Remove duplicate description about Experimenter classes

[ovs.git] / lib / meta-flow.xml

<?xml version="1.0" encoding="utf-8"?>
<fields>
  <h1>Introduction</h1>

  <p>
    This document aims to comprehensively document all of the fields,
    both standard and non-standard, supported by OpenFlow or Open
    vSwitch, regardless of origin.
  </p>

  <h2>Fields</h2>

  <p>
    A <dfn>field</dfn> is a property of a packet.  Most familiarly, <dfn>data
    fields</dfn> are fields that can be extracted from a packet.  Most data
    fields are copied directly from protocol headers, e.g. at layer 2, the
    Ethernet source and destination addresses, or the VLAN ID; at layer 3, the
    IPv4 or IPv6 source and destination; and at layer 4, the TCP or UDP ports.
    Other data fields are computed, e.g. <ref field="ip_frag"/> describes
    whether a packet is a fragment but it is not copied directly from the IP
    header.
  </p>

  <p>
    Data fields that are always present as a consequence of the basic
    networking technology in use are called called <dfn>root fields</dfn>.
    Open vSwitch 2.7 and earlier considered Ethernet fields to be root fields,
    and this remains the default mode of operation for Open vSwitch bridges.
    When a packet is received from a non-Ethernet interfaces, such as a layer-3
    LISP tunnel, Open vSwitch 2.7 and earlier force-fit the packet to this
    Ethernet-centric point of view by pretending that an Ethernet header is
    present whose Ethernet type that indicates the packet's actual type (and
    whose source and destination addresses are all-zero).
  </p>

  <p>
    Open vSwitch 2.8 and later implement the ``packet type-aware pipeline''
    concept introduced in OpenFlow 1.5.  Such a pipeline does not have any root
    fields.  Instead, a new metadata field, <ref field="packet_type"/>,
    indicates the basic type of the packet, which can be Ethernet, IPv4, IPv6,
    or another type.  For backward compatibility, by default Open vSwitch 2.8
    imitates the behavior of Open vSwitch 2.7 and earlier.  Later versions of
    Open vSwitch may change the default, and in the meantime controllers can
    turn off this legacy behavior, on a port-by-port basis, by setting
    <code>options:packet_type</code> to <code>ptap</code> in the
    <code>Interface</code> table.  This is significant only for ports that can
    handle non-Ethernet packets, which is currently just LISP, VXLAN-GPE, and
    GRE tunnel ports.  See <code>ovs-vwitchd.conf.db</code>(5) for more
    information.
  </p>

  <p>
    Non-root data fields are not always present.  A packet contains ARP
    fields, for example, only when its packet type is ARP or when it is an
    Ethernet packet whose Ethernet header indicates the Ethertype for ARP,
    0x0806.  In this documentation, we say that a field is
    <dfn>applicable</dfn> when it is present in a packet, and
    <dfn>inapplicable</dfn> when it is not.  (These are not standard terms.)
    We refer to the conditions that determine whether a field is applicable as
    <dfn>prerequisites</dfn>.  Some VLAN-related fields are a special case:
    these fields are always applicable for Ethernet packets, but have a
    designated value or bit that indicates whether a VLAN header is present,
    with the remaining values or bits indicating the VLAN header's content
    (if it is present).  <!-- XXX also ethertype -->
  </p>

  <p>
    An inapplicable field does not have a value, not even a nominal
    ``value'' such as all-zero-bits.  In many circumstances, OpenFlow
    and Open vSwitch allow references only to applicable fields.  For
    example, one may match (see <cite>Matching</cite>, below) a given
    field only if the match includes the field's prerequisite,
    e.g. matching an ARP field is only allowed if one also matches on
    Ethertype 0x0806 or the <ref field="packet_type"/> for ARP in a packet
    type-aware bridge.
  </p>

  <p>
    Sometimes a packet may contain multiple instances of a header.
    For example, a packet may contain multiple VLAN or MPLS headers,
    and tunnels can cause any data field to recur.  OpenFlow and Open
    vSwitch do not address these cases uniformly.  For VLAN and MPLS
    headers, only the outermost header is accessible, so that inner
    headers may be accessed only by ``popping'' (removing) the outer
    header.  (Open vSwitch supports only a single VLAN header in any
    case.)  For tunnels, e.g. GRE or VXLAN, the outer header and inner
    headers are treated as different data fields.
  </p>

  <p>
    Many network protocols are built in layers as a stack of concatenated
    headers.  Each header typically contains a ``next type'' field that
    indicates the type of the protocol header that follows, e.g. Ethernet
    contains an Ethertype and IPv4 contains a IP protocol type.  The
    exceptional cases, where protocols are layered but an outer layer does not
    indicate the protocol type for the inner layer, or gives only an ambiguous
    indication, are troublesome.  An MPLS header, for example, only indicates
    whether another MPLS header or some other protocol follows, and in the
    latter case the inner protocol must be known from the context.  In these
    exceptional cases, OpenFlow and Open vSwitch cannot provide insight into
    the inner protocol data fields without additional context, and thus they
    treat all later data fields as inapplicable until an OpenFlow action
    explicitly specifies what protocol follows.  In the case of MPLS, the
    OpenFlow ``pop MPLS'' action that removes the last MPLS header from a
    packet provides this context, as the Ethertype of the payload.  See
    <cite>Layer 2.5: MPLS</cite> for more information.
  </p>

  <p>
    OpenFlow and Open vSwitch support some fields other than data
    fields.  <dfn>Metadata fields</dfn> relate to the origin or
    treatment of a packet, but they are not extracted from the packet
    data itself.  One example is the physical port on which a packet
    arrived at the switch.  <dfn>Register fields</dfn> act like
    variables: they give an OpenFlow switch space for temporary
    storage while processing a packet.  Existing metadata and register
    fields have no prerequisites.
  </p>

  <p>
    A field's value consists of an integral number of bytes.  For data
    fields, sometimes those bytes are taken directly from the packet.
    Other data fields are copied from a packet with padding (usually
    with zeros and in the most significant positions).  The remaining
    data fields are transformed in other ways as they are copied from
    the packets, to make them more useful for matching.
  </p>

  <h2>Matching</h2>

  <p>
    The most important use of fields in OpenFlow is
    <dfn>matching</dfn>, to determine whether particular field values
    agree with a set of constraints called a <dfn>match</dfn>.  A
    match consists of zero or more constraints on individual fields,
    all of which must be met to satisfy the match.  (A match that
    contains no constraints is always satisfied.)  OpenFlow and Open
    vSwitch support a number of forms of matching on individual
    fields:
  </p>

  <dl>
    <dt><dfn>Exact match</dfn>, e.g. <code>nw_src=10.1.2.3</code></dt>
    <dd>
      <p>
        Only a particular value of the field is matched; for example, only one
        particular source IP address.  Exact matches are written as
        <code><var>field</var>=<var>value</var></code>.  The forms accepted for
        <var>value</var> depend on the field.
      </p>

      <p>
        All fields support exact matches.
      </p>
    </dd>

    <dt>
      <dfn>Bitwise match</dfn>, e.g. <code>nw_src=10.1.0.0/255.255.0.0</code>
    </dt>
    <dd>
      <p>
        Specific bits in the field must have specified values; for example,
        only source IP addresses in a particular subnet.  Bitwise matches are
        written as
        <code><var>field</var>=<var>value</var>/<var>mask</var></code>, where
        <var>value</var> and <var>mask</var> take one of the forms accepted for
        an exact match on <var>field</var>.  Some fields accept other forms for
        bitwise matches; for example, <code>nw_src=10.1.0.0/255.255.0.0</code>
        may also be written <code>nw_src=10.1.0.0/16</code>.
      </p>

      <p>
        Most OpenFlow switches do not allow every bitwise matching on every
        field (and before OpenFlow 1.2, the protocol did not even provide for
        the possibility for most fields).  Even switches that do allow bitwise
        matching on a given field may restrict the masks that are allowed, e.g.
        by allowing matches only on contiguous sets of bits starting from the
        most significant bit, that is, ``CIDR'' masks [RFC 4632].  Open vSwitch
        does not allows bitwise matching on every field, but it allows
        arbitrary bitwise masks on any field that does support bitwise
        matching.  (Older versions had some restrictions, as documented in the
        descriptions of individual fields.)
      </p>
    </dd>

    <dt><dfn>Wildcard</dfn>, e.g. ``any <code>nw_src</code>''</dt>
    <dd>
      <p>
        The value of the field is not constrained.  Wildcarded fields may be
        written as <code><var>field</var>=*</code>, although it is unusual to
        mention them at all.  (When specifying a wildcard explicitly in a
        command invocation, be sure to using quoting to protect against shell
        expansion.)
      </p>

      <p>
        There is a tiny difference between wildcarding a field and not
        specifying any match on a field: wildcarding a field requires
        satisfying the field's prerequisites.
      </p>
    </dd>
  </dl>

  <p>
    Some types of matches on individual fields cannot be expressed directly
    with OpenFlow and Open vSwitch.  These can be expressed indirectly:
  </p>

  <dl>
    <dt><dfn>Set match</dfn>, e.g. ``<code>tcp_dst</code> ∈ {80, 443,
    8080}''</dt>
    <dd>
      <p>
        The value of a field is one of a specified set of values; for
        example, the TCP destination port is 80, 443, or 8080.
      </p>

      <p>
        For matches used in flows (see <cite>Flows</cite>, below), multiple
        flows can simulate set matches.
      </p>
    </dd>

    <dt><dfn>Range match</dfn>, e.g. ``1000 ≤ <code>tcp_dst</code> ≤
    1999''</dt>
    <dd>
      <p>
        The value of the field must lie within a numerical range, for
        example, TCP destination ports between 1000 and 1999.
      </p>

      <p>
        Range matches can be expressed as a collection of bitwise matches.  For
        example, suppose that the goal is to match TCP source ports 1000 to
        1999, inclusive.  The binary representations of 1000 and 1999 are:
      </p>

      <pre fixed="yes">
01111101000
11111001111
      </pre>

      <p>
        The following series of bitwise matches will match 1000 and
        1999 and all the values in between:
      </p>

      <pre fixed="yes">
01111101xxx
0111111xxxx
10xxxxxxxxx
110xxxxxxxx
1110xxxxxxx
11110xxxxxx
1111100xxxx
      </pre>

      <p>
        which can be written as the following matches:
      </p>

      <pre>
tcp,tp_src=0x03e8/0xfff8
tcp,tp_src=0x03f0/0xfff0
tcp,tp_src=0x0400/0xfe00
tcp,tp_src=0x0600/0xff00
tcp,tp_src=0x0700/0xff80
tcp,tp_src=0x0780/0xffc0
tcp,tp_src=0x07c0/0xfff0
      </pre>
    </dd>

    <dt><dfn>Inequality match</dfn>, e.g. ``<code>tcp_dst</code> ≠ 80''</dt>
    <dd>
      <p>
        The value of the field differs from a specified value, for
        example, all TCP destination ports except 80.
      </p>

      <p>
        An inequality match on an <var>n</var>-bit field can be expressed as a
        disjunction of <var>n</var> 1-bit matches.  For example, the inequality
        match ``<code>vlan_pcp</code> ≠ 5'' can be expressed as
        ``<code>vlan_pcp</code> = 0/4 or <code>vlan_pcp</code> = 2/2 or
        <code>vlan_pcp</code> = 0/1.''  For matches used in flows (see
        <cite>Flows</cite>, below), sometimes one can more compactly express
        inequality as a higher-priority flow that matches the exceptional case
        paired with a lower-priority flow that matches the general case.
      </p>

      <p>
        Alternatively, an inequality match may be converted to a pair of range
        matches, e.g. <code>tcp_src ≠ 80</code> may be expressed as ``0 ≤
        <code>tcp_src</code> &lt; 80 or 80 &lt; <code>tcp_src</code> ≤ 65535'',
        and then each range match may in turn be converted to a bitwise match.
      </p>
    </dd>

    <dt><dfn>Conjunctive match</dfn>, e.g. ``<code>tcp_src</code> ∈ {80, 443, 8080} and <code>tcp_dst</code> ∈ {80, 443, 8080}''</dt>
    <dd>
      As an OpenFlow extension, Open vSwitch supports matching on conditions on
      conjunctions of the previously mentioned forms of matching.  See the
      documentation for <ref field="conj_id"/> for more information.
    </dd>
  </dl>

  <p>
    All of these supported forms of matching are special cases of bitwise
    matching.  In some cases this influences the design of field values.  <ref
    field="ip_frag"/> is the most prominent example: it is designed to make all
    of the practically useful checks for IP fragmentation possible as a single
    bitwise match.
  </p>

  <h3>Shorthands</h3>

  <p>
    Some matches are very commonly used, so Open vSwitch accepts shorthand
    notations.  In some cases, Open vSwitch also uses shorthand notations when
    it displays matches.  The following shorthands are defined, with their long
    forms shown on the right side:
  </p>

  <dl>
    <dt><code>eth</code></dt>
    <dd><code>packet_type=(0,0)</code> (Open vSwitch 2.8 and later)</dd>
    <dt><code>ip</code></dt>     <dd><code>eth_type=0x0800</code></dd>
    <dt><code>ipv6</code></dt>   <dd><code>eth_type=0x86dd</code></dd>
    <dt><code>icmp</code></dt>   <dd><code>eth_type=0x0800,ip_proto=1</code></dd>
    <dt><code>icmp6</code></dt>  <dd><code>eth_type=0x86dd,ip_proto=58</code></dd>
    <dt><code>tcp</code></dt>    <dd><code>eth_type=0x0800,ip_proto=6</code></dd>
    <dt><code>tcp6</code></dt>   <dd><code>eth_type=0x86dd,ip_proto=6</code></dd>
    <dt><code>udp</code></dt>    <dd><code>eth_type=0x0800,ip_proto=17</code></dd>
    <dt><code>udp6</code></dt>   <dd><code>eth_type=0x86dd,ip_proto=17</code></dd>
    <dt><code>sctp</code></dt>   <dd><code>eth_type=0x0800,ip_proto=132</code></dd>
    <dt><code>sctp6</code></dt>  <dd><code>eth_type=0x86dd,ip_proto=132</code></dd>
    <dt><code>arp</code></dt>    <dd><code>eth_type=0x0806</code></dd>
    <dt><code>rarp</code></dt>   <dd><code>eth_type=0x8035</code></dd>
    <dt><code>mpls</code></dt>   <dd><code>eth_type=0x8847</code></dd>
    <dt><code>mplsm</code></dt>  <dd><code>eth_type=0x8848</code></dd>
  </dl>


  <h2>Evolution of OpenFlow Fields</h2>

  <p>
    The discussion so far applies to all OpenFlow and Open vSwitch
    versions.  This section starts to draw in specific information by
    explaining, in broad terms, the treatment of fields and matches in
    each OpenFlow version.
  </p>

  <h3>OpenFlow 1.0</h3>

  <p>
    OpenFlow 1.0 defined the OpenFlow protocol format of a match as a
    fixed-length data structure that could match on the following
    fields:
  </p>

  <ul>
    <li>Ingress port.</li>
    <li>Ethernet source and destination MAC.</li>
    <li>Ethertype (with a special value to match frames that lack an
    Ethertype).</li>
    <li>VLAN ID and priority.</li>
    <li>IPv4 source, destination, protocol, and DSCP.</li>
    <li>TCP source and destination port.</li>
    <li>UDP source and destination port.</li>
    <li>ICMPv4 type and code.</li>
    <li>ARP IPv4 addresses (SPA and TPA) and opcode.</li>
  </ul>

  <p>
    Each supported field corresponded to some member of the data
    structure.  Some members represented multiple fields, in the case
    of the TCP, UDP, ICMPv4, and ARP fields whose presence is mutually
    exclusive.  This also meant that some members were poor fits for
    their fields: only the low 8 bits of the 16-bit ARP opcode could
    be represented, and the ICMPv4 type and code were padded with 8 bits
    of zeros to fit in the 16-bit members primarily meant for TCP and
    UDP ports.  An additional bitmap member indicated, for each
    member, whether its field should be an ``exact'' or ``wildcarded''
    match (see <cite>Matching</cite>), with additional support for
    CIDR prefix matching on the IPv4 source and destination fields.
  </p>

  <p>
    Simplicity was recognized early on as the main virtue of this
    approach.  Obviously, any fixed-length data structure cannot
    support matching new protocols that do not fit.  There was no
    room, for example, for matching IPv6 fields, which was not a
    priority at the time.  Lack of room to support matching the
    Ethernet addresses inside ARP packets actually caused more of a
    design problem later, leading to an Open vSwitch extension action
    specialized for dropping ``spoofed'' ARP packets in which the
    frame and ARP Ethernet source addressed differed.  (This extension
    was never standardized.  Open vSwitch dropped support for it a few
    releases after it added support for full ARP matching.)
  </p>

  <p>
    The design of the OpenFlow fixed-length matches also illustrates
    compromises, in both directions, between the strengths and
    weaknesses of software and hardware that have always influenced
    the design of OpenFlow.  Support for matching ARP fields that do
    fit in the data structure was only added late in the design
    process (and remained optional in OpenFlow 1.0), for example,
    because common switch ASICs did not support matching these fields.
  </p>

  <p>
    The compromises in favor of software occurred for more complicated
    reasons.  The OpenFlow designers did not know how to implement
    matching in software that was fast, dynamic, and general.  (A way
    was later found [Srinivasan].)  Thus, the designers sought to
    support dynamic, general matching that would be fast in realistic
    special cases, in particular when all of the matches were
    <dfn>microflows</dfn>, that is, matches that specify every field
    present in a packet, because such matches can be implemented as a
    single hash table lookup.  Contemporary research supported the
    feasibility of this approach: the number of microflows in a campus
    network had been measured to peak at about 10,000 [Casado, section
    3.2].  (Calculations show that this can only be true in a lightly
    loaded network [Pepelnjak].)
  </p>

  <p>
    As a result, OpenFlow 1.0 required switches to treat microflow
    matches as the highest possible priority.  This let software
    switches perform the microflow hash table lookup first. Only on
    failure to match a microflow did the switch need to fall back to
    checking the more general and presumed slower matches.  Also, the
    OpenFlow 1.0 flow match was minimally flexible, with no support
    for general bitwise matching, partly on the basis that this seemed
    more likely amenable to relatively efficient software
    implementation. (CIDR masking for IPv4 addresses was added
    relatively late in the OpenFlow 1.0 design process.)
  </p>

  <p>
    Microflow matching was later discovered to aid some hardware
    implementations.  The TCAM chips used for matching in hardware do
    not support priority in the same way as OpenFlow but instead tie
    priority to ordering [Pagiamtzis].  Thus, adding a new match with
    a priority between the priorities of existing matches can require
    reordering an arbitrary number of TCAM entries.  On the other
    hand, when microflows are highest priority, they can be managed as
    a set-aside portion of the TCAM entries.
  </p>

  <p>
    The emphasis on matching microflows also led designers to
    carefully consider the bandwidth requirements between switch and
    controller: to maximize the number of microflow setups per second,
    one must minimize the size of each flow's description.  This
    favored the fixed-length format in use, because it expressed
    common TCP and UDP microflows in fewer bytes than more flexible
    ``type-length-value'' (TLV) formats.  (Early versions of OpenFlow
    also avoided TLVs in general to head off protocol fragmentation.)
  </p>

  <h4>Inapplicable Fields</h4>

  <p>
    OpenFlow 1.0 does not clearly specify how to treat inapplicable
    fields.  The members for inapplicable fields are always present in
    the match data structure, as are the bits that indicate whether
    the fields are matched, and the ``correct'' member and bit values
    for inapplicable fields is unclear.  OpenFlow 1.0 implementations
    changed their behavior over time as priorities shifted.  The early
    OpenFlow reference implementation, motivated to make every flow a
    microflow to enable hashing, treated inapplicable fields as exact
    matches on a value of 0.  Initially, this behavior was implemented
    in the reference controller only.
  </p>

  <p>
    Later, the reference switch was also changed to actually force any
    wildcarded inapplicable fields into exact matches on 0.  The
    latter behavior sometimes caused problems, because the modified
    flow was the one reported back to the controller later when it
    queried the flow table, and the modifications sometimes meant that
    the controller could not properly recognize the flow that it had
    added.  In retrospect, perhaps this problem should have alerted
    the designers to a design error, but the ability to use a single
    hash table was held to be more important than almost every other
    consideration at the time.
  </p>

  <p>
    When more flexible match formats were introduced much later, they
    disallowed any mention of inapplicable fields as part of a match.
    This raised the question of how to translate between this new
    format and the OpenFlow 1.0 fixed format.  It seemed somewhat
    inconsistent and backward to treat fields as exact-match in one
    format and forbid matching them in the other, so instead the
    treatment of inapplicable fields in the fixed-length format was
    changed from exact match on 0 to wildcarding.  (A better
    classifier had by now eliminated software performance problems
    with wildcards.)
  </p>

  <p>
    The OpenFlow 1.0.1 errata (released only in 2012) added some
    additional explanation [OpenFlow 1.0.1, section 3.4], but it did
    not mandate specific behavior because of variation among
    implementations.
  </p>

  <h3>OpenFlow 1.1</h3>

  <p>
    The OpenFlow 1.1 protocol match format was designed as a type/length/value
    (TLV) format to allow for future flexibility.  The specification
    standardized only a single type <code>OFPMT_STANDARD</code> (0) with a
    fixed-size payload, described here.  The additional fields and bitwise
    masks in OpenFlow 1.1 cause this match structure to be over twice as large
    as in OpenFlow 1.0, 88 bytes versus 40.
  </p>

  <p>
    OpenFlow 1.1 added support for the following fields:
  </p>

  <ul>
    <li>SCTP source and destination port.</li>
    <li>MPLS label and traffic control (TC) fields.</li>
    <li>One 64-bit register (named ``metadata'').</li>
  </ul>

  <p>
    OpenFlow 1.1 increased the width of the ingress port number field (and all
    other port numbers in the protocol) from 16 bits to 32 bits.
  </p>

  <p>
    OpenFlow 1.1 increased matching flexibility by introducing
    arbitrary bitwise matching on Ethernet and IPv4 address fields and
    on the new ``metadata'' register field.  Switches were not
    required to support all possible masks [OpenFlow 1.1, section
    4.3].
  </p>

  <p>
    By a strict reading of the specification, OpenFlow 1.1 removed
    support for matching ICMPv4 type and code [OpenFlow 1.1, section
    A.2.3], but this is likely an editing error because ICMP
    matching is described elsewhere [OpenFlow 1.1, Table 3, Table 4,
    Figure 4].  Open vSwitch does support ICMPv4 type and code
    matching with OpenFlow 1.1.
  </p>

  <p>
    OpenFlow 1.1 avoided the pitfalls of inapplicable fields that
    OpenFlow 1.0 encountered, by requiring the switch to ignore the
    specified field values [OpenFlow 1.1, section A.2.3].  It also
    implied that the switch should ignore the bits that indicate
    whether to match inapplicable fields.
  </p>

  <h4>Physical Ingress Port</h4>

  <p>
    OpenFlow 1.1 introduced a new pseudo-field, the physical ingress port.  The
    physical ingress port is only a pseudo-field because it cannot be used for
    matching.  It appears only one place in the protocol, in the ``packet-in''
    message that passes a packet received at the switch to an OpenFlow
    controller.
  </p>

  <p>
    A packet's ingress port and physical ingress port are identical except for
    packets processed by a switch feature such as bonding or tunneling that
    makes a packet appear to arrive on a ``virtual'' port associated with the
    bond or the tunnel.  For such packets, the ingress port is the virtual port
    and the physical ingress port is, naturally, the physical port.  Open
    vSwitch implements both bonding and tunneling, but its bonding
    implementation does not use virtual ports and its tunnels are typically not
    on the same OpenFlow switch as their physical ingress ports (which need not
    be part of any switch), so the ingress port and physical ingress port are
    always the same in Open vSwitch.
  </p>

  <h3>OpenFlow 1.2</h3>

  <p>
    OpenFlow 1.2 abandoned the fixed-length approach to matching.  One reason
    was size, since adding support for IPv6 address matching (now seen as
    important), with bitwise masks, would have added 64 bytes to the match
    length, increasing it from 88 bytes in OpenFlow 1.1 to over 150 bytes.
    Extensibility had also become important as controller writers increasingly
    wanted support for new fields without having to change messages throughout
    the OpenFlow protocol.  The challenges of carefully defining fixed-length
    matches to avoid problems with inapplicable fields had also become clear
    over time.
  </p>

  <p>
    Therefore, OpenFlow 1.2 adopted a flow format using a flexible
    type-length-value (TLV) representation, in which each TLV expresses a match
    on one field.  These TLVs were in turn encapsulated inside the outer TLV
    wrapper introduced in OpenFlow 1.1 with the new identifier
    <code>OFPMT_OXM</code> (1).  (This wrapper fulfilled its intended purpose
    of reducing the amount of churn in the protocol when changing match
    formats; some messages that included matches remained unchanged from
    OpenFlow 1.1 to 1.2 and later versions.)
  </p>

  <p>
    OpenFlow 1.2 added support for the following fields:
  </p>

  <ul>
    <li>ARP hardware addresses (SHA and THA).</li>
    <li>IPv4 ECN.</li>
    <li>IPv6 source and destination addresses, flow label, DSCP, ECN,
    and protocol.</li>
    <li>TCP, UDP, and SCTP port numbers when encapsulated inside IPv6.</li>
    <li>ICMPv6 type and code.</li>
    <li>ICMPv6 Neighbor Discovery target address and source and target
    Ethernet addresses.</li>
  </ul>

  <!-- mention tun_id_from_cookie extension? -->

  <p>
    The OpenFlow 1.2 format, called <dfn>OXM</dfn> (<dfn>OpenFlow Extensible
    Match</dfn>), was modeled closely on an extension to OpenFlow 1.0
    introduced in Open vSwitch 1.1 called <dfn>NXM</dfn> (<dfn>Nicira Extended
    Match</dfn>).  Each OXM or NXM TLV has the following format:
  </p>

  <diagram>
    <header name="type">
      <bits name="vendor/class" above="16" width=".75"/>
      <bits name="field" above="7" width=".4"/>
    </header>
    <nospace/>
    <header name="">
      <bits name="HM" above="1" width=".25"/>
      <bits name="length" above="8" width=".4"/>
    </header>
    <header name="">
      <bits name="body" above="length bytes" width="1.7"/>
    </header>
  </diagram>

  <p>
    The most significant 16 bits of the NXM or OXM header, called
    <code>vendor</code> by NXM and <code>class</code> by OXM, identify
    an organization permitted to allocate identifiers for fields.  NXM
    allocates only two vendors, 0x0000 for fields supported by
    OpenFlow 1.0 and 0x0001 for fields implemented as an Open vSwitch
    extension.  OXM assigns classes as follows:
  </p>

  <dl>
    <dt>0x0000 (<code>OFPXMC_NXM_0</code>).</dt>
    <dt>0x0001 (<code>OFPXMC_NXM_1</code>).</dt>
    <dd>Reserved for NXM compatibility.</dd>

    <dt>0x0002 to 0x7fff</dt>
    <dd>
      Reserved for allocation to ONF members, but none yet assigned.
    </dd>

    <dt>0x8000 (<code>OFPXMC_OPENFLOW_BASIC</code>)</dt>
    <dd>
      Used for most standard OpenFlow fields.
    </dd>

    <dt>0x8001 (<code>OFPXMC_PACKET_REGS</code>)</dt>
    <dd>
      Used for packet register fields in OpenFlow 1.5 and later.
    </dd>

    <dt>0x8002 to 0xfffe</dt>
    <dd>
      Reserved for the OpenFlow specification.
    </dd>

    <dt>0xffff (<code>OFPXMC_EXPERIMENTER</code>)</dt>
    <dd>Experimental use.</dd>
  </dl>

  <p>
    When <code>class</code> is 0xffff, the OXM header is extended to 64 bits by
    using the first 32 bits of the body as an <code>experimenter</code> field
    whose most significant byte is zero and whose remaining bytes are an
    Organizationally Unique Identifier (OUI) assigned by the IEEE [IEEE OUI],
    as shown below.
  </p>

  <diagram>
    <header name="type">
      <bits name="class" above="16" below="0xffff" width=".75"/>
      <bits name="field" above="7" width=".4"/>
    </header>
    <nospace/>
    <header name="">
      <bits name="HM" above="1" width=".25"/>
      <bits name="length" above="8" width=".4"/>
    </header>

    <header name="experimenter">
      <bits name="zero" above="8" below="0x00" width=".4"/>
      <bits name="OUI" above="24" width="1"/>
    </header>
    <header name="">
      <bits name="body" above="(length - 4) bytes" width="1.7"/>
    </header>
  </diagram>

  <p>
    OpenFlow says that support for experimenter fields is optional.  Open
    vSwitch 2.4 and later does support them, so that it can support the
    following experimenter classes:
  </p>

  <dl>
    <dt>0x4f4e4600 (<code>ONFOXM_ET</code>)</dt>
    <dd>
      Used by official Open Networking Foundation extensions in OpenFlow 1.3
      and later.
      e.g. [TCP Flags Match Field Extension].
    </dd>

    <dt>0x005ad650 (<code>NXOXM_NSH</code>)</dt>
    <dd>
      Used by Open vSwitch for NSH extensions, in the absence of an official
      ONF-assigned class.  (This OUI is randomly generated.)
    </dd>
  </dl>

  <p>
    Taken as a unit, <code>class</code> (or <code>vendor</code>),
    <code>field</code>, and <code>experimenter</code> (when present) uniquely
    identify a particular field.
  </p>

  <p>
    When <code>hasmask</code> (abbreviated <code>HM</code> above) is 0, the OXM
    is an exact match on an entire field.  In this case, the body (excluding
    the experimenter field, if present) is a single value to be matched.
  </p>

  <p>
    When <code>hasmask</code> is 1, the OXM is a bitwise match.  The body
    (excluding the experimenter field) consists of a value to match, followed
    by the bitwise mask to apply.  A 1-bit in the mask indicates that the
    corresponding bit in the value should be matched and a 0-bit that it should
    be ignored.  For example, for an IP address field, a value of 192.168.0.0
    followed by a mask of 255.255.0.0 would match addresses in the
    196.168.0.0/16 subnet.
  </p>

  <ul>
    <li>
      Some fields might not support masking at all, and some fields that do
      support masking might restrict it to certain patterns.  For example,
      fields that have IP address values might be restricted to CIDR masks.
      The descriptions of individual fields note these restrictions.
    </li>

    <li>
      An OXM TLV with a mask that is all zeros is not useful (although it is
      not forbidden), because it is has the same effect as omitting the TLV
      entirely.
    </li>

    <li>
      It is not meaningful to pair a 0-bit in an OXM mask with a 1-bit in its
      value, and Open vSwitch rejects such an OXM with the error
      <code>OFPBMC_BAD_WILDCARDS</code>, as required by OpenFlow 1.3 and later.
    </li>
  </ul>

  <p>
    The <code>length</code> identifies the number of bytes in the body,
    including the 4-byte <code>experimenter</code> header, if it is present.
    Each OXM TLV has a fixed length; that is, given <code>class</code>,
    <code>field</code>, <code>experimenter</code> (if present), and
    <code>hasmask</code>, <code>length</code> is a constant.  The
    <code>length</code> is included explicitly to allow software to minimally
    parse OXM TLVs of unknown types.
  </p>

  <p>
    OXM TLVs must be ordered so that a field's prerequisites are satisfied
    before it is parsed.  For example, an OXM TLV that matches on the IPv4
    source address field is only allowed following an OXM TLV that matches on
    the Ethertype for IPv4.  Similarly, an OXM TLV that matches on the TCP
    source port must follow a TLV that matches an Ethertype of IPv4 or IPv6 and
    one that matches an IP protocol of TCP (in that order).  The order of OXM
    TLVs is not otherwise restricted; no canonical ordering is defined.
  </p>

  <p>
    A given field may be matched only once in a series of OXM TLVs.
  </p>

  <!-- EXT-482? -->

  <h3>OpenFlow 1.3</h3>

  <p>
    OpenFlow 1.3 showed OXM to be largely successful, by adding new fields
    without making any changes to how flow matches otherwise worked.  It added
    OXMs for the following fields supported by Open vSwitch:
  </p>

  <ul>
    <li>Tunnel ID for ports associated with e.g. VXLAN or keyed GRE.</li>
    <li>MPLS ``bottom of stack'' (BOS) bit.</li>
  </ul>

  <p>
    OpenFlow 1.3 also added OXMs for the following fields not documented here
    and not yet implemented by Open vSwitch:
  </p>

  <ul>
    <li>IPv6 extension header handling.</li>
    <li>PBB I-SID.</li>
  </ul>

  <h3>OpenFlow 1.4</h3>

  <p>
    OpenFlow 1.4 added OXMs for the following fields not documented here and
    not yet implemented by Open vSwitch:
  </p>

  <ul>
    <li>PBB UCA.</li>
  </ul>

  <h3>OpenFlow 1.5</h3>

  <p>
    OpenFlow 1.5 added OXMs for the following fields supported by Open vSwitch:
  </p>

  <ul>
    <li>Packet type.</li>
    <li>TCP flags.</li>
    <li>Packet registers.</li>
    <li>The output port in the OpenFlow action set.</li>
  </ul>

  <h1>Fields Reference</h1>

  <p>
    The following sections document the fields that Open vSwitch supports.
    Each section provides introductory material on a group of related fields,
    followed by information on each individual field.  In addition to
    field-specific information, each field begins with a table with entries for
    the following important properties:
  </p>

  <dl>
    <dt>Name</dt>
    <dd>
      The field's name, used for parsing and formatting the field, e.g. in
      <code>ovs-ofctl</code> commands.  For historical reasons, some fields
      have an additional name that is accepted as an alternative in parsing.
      This name, when there is one, is listed as well, e.g. ``<code>tun</code>
      (aka <code>tunnel_id</code>).''
    </dd>

    <dt>Width</dt>
    <dd>
      The field's width, always a multiple of 8 bits.  Some fields don't use
      all of the bits, so this may be accompanied by an explanation.  For
      example, OpenFlow embeds the 2-bit IP ECN field as as the low bits in an
      8-bit byte, and so its width is expressed as ``8 bits (only the
      least-significant 2 bits may be nonzero).''
    </dd>

    <dt>Format</dt>
    <dd>
      <p>
        How a value for the field is formatted or parsed by, e.g.,
        <code>ovs-ofctl</code>.  Some possibilities are generic:
      </p>

      <dl>
        <dt>decimal</dt>
        <dd>
          Formats as a decimal number.  On input, accepts decimal numbers or
          hexadecimal numbers prefixed by <code>0x</code>.
        </dd>

        <dt>hexadecimal</dt>
        <dd>
          Formats as a hexadecimal number prefixed by <code>0x</code>.  On
          input, accepts decimal numbers or hexadecimal numbers prefixed by
          <code>0x</code>.  (The default for parsing is <em>not</em>
          hexadecimal: only a <code>0x</code> prefix causes input to be treated
          as hexadecimal.)
        </dd>

        <dt>Ethernet</dt>
        <dd>
          Formats and accepts the common Ethernet address format
          <code><var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var>:<var>xx</var></code>.
        </dd>

        <dt>IPv4</dt>
        <dd>
          Formats and accepts the dotted-quad format
          <code><var>a</var>.<var>b</var>.<var>c</var>.<var>d</var></code>.
          For bitwise matches, formats and accepts
          <code><var>address</var>/<var>length</var></code> CIDR notation in
          addition to <code><var>address</var>/<var>mask</var></code>.
        </dd>

        <dt>IPv6</dt>
        <dd>
          Formats and accepts the common IPv6 address formats, plus CIDR
          notation for bitwise matches.
        </dd>

        <dt>OpenFlow 1.0 port</dt>
        <dd>
          Accepts 16-bit port numbers in decimal, plus OpenFlow well-known port
          names (e.g. <code>IN_PORT</code>) in uppercase or lowercase.
        </dd>

        <dt>OpenFlow 1.1+ port</dt>
        <dd>
          Same syntax as OpenFlow 1.0 ports but for 32-bit OpenFlow 1.1+ port
          number fields.
        </dd>
      </dl>

      <p>
        Other, field-specific formats are explained along with their fields.
      </p>
    </dd>

    <dt>Masking</dt>
    <dd>
      For most fields, this says ``arbitrary bitwise masks,'' meaning that a
      flow may match any combination of bits in the field.  Some fields
      instead say ``exact match only,'' which means that a flow that matches
      on this field must match on the whole field instead of just certain
      bits.  Either way, this reports masking support for the latest version
      of Open vSwitch using OXM or NXM (that is, either OpenFlow 1.2+ or
      OpenFlow 1.0 plus Open vSwitch NXM extensions).  In particular,
      OpenFlow 1.0 (without NXM) and 1.1 don't always support masking even if
      Open vSwitch itself does; refer to the <em>OpenFlow 1.0</em> and
      <em>OpenFlow 1.1</em> rows to learn about masking with these protocol
      versions.
    </dd>

    <dt>Prerequisites</dt>
    <dd>
      <p>
        Requirements that must be met to match on this field.  For example,
        <ref field="ip_src"/> has IPv4 as a prerequisite, meaning that a match
        must include <code>eth_type=0x0800</code> to match on the IPv4 source
        address.  The following prerequisites, with their requirements, are
        currently in use:
      </p>

      <dl>
        <dt>none</dt>
        <dd>(no requirements)</dd>

        <dt>VLAN VID</dt>
        <dd><code>vlan_tci=0x1000/0x1000</code> (i.e. a VLAN header is
        present)</dd>

        <dt>ARP</dt>
        <dd><code>eth_type=0x0806</code> (ARP) or <code>eth_type=0x8035</code> (RARP)</dd>

        <dt>IPv4</dt>
        <dd><code>eth_type=0x0800</code></dd>

        <dt>IPv6</dt>
        <dd><code>eth_type=0x86dd</code></dd>

        <dt>IPv4/IPv6</dt>
        <dd>IPv4 or IPv6</dd>

        <dt>MPLS</dt>
        <dd><code>eth_type=0x8847</code> or <code>eth_type=0x8848</code></dd>

        <dt>TCP</dt>
        <dd>IPv4/IPv6 and <code>ip_proto=6</code></dd>

        <dt>UDP</dt>
        <dd>IPv4/IPv6 and <code>ip_proto=17</code></dd>

        <dt>SCTP</dt>
        <dd>IPv4/IPv6 and <code>ip_proto=132</code></dd>

        <dt>ICMPv4</dt>
        <dd>IPv4 and <code>ip_proto=1</code></dd>

        <dt>ICMPv6</dt>
        <dd>IPv6 and <code>ip_proto=58</code></dd>

        <dt>ND solicit</dt>
        <dd>ICMPv6 and <code>icmp_type=135</code> and <code>icmp_code=0</code></dd>

        <dt>ND advert</dt>
        <dd>ICMPv6 and <code>icmp_type=136</code> and <code>icmp_code=0</code></dd>

        <dt>ND</dt>
        <dd>ND solicit or ND advert</dd>
      </dl>

      <p>
        The TCP, UDP, and SCTP prerequisites also have the special requirement
        that <code>nw_frag</code> is not being used to select ``later
        fragments.''  This is because only the first fragment of a fragmented
        IPv4 or IPv6 datagram contains the TCP or UDP header.
      </p>
    </dd>

    <dt>Access</dt>
    <dd>
      Most fields are ``read/write,'' which means that common OpenFlow actions
      like <code>set_field</code> can modify them.  Fields that are
      ``read-only'' cannot be modified in these general-purpose ways, although
      there may be other ways that actions can modify them.
    </dd>

    <dt>OpenFlow 1.0</dt>
    <dt>OpenFlow 1.1</dt>
    <dd>
      These rows report the level of support that OpenFlow 1.0 or OpenFlow 1.1,
      respectively, has for a field.  For OpenFlow 1.0, supported fields are
      reported as either ``yes (exact match only)'' for fields that do not
      support any bitwise masking or ``yes (CIDR match only)'' for fields that
      support CIDR masking.  OpenFlow 1.1 supported fields report either ``yes
      (exact match only)'' or simply ``yes'' for fields that do support
      arbitrary masks.  These OpenFlow versions supported a fixed collection of
      fields that cannot be extended, so many more fields are reported as ``not
      supported.''
    </dd>

    <dt>OXM</dt>
    <dt>NXM</dt>
    <dd>
      <p>
        These rows report the OXM and NXM code points that correspond to a
        given field.  Either or both may be ``none.''
      </p>

      <p>
        A field that has only an OXM code point is usually one that was
        standardized before it was added to Open vSwitch.  A field that has
        only an NXM code point is usually one that is not yet standardized.
        When a field has both OXM and NXM code points, it usually indicates
        that it was introduced as an Open vSwitch extension under the NXM code
        point, then later standardized under the OXM code point.  A field can
        have more than one OXM code point if it was standardized in OpenFlow
        1.4 or later and additionally introduced as an official ONF extension
        for OpenFlow 1.3.  (A field that has neither OXM nor NXM code point is
        typically an obsolete field that is supported in some other form using
        OXM or NXM.)
      </p>

      <p>
        Each code point in these rows is described in the form
        ``<code>NAME</code> (<var>number</var>) since OpenFlow <var>spec</var>
        and Open vSwitch <var>version</var>,''
        e.g. ``<code>OXM_OF_ETH_TYPE</code> (5) since OpenFlow 1.2 and Open
        vSwitch 1.7.''  First, <code>NAME</code>, which specifies a name for
        the code point, starts with a prefix that designates a class and, in
        some cases, a vendor, as listed in the following table:
      </p>

