What’s New with OVS and OVN 2.8
This document is about what was added in Open vSwitch 2.8, which was released at the end of August 2017, concentrating on the new features in OVN. It also covers some of what is coming up in Open vSwitch and OVN 2.9, which is due to be released in February 2018. OVN has many features, and this document does not cover every new or enhanced feature (but contributions are welcome).
This document assumes a basic familiarity with Open vSwitch, OVN, and their
associated tools. For more information, please refer to the Open vSwitch and
OVN documentation, such as the
Debugging and Troubleshooting
Before version 2.8, Open vSwitch command-line tools were far more painful to use than they needed to be. This section covers the improvements made to the CLI in the 2.8 release.
The OVN CLI, through
full-length UUIDs almost everywhere. It didn’t even provide any assistance
with completion, etc., which in practice meant always cutting and pasting UUIDs
from one command or window to another. This problem wasn’t limited to the
places where one would expect to have to see or use a UUID, either. In many
places where one would expect to be able to use a network, router, or port
name, a UUID was required instead. In many places where one would want to see
a name, the UUID was displayed instead. More than anything else, these
shortcomings made the CLI user-hostile.
There was an underlying problem that the southbound database didn’t actually contain all the information needed to provide a decent user interface. In some cases, for example, the human-friendly names that one would want to use for entities simply weren’t part of the database. These names weren’t necessary for correctness, only for usability.
OVN 2.8 eased many of these problems. Most parts of the CLI now allow the user
to abbreviate UUIDs, as long as the abbreviations are unique within the
database. Some parts of the CLI where full-length UUIDs make output hard to
read now abbreviate them themselves. Perhaps more importantly, in many places
the OVN CLI now displays and accepts human-friendly names for networks,
routers, ports, and other entities. In the places where the names were not
previously available, OVN (through
ovn-northd) now copies the names into
the southbound database.
The CLIs for layers below OVN, at the OpenFlow and datapath layers with
ovs-dpctl, respectively, had some similar problems in
which numbers were used for entities that had human-friendly names. Open
vSwitch 2.8 also solves some of those problems. Other than that, the most
notable enhancement in this area was the
--no-stats option to
dump-flows, which made that command’s output more readable for the cases
where per-flow statistics were not interesting to the reader.
Connections Between Levels
OVN and Open vSwitch work almost like a stack of compilers: the OVN Neutron
plugin translates Neutron configuration into OVN northbound configuration,
ovn-northd translates into logical flows, which
translates into OpenFlow flows, which
ovs-vswitchd translates into datapath
flows. For debugging and troubleshooting it is often necessary to understand
exactly how these translations work. The relationship from a logical flow to
its OpenFlow flows, or in the other direction, from an OpenFlow flow back to
the logical flow that produced it, was often of particular interest, but OVN
didn’t provide good tools for the job.
OVN 2.8 added some new features that ease these jobs.
has a new option
--ovs that lists the OpenFlow flows on a particular
chassis that were generated from the logical flows that it lists.
ovn-trace also added a similar
--ovs option that applies to the logical
flows it traces.
In the other direction, OVN 2.8 added a new utility
ovn-detrace that, given
an Open vSwitch trace of OpenFlow flows, annotates it with the logical flows
that yielded those OpenFlow flows.
OVN supports a distributed firewall with stateful connection tracking to ensure
that only packets for established connections, or those that the configuration
explicitly allows, can ingress a given VM or container. Neutron uses this
feature by default. Most packets in an OpenStack environment pass through it
twice, once after egress from the packet’s source VM and once before ingress
into its destination VM. Before OVN 2.8, the
ovn-trace program, which
shows the path of a packet through an OVN logical network, did not support the
logical firewall, which in practice made it almost useless for Neutron.
In OVN 2.8,
ovn-trace adds support for the logical firewall. By default it
assumes that packets are part of an established connection, which is usually
what the user wants as part of the trace. It also accepts command-line options
to override that assumption, which allows the user to discover the treatment of
packets that the firewall should drop.
At the next level deeper, prior to Open vSwitch 2.8, the OpenFlow tracing
ofproto/trace also supported neither the connection tracking
feature underlying the OVN distributed firewall nor the “recirculation” feature
that accompanied it. This meant that, even if the user tried to look deeper
into the distributed firewall mechanism, he or she would encounter a further
roadblock. Open vSwitch 2.8 added support for both of these features as well.
ovn-nbctl show and
ovn-sbctl show, for showing an overview of the OVN
configuration, didn’t show a lot of important information. OVN adds some more
useful information here.
DNS, and IPAM
OVN 2.8 adds a built-in DNS server designed for assigning names to VMs and
containers within an OVN logical network. DNS names are assigned using records
in the OVN northbound database and, like other OVN features, translated into
logical flows at the OVN southbound layer. DNS requests directed to the OVN
DNS server never leave the hypervisor from which the request is sent; instead,
OVN processes and replies to the request from its
agent. The OVN DNS server is not a general-purpose DNS server and cannot be
used for that purpose.
OVN includes simple built-in support for IP address management (IPAM), in which OVN assigns IP addresses to VMs or containers from a pool or pools of IP addresses delegated to it by the administrator. Before OVN 2.8, OVN IPAM only supported IPv4 addresses; OVN 2.8 adds support for IPv6. OVN 2.8 also enhances the address pool support to allow specific addresses to be excluded. Neutron assigns IP addresses itself and does not use OVN IPAM.
As a distributed system, in OVN a lot can go wrong. As OVN advances, it adds redundancy in places where currently a single failure could disrupt the functioning of the system as a whole. OVN 2.8 adds two new kinds of high availability.
ovn-northd program sits between the OVN northbound and southbound
databases and translates from a logical network configuration into logical
ovn-northd itself or the host on which it runs fails, then
updates to the OVN northbound configuration will not propagate to the
hypervisors and the OVN configuration freezes in place until
OVN 2.8 adds support for active-backup HA to
ovn-northd. When more than
ovn-northd instance runs, it uses an OVSDB locking feature to
automatically choose a single active instance. When that instance dies or
becomes nonresponsive, the OVSDB server automatically choose one of the
remaining instance(s) to take over.
L3 Gateway HA
In OVN 2.8, multiple chassis may now be specified for L3 gateways. When more than one chassis is specified, OVN manages high availability for that gateway. Each hypervisor uses the BFD protocol to keep track of the gateway nodes that are currently up. At any given time, a hypervisor uses the highest-priority gateway node that is currently up.
The OVN architecture relies heavily on OVSDB, the Open vSwitch database, for hosting the northbound and southbound databases. OVSDB was originally selected for this purpose because it was already used in Open vSwitch for configuring OVS itself and, thus, it was well integrated with OVS and well supported in C and Python, the two languages that are used in Open vSwitch.
OVSDB was well designed for its original purpose of configuring Open vSwitch. It supports ACID transactions, has a small, efficient server, a flexible schema system, and good support for troubleshooting and debugging. However, it lacked several features that are important for OVN but not for Open vSwitch. As OVN advances, these missing features have become more and more of a problem. One option would be to switch to a different database that already has many of these features, but despite a careful search, no ideal existing database was identified, so the project chose instead to improve OVSDB where necessary to bring it up to speed. The following sections talk more about recent and future improvements.
ovsdb-server was only used for OVS configuration, high availability
was not important.
ovsdb-server was capable of restarting itself
automatically if it crashed, and if the whole system went down then Open
vSwitch itself was dead too, so the database server’s failure was not
In contrast, the northbound and southbound databases are centralized components
of a distributed system, so it is important that they not be a single point of
failure for the system as a whole. In released versions of OVN,
ovsdb-server supports only “active-backup replication” across a pair of
servers. This means that if one server goes down, the other can pick it back
up approximately where the other one left off. The servers do not have
built-in support for deciding at any given time which is the active and which
the backup, so the administrator must configure an external agent to do this
Active-backup replication is not entirely satisfactory, for multiple reasons. Replication is only approximate. Configuring the external agent requires extra work. There is no benefit from the backup server except when the active server fails. At most two servers can be used.
A new form of high availability for OVSDB is under development for the OVN 2.9
release, based on the Raft algorithm for distributed consensus. Whereas
replication uses two servers, clustering using Raft requires three or more
(typically an odd number) and continues functioning as long as more than half
of the servers are up. The clustering implementation is built into
ovsdb-server and does not require an external agent. Clustering preserves
the ACID properties of the database, so that a transaction that commits is
guaranteed to persist. Finally, reads (which are the bulk of the OVN workload)
scale with the size of the cluster, so that adding more servers should improve
performance as the number of hypervisors in an OVN deployment increases. As of
this writing, OVSDB support for clustering is undergoing development and early
Until Open vSwitch 2.8,
ovsdb-server had little support for access control
within a database. If an OVSDB client could modify the database at all, it
could make arbitrary changes. This was sufficient for most uses case to that
Hypervisors in an OVN deployment need access to the OVN southbound database. Most of their access is reads, to find out about the OVN configuration. Hypervisors do need some write access to the southbound database, primarily to let the other hypervisors know what VMs and containers they are running and how to reach them. Thus, OVN gives all of the hypervisors in the OVN deployment write access to the OVN southbound database. This is fine when all is well, but if any of the hypervisors were compromised then they could disrupt the entire OVN deployment by corrupting the database.
The OVN developers considered a few ways to solve this problem. One way would
be to introduce a new central service (perhaps in
ovn-northd) that provided
only the kinds of writes that the hypervisors legitimately need, and then grant
hypervisors direct access to the southbound database only for reads. But
ultimately the developers decided to introduce a new form of more access
control for OVSDB, called the OVSDB RBAC (role-based access control) feature.
OVSDB RBAC allows for granular enough control over access that hypervisors can
be granted only the ability to add, modify, and delete the records that relate
to themselves, preventing them from corrupting the database as a whole.
For more information about new features in OVN and Open vSwitch, please refer to the NEWS file distributed with the source tree. If you have questions about Open vSwitch or OVN features, please feel free to write to the Open vSwitch discussion mailing list at email@example.com.