ovs-fields - Man Page

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.

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.

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

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 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.

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 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:

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.

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 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 added OXMs for the following fields not documented here and not yet implemented by Open vSwitch:

• PBB UCA.

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.

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 a, b, c, or d to the OpenFlow controller:

      ip,ip_src=a actions=controller
      ip,ip_src=b actions=controller
      ip,ip_src=c actions=controller
      ip,ip_src=d actions=controller

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

      ip,ip_dst=e actions=controller
      ip,ip_dst=f actions=controller
      ip,ip_dst=g actions=controller
      ip,ip_dst=h actions=controller

Installing all of the above flows in a single flow table yields a disjunctive effect: a packet is sent to the controller if ip_src ∈ {a,b,c,d} or ip_dst ∈ {e,f,g,h} (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.)

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

The conjunction action is a solution for conjunctive matches that is built into Open vSwitch. A conjunction 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.

To implement a conjunctive flow with conjunction, assign the conjunctive flow a 32-bit id, which must be unique within an OpenFlow table. Assign each of the n ≥ 2 dimensions a unique number from 1 to n; the ordering is unimportant. Add one flow to the OpenFlow flow table for each possible value of each dimension with conjunction(id, k/n) as the flow’s actions, where k 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 conj_id to the specified id and then again searching the flow table.

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), and 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 conj_id against the conjunction ID (1234), finding the first flow listed below.

      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)

Many subtleties exist:

• In the example above, every flow in a single dimension has the same form, that is, dimension 1 matches on ip_src and dimension 2 on ip_dst, 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).
• 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.
• A flow may have multiple conjunction actions, with different id values. This is useful for multiple conjunctive flows with overlapping sets. If one conjunctive flow matches packets with both ip_src ∈ {a,b} and ip_dst ∈ {d,e} and a second conjunctive flow matches ip_src ∈ {b,c} and ip_dst ∈ {f,g}, for example, then the flow that matches ip_src=b would have two conjunction actions, one for each conjunctive flow. The order of conjunction actions within a list of actions is not significant.
• A flow with conjunction actions may also include note actions for annotations, but not any other kind of actions. (They would not be useful because they would never be executed.)
• All of the flows that constitute a conjunctive flow with a given id must have the same priority. (Flows with the same id but different priorities are currently treated as different conjunctive flows, that is, currently id 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 id values unique within an OpenFlow table.)
• 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. (The flows that constitute a conjunctive flow may overlap with those that constitute the same or another conjunctive flow.)
• Following a conjunctive flow match, the search for the flow with conj_id=id is done in the same general-purpose way as other flow table searches, so one can use flows with conj_id=id to act differently depending on circumstances. (One exception is that the search for the conj_id=id flow itself ignores conjunctive flows, to avoid recursion.) If the search with conj_id=id fails, Open vSwitch acts as if the conjunctive flow had not matched at all, and continues searching the flow table for other matching flows.

• OpenFlow prerequisite checking occurs for the flow with conj_id=id in the same way as any other flow, e.g. in an OpenFlow 1.1+ context, putting a mod_nw_src action into the example above would require adding an ip match, like this:

            conj_id=1234,ip actions=mod_nw_src:1.2.3.4,controller

• OpenFlow prerequisite checking also occurs for the individual flows that comprise a conjunctive match in the same way as any other flow.
• 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.)

• 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 ip_src ∈ {a,b,c,d} and ip_dst ∈ {e,f,g,h} and tcp_dst = i. One way to implement this is to add the new constraint to the conj_id flow, like this:

            conj_id=1234,tcp,tcp_dst=i actions=mod_nw_src:1.2.3.4,controller

  but this is not recommended 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.

• A conjunctive match must have n ≥ 2 dimensions (otherwise a conjunctive match is not necessary). Open vSwitch enforces this.
• Each dimension within a conjunctive match should ordinarily have more than one flow. Open vSwitch does not enforce this.

Conjunction ID Field

Used for conjunctive matching. See above for more information.

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.

Open vSwitch supports a spectrum of usage models for mapping tunnels to OpenFlow ports:

``Port-based’’ tunnels

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, 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.

The following Open vSwitch commands create a bridge br-int, add port tap0 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 tap0 to the tunnel and vice versa:

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

``Flow-based’’ tunnels

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, 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.

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

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 allgre 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’

Mixed models.

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 gre0 and allgre tunnel ports described above.

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

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

Intermediate models.

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.

ovs-vswitchd.conf.db(5) describes all the details of tunnel configuration.

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

These fields are zeros for packets that did not arrive on a tunnel.

Tunnel ID Field

Many kinds of tunnels support a tunnel ID:

• VXLAN and Geneve have a 24-bit virtual network identifier (VNI).
• LISP has a 24-bit instance ID.
• GRE has an optional 32-bit key.
• STT has a 64-bit key.
• ERSPAN has a 10-bit key (Session ID).
• GTPU has a 32-bit key (Tunnel Endpoint ID).

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.

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.

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

Tunnel IPv4 Source Field

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.

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.

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

Tunnel IPv4 Destination Field

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.

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

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

Tunnel IPv6 Source Field

Similar to tun_src, but for tunnels over IPv6.

Tunnel IPv6 Destination Field

Similar to tun_dst, but for tunnels over IPv6.

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

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:

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.

VXLAN Group-Based Policy ID Field

For a packet tunneled over VXLAN with the Group-Based Policy (GBP) extension, this field represents the GBP policy ID, as shown above.

VXLAN Group-Based Policy Flags Field

For a packet tunneled over VXLAN with the Group-Based Policy (GBP) extension, this field represents the GBP policy flags, as shown above.

The field has the format shown below:

Unlabeled bits are reserved and must be transmitted as 0. The VXLAN GBP draft defines the other bits’ meanings as:

D (Don’t Learn)

When set, this bit indicates that the egress tunnel endpoint must not learn the source address of the encapsulated frame.

A (Applied)

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.

These fields provide access to features in the ERSPAN tunneling protocol [ERSPAN], which has two major versions: version 1 (aka type II) and version 2 (aka type III).

Regardless of version, ERSPAN is encapsulated within a fixed 8-byte GRE header that consists of a 4-byte GRE base header and a 4-byte sequence number. The ERSPAN version 1 header format is:

The ERSPAN version 2 header format is:

ERSPAN Version Field

ERSPAN version number: 1 for version 1, or 2 for version 2.

ERSPAN Index Field

This field is a 20-bit index/port number associated with the ERSPAN traffic’s source port and direction (ingress/egress). This field is platform dependent.

ERSPAN Direction Field

For ERSPAN v2, the mirrored traffic’s direction: 0 for ingress traffic, 1 for egress traffic.

ERSPAN Hardware ID Field

A 6-bit unique identifier of an ERSPAN v2 engine within a system.

These fields provide access to set-up GPRS Tunnelling Protocol for User Plane (GTPv1-U), based on 3GPP TS 29.281. A GTP-U header has the following format:

The flags and message type have the Open vSwitch GTP-U specific fields described below. Open vSwitch makes the TEID (Tunnel Endpoint Identifier), which identifies a tunnel endpoint in the receiving GTP-U protocol entity, available via tun_id.

GTP-U Flags Field

This field holds the 8-bit GTP-U flags, encoded as:

The flags are:

version

Used to determine the version of the GTP-U protocol, which should be set to 1.

PT

Protocol type, used as a protocol discriminator between GTP (1) and GTP’ (0).

rsv

Reserved. Must be zero.

E

If 1, indicates the presence of a meaningful value of the Next Extension Header field.

S

If 1, indicates the presence of a meaningful value of the Sequence Number field.

PN

If 1, indicates the presence of a meaningful value of the N-PDU Number field.

GTP-U Message Type Field

This field indicates whether it’s a signalling message used for path management, or a user plane message which carries the original packet. The complete range of message types can be referred to [3GPP TS 29.281].

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.

Generic Tunnel Option 0 Field

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.

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:

Taken together, the class and type 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.

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

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

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:

ovs-ofctl add-flow br0 tun_metadata0=1234,actions=controller

An option not received or not bound is matched as all zeros.

Tunnel Flags Field

Flags indicating various aspects of the tunnel encapsulation.

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

Currently, only one flag is defined:

oam

The tunnel protocol indicated that this is an OAM (Operations and Management) control packet.

The switch may reject matches against unknown flags.

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.

For non-tunneled packets, the value is 0.

These fields relate to the origin or treatment of a packet, but they are not extracted from the packet data itself.

Ingress Port Field

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:

1 through 0xfeff (65,279), inclusive.

Conventional OpenFlow port numbers.

OFPP_LOCAL (0xfffe or 65,534).

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.

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

OFPP_NONE (OpenFlow 1.0) or OFPP_ANY (OpenFlow 1.1+) (0xffff or 65,535).

OFPP_CONTROLLER (0xfffd or 65,533).

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.

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

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

0

Zero is not a valid OpenFlow port number.

OFPP_MAX (0xff00 or 65,280).

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 OFPP_MAX to be used as a port number, so packets will never be received on this port. (Other OpenFlow switches, of course, might use it.)

OFPP_UNSET (0xfff7 or 65,527)

OFPP_IN_PORT (0xfff8 or 65,528)

OFPP_TABLE (0xfff9 or 65,529)

OFPP_NORMAL (0xfffa or 65,530)

OFPP_FLOOD (0xfffb or 65,531)

OFPP_ALL (0xfffc or 65,532)

These port numbers are used only in output actions and never appear as ingress ports.

Most of these port numbers were defined in OpenFlow 1.0, but OFPP_UNSET was only introduced in OpenFlow 1.5.

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:

• The resubmit Open vSwitch extension action allows a flow table lookup with an arbitrary ingress port.
• An action that modifies the ingress port field (see below), such as e.g. load or set_field, followed by an action or instruction that performs another flow table lookup, such as resubmit or goto_table.

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

• OpenFlow requires suppressing output actions to in_port. That is, the following two flows both drop all packets that arrive on port 1:

  in_port=1,actions=1
  in_port=1,actions=drop

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

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

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, actions=load:0->NXM_OF_IN_PORT[],output:123 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:

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

or, in Open vSwitch 2.7 or later, use the clone action to save and restore it:

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

The ability to modify the ingress port is an Open vSwitch extension to OpenFlow.

OXM Ingress Port Field

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:

• OpenFlow 1.0 ports 0x0000 to 0xfeff, inclusive, map to OpenFlow 1.1 port numbers with the same values.
• OpenFlow 1.0 ports 0xff00 to 0xffff, inclusive, map to OpenFlow 1.1 port numbers 0xffffff00 to 0xffffffff.
• OpenFlow 1.1 ports 0x0000ff00 to 0xfffffeff are not mapped and not supported.

in_port and in_port_oxm are two views of the same information, so all of the comments on in_port apply to in_port_oxm too. Modifying in_port changes in_port_oxm, and vice versa.

Setting in_port_oxm to an unsupported value yields unspecified behavior.

Output Queue Field

Future Directions: 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 set_queue, enqueue, and pop_queue actions currently influence the output queue.

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.

Configuring queues on OpenFlow is not well standardized. On Linux, Open vSwitch supports queue configuration via OVSDB, specifically the QoS and Queue tables (see ovs-vswitchd.conf.db(5) 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. tc). 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.

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.

OpenFlow 1.0 only allowed output queues to be specified as part of an enqueue 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.

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.

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.

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:

• A hop across an Open vSwitch ``patch port’’ (which does not actually involve queuing) preserves the output queue.

• 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).

  (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.)

Packet Mark Field

Packet mark comes to Open vSwitch from the Linux kernel, in which the sk_buff data structure that represents a packet contains a 32-bit member named skb_mark. The value of skb_mark 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 iptables, for example, one can mark some traffic specially at ingress and then handle that traffic differently at egress based on the marked value.

Packet mark is an attempt at a generalization of the skb_mark concept beyond Linux, at least through more generic naming. Like skb_priority, packet mark is preserved across forwarding steps within a machine. Unlike 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 iptables configuration mentioned above.

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.

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.

Action Set Output Port Field

Holds the output port currently in the OpenFlow action set (i.e. from an output action within a write_actions 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 OFPP_UNSET.

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).

Packet Type Field

The type of the packet in the format specified in OpenFlow 1.5:

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

Open vSwitch currently supports the following classes of packet types for matching:

(0,0)

Ethernet.

(1,ethertype)

The specified ethertype. Open vSwitch can forward packets with any ethertype, but it can only match on and process data fields for the following supported packet types:

(1,0x800)

IPv4

(1,0x806)

ARP

(1,0x86dd)

IPv6

(1,0x8847)

MPLS

(1,0x8848)

MPLS multicast

(1,0x8035)

RARP

(1,0x894f)

NSH

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

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

Matching on 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 packet_type match, Open vSwitch acts as though a match on (0,0) (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.

Open vSwitch supports ``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.

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 ct_state. A packet is untracked at the beginning of the Open vSwitch pipeline and continues to be untracked until the pipeline invokes the ct action. The connection tracking fields are all zeroes in an untracked packet. When a flow in the Open vSwitch pipeline invokes the ct action, the action initializes the connection tracking fields and the packet becomes tracked for the remainder of its processing.

The connection tracker stores connection state in an internal table, but it only adds a new entry to this table when a ct action for a new connection invokes ct with the commit parameter. For a given connection, when a pipeline has executed ct, but not yet with commit, the connection is said to be uncommitted. 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.

Connection tracking is an Open vSwitch extension to OpenFlow. Open vSwitch 2.5 added the initial support for connection tracking. Subsequent versions of Open vSwitch added many refinements and extensions to the initial support. Many of these capabilities depend on the Open vSwitch datapath rather than simply the userspace version. The capabilities column in the Datapath table (see ovs-vswitchd.conf.db(5)) reports the detailed capabilities of a particular Open vSwitch datapath.

Connection Tracking State Field

This field holds several flags that can be used to determine the state of the connection to which the packet belongs.

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

The following flags are defined:

new (0x01)

A new connection. Set to 1 if this is an uncommitted connection.

est (0x02)

Part of an existing connection. Set to 1 if packets of a committed connection have been seen by conntrack from both directions.

rel (0x04)

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.

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

rpl (0x08)

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.

inv (0x10)

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:

• L3/L4 protocol handler is not loaded/unavailable. With the Linux kernel datapath, this may mean that the nf_conntrack_ipv4 or nf_conntrack_ipv6 modules are not loaded.
• L3/L4 protocol handler determines that the packet is malformed.
• Packets are unexpected length for protocol.

trk (0x20)

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.

snat (0x40)

This packet was transformed by source address/port translation by a preceding ct action. Open vSwitch 2.6 added this flag.

dnat (0x80)

This packet was transformed by destination address/port translation by a preceding ct action. Open vSwitch 2.6 added this flag.

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

1. If trk is unset, no other flags are set.
2. If trk is set, one or more other flags may be set.
3. If inv is set, only the trk flag is also set.
4. new and est are mutually exclusive.
5. new and rpl are mutually exclusive.
6. rel may be set in conjunction with any other flags.

Future versions of Open vSwitch may define new flags.

Connection Tracking Zone Field

A connection tracking zone, the zone value passed to the most recent ct action. Each zone is an independent connection tracking context, so tracking the same packet in multiple contexts requires using the ct action multiple times.

Connection Tracking Mark Field

The metadata committed, by an action within the exec parameter to the ct action, to the connection to which the current packet belongs.

Connection Tracking Label Field

The label committed, by an action within the exec parameter to the ct action, to the connection to which the current packet belongs.

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

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 nat 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 parent connection, e.g., an FTP control connection.

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 ct_state=+new, ct_state=+est, ct_state=+rel, and ct_state=+trk-inv.

Connection Tracking Original Direction IPv4 Source Address Field

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.

Connection Tracking Original Direction IPv4 Destination Address Field

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.

Connection Tracking Original Direction IPv6 Source Address Field

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.

Connection Tracking Original Direction IPv6 Destination Address Field

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.

Connection Tracking Original Direction IP Protocol Field

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.

Connection Tracking Original Direction Transport Layer Source Port Field

Bitwise match on the conntrack original direction tuple transport source, when MFF_CT_NW_PROTO has value 6 for TCP, 17 for UDP, or 132 for SCTP. When MFF_CT_NW_PROTO has value 1 for ICMP, or 58 for ICMPv6, the lower 8 bits of MFF_CT_TP_SRC 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.

Connection Tracking Original Direction Transport Layer Source Port Field

Bitwise match on the conntrack original direction tuple transport destination port, when MFF_CT_NW_PROTO has value 6 for TCP, 17 for UDP, or 132 for SCTP. When MFF_CT_NW_PROTO has value 1 for ICMP, or 58 for ICMPv6, the lower 8 bits of MFF_CT_TP_DST 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.

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).

OpenFlow Metadata Field

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.

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.

Register 0 Field

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.

Extended Register 0 Field

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).

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

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.

Double-Extended Register 0 Field

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: xxreg0 overlays reg0 through reg3, with reg0 supplying the most-significant bits of xxreg0 and reg3 the least-significant. xxreg1 similarly overlays reg4 through reg7, and so on.

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.

Ethernet Source Field

The Ethernet source address:

Ethernet Destination Field

The Ethernet destination address:

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:

01:00:00:00:00:00

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

fe:ff:ff:ff:ff:ff

Match all bits except the multicast bit. This is probably not useful.

ff:ff:ff:ff:ff:ff

Exact match (equivalent to omitting the mask).

00:00:00:00:00:00

Wildcard all bits (equivalent to dl_dst=*).

Ethernet Type Field

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:

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 0xaa and control byte 0x03, which indicate that a SNAP header follows the LLC header. In turn, OpenFlow and Open vSwitch only support a SNAP header with organization 0x000000. In such a case, this field is copied from the type field in the SNAP header, like this:

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:

LLC and SNAP encapsulation look like this with an 802.1Q header:

When a packet doesn’t match any of the header formats described above, Open vSwitch and OpenFlow set this field to 0x5ff (OFP_DL_TYPE_NOT_ETH_TYPE).

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.

An 802.1Q VLAN header consists of two 16-bit fields:

The first 16 bits of the VLAN header, the TPID (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 0x8100. OpenFlow 1.0 explicitly supports only TPID 0x8100. OpenFlow 1.1, but not earlier or later versions, also requires support for TPID 0x88a8 (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 0x8100 and 0x88a8 should be pushed). No version of OpenFlow provides a way to distinguish or match on the TPID.

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

• PCP (Priority Control Point), is a 3-bit 802.1p priority. The lowest priority is value 1, the second-lowest is value 0, and priority increases from 2 up to highest priority 7.
• CFI (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 DEI (Drop Eligibility Indicator). By either name, OpenFlow and Open vSwitch don’t provide any way to match or set this bit.
• VID (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 priority tag because it is only meaningful for assigning the frame a priority. VID 0xfff (4,095) is reserved.

See eth_type for illustrations of a complete Ethernet frame with 802.1Q tag included.

Open vSwitch can match only a single VLAN header. If more than one VLAN header is present, then 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.

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.

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.

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

• When both dl_vlan and dl_vlan_pcp are wildcarded, the flow matches packets without an 802.1Q header or with any 802.1Q header.
• The match dl_vlan=0xffff causes a flow to match only packets without an 802.1Q header. Such a flow should also wildcard dl_vlan_pcp, 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.

• Otherwise, the flow matches only packets with an 802.1Q header. If dl_vlan is not wildcarded, then the flow only matches packets with the VLAN ID specified in dl_vlan’s low 12 bits. If dl_vlan_pcp is not wildcarded, then the flow only matches packets with the priority specified in dl_vlan_pcp’s low 3 bits.

  OpenFlow does not specify how to interpret the high 4 bits of dl_vlan or the high 5 bits of dl_vlan_pcp. Open vSwitch ignores them.

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

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

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

OpenFlow 1.2+ VLAN ID Field

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 (0x1000 aka OFPVID_PRESENT) also set to 1. The three most significant bits are always zero:

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:

Fully wildcarded

Matches any packet, that is, one without an 802.1Q header or with an 802.1Q header with any TCI value.

Value 0x0000 (OFPVID_NONE), mask 0xffff (or no mask)

Matches only packets without an 802.1Q header.

Value 0x1000, mask 0x1000

Matches any packet with an 802.1Q header, regardless of VLAN ID.

Value 0x1009, mask 0xffff (or no mask)

Match only packets with an 802.1Q header with VLAN ID 9.

Value 0x1001, mask 0x1001

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.)

OpenFlow 1.2+ VLAN Priority Field

The 3 least significant bits may be used to match the PCP bits in an 802.1Q header. Other bits are always zero:

This field may only be used when vlan_vid is not wildcarded and does not exact match on 0 (which only matches when there is no 802.1Q header).

See VLAN Comparison Chart, below, for some examples.

The vlan_tci extension can describe more kinds of VLAN matches than the other variants. It is also simpler than the other variants.

VLAN TCI Field

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 P for ``present’’ below) forced to 1. Thus, for a packet in VLAN 9 with priority 7, it has the value 0xf009:

Usage examples:

vlan_tci=0

Match packets without an 802.1Q header.

vlan_tci=0x1000/0x1000

Match packets with an 802.1Q header, regardless of VLAN and priority values.

vlan_tci=0xf123

Match packets tagged with priority 7 in VLAN 0x123.

vlan_tci=0x1123/0x1fff

Match packets tagged with VLAN 0x123 (and any priority).

vlan_tci=0x5000/0xf000

Match packets tagged with priority 2 (in any VLAN).

vlan_tci=0/0xfff

Match packets with no 802.1Q header or tagged with VLAN 0 (and any priority).

vlan_tci=0x5000/0xe000

Match packets with no 802.1Q header or tagged with priority 2 (in any VLAN).

vlan_tci=0/0xefff

Match packets with no 802.1Q header or tagged with VLAN 0 and priority 0.

See VLAN Comparison Chart, below, for more examples.

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:

All numbers in the table are expressed in hexadecimal. The columns in the table are interpreted as follows:

Criteria

See the list below.

OpenFlow 1.0

OpenFlow 1.1

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

OpenFlow 1.2+

xxxx/yyyy,zz means vlan_vid with value xxxx and mask yyyy, and vlan_pcp (which is not maskable) with value zz. -- means that vlan_pcp is omitted. ``(none)’’ means that OpenFlow 1.2 cannot match with these criteria.

NXM

xxxx/yyyy means vlan_tci with value xxxx and mask yyyy.

The matching criteria described by the table are:

[1]

Matches any packet, that is, one without an 802.1Q header or with an 802.1Q header with any TCI value.

[2]

Matches only packets without an 802.1Q header.

OpenFlow 1.0 doesn’t define the behavior if dl_vlan is set to 0xffff and dl_vlan_pcp is not wildcarded. (Open vSwitch always ignores dl_vlan_pcp when dl_vlan is set to 0xffff.)

OpenFlow 1.1 says explicitly to ignore dl_vlan_pcp when dl_vlan is set to 0xffff.

OpenFlow 1.2 doesn’t say how to interpret a match with vlan_vid value 0 and a mask with OFPVID_PRESENT (0x1000) 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 0x1000.

Any NXM match with vlan_tci value 0 and the CFI bit set to 1 in the mask is equivalent to the one listed in the table.

[3]

Matches only packets that have an 802.1Q header with VID xxx (and any PCP).

[4]

Matches only packets that have an 802.1Q header with PCP y (and any VID).

OpenFlow 1.0 doesn’t clearly define the behavior for this case. Open vSwitch implements it this way.

In the NXM value, z equals (y << 1) | 1.

[5]

Matches only packets that have an 802.1Q header with VID xxx and PCP y.

In the NXM value, z equals (y << 1) | 1.

[6]

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.)

[7]

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.)

[8]

Matches packets with no 802.1Q header or with an 802.1Q header with a VID of 0. Only possible with NXM.

[9]

Matches packets with no 802.1Q header or with an 802.1Q header with a PCP of 0. Only possible with NXM.

[10]

Matches packets with no 802.1Q header or with an 802.1Q header with both VID and PCP of 0. Only possible with NXM.

One or more MPLS headers (more commonly called MPLS labels) follow an Ethernet type field that specifies an MPLS Ethernet type [RFC 3032]. Ethertype 0x8847 is used for all unicast. Multicast MPLS is divided into two specific classes, one of which uses Ethertype 0x8847 and the other 0x8848 [RFC 5332].

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

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

The fields within an MPLS label are:

Label, 20 bits.

An identifier.

Traffic control (TC), 3 bits.

Used for quality of service.

Bottom of stack (BOS), 1 bit (labeled just ``S’’ above).

0 indicates that another MPLS label follows this one.

1 indicates that this MPLS label is the last one in the stack, so that some other protocol follows this one.

Time to live (TTL), 8 bits.

Each hop across an MPLS network decrements the TTL by 1. If it reaches 0, the packet is discarded.

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.

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.

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.

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]:

• 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.
• Some deployments use a single inner protocol consistently.
• In some deployments, the inner protocol must be inferred from the innermost label.
• 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 4 or 6. OpenFlow and Open vSwitch do not currently support these cases.

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 pop_mpls action.

MPLS Label Field

The least significant 20 bits hold the ``label’’ field from the MPLS label. Other bits are zero:

Most label values are available for any use by deployments. Values under 16 are reserved.

MPLS Traffic Class Field

The least significant 3 bits hold the TC field from the MPLS label. Other bits are zero:

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

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

MPLS Bottom of Stack Field

The least significant bit holds the BOS field from the MPLS label. Other bits are zero:

This field is useful as part of processing a series of incoming MPLS labels. A flow that includes a pop_mpls action should generally match on mpls_bos:

• When mpls_bos is 0, there is another MPLS label following this one, so the Ethertype passed to pop_mpls should be an MPLS Ethertype. For example: table=0, dl_type=0x8847, mpls_bos=0, actions=pop_mpls:0x8847, goto_table:1
• When mpls_bos is 1, this MPLS label is the last one, so the Ethertype passed to pop_mpls should be a non-MPLS Ethertype such as IPv4. For example: table=1, dl_type=0x8847, mpls_bos=1, actions=pop_mpls:0x0800, goto_table:2

MPLS Time-to-Live Field

Holds the 8-bit time-to-live field from the MPLS label:

These fields are applicable only to IPv4 flows, that is, flows that match on the IPv4 Ethertype 0x0800.

IPv4 Source Address Field

The source address from the IPv4 header:

For historical reasons, in an ARP or RARP flow, Open vSwitch interprets matches on nw_src as actually referring to the ARP SPA.

IPv4 Destination Address Field

The destination address from the IPv4 header:

For historical reasons, in an ARP or RARP flow, Open vSwitch interprets matches on nw_dst as actually referring to the ARP TPA.

These fields apply only to IPv6 flows, that is, flows that match on the IPv6 Ethertype 0x86dd.

IPv6 Source Address Field

The source address from the IPv6 header:

Open vSwitch 1.8 added support for bitwise matching; earlier versions supported only CIDR masks.

IPv6 Destination Address Field

The destination address from the IPv6 header:

Open vSwitch 1.8 added support for bitwise matching; earlier versions supported only CIDR masks.

IPv6 Flow Label Field

The least significant 20 bits hold the flow label field from the IPv6 header. Other bits are zero:

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 0x0800 or the IPv6 Ethertype 0x86dd may match on these fields.

IPv4/v6 Protocol Field

Matches the IPv4 or IPv6 protocol type.

For historical reasons, in an ARP or RARP flow, Open vSwitch interprets matches on nw_proto 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.

In the case of fragmented traffic, a difference exists in the way the field acts for IPv4 and IPv6 later fragments. For IPv6 fragments with nonzero offset, nw_proto is set to the IPv6 protocol type for fragments (44). Conversely, for IPv4 later fragments, the field is set based on the protocol type present in the header.

IPv4/v6 TTL/Hop Limit Field

The main reason to match on the TTL or hop limit field is to detect whether a dec_ttl action will fail due to a TTL exceeded error. Another way that a controller can detect TTL exceeded is to listen for OFPR_INVALID_TTL ``packet-in’’ messages via OpenFlow.

IPv4/v6 Fragment Bitmask Field

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:

no

Match only non-fragmented packets.

yes

Matches all fragments.

first

Matches only fragments with offset 0.

later

Matches only fragments with nonzero offset.

not_later

Matches non-fragmented packets and fragments with zero offset.

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:

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

• A packet that is not an IP fragment has value 0.
• A packet that is an IP fragment with offset 0 (the first fragment) has bit 0 set and thus value 1.
• A packet that is an IP fragment with nonzero offset has bits 0 and 1 set and thus value 3.

The switch may reject matches against values that can never appear.

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

• In OFPC_FRAG_DROP 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.
• Open vSwitch does not implement OFPC_FRAG_REASM 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.
• In OFPC_FRAG_NORMAL 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.
• In OFPC_FRAG_NX_MATCH mode, all three values are possible. For fragments with offset 0, Open vSwitch makes L4 header information available.

Thus, this field is likely to be most useful for an Open vSwitch switch configured in OFPC_FRAG_NX_MATCH mode. See the description of the set-frags command in ovs-ofctl(8), for more details.

IPv4 and IPv6 contain a one-byte ``type of service’’ or TOS field that has the following format:

IPv4/v6 DSCP (Bits 2-7) Field

This field is the TOS byte with the two ECN bits cleared to 0:

IPv4/v6 DSCP (Bits 0-5) Field

This field is the TOS byte shifted right to put the DSCP bits in the 6 least-significant bits:

IPv4/v6 ECN Field

This field is the TOS byte with the DSCP bits cleared to 0:

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:

The ARP fields are also used for RARP, the Reverse Address Resolution Protocol, which shares ARP’s wire format.

ARP Opcode Field

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.

ARP Source IPv4 Address Field

ARP Target IPv4 Address Field

ARP Source Ethernet Address Field

ARP Target Ethernet Address Field

Service functions are widely deployed and essential in many networks. These service functions provide a range of features such as security, WAN acceleration, and server load balancing. Service functions may be instantiated at different points in the network infrastructure such as the wide area network, data center, and so forth.

Prior to development of the SFC architecture [RFC 7665] and the protocol specified in this document, current service function deployment models have been relatively static and bound to topology for insertion and policy selection. Furthermore, they do not adapt well to elastic service environments enabled by virtualization.

New data center network and cloud architectures require more flexible service function deployment models. Additionally, the transition to virtual platforms demands an agile service insertion model that supports dynamic and elastic service delivery. Specifically, the following functions are necessary:

1. The movement of service functions and application workloads in the network.
2. The ability to easily bind service policy to granular information, such as per-subscriber state.
3. The capability to steer traffic to the requisite service function(s).

The Network Service Header (NSH) specification defines a new data plane protocol, which is an encapsulation for service function chains. The NSH is designed to encapsulate an original packet or frame, and in turn be encapsulated by an outer transport encapsulation (which is used to deliver the NSH to NSH-aware network elements), as shown below:

The NSH is composed of the following elements:

1. Service Function Path identification.
2. Indication of location within a Service Function Path.
3. Optional, per packet metadata (fixed length or variable).

[RFC 7665] provides an overview of a service chaining architecture that clearly defines the roles of the various elements and the scope of a service function chaining encapsulation. Figure 3 of [RFC 7665] depicts the SFC architectural components after classification. The NSH is the SFC encapsulation referenced in [RFC 7665].

flags field (2 bits) Field

TTL field (6 bits) Field

mdtype field (8 bits) Field

np (next protocol) field (8 bits) Field

spi (service path identifier) field (24 bits) Field

si (service index) field (8 bits) Field

c1 (Network Platform Context) field (32 bits) Field

c2 (Network Shared Context) field (32 bits) Field

c3 (Service Platform Context) field (32 bits) Field

c4 (Service Shared Context) field (32 bits) Field

For matching purposes, no distinction is made whether these protocols are encapsulated within IPv4 or IPv6.

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

TCP Source Port Field

Open vSwitch 1.6 added support for bitwise matching.

TCP Destination Port Field

Open vSwitch 1.6 added support for bitwise matching.

TCP Flags Field

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].

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

The flag bits are:

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:

UDP Source Port Field

UDP Destination Port Field

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:

SCTP Source Port Field

SCTP Destination Port Field