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Mirage OpenFlow Implementation

OpenFlow is a switching standard and open protocol enabling distributed control of the flow tables contained within Ethernet switches in a network. Each OpenFlow switch has three parts:

  • A datapath, containing a flow table, associating set of actions with each flow entry;
  • A secure channel, connecting to a controller; and
  • The OpenFlow protocol, used by the controller to talk to switches.

Following this standard model, the implementation comprises three parts:

  • Openflow library, contains a complete parsing library in pure Ocaml and a minimal controller library using an event-driven model.
  • Openflow.switch library, provides a skeleton OpenFlow switch supporting most elementary switch functionality.
  • Openflow.flv library, implements a basic FLowVisor reimplementation in ocaml.

N.B. There are two versions of the OpenFlow protocol: v1.0.0 (0x01 on the wire) and v1.1.0 (0x02 on the wire). The implementation supports wire protocol 0x01 as this is what is implemented in Open vSwitch, used for debugging.


The file begins with some utility functions, operators, types. The bulk of the code is organised following the v1.0.0 protocol specification, as implemented by Open vSwitch v1.2. Each set of messages is contained within its own module, most of which contain a type t representing the entity named by the module, plus relevant parsers to convert a bitstring to a type (parse_*) and pretty printers for the type (string_of_*). At the end of the file, in the root Ofpacket module scope, are definitions for interacting with the protocol as a whole, e.g., error codes, OpenFlow message types and standard header, root OpenFlow parser, OpenFlow packet builders.

Queue, Port, Switch

The Queue module is really a placeholder currently. OpenFlow defines limited quality-of-service support via a simple queueing mechanism. Flows are mapped to queues attached to ports, and each queue is then configured as desired. The specification currently defines just a minimum rate, although specific implementations may provide more.

The Port module wraps several port related elements:

  • t, where that is either simply the index of the port in the switch, or the special indexes (> 0xff00) representing the controller, flooding, etc.
  • config, a specific port's configuration (up/down, STP supported, etc).
  • features, a port's feature set (rate, fiber/copper, etc).
  • state, a port's current state (up/down, STP learning mode, etc).
  • phy, a port's physical details (index, address, name, etc).
  • stats, current statistics of the port (packet and byte counters, collisions, etc).
  • reason and status, for reporting changes to a port's configuration; reason is one of ADD|DEL|MOD.

Finally, Switch wraps elements pertaining to a whole switch, that is a collection of ports, tables (including the group table), and the connection to the controller.

  • capabilities, the switch's capabilities in terms of supporting IP fragment reassembly, various statistics, etc.
  • action, the types of action the switch's ports support (setting various fields, etc).
  • features, the switch's id, number of buffers, tables, port list etc.
  • config, for masking against handling of IP fragments: no special handling, drop, reassemble.

Wildcards, Match, Flow

The Wildcards and Match modules both simply wrap types respectively representing the fields to wildcard in a flow match, and the flow match specification itself.

The Flow module then contains structures representing:

  • t, the flow itself (its age, activity, priority, etc); and
  • stats, extended statistics association with a flow identified by a 64 bit cookie.

Packet_in, Packet_out

These represent messages associated with receipt or transmission of a packet in response to a controller initiated action.

Packet_in is used where a packet arrives at the switch and is forwarded to the controller, either due to lack of matching entry, or an explicit action.

Packet_out contains the structure used by the controller to indicate to the switch that a packet it has been buffering must now have some actions performed on it, typically culminating in it being forward out of one or more ports.

Flow_mod, Port_mod

These represent modification messages to existing flow and port state in the switch.


Finally, the Stats module contains structures representing the different statistics messages available through OpenFlow, as well as the request and response messages that transport them.


A simple module to create an openflow channel abstraction over a serires of different transport mechanisms. At the moment the library contains support of Channel.t connections and Lwt_stream streams. The protocol ensures to read from the socket full Openflow pdus and transform them to appropriate Ofpacket structures.


Initially modelled after NOX, this is a skeleton controller that provides a simple event based wrapper around the OpenFlow protocol. It currently provides the minimal set of events corresponding to basic switch operation:

  • DATAPATH_JOIN, representing the connection of a datapath to the controller, i.e., notification of the existence of a switch.
  • DATAPATH_LEAVE, representing the disconnection of a datapath from the controller, i.e., notification of the destruction of a switch.
  • PACKET_IN, representing the forwarding of a packet to the controller, whether through an explicit action corresponding to a flow match, or simply as the default when flow match is found.
  • FLOW_REMOVED, i.e., representing the switch notification regarding the removal of a flow from the flow table.
  • FLOW_STATS_REPLY, i.e., represents the replies transmitted by the switch after a flow_stats_req.
  • AGGR_FLOW_STATS_REPLY, i.e., representing the reply transmitted by the switch to an aggr_flow_stats_req.
  • DESC_STATS_REPLY, i.e., representing the reply of a switch to desc_stats request.
  • PORT_STATS_REPLY, i.e., representing the replt of a switch to a port_stats request providing port level counter and the state of the switch.
  • TABLE_STATS_REPLY, i.e., representing the reply of a switch to a table_stats request.
  • PORT_STATUS_REPLY, i.e., representing the notification send by the switch when the state of a port of the switch is changed.

The controller state is mutable and modelled as:

  • A list of callbacks per event, each taking the current state, the originating datapath, and the event;
  • Mappings from switch (datapath_id) to a Mirage communications channel (Channel.t); and

The main work of the controller is carried out in process_of_packet which processes each received packet within the context given by the current state of the switch: this is where the OpenFlow state machine is implemented.

The controller entry point is via the listen, local_connect or connect function which effectively creates a receiving channel to parse OpenFlow packets, and pass them to process_of_packet which handles a range of standard protocol-level interactions, e.g., ECHO_REQ, FEATURES_RESP, generating Mirage events as appropriate. Specifically, controller is passed as callback to the respective connection method, and recursively evaluates read_packet to read the incoming packet and pass it to process_of_packet.


An OpenFlow switch or datapath consists of a flow table, a group table (in later versions, not supported in v1.0.0), and a channel back to the controller. Communication over the channel is via the OpenFlow protocol, and is how the controller manages the switch.

In short, each table contains flow entries consisting of match fields, counters, and instructions to apply to packets. Starting with the first flow table, if an incoming packet matches an entry, the counters are updated and the instructions carried out. If no entry in the first table matches, (part of) the packet is forwarded to the controller, or it is dropped, or it proceeds to the next flow table.

At the current point the switch doesn't support any queue principles.

Skeleton code is as follows:


Represents a single flow table entry. Each entry consists of:

  • counters, to keep statistics per-table, -flow, -port, -queue (Entry.table_counter list, Entry.flow_counter list, Entry.port_counter list, Entry.queue_counter list); and
  • actions, to perform on packets matching the fields (Entry.action list).


A simple module representing a table of flow entries. Currently just an id (tid) , a hashtbl of entries ((OP.Match.t, Entry.t) Hashtbl.t), a list of exact match entries to reduce the lookup time for wildcard entries and a the table counter.


Encapsulating the switch (or datapath) itself. Currently defines a port as:

  • details, a physical port configuration (Ofpacket.Port.phy); and
  • device, some handle to the physical device (mocked out as a string).

The switch is then modelled as:

  • ports, a list of physical ports (Switch.port list);
  • table, the table of flow entries for this switch;
  • stats, a set of per-switch counters (Switch.stats); and
  • p_sflow, the probability in use when sFlow sampling.

Note that the vocabulary of a number of these changes with v1.1.0, in addition to the table structure becoming more complex (support for chains of tables, forwarding to tables, and the group table).


What's the best way to structure the controller so that application code can introduce generation and consumption of new events? NOX permits this within a single event-handling framework -- is this simply out-of-scope here, or should we have a separate event-based programming framework available, or is there a straightforward Ocaml-ish way to incorporate this into the OpenFlow Controller?

What's the best way to expose parsing as a separate activity to reading data off the wire? Specifically, I'd really like to reuse functions from Net.Ethif, Net.Ipv4, etc to recover structure from the bitstring without need to have OfPacket.Match.parse_from_raw_packet. Previously I have found having parsers that return structured data and then wrapping up the packet structure as a nested type, e.g., PCAP(pcaph, ETH(ethh, IPv4(iph, payload))) or ...TCP(tcph, payload)))) worked well, permitting fairly natural pattern matching. The depth to which the packet was deumltiplexed was controlled by a parameter to the entry-point parser.

The Switch design is almost certainly very inefficient, and needs working on. This is waiting on implementation -- although sketched out, waiting on network driver model to actually be able to get hold of physical devices and frames. When we can, also need to consider how to control packet parsing, and demultiplexing of frames for switching from frames comprising the TCP stream carrying the controller channel. Ideally, it would be transparent to have a Channel for the controller's OpenFlow messages and a per-device frame handler for everything else. That is, Mirage would do the necessary demultiplexing -- but only what's necessary -- passing non-OpenFlow frames to the switch to be matched, but reassembling the TCP flow carrying the controller's OpenFlow traffic.


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