      <oxm_classes/>

      <p>
        For more information on OXM/NXM classes and vendors, refer back to
        <em>OpenFlow 1.2</em> under <em>Evolution of OpenFlow Fields</em>.  The
        <var>number</var> is the field number within the class and vendor.  The
        OpenFlow <var>spec</var> is the version of OpenFlow that standardized
        the code point.  It is omitted for NXM code points because they are
        nonstandard.  The <var>version</var> is the version of Open vSwitch
        that first supported the code point.
      </p>
    </dd>
  </dl>

  <group title="Conjunctive Match">
    <p>
      An individual OpenFlow flow can match only a single value for each field.
      However, situations often arise where one wants to match one of a set of
      values within a field or fields.  For matching a single field against a
      set, it is straightforward and efficient to add multiple flows to the
      flow table, one for each value in the set.  For example, one might use
      the following flows to send packets with IP source address <var>a</var>,
      <var>b</var>, <var>c</var>, or <var>d</var> to the OpenFlow controller:
    </p>

    <pre>
      ip,ip_src=<var>a</var> actions=controller
      ip,ip_src=<var>b</var> actions=controller
      ip,ip_src=<var>c</var> actions=controller
      ip,ip_src=<var>d</var> actions=controller
    </pre>

    <p>
      Similarly, these flows send packets with IP destination address
      <var>e</var>, <var>f</var>, <var>g</var>, or <var>h</var> to the OpenFlow
      controller:
    </p>

    <pre>
      ip,ip_dst=<var>e</var> actions=controller
      ip,ip_dst=<var>f</var> actions=controller
      ip,ip_dst=<var>g</var> actions=controller
      ip,ip_dst=<var>h</var> actions=controller
    </pre>

    <p>
      Installing all of the above flows in a single flow table yields a
      disjunctive effect: a packet is sent to the controller if
      <code>ip_src</code> ∈ {<var>a</var>,<var>b</var>,<var>c</var>,<var>d</var>}
      or <code>ip_dst</code> ∈
      {<var>e</var>,<var>f</var>,<var>g</var>,<var>h</var>} (or both).
      (Pedantically, if both of the above sets of flows are present in the flow
      table, they should have different priorities, because OpenFlow says that
      the results are undefined when two flows with same priority can both match
      a single packet.)
    </p>

    <p>
      Suppose, on the other hand, one wishes to match conjunctively, that is, to
      send a packet to the controller only if both <code>ip_src</code> ∈
      {<var>a</var>,<var>b</var>,<var>c</var>,<var>d</var>} and
      <code>ip_dst</code> ∈
      {<var>e</var>,<var>f</var>,<var>g</var>,<var>h</var>}.  This requires 4 × 4
      = 16 flows, one for each possible pairing of <code>ip_src</code> and
      <code>ip_dst</code>.  That is acceptable for our small example, but it does
      not gracefully extend to larger sets or greater numbers of dimensions.
    </p>

    <p>
      The <code>conjunction</code> action is a solution for conjunctive matches
      that is built into Open vSwitch.  A <code>conjunction</code> action ties groups of
      individual OpenFlow flows into higher-level ``conjunctive flows''.  Each
      group corresponds to one dimension, and each flow within the group matches
      one possible value for the dimension.  A packet that matches one flow from
      each group matches the conjunctive flow.
    </p>

    <p>
      To implement a conjunctive flow with <code>conjunction</code>, assign the
      conjunctive flow a 32-bit <var>id</var>, which must be unique within an
      OpenFlow table.  Assign each of the <var>n</var> ≥ 2 dimensions a unique
      number from 1 to <var>n</var>; the ordering is unimportant.  Add one flow
      to the OpenFlow flow table for each possible value of each dimension with
      <code>conjunction(<var>id</var>, <var>k</var>/<var>n</var>)</code> as the
      flow's actions, where <var>k</var> is the number assigned to the flow's
      dimension.  Together, these flows specify the conjunctive flow's match
      condition.  When the conjunctive match condition is met, Open vSwitch looks
      up one more flow that specifies the conjunctive flow's actions and receives
      its statistics.  This flow is found by setting <code>conj_id</code> to the
      specified <var>id</var> and then again searching the flow table.
    </p>

    <p>
      The following flows provide an example.  Whenever the IP source is one of
      the values in the flows that match on the IP source (dimension 1 of 2),
      <em>and</em> the IP destination is one of the values in the flows that
      match on IP destination (dimension 2 of 2), Open vSwitch searches for a
      flow that matches <code>conj_id</code> against the conjunction ID (1234),
      finding the first flow listed below.
    </p>

    <pre>
      conj_id=1234 actions=controller
      ip,ip_src=10.0.0.1 actions=conjunction(1234, 1/2)
      ip,ip_src=10.0.0.4 actions=conjunction(1234, 1/2)
      ip,ip_src=10.0.0.6 actions=conjunction(1234, 1/2)
      ip,ip_src=10.0.0.7 actions=conjunction(1234, 1/2)
      ip,ip_dst=10.0.0.2 actions=conjunction(1234, 2/2)
      ip,ip_dst=10.0.0.5 actions=conjunction(1234, 2/2)
      ip,ip_dst=10.0.0.7 actions=conjunction(1234, 2/2)
      ip,ip_dst=10.0.0.8 actions=conjunction(1234, 2/2)
    </pre>

    <p>
      Many subtleties exist:
    </p>

    <ul>
      <li>
        In the example above, every flow in a single dimension has the same form,
        that is, dimension 1 matches on <code>ip_src</code> and dimension 2 on
        <code>ip_dst</code>, but this is not a requirement.  Different flows
        within a dimension may match on different bits within a field (e.g. IP
        network prefixes of different lengths, or TCP/UDP port ranges as bitwise
        matches), or even on entirely different fields (e.g. to match packets for
        TCP source port 80 or TCP destination port 80).
      </li>

      <li>
        The flows within a dimension can vary their matches across more than
        one field, e.g. to match only specific pairs of IP source and
        destination addresses or L4 port numbers.
      </li>

      <li>
        A flow may have multiple <code>conjunction</code> actions, with different
        <code>id</code> values.  This is useful for multiple conjunctive flows with
        overlapping sets.  If one conjunctive flow matches packets with both
        <code>ip_src</code> ∈ {<var>a</var>,<var>b</var>} and <code>ip_dst</code> ∈
        {<var>d</var>,<var>e</var>} and a second conjunctive flow matches <code>ip_src</code>
        ∈ {<var>b</var>,<var>c</var>} and <code>ip_dst</code> ∈ {<var>f</var>,<var>g</var>}, for
        example, then the flow that matches <code>ip_src=</code><var>b</var> would have two
        <code>conjunction</code> actions, one for each conjunctive flow.  The order
        of <code>conjunction</code> actions within a list of actions is not
        significant.
      </li>
      <li>
        A flow with <code>conjunction</code> actions may also include <code>note</code>
        actions for annotations, but not any other kind of actions.  (They
        would not be useful because they would never be executed.)
      </li>
      <li>
        All of the flows that constitute a conjunctive flow with a given
        <var>id</var> must have the same priority.  (Flows with the same <var>id</var>
        but different priorities are currently treated as different
        conjunctive flows, that is, currently <var>id</var> values need only be
        unique within an OpenFlow table at a given priority.  This behavior
        isn't guaranteed to stay the same in later releases, so please use
        <var>id</var> values unique within an OpenFlow table.)
      </li>
      <li>
        Conjunctive flows must not overlap with each other, at a given
        priority, that is, any given packet must be able to match at most one
        conjunctive flow at a given priority.  Overlapping conjunctive flows
        yield unpredictable results.
      </li>
      <li>
        Following a conjunctive flow match, the search for the flow with
        <code>conj_id=</code><var>id</var> is done in the same general-purpose way as
        other flow table searches, so one can use flows with
        <code>conj_id=</code><var>id</var> to act differently depending on
        circumstances.  (One exception is that the search for the
        <code>conj_id=</code><var>id</var> flow itself ignores conjunctive flows, to
        avoid recursion.) If the search with <code>conj_id=</code><var>id</var> fails,
        Open vSwitch acts as if the conjunctive flow had not matched at all, and
        continues searching the flow table for other matching flows.
      </li>
      <li>
        <p>
          OpenFlow prerequisite checking occurs for the flow with
          <code>conj_id=</code><var>id</var> in the same way as any other flow, e.g. in
          an OpenFlow 1.1+ context, putting a <code>mod_nw_src</code> action into the example
          above would require adding an <code>ip</code> match, like this:
        </p>
        <pre>
          conj_id=1234,ip actions=mod_nw_src:1.2.3.4,controller
        </pre>
      </li>
      <li>
        OpenFlow prerequisite checking also occurs for the individual flows
        that comprise a conjunctive match in the same way as any other flow.
      </li>
      <li>
        The flows that constitute a conjunctive flow do not have useful
        statistics.  They are never updated with byte or packet counts, and so
        on.  (For such a flow, therefore, the idle and hard timeouts work much
        the same way.)
      </li>
      <li>
        <p>
          Sometimes there is a choice of which flows include a particular match.
          For example, suppose that we added an extra constraint to our example,
          to match on <code>ip_src</code> ∈
          {<var>a</var>,<var>b</var>,<var>c</var>,<var>d</var>} and
          <code>ip_dst</code> ∈
          {<var>e</var>,<var>f</var>,<var>g</var>,<var>h</var>} and
          <code>tcp_dst</code> = <var>i</var>.  One way to implement this is to
          add the new constraint to the <code>conj_id</code> flow, like this:
        </p>
        <pre>
          conj_id=1234,tcp,tcp_dst=<var>i</var> actions=mod_nw_src:1.2.3.4,controller
        </pre>
        <p>
          but <em>this is not recommended</em> because of the cost of the extra
          flow table lookup.  Instead, add the constraint to the individual
          flows, either in one of the dimensions or (slightly better) all of
          them.
        </p>
      </li>
      <li>
        A conjunctive match must have <var>n</var> ≥ 2 dimensions (otherwise a
        conjunctive match is not necessary).  Open vSwitch enforces this.
      </li>
      <li>
        Each dimension within a conjunctive match should ordinarily have more
        than one flow.  Open vSwitch does not enforce this.
      </li>
    </ul>

    <field id="MFF_CONJ_ID" title="Conjunction ID">
      Used for conjunctive matching.  See above for more information.
    </field>
  </group>

  <group title="Network Service Header">
    <field id="MFF_NSH_FLAGS"
        title="flags field (8 bits)"/>
    <field id="MFF_NSH_MDTYPE"
        title="mdtype field (8 bits)"/>
    <field id="MFF_NSH_NP"
        title="np (next protocol) field (8 bits)"/>
    <field id="MFF_NSH_SPI"
        title="spi (service path identifier) field (24 bits)"/>
    <field id="MFF_NSH_SI"
        title="si (service index) field (8 bits)"/>
    <field id="MFF_NSH_C1"
        title="c1 (Network Platform Context) field (32 bits)"/>
    <field id="MFF_NSH_C2"
        title="c2 (Network Shared Context) field (32 bits)"/>
    <field id="MFF_NSH_C3"
        title="c3 (Service Platform Context) field (32 bits)"/>
    <field id="MFF_NSH_C4"
        title="c4 (Service Shared Context) field (32 bits)"/>
  </group>

  <group title="Tunnel">
    <p>
      The fields in this group relate to tunnels, which Open vSwitch
      supports in several forms (GRE, VXLAN, and so on).  Most of
      these fields do appear in the wire format of a packet, so they
      are data fields from that point of view, but they are metadata
      from an OpenFlow flow table point of view because they do not
      appear in packets that are forwarded to the controller or to
      ordinary (non-tunnel) output ports.
    </p>

    <p>
      Open vSwitch supports a spectrum of usage models for mapping
      tunnels to OpenFlow ports:
    </p>

    <dl>
      <dt>``Port-based'' tunnels</dt>
      <dd>
        <p>
          In this model, an OpenFlow port represents one tunnel: it matches a
          particular type of tunnel traffic between two IP endpoints, with a
          particular tunnel key (if keys are in use).  In this situation, <ref
          field="in_port"/> suffices to distinguish one tunnel from another, so
          the tunnel header fields have little importance for OpenFlow
          processing.  (They are still populated and may be used if it is
          convenient.)  The tunnel header fields play no role in sending
          packets out such an OpenFlow port, either, because the OpenFlow port
          itself fully specifies the tunnel headers.
        </p>

        <p>
          The following Open vSwitch commands create a bridge
          <code>br-int</code>, add port <code>tap0</code> to the bridge as
          OpenFlow port 1, establish a port-based GRE tunnel between the local
          host and remote IP 192.168.1.1 using GRE key 5001 as OpenFlow port 2,
          and arranges to forward all traffic from <code>tap0</code> to the
          tunnel and vice versa:
        </p>

        <pre>
ovs-vsctl add-br br-int
ovs-vsctl add-port br-int tap0 -- set interface tap0 ofport_request=1
ovs-vsctl add-port br-int gre0 --
    set interface gre0 ofport_request=2 type=gre \
                       options:remote_ip=192.168.1.1 options:key=5001
ovs-ofctl add-flow br-int in_port=1,actions=2
ovs-ofctl add-flow br-int in_port=2,actions=1
        </pre>
      </dd>

      <dt>``Flow-based'' tunnels</dt>
      <dd>
        <p>
          In this model, one OpenFlow port represents all possible tunnels of a
          given type with an endpoint on the current host, for example, all GRE
          tunnels.  In this situation, <ref field="in_port"/> only indicates
          that traffic was received on the particular kind of tunnel.  This is
          where the tunnel header fields are most important: they allow the
          OpenFlow tables to discriminate among tunnels based on their IP
          endpoints or keys.  Tunnel header fields also determine the IP
          endpoints and keys of packets sent out such a tunnel port.
        </p>

        <p>
          The following Open vSwitch commands create a bridge
          <code>br-int</code>, add port <code>tap0</code> to the
          bridge as OpenFlow port 1, establish a flow-based GRE tunnel
          port 3, and arranges to forward all traffic from
          <code>tap0</code> to remote IP 192.168.1.1 over a GRE tunnel
          with key 5001 and vice versa:
        </p>

        <pre>
ovs-vsctl add-br br-int
ovs-vsctl add-port br-int tap0 -- set interface tap0 ofport_request=1
ovs-vsctl add-port br-int allgre --
    set interface gre0 ofport_request=3 type=gre \
                       options:remote_ip=flow options:key=flow
ovs-ofctl add-flow br-int \
    'in_port=1 actions=set_tunnel:5001,set_field:192.168.1.1->tun_dst,3'
ovs-ofctl add-flow br-int 'in_port=3,tun_src=192.168.1.1,tun_id=5001 actions=1'
        </pre>
      </dd>

      <dt>Mixed models.</dt>
      <dd>
        <p>
          One may define both flow-based and port-based tunnels at the
          same time.  For example, it is valid and possibly useful to
          create and configure both <code>gre0</code> and
          <code>allgre</code> tunnel ports described above.
        </p>

        <p>
          Traffic is attributed on ingress to the most specific
          matching tunnel.  For example, <code>gre0</code> is more
          specific than <code>allgre</code>.  Therefore, if both
          exist, then <code>gre0</code> will be the ingress port for any
          GRE traffic received from 192.168.1.1 with key 5001.
        </p>

        <p>
          On egress, traffic may be directed to any appropriate tunnel
          port.  If both <code>gre0</code> and <code>allgre</code> are
          configured as already described, then the actions
          <code>2</code> and
          <code>set_tunnel:5001,set_field:192.168.1.1->tun_dst,3</code>
          send the same tunnel traffic.
        </p>
      </dd>

      <dt>Intermediate models.</dt>
      <dd>
        Ports may be configured as partially flow-based.  For example,
        one may define an OpenFlow port that represents tunnels
        between a pair of endpoints but leaves the flow table to
        discriminate on the flow key.
      </dd>
    </dl>

    <p>
      <code>ovs-vswitchd.conf.db</code>(5) describes all the details of tunnel
      configuration.
    </p>

    <p>
      These fields do not have any prerequisites, which means that a
      flow may match on any or all of them, in any combination.
    </p>

    <p>
      These fields are zeros for packets that did not arrive on a tunnel.
    </p>

    <field id="MFF_TUN_ID" title="Tunnel ID">
      <p>
        Many kinds of tunnels support a tunnel ID:
      </p>

      <ul>
        <li>
          VXLAN and Geneve have a 24-bit virtual network identifier (VNI).
        </li>
        <li>LISP has a 24-bit instance ID.</li>
        <li>GRE has an optional 32-bit key.</li>
        <li>STT has a 64-bit key.</li>
      </ul>

      <p>
        When a packet is received from a tunnel, this field holds the
        tunnel ID in its least significant bits, zero-extended to fit.
        This field is zero if the tunnel does not support an ID, or if
        no ID is in use for a tunnel type that has an optional ID, or
        if an ID of zero received, or if the packet was not received
        over a tunnel.
      </p>

      <p>
        When a packet is output to a tunnel port, the tunnel
        configuration determines whether the tunnel ID is taken from
        this field or bound to a fixed value.  See the earlier
        description of ``port-based'' and ``flow-based'' tunnels for
        more information.
      </p>

      <p>
        The following diagram shows the origin of this field in a
        typical keyed GRE tunnel:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" below="47" width="0.4"/>
          <bits name="src" above="32" width="0.4"/>
          <bits name="dst" above="32" width="0.4"/>
        </header>
        <header name="GRE">
          <bits name="..." above="16" width="0.4"/>
          <bits name="type" above="16" below="0x6558" width="0.4"/>
          <bits name="key" above="32" width=".4" fill="yes"/>
        </header>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" width="0.4"/>
        </header>
        <dots/>
      </diagram>
    </field>

    <field id="MFF_TUN_SRC" title="Tunnel IPv4 Source">
      <p>
        When a packet is received from a tunnel, this field is the
        source address in the outer IP header of the tunneled packet.
        This field is zero if the packet was not received over a
        tunnel.
      </p>

      <p>
        When a packet is output to a flow-based tunnel port, this
        field influences the IPv4 source address used to send the
        packet.  If it is zero, then the kernel chooses an appropriate
        IP address based using the routing table.
      </p>

      <p>
        The following diagram shows the origin of this field in a
        typical keyed GRE tunnel:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" below="47" width="0.4"/>
          <bits name="src" above="32" width="0.4" fill="yes"/>
          <bits name="dst" above="32" width="0.4"/>
        </header>
        <header name="GRE">
          <bits name="..." above="16" width="0.4"/>
          <bits name="type" above="16" below="0x6558" width="0.4"/>
          <bits name="key" above="32" width=".4"/>
        </header>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" width="0.4"/>
        </header>
        <dots/>
      </diagram>
    </field>

    <field id="MFF_TUN_DST" title="Tunnel IPv4 Destination">
      <p>
        When a packet is received from a tunnel, this field is the
        destination address in the outer IP header of the tunneled
        packet.  This field is zero if the packet was not received
        over a tunnel.
      </p>

      <p>
        When a packet is output to a flow-based tunnel port, this
        field specifies the destination to which the tunnel packet is
        sent.
      </p>

      <p>
        The following diagram shows the origin of this field in a
        typical keyed GRE tunnel:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" below="47" width="0.4"/>
          <bits name="src" above="32" width="0.4"/>
          <bits name="dst" above="32" width="0.4" fill="yes"/>
        </header>
        <header name="GRE">
          <bits name="..." above="16" width="0.4"/>
          <bits name="type" above="16" below="0x6558" width="0.4"/>
          <bits name="key" above="32" width=".4"/>
        </header>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" width="0.4"/>
        </header>
        <dots/>
      </diagram>
    </field>

    <field id="MFF_TUN_IPV6_SRC" title="Tunnel IPv6 Source">
      Similar to <ref field="tun_src"/>, but for tunnels over IPv6.
    </field>

    <field id="MFF_TUN_IPV6_DST" title="Tunnel IPv6 Destination">
      Similar to <ref field="tun_dst"/>, but for tunnels over IPv6.
    </field>

    <h2>VXLAN Group-Based Policy Fields</h2>

    <p>
      The VXLAN header is defined as follows [RFC 7348], where the
      <code>I</code> bit must be set to 1, unlabeled bits or those labeled
      <code>reserved</code> must be set to 0, and Open vSwitch makes the VNI
      available via <ref field="tun_id"/>:
    </p>

    <diagram>
      <header name="VXLAN flags">
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="I" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
      </header>
      <nospace/>
      <header>
        <bits name="reserved" above="24" width="1.2"/>
        <bits name="VNI" above="24" width="1.2"/>
        <bits name="reserved" above="8" width=".5"/>
      </header>
    </diagram>

    <p>
      VXLAN Group-Based Policy [VXLAN Group Policy Option] adds new
      interpretations to existing bits in the VXLAN header, reinterpreting it
      as follows, with changes highlighted:
    </p>

    <diagram>
      <header name="GBP flags">
        <bits name="" above="1" width="0.15"/>
        <bits name="D" above="1" width="0.15" fill="yes"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="A" above="1" width="0.15" fill="yes"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
        <bits name="" above="1" width="0.15"/>
      </header>
      <nospace/>
      <header>
        <bits name="group policy ID" above="24" width="1.2" fill="yes"/>
        <bits name="VNI" above="24" width="1.2"/>
        <bits name="reserved" above="8" width=".5"/>
      </header>
    </diagram>

