Ubuntu Manpage: ovn-architecture - Open Virtual Network architecture

NAME

       ovn-architecture - Open Virtual Network architecture

DESCRIPTION

OVN, the Open Virtual Network, is a system to support logical network abstraction in
virtual machine and container environments. OVN complements the existing capabilities of
OVS to add native support for logical network abstractions, such as logical 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.

A physical network comprises physical wires, switches, and routers. A virtual network
extends a physical network into a hypervisor or container platform, bridging VMs or
containers into the physical network. An OVN logical network is a network implemented in
software that is insulated from physical (and thus virtual) networks by tunnels or other
encapsulations. This allows IP and other address spaces used in logical networks to
overlap with those used on physical networks without causing conflicts. Logical network
topologies can be arranged without regard for the topologies of the physical networks on
which they run. Thus, VMs that are part of a logical network can migrate from one physical
machine to another without network disruption. See Logical Networks, below, for more
information.

The encapsulation layer prevents VMs and containers connected to a logical network from
communicating with nodes on physical networks. For clustering VMs and containers, this can
be acceptable or even desirable, but in many cases VMs and containers do need connectivity
to physical networks. OVN provides multiple forms of gateways for this purpose. See
Gateways, below, for more information.

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.

• An OVN Database physical or virtual node (or, eventually, cluster) installed
in a central location.

• One or more (usually many) hypervisors. Hypervisors must run Open vSwitch
and implement the interface described in
Documentation/topics/integration.rst in the Open vSwitch 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.

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 Cloud Management System, as defined above.

• 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 |
| | | |
+-------------------------------+ +-------------------------------+

Information Flow in OVN
Configuration data in OVN flows from north to south. The CMS, through its OVN/CMS plugin,
passes the logical network configuration to ovn-northd via the northbound database. In
turn, ovn-northd compