the same time.
      </column>

      <column name="options" key="tos">
        Optional.  The value of the ToS bits to be set on the encapsulating
        packet.  It may also be the word <code>inherit</code>, in which case
        the ToS will be copied from the inner packet if it is IPv4 or IPv6
        (otherwise it will be 0).  The ECN fields are always inherited.
        Default is 0.
      </column>

      <column name="options" key="ttl">
        Optional.  The TTL to be set on the encapsulating packet.  It may also
        be the word <code>inherit</code>, in which case the TTL will be copied
        from the inner packet if it is IPv4 or IPv6 (otherwise it will be the
        system default, typically 64).  Default is the system default TTL.
      </column>
      
      <column name="options" key="df_inherit" type='{"type": "boolean"}'>
        Optional.  If enabled, the Don't Fragment bit will be copied from the
        inner IP headers (those of the encapsulated traffic) to the outer
        (tunnel) headers.  Default is disabled; set to <code>true</code> to
        enable.
      </column>

      <column name="options" key="df_default"
              type='{"type": "boolean"}'>
        Optional.  If enabled, the Don't Fragment bit will be set by default on
        tunnel headers if the <code>df_inherit</code> option is not set, or if
        the encapsulated packet is not IP.  Default is enabled; set to
        <code>false</code> to disable.
      </column>

      <column name="options" key="pmtud" type='{"type": "boolean"}'>
        Optional.  Enable tunnel path MTU discovery.  If enabled ``ICMP
        Destination Unreachable - Fragmentation Needed'' messages will be
        generated for IPv4 packets with the DF bit set and IPv6 packets above
        the minimum MTU if the packet size exceeds the path MTU minus the size
        of the tunnel headers.  Note that this option causes behavior that is
        typically reserved for routers and therefore is not entirely in
        compliance with the IEEE 802.1D specification for bridges.  Default is
        enabled; set to <code>false</code> to disable.
      </column>

      <group title="Tunnel Options: gre only">
        <p>
          Only <code>gre</code> interfaces support these options.
        </p>

        <column name="options" key="header_cache" type='{"type": "boolean"}'>
          Enable caching of tunnel headers and the output path.  This can lead
          to a significant performance increase without changing behavior.  In
          general it should not be necessary to adjust this setting.  However,
          the caching can bypass certain components of the IP stack (such as
          <code>iptables</code>) and it may be useful to disable it if these
          features are required or as a debugging measure.  Default is enabled,
          set to <code>false</code> to disable.
        </column>
      </group>

      <group title="Tunnel Options: gre and ipsec_gre only">
        <p>
          Only <code>gre</code> and <code>ipsec_gre</code> interfaces support
          these options.
        </p>

        <column name="options" key="csum" type='{"type": "boolean"}'>
          <p>
            Optional.  Compute GRE checksums on outgoing packets.  Default is
            disabled, set to <code>true</code> to enable.  Checksums present on
            incoming packets will be validated regardless of this setting.
          </p>

          <p>
            GRE checksums impose a significant performance penalty because they
            cover the entire packet.  The encapsulated L3, L4, and L7 packet
            contents typically have their own checksums, so this additional
            checksum only adds value for the GRE and encapsulated L2 headers.
          </p>

          <p>
            This option is supported for <code>ipsec_gre</code>, but not useful
            because GRE checksums are weaker than, and redundant with, IPsec
            payload authentication.
          </p>
        </column>
      </group>

      <group title="Tunnel Options: ipsec_gre only">
        <p>
          Only <code>ipsec_gre</code> interfaces support these options.
        </p>

        <column name="options" key="peer_cert">
          Required for certificate authentication.  A string containing the
          peer's certificate in PEM format.  Additionally the host's
          certificate must be specified with the <code>certificate</code>
          option.
        </column>

        <column name="options" key="certificate">
          Required for certificate authentication.  The name of a PEM file
          containing a certificate that will be presented to the peer during
          authentication.
        </column>

        <column name="options" key="private_key">
          Optional for certificate authentication.  The name of a PEM file
          containing the private key associated with <code>certificate</code>.
          If <code>certificate</code> contains the private key, this option may
          be omitted.
        </column>

        <column name="options" key="psk">
          Required for pre-shared key authentication.  Specifies a pre-shared
          key for authentication that must be identical on both sides of the
          tunnel.
        </column>
      </group>
    </group>

    <group title="Patch Options">
      <p>
        Only <code>patch</code> interfaces support these options.
      </p>

      <column name="options" key="peer">
        The <ref column="name"/> of the <ref table="Interface"/> for the other
        side of the patch.  The named <ref table="Interface"/>'s own
        <code>peer</code> option must specify this <ref table="Interface"/>'s
        name.  That is, the two patch interfaces must have reversed <ref
        column="name"/> and <code>peer</code> values.
      </column>
    </group>

    <group title="Interface Status">
      <p>
        Status information about interfaces attached to bridges, updated every
        5 seconds.  Not all interfaces have all of these properties; virtual
        interfaces don't have a link speed, for example.  Non-applicable
        columns will have empty values.
      </p>
      <column name="admin_state">
        <p>
          The administrative state of the physical network link.
        </p>
      </column>

      <column name="link_state">
        <p>
          The observed state of the physical network link.  This is ordinarily
          the link's carrier status.  If the interface's <ref table="Port"/> is
          a bond configured for miimon monitoring, it is instead the network
          link's miimon status.
        </p>
      </column>

      <column name="link_resets">
        <p>
          The number of times Open vSwitch has observed the
          <ref column="link_state"/> of this <ref table="Interface"/> change.
        </p>
      </column>

      <column name="link_speed">
        <p>
          The negotiated speed of the physical network link.
          Valid values are positive integers greater than 0.
        </p>
      </column>

      <column name="duplex">
        <p>
          The duplex mode of the physical network link.
        </p>
      </column>

      <column name="mtu">
        <p>
          The MTU (maximum transmission unit); i.e. the largest
          amount of data that can fit into a single Ethernet frame.
          The standard Ethernet MTU is 1500 bytes.  Some physical media
          and many kinds of virtual interfaces can be configured with
          higher MTUs.
        </p>
        <p>
          This column will be empty for an interface that does not
          have an MTU as, for example, some kinds of tunnels do not.
        </p>
      </column>

      <column name="lacp_current">
        Boolean value indicating LACP status for this interface.  If true, this
        interface has current LACP information about its LACP partner.  This
        information may be used to monitor the health of interfaces in a LACP
        enabled port.  This column will be empty if LACP is not enabled.
      </column>

      <column name="status">
        Key-value pairs that report port status.  Supported status values are
        <ref column="type"/>-dependent; some interfaces may not have a valid
        <ref column="status" key="driver_name"/>, for example.
      </column>

      <column name="status" key="driver_name">
        The name of the device driver controlling the network adapter.
      </column>

      <column name="status" key="driver_version">
        The version string of the device driver controlling the network
        adapter.
      </column>

      <column name="status" key="firmware_version">
        The version string of the network adapter's firmware, if available.
      </column>

      <column name="status" key="source_ip">
        The source IP address used for an IPv4 tunnel end-point, such as
        <code>gre</code> or <code>capwap</code>.
      </column>

      <column name="status" key="tunnel_egress_iface">
        Egress interface for tunnels.  Currently only relevant for GRE and
        CAPWAP tunnels.  On Linux systems, this column will show the name of
        the interface which is responsible for routing traffic destined for the
        configured <ref column="options" key="remote_ip"/>.  This could be an
        internal interface such as a bridge port.
      </column>

      <column name="status" key="tunnel_egress_iface_carrier"
              type='{"type": "string", "enum": ["set", ["down", "up"]]}'>
        Whether carrier is detected on <ref column="status"
        key="tunnel_egress_iface"/>.
      </column>
    </group>

    <group title="Statistics">
      <p>
        Key-value pairs that report interface statistics.  The current
        implementation updates these counters periodically.  Future
        implementations may update them when an interface is created, when they
        are queried (e.g. using an OVSDB <code>select</code> operation), and
        just before an interface is deleted due to virtual interface hot-unplug
        or VM shutdown, and perhaps at other times, but not on any regular
        periodic basis.
      </p>
      <p>
        These are the same statistics reported by OpenFlow in its <code>struct
        ofp_port_stats</code> structure.  If an interface does not support a
        given statistic, then that pair is omitted.
      </p>
      <group title="Statistics: Successful transmit and receive counters">
        <column name="statistics" key="rx_packets">
          Number of received packets.
        </column>
        <column name="statistics" key="rx_bytes">
          Number of received bytes.
        </column>
        <column name="statistics" key="tx_packets">
          Number of transmitted packets.
        </column>
        <column name="statistics" key="tx_bytes">
          Number of transmitted bytes.
        </column>
      </group>
      <group title="Statistics: Receive errors">
        <column name="statistics" key="rx_dropped">
          Number of packets dropped by RX.
        </column>
        <column name="statistics" key="rx_frame_err">
          Number of frame alignment errors.
        </column>
        <column name="statistics" key="rx_over_err">
          Number of packets with RX overrun.
        </column>
        <column name="statistics" key="rx_crc_err">
          Number of CRC errors.
        </column>
        <column name="statistics" key="rx_errors">
          Total number of receive errors, greater than or equal to the sum of
          the above.
        </column>
      </group>              
      <group title="Statistics: Transmit errors">
        <column name="statistics" key="tx_dropped">
          Number of packets dropped by TX.
        </column>
        <column name="statistics" key="collisions">
          Number of collisions.
        </column>
        <column name="statistics" key="tx_errors">
          Total number of transmit errors, greater than or equal to the sum of
          the above.
        </column>
      </group>
    </group>

    <group title="Ingress Policing">
      <p>
        These settings control ingress policing for packets received on this
        interface.  On a physical interface, this limits the rate at which
        traffic is allowed into the system from the outside; on a virtual
        interface (one connected to a virtual machine), this limits the rate at
        which the VM is able to transmit.
      </p>
      <p>
        Policing is a simple form of quality-of-service that simply drops
        packets received in excess of the configured rate.  Due to its
        simplicity, policing is usually less accurate and less effective than
        egress QoS (which is configured using the <ref table="QoS"/> and <ref
        table="Queue"/> tables).
      </p>
      <p>
        Policing is currently implemented only on Linux.  The Linux
        implementation uses a simple ``token bucket'' approach:
      </p>
      <ul>
        <li>
          The size of the bucket corresponds to <ref
          column="ingress_policing_burst"/>.  Initially the bucket is full.
        </li>
        <li>
          Whenever a packet is received, its size (converted to tokens) is
          compared to the number of tokens currently in the bucket.  If the
          required number of tokens are available, they are removed and the
          packet is forwarded.  Otherwise, the packet is dropped.
        </li>
        <li>
          Whenever it is not full, the bucket is refilled with tokens at the
          rate specified by <ref column="ingress_policing_rate"/>.
        </li>
      </ul>
      <p>
        Policing interacts badly with some network protocols, and especially
        with fragmented IP packets.  Suppose that there is enough network
        activity to keep the bucket nearly empty all the time.  Then this token
        bucket algorithm will forward a single packet every so often, with the
        period depending on packet size and on the configured rate.  All of the
        fragments of an IP packets are normally transmitted back-to-back, as a
        group.  In such a situation, therefore, only one of these fragments
        will be forwarded and the rest will be dropped.  IP does not provide
        any way for the intended recipient to ask for only the remaining
        fragments.  In such a case there are two likely possibilities for what
        will happen next: either all of the fragments will eventually be
        retransmitted (as TCP will do), in which case the same problem will
        recur, or the sender will not realize that its packet has been dropped
        and data will simply be lost (as some UDP-based protocols will do).
        Either way, it is possible that no forward progress will ever occur.
      </p>
      <column name="ingress_policing_rate">
        <p>
          Maximum rate for data received on this interface, in kbps.  Data
          received faster than this rate is dropped.  Set to <code>0</code>
          (the default) to disable policing.
        </p>
      </column>

      <column name="ingress_policing_burst">
        <p>Maximum burst size for data received on this interface, in kb.  The
        default burst size if set to <code>0</code> is 1000 kb.  This value
        has no effect if <ref column="ingress_policing_rate"/>
        is <code>0</code>.</p>
        <p>
          Specifying a larger burst size lets the algorithm be more forgiving,
          which is important for protocols like TCP that react severely to
          dropped packets.  The burst size should be at least the size of the
          interface's MTU.  Specifying a value that is numerically at least as
          large as 10% of <ref column="ingress_policing_rate"/> helps TCP come
          closer to achieving the full rate.
        </p>
      </column>
    </group>

    <group title="Connectivity Fault Management">
      <p>
        802.1ag Connectivity Fault Management (CFM) allows a group of
        Maintenance Points (MPs) called a Maintenance Association (MA) to
        detect connectivity problems with each other.  MPs within a MA should
        have complete and exclusive interconnectivity.  This is verified by
        occasionally broadcasting Continuity Check Messages (CCMs) at a
        configurable transmission interval.
      </p>

      <p>
        According to the 802.1ag specification, each Maintenance Point should
        be configured out-of-band with a list of Remote Maintenance Points it
        should have connectivity to.  Open vSwitch differs from the
        specification in this area.  It simply assumes the link is faulted if
        no Remote Maintenance Points are reachable, and considers it not
        faulted otherwise.
      </p>

      <column name="cfm_mpid">
        A Maintenance Point ID (MPID) uniquely identifies each endpoint within
        a Maintenance Association.  The MPID is used to identify this endpoint
        to other Maintenance Points in the MA.  Each end of a link being
        monitored should have a different MPID.  Must be configured to enable
        CFM on this <ref table="Interface"/>.
      </column>

      <column name="cfm_fault">
        <p>
          Indicates a connectivity fault triggered by an inability to receive
          heartbeats from any remote endpoint.  When a fault is triggered on
          <ref table="Interface"/>s participating in bonds, they will be
          disabled.
        </p>
        <p>
          Faults can be triggered for several reasons.  Most importantly they
          are triggered when no CCMs are received for a period of 3.5 times the
          transmission interval. Faults are also triggered when any CCMs
          indicate that a Remote Maintenance Point is not receiving CCMs but
          able to send them.  Finally, a fault is triggered if a CCM is
          received which indicates unexpected configuration.  Notably, this
          case arises when a CCM is received which advertises the local MPID.
        </p>
      </column>

      <column name="cfm_remote_mpids">
        When CFM is properly configured, Open vSwitch will occasionally
        receive CCM broadcasts.  These broadcasts contain the MPID of the
        sending Maintenance Point.  The list of MPIDs from which this
        <ref table="Interface"/> is receiving broadcasts from is regularly
        collected and written to this column.
      </column>

      <column name="other_config" key="cfm_interval"
              type='{"type": "integer"}'>
        The interval, in milliseconds, between transmissions of CFM heartbeats.
        Three missed heartbeat receptions indicate a connectivity fault.
        Defaults to 1000.
      </column>

      <column name="other_config" key="cfm_extended"
              type='{"type": "boolean"}'>
        When <code>true</code>, the CFM module operates in extended mode. This
        causes it to use a nonstandard destination address to avoid conflicting
        with compliant implementations which may be running concurrently on the
        network. Furthermore, extended mode increases the accuracy of the
        <code>cfm_interval</code> configuration parameter by breaking wire
        compatibility with 802.1ag compliant implementations.  Defaults to
        <code>false</code>.
      </column>
      <column name="other_config" key="cfm_opstate"
              type='{"type": "string", "enum": ["set", ["down", "up"]]}'>
        When <code>down</code>, the CFM module marks all CCMs it generates as
        operationally down without triggering a fault.  This allows remote
        maintenance points to choose not to forward traffic to the
        <ref table="Interface"/> on which this CFM module is running.
        Currently, in Open vSwitch, the opdown bit of CCMs affects
        <ref table="Interface"/>s participating in bonds, and the bundle
        OpenFlow action. This setting is ignored when CFM is not in extended
        mode.  Defaults to <code>up</code>.
      </column>

      <column name="other_config" key="cfm_ccm_vlan"
        type='{"type": "integer", "minInteger": 1, "maxInteger": 4095}'>
        When set, the CFM module will apply a VLAN tag to all CCMs it generates
        with the given value.
      </column>

    </group>

    <group title="Bonding Configuration">
      <column name="other_config" key="bond-stable-id"
              type='{"type": "integer", "minInteger": 1}'>
        Used in <code>stable</code> bond mode to make slave
        selection decisions.  Allocating <ref column="other_config"
        key="bond-stable-id"/> values consistently across interfaces
        participating in a bond will guarantee consistent slave selection
        decisions across <code>ovs-vswitchd</code> instances when using
        <code>stable</code> bonding mode.
      </column>

      <column name="other_config" key="lacp-port-id"
              type='{"type": "integer", "minInteger": 1, "maxInteger": 65535}'>
        The LACP port ID of this <ref table="Interface"/>.  Port IDs are
        used in LACP negotiations to identify individual ports
        participating in a bond.
      </column>

      <column name="other_config" key="lacp-port-priority"
              type='{"type": "integer", "minInteger": 1, "maxInteger": 65535}'>
        The LACP port priority of this <ref table="Interface"/>.  In LACP
        negotiations <ref table="Interface"/>s with numerically lower
        priorities are preferred for aggregation.
      </column>

      <column name="other_config" key="lacp-aggregation-key"
              type='{"type": "integer", "minInteger": 1, "maxInteger": 65535}'>
        The LACP aggregation key of this <ref table="Interface"/>.  <ref
        table="Interface"/>s with different aggregation keys may not be active
        within a given <ref table="Port"/> at the same time.
      </column>
    </group>

    <group title="Virtual Machine Identifiers">
      <p>
        These key-value pairs specifically apply to an interface that
        represents a virtual Ethernet interface connected to a virtual
        machine.  These key-value pairs should not be present for other types
        of interfaces.  Keys whose names end in <code>-uuid</code> have
        values that uniquely identify the entity in question.  For a Citrix
        XenServer hypervisor, these values are UUIDs in RFC 4122 format.
        Other hypervisors may use other formats.
      </p>

      <column name="external_ids" key="attached-mac">
        The MAC address programmed into the ``virtual hardware'' for this
        interface, in the form
        <var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>.
        For Citrix XenServer, this is the value of the <code>MAC</code> field
        in the VIF record for this interface.
      </column>

      <column name="external_ids" key="iface-id">
        A system-unique identifier for the interface.  On XenServer, this will
        commonly be the same as <ref column="external_ids" key="xs-vif-uuid"/>.
      </column>

      <column name="external_ids" key="xs-vif-uuid">
        The virtual interface associated with this interface.
      </column>

      <column name="external_ids" key="xs-network-uuid">
        The virtual network to which this interface is attached.
      </column>

      <column name="external_ids" key="xs-vm-uuid">
        The VM to which this interface belongs.
      </column>
    </group>

    <group title="VLAN Splinters">
      <p>
        The ``VLAN splinters'' feature increases Open vSwitch compatibility
        with buggy network drivers in old versions of Linux that do not
        properly support VLANs when VLAN devices are not used, at some cost
        in memory and performance.
      </p>

      <p>
        When VLAN splinters are enabled on a particular interface, Open vSwitch
        creates a VLAN device for each in-use VLAN.  For sending traffic tagged
        with a VLAN on the interface, it substitutes the VLAN device.  Traffic
        received on the VLAN device is treated as if it had been received on
        the interface on the particular VLAN.
      </p>

      <p>
        VLAN splinters consider a VLAN to be in use if:
      </p>

      <ul>
        <li>
          The VLAN is the <ref table="Port" column="tag"/> value in any <ref
          table="Port"/> record.
        </li>

        <li>
          The VLAN is listed within the <ref table="Port" column="trunks"/>
          column of the <ref table="Port"/> record of an interface on which
          VLAN splinters are enabled.

          An empty <ref table="Port" column="trunks"/> does not influence the
          in-use VLANs: creating 4,096 VLAN devices is impractical because it
          will exceed the current 1,024 port per datapath limit.
        </li>

        <li>
          An OpenFlow flow within any bridge matches the VLAN.
        </li>
      </ul>

      <p>
        The same set of in-use VLANs applies to every interface on which VLAN
        splinters are enabled.  That is, the set is not chosen separately for
        each interface but selected once as the union of all in-use VLANs based
        on the rules above.
      </p>

      <p>
        It does not make sense to enable VLAN splinters on an interface for an
        access port, or on an interface that is not a physical port.
      </p>

      <p>
        VLAN splinters are deprecated.  When broken device drivers are no
        longer in widespread use, we will delete this feature.
      </p>

      <column name="other_config" key="enable-vlan-splinters"
              type='{"type": "boolean"}'>
        <p>
          Set to <code>true</code> to enable VLAN splinters on this interface.
          Defaults to <code>false</code>.
        </p>

        <p>
          VLAN splinters increase kernel and userspace memory overhead, so do
          not use them unless they are needed.
        </p>
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Flow_Table" title="OpenFlow table configuration">
    <p>Configuration for a particular OpenFlow table.</p>

    <column name="name">
      The table's name.  Set this column to change the name that controllers
      will receive when they request table statistics, e.g. <code>ovs-ofctl
      dump-tables</code>.  The name does not affect switch behavior.
    </column>

    <column name="flow_limit">
      If set, limits the number of flows that may be added to the table.  Open
      vSwitch may limit the number of flows in a table for other reasons,
      e.g. due to hardware limitations or for resource availability or
      performance reasons.
    </column>

    <column name="overflow_policy">
      <p>
        Controls the switch's behavior when an OpenFlow flow table modification
        request would add flows in excess of <ref column="flow_limit"/>.  The
        supported values are:
      </p>

      <dl>
        <dt><code>refuse</code></dt>
        <dd>
          Refuse to add the flow or flows.  This is also the default policy
          when <ref column="overflow_policy"/> is unset.
        </dd>

        <dt><code>evict</code></dt>
        <dd>
          Delete the flow that will expire soonest.  See <ref column="groups"/>
          for details.
        </dd>
      </dl>
    </column>

    <column name="groups">
      <p>
        When <ref column="overflow_policy"/> is <code>evict</code>, this
        controls how flows are chosen for eviction when the flow table would
        otherwise exceed <ref column="flow_limit"/> flows.  Its value is a set
        of NXM fields or sub-fields, each of which takes one of the forms
        <code><var>field</var>[]</code> or
        <code><var>field</var>[<var>start</var>..<var>end</var>]</code>,
        e.g. <code>NXM_OF_IN_PORT[]</code>.  Please see
        <code>nicira-ext.h</code> for a complete list of NXM field names.
      </p>

      <p>
        When a flow must be evicted due to overflow, the flow to evict is
        chosen through an approximation of the following algorithm:
      </p>

      <ol>
        <li>
          Divide the flows in the table into groups based on the values of the
          specified fields or subfields, so that all of the flows in a given
          group have the same values for those fields.  If a flow does not
          specify a given field, that field's value is treated as 0.
        </li>

        <li>
          Consider the flows in the largest group, that is, the group that
          contains the greatest number of flows.  If two or more groups all
          have the same largest number of flows, consider the flows in all of
          those groups.
        </li>

        <li>
          Among the flows under consideration, choose the flow that expires
          soonest for eviction.
        </li>
      </ol>

      <p>
        The eviction process only considers flows that have an idle timeout or
        a hard timeout.  That is, eviction never deletes permanent flows.
        (Permanent flows do count against <ref column="flow_limit"/>.
      </p>

      <p>
        Open vSwitch ignores any invalid or unknown field specifications.
      </p>

      <p>
        When <ref column="overflow_policy"/> is not <code>evict</code>, this
        column has no effect.
      </p>
    </column>
  </table>

  <table name="QoS" title="Quality of Service configuration">
    <p>Quality of Service (QoS) configuration for each Port that
    references it.</p>

    <column name="type">
      <p>The type of QoS to implement.  The <ref table="Open_vSwitch"
      column="capabilities"/> column in the <ref table="Open_vSwitch"/> table
      identifies the types that a switch actually supports.  The currently
      defined types are listed below:</p>
      <dl>
        <dt><code>linux-htb</code></dt>
        <dd>
          Linux ``hierarchy token bucket'' classifier.  See tc-htb(8) (also at
          <code>http://linux.die.net/man/8/tc-htb</code>) and the HTB manual
          (<code>http://luxik.cdi.cz/~devik/qos/htb/manual/userg.htm</code>)
          for information on how this classifier works and how to configure it.
        </dd>
      </dl>
      <dl>
        <dt><code>linux-hfsc</code></dt>
        <dd>
          Linux "Hierarchical Fair Service Curve" classifier.
          See <code>http://linux-ip.net/articles/hfsc.en/</code> for
          information on how this classifier works.
        </dd>
      </dl>
    </column>

    <column name="queues">
      <p>A map from queue numbers to <ref table="Queue"/> records.  The
      supported range of queue numbers depend on <ref column="type"/>.  The
      queue numbers are the same as the <code>queue_id</code> used in
      OpenFlow in <code>struct ofp_action_enqueue</code> and other
      structures.  Queue 0 is used by OpenFlow output actions that do not
      specify a specific queue.</p>
    </column>

    <group title="Configuration for linux-htb and linux-hfsc">
      <p>
        The <code>linux-htb</code> and <code>linux-hfsc</code> classes support
        the following key-value pair:
      </p>
      
      <column name="other_config" key="max-rate" type='{"type": "integer"}'>
        Maximum rate shared by all queued traffic, in bit/s.  Optional.  If not
        specified, for physical interfaces, the default is the link rate.  For
        other interfaces or if the link rate cannot be determined, the default
        is currently 100 Mbps.
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Queue" title="QoS output queue.">
    <p>A configuration for a port output queue, used in configuring Quality of
    Service (QoS) features.  May be referenced by <ref column="queues"
    table="QoS"/> column in <ref table="QoS"/> table.</p>

    <column name="dscp">
      If set, Open vSwitch will mark all traffic egressing this
      <ref table="Queue"/> with the given DSCP bits.  Traffic egressing the
      default <ref table="Queue"/> is only marked if it was explicitly selected
      as the <ref table="Queue"/> at the time the packet was output.  If unset,
      the DSCP bits of traffic egressing this <ref table="Queue"/> will remain
      unchanged.
    </column>

    <group title="Configuration for linux-htb QoS">
      <p>
        <ref table="QoS"/> <ref table="QoS" column="type"/>
        <code>linux-htb</code> may use <code>queue_id</code>s less than 61440.
        It has the following key-value pairs defined.
      </p>
      
      <column name="other_config" key="min-rate"
              type='{"type": "integer", "minInteger": 1}'>
        Minimum guaranteed bandwidth, in bit/s.
      </column>

      <column name="other_config" key="max-rate"
              type='{"type": "integer", "minInteger": 1}'>
        Maximum allowed bandwidth, in bit/s.  Optional.  If specified, the
        queue's rate will not be allowed to exceed the specified value, even
        if excess bandwidth is available.  If unspecified, defaults to no
        limit.
      </column>

      <column name="other_config" key="burst"
              type='{"type": "integer", "minInteger": 1}'>
        Burst size, in bits.  This is the maximum amount of ``credits'' that a
        queue can accumulate while it is idle.  Optional.  Details of the
        <code>linux-htb</code> implementation require a minimum burst size, so
        a too-small <code>burst</code> will be silently ignored.
      </column>

      <column name="other_config" key="priority"
              type='{"type": "integer", "minInteger": 0, "maxInteger": 4294967295}'>
        A queue with a smaller <code>priority</code> will receive all the
        excess bandwidth that it can use before a queue with a larger value
        receives any.  Specific priority values are unimportant; only relative
        ordering matters.  Defaults to 0 if unspecified.
      </column>
    </group>

    <group title="Configuration for linux-hfsc QoS">
      <p>
        <ref table="QoS"/> <ref table="QoS" column="type"/>
        <code>linux-hfsc</code> may use <code>queue_id</code>s less than 61440.
        It has the following key-value pairs defined.
      </p>
      
      <column name="other_config" key="min-rate"
              type='{"type": "integer", "minInteger": 1}'>
        Minimum guaranteed bandwidth, in bit/s.
      </column>
      
      <column name="other_config" key="max-rate"
              type='{"type": "integer", "minInteger": 1}'>
        Maximum allowed bandwidth, in bit/s.  Optional.  If specified, the
        queue's rate will not be allowed to exceed the specified value, even if
        excess bandwidth is available.  If unspecified, defaults to no
        limit.
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="other_config"/>
      <column name="external_ids"/>
    </group>
  </table>

  <table name="Mirror" title="Port mirroring.">
    <p>A port mirror within a <ref table="Bridge"/>.</p>
    <p>A port mirror configures a bridge to send selected frames to special
    ``mirrored'' ports, in addition to their normal destinations.  Mirroring
    traffic may also be referred to as SPAN or RSPAN, depending on how
    the mirrored traffic is sent.</p>

    <column name="name">
      Arbitrary identifier for the <ref table="Mirror"/>.
    </column>

    <group title="Selecting Packets for Mirroring">
      <p>
        To be selected for mirroring, a given packet must enter or leave the
        bridge through a selected port and it must also be in one of the
        selected VLANs.
      </p>

      <column name="select_all">
        If true, every packet arriving or departing on any port is
        selected for mirroring.
      </column>

      <column name="select_dst_port">
        Ports on which departing packets are selected for mirroring.
      </column>

      <column name="select_src_port">
        Ports on which arriving packets are selected for mirroring.
      </column>

      <column name="select_vlan">
        VLANs on which packets are selected for mirroring.  An empty set
        selects packets on all VLANs.
      </column>
    </group>

    <group title="Mirroring Destination Configuration">
      <p>
        These columns are mutually exclusive.  Exactly one of them must be
        nonempty.
      </p>

      <column name="output_port">
        <p>Output port for selected packets, if nonempty.</p>
        <p>Specifying a port for mirror output reserves that port exclusively
        for mirroring.  No frames other than those selected for mirroring
        via this column
        will be forwarded to the port, and any frames received on the port
        will be discarded.</p>
        <p>
          The output port may be any kind of port supported by Open vSwitch.
          It may be, for example, a physical port (sometimes called SPAN) or a
          GRE tunnel.
        </p>
      </column>

      <column name="output_vlan">
        <p>Output VLAN for selected packets, if nonempty.</p>
        <p>The frames will be sent out all ports that trunk
        <ref column="output_vlan"/>, as well as any ports with implicit VLAN
        <ref column="output_vlan"/>.  When a mirrored frame is sent out a
        trunk port, the frame's VLAN tag will be set to
        <ref column="output_vlan"/>, replacing any existing tag; when it is
        sent out an implicit VLAN port, the frame will not be tagged.  This
        type of mirroring is sometimes called RSPAN.</p>
        <p>
          The following destination MAC addresses will not be mirrored to a
          VLAN to avoid confusing switches that interpret the protocols that
          they represent:
        </p>
        <dl>
          <dt><code>01:80:c2:00:00:00</code></dt>
          <dd>IEEE 802.1D Spanning Tree Protocol (STP).</dd>

          <dt><code>01:80:c2:00:00:01</code></dt>
          <dd>IEEE Pause frame.</dd>

          <dt><code>01:80:c2:00:00:0<var>x</var></code></dt>
          <dd>Other reserved protocols.</dd>

          <dt><code>01:00:0c:cc:cc:cc</code></dt>
          <dd>
            Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP),
            Dynamic Trunking Protocol (DTP), Port Aggregation Protocol (PAgP),
            and others.
          </dd>

          <dt><code>01:00:0c:cc:cc:cd</code></dt>
          <dd>Cisco Shared Spanning Tree Protocol PVSTP+.</dd>

          <dt><code>01:00:0c:cd:cd:cd</code></dt>
          <dd>Cisco STP Uplink Fast.</dd>

          <dt><code>01:00:0c:00:00:00</code></dt>
          <dd>Cisco Inter Switch Link.</dd>
        </dl>
        <p><em>Please note:</em> Mirroring to a VLAN can disrupt a network that
        contains unmanaged switches.  Consider an unmanaged physical switch
        with two ports: port 1, connected to an end host, and port 2,
        connected to an Open vSwitch configured to mirror received packets
        into VLAN 123 on port 2.  Suppose that the end host sends a packet on
        port 1 that the physical switch forwards to port 2.  The Open vSwitch
        forwards this packet to its destination and then reflects it back on
        port 2 in VLAN 123.  This reflected packet causes the unmanaged
        physical switch to replace the MAC learning table entry, which
        correctly pointed to port 1, with one that incorrectly points to port
        2.  Afterward, the physical switch will direct packets destined for
        the end host to the Open vSwitch on port 2, instead of to the end
        host on port 1, disrupting connectivity.  If mirroring to a VLAN is
        desired in this scenario, then the physical switch must be replaced
        by one that learns Ethernet addresses on a per-VLAN basis.  In
        addition, learning should be disabled on the VLAN containing mirrored
        traffic. If this is not done then intermediate switches will learn
        the MAC address of each end host from the mirrored traffic.  If
        packets being sent to that end host are also mirrored, then they will
        be dropped since the switch will attempt to send them out the input
        port. Disabling learning for the VLAN will cause the switch to
        correctly send the packet out all ports configured for that VLAN.  If
        Open vSwitch is being used as an intermediate switch, learning can be
        disabled by adding the mirrored VLAN to <ref column="flood_vlans"/>
        in the appropriate <ref table="Bridge"/> table or tables.</p>
        <p>
          Mirroring to a GRE tunnel has fewer caveats than mirroring to a
          VLAN and should generally be preferred.
        </p>
      </column>
    </group>

    <group title="Statistics: Mirror counters">
      <p>
        Key-value pairs that report mirror statistics.
      </p>
      <column name="statistics" key="tx_packets">
        Number of packets transmitted through this mirror.
      </column>
      <column name="statistics" key="tx_bytes">
        Number of bytes transmitted through this mirror.
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="Controller" title="OpenFlow controller configuration.">
    <p>An OpenFlow controller.</p>

    <p>
      Open vSwitch supports two kinds of OpenFlow controllers:
    </p>

    <dl>
      <dt>Primary controllers</dt>
      <dd>
        <p>
          This is the kind of controller envisioned by the OpenFlow 1.0
          specification.  Usually, a primary controller implements a network
          policy by taking charge of the switch's flow table.
        </p>

        <p>
          Open vSwitch initiates and maintains persistent connections to
          primary controllers, retrying the connection each time it fails or
          drops.  The <ref table="Bridge" column="fail_mode"/> column in the
          <ref table="Bridge"/> table applies to primary controllers.
        </p>

        <p>
          Open vSwitch permits a bridge to have any number of primary
          controllers.  When multiple controllers are configured, Open
          vSwitch connects to all of them simultaneously.  Because
          OpenFlow 1.0 does not specify how multiple controllers
          coordinate in interacting with a single switch, more than
          one primary controller should be specified only if the
          controllers are themselves designed to coordinate with each
          other.  (The Nicira-defined <code>NXT_ROLE</code> OpenFlow
          vendor extension may be useful for this.)
        </p>
      </dd>
      <dt>Service controllers</dt>
      <dd>
        <p>
          These kinds of OpenFlow controller connections are intended for
          occasional support and maintenance use, e.g. with
          <code>ovs-ofctl</code>.  Usually a service controller connects only
          briefly to inspect or modify some of a switch's state.
        </p>

        <p>
          Open vSwitch listens for incoming connections from service
          controllers.  The service controllers initiate and, if necessary,
          maintain the connections from their end.  The <ref table="Bridge"
          column="fail_mode"/> column in the <ref table="Bridge"/> table does
          not apply to service controllers.
        </p>

        <p>
          Open vSwitch supports configuring any number of service controllers.
        </p>
      </dd>
    </dl>

    <p>
      The <ref column="target"/> determines the type of controller.
    </p>

    <group title="Core Features">
      <column name="target">
        <p>Connection method for controller.</p>
        <p>
          The following connection methods are currently supported for primary
          controllers:
        </p>
        <dl>
          <dt><code>ssl:<var>ip</var></code>[<code>:<var>port</var></code>]</dt>
          <dd>
            <p>The specified SSL <var>port</var> (default: 6633) on the host at
            the given <var>ip</var>, which must be expressed as an IP address
            (not a DNS name).  The <ref table="Open_vSwitch" column="ssl"/>
            column in the <ref table="Open_vSwitch"/> table must point to a
            valid SSL configuration when this form is used.</p>
            <p>SSL support is an optional feature that is not always built as
            part of Open vSwitch.</p>
          </dd>
          <dt><code>tcp:<var>ip</var></code>[<code>:<var>port</var></code>]</dt>
          <dd>The specified TCP <var>port</var> (default: 6633) on the host at
          the given <var>ip</var>, which must be expressed as an IP address
          (not a DNS name).</dd>
        </dl>
        <p>
          The following connection methods are currently supported for service
          controllers:
        </p>
        <dl>
          <dt><code>pssl:</code>[<var>port</var>][<code>:<var>ip</var></code>]</dt>
          <dd>
            <p>
              Listens for SSL connections on the specified TCP <var>port</var>
              (default: 6633).  If <var>ip</var>, which must be expressed as an
              IP address (not a DNS name), is specified, then connections are
              restricted to the specified local IP address.
            </p>
            <p>
              The <ref table="Open_vSwitch" column="ssl"/> column in the <ref
              table="Open_vSwitch"/> table must point to a valid SSL
              configuration when this form is used.
            </p>
            <p>SSL support is an optional feature that is not always built as
            part of Open vSwitch.</p>
          </dd>
          <dt><code>ptcp:</code>[<var>port</var>][<code>:<var>ip</var></code>]</dt>
          <dd>
            Listens for connections on the specified TCP <var>port</var>
            (default: 6633).  If <var>ip</var>, which must be expressed as an
            IP address (not a DNS name), is specified, then connections are
            restricted to the specified local IP address.
          </dd>
        </dl>
        <p>When multiple controllers are configured for a single bridge, the
        <ref column="target"/> values must be unique.  Duplicate
        <ref column="target"/> values yield unspecified results.</p>
      </column>

      <column name="connection_mode">
        <p>If it is specified, this setting must be one of the following
        strings that describes how Open vSwitch contacts this OpenFlow
        controller over the network:</p>

        <dl>
          <dt><code>in-band</code></dt>
          <dd>In this mode, this controller's OpenFlow traffic travels over the
          bridge associated with the controller.  With this setting, Open
          vSwitch allows traffic to and from the controller regardless of the
          contents of the OpenFlow flow table.  (Otherwise, Open vSwitch
          would never be able to connect to the controller, because it did
          not have a flow to enable it.)  This is the most common connection
          mode because it is not necessary to maintain two independent
          networks.</dd>
          <dt><code>out-of-band</code></dt>
          <dd>In this mode, OpenFlow traffic uses a control network separate
          from the bridge associated with this controller, that is, the
          bridge does not use any of its own network devices to communicate
          with the controller.  The control network must be configured
          separately, before or after <code>ovs-vswitchd</code> is started.
          </dd>
        </dl>

        <p>If not specified, the default is implementation-specific.</p>
      </column>
    </group>

    <group title="Controller Failure Detection and Handling">
      <column name="max_backoff">
        Maximum number of milliseconds to wait between connection attempts.
        Default is implementation-specific.
      </column>

      <column name="inactivity_probe">
        Maximum number of milliseconds of idle time on connection to
        controller before sending an inactivity probe message.  If Open
        vSwitch does not communicate with the controller for the specified
        number of seconds, it will send a probe.  If a response is not
        received for the same additional amount of time, Open vSwitch
        assumes the connection has been broken and attempts to reconnect.
        Default is implementation-specific.  A value of 0 disables
        inactivity probes.
      </column>
    </group>

    <group title="OpenFlow Rate Limiting">
      <column name="controller_rate_limit">
        <p>The maximum rate at which packets in unknown flows will be
        forwarded to the OpenFlow controller, in packets per second.  This
        feature prevents a single bridge from overwhelming the controller.
        If not specified, the default is implementation-specific.</p>
        <p>In addition, when a high rate triggers rate-limiting, Open
        vSwitch queues controller packets for each port and transmits
        them to the controller at the configured rate.  The number of
        queued packets is limited by
        the <ref column="controller_burst_limit"/> value.  The packet
        queue is shared fairly among the ports on a bridge.</p><p>Open
        vSwitch maintains two such packet rate-limiters per bridge.
        One of these applies to packets sent up to the controller
        because they do not correspond to any flow.  The other applies
        to packets sent up to the controller by request through flow
        actions. When both rate-limiters are filled with packets, the
        actual rate that packets are sent to the controller is up to
        twice the specified rate.</p>
      </column>

      <column name="controller_burst_limit">
        In conjunction with <ref column="controller_rate_limit"/>,
        the maximum number of unused packet credits that the bridge will
        allow to accumulate, in packets.  If not specified, the default
        is implementation-specific.
      </column>
    </group>

    <group title="Additional In-Band Configuration">
      <p>These values are considered only in in-band control mode (see
      <ref column="connection_mode"/>).</p>

      <p>When multiple controllers are configured on a single bridge, there
      should be only one set of unique values in these columns.  If different
      values are set for these columns in different controllers, the effect
      is unspecified.</p>

      <column name="local_ip">
        The IP address to configure on the local port,
        e.g. <code>192.168.0.123</code>.  If this value is unset, then
        <ref column="local_netmask"/> and <ref column="local_gateway"/> are
        ignored.
      </column>

      <column name="local_netmask">
        The IP netmask to configure on the local port,
        e.g. <code>255.255.255.0</code>.  If <ref column="local_ip"/> is set
        but this value is unset, then the default is chosen based on whether
        the IP address is class A, B, or C.
      </column>

      <column name="local_gateway">
        The IP address of the gateway to configure on the local port, as a
        string, e.g. <code>192.168.0.1</code>.  Leave this column unset if
        this network has no gateway.
      </column>
    </group>

    <group title="Controller Status">
      <column name="is_connected">
        <code>true</code> if currently connected to this controller,
        <code>false</code> otherwise.
      </column>

      <column name="role"
              type='{"type": "string", "enum": ["set", ["other", "master", "slave"]]}'>
        <p>The level of authority this controller has on the associated
        bridge. Possible values are:</p>
        <dl>
          <dt><code>other</code></dt>
          <dd>Allows the controller access to all OpenFlow features.</dd>
          <dt><code>master</code></dt>
          <dd>Equivalent to <code>other</code>, except that there may be at
          most one master controller at a time.  When a controller configures
          itself as <code>master</code>, any existing master is demoted to
          the <code>slave</code>role.</dd>
          <dt><code>slave</code></dt>
          <dd>Allows the controller read-only access to OpenFlow features.
          Attempts to modify the flow table will be rejected with an
          error.  Slave controllers do not receive OFPT_PACKET_IN or
          OFPT_FLOW_REMOVED messages, but they do receive OFPT_PORT_STATUS
          messages.</dd>
        </dl>
      </column>

      <column name="status" key="last_error">
        A human-readable description of the last error on the connection
        to the controller; i.e. <code>strerror(errno)</code>.  This key
        will exist only if an error has occurred.
      </column>

      <column name="status" key="state"
              type='{"type": "string", "enum": ["set", ["VOID", "BACKOFF", "CONNECTING", "ACTIVE", "IDLE"]]}'>
        <p>
          The state of the connection to the controller:
        </p>
        <dl>
          <dt><code>VOID</code></dt>
          <dd>Connection is disabled.</dd>

          <dt><code>BACKOFF</code></dt>
          <dd>Attempting to reconnect at an increasing period.</dd>

          <dt><code>CONNECTING</code></dt>
          <dd>Attempting to connect.</dd>

          <dt><code>ACTIVE</code></dt>
          <dd>Connected, remote host responsive.</dd>

          <dt><code>IDLE</code></dt>
          <dd>Connection is idle.  Waiting for response to keep-alive.</dd>
        </dl>
        <p>
          These values may change in the future.  They are provided only for
          human consumption.
        </p>
      </column>
      
      <column name="status" key="sec_since_connect"
              type='{"type": "integer", "minInteger": 0}'>
        The amount of time since this controller last successfully connected to
        the switch (in seconds).  Value is empty if controller has never
        successfully connected.
      </column>
      
      <column name="status" key="sec_since_disconnect"
              type='{"type": "integer", "minInteger": 1}'>
        The amount of time since this controller last disconnected from
        the switch (in seconds). Value is empty if controller has never
        disconnected.
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="Manager" title="OVSDB management connection.">
    <p>
      Configuration for a database connection to an Open vSwitch database
      (OVSDB) client.
    </p>

    <p>
      This table primarily configures the Open vSwitch database
      (<code>ovsdb-server</code>), not the Open vSwitch switch
      (<code>ovs-vswitchd</code>).  The switch does read the table to determine
      what connections should be treated as in-band.
    </p>

    <p>
      The Open vSwitch database server can initiate and maintain active
      connections to remote clients.  It can also listen for database
      connections.
    </p>

    <group title="Core Features">
      <column name="target">
        <p>Connection method for managers.</p>
        <p>
          The following connection methods are currently supported:
        </p>
        <dl>
          <dt><code>ssl:<var>ip</var></code>[<code>:<var>port</var></code>]</dt>
          <dd>
            <p>
              The specified SSL <var>port</var> (default: 6632) on the host at
              the given <var>ip</var>, which must be expressed as an IP address
              (not a DNS name).  The <ref table="Open_vSwitch" column="ssl"/>
              column in the <ref table="Open_vSwitch"/> table must point to a
              valid SSL configuration when this form is used.
            </p>
            <p>
              SSL support is an optional feature that is not always built as
              part of Open vSwitch.
            </p>
          </dd>

          <dt><code>tcp:<var>ip</var></code>[<code>:<var>port</var></code>]</dt>
          <dd>
            The specified TCP <var>port</var> (default: 6632) on the host at
            the given <var>ip</var>, which must be expressed as an IP address
            (not a DNS name).
          </dd>
          <dt><code>pssl:</code>[<var>port</var>][<code>:<var>ip</var></code>]</dt>
          <dd>
            <p>
              Listens for SSL connections on the specified TCP <var>port</var>
              (default: 6632).  If <var>ip</var>, which must be expressed as an
              IP address (not a DNS name), is specified, then connections are
              restricted to the specified local IP address.
            </p>
            <p>
              The <ref table="Open_vSwitch" column="ssl"/> column in the <ref
              table="Open_vSwitch"/> table must point to a valid SSL
              configuration when this form is used.
            </p>
            <p>
              SSL support is an optional feature that is not always built as
              part of Open vSwitch.
            </p>
          </dd>
          <dt><code>ptcp:</code>[<var>port</var>][<code>:<var>ip</var></code>]</dt>
          <dd>
            Listens for connections on the specified TCP <var>port</var>
            (default: 6632).  If <var>ip</var>, which must be expressed as an
            IP address (not a DNS name), is specified, then connections are
            restricted to the specified local IP address.
          </dd>
        </dl>
        <p>When multiple managers are configured, the <ref column="target"/>
        values must be unique.  Duplicate <ref column="target"/> values yield
        unspecified results.</p>
      </column>

      <column name="connection_mode">
        <p>
          If it is specified, this setting must be one of the following strings
          that describes how Open vSwitch contacts this OVSDB client over the
          network:
        </p>

        <dl>
          <dt><code>in-band</code></dt>
          <dd>
            In this mode, this connection's traffic travels over a bridge
            managed by Open vSwitch.  With this setting, Open vSwitch allows
            traffic to and from the client regardless of the contents of the
            OpenFlow flow table.  (Otherwise, Open vSwitch would never be able
            to connect to the client, because it did not have a flow to enable
            it.)  This is the most common connection mode because it is not
            necessary to maintain two independent networks.
          </dd>
          <dt><code>out-of-band</code></dt>
          <dd>
            In this mode, the client's traffic uses a control network separate
            from that managed by Open vSwitch, that is, Open vSwitch does not
            use any of its own network devices to communicate with the client.
            The control network must be configured separately, before or after
            <code>ovs-vswitchd</code> is started.
          </dd>
        </dl>

        <p>
          If not specified, the default is implementation-specific.
        </p>
      </column>
    </group>

    <group title="Client Failure Detection and Handling">
      <column name="max_backoff">
        Maximum number of milliseconds to wait between connection attempts.
        Default is implementation-specific.
      </column>

      <column name="inactivity_probe">
        Maximum number of milliseconds of idle time on connection to the client
        before sending an inactivity probe message.  If Open vSwitch does not
        communicate with the client for the specified number of seconds, it
        will send a probe.  If a response is not received for the same
        additional amount of time, Open vSwitch assumes the connection has been
        broken and attempts to reconnect.  Default is implementation-specific.
        A value of 0 disables inactivity probes.
      </column>
    </group>

    <group title="Status">
      <column name="is_connected">
        <code>true</code> if currently connected to this manager,
        <code>false</code> otherwise.
      </column>

      <column name="status" key="last_error">
        A human-readable description of the last error on the connection
        to the manager; i.e. <code>strerror(errno)</code>.  This key
        will exist only if an error has occurred.
      </column>

      <column name="status" key="state"
              type='{"type": "string", "enum": ["set", ["VOID", "BACKOFF", "CONNECTING", "ACTIVE", "IDLE"]]}'>
        <p>
          The state of the connection to the manager:
        </p>
        <dl>
          <dt><code>VOID</code></dt>
          <dd>Connection is disabled.</dd>

          <dt><code>BACKOFF</code></dt>
          <dd>Attempting to reconnect at an increasing period.</dd>

          <dt><code>CONNECTING</code></dt>
          <dd>Attempting to connect.</dd>

          <dt><code>ACTIVE</code></dt>
          <dd>Connected, remote host responsive.</dd>

