This document aims to comprehensively document all of the fields,
both standard and non-standard, supported by OpenFlow or Open
vSwitch, regardless of origin.
Fields
A field is a property of a packet. Most familiarly, data fields
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.
ip_frag describes whether a packet is a fragment but it is not
copied directly from the IP header.
Data fields that are always present as a consequence of the basic
networking technology in use are called called root fields. 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).
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,
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
options:packet_type to ptap in the Interface 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 ovs-vwitchd.conf.db(5) for more information.
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 applicable when it is present in a packet, and
inapplicable when it is not. (These are not standard terms.) We
refer to the conditions that determine whether a field is
applicable as prerequisites. 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).
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 Matching, 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 packet_type for ARP in a packet type-aware bridge.
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.
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 Layer 2.5:
MPLS for more information.
OpenFlow and Open vSwitch support some fields other than data
fields. Metadata fields 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. Register fields act like variables: they give an OpenFlow
switch space for temporary storage while processing a packet.
Existing metadata and register fields have no prerequisites.
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.
Matching
The most important use of fields in OpenFlow is matching, to
determine whether particular field values agree with a set of
constraints called a match. 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:
Exact match, e.g. nw_src=10.1.2.3
Only a particular value of the field is matched;
for example, only one particular source IP address.
Exact matches are written as field=value. The forms
accepted for value depend on the field.
All fields support exact matches.
Bitwise match, e.g. nw_src=10.1.0.0/255.255.0.0
Specific bits in the field must have specified
values; for example, only source IP addresses in a
particular subnet. Bitwise matches are written as
field=value/mask, where value and mask take one of
the forms accepted for an exact match on field.
Some fields accept other forms for bitwise matches;
for example, nw_src=10.1.0.0/255.255.0.0 may also
be written nw_src=10.1.0.0/16.
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.)
Wildcard, e.g. ``any nw_src''
The value of the field is not constrained.
Wildcarded fields may be written as field=*,
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.)
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.
Some types of matches on individual fields cannot be expressed
directly with OpenFlow and Open vSwitch. These can be expressed
indirectly:
Set match, e.g. ``tcp_dst ∈ {80, 443, 8080}''
The value of a field is one of a specified set of
values; for example, the TCP destination port is
80, 443, or 8080.
For matches used in flows (see Flows, below),
multiple flows can simulate set matches.
Range match, e.g. ``1000 ≤ tcp_dst ≤ 1999''
The value of the field must lie within a numerical
range, for example, TCP destination ports between
1000 and 1999.
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:
01111101000
11111001111
The following series of bitwise matches will match
1000 and 1999 and all the values in between:
01111101xxx
0111111xxxx
10xxxxxxxxx
110xxxxxxxx
1110xxxxxxx
11110xxxxxx
1111100xxxx
which can be written as the following matches:
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
Inequality match, e.g. ``tcp_dst ≠ 80''
The value of the field differs from a specified
value, for example, all TCP destination ports
except 80.
An inequality match on an n-bit field can be
expressed as a disjunction of n 1-bit matches. For
example, the inequality match ``vlan_pcp ≠ 5'' can
be expressed as ``vlan_pcp = 0/4 or vlan_pcp = 2/2
or vlan_pcp = 0/1.'' For matches used in flows (see
Flows, 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.
Alternatively, an inequality match may be converted
to a pair of range matches, e.g. tcp_src ≠ 80 may
be expressed as ``0 ≤ tcp_src < 80 or 80 < tcp_src
≤ 65535'', and then each range match may in turn be
converted to a bitwise match.
Conjunctive match, e.g. ``tcp_src ∈ {80, 443, 8080} and
tcp_dst ∈ {80, 443, 8080}''
As an OpenFlow extension, Open vSwitch supports
matching on conditions on conjunctions of the
previously mentioned forms of matching. See the
documentation for conj_id for more information.
All of these supported forms of matching are special cases of
bitwise matching. In some cases this influences the design of
field values. 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.
Shorthands
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:
eth packet_type=(0,0) (Open vSwitch 2.8 and later)
ip eth_type=0x0800
ipv6 eth_type=0x86dd
icmp eth_type=0x0800,ip_proto=1
icmp6 eth_type=0x86dd,ip_proto=58
tcp eth_type=0x0800,ip_proto=6
tcp6 eth_type=0x86dd,ip_proto=6
udp eth_type=0x0800,ip_proto=17
udp6 eth_type=0x86dd,ip_proto=17
sctp eth_type=0x0800,ip_proto=132
sctp6 eth_type=0x86dd,ip_proto=132
arp eth_type=0x0806
rarp eth_type=0x8035
mpls eth_type=0x8847
mplsm eth_type=0x8848
Evolution of OpenFlow Fields
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.
