ovn-architecture -- Open Virtual Network architecture
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. Services such as DHCP are also desirable features. Just like OVS, OVN's design goal is to have a production-quality implementation that can operate at significant scale.
An OVN deployment consists of several components:
A Cloud Management System (CMS), which is OVN's ultimate client (via its users and administrators). OVN integration requires installing a CMS-specific plugin and related software (see below). OVN initially targets OpenStack as CMS.
We generally speak of ``the'' CMS, but one can imagine scenarios in which multiple CMSes manage different parts of an OVN deployment.
IntegrationGuide.md
in the OVS source tree. Any hypervisor
platform supported by Open vSwitch is acceptable.
Zero or more gateways. A gateway extends a tunnel-based
logical network into a physical network by bidirectionally forwarding
packets between tunnels and a physical Ethernet port. This allows
non-virtualized machines to participate in logical networks. A gateway
may be a physical host, a virtual machine, or an ASIC-based hardware
switch that supports the vtep
(5) schema. (Support for the
latter will come later in OVN implementation.)
Hypervisors and gateways are together called transport node or chassis.
The diagram below shows how the major components of OVN and related software interact. Starting at the top of the diagram, we have:
The OVN/CMS Plugin is the component of the CMS that interfaces to OVN. In OpenStack, this is a Neutron plugin. The plugin's main purpose is to translate the CMS's notion of logical network configuration, stored in the CMS's configuration database in a CMS-specific format, into an intermediate representation understood by OVN.
This component is necessarily CMS-specific, so a new plugin needs to be developed for each CMS that is integrated with OVN. All of the components below this one in the diagram are CMS-independent.
The OVN Northbound Database receives the intermediate
representation of logical network configuration passed down by the
OVN/CMS Plugin. The database schema is meant to be ``impedance
matched'' with the concepts used in a CMS, so that it directly supports
notions of logical switches, routers, ACLs, and so on. See
ovn-nb
(5) for details.
The OVN Northbound Database has only two clients: the OVN/CMS Plugin
above it and ovn-northd
below it.
ovn-northd
(8) connects to the OVN Northbound Database
above it and the OVN Southbound Database below it. It translates the
logical network configuration in terms of conventional network
concepts, taken from the OVN Northbound Database, into logical
datapath flows in the OVN Southbound Database below it.
The OVN Southbound Database is the center of the system.
Its clients are ovn-northd
(8) above it and
ovn-controller
(8) on every transport node below it.
The OVN Southbound Database contains three kinds of data: Physical
Network (PN) tables that specify how to reach hypervisor and
other nodes, Logical Network (LN) tables that describe the
logical network in terms of ``logical datapath flows,'' and
Binding tables that link logical network components'
locations to the physical network. The hypervisors populate the PN and
Port_Binding tables, whereas ovn-northd
(8) populates the
LN tables.
OVN Southbound Database performance must scale with the number of
transport nodes. This will likely require some work on
ovsdb-server
(1) as we encounter bottlenecks.
Clustering for availability may be needed.
The remaining components are replicated onto each hypervisor:
ovn-controller
(8) is OVN's agent on each hypervisor and
software gateway. Northbound, it connects to the OVN Southbound
Database to learn about OVN configuration and status and to
populate the PN table and the Chassis
column in
Binding
table with the hypervisor's status.
Southbound, it connects to ovs-vswitchd
(8) as an
OpenFlow controller, for control over network traffic, and to the
local ovsdb-server
(1) to allow it to monitor and
control Open vSwitch configuration.
ovs-vswitchd
(8) and ovsdb-server
(1) are
conventional components of Open vSwitch.