    <p>
      Open vSwitch makes GBP fields and flags available through the following
      fields.  Only packets that arrive over a VXLAN tunnel with the GBP
      extension enabled have these fields set.  In other packets they are zero
      on receive and ignored on transmit.
    </p>

    <field id="MFF_TUN_GBP_ID" title="VXLAN Group-Based Policy ID">
      <p>
        For a packet tunneled over VXLAN with the Group-Based Policy (GBP)
        extension, this field represents the GBP policy ID, as shown above.
      </p>
    </field>

    <field id="MFF_TUN_GBP_FLAGS" title="VXLAN Group-Based Policy Flags">
      <p>
        For a packet tunneled over VXLAN with the Group-Based Policy (GBP)
        extension, this field represents the GBP policy flags, as shown above.
      </p>

      <p>
        The field has the format shown below:
      </p>

      <diagram>
        <header name="GBP Flags">
          <bits name="" above="1" width="0.15"/>
          <bits name="D" above="1" width="0.15"/>
          <bits name="" above="1" width="0.15"/>
          <bits name="" above="1" width="0.15"/>
          <bits name="A" above="1" width="0.15"/>
          <bits name="" above="1" width="0.15"/>
          <bits name="" above="1" width="0.15"/>
          <bits name="" above="1" width="0.15"/>
        </header>
      </diagram>

      <p>
        Unlabeled bits are reserved and must be transmitted as 0.  The VXLAN
        GBP draft defines the other bits' meanings as:
      </p>

      <dl>
        <dt><code>D</code> (Don't Learn)</dt>
        <dd>
          When set, this bit indicates that the egress tunnel endpoint must not
          learn the source address of the encapsulated frame.
        </dd>

        <dt><code>A</code> (Applied)</dt>
        <dd>
          When set, indicates that the group policy has already been applied to
          this packet.  Devices must not apply policies when the A bit is set.
        </dd>
      </dl>
    </field>

    <h2>Geneve Fields</h2>

    <p>
      These fields provide access to additional features in the Geneve
      tunneling protocol [Geneve].  Their names are somewhat generic in the
      hope that the same fields could be reused for other protocols in the
      future; for example, the NSH protocol [NSH] supports TLV options whose
      form is identical to that for Geneve options.
    </p>

    <field id="MFF_TUN_METADATA0" title="Generic Tunnel Option 0">
      <p>
        The above information specifically covers generic tunnel option 0, but
        Open vSwitch supports 64 options, numbered 0 through 63, whose
        NXM field numbers are 40 through 103.
      </p>

      <p>
        These fields provide OpenFlow access to the generic type-length-value
        options defined by the Geneve tunneling protocol or other protocols
        with options in the same TLV format as Geneve options.  Each of these
        options has the following wire format:
      </p>

      <diagram>
        <header name="header">
          <bits name="class" above="16" width="0.6"/>
          <bits name="type" above="8" width="0.5"/>
          <bits name="res" above="3" below="0" width="0.25"/>
          <bits name="length" above="5" width="0.4"/>
        </header>
        <nospace/>
        <header name="body">
          <bits name="value" above="4×(length - 1) bytes" width="1.7"/>
        </header>
      </diagram>

      <p>
        Taken together, the <code>class</code> and <code>type</code> in the
        option format mean that there are about 16 million distinct kinds of
        TLV options, too many to give individual OXM code points.  Thus, Open
        vSwitch requires the user to define the TLV options of interest, by
        binding up to 64 TLV options to generic tunnel option NXM code points.
        Each option may have up to 124 bytes in its body, the maximum allowed
        by the TLV format, but bound options may total at most 252 bytes of
        body.
      </p>

      <p>
        Open vSwitch extensions to the OpenFlow protocol bind TLV options to
        NXM code points.  The <code>ovs-ofctl</code>(8) program offers one way
        to use these extensions, e.g. to configure a mapping from a TLV option
        with <code>class</code> <code>0xffff</code>, <code>type</code>
        <code>0</code>, and a body length of 4 bytes:
      </p>

      <pre>
ovs-ofctl add-tlv-map br0 "{class=0xffff,type=0,len=4}->tun_metadata0"
      </pre>

      <p>
        Once a TLV option is properly bound, it can be accessed and modified
        like any other field, e.g. to send packets that have value 1234 for the
        option described above to the controller:
      </p>

      <pre>
ovs-ofctl add-flow br0 tun_metadata0=1234,actions=controller
      </pre>

      <p>
        An option not received or not bound is matched as all zeros.
      </p>
    </field>
    <!--- XXX need a way to define a range of OXMs -->
    <field id="MFF_TUN_METADATA1" title="Generic Tunnel Option 1" hidden="yes"/>
    <field id="MFF_TUN_METADATA2" title="Generic Tunnel Option 2" hidden="yes"/>
    <field id="MFF_TUN_METADATA3" title="Generic Tunnel Option 3" hidden="yes"/>
    <field id="MFF_TUN_METADATA4" title="Generic Tunnel Option 4" hidden="yes"/>
    <field id="MFF_TUN_METADATA5" title="Generic Tunnel Option 5" hidden="yes"/>
    <field id="MFF_TUN_METADATA6" title="Generic Tunnel Option 6" hidden="yes"/>
    <field id="MFF_TUN_METADATA7" title="Generic Tunnel Option 7" hidden="yes"/>
    <field id="MFF_TUN_METADATA8" title="Generic Tunnel Option 8" hidden="yes"/>
    <field id="MFF_TUN_METADATA9" title="Generic Tunnel Option 9" hidden="yes"/>
    <field id="MFF_TUN_METADATA10" title="Generic Tunnel Option 10" hidden="yes"/>
    <field id="MFF_TUN_METADATA11" title="Generic Tunnel Option 11" hidden="yes"/>
    <field id="MFF_TUN_METADATA12" title="Generic Tunnel Option 12" hidden="yes"/>
    <field id="MFF_TUN_METADATA13" title="Generic Tunnel Option 13" hidden="yes"/>
    <field id="MFF_TUN_METADATA14" title="Generic Tunnel Option 14" hidden="yes"/>
    <field id="MFF_TUN_METADATA15" title="Generic Tunnel Option 15" hidden="yes"/>
    <field id="MFF_TUN_METADATA16" title="Generic Tunnel Option 16" hidden="yes"/>
    <field id="MFF_TUN_METADATA17" title="Generic Tunnel Option 17" hidden="yes"/>
    <field id="MFF_TUN_METADATA18" title="Generic Tunnel Option 18" hidden="yes"/>
    <field id="MFF_TUN_METADATA19" title="Generic Tunnel Option 19" hidden="yes"/>
    <field id="MFF_TUN_METADATA20" title="Generic Tunnel Option 20" hidden="yes"/>
    <field id="MFF_TUN_METADATA21" title="Generic Tunnel Option 21" hidden="yes"/>
    <field id="MFF_TUN_METADATA22" title="Generic Tunnel Option 22" hidden="yes"/>
    <field id="MFF_TUN_METADATA23" title="Generic Tunnel Option 23" hidden="yes"/>
    <field id="MFF_TUN_METADATA24" title="Generic Tunnel Option 24" hidden="yes"/>
    <field id="MFF_TUN_METADATA25" title="Generic Tunnel Option 25" hidden="yes"/>
    <field id="MFF_TUN_METADATA26" title="Generic Tunnel Option 26" hidden="yes"/>
    <field id="MFF_TUN_METADATA27" title="Generic Tunnel Option 27" hidden="yes"/>
    <field id="MFF_TUN_METADATA28" title="Generic Tunnel Option 28" hidden="yes"/>
    <field id="MFF_TUN_METADATA29" title="Generic Tunnel Option 29" hidden="yes"/>
    <field id="MFF_TUN_METADATA30" title="Generic Tunnel Option 30" hidden="yes"/>
    <field id="MFF_TUN_METADATA31" title="Generic Tunnel Option 31" hidden="yes"/>
    <field id="MFF_TUN_METADATA32" title="Generic Tunnel Option 32" hidden="yes"/>
    <field id="MFF_TUN_METADATA33" title="Generic Tunnel Option 33" hidden="yes"/>
    <field id="MFF_TUN_METADATA34" title="Generic Tunnel Option 34" hidden="yes"/>
    <field id="MFF_TUN_METADATA35" title="Generic Tunnel Option 35" hidden="yes"/>
    <field id="MFF_TUN_METADATA36" title="Generic Tunnel Option 36" hidden="yes"/>
    <field id="MFF_TUN_METADATA37" title="Generic Tunnel Option 37" hidden="yes"/>
    <field id="MFF_TUN_METADATA38" title="Generic Tunnel Option 38" hidden="yes"/>
    <field id="MFF_TUN_METADATA39" title="Generic Tunnel Option 39" hidden="yes"/>
    <field id="MFF_TUN_METADATA40" title="Generic Tunnel Option 40" hidden="yes"/>
    <field id="MFF_TUN_METADATA41" title="Generic Tunnel Option 41" hidden="yes"/>
    <field id="MFF_TUN_METADATA42" title="Generic Tunnel Option 42" hidden="yes"/>
    <field id="MFF_TUN_METADATA43" title="Generic Tunnel Option 43" hidden="yes"/>
    <field id="MFF_TUN_METADATA44" title="Generic Tunnel Option 44" hidden="yes"/>
    <field id="MFF_TUN_METADATA45" title="Generic Tunnel Option 45" hidden="yes"/>
    <field id="MFF_TUN_METADATA46" title="Generic Tunnel Option 46" hidden="yes"/>
    <field id="MFF_TUN_METADATA47" title="Generic Tunnel Option 47" hidden="yes"/>
    <field id="MFF_TUN_METADATA48" title="Generic Tunnel Option 48" hidden="yes"/>
    <field id="MFF_TUN_METADATA49" title="Generic Tunnel Option 49" hidden="yes"/>
    <field id="MFF_TUN_METADATA50" title="Generic Tunnel Option 50" hidden="yes"/>
    <field id="MFF_TUN_METADATA51" title="Generic Tunnel Option 51" hidden="yes"/>
    <field id="MFF_TUN_METADATA52" title="Generic Tunnel Option 52" hidden="yes"/>
    <field id="MFF_TUN_METADATA53" title="Generic Tunnel Option 53" hidden="yes"/>
    <field id="MFF_TUN_METADATA54" title="Generic Tunnel Option 54" hidden="yes"/>
    <field id="MFF_TUN_METADATA55" title="Generic Tunnel Option 55" hidden="yes"/>
    <field id="MFF_TUN_METADATA56" title="Generic Tunnel Option 56" hidden="yes"/>
    <field id="MFF_TUN_METADATA57" title="Generic Tunnel Option 57" hidden="yes"/>
    <field id="MFF_TUN_METADATA58" title="Generic Tunnel Option 58" hidden="yes"/>
    <field id="MFF_TUN_METADATA59" title="Generic Tunnel Option 59" hidden="yes"/>
    <field id="MFF_TUN_METADATA60" title="Generic Tunnel Option 60" hidden="yes"/>
    <field id="MFF_TUN_METADATA61" title="Generic Tunnel Option 61" hidden="yes"/>
    <field id="MFF_TUN_METADATA62" title="Generic Tunnel Option 62" hidden="yes"/>
    <field id="MFF_TUN_METADATA63" title="Generic Tunnel Option 63" hidden="yes"/>

    <field id="MFF_TUN_FLAGS" title="Tunnel Flags">
      <p>
        Flags indicating various aspects of the tunnel encapsulation.
      </p>

      <p>
        Matches on this field are most conveniently written in terms of
        symbolic names (given in the diagram below), each preceded by either
        <code>+</code> for a flag that must be set, or <code>-</code> for a
        flag that must be unset, without any other delimiters between the
        flags.  Flags not mentioned are wildcarded.  For example,
        <code>tun_flags=+oam</code> matches only OAM packets.  Matches can also
        be written as <code><var>flags</var>/<var>mask</var></code>, where
        <var>flags</var> and <var>mask</var> are 16-bit numbers in decimal or
        in hexadecimal prefixed by <code>0x</code>.
      </p>

      <p>
        Currently, only one flag is defined:
      </p>

      <dl>
        <dt><code>oam</code></dt>
        <dd>
          The tunnel protocol indicated that this is an OAM (Operations and
          Management) control packet.
        </dd>
      </dl>

      <p>
        The switch may reject matches against unknown flags.
      </p>

      <p>
        Newer versions of Open vSwitch may introduce additional flags with new
        meanings.  It is therefore not recommended to use an exact match on
        this field since the behavior of these new flags is unknown and should
        be ignored.
      </p>

      <p>
        For non-tunneled packets, the value is 0.
      </p>
    </field>

    <!-- Open vSwitch uses the following fields internally, but it
         does not expose them to the user via OpenFlow, so we do not
         document them. -->
    <field id="MFF_TUN_TTL" title="Tunnel IPv4 Time-to-Live" internal="yes"/>
    <field id="MFF_TUN_TOS" title="Tunnel IPv4 Type of Service" internal="yes"/>
  </group>

  <group title="Metadata">
    <p>
      These fields relate to the origin or treatment of a packet, but
      they are not extracted from the packet data itself.
    </p>

    <field id="MFF_IN_PORT" title="Ingress Port">
      <p>
        The OpenFlow port on which the packet being processed arrived.
        This is a 16-bit field that holds an OpenFlow 1.0 port number.
        For receiving a packet, the only values that appear in this
        field are:
      </p>

      <dl>
        <dt>1 through <code>0xfeff</code> (65,279), inclusive.</dt>
        <dd>
          Conventional OpenFlow port numbers.
        </dd>

        <dt><code>OFPP_LOCAL</code> (<code>0xfffe</code> or 65,534).</dt>
        <dd>
          <p>
            The ``local'' port, which in Open vSwitch is always named
            the same as the bridge itself.  This represents a
            connection between the switch and the local TCP/IP stack.
            This port is where an IP address is most commonly
            configured on an Open vSwitch switch.
          </p>

          <p>
            OpenFlow does not require a switch to have a local port,
            but all existing versions of Open vSwitch have always
            included a local port.  <b>Future Directions:</b> Future
            versions of Open vSwitch might be able to optionally omit
            the local port, if someone submits code to implement such
            a feature.
          </p>
        </dd>

        <dt><code>OFPP_NONE</code> (OpenFlow 1.0) or <code>OFPP_ANY</code> (OpenFlow 1.1+) (<code>0xffff</code> or 65,535).</dt>
        <dt><code>OFPP_CONTROLLER</code> (<code>0xfffd</code> or 65,533).</dt>
        <dd>
          <p>
            When a controller injects a packet into an OpenFlow switch
            with a ``packet-out'' request, it can specify one of these
            ingress ports to indicate that the packet was generated
            internally rather than having been received on some port.
          </p>

          <p>
            OpenFlow 1.0 specified <code>OFPP_NONE</code> for this
            purpose.  Despite that, some controllers used
            <code>OFPP_CONTROLLER</code>, and some switches only
            accepted <code>OFPP_CONTROLLER</code>, so OpenFlow 1.0.2
            required support for both ports.  OpenFlow 1.1 and later
            were more clearly drafted to allow only
            <code>OFPP_CONTROLLER</code>.  For maximum compatibility,
            Open vSwitch allows both ports with all OpenFlow versions.
          </p>
        </dd>
      </dl>

      <p>
        Values not mentioned above will never appear when receiving a
        packet, including the following notable values:
      </p>

      <dl>
        <dt>0</dt>
        <dd>
          Zero is not a valid OpenFlow port number.
        </dd>

        <dt><code>OFPP_MAX</code> (<code>0xff00</code> or 65,280).</dt>
        <dd>
          This value has only been clearly specified as a valid port
          number as of OpenFlow 1.3.3.  Before that, its status was
          unclear, and so Open vSwitch has never allowed
          <code>OFPP_MAX</code> to be used as a port number, so
          packets will never be received on this port.  (Other
          OpenFlow switches, of course, might use it.)
        </dd>

        <dt><code>OFPP_UNSET</code> (<code>0xfff7</code> or 65,527)</dt>
        <dt><code>OFPP_IN_PORT</code> (<code>0xfff8</code> or 65,528)</dt>
        <dt><code>OFPP_TABLE</code> (<code>0xfff9</code> or 65,529)</dt>
        <dt><code>OFPP_NORMAL</code> (<code>0xfffa</code> or 65,530)</dt>
        <dt><code>OFPP_FLOOD</code> (<code>0xfffb</code> or 65,531)</dt>
        <dt><code>OFPP_ALL</code> (<code>0xfffc</code> or 65,532)</dt>
        <dd>
          <p>
            These port numbers are used only in output actions and never
            appear as ingress ports.
          </p>

          <p>
            Most of these port numbers were defined in OpenFlow 1.0, but
            <code>OFPP_UNSET</code> was only introduced in OpenFlow 1.5.
          </p>
        </dd>
      </dl>

      <p>
        Values that will never appear when receiving a packet may
        still be matched against in the flow table.  There are still
        circumstances in which those flows can be matched:
      </p>

      <ul>
        <li>
          The <code>resubmit</code> Open vSwitch extension action allows a
          flow table lookup with an arbitrary ingress port.
        </li>

        <li>
          An action that modifies the ingress port field (see below),
          such as e.g. <code>load</code> or <code>set_field</code>,
          followed by an action or instruction that performs another
          flow table lookup, such as <code>resubmit</code> or
          <code>goto_table</code>.
        </li>
      </ul>

      <p>
        This field is heavily used for matching in OpenFlow tables,
        but for packet egress, it has only very limited roles:
      </p>

      <ul>
        <li>
          <p>
            OpenFlow requires suppressing output actions to <ref
            field="in_port"/>.  That is, the following two flows both drop all
            packets that arrive on port 1:
          </p>

          <pre>
in_port=1,actions=1
in_port=1,actions=drop
          </pre>

          <p>
            (This behavior is occasionally useful for flooding to a
            subset of ports.  Specifying <code>actions=1,2,3,4</code>,
            for example, outputs to ports 1, 2, 3, and 4, omitting the
            ingress port.)
          </p>
        </li>

        <li>
          OpenFlow has a special port <code>OFPP_IN_PORT</code> (with
          value 0xfff8) that outputs to the ingress port.  For example,
          in a switch that has four ports numbered 1 through 4,
          <code>actions=1,2,3,4,in_port</code> outputs to ports 1, 2,
          3, and 4, including the ingress port.
        </li>
      </ul>

      <p>
        Because the ingress port field has so little influence on packet
        processing, it does not ordinarily make sense to modify the
        ingress port field.  The field is writable only to support the
        occasional use case where the ingress port's roles in packet
        egress, described above, become troublesome.  For example,
        <code>actions=load:0-&gt;NXM_OF_IN_PORT[],output:123</code>
        will output to port 123 regardless of whether it is in the
        ingress port.  If the ingress port is important, then one may save
        and restore it on the stack:
      </p>

      <pre>
actions=push:NXM_OF_IN_PORT[],load:0->NXM_OF_IN_PORT[],output:123,pop:NXM_OF_IN_PORT[]
      </pre>

      <p>
        or, in Open vSwitch 2.7 or later, use the <code>clone</code> action to
        save and restore it:
      </p>

      <pre>
actions=clone(load:0->NXM_OF_IN_PORT[],output:123)
      </pre>

      <p>
        The ability to modify the ingress port is an Open vSwitch
        extension to OpenFlow.
      </p>
    </field>

    <field id="MFF_IN_PORT_OXM" title="OXM Ingress Port">
      <p>
        OpenFlow 1.1 and later use a 32-bit port number, so this field
        supplies a 32-bit view of the ingress port.  Current versions of
        Open vSwitch support only a 16-bit range of ports:
      </p>

      <ul>
        <li>
          OpenFlow 1.0 ports <code>0x0000</code> to
          <code>0xfeff</code>, inclusive, map to OpenFlow 1.1
          port numbers with the same values.
        </li>

        <li>
          OpenFlow 1.0 ports <code>0xff00</code> to
          <code>0xffff</code>, inclusive, map to OpenFlow 1.1 port
          numbers <code>0xffffff00</code> to <code>0xffffffff</code>.
        </li>

        <li>
          OpenFlow 1.1 ports <code>0x0000ff00</code> to
          <code>0xfffffeff</code> are not mapped and not supported.
        </li>
      </ul>

      <p>
        <ref field="in_port"/> and <ref field="in_port_oxm"/> are two views of
        the same information, so all of the comments on <ref field="in_port"/>
        apply to <ref field="in_port_oxm"/> too.  Modifying <ref
        field="in_port"/> changes <ref field="in_port_oxm"/>, and vice versa.
      </p>

      <p>
        Setting <ref field="in_port_oxm"/> to an unsupported value yields
        unspecified behavior.
      </p>
    </field>

    <field id="MFF_SKB_PRIORITY" title="Output Queue">
      <p>
        <b>Future Directions:</b> Open vSwitch implements the output queue as a
        field, but does not currently expose it through OXM or NXM for matching
        purposes.  If this turns out to be a useful feature, it could be
        implemented in future versions.  Only the <code>set_queue</code>,
        <code>enqueue</code>, and <code>pop_queue</code> actions currently
        influence the output queue.
      </p>

      <p>
        This field influences how packets in the flow will be queued,
        for quality of service (QoS) purposes, when they egress the
        switch.  Its range of meaningful values, and their meanings,
        varies greatly from one OpenFlow implementation to another.
        Even within a single implementation, there is no guarantee
        that all OpenFlow ports have the same queues configured or
        that all OpenFlow ports in an implementation can be configured
        the same way queue-wise.
      </p>

      <p>
        Configuring queues on OpenFlow is not well standardized.  On
        Linux, Open vSwitch supports queue configuration via OVSDB,
        specifically the <code>QoS</code> and <code>Queue</code>
        tables (see <code>ovs-vswitchd.conf.db(5)</code> for details).
        Ports of Open vSwitch to other platforms might require queue
        configuration through some separate protocol (such as a CLI).
        Even on Linux, Open vSwitch exposes only a fraction of the
        kernel's queuing features through OVSDB, so advanced or
        unusual uses might require use of separate utilities
        (e.g. <code>tc</code>).  OpenFlow switches other than Open
        vSwitch might use OF-CONFIG or any of the configuration
        methods mentioned above.  Finally, some OpenFlow switches have
        a fixed number of fixed-function queues (e.g. eight queues
        with strictly defined priorities) and others do not support
        any control over queuing.
      </p>

      <p>
        The only output queue that all OpenFlow implementations must
        support is zero, to identify a default queue, whose properties
        are implementation-defined.  Outputting a packet to a queue
        that does not exist on the output port yields unpredictable
        behavior: among the possibilities are that the packet might be
        dropped or transmitted with a very high or very low priority.
      </p>

      <p>
        OpenFlow 1.0 only allowed output queues to be specified as part of an
        <code>enqueue</code> action that specified both a queue and an output
        port.  That is, OpenFlow 1.0 treats the queue as an argument to an
        action, not as a field.
      </p>

      <p>
        To increase flexibility, OpenFlow 1.1 added an action to set the output
        queue.  This model was carried forward, without change, through
        OpenFlow 1.5.
      </p>

      <p>
        Open vSwitch implements the native queuing model of each
        OpenFlow version it supports.  Open vSwitch also includes an
        extension for setting the output queue as an action in
        OpenFlow 1.0.
      </p>

      <p>
        When a packet ingresses into an OpenFlow switch, the output
        queue is ordinarily set to 0, indicating the default queue.
        However, Open vSwitch supports various ways to forward a
        packet from one OpenFlow switch to another within a single
        host.  In these cases, Open vSwitch maintains the output queue
        across the forwarding step.  For example:
      </p>

      <ul>
        <li>
          A hop across an Open vSwitch ``patch port'' (which does not
          actually involve queuing) preserves the output queue.
        </li>

        <li>
          <p>
            When a flow sets the output queue then outputs to an
            OpenFlow tunnel port, the encapsulation preserves the
            output queue.  If the kernel TCP/IP stack routes the
            encapsulated packet directly to a physical interface, then
            that output honors the output queue.  Alternatively, if
            the kernel routes the encapsulated packet to another Open
            vSwitch bridge, then the output queue set previously
            becomes the initial output queue on ingress to the second
            bridge and will thus be used for further output actions
            (unless overridden by a new ``set queue'' action).
          </p>

          <p>
            (This description reflects the current behavior of Open
            vSwitch on Linux.  This behavior relies on details of the
            Linux TCP/IP stack.  It could be difficult to make ports
            to other operating systems behave the same way.)
          </p>
        </li>
      </ul>
    </field>

    <field id="MFF_PKT_MARK" title="Packet Mark">
      <p>
        Packet mark comes to Open vSwitch from the Linux kernel, in
        which the <code>sk_buff</code> data structure that represents
        a packet contains a 32-bit member named <code>skb_mark</code>.
        The value of <code>skb_mark</code> propagates along with the
        packet it accompanies wherever the packet goes in the kernel.
        It has no predefined semantics but various kernel-user
        interfaces can set and match on it, which makes it suitable
        for ``marking'' packets at one point in their handling and
        then acting on the mark later.  With <code>iptables</code>,
        for example, one can mark some traffic specially at ingress
        and then handle that traffic differently at egress based on
        the marked value.
      </p>

      <p>
        Packet mark is an attempt at a generalization of the
        <code>skb_mark</code> concept beyond Linux, at least through more
        generic naming.  Like <ref field="skb_priority"/>, packet mark is
        preserved across forwarding steps within a machine.  Unlike <ref
        field="skb_priority"/>, packet mark has no direct effect on packet
        forwarding: the value set in packet mark does not matter unless some
        later OpenFlow table or switch matches on packet mark, or unless the
        packet passes through some other kernel subsystem that has been
        configured to interpret packet mark in specific ways, e.g. through
        <code>iptables</code> configuration mentioned above.
      </p>

      <p>
        Preserving packet mark across kernel forwarding steps relies
        heavily on kernel support, which ports to non-Linux operating
        systems may not have.  Regardless of operating system support,
        Open vSwitch supports packet mark within a single bridge and
        across patch ports.
      </p>

      <p>
        The value of packet mark when a packet ingresses into the
        first Open vSwich bridge is typically zero, but it could be
        nonzero if its value was previously set by some kernel
        subsystem.
      </p>
    </field>

    <field id="MFF_ACTSET_OUTPUT" title="Action Set Output Port">
      <p>
        Holds the output port currently in the OpenFlow action set (i.e. from
        an <code>output</code> action within a <code>write_actions</code>
        instruction).  Its value is an OpenFlow port number.  If there is no
        output port in the OpenFlow action set, or if the output port will be
        ignored (e.g. because there is an output group in the OpenFlow action
        set), then the value will be <code>OFPP_UNSET</code>.
      </p>

      <p>
        Open vSwitch allows any table to match this field.  OpenFlow, however,
        only requires this field to be matchable from within an OpenFlow egress
        table (a feature that Open vSwitch does not yet implement).
      </p>
    </field>

    <field id="MFF_DP_HASH" title="Datapath Hash" internal="yes"/>
    <field id="MFF_RECIRC_ID" title="Datapath Recirculation ID" internal="yes"/>

    <field id="MFF_PACKET_TYPE" title="Packet Type">
      <p>
        The type of the packet in the format specified in OpenFlow 1.5:
      </p>

      <diagram>
        <header name="Packet type">
          <bits name="ns" above="16" width=".75"/>
          <bits name="ns_type" above="16" width=".75"/>
        </header>
        <dots/>
      </diagram>

      <p>
        The upper 16 bits, <var>ns</var>, are a namespace.  The meaning of
        <var>ns_type</var> depends on the namespace.  The packet type field is
        specified and displayed in the format
        <code>(<var>ns</var>,<var>ns_type</var>)</code>.
      </p>

      <p>
        Open vSwitch currently supports the following classes of packet types
        for matching:
        <dl>
          <dt><code>(0,0)</code></dt>
            <dd>Ethernet.</dd>
          <dt><code>(1,<var>ethertype</var>)</code></dt>
          <dd>
            <p>
              The specified <var>ethertype</var>.  Open vSwitch can forward
              packets with any <var>ethertype</var>, but it can only match on
              and process data fields for the following supported packet types:
            </p>
            <dl>
              <dt><code>(1,0x800)</code></dt>  <dd>IPv4</dd>
              <dt><code>(1,0x806)</code></dt>  <dd>ARP</dd>
              <dt><code>(1,0x86dd)</code></dt> <dd>IPv6</dd>
              <dt><code>(1,0x8847)</code></dt> <dd>MPLS</dd>
              <dt><code>(1,0x8848)</code></dt> <dd>MPLS multicast</dd>
              <dt><code>(1,0x8035)</code></dt> <dd>RARP</dd>
              <dt><code>(1,0x894f)</code></dt> <dd>NSH</dd>
            </dl>
          </dd>
        </dl>
      </p>

      <p>
        Consider the distinction between a packet with <code>packet_type=(0,0),
        dl_type=0x800</code> and one with <code>packet_type=(1,0x800)</code>.
        The former is an Ethernet frame that contains an IPv4 packet, like
        this:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" width="0.4"/>
          <bits name="src" above="32" width="0.4"/>
          <bits name="dst" above="32" width="0.4"/>
        </header>
        <dots/>
      </diagram>

      <p>
        The latter is an IPv4 packet not encapsulated inside any outer frame,
        like this:
      </p>

      <diagram>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" width="0.4"/>
          <bits name="src" above="32" width="0.4"/>
          <bits name="dst" above="32" width="0.4"/>
        </header>
        <dots/>
      </diagram>

      <p>
        Matching on <ref field="packet_type"/> is a pre-requisite for matching
        on any data field, but for backward compatibility, when a match on a
        data field is present without a <ref field="packet_type"/> match, Open
        vSwitch acts as though a match on <code>(0,0)</code> (Ethernet) had
        been supplied.  Similarly, when Open vSwitch sends flow match
        information to a controller, e.g. in a reply to a request to dump the
        flow table, Open vSwitch omits a match on packet type (0,0) if it would
        be implied by a data field match.
      </p>
    </field>

  </group>

  <group title="Connection Tracking">
    <p>
      Open vSwitch 2.5 and later support ``connection tracking,'' which allows
      bidirectional streams of packets to be statefully grouped into
      connections.  Open vSwitch connection tracking, for example, identifies
      the patterns of TCP packets that indicates a successfully initiated
      connection, as well as those that indicate that a connection has been
      torn down.  Open vSwitch connection tracking can also identify related
      connections, such as FTP data connections spawned from FTP control
      connections.
    </p>

    <p>
      An individual packet passing through the pipeline may be in one of two
      states, ``untracked'' or ``tracked,'' which may be distinguished via the
      ``trk'' flag in <ref field="ct_state"/>.  A packet is
      <dfn>untracked</dfn> at the beginning of the Open vSwitch pipeline and
      continues to be untracked until the pipeline invokes the <code>ct</code>
      action.  The connection tracking fields are all zeroes in an untracked
      packet.  When a flow in the Open vSwitch pipeline invokes the
      <code>ct</code> action, the action initializes the connection tracking
      fields and the packet becomes <dfn>tracked</dfn> for the remainder of its
      processing.
    </p>

    <p>
      The connection tracker stores connection state in an internal table, but
      it only adds a new entry to this table when a <code>ct</code> action for
      a new connection invokes <code>ct</code> with the <code>commit</code>
      parameter.  For a given connection, when a pipeline has executed
      <code>ct</code>, but not yet with <code>commit</code>, the connection is
      said to be <dfn>uncommitted</dfn>.  State for an uncommitted connection
      is ephemeral and does not persist past the end of the pipeline, so some
      features are only available to committed connections.  A connection would
      typically be left uncommitted as a way to drop its packets.
    </p>

    <p>
      Connection tracking is an Open vSwitch extension to OpenFlow.
    </p>

    <field id="MFF_CT_STATE" title="Connection Tracking State">
      <p>
        This field holds several flags that can be used to determine the state
        of the connection to which the packet belongs.
      </p>

      <p>
        Matches on this field are most conveniently written in terms of
        symbolic names (listed below), each preceded by either <code>+</code>
        for a flag that must be set, or <code>-</code> for a flag that must be
        unset, without any other delimiters between the flags.  Flags not
        mentioned are wildcarded.  For example,
        <code>tcp,ct_state=+trk-new</code> matches TCP packets that have been
        run through the connection tracker and do not establish a new
        connection.  Matches can also be written as
        <code><var>flags</var>/<var>mask</var></code>, where <var>flags</var>
        and <var>mask</var> are 32-bit numbers in decimal or in hexadecimal
        prefixed by <code>0x</code>.
      </p>

      <p>
        The following flags are defined:
      </p>

      <dl>
        <dt><code>new</code> (0x01)</dt>
        <dd>
          A new connection.  Set to 1 if this is an uncommitted connection.
        </dd>

        <dt><code>est</code> (0x02)</dt>
        <dd>
          Part of an existing connection.  Set to 1 if this is a committed
          connection.
        </dd>

        <dt><code>rel</code> (0x04)</dt>
        <dd>
          <p>
            Related to an existing connection, e.g. an ICMP ``destination
            unreachable'' message or an FTP data connections.  This flag will
            only be 1 if the connection to which this one is related is
            committed.
          </p>

          <p>
            Connections identified as <code>rel</code> are separate from the
            originating connection and must be committed separately.  All
            packets for a related connection will have the <code>rel</code>
            flag set, not just the initial packet.
          </p>
        </dd>

        <dt><code>rpl</code> (0x08)</dt>
        <dd>
          This packet is in the reply direction, meaning that it is in the
          opposite direction from the packet that initiated the connection.
          This flag will only be 1 if the connection is committed.
        </dd>

        <dt><code>inv</code> (0x10)</dt>
        <dd>
          <p>
            The state is invalid, meaning that the connection tracker couldn't
            identify the connection. This flag is a catch-all for problems
            in the connection or the connection tracker, such as:
          </p>

          <ul>
            <li>
              L3/L4 protocol handler is not loaded/unavailable.  With the Linux
              kernel datapath, this may mean that the
              <code>nf_conntrack_ipv4</code> or <code>nf_conntrack_ipv6</code>
              modules are not loaded.
            </li>

            <li>
              L3/L4 protocol handler determines that the packet is malformed.
            </li>

            <li>
              Packets are unexpected length for protocol.
            </li>
          </ul>
        </dd>

        <dt><code>trk</code> (0x20)</dt>
        <dd>
          This packet is tracked, meaning that it has previously traversed the
          connection tracker.  If this flag is not set, then no other flags
          will be set.  If this flag is set, then the packet is tracked and
          other flags may also be set.
        </dd>

        <dt><code>snat</code> (0x40)</dt>
        <dd>
          This packet was transformed by source address/port translation by a
          preceding <code>ct</code> action.  Open vSwitch 2.6 added this flag.
        </dd>

        <dt><code>dnat</code> (0x80)</dt>
        <dd>
          This packet was transformed by destination address/port translation
          by a preceding <code>ct</code> action.  Open vSwitch 2.6 added this
          flag.
        </dd>
      </dl>

      <p>
        There are additional constraints on these flags, listed in decreasing
        order of precedence below:
      </p>

      <ol>
        <li>
          If <code>trk</code> is unset, no other flags are set.
        </li>

        <li>
          If <code>trk</code> is set, one or more other flags may be set.
        </li>

        <li>
          If <code>inv</code> is set, only the <code>trk</code> flag is also
          set.
        </li>

        <li>
          <code>new</code> and <code>est</code> are mutually exclusive.
        </li>

        <li>
          <code>new</code> and <code>rpl</code> are mutually exclusive.
        </li>

        <li>
          <code>rel</code> may be set in conjunction with any other flags.
        </li>
      </ol>

      <p>
        Future versions of Open vSwitch may define new flags.
      </p>
    </field>

    <field id="MFF_CT_ZONE" title="Connection Tracking Zone">
      A connection tracking zone, the zone value passed to the most recent
      <code>ct</code> action.  Each zone is an independent connection tracking
      context, so tracking the same packet in multiple contexts requires using
      the <code>ct</code> action multiple times.
    </field>

    <field id="MFF_CT_MARK" title="Connection Tracking Mark">
      The metadata committed, by an action within the <code>exec</code>
      parameter to the <code>ct</code> action, to the connection to which the
      current packet belongs.
    </field>

    <field id="MFF_CT_LABEL" title="Connection Tracking Label">
      The label committed, by an action within the <code>exec</code>
      parameter to the <code>ct</code> action, to the connection to which the
      current packet belongs.
    </field>

    <p>
      Open vSwitch 2.8 introduced the matching support for connection
      tracker original direction 5-tuple fields.
    </p>

    <p>
      For non-committed non-related connections the conntrack original
      direction tuple fields always have the same values as the
      corresponding headers in the packet itself.  For any other packets of
      a committed connection the conntrack original direction tuple fields
      reflect the values from that initial non-committed non-related packet,
      and thus may be different from the actual packet headers, as the
      actual packet headers may be in reverse direction (for reply packets),
      transformed by NAT (when \fBnat\fR option was applied to the
      connection), or be of different protocol (i.e., when an ICMP response
      is sent to an UDP packet).  In case of related connections, e.g., an
      FTP data connection, the original direction tuple contains the
      original direction headers from the master connection, e.g., an FTP
      control connection.
    </p>

    <p>
      The following fields are populated by the ct action, and require a
      match to a valid connection tracking state as a prerequisite, in
      addition to the IP or IPv6 ethertype match.  Examples of valid
      connection tracking state matches include \fBct_state=+new\fR,
      \fBct_state=+est\fR, \fBct_state=+rel\fR, and \fBct_state=+trk-inv\fR.
    </p>

    <field id="MFF_CT_NW_SRC" title="Connection Tracking Original Direction IPv4 Source Address">
      Matches IPv4 conntrack original direction tuple source address.
      See the paragraphs above for general description to the
      conntrack original direction tuple.  Introduced in Open vSwitch
      2.8.
    </field>

    <field id="MFF_CT_NW_DST" title="Connection Tracking Original Direction IPv4 Destination Address">
      Matches IPv4 conntrack original direction tuple destination address.
      See the paragraphs above for general description to the
      conntrack original direction tuple.  Introduced in Open vSwitch
      2.8.
    </field>

    <field id="MFF_CT_IPV6_SRC" title="Connection Tracking Original Direction IPv6 Source Address">
      Matches IPv6 conntrack original direction tuple source address.
      See the paragraphs above for general description to the
      conntrack original direction tuple.  Introduced in Open vSwitch
      2.8.
    </field>

    <field id="MFF_CT_IPV6_DST" title="Connection Tracking Original Direction IPv6 Destination Address">
      Matches IPv6 conntrack original direction tuple destination address.
      See the paragraphs above for general description to the
      conntrack original direction tuple.  Introduced in Open vSwitch
      2.8.
    </field>

    <field id="MFF_CT_NW_PROTO" title="Connection Tracking Original Direction IP Protocol">
      Matches conntrack original direction tuple IP protocol type,
      which is specified as a decimal number between 0 and 255,
      inclusive (e.g. 1 to match ICMP packets or 6 to match TCP
      packets).  In case of, for example, an ICMP response to an UDP
      packet, this may be different from the IP protocol type of the
      packet itself.  See the paragraphs above for general description
      to the conntrack original direction tuple.  Introduced in Open
      vSwitch 2.8.
    </field>

    <field id="MFF_CT_TP_SRC" title="Connection Tracking Original Direction Transport Layer Source Port">
      Bitwise match on the conntrack original direction tuple
      transport source, when
      <code>MFF_CT_NW_PROTO</code> has value 6 for TCP, 17 for UDP, or
      132 for SCTP.  When <code>MFF_CT_NW_PROTO</code> has value 1 for
      ICMP, or 58 for ICMPv6, the lower 8 bits of
      <code>MFF_CT_TP_SRC</code> matches the conntrack original
      direction ICMP type.  See the paragraphs above for general
      description to the conntrack original direction
      tuple. Introduced in Open vSwitch 2.8.
    </field>

    <field id="MFF_CT_TP_DST" title="Connection Tracking Original Direction Transport Layer Source Port">
      Bitwise match on the conntrack original direction tuple
      transport destination port, when
      <code>MFF_CT_NW_PROTO</code> has value 6 for TCP, 17 for UDP, or
      132 for SCTP.  When <code>MFF_CT_NW_PROTO</code> has value 1 for
      ICMP, or 58 for ICMPv6, the lower 8 bits of
      <code>MFF_CT_TP_DST</code> matches the conntrack original
      direction ICMP code.  See the paragraphs above for general
      description to the conntrack original direction
      tuple. Introduced in Open vSwitch 2.8.
    </field>
  </group>

  <group title="Register">
    <p>
      These fields give an OpenFlow switch space for temporary storage while
      the pipeline is running.  Whereas metadata fields can have a meaningful
      initial value and can persist across some hops across OpenFlow switches,
      registers are always initially 0 and their values never persist across
      inter-switch hops (not even across patch ports).
    </p>

    <field id="MFF_METADATA" title="OpenFlow Metadata">
      <p>
        This field is the oldest standardized OpenFlow register field,
        introduced in OpenFlow 1.1.  It was introduced to model the limited
        number of user-defined bits that some ASIC-based switches can carry
        through their pipelines.  Because of hardware limitations, OpenFlow
        allows switches to support writing and masking only an
        implementation-defined subset of bits, even no bits at all.  The Open
        vSwitch software switch always supports all 64 bits, but of course an
        Open vSwitch port to an ASIC would have the same restriction as the
        ASIC itself.
      </p>

      <p>
        This field has an OXM code point, but OpenFlow 1.4 and earlier allow it
        to be modified only with a specialized instruction, not with a
        ``set-field'' action.  OpenFlow 1.5 removes this restriction.  Open
        vSwitch does not enforce this restriction, regardless of OpenFlow
        version.
      </p>
    </field>

    <field id="MFF_REG0" title="Register 0">
      This is the first of several Open vSwitch registers, all of which have
      the same properties.  Open vSwitch 1.1 introduced registers 0, 1, 2, and
      3, version 1.3 added register 4, version 1.7 added registers 5, 6, and 7,
      and version 2.6 added registers 8 through 15.
    </field>
    <!-- XXX series -->
    <field id="MFF_REG1" title="Register 1" hidden="yes"/>
    <field id="MFF_REG2" title="Register 2" hidden="yes"/>
    <field id="MFF_REG3" title="Register 3" hidden="yes"/>
    <field id="MFF_REG4" title="Register 4" hidden="yes"/>
    <field id="MFF_REG5" title="Register 5" hidden="yes"/>
    <field id="MFF_REG6" title="Register 6" hidden="yes"/>
    <field id="MFF_REG7" title="Register 7" hidden="yes"/>
    <field id="MFF_REG8" title="Register 8" hidden="yes"/>
    <field id="MFF_REG9" title="Register 9" hidden="yes"/>
    <field id="MFF_REG10" title="Register 10" hidden="yes"/>
    <field id="MFF_REG11" title="Register 11" hidden="yes"/>
    <field id="MFF_REG12" title="Register 12" hidden="yes"/>
    <field id="MFF_REG13" title="Register 13" hidden="yes"/>
    <field id="MFF_REG14" title="Register 14" hidden="yes"/>
    <field id="MFF_REG15" title="Register 15" hidden="yes"/>

    <field id="MFF_XREG0" title="Extended Register 0">
      <p>
        This is the first of the registers introduced in OpenFlow 1.5.
        OpenFlow 1.5 calls these fields just the ``packet registers,'' but Open
        vSwitch already had 32-bit registers by that name, so Open vSwitch uses
        the name ``extended registers'' in an attempt to reduce confusion.  The
        standard allows for up to 128 registers, each 64 bits wide, but Open
        vSwitch only implements 4 (in versions 2.4 and 2.5) or 8 (in version
        2.6 and later).
      </p>

      <p>
        Each of the 64-bit extended registers overlays two of the 32-bit
        registers: <code>xreg0</code> overlays <code>reg0</code> and
        <code>reg1</code>, with <code>reg0</code> supplying the
        most-significant bits of <code>xreg0</code> and <code>reg1</code> the
        least-significant.  Similarly, <code>xreg1</code> overlays
        <code>reg2</code> and <code>reg3</code>, and so on.
      </p>

      <p>
        The OpenFlow specification says, ``In most cases, the packet registers
        can not be matched in tables, i.e. they usually can not be used in the
        flow entry match structure'' [OpenFlow 1.5, section 7.2.3.10], but
        there is no reason for a software switch to impose such a restriction,
        and Open vSwitch does not.
      </p>
    </field>

    <!-- XXX series -->
    <field id="MFF_XREG1" title="Extended Register 1" hidden="yes"/>
    <field id="MFF_XREG2" title="Extended Register 2" hidden="yes"/>
    <field id="MFF_XREG3" title="Extended Register 3" hidden="yes"/>
    <field id="MFF_XREG4" title="Extended Register 4" hidden="yes"/>
    <field id="MFF_XREG5" title="Extended Register 5" hidden="yes"/>
    <field id="MFF_XREG6" title="Extended Register 6" hidden="yes"/>
    <field id="MFF_XREG7" title="Extended Register 7" hidden="yes"/>

    <field id="MFF_XXREG0" title="Double-Extended Register 0">
      <p>
        This is the first of the double-extended registers introduce in Open
        vSwitch 2.6. Each of the 128-bit extended registers overlays four of
        the 32-bit registers: <code>xxreg0</code> overlays <code>reg0</code>
        through <code>reg3</code>, with <code>reg0</code> supplying the
        most-significant bits of <code>xxreg0</code> and <code>reg3</code> the
        least-significant.  <code>xxreg1</code> similarly overlays
        <code>reg4</code> through <code>reg7</code>, and so on.
      </p>
    </field>

    <!-- XXX series -->
    <field id="MFF_XXREG1" title="Double-Extended Register 1" hidden="yes"/>
    <field id="MFF_XXREG2" title="Double-Extended Register 2" hidden="yes"/>
    <field id="MFF_XXREG3" title="Double-Extended Register 3" hidden="yes"/>
  </group>

  <group title="Layer 2 (Ethernet)">
    <p>
      Ethernet is the only layer-2 protocol that Open vSwitch
      supports.  As with most software, Open vSwitch and OpenFlow
      regard an Ethernet frame to begin with the 14-byte header and
      end with the final byte of the payload; that is, the frame check
      sequence is not considered part of the frame.
    </p>

    <field id="MFF_ETH_SRC" title="Ethernet Source">
      <p>
        The Ethernet source address:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75"/>
          <bits name="src" above="48" width=".75" fill="yes"/>
          <bits name="type" above="16" width="0.4"/>
        </header>
        <dots/>
      </diagram>
    </field>

    <field id="MFF_ETH_DST" title="Ethernet Destination">
      <p>
        The Ethernet destination address:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75" fill="yes"/>
          <bits name="src" above="48" width=".75"/>
          <bits name="type" above="16" width="0.4"/>
        </header>
        <dots/>
      </diagram>

      <p>
        Open vSwitch 1.8 and later support arbitrary masks for source and/or
        destination.  Earlier versions only support masking the destination
        with the following masks:
      </p>

      <dl>
        <dt><code>01:00:00:00:00:00</code></dt>
        <dd>
          Match only the multicast bit.  Thus,
          <code>dl_dst=01:00:00:00:00:00/01:00:00:00:00:00</code> matches all
          multicast (including broadcast) Ethernet packets, and
          <code>dl_dst=00:00:00:00:00:00/01:00:00:00:00:00</code> matches all
          unicast Ethernet packets.
        </dd>

        <dt><code>fe:ff:ff:ff:ff:ff</code></dt>
        <dd>
          Match all bits except the multicast bit.  This is probably not
          useful.
        </dd>

        <dt><code>ff:ff:ff:ff:ff:ff</code></dt>
        <dd>
          Exact match (equivalent to omitting the mask).
        </dd>

        <dt><code>00:00:00:00:00:00</code></dt>
        <dd>
          Wildcard all bits (equivalent to <code>dl_dst=*</code>).
        </dd>
      </dl>
    </field>

    <field id="MFF_ETH_TYPE" title="Ethernet Type">
      <p>
        The most commonly seen Ethernet frames today use a format
        called ``Ethernet II,'' in which the last two bytes of the
        Ethernet header specify the Ethertype.  For such a frame, this
        field is copied from those bytes of the header, like so:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75"/>
          <bits name="src" above="48" width=".75"/>
          <bits name="type" above="16" below="\[&gt;=]0x600" width="0.4" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        Every Ethernet type has a value 0x600 (1,536) or greater.
        When the last two bytes of the Ethernet header have a value
        too small to be an Ethernet type, then the value found there
        is the total length of the frame in bytes, excluding the
        Ethernet header.  An 802.2 LLC header typically follows the
        Ethernet header.  OpenFlow and Open vSwitch only support LLC
        headers with DSAP and SSAP <code>0xaa</code> and control byte
        <code>0x03</code>, which indicate that a SNAP header follows
        the LLC header.  In turn, OpenFlow and Open vSwitch only
        support a SNAP header with organization <code>0x000000</code>.
        In such a case, this field is copied from the type field in
        the SNAP header, like this:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75"/>
          <bits name="src" above="48" width=".75"/>
          <bits name="type" above="16" below="&lt;0x600" width="0.4"/>
        </header>
        <header name="LLC">
          <bits name="DSAP" above="8" below="0xaa" width=".4"/>
          <bits name="SSAP" above="8" below="0xaa" width=".4"/>
          <bits name="cntl" above="8" below="0x03" width=".4"/>
        </header>
        <header name="SNAP">
          <bits name="org" above="24" below="0x000000" width=".75"/>
          <bits name="type" above="16" below="\[&gt;=]0x600" width=".4" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        When an 802.1Q header is inserted after the Ethernet source
        and destination, this field is populated with the encapsulated
        Ethertype, not the 802.1Q Ethertype.  With an Ethernet II
        inner frame, the result looks like this:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75"/>
          <bits name="src" above="48" width=".75"/>
        </header>
        <header name="802.1Q">
          <bits name="TPID" above="16" below="0x8100" width=".4"/>
          <bits name="TCI" above="16" width=".4"/>
        </header>
        <header name="Ethertype">
          <bits name="type" above="16" below="\[&gt;=]0x600" width=".4" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        LLC and SNAP encapsulation look like this with an 802.1Q header:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width=".75"/>
          <bits name="src" above="48" width=".75"/>
        </header>
        <header name="802.1Q">
          <bits name="TPID" above="16" below="0x8100" width=".4"/>
          <bits name="TCI" above="16" width=".4"/>
        </header>
        <header name="Ethertype">
          <bits name="type" above="16" below="&lt;0x600" width="0.4"/>
        </header>
        <header name="LLC">
          <bits name="DSAP" above="8" below="0xaa" width=".4"/>
          <bits name="SSAP" above="8" below="0xaa" width=".4"/>
          <bits name="cntl" above="8" below="0x03" width=".4"/>
        </header>
        <header name="SNAP">
          <bits name="org" above="24" below="0x000000" width=".75"/>
          <bits name="type" above="16" below="\[&gt;=]0x600" width=".4" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        When a packet doesn't match any of the header formats described
        above, Open vSwitch and OpenFlow set this field to
        <code>0x5ff</code> (<code>OFP_DL_TYPE_NOT_ETH_TYPE</code>).
      </p>
    </field>
  </group>

  <group title="VLAN">
    <p>
      The 802.1Q VLAN header causes more trouble than any other 4
      bytes in networking.  OpenFlow 1.0, 1.1, and 1.2+ all treat VLANs
      differently.  Open vSwitch extensions add another variant to the mix.
      Open vSwitch reconciles all four treatments as best it can.
    </p>

    <h2>VLAN Header Format</h2>

    <p>
      An 802.1Q VLAN header consists of two 16-bit fields:
    </p>

    <diagram>
      <header name="TPID">
        <bits name="Ethertype" above="16" below="0x8100" width="1.8"/>
      </header>
      <nospace/>
      <header name="TCI">
        <bits name="PCP" above="3" width=".6"/>
        <bits name="CFI" above="1" below="0" width=".3"/>
        <bits name="VID" above="12" width=".9"/>
      </header>
    </diagram>

    <p>
      The first 16 bits of the VLAN header, the <dfn>TPID</dfn> (Tag Protocol
      IDentifier), is an Ethertype.  When the VLAN header is inserted just
      after the source and destination MAC addresses in a Ethertype frame, the
      TPID serves to identify the presence of the VLAN.  The standard TPID, the
      only one that Open vSwitch supports, is <code>0x8100</code>.  OpenFlow
      1.0 explicitly supports only TPID <code>0x8100</code>.  OpenFlow 1.1, but
      not earlier or later versions, also requires support for TPID
      <code>0x88a8</code> (Open vSwitch does not support this).  OpenFlow 1.2
      through 1.5 do not require support for specific TPIDs (the ``push vlan
      header'' action does say that only <code>0x8100</code> and
      <code>0x88a8</code> should be pushed).  No version of OpenFlow provides a
      way to distinguish or match on the TPID.
    </p>

    <p>
      The remaining 16 bits of the VLAN header, the <dfn>TCI</dfn>
      (Tag Control Information), is subdivided into three subfields:
    </p>

    <ul>
      <li>
        <dfn>PCP</dfn> (Priority Control Point), is a 3-bit 802.1p
        <dfn>priority</dfn>.  The lowest priority is value 1, the
        second-lowest is value 0, and priority increases from 2 up to
        highest priority 7.
      </li>

      <li>
        <p>
          <dfn>CFI</dfn> (Canonical Format Indicator), is a 1-bit field.  On an
          Ethernet network, its value is always 0.  This led to it later being
          repurposed under the name <dfn>DEI</dfn> (Drop Eligibility
          Indicator).  By either name, OpenFlow and Open vSwitch don't provide
          any way to match or set this bit.
        </p>
      </li>

      <li>
        <dfn>VID</dfn> (VLAN IDentifier), is a 12-bit VLAN.  If the
        VID is 0, then the frame is not part of a VLAN.  In that case,
        the VLAN header is called a <dfn>priority tag</dfn> because it
        is only meaningful for assigning the frame a priority.  VID
        <code>0xfff</code> (4,095) is reserved.
      </li>
    </ul>

    <p>
      See <ref field="eth_type"/> for illustrations of a complete Ethernet
      frame with 802.1Q tag included.
    </p>

    <h2>Multiple VLANs</h2>

    <p>
      Open vSwitch can match only a single VLAN header.  If more than
      one VLAN header is present, then <ref field="eth_type"/>
      holds the TPID of the inner VLAN header.  Open vSwitch stops
      parsing the packet after the inner TPID, so matching further
      into the packet (e.g. on the inner TCI or L3 fields) is not
      possible.
    </p>

    <p>
      OpenFlow only directly supports matching a single VLAN header.  In
      OpenFlow 1.1 or later, one OpenFlow table can match on the outermost VLAN
      header and pop it off, and a later OpenFlow table can match on the next
      outermost header.  Open vSwitch does not support this.
    </p>

    <h2>VLAN Field Details</h2>

    <p>
      The four variants have three different levels of expressiveness: OpenFlow
      1.0 and 1.1 VLAN matching are less powerful than OpenFlow 1.2+ VLAN
      matching, which is less powerful than Open vSwitch extension VLAN
      matching.
    </p>

    <h2>OpenFlow 1.0 VLAN Fields</h2>

    <p>
      OpenFlow 1.0 uses two fields, called <code>dl_vlan</code> and
      <code>dl_vlan_pcp</code>, each of which can be either exact-matched or
      wildcarded, to specify VLAN matches:
    </p>

    <ul>
      <li>
        When both <code>dl_vlan</code> and <code>dl_vlan_pcp</code> are
        wildcarded, the flow matches packets without an 802.1Q header or
        with any 802.1Q header.
      </li>

      <li>
        The match <code>dl_vlan=0xffff</code> causes a flow to match only
        packets without an 802.1Q header.  Such a flow should also wildcard
        <code>dl_vlan_pcp</code>, since a packet without an 802.1Q header does
        not have a PCP.  OpenFlow does not specify what to do if a match on PCP
        is actually present, but Open vSwitch ignores it.
      </li>

      <li>
        <p>
          Otherwise, the flow matches only packets with an 802.1Q
          header.  If <code>dl_vlan</code> is not wildcarded, then the
          flow only matches packets with the VLAN ID specified in
          <code>dl_vlan</code>'s low 12 bits.  If
          <code>dl_vlan_pcp</code> is not wildcarded, then the flow
          only matches packets with the priority specified in
          <code>dl_vlan_pcp</code>'s low 3 bits.
        </p>

        <p>
          OpenFlow does not specify how to interpret the high 4 bits of
          <code>dl_vlan</code> or the high 5 bits of <code>dl_vlan_pcp</code>.
          Open vSwitch ignores them.
        </p>
      </li>
    </ul>

    <field id="MFF_DL_VLAN" title="OpenFlow 1.0 VLAN ID" hidden="yes"/>
    <field id="MFF_DL_VLAN_PCP" title="OpenFlow 1.0 VLAN Priority"
           hidden="yes"/>

    <h2>OpenFlow 1.1 VLAN Fields</h2>

    <p>
      VLAN matching in OpenFlow 1.1 is similar to OpenFlow 1.0.
      The one refinement is that when <code>dl_vlan</code> matches on
      <code>0xfffe</code> (<code>OFVPID_ANY</code>), the flow matches
      only packets with an 802.1Q header, with any VLAN ID.  If
      <code>dl_vlan_pcp</code> is wildcarded, the flow matches any
      packet with an 802.1Q header, regardless of VLAN ID or priority.
      If <code>dl_vlan_pcp</code> is not wildcarded, then the flow
      only matches packets with the priority specified in
      <code>dl_vlan_pcp</code>'s low 3 bits.
    </p>

    <p>
      OpenFlow 1.1 uses the name <code>OFPVID_NONE</code>, instead of
      <code>OFP_VLAN_NONE</code>, for a <code>dl_vlan</code> of
      <code>0xffff</code>, but it has the same meaning.
    </p>

    <p>
      In OpenFlow 1.1, Open vSwitch reports error
      <code>OFPBMC_BAD_VALUE</code> for an attempt to match on
      <code>dl_vlan</code> between 4,096 and <code>0xfffd</code>,
      inclusive, or <code>dl_vlan_pcp</code> greater than 7.
    </p>

    <h2>OpenFlow 1.2 VLAN Fields</h2>

    <field id="MFF_VLAN_VID" title="OpenFlow 1.2+ VLAN ID">
      <p>
        The OpenFlow standard describes this field as consisting of
        ``12+1'' bits.  On ingress, its value is 0 if no 802.1Q header
        is present, and otherwise it holds the VLAN VID in its least
        significant 12 bits, with bit 12 (<code>0x1000</code> aka
        <code>OFPVID_PRESENT</code>) also set to 1.  The three most
        significant bits are always zero:
      </p>

      <diagram>
        <header name="OXM_OF_VLAN_VID">
          <bits name="" above="3" below="0" width=".6"/>
          <bits name="P" above="1" width=".1"/>
          <bits name="VLAN ID" above="12" width=".9"/>
        </header>
      </diagram>

      <p>
        As a consequence of this field's format, one may use it to match the
        VLAN ID in all of the ways available with the OpenFlow 1.0 and 1.1
        formats, and a few new ways:
      </p>

      <dl>
        <dt>Fully wildcarded</dt>
        <dd>
          Matches any packet, that is, one without an 802.1Q header or
          with an 802.1Q header with any TCI value.
        </dd>

        <dt>
          Value <code>0x0000</code> (<code>OFPVID_NONE</code>), mask
          <code>0xffff</code> (or no mask)
        </dt>
        <dd>
          Matches only packets without an 802.1Q header.
        </dd>

        <dt>
          Value <code>0x1000</code>, mask <code>0x1000</code>
        </dt>
        <dd>
          Matches any packet with an 802.1Q header, regardless of VLAN
          ID.
        </dd>

        <dt>
          Value <code>0x1009</code>, mask <code>0xffff</code> (or no mask)
        </dt>
        <dd>
          Match only packets with an 802.1Q header with VLAN ID 9.
        </dd>

        <dt>Value <code>0x1001</code>, mask <code>0x1001</code></dt>
        <dd>
          Matches only packets that have an 802.1Q header with an
          odd-numbered VLAN ID.  (This is just an example; one can
          match on any desired VLAN ID bit pattern.)
        </dd>
      </dl>
    </field>

    <field id="MFF_VLAN_PCP" title="OpenFlow 1.2+ VLAN Priority">
      <p>
        The 3 least significant bits may be used to match the PCP bits
        in an 802.1Q header.  Other bits are always zero:
      </p>

      <diagram>
        <header name="OXM_OF_VLAN_VID">
          <bits name="zero" above="5" below="0" width="1.0"/>
          <bits name="PCP" above="3" width=".6"/>
        </header>
      </diagram>

      <p>
        This field may only be used when <ref field="vlan_vid"/> is not
        wildcarded and does not exact match on 0 (which only matches
        when there is no 802.1Q header).
      </p>

      <p>
        See <cite>VLAN Comparison Chart</cite>, below, for some examples.
      </p>
    </field>

    <h2>Open vSwitch Extension VLAN Field</h2>

    <p>
      The <ref field="vlan_tci"/> extension can describe more kinds of VLAN
      matches than the other variants.  It is also simpler than the other
      variants.
    </p>

    <field id="MFF_VLAN_TCI" title="VLAN TCI">
      <p>
        For a packet without an 802.1Q header, this field is zero.  For a
        packet with an 802.1Q header, this field is the TCI with the bit in
        CFI's position (marked <code>P</code> for ``present'' below) forced to
        1.  Thus, for a packet in VLAN 9 with priority 7, it has the value
        <code>0xf009</code>:
      </p>

      <diagram>
        <header name="NXM_VLAN_TCI">
          <bits name="PCP" above="3" below="7" width=".6"/>
          <bits name="P" above="1" below="1" width=".2"/>
          <bits name="VID" above="12" below="9" width=".9"/>
          </header>
      </diagram>

      <p>
        Usage examples:
      </p>

      <dl>
        <dt><code>vlan_tci=0</code></dt>
        <dd>
          Match packets without an 802.1Q header.
        </dd>

        <dt><code>vlan_tci=0x1000/0x1000</code></dt>
        <dd>
          Match packets with an 802.1Q header, regardless  of  VLAN
          and priority values.
        </dd>

        <dt><code>vlan_tci=0xf123</code></dt>
        <dd>
          Match packets tagged with priority 7 in VLAN 0x123.
        </dd>

        <dt><code>vlan_tci=0x1123/0x1fff</code></dt>
        <dd>
          Match packets tagged with VLAN 0x123 (and any priority).
        </dd>