OpenFlow 1.0
OpenFlow 1.0 defined the OpenFlow protocol format of a match as a
fixed-length data structure that could match on the following
fields:
• Ingress port.
• Ethernet source and destination MAC.
• Ethertype (with a special value to match frames
that lack an Ethertype).
• VLAN ID and priority.
• IPv4 source, destination, protocol, and DSCP.
• TCP source and destination port.
• UDP source and destination port.
• ICMPv4 type and code.
• ARP IPv4 addresses (SPA and TPA) and opcode.
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 Matching), with additional
support for CIDR prefix matching on the IPv4 source and
destination fields.
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.)
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.
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 microflows, 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].)
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.)
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.
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.)
Inapplicable Fields
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.
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.
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.)
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.
OpenFlow 1.1
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 OFPMT_STANDARD
(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.
OpenFlow 1.1 added support for the following fields:
• SCTP source and destination port.
• MPLS label and traffic control (TC) fields.
• One 64-bit register (named ``metadata'').
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.
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].
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.
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.
Physical Ingress Port
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.
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.
OpenFlow 1.2
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.
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 OFPMT_OXM (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.)
OpenFlow 1.2 added support for the following fields:
• ARP hardware addresses (SHA and THA).
• IPv4 ECN.
• IPv6 source and destination addresses, flow label,
DSCP, ECN, and protocol.
• TCP, UDP, and SCTP port numbers when encapsulated
inside IPv6.
• ICMPv6 type and code.
• ICMPv6 Neighbor Discovery target address and source
and target Ethernet addresses.
The OpenFlow 1.2 format, called OXM (OpenFlow Extensible Match),
was modeled closely on an extension to OpenFlow 1.0 introduced in
Open vSwitch 1.1 called NXM (Nicira Extended Match). Each OXM or
NXM TLV has the following format:
type
<---------------->
16 7 1 8 length bytes
+------------+-----+--+------+ +------------+
|vendor/class|field|HM|length| | body |
+------------+-----+--+------+ +------------+
The most significant 16 bits of the NXM or OXM header, called
vendor by NXM and class 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:
0x0000 (OFPXMC_NXM_0).
0x0001 (OFPXMC_NXM_1).
Reserved for NXM compatibility.
0x0002 to 0x7fff
Reserved for allocation to ONF members, but none yet
assigned.
0x8000 (OFPXMC_OPENFLOW_BASIC)
Used for most standard OpenFlow fields.
0x8001 (OFPXMC_PACKET_REGS)
Used for packet register fields in OpenFlow 1.5 and
later.
0x8002 to 0xfffe
Reserved for the OpenFlow specification.
0xffff (OFPXMC_EXPERIMENTER)
Experimental use.
When class is 0xffff, the OXM header is extended to 64 bits by
using the first 32 bits of the body as an experimenter 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.
type experimenter
<----------> <---------->
16 7 1 8 8 24 (length - 4) bytes
+------+-----+--+------+ +------+-----+ +------------------+
|class |field|HM|length| | zero | OUI | | body |
+------+-----+--+------+ +------+-----+ +------------------+
0xffff 0x00
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:
0x4f4e4600 (ONFOXM_ET)
Used by official Open Networking Foundation
extensions in OpenFlow 1.3 and later. e.g. [TCP
Flags Match Field Extension].
0x005ad650 (NXOXM_NSH)
Used by Open vSwitch for NSH extensions, in the
absence of an official ONF-assigned class. (This
OUI is randomly generated.)
Taken as a unit, class (or vendor), field, and experimenter (when
present) uniquely identify a particular field.
When hasmask (abbreviated HM 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.
When hasmask 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.
• 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.
• 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.
• 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 OFPBMC_BAD_WILDCARDS, as
required by OpenFlow 1.3 and later.
The length identifies the number of bytes in the body, including
the 4-byte experimenter header, if it is present. Each OXM TLV
has a fixed length; that is, given class, field, experimenter (if
present), and hasmask, length is a constant. The length is
included explicitly to allow software to minimally parse OXM TLVs
of unknown types.
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.
A given field may be matched only once in a series of OXM TLVs.
OpenFlow 1.3
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:
• Tunnel ID for ports associated with e.g. VXLAN or
keyed GRE.
• MPLS ``bottom of stack'' (BOS) bit.
OpenFlow 1.3 also added OXMs for the following fields not
documented here and not yet implemented by Open vSwitch:
• IPv6 extension header handling.
• PBB I-SID.
OpenFlow 1.4
OpenFlow 1.4 added OXMs for the following fields not documented
here and not yet implemented by Open vSwitch:
• PBB UCA.
OpenFlow 1.5
OpenFlow 1.5 added OXMs for the following fields supported by
Open vSwitch:
• Packet type.
• TCP flags.
• Packet registers.
• The output port in the OpenFlow action set.