CMS | | +-----------|-----------+ | | | | OVN/CMS Plugin | | | | | | | | OVN Northbound DB | | | | | | | | ovn-northd | | | | +-----------|-----------+ | | +-------------------+ | OVN Southbound DB | +-------------------+ | | +------------------+------------------+ | | | HV 1 | | HV n | +---------------|---------------+ . +---------------|---------------+ | | | . | | | | ovn-controller | . | ovn-controller | | | | | . | | | | | | | | | | | | | ovs-vswitchd ovsdb-server | | ovs-vswitchd ovsdb-server | | | | | +-------------------------------+ +-------------------------------+
Each chassis in an OVN deployment must be configured with an Open vSwitch
bridge dedicated for OVN's use, called the integration bridge.
System startup scripts may create this bridge prior to starting
ovn-controller
if desired. If this bridge does not exist when
ovn-controller starts, it will be created automatically with the default
configuration suggested below. The ports on the integration bridge include:
ovn-controller
adds, updates, and removes
these tunnel ports.
IntegrationGuide.md
) takes care of
this. (This is not part of OVN or new to OVN; this is pre-existing
integration work that has already been done on hypervisors that support
OVS.)
ovn-controller
. This can be a patch port to another bridge,
instead of a physical port, in more sophisticated setups.
Other ports should not be attached to the integration bridge. In particular, physical ports attached to the underlay network (as opposed to gateway ports, which are physical ports attached to logical networks) must not be attached to the integration bridge. Underlay physical ports should instead be attached to a separate Open vSwitch bridge (they need not be attached to any bridge at all, in fact).
The integration bridge should be configured as described below.
The effect of each of these settings is documented in
ovs-vswitchd.conf.db
(5):
fail-mode=secure
ovn-controller
starts up. See Controller Failure
Settings
in ovs-vsctl
(8) for more information.
other-config:disable-in-band=true
In-Band
Control
in DESIGN.md
for more information.
The customary name for the integration bridge is br-int
, but
another name may be used.
A logical network implements the same concepts as physical networks, but they are insulated from the physical network with tunnels or other encapsulations. This allows logical networks to have separate IP and other address spaces that overlap, without conflicting, with those used for physical networks. Logical network topologies can be arranged without regard for the topologies of the physical networks on which they run.
Logical network concepts in OVN include:
Tables and their schemas presented in isolation are difficult to understand. Here's an example.
A VIF on a hypervisor is a virtual network interface attached either to a VM or a container running directly on that hypervisor (This is different from the interface of a container running inside a VM).
The steps in this example refer often to details of the OVN and OVN
Northbound database schemas. Please see ovn-sb
(5) and
ovn-nb
(5), respectively, for the full story on these
databases.
Logical_Port
table. In the new
row, name
is vif-id, mac
is
mac, switch
points to the OVN logical switch's
Logical_Switch record, and other columns are initialized appropriately.
ovn-northd
receives the OVN Northbound database update. In
turn, it makes the corresponding updates to the OVN Southbound database,
by adding rows to the OVN Southbound database Logical_Flow
table to reflect the new port, e.g. add a flow to recognize that packets
destined to the new port's MAC address should be delivered to it, and
update the flow that delivers broadcast and multicast packets to include
the new port. It also creates a record in the Binding
table
and populates all its columns except the column that identifies the
chassis
.
ovn-controller
receives the
Logical_Flow
table updates that ovn-northd
made
in the previous step. As long as the VM that owns the VIF is powered
off, ovn-controller
cannot do much; it cannot, for example,
arrange to send packets to or receive packets from the VIF, because the
VIF does not actually exist anywhere.
IntegrationGuide.md
) adds the VIF
to the OVN integration bridge and stores vif-id in
external-ids
:iface-id
to indicate that the
interface is an instantiation of the new VIF. (None of this code is new
in OVN; this is pre-existing integration work that has already been done
on hypervisors that support OVS.)
ovn-controller
notices external-ids
:iface-id
in the new
Interface. In response, it updates the local hypervisor's OpenFlow
tables so that packets to and from the VIF are properly handled.
Afterward, in the OVN Southbound DB, it updates the
Binding
table's chassis
column for the
row that links the logical port from
external-ids
:iface-id
to the hypervisor.
ovn-northd
notices
the chassis
column updated for the row in
Binding
table and pushes this upward by updating the
column in the OVN
Northbound database's table to
indicate that the VIF is now up. The CMS, if it uses this feature, can
then
react by allowing the VM's execution to proceed.
ovn-controller
notices the completely populated row in the
Binding
table. This provides ovn-controller
the physical location of the logical port, so each instance updates the
OpenFlow tables of its switch (based on logical datapath flows in the OVN
DB Logical_Flow
table) so that packets to and from the VIF
can be properly handled via tunnels.
ovn-controller
notices that the VIF was deleted. In
response, it removes the Chassis
column content in the
Binding
table for the logical port.
ovn-controller
notices the empty
Chassis
column in the Binding
table's row
for the logical port. This means that ovn-controller
no
longer knows the physical location of the logical port, so each instance
updates its OpenFlow table to reflect that.
Logical_Port
table.
ovn-northd
receives the OVN Northbound update and in turn
updates the OVN Southbound database accordingly, by removing or updating
the rows from the OVN Southbound database Logical_Flow
table
and Binding
table that were related to the now-destroyed
VIF.
ovn-controller
receives the
Logical_Flow
table updates that ovn-northd
made
in the previous step. ovn-controller
updates OpenFlow
tables to reflect the update, although there may not be much to do, since
the VIF had already become unreachable when it was removed from the
Binding
table in a previous step.
OVN provides virtual network abstractions by converting information written in OVN_NB database to OpenFlow flows in each hypervisor. Secure virtual networking for multi-tenants can only be provided if OVN controller is the only entity that can modify flows in Open vSwitch. When the Open vSwitch integration bridge resides in the hypervisor, it is a fair assumption to make that tenant workloads running inside VMs cannot make any changes to Open vSwitch flows.
If the infrastructure provider trusts the applications inside the containers not to break out and modify the Open vSwitch flows, then containers can be run in hypervisors. This is also the case when containers are run inside the VMs and Open vSwitch integration bridge with flows added by OVN controller resides in the same VM. For both the above cases, the workflow is the same as explained with an example in the previous section ("Life Cycle of a VIF").
This section talks about the life cycle of a container interface (CIF) when containers are created in the VMs and the Open vSwitch integration bridge resides inside the hypervisor. In this case, even if a container application breaks out, other tenants are not affected because the containers running inside the VMs cannot modify the flows in the Open vSwitch integration bridge.
When multiple containers are created inside a VM, there are multiple CIFs associated with them. The network traffic associated with these CIFs need to reach the Open vSwitch integration bridge running in the hypervisor for OVN to support virtual network abstractions. OVN should also be able to distinguish network traffic coming from different CIFs. There are two ways to distinguish network traffic of CIFs.
One way is to provide one VIF for every CIF (1:1 model). This means that there could be a lot of network devices in the hypervisor. This would slow down OVS because of all the additional CPU cycles needed for the management of all the VIFs. It would also mean that the entity creating the containers in a VM should also be able to create the corresponding VIFs in the hypervisor.
The second way is to provide a single VIF for all the CIFs (1:many model). OVN could then distinguish network traffic coming from different CIFs via a tag written in every packet. OVN uses this mechanism and uses VLAN as the tagging mechanism.
Logical_Port
table. In the new row, name
is
any unique identifier, parent_name
is the vif-id
of the VM through which the CIF's network traffic is expected to go
through and the tag
is the VLAN tag that identifies the
network traffic of that CIF.
ovn-northd
receives the OVN Northbound database update. In
turn, it makes the corresponding updates to the OVN Southbound database,
by adding rows to the OVN Southbound database's Logical_Flow
table to reflect the new port and also by creating a new row in the
Binding
table and populating all its columns except the
column that identifies the chassis
.
ovn-controller
subscribes to the
changes in the Binding
table. When a new row is created
by ovn-northd
that includes a value in
parent_port
column of Binding
table, the
ovn-controller
in the hypervisor whose OVN integration bridge
has that same value in vif-id in
external-ids
:iface-id
updates the local hypervisor's OpenFlow tables so that packets to and
from the VIF with the particular VLAN tag
are properly
handled. Afterward it updates the chassis
column of
the Binding
to reflect the physical location.
ovn-northd
notices the updated chassis
column in Binding
table and updates the column in the OVN Northbound database's
table to indicate that the
CIF is now up. The entity responsible to start the container application
queries this value and starts the application.
Logical_Port
table.
ovn-northd
receives the OVN Northbound update and in turn
updates the OVN Southbound database accordingly, by removing or updating
the rows from the OVN Southbound database Logical_Flow
table
that were related to the now-destroyed CIF. It also deletes the row in
the Binding
table for that CIF.
ovn-controller
receives the
Logical_Flow
table updates that ovn-northd
made
in the previous step. ovn-controller
updates OpenFlow
tables to reflect the update.
This section describes how a packet travels from one virtual machine or
container to another through OVN. This description focuses on the physical
treatment of a packet; for a description of the logical life cycle of a
packet, please refer to the Logical_Flow
table in
ovn-sb
(5).
This section mentions several data and metadata fields, for clarity summarized here:
Tunnel Encapsulations
, below, for details.
A field that denotes the logical port from which the packet entered the logical datapath. OVN stores this in Nicira extension register number 6.
Geneve and STT tunnels pass this field as part of the tunnel key. Although VXLAN tunnels do not explicitly carry a logical input port, OVN only uses VXLAN to communicate with gateways that from OVN's perspective consist of only a single logical port, so that OVN can set the logical input port field to this one on ingress to the OVN logical pipeline.
A field that denotes the logical port from which the packet will leave the logical datapath. This is initialized to 0 at the beginning of the logical ingress pipeline. OVN stores this in Nicira extension register number 7.
Geneve and STT tunnels pass this field as part of the tunnel key. VXLAN tunnels do not transmit the logical output port field.
Life Cycle of a container interface inside a
VM
, above, for more information).
Initially, a VM or container on the ingress hypervisor sends a packet on a port attached to the OVN integration bridge. Then:
OpenFlow table 0 performs physical-to-logical translation. It matches the packet's ingress port. Its actions annotate the packet with logical metadata, by setting the logical datapath field to identify the logical datapath that the packet is traversing and the logical input port field to identify the ingress port. Then it resubmits to table 16 to enter the logical ingress pipeline.
Packets that originate from a container nested within a VM are treated in a slightly different way. The originating container can be distinguished based on the VIF-specific VLAN ID, so the physical-to-logical translation flows additionally match on VLAN ID and the actions strip the VLAN header. Following this step, OVN treats packets from containers just like any other packets.
Table 0 also processes packets that arrive from other chassis. It
distinguishes them from other packets by ingress port, which is a
tunnel. As with packets just entering the OVN pipeline, the actions
annotate these packets with logical datapath and logical ingress port
metadata. In addition, the actions set the logical output port field,
which is available because in OVN tunneling occurs after the logical
output port is known. These three pieces of information are obtained
from the tunnel encapsulation metadata (see Tunnel
Encapsulations
for encoding details). Then the actions resubmit
to table 33 to enter the logical egress pipeline.
OpenFlow tables 16 through 31 execute the logical ingress pipeline from
the Logical_Flow
table in the OVN Southbound database.
These tables are expressed entirely in terms of logical concepts like
logical ports and logical datapaths. A big part of
ovn-controller
's job is to translate them into equivalent
OpenFlow (in particular it translates the table numbers:
Logical_Flow
tables 0 through 15 become OpenFlow tables 16
through 31).
Most OVN actions have fairly obvious implementations in OpenFlow (with
OVS extensions), e.g. next;
is implemented as
resubmit
, field =
constant;
as set_field
. A few are worth
describing in more detail:
output:
output
action, then each one is
separately resubmitted to table 32. This can be used to send
multiple copies of the packet to multiple ports. (If the packet was
not modified between the output
actions, and some of the
copies are destined to the same hypervisor, then using a logical
multicast output port would save bandwidth between hypervisors.)
get_arp(P, A);
Implemented by storing arguments into OpenFlow fields, then
resubmitting to table 65, which ovn-controller
populates with flows generated from the MAC_Binding
table in the OVN Southbound database. If there is a match in table
65, then its actions store the bound MAC in the Ethernet
destination address field.