          <dt><code>IDLE</code></dt>
          <dd>Connection is idle.  Waiting for response to keep-alive.</dd>
        </dl>
        <p>
          These values may change in the future.  They are provided only for
          human consumption.
        </p>
      </column>

      <column name="status" key="sec_since_connect"
              type='{"type": "integer", "minInteger": 0}'>
        The amount of time since this manager last successfully connected
        to the database (in seconds). Value is empty if manager has never
        successfully connected.
      </column>

      <column name="status" key="sec_since_disconnect"
              type='{"type": "integer", "minInteger": 0}'>
        The amount of time since this manager last disconnected from the
        database (in seconds). Value is empty if manager has never
        disconnected.
      </column>

      <column name="status" key="locks_held">
        Space-separated list of the names of OVSDB locks that the connection
        holds.  Omitted if the connection does not hold any locks.
      </column>

      <column name="status" key="locks_waiting">
        Space-separated list of the names of OVSDB locks that the connection is
        currently waiting to acquire.  Omitted if the connection is not waiting
        for any locks.
      </column>

      <column name="status" key="locks_lost">
        Space-separated list of the names of OVSDB locks that the connection
        has had stolen by another OVSDB client.  Omitted if no locks have been
        stolen from this connection.
      </column>

      <column name="status" key="n_connections"
              type='{"type": "integer", "minInteger": 2}'>
        <p>
          When <ref column="target"/> specifies a connection method that
          listens for inbound connections (e.g. <code>ptcp:</code> or
          <code>pssl:</code>) and more than one connection is actually active,
          the value is the number of active connections.  Otherwise, this
          key-value pair is omitted.
        </p>
        <p>
          When multiple connections are active, status columns and key-value
          pairs (other than this one) report the status of one arbitrarily
          chosen connection.
        </p>
      </column>
    </group>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="NetFlow">
    A NetFlow target.  NetFlow is a protocol that exports a number of
    details about terminating IP flows, such as the principals involved
    and duration.

    <column name="targets">
      NetFlow targets in the form
      <code><var>ip</var>:<var>port</var></code>.  The <var>ip</var>
      must be specified numerically, not as a DNS name.
    </column>

    <column name="engine_id">
      Engine ID to use in NetFlow messages.  Defaults to datapath index
      if not specified.
    </column>

    <column name="engine_type">
      Engine type to use in NetFlow messages.  Defaults to datapath
      index if not specified.
    </column>

    <column name="active_timeout">
      The interval at which NetFlow records are sent for flows that are
      still active, in seconds.  A value of <code>0</code> requests the
      default timeout (currently 600 seconds); a value of <code>-1</code>
      disables active timeouts.
    </column>

    <column name="add_id_to_interface">
      <p>If this column's value is <code>false</code>, the ingress and egress
      interface fields of NetFlow flow records are derived from OpenFlow port
      numbers.  When it is <code>true</code>, the 7 most significant bits of
      these fields will be replaced by the least significant 7 bits of the
      engine id.  This is useful because many NetFlow collectors do not
      expect multiple switches to be sending messages from the same host, so
      they do not store the engine information which could be used to
      disambiguate the traffic.</p>
      <p>When this option is enabled, a maximum of 508 ports are supported.</p>
    </column>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="SSL">
    SSL configuration for an Open_vSwitch.

    <column name="private_key">
      Name of a PEM file containing the private key used as the switch's
      identity for SSL connections to the controller.
    </column>

    <column name="certificate">
      Name of a PEM file containing a certificate, signed by the
      certificate authority (CA) used by the controller and manager,
      that certifies the switch's private key, identifying a trustworthy
      switch.
    </column>

    <column name="ca_cert">
      Name of a PEM file containing the CA certificate used to verify
      that the switch is connected to a trustworthy controller.
    </column>

    <column name="bootstrap_ca_cert">
      If set to <code>true</code>, then Open vSwitch will attempt to
      obtain the CA certificate from the controller on its first SSL
      connection and save it to the named PEM file. If it is successful,
      it will immediately drop the connection and reconnect, and from then
      on all SSL connections must be authenticated by a certificate signed
      by the CA certificate thus obtained.  <em>This option exposes the
      SSL connection to a man-in-the-middle attack obtaining the initial
      CA certificate.</em>  It may still be useful for bootstrapping.
    </column>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="sFlow">
    <p>An sFlow(R) target.  sFlow is a protocol for remote monitoring
    of switches.</p>

    <column name="agent">
      Name of the network device whose IP address should be reported as the
      ``agent address'' to collectors.  If not specified, the agent device is
      figured from the first target address and the routing table.  If the
      routing table does not contain a route to the target, the IP address
      defaults to the <ref table="Controller" column="local_ip"/> in the
      collector's <ref table="Controller"/>.  If an agent IP address cannot be
      determined any of these ways, sFlow is disabled.
    </column>

    <column name="header">
      Number of bytes of a sampled packet to send to the collector.
      If not specified, the default is 128 bytes.
    </column>

    <column name="polling">
      Polling rate in seconds to send port statistics to the collector.
      If not specified, defaults to 30 seconds.
    </column>

    <column name="sampling">
      Rate at which packets should be sampled and sent to the collector.
      If not specified, defaults to 400, which means one out of 400
      packets, on average, will be sent to the collector.
    </column>

    <column name="targets">
      sFlow targets in the form
      <code><var>ip</var>:<var>port</var></code>.
    </column>

    <group title="Common Columns">
      The overall purpose of these columns is described under <code>Common
      Columns</code> at the beginning of this document.

      <column name="external_ids"/>
    </group>
  </table>

  <table name="Capability">
    <p>Records in this table describe functionality supported by the hardware
    and software platform on which this Open vSwitch is based.  Clients
    should not modify this table.</p>

    <p>A record in this table is meaningful only if it is referenced by the
    <ref table="Open_vSwitch" column="capabilities"/> column in the
    <ref table="Open_vSwitch"/> table.  The key used to reference it, called
    the record's ``category,'' determines the meanings of the
    <ref column="details"/> column.  The following general forms of
    categories are currently defined:</p>

    <dl>
      <dt><code>qos-<var>type</var></code></dt>
      <dd><var>type</var> is supported as the value for
      <ref column="type" table="QoS"/> in the <ref table="QoS"/> table.
      </dd>
    </dl>

    <column name="details">
      <p>Key-value pairs that describe capabilities.  The meaning of the pairs
      depends on the category key that the <ref table="Open_vSwitch"
      column="capabilities"/> column in the <ref table="Open_vSwitch"/> table
      uses to reference this record, as described above.</p>

      <p>The presence of a record for category <code>qos-<var>type</var></code>
      indicates that the switch supports <var>type</var> as the value of
      the <ref table="QoS" column="type"/> column in the <ref table="QoS"/>
      table.  The following key-value pairs are defined to further describe
      QoS capabilities:</p>

      <dl>
        <dt><code>n-queues</code></dt>
        <dd>Number of supported queues, as a positive integer.  Keys in the
        <ref table="QoS" column="queues"/> column for <ref table="QoS"/>
        records whose <ref table="QoS" column="type"/> value
        equals <var>type</var> must range between 0 and this value minus one,
        inclusive.</dd>
      </dl>
    </column>
  </table>

</database>