        <dt><code>vlan_tci=0x5000/0xf000</code></dt>
        <dd>
          Match packets tagged with priority 2 (in any VLAN).
        </dd>

        <dt><code>vlan_tci=0/0xfff</code></dt>
        <dd>
          Match packets with no 802.1Q header or tagged with VLAN 0
          (and any priority).
        </dd>

        <dt><code>vlan_tci=0x5000/0xe000</code></dt>
        <dd>
          Match packets with no 802.1Q header or tagged with priority 2 (in any VLAN).
        </dd>

        <dt><code>vlan_tci=0/0xefff</code></dt>
        <dd>
          Match packets with no 802.1Q header or tagged with VLAN 0
          and priority 0.
        </dd>
      </dl>

      <p>
        See <cite>VLAN Comparison Chart</cite>, below, for more examples.
      </p>
    </field>

    <h2>VLAN Comparison Chart</h2>

    <p>
      The following table describes each of several possible matching
      criteria on 802.1Q header may be expressed with each variation
      of the VLAN matching fields:
    </p>

    <tbl>
r r r r r.
Criteria        OpenFlow 1.0    OpenFlow 1.1    OpenFlow 1.2+   NXM
\_      \_      \_      \_      \_
[1]     \fL????\fR/\fL1\fR,\fL??\fR/\fL?\fR     \fL????\fR/\fL1\fR,\fL??\fR/\fL?\fR     \fL0000\fR/\fL0000\fR,\fL--\fR  \fL0000\fR/\fL0000\fR
[2]     \fLffff\fR/\fL0\fR,\fL??\fR/\fL?\fR     \fLffff\fR/\fL0\fR,\fL??\fR/\fL?\fR     \fL0000\fR/\fLffff\fR,\fL--\fR  \fL0000\fR/\fLffff\fR
[3]     \fL0xxx\fR/\fL0\fR,\fL??\fR/\fL1\fR     \fL0xxx\fR/\fL0\fR,\fL??\fR/\fL1\fR     \fL1xxx\fR/\fLffff\fR,\fL--\fR  \fL1xxx\fR/\fL1fff\fR
[4]     \fL????\fR/\fL1\fR,\fL0y\fR/\fL0\fR     \fLfffe\fR/\fL0\fR,\fL0y\fR/\fL0\fR     \fL1000\fR/\fL1000\fR,\fL0y\fR  \fLz000\fR/\fLf000\fR
[5]     \fL0xxx\fR/\fL0\fR,\fL0y\fR/\fL0\fR     \fL0xxx\fR/\fL0\fR,\fL0y\fR/\fL0\fR     \fL1xxx\fR/\fLffff\fR,\fL0y\fR  \fLzxxx\fR/\fLffff\fR
.T&amp;
r r c c r.
[6]     (none)  (none)  \fL1001\fR/\fL1001\fR,\fL--\fR  \fL1001\fR/\fL1001\fR
.T&amp;
r r c c c.
[7]     (none)  (none)  (none)  \fL3000\fR/\fL3000\fR
[8]     (none)  (none)  (none)  \fL0000\fR/\fL0fff\fR
[9]     (none)  (none)  (none)  \fL0000\fR/\fLf000\fR
[10]    (none)  (none)  (none)  \fL0000\fR/\fLefff\fR
    </tbl>

    <p>
      All numbers in the table are expressed in hexadecimal.  The
      columns in the table are interpreted as follows:
    </p>

    <dl>
      <dt>Criteria</dt>
      <dd>See the list below.</dd>

      <dt>OpenFlow 1.0</dt>
      <dt>OpenFlow 1.1</dt>
      <dd>
        <literal>wwww/x,yy/z</literal> means VLAN ID match value
        <literal>wwww</literal> with wildcard bit <literal>x</literal>
        and VLAN PCP match value <literal>yy</literal> with wildcard
        bit <literal>z</literal>.  <literal>?</literal> means that the
        given bits are ignored (and conventionally
        <literal>0</literal> for <literal>wwww</literal> or
        <literal>yy</literal>, conventionally <literal>1</literal> for
        <literal>x</literal> or <literal>z</literal>).  ``(none)''
        means that OpenFlow 1.0 (or 1.1) cannot match with these
        criteria.
      </dd>

      <dt>OpenFlow 1.2+</dt>
      <dd>
        <literal>xxxx/yyyy,zz</literal> means <ref field="vlan_vid"/> with
        value <literal>xxxx</literal> and mask <literal>yyyy</literal>, and
        <ref field="vlan_pcp"/> (which is not maskable) with value
        <literal>zz</literal>.  <literal>--</literal> means that <ref
        field="vlan_pcp"/> is omitted.  ``(none)'' means that OpenFlow 1.2
        cannot match with these criteria.
      </dd>

      <dt>NXM</dt>
      <dd>
        <literal>xxxx/yyyy</literal> means <ref field="vlan_tci"/> with value
        <literal>xxxx</literal> and mask <literal>yyyy</literal>.
      </dd>
    </dl>

    <p>
      The matching criteria described by the table are:
    </p>

    <dl>
      <dt>[1]</dt>
      <dd>
        Matches any packet, that is, one without an 802.1Q header or
        with an 802.1Q header with any TCI value.
      </dd>

      <dt>[2]</dt>
      <dd>
        <p>
          Matches only packets without an 802.1Q header.
        </p>

        <p>
          OpenFlow 1.0 doesn't define the behavior if <ref field="dl_vlan"/> is
          set to <code>0xffff</code> and <ref field="dl_vlan_pcp"/> is not
          wildcarded.  (Open vSwitch always ignores <ref field="dl_vlan_pcp"/>
          when <ref field="dl_vlan"/> is set to <code>0xffff</code>.)
        </p>

        <p>
          OpenFlow 1.1 says explicitly to ignore <ref field="dl_vlan_pcp"/>
          when <ref field="dl_vlan"/> is set to <code>0xffff</code>.
        </p>

        <p>
          OpenFlow 1.2 doesn't say how to interpret a match with <ref
          field="vlan_vid"/> value 0 and a mask with
          <code>OFPVID_PRESENT</code> (<code>0x1000</code>) set to 1 and some
          other bits in the mask set to 1 also.  Open vSwitch interprets it the
          same way as a mask of <code>0x1000</code>.
        </p>

        <p>
          Any NXM match with <ref field="vlan_tci"/> value 0 and the CFI bit
          set to 1 in the mask is equivalent to the one listed in the table.
        </p>
      </dd>

      <dt>[3]</dt>
      <dd>
        Matches only packets that have an 802.1Q header with VID
        <literal>xxx</literal> (and any PCP).
      </dd>

      <dt>[4]</dt>
      <dd>
        <p>
          Matches only packets that have an 802.1Q header with PCP
          <literal>y</literal> (and any VID).
        </p>

        <p>
          OpenFlow 1.0 doesn't clearly define the behavior for this
          case.  Open vSwitch implements it this way.
        </p>

        <p>
          In the NXM value, <literal>z</literal> equals
          (<literal>y</literal> &lt;&lt; 1) | 1.
        </p>
      </dd>

      <dt>[5]</dt>
      <dd>
        <p>
          Matches only packets that have an 802.1Q header with VID
          <literal>xxx</literal> and PCP <literal>y</literal>.
        </p>

        <p>
          In the NXM value, <literal>z</literal> equals
          (<literal>y</literal> &lt;&lt; 1) | 1.
        </p>
      </dd>

      <dt>[6]</dt>
      <dd>
        Matches only packets that have an 802.1Q header with an
        odd-numbered VID (and any PCP).  Only possible with OpenFlow
        1.2 and NXM.  (This is just an example; one can match on any
        desired VID bit pattern.)
      </dd>

      <dt>[7]</dt>
      <dd>
        Matches only packets that have an 802.1Q header with an
        odd-numbered PCP (and any VID).  Only possible with NXM.
        (This is just an example; one can match on any desired VID bit
        pattern.)
      </dd>

      <dt>[8]</dt>
      <dd>
        Matches packets with no 802.1Q header or with an 802.1Q header
        with a VID of 0.  Only possible with NXM.
      </dd>

      <dt>[9]</dt>
      <dd>
        Matches packets with no 802.1Q header or with an 802.1Q header
        with a PCP of 0.  Only possible with NXM.
      </dd>

      <dt>[10]</dt>
      <dd>
        Matches packets with no 802.1Q header or with an 802.1Q header
        with both VID and PCP of 0.  Only possible with NXM.
      </dd>
    </dl>
  </group>

  <group title="Layer 2.5: MPLS">
    <p>
      One or more MPLS headers (more commonly called <dfn>MPLS
      labels</dfn>) follow an Ethernet type field that specifies an
      MPLS Ethernet type [RFC 3032].  Ethertype <code>0x8847</code> is
      used for all unicast.  Multicast MPLS is divided into two
      specific classes, one of which uses Ethertype
      <code>0x8847</code> and the other <code>0x8848</code> [RFC
      5332].
    </p>

    <p>
      The most common overall packet format is Ethernet II, shown
      below (SNAP encapsulation may be used but is not ordinarily seen
      in Ethernet networks):
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.75"/>
        <bits name="src" above="48" width="0.75"/>
        <bits name="type" above="16" below="0x8847" width="0.4"/>
      </header>
      <header name="MPLS">
        <bits name="label" above="20" width=".6"/>
        <bits name="TC" above="3" width=".3"/>
        <bits name="S" above="1" width=".1"/>
        <bits name="TTL" above="8" width=".4"/>
      </header>
      <dots/>
    </diagram>

    <p>
      MPLS can be encapsulated inside an 802.1Q header, in which case
      the combination looks like this:
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width=".75"/>
        <bits name="src" above="48" width=".75"/>
      </header>
      <header name="802.1Q">
        <bits name="TPID" above="16" below="0x8100" width=".4"/>
        <bits name="TCI" above="16" width=".4"/>
      </header>
      <header name="Ethertype">
        <bits name="type" above="16" below="0x8847" width=".4"/>
      </header>
      <header name="MPLS">
        <bits name="label" above="20" width=".6"/>
        <bits name="TC" above="3" width=".3"/>
        <bits name="S" above="1" width=".1"/>
        <bits name="TTL" above="8" width=".4"/>
      </header>
      <dots/>
    </diagram>

    <p>
      The fields within an MPLS label are:
    </p>

    <dl>
      <dt>Label, 20 bits.</dt>
      <dd>
        An identifier.
      </dd>

      <dt>Traffic control (TC), 3 bits.</dt>
      <dd>
        Used for quality of service.
      </dd>

      <dt>Bottom of stack (BOS), 1 bit (labeled just ``S'' above).</dt>
      <dd>
        <p>
          0 indicates that another MPLS label follows this one.
        </p>

        <p>
          1 indicates that this MPLS label is the last one in the
          stack, so that some other protocol follows this one.
        </p>
      </dd>

      <dt>Time to live (TTL), 8 bits.</dt>
      <dd>
        <p>
          Each hop across an MPLS network decrements the TTL by 1.  If
          it reaches 0, the packet is discarded.
        </p>

        <p>
          OpenFlow does not make the MPLS TTL available as a match field, but
          actions are available to set and decrement the TTL.  Open vSwitch 2.6
          and later makes the MPLS TTL available as an extension.
        </p>
      </dd>
    </dl>

    <h2>MPLS Label Stacks</h2>

    <p>
      Unlike the other encapsulations supported by OpenFlow and Open vSwitch,
      MPLS labels are routinely used in ``stacks'' two or three deep and
      sometimes even deeper.  Open vSwitch currently supports up to three
      labels.
    </p>

    <p>
      The OpenFlow specification only supports matching on the outermost MPLS
      label at any given time.  To match on the second label, one must first
      ``pop'' the outer label and advance to another OpenFlow table, where the
      inner label may be matched.  To match on the third label, one must pop
      the two outer labels, and so on.  The Open Networking Foundation is
      considering support for directly matching on multiple MPLS labels for
      OpenFlow 1.6.<!-- XXX add EXT-* link -->
    </p>

    <h2>MPLS Inner Protocol</h2>

    <p>
      Unlike all other forms of encapsulation that Open vSwitch and
      OpenFlow support, an MPLS label does not indicate what inner
      protocol it encapsulates.  Different deployments determine the
      inner protocol in different ways [RFC 3032]:
    </p>

    <ul>
      <li>
        A few reserved label values do indicate an inner protocol.
        Label 0, the ``IPv4 Explicit NULL Label,'' indicates inner
        IPv4.  Label 2, the ``IPv6 Explicit NULL Label,'' indicates
        inner IPv6.
      </li>

      <li>
        Some deployments use a single inner protocol consistently.
      </li>

      <li>
        In some deployments, the inner protocol must be inferred from
        the innermost label.
      </li>

      <li>
        In some deployments, the inner protocol must be inferred from
        the innermost label and the encapsulated data, e.g. to
        distinguish between inner IPv4 and IPv6 based on whether the
        first nibble of the inner protocol data are <code>4</code> or
        <code>6</code>.  OpenFlow and Open vSwitch do not currently
        support these cases.
      </li>
    </ul>

    <p>
      Open vSwitch and OpenFlow do not infer the inner protocol, even if
      reserved label values are in use.  Instead, the flow table must specify
      the inner protocol at the time it pops the bottommost MPLS label, using
      the Ethertype argument to the <code>pop_mpls</code> action.
    </p>

    <h2>Field Details</h2>

    <field id="MFF_MPLS_LABEL" title="MPLS Label">
      <p>
        The least significant 20 bits hold the ``label'' field from
        the MPLS label.  Other bits are zero:
      </p>

      <diagram>
        <header name="OXM_OF_MPLS_LABEL">
          <bits name="zero" above="12" below="0" width=".6"/>
          <bits name="label" above="20" width="1.0"/>
        </header>
      </diagram>

      <p>
        Most label values are available for any use by deployments.
        Values under 16 are reserved.
      </p>
    </field>

    <field id="MFF_MPLS_TC" title="MPLS Traffic Class">
      <p>
        The least significant 3 bits hold the TC field from the MPLS
        label.  Other bits are zero:
      </p>

      <diagram>
        <header name="OXM_OF_MPLS_TC">
          <bits name="zero" above="5" below="0" width="1.0"/>
          <bits name="TC" above="3" width=".6"/>
        </header>
      </diagram>

      <p>
        This field is intended for use for Quality of Service (QoS)
        and Explicit Congestion Notification purposes, but its
        particular interpretation is deployment specific.
      </p>

      <p>
        Before 2009, this field was named EXP and reserved for
        experimental use [RFC 5462].
      </p>
    </field>

    <field id="MFF_MPLS_BOS" title="MPLS Bottom of Stack">
      <p>
        The least significant bit holds the BOS field from the MPLS
        label.  Other bits are zero:
      </p>

      <diagram>
        <header name="OXM_OF_MPLS_BOS">
          <bits name="zero" above="7" below="0" width="1.3"/>
          <bits name="BOS" above="1" width=".3"/>
        </header>
      </diagram>

      <p>
        This field is useful as part of processing a series of incoming MPLS
        labels.  A flow that includes a <code>pop_mpls</code> action should
        generally match on <ref field="mpls_bos"/>:
      </p>

      <ul>
        <li>
          When <ref field="mpls_bos"/> is 1, there is another MPLS label
          following this one, so the Ethertype passed to <code>pop_mpls</code>
          should be an MPLS Ethertype.  For example: <code>table=0,
          dl_type=0x8847, mpls_bos=1, actions=pop_mpls:0x8847,
          goto_table:1</code>
        </li>

        <li>
          When <ref field="mpls_bos"/> is 0, this MPLS label is the last one,
          so the Ethertype passed to <code>pop_mpls</code> should be a non-MPLS
          Ethertype such as IPv4.  For example: <code>table=1, dl_type=0x8847,
          mpls_bos=0, actions=pop_mpls:0x0800, goto_table:2</code>
        </li>
      </ul>
    </field>

    <field id="MFF_MPLS_TTL" title="MPLS Time-to-Live">
      <p>
        Holds the 8-bit time-to-live field from the MPLS label:
      </p>

      <diagram>
        <header name="NXM_NX_MPLS_TTL">
          <bits name="TTL" above="8" width=".4"/>
        </header>
      </diagram>
    </field>
  </group>

  <group title="Layer 3: IPv4 and IPv6">
    <h2>IPv4 Specific Fields</h2>

    <p>
      These fields are applicable only to IPv4 flows, that is, flows that match
      on the IPv4 Ethertype <code>0x0800</code>.
    </p>

    <field id="MFF_IPV4_SRC" title="IPv4 Source Address">
      <p>
        The source address from the IPv4 header:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" width="0.4"/>
          <bits name="src" above="32" width="0.4" fill="yes"/>
          <bits name="dst" above="32" width="0.4"/>
        </header>
        <dots/>
      </diagram>

      <p>
        For historical reasons, in an ARP or RARP flow, Open vSwitch interprets
        matches on <code>nw_src</code> as actually referring to the ARP SPA.
      </p>
    </field>

    <field id="MFF_IPV4_DST" title="IPv4 Destination Address">
      <p>
      The destination address from the IPv4 header:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x800" width="0.4"/>
        </header>
        <header name="IPv4">
          <bits name="..." width="0.4"/>
          <bits name="proto" above="8" width="0.4"/>
          <bits name="src" above="32" width="0.4"/>
          <bits name="dst" above="32" width="0.4" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        For historical reasons, in an ARP or RARP flow, Open vSwitch interprets
        matches on <code>nw_dst</code> as actually referring to the ARP TPA.
      </p>
    </field>

    <h2>IPv6 Specific Fields</h2>

    <p>
      These fields apply only to IPv6 flows, that is, flows that match
      on the IPv6 Ethertype <code>0x86dd</code>.
    </p>

    <field id="MFF_IPV6_SRC" title="IPv6 Source Address">
      <p>
        The source address from the IPv6 header:
      </p>

      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x86dd" width="0.4"/>
        </header>
        <header name="IPv6">
          <bits name="..." width="0.4"/>
          <bits name="next" above="8" width="0.3"/>
          <bits name="src" above="128" width="0.8" fill="yes"/>
          <bits name="dst" above="128" width="0.8"/>
        </header>
        <dots/>
      </diagram>

      <p>
        Open vSwitch 1.8 added support for bitwise matching; earlier versions
        supported only CIDR masks.
      </p>
    </field>
    <field id="MFF_IPV6_DST" title="IPv6 Destination Address">
      <p>
        The destination address from the IPv6 header:
      </p>
      <diagram>
        <header name="Ethernet">
          <bits name="dst" above="48" width="0.4"/>
          <bits name="src" above="48" width="0.4"/>
          <bits name="type" above="16" below="0x86dd" width="0.4"/>
        </header>
        <header name="IPv6">
          <bits name="..." width="0.4"/>
          <bits name="next" above="8" width="0.3"/>
          <bits name="src" above="128" width="0.8"/>
          <bits name="dst" above="128" width="0.8" fill="yes"/>
        </header>
        <dots/>
      </diagram>

      <p>
        Open vSwitch 1.8 added support for bitwise matching; earlier versions
        supported only CIDR masks.
      </p>
    </field>
    <field id="MFF_IPV6_LABEL" title="IPv6 Flow Label">
      <p>
        The least significant 20 bits hold the flow label field from
        the IPv6 header.  Other bits are zero:
      </p>

      <diagram>
        <header name="OXM_OF_IPV6_FLABEL">
          <bits name="zero" above="12" below="0" width=".6"/>
          <bits name="label" above="20" width="1.0"/>
        </header>
      </diagram>
    </field>

    <h2>IPv4/IPv6 Fields</h2>

    <p>
      These fields exist with at least approximately the same meaning in both
      IPv4 and IPv6, so they are treated as a single field for matching
      purposes.  Any flow that matches on the IPv4 Ethertype
      <code>0x0800</code> or the IPv6 Ethertype <code>0x86dd</code> may match
      on these fields.
    </p>

    <field id="MFF_IP_PROTO" title="IPv4/v6 Protocol">
      <p>
        Matches the IPv4 or IPv6 protocol type.
      </p>

      <p>
        For historical reasons, in an ARP or RARP flow, Open vSwitch interprets
        matches on <code>nw_proto</code> as actually referring to the ARP
        opcode.  The ARP opcode is a 16-bit field, so for matching purposes ARP
        opcodes greater than 255 are treated as 0; this works adequately
        because in practice ARP and RARP only use opcodes 1 through 4.
      </p>
    </field>

    <field id="MFF_IP_TTL" title="IPv4/v6 TTL/Hop Limit">
      The main reason to match on the TTL or hop limit field is to detect
      whether a <code>dec_ttl</code> action will fail due to a TTL exceeded
      error.  Another way that a controller can detect TTL exceeded is to
      listen for <code>OFPR_INVALID_TTL</code> ``packet-in'' messages via
      OpenFlow.
    </field>
    
    <field id="MFF_IP_FRAG" title="IPv4/v6 Fragment Bitmask">
      <p>
        Specifies what kinds of IP fragments or non-fragments to match.  The
        value for this field is most conveniently specified as one of the
        following:
      </p>