(The OpenFlow actions save and restore the OpenFlow fields used for the arguments, so that the OVN actions do not have to be aware of this temporary use.)
put_arp(P, A, E);
Implemented by storing the arguments into OpenFlow fields, then
outputting a packet to ovn-controller
, which updates
the MAC_Binding
table.
(The OpenFlow actions save and restore the OpenFlow fields used for the arguments, so that the OVN actions do not have to be aware of this temporary use.)
OpenFlow tables 32 through 47 implement the output
action
in the logical ingress pipeline. Specifically, table 32 handles
packets to remote hypervisors, table 33 handles packets to the local
hypervisor, and table 34 discards packets whose logical ingress and
egress port are the same.
Logical patch ports are a special case. Logical patch ports do not have a physical location and effectively reside on every hypervisor. Thus, flow table 33, for output to ports on the local hypervisor, naturally implements output to unicast logical patch ports too. However, applying the same logic to a logical patch port that is part of a logical multicast group yields packet duplication, because each hypervisor that contains a logical port in the multicast group will also output the packet to the logical patch port. Thus, multicast groups implement output to logical patch ports in table 32.
Each flow in table 32 matches on a logical output port for unicast or multicast logical ports that include a logical port on a remote hypervisor. Each flow's actions implement sending a packet to the port it matches. For unicast logical output ports on remote hypervisors, the actions set the tunnel key to the correct value, then send the packet on the tunnel port to the correct hypervisor. (When the remote hypervisor receives the packet, table 0 there will recognize it as a tunneled packet and pass it along to table 33.) For multicast logical output ports, the actions send one copy of the packet to each remote hypervisor, in the same way as for unicast destinations. If a multicast group includes a logical port or ports on the local hypervisor, then its actions also resubmit to table 33. Table 32 also includes a fallback flow that resubmits to table 33 if there is no other match.
Flows in table 33 resemble those in table 32 but for logical ports that reside locally rather than remotely. For unicast logical output ports on the local hypervisor, the actions just resubmit to table 34. For multicast output ports that include one or more logical ports on the local hypervisor, for each such logical port P, the actions change the logical output port to P, then resubmit to table 34.
A special case is that when a localnet port exists on the datapath, remote port is connected by switching to the localnet port. In this case, instead of adding a flow in table 32 to reach the remote port, a flow is added in table 33 to switch the logical outport to the localnet port, and resubmit to table 33 as if it were unicasted to a logical port on the local hypervisor.
Table 34 matches and drops packets for which the logical input and output ports are the same. It resubmits other packets to table 48.
OpenFlow tables 48 through 63 execute the logical egress pipeline from
the Logical_Flow
table in the OVN Southbound database.
The egress pipeline can perform a final stage of validation before
packet delivery. Eventually, it may execute an output
action, which ovn-controller
implements by resubmitting to
table 64. A packet for which the pipeline never executes
output
is effectively dropped (although it may have been
transmitted through a tunnel across a physical network).
The egress pipeline cannot change the logical output port or cause further tunneling.
OpenFlow table 64 performs logical-to-physical translation, the opposite of table 0. It matches the packet's logical egress port. Its actions output the packet to the port attached to the OVN integration bridge that represents that logical port. If the logical egress port is a container nested with a VM, then before sending the packet the actions push on a VLAN header with an appropriate VLAN ID.
If the logical egress port is a logical patch port, then table 64 outputs to an OVS patch port that represents the logical patch port. The packet re-enters the OpenFlow flow table from the OVS patch port's peer in table 0, which identifies the logical datapath and logical input port based on the OVS patch port's OpenFlow port number.
