Implementation Details

Q: I hear OVS has a couple of kinds of flows. Can you tell me about them?

A: Open vSwitch uses different kinds of flows for different purposes:

  • OpenFlow flows are the most important kind of flow. OpenFlow controllers use these flows to define a switch’s policy. OpenFlow flows support wildcards, priorities, and multiple tables.

    When in-band control is in use, Open vSwitch sets up a few “hidden” flows, with priority higher than a controller or the user can configure, that are not visible via OpenFlow. (See the “Controller” section of the FAQ for more information about hidden flows.)

  • The Open vSwitch software switch implementation uses a second kind of flow internally. These flows, called “datapath” or “kernel” flows, do not support priorities and comprise only a single table, which makes them suitable for caching. (Like OpenFlow flows, datapath flows do support wildcarding, in Open vSwitch 1.11 and later.) OpenFlow flows and datapath flows also support different actions and number ports differently.

    Datapath flows are an implementation detail that is subject to change in future versions of Open vSwitch. Even with the current version of Open vSwitch, hardware switch implementations do not necessarily use this architecture.

Users and controllers directly control only the OpenFlow flow table. Open vSwitch manages the datapath flow table itself, so users should not normally be concerned with it.

Q: Why are there so many different ways to dump flows?

A: Open vSwitch has two kinds of flows (see the previous question), so it has commands with different purposes for dumping each kind of flow:

  • ovs-ofctl dump-flows <br> dumps OpenFlow flows, excluding hidden flows. This is the most commonly useful form of flow dump. (Unlike the other commands, this should work with any OpenFlow switch, not just Open vSwitch.)
  • ovs-appctl bridge/dump-flows <br> dumps OpenFlow flows, including hidden flows. This is occasionally useful for troubleshooting suspected issues with in-band control.
  • ovs-dpctl dump-flows [dp] dumps the datapath flow table entries for a Linux kernel-based datapath. In Open vSwitch 1.10 and later, ovs-vswitchd merges multiple switches into a single datapath, so it will show all the flows on all your kernel-based switches. This command can occasionally be useful for debugging.
  • ovs-appctl dpif/dump-flows <br>, new in Open vSwitch 1.10, dumps datapath flows for only the specified bridge, regardless of the type.

Q: How does multicast snooping works with VLANs?

A: Open vSwitch maintains snooping tables for each VLAN.

Q: Can OVS populate the kernel flow table in advance instead of in reaction to packets?

A: No. There are several reasons:

  • Kernel flows are not as sophisticated as OpenFlow flows, which means that some OpenFlow policies could require a large number of kernel flows. The “conjunctive match” feature is an extreme example: the number of kernel flows it requires is the product of the number of flows in each dimension.
  • With multiple OpenFlow flow tables and simple sets of actions, the number of kernel flows required can be as large as the product of the number of flows in each dimension. With more sophisticated actions, the number of kernel flows could be even larger.
  • Open vSwitch is designed so that any version of OVS userspace interoperates with any version of the OVS kernel module. This forward and backward compatibility requires that userspace observe how the kernel module parses received packets. This is only possible in a straightforward way when userspace adds kernel flows in reaction to received packets.

For more relevant information on the architecture of Open vSwitch, please read “The Design and Implementation of Open vSwitch”, published in USENIX NSDI 2015.

Q: How many packets does OVS buffer?

A: Open vSwitch fast path packet processing uses a “run to completion” model in which every packet is completely handled in a single pass. Therefore, in the common case where a packet just passes through the fast path, Open vSwitch does not buffer packets itself. The operating system and the network drivers involved in receiving and later in transmitting the packet do often include buffering. Open vSwitch is only a middleman between these and does not have direct access or influence over their buffers.

Outside the common case, Open vSwitch does sometimes buffer packets. When the OVS fast path processes a packet that does not match any of the flows in its megaflow cache, it passes that packet to the Open vSwitch slow path. This procedure queues a copy of the packet to the Open vSwitch userspace which processes it and, if necessary, passes it back to the kernel module. Queuing the packet to userspace as part of this process involves buffering. (Going the opposite direction does not, because the kernel actually processes the request synchronously.) A few other exceptional cases also queue packets to userspace for processing; most of these are due to OpenFlow actions that the fast path cannot handle and that must therefore be handled by the slow path instead.

OpenFlow also has a concept of packet buffering. When an OpenFlow switch sends a packet to a controller, it may opt to retain a copy of the packet in an OpenFlow “packet buffer”. Later, if the controller wants to tell the switch to forward a copy of that packet, it can refer to the packet through its assigned buffer, instead of sending the whole packet back to the switch, thereby saving bandwidth in the OpenFlow control channel. Before Open vSwitch 2.7, OVS implemented such buffering; Open vSwitch 2.7 and later do not.