      <dl>
        <dt><code>no</code></dt>
        <dd>
          Match only non-fragmented packets.
        </dd>

        <dt><code>yes</code></dt>
        <dd>
          Matches all fragments.
        </dd>

        <dt><code>first</code></dt>
        <dd>
          Matches only fragments with offset 0.
        </dd>

        <dt><code>later</code></dt>
        <dd>
          Matches only fragments with nonzero offset.
        </dd>

        <dt><code>not_later</code></dt>
        <dd>
          Matches non-fragmented packets and fragments with zero offset.
        </dd>
      </dl>

      <p>
        The field is internally formatted as 2 bits: bit 0 is 1 for an IP
        fragment with any offset (and otherwise 0), and bit 1 is 1 for an IP
        fragment with nonzero offset (and otherwise 0), like so:
      </p>

      <diagram>
        <header name="NXM_NX_IP_FRAG">
          <bits name="zero" above="6" below="0" width=".9"/>
          <bits name="later" above="1" width=".3"/>
          <bits name="any" above="1" width=".3"/>
        </header>
      </diagram>

      <p>
        Even though 2 bits have 4 possible values, this field only uses 3 of
        them:
      </p>

      <ul>
        <li>
          A packet that is not an IP fragment has value 0.
        </li>

        <li>
          A packet that is an IP fragment with offset 0 (the first fragment)
          has bit 0 set and thus value 1.
        </li>

        <li>
          A packet that is an IP fragment with nonzero offset has bits 0 and 1
          set and thus value 3.
        </li>
      </ul>

      <p>
        The switch may reject matches against values that can never appear.
      </p>

      <p>
        It is important to understand how this field interacts with the
        OpenFlow fragment handling mode:
      </p>

      <ul>
        <li>
          In <code>OFPC_FRAG_DROP</code> mode, the OpenFlow switch drops all IP
          fragments before they reach the flow table, so every packet that is
          available for matching will have value 0 in this field.
        </li>

        <li>
          Open vSwitch does not implement <code>OFPC_FRAG_REASM</code> mode,
          but if it did then IP fragments would be reassembled before they
          reached the flow table and again every packet available for matching
          would always have value 0.
        </li>

        <li>
          In <code>OFPC_FRAG_NORMAL</code> mode, all three values are possible,
          but OpenFlow 1.0 says that fragments' transport ports are always 0,
          even for the first fragment, so this does not provide much extra
          information.
        </li>

        <li>
          In <code>OFPC_FRAG_NX_MATCH</code> mode, all three values are
          possible.  For fragments with offset 0, Open vSwitch makes L4 header
          information available.
        </li>
      </ul>
      
      <p>
        Thus, this field is likely to be most useful for an Open vSwitch switch
        configured in <code>OFPC_FRAG_NX_MATCH</code> mode.  See the
        description of the <code>set-frags</code> command in
        <code>ovs-ofctl</code>(8), for more details.
      </p>
    </field>

    <h3>IPv4/IPv6 TOS Fields</h3>

    <p>
      IPv4 and IPv6 contain a one-byte ``type of service'' or TOS field that
      has the following format:
    </p>

    <diagram>
      <header name="type of service">
        <bits name="DSCP" above="6" width=".9"/>
        <bits name="ECN" above="2" width=".3"/>
      </header>
    </diagram>

    <field id="MFF_IP_DSCP" title="IPv4/v6 DSCP (Bits 2-7)">
      <p>
        This field is the TOS byte with the two ECN bits cleared to 0:
      </p>

      <diagram>
        <header name="NXM_OF_IP_TOS">
          <bits name="DSCP" above="6" width=".9"/>
          <bits name="zero" above="2" below="0" width=".3"/>
        </header>
      </diagram>
    </field>
    <field id="MFF_IP_DSCP_SHIFTED" title="IPv4/v6 DSCP (Bits 0-5)">
      <p>
        This field is the TOS byte shifted right to put the DSCP bits in the
        6 least-significant bits:
      </p>

      <diagram>
        <header name="OXM_OF_IP_DSCP">
          <bits name="zero" above="2" below="0" width=".3"/>
          <bits name="DSCP" above="6" width=".9"/>
        </header>
      </diagram>
    </field>
    <field id="MFF_IP_ECN" title="IPv4/v6 ECN">
      <p>
        This field is the TOS byte with the DSCP bits cleared to 0:
      </p>

      <diagram>
        <header name="OXM_OF_IP_ECN">
          <bits name="zero" above="6" below="0" width=".9"/>
          <bits name="ECN" above="2" width=".35"/>
        </header>
      </diagram>
    </field>

  </group>

  <group title="Layer 3: ARP">
    <p>
      In theory, Address Resolution Protocol, or ARP, is a generic protocol
      generic protocol that can be used to obtain the hardware address that
      corresponds to any higher-level protocol address.  In contemporary usage,
      ARP is used only in Ethernet networks to obtain the Ethernet address for
      a given IPv4 address.  OpenFlow and Open vSwitch only support this usage
      of ARP.  For this use case, an ARP packet has the following format, with
      the ARP fields exposed as Open vSwitch fields highlighted:
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x806" width="0.4"/>
      </header>
      <header name="ARP">
        <bits name="hrd" above="16" below="1" width=".3"/>
        <bits name="pro" above="16" below="0x800" width=".3"/>
        <bits name="hln" above="8" below="6" width=".2"/>
        <bits name="pln" above="8" below="4" width=".2"/>
        <bits name="op" above="16" width=".2" fill="yes"/>
        <bits name="sha" above="48" width="0.5" fill="yes"/>
        <bits name="spa" above="16" width="0.3" fill="yes"/>
        <bits name="tha" above="48" width="0.5" fill="yes"/>
        <bits name="tpa" above="16" width="0.3" fill="yes"/>
      </header>
    </diagram>

    <p>
      The ARP fields are also used for RARP, the Reverse Address Resolution
      Protocol, which shares ARP's wire format.
    </p>

    <field id="MFF_ARP_OP" title="ARP Opcode">
      Even though this is a 16-bit field, Open vSwitch does not support ARP
      opcodes greater than 255; it treats them to zero.  This works adequately
      because in practice ARP and RARP only use opcodes 1 through 4.
    </field>

    <field id="MFF_ARP_SPA" title="ARP Source IPv4 Address"/>
    <field id="MFF_ARP_TPA" title="ARP Target IPv4 Address"/>
    <field id="MFF_ARP_SHA" title="ARP Source Ethernet Address"/>
    <field id="MFF_ARP_THA" title="ARP Target Ethernet Address"/>
  </group>

  <group title="Layer 4: TCP, UDP, and SCTP">
    <p>
      For matching purposes, no distinction is made whether these protocols are
      encapsulated within IPv4 or IPv6.
    </p>

    <h2>TCP</h2>

    <p>
      The following diagram shows TCP within IPv4.  Open vSwitch also supports
      TCP in IPv6.  Only TCP fields implemented as Open vSwitch fields are
      shown:
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x800" width="0.4"/>
      </header>
      <header name="IPv4">
        <bits name="..." width="0.4"/>
        <bits name="proto" above="8" below="6" width="0.3"/>
        <bits name="src" above="32" width="0.4"/>
        <bits name="dst" above="32" width="0.4"/>
      </header>
      <header name="TCP">
        <bits name="src" above="16" width=".2"/>
        <bits name="dst" above="16" width=".2"/>
        <bits name="..." width=".75"/>
        <bits name="flags" above="12" width=".3"/>
        <bits name="..." width=".6"/>
      </header>
      <dots/>
    </diagram>
    <field id="MFF_TCP_SRC" title="TCP Source Port">
      Open vSwitch 1.6 added support for bitwise matching.
    </field>
    <field id="MFF_TCP_DST" title="TCP Destination Port">
      Open vSwitch 1.6 added support for bitwise matching.
    </field>
    <field id="MFF_TCP_FLAGS" title="TCP Flags">
      <p>
        This field holds the TCP flags.  TCP currently defines 9 flag bits.  An
        additional 3 bits are reserved.  For more information, see [RFC 793],
        [RFC 3168], and [RFC 3540].
      </p>

      <p>
        Matches on this field are most conveniently written in terms of
        symbolic names (given in the diagram below), each preceded by either
        <code>+</code> for a flag that must be set, or <code>-</code> for a
        flag that must be unset, without any other delimiters between the
        flags.  Flags not mentioned are wildcarded.  For example,
        <code>tcp,tcp_flags=+syn-ack</code> matches TCP SYNs that are not ACKs,
        and <code>tcp,tcp_flags=+[200]</code> matches TCP packets with the
        reserved [200] flag set.  Matches can also be written as
        <code><var>flags</var>/<var>mask</var></code>, where <var>flags</var>
        and <var>mask</var> are 16-bit numbers in decimal or in hexadecimal
        prefixed by <code>0x</code>.
      </p>

      <p>
        The flag bits are:
      </p>

      <diagram>
        <header>
          <bits name="zero" above="4" below="0" width=".9"/>
        </header>
        <nospace/>
        <header name="reserved">
          <bits name="[800]" above="1" width=".35"/>
          <bits name="[400]" above="1" width=".35"/>
          <bits name="[200]" above="1" width=".35"/>
        </header>
        <nospace/>
        <header name="later RFCs">
          <bits name="NS" above="1" width=".35"/>
          <bits name="CWR" above="1" width=".35"/>
          <bits name="ECE" above="1" width=".35"/>
        </header>
        <nospace/>
        <header name="RFC 793">
          <bits name="URG" above="1" width=".35"/>
          <bits name="ACK" above="1" width=".35"/>
          <bits name="PSH" above="1" width=".35"/>
          <bits name="RST" above="1" width=".35"/>
          <bits name="SYN" above="1" width=".35"/>
          <bits name="FIN" above="1" width=".35"/>
        </header>
      </diagram>
    </field>

    <h2>UDP</h2>

    <p>
      The following diagram shows UDP within IPv4.  Open vSwitch also supports
      UDP in IPv6.  Only UDP fields that Open vSwitch exposes as fields are
      shown:
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x800" width="0.4"/>
      </header>
      <header name="IPv4">
        <bits name="..." width="0.4"/>
        <bits name="proto" above="8" below="17" width="0.3"/>
        <bits name="src" above="32" width="0.4"/>
        <bits name="dst" above="32" width="0.4"/>
      </header>
      <header name="UDP">
        <bits name="src" above="16" width=".2"/>
        <bits name="dst" above="16" width=".2"/>
        <bits name="..." width=".4"/>
      </header>
      <dots/>
    </diagram>
    <field id="MFF_UDP_SRC" title="UDP Source Port"/>
    <field id="MFF_UDP_DST" title="UDP Destination Port"/>

    <h2>SCTP</h2>

    <p>
      The following diagram shows SCTP within IPv4.  Open vSwitch also supports
      SCTP in IPv6.  Only SCTP fields that Open vSwitch exposes as fields are
      shown:
    </p>

    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x800" width="0.4"/>
      </header>
      <header name="IPv4">
        <bits name="..." width="0.4"/>
        <bits name="proto" above="8" below="132" width="0.3"/>
        <bits name="src" above="32" width="0.4"/>
        <bits name="dst" above="32" width="0.4"/>
      </header>
      <header name="SCTP">
        <bits name="src" above="16" width=".2"/>
        <bits name="dst" above="16" width=".2"/>
        <bits name="..." width=".8"/>
      </header>
      <dots/>
    </diagram>
    <field id="MFF_SCTP_SRC" title="SCTP Source Port"/>
    <field id="MFF_SCTP_DST" title="SCTP Destination Port"/>
  </group>

  <group title="Layer 4: ICMPv4 and ICMPv6">
    <h2>ICMPv4</h2>
    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x800" width="0.4"/>
      </header>
      <header name="IPv4">
        <bits name="..." width="0.4"/>
        <bits name="proto" above="8" below="1" width="0.3"/>
        <bits name="src" above="32" width="0.4"/>
        <bits name="dst" above="32" width="0.4"/>
      </header>
      <header name="ICMPv4">
        <bits name="type" above="8" width=".3"/>
        <bits name="code" above="8" width=".3"/>
        <bits name="..." width=".8"/>
      </header>
      <dots/>
    </diagram>
    <field id="MFF_ICMPV4_TYPE" title="ICMPv4 Type">
      <p>
        For historical reasons, in an ICMPv4 flow, Open vSwitch interprets
        matches on <code>tp_src</code> as actually referring to the ICMP type.
      </p>
    </field>
    <field id="MFF_ICMPV4_CODE" title="ICMPv4 Code">
      <p>
        For historical reasons, in an ICMPv4 flow, Open vSwitch interprets
        matches on <code>tp_dst</code> as actually referring to the ICMP code.
      </p>
    </field>

    <h2>ICMPv6</h2>
    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x86dd" width="0.4"/>
      </header>
      <header name="IPv6">
        <bits name="..." width="0.2"/>
        <bits name="next" above="8" below="58" width="0.3"/>
        <bits name="src" above="128" width="0.4"/>
        <bits name="dst" above="128" width="0.4"/>
      </header>
      <header name="ICMPv6">
        <bits name="type" above="8" width=".3"/>
        <bits name="code" above="8" width=".3"/>
        <bits name="..." width=".8"/>
      </header>
      <dots/>
    </diagram>
    <field id="MFF_ICMPV6_TYPE" title="ICMPv6 Type"/>
    <field id="MFF_ICMPV6_CODE" title="ICMPv6 Code"/>

    <h2>ICMPv6 Neighbor Discovery</h2>
    <diagram>
      <header name="Ethernet">
        <bits name="dst" above="48" width="0.4"/>
        <bits name="src" above="48" width="0.4"/>
        <bits name="type" above="16" below="0x86dd" width="0.4"/>
      </header>
      <header name="IPv6">
        <bits name="..." width="0.2"/>
        <bits name="next" above="8" below="58" width="0.3"/>
        <bits name="src" above="128" width="0.4"/>
        <bits name="dst" above="128" width="0.4"/>
      </header>
      <header name="ICMPv6">
        <bits name="type" above="8" below="135/136" width=".3"/>
        <bits name="code" above="8" below="0" width=".3"/>
        <bits name="..." width=".8"/>
      </header>
      <header name="ICMPv6 ND">
        <bits name="target" above="128" width=".4"/>
        <bits name="option ..." width=".6"/>
      </header>
    </diagram>
    <field id="MFF_ND_TARGET" title="ICMPv6 Neighbor Discovery Target IPv6"/>
    <field id="MFF_ND_SLL"
           title="ICMPv6 Neighbor Discovery Source Ethernet Address"/>
    <field id="MFF_ND_TLL"
           title="ICMPv6 Neighbor Discovery Target Ethernet Address"/>
  </group>

  <h1>References</h1>

  <dl>
    <dt>Casado</dt>
    <dd>
      M. Casado, M. J. Freedman, J. Pettit, J. Luo, N. McKeown, and
      S. Shenker, ``Ethane: Taking Control of the Enterprise,''
      Computer Communications Review, October 2007.
    </dd>

    <dt>EXT-56</dt>
    <dd>
      J. Tonsing, ``Permit one of a set of prerequisites to apply, e.g. don't
      preclude non-Ethernet media,'' <url
      href="https://rs.opennetworking.org/bugs/browse/EXT-56"/> (ONF
      members only).
    </dd>

    <dt>EXT-112</dt>
    <dd>
      J. Tourrilhes, ``Support non-Ethernet packets throughout the
      pipeline,'' <url
      href="https://rs.opennetworking.org/bugs/browse/EXT-112"/> (ONF
      members only).
    </dd>

    <dt>EXT-134</dt>
    <dd>
      J. Tourrilhes, ``Match first nibble of the MPLS payload,'' <url
      href="https://rs.opennetworking.org/bugs/browse/EXT-134"/> (ONF
      members only).
    </dd>

    <dt>Geneve</dt>
    <dd>
      J. Gross, I. Ganga, and T. Sridhar, editors, ``Geneve: Generic Network
      Virtualization Encapsulation,'' <url
      href="https://datatracker.ietf.org/doc/draft-ietf-nvo3-geneve/"/>.
    </dd>

    <dt>IEEE OUI</dt>
    <dd>
      IEEE Standards Association, ``MAC Address Block Large (MA-L),''
      <url
      href="https://standards.ieee.org/develop/regauth/oui/index.html"/>.
    </dd>

    <dt>NSH</dt>
    <dd>
      P. Quinn and U. Elzur, editors, ``Network Service Header,'' <url
      href="https://datatracker.ietf.org/doc/draft-ietf-sfc-nsh/"/>.
    </dd>

    <dt>OpenFlow 1.0.1</dt>
    <dd>
      Open Networking Foundation, ``OpenFlow Switch Errata, Version
      1.0.1,'' June 2012.
    </dd>

    <dt>OpenFlow 1.1</dt>
    <dd>
      OpenFlow Consortium, ``OpenFlow Switch Specification Version
      1.1.0 Implemented (Wire Protocol 0x02),'' February 2011.
    </dd>

    <dt>OpenFlow 1.5</dt>
    <dd>
      Open Networking Foundation, ``OpenFlow Switch Specification Version
      1.5.0 (Protocol version 0x06),'' December 2014.
    </dd>

    <dt>OpenFlow Extensions 1.3.x Package 2</dt>
    <dd>
      Open Networking Foundation, ``OpenFlow Extensions 1.3.x Package 2,''
      December 2013.
    </dd>

    <dt>TCP Flags Match Field Extension</dt>
    <dd>
      Open Networking Foundation, ``TCP flags match field Extension,'' December
      2014.  In [OpenFlow Extensions 1.3.x Package 2].
    </dd>

    <dt>Pepelnjak</dt>
    <dd>
      I. Pepelnjak, ``OpenFlow and Fermi Estimates,'' <url
      href="http://blog.ipspace.net/2013/09/openflow-and-fermi-estimates.html"/>.
    </dd>

    <dt>RFC 793</dt>
    <dd>
      ``Transmission Control Protocol,'' <url
      href="http://www.ietf.org/rfc/rfc793.txt"/>.
    </dd>

    <dt>RFC 3032</dt>
    <dd>
       E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D. Farinacci,
       T. Li, and A. Conta, ``MPLS Label Stack Encoding,'' <url
       href="http://www.ietf.org/rfc/rfc3032.txt"/>.
    </dd>

    <dt>RFC 3168</dt>
    <dd>
      K. Ramakrishnan, S. Floyd, and D. Black, ``The Addition of Explicit
      Congestion Notification (ECN) to IP,'' <url href="https://tools.ietf.org/html/rfc3168"/>.
    </dd>

    <dt>RFC 3540</dt>
    <dd>
      N. Spring, D. Wetherall, and D. Ely, ``Robust Explicit Congestion
      Notification (ECN) Signaling with Nonces,'' <url
      href="https://tools.ietf.org/html/rfc3540"/>.
    </dd>

    <dt>RFC 4632</dt>
    <dd>
      V. Fuller and T. Li, ``Classless Inter-domain Routing (CIDR): The
      Internet Address Assignment and Aggregation Plan,'' <url
      href="https://tools.ietf.org/html/rfc4632"/>.
    </dd>

    <dt>RFC 5462</dt>
    <dd>
      L. Andersson and R. Asati, ``Multiprotocol Label Switching
      (MPLS) Label Stack Entry: ``EXP'' Field Renamed to ``Traffic
      Class'' Field,'' <url
      href="http://www.ietf.org/rfc/rfc5462.txt"/>.
    </dd>

    <dt>RFC 6830</dt>
    <dd>
      D. Farinacci, V. Fuller, D. Meyer, and D. Lewis, ``The
      Locator/ID Separation Protocol (LISP),'' <url
      href="http://www.ietf.org/rfc/rfc6830.txt"/>.
    </dd>

    <dt>RFC 7348</dt>
    <dd>
      M. Mahalingam, D. Dutt, K. Duda, P. Agarwal, L. Kreeger, T. Sridhar,
      M. Bursell, and C. Wright, ``Virtual eXtensible Local Area Network
      (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over
      Layer 3 Networks, '' <url href="https://tools.ietf.org/html/rfc7348"/>.
    </dd>

    <dt>Srinivasan</dt>
    <dd>
      V. Srinivasan, S. Suriy, and G. Varghese, ``Packet
      Classification using Tuple Space Search,'' SIGCOMM 1999.
    </dd>

    <dt>Pagiamtzis</dt>
    <dd>
      K. Pagiamtzis and A. Sheikholeslami, ``Content-addressable
      memory (CAM) circuits and architectures: A tutorial and
      survey,'' IEEE Journal of Solid-State Circuits, vol. 41, no. 3,
      pp. 712-727, March 2006.
    </dd>

    <dt>VXLAN Group Policy Option</dt>
    <dd>
      M. Smith and L. Kreeger, `` VXLAN Group Policy Option.'' Internet-Draft.
      <url href="https://tools.ietf.org/html/draft-smith-vxlan-group-policy"/>.
    </dd>
  </dl>

  <h1>Authors</h1>

  <p>
    Ben Pfaff, with advice from Justin Pettit and Jean Tourrilhes.
  </p>

</fields>

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