A gateway is a chassis that forwards traffic between the OVN-managed part of a logical network and a physical VLAN, extending a tunnel-based logical network into a physical network.
The steps below refer often to details of the OVN and VTEP database
schemas. Please see ovn-sb
(5), ovn-nb
(5)
and vtep
(5), respectively, for the full story on these
databases.
Physical_Switch
table entry in the
VTEP
database. The ovn-controller-vtep
connected to this VTEP database, will recognize the new VTEP gateway
and create a new Chassis
table entry for it in the
OVN_Southbound
database.
Logical_Switch
table entry, and bind a particular vlan on a VTEP gateway's port to
any VTEP logical switch. Once a VTEP logical switch is bound to
a VTEP gateway, the ovn-controller-vtep
will detect
it and add its name to the vtep_logical_switches
column of the Chassis
table in the
OVN_Southbound
database. Note, the tunnel_key
column of VTEP logical switch is not filled at creation. The
ovn-controller-vtep
will set the column when the
correponding vtep logical switch is bound to an OVN logical network.
Logical_Port
table entry in the
OVN_Northbound
database. Then, the type column
of this entry must be set to "vtep". Next, the
vtep-logical-switch and vtep-physical-switch keys
in the options column must also be specified, since
multiple VTEP gateways can attach to the same VTEP logical switch.
OVN_Northbound
database and its configuration will be passed down to the
OVN_Southbound
database as a new Port_Binding
table entry. The ovn-controller-vtep
will recognize the
change and bind the logical port to the corresponding VTEP gateway
chassis. Configuration of binding the same VTEP logical switch to
a different OVN logical networks is not allowed and a warning will be
generated in the log.
ovn-controller-vtep
will update the tunnel_key
column of the VTEP logical switch to the corresponding
Datapath_Binding
table entry's tunnel_key for the
bound OVN logical network.
ovn-controller-vtep
will keep reacting to the
configuration change in the Port_Binding
in the
OVN_Northbound
database, and updating the
Ucast_Macs_Remote
table in the VTEP
database.
This allows the VTEP gateway to understand where to forward the unicast
traffic coming from the extended external network.
VTEP
database.
The ovn-controller-vtep
will recognize the event and
remove all related configurations (Chassis
table entry
and port bindings) in the OVN_Southbound
database.
ovn-controller-vtep
is terminated, all related
configurations in the OVN_Southbound
database and
the VTEP
database will be cleaned, including
Chassis
table entries for all registered VTEP gateways
and their port bindings, and all Ucast_Macs_Remote
table
entries and the Logical_Switch
tunnel keys.
OVN annotates logical network packets that it sends from one hypervisor to another with the following three pieces of metadata, which are encoded in an encapsulation-specific fashion:
tunnel_key
column in the OVN Southbound Datapath_Binding
table.
tunnel_key
column in the OVN
Southbound Port_Binding
table).
tunnel_key
column in the OVN Southbound
Multicast_Group
table).
For hypervisor-to-hypervisor traffic, OVN supports only Geneve and STT encapsulations, for the following reasons:
Due to its flexibility, the preferred encapsulation between hypervisors is Geneve. For Geneve encapsulation, OVN transmits the logical datapath identifier in the Geneve VNI. OVN transmits the logical ingress and logical egress ports in a TLV with class 0x0102, type 0, and a 32-bit value encoded as follows, from MSB to LSB:
Environments whose NICs lack Geneve offload may prefer STT encapsulation for performance reasons. For STT encapsulation, OVN encodes all three pieces of logical metadata in the STT 64-bit tunnel ID as follows, from MSB to LSB:
For connecting to gateways, in addition to Geneve and STT, OVN supports VXLAN, because only VXLAN support is common on top-of-rack (ToR) switches. Currently, gateways have a feature set that matches the capabilities as defined by the VTEP schema, so fewer bits of metadata are necessary. In the future, gateways that do not support encapsulations with large amounts of metadata may continue to have a reduced feature set.