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General

Q: What is OVN?

A: OVN, the Open Virtual Network, is a system to support virtual network abstraction. OVN complements the existing capabilities of OVS to add native support for virtual network abstractions, such as virtual L2 and L3 overlays and security groups.

OVN is intended to be used by cloud management software (CMS). For details about the architecture of OVN, see the ovn-architecture manpage. Some high-level features offered by OVN include

  • Distributed virtual routers
  • Distributed logical switches
  • Access Control Lists
  • DHCP
  • DNS server

Q: How can I try OVN?

A: The OVN source code can be built on a Linux system. You can build and experiment with OVN on any Linux machine. Packages for various Linux distributions are available on many platforms, including: Debian, Ubuntu, Fedora.

Q: Why does OVN use STT and Geneve instead of VLANs or VXLAN (or GRE)?

A: OVN implements a fairly sophisticated packet processing pipeline in "logical datapaths" that can implement switching or routing functionality. A logical datapath has an ingress pipeline and an egress pipeline, and each of these pipelines can include logic based on packet fields as well as packet metadata such as the logical ingress and egress ports (the latter only in the egress pipeline).

The processing for a logical datapath can be split across hypervisors. In particular, when a logical ingress pipeline executes an "output" action, OVN passes the packet to the egress pipeline on the hypervisor (or, in the case of output to a logical multicast group, hypervisors) on which the logical egress port is located. If this hypervisor is not the same as the ingress hypervisor, then the packet has to be transmitted across a physical network.

This situation is where tunneling comes in. To send the packet to another hypervisor, OVN encapsulates it with a tunnel protocol and sends the encapsulated packet across the physical network. When the remote hypervisor receives the tunnel packet, it decapsulates it and passes it through the logical egress pipeline. To do so, it also needs the metadata, that is, the logical ingress and egress ports.

Thus, to implement OVN logical packet processing, at least the following metadata must pass across the physical network:

  • Logical datapath ID, a 24-bit identifier. In Geneve, OVN uses the VNI to hold the logical datapath ID; in STT, OVN uses 24 bits of STT's 64-bit context ID.
  • Logical ingress port, a 15-bit identifier. In Geneve, OVN uses an option to hold the logical ingress port; in STT, 15 bits of the context ID.
  • Logical egress port, a 16-bit identifier. In Geneve, OVN uses an option to hold the logical egress port; in STT, 16 bits of the context ID.

See ovn-architecture(7), under "Tunnel Encapsulations", for details.

Together, these metadata require 24 + 15 + 16 = 55 bits. GRE provides 32 bits, VXLAN provides 24, and VLAN only provides 12. Most notably, if logical egress pipelines do not match on the logical ingress port, thereby restricting the class of ACLs available to users, then this eliminates 15 bits, bringing the requirement down to 40 bits. At this point, one can choose to limit the size of the OVN logical network in various ways, e.g.:

  • 16 bits of logical datapaths + 16 bits of logical egress ports. This combination fits within a 32-bit GRE tunnel key.
  • 12 bits of logical datapaths + 12 bits of logical egress ports. This combination fits within a 24-bit VXLAN VNI.
  • It's difficult to identify an acceptable compromise for a VLAN-based deployment.

These compromises wouldn't suit every site, since some deployments may need to allocate more bits to the datapath or egress port identifiers.

As a side note, OVN does support VXLAN for use with ASIC-based top of rack switches, using ovn-controller-vtep(8) and the OVSDB VTEP schema described in vtep(5), but this limits the features available from OVN to the subset available from the VTEP schema.

Q: How can I contribute to the OVN Community?

A: You can start by joining the mailing lists and helping to answer questions. You can also suggest improvements to documentation. If you have a feature or bug you would like to work on, send a mail to one of the :doc:`mailing lists </internals/mailing-lists>`.

Q: What does it mean when a feature is marked "experimental"?

A: Experimental features are marked this way because of one of several reasons:

  • The developer was only able to test the feature in a limited environment. Therefore the feature may not always work as intended in all environments.
  • During review, the potential for failure was noticed, but the circumstances that would lead to that failure were hard to nail down or were strictly theoretical.
  • What exists in OVN may be an early version of a more fleshed-out feature to come in a later version.
  • The feature was developed against a draft RFC that is subject to change when the RFC is published.
  • The feature was developed based on observations of how a specific vendor implements a feature, rather than using IETF standards or other documentated specifications.

A feature may be declared experimental for other reasons as well, but the above are the most common. When a feature is marked experimental, it has the following properties:

  • The feature must be opt-in. The feature must be disabled by default. When the feature is disabled, it must have no bearing on other OVN functionality.
  • Configuration and implementation details of the feature are subject to change between major or minor versions of OVN.
  • Users make use of this feature at their own risk. Users are free to file issues against the feature, but developers are more likely to prioritize work on non-experimental features first.
  • Experimental features may be removed. For instance, if an experimental feature exposes a security risk, it may be removed rather than repaired.

The hope is that experimental features will eventually lose the "experimental" marker and become a core feature. However, there is no specific test or process defined for when a feature no longer needs to be considered experimental. This typically will be decided collectively by OVN maintainers.

Q: How is a feature marked "experimental"?

A: Experimental features must contain the following note in their man pages (ovn-nb.5, ovn-sb.5, ovn-controller.8, etc): "NOTE: this feature is experimental and may be subject to removal/change in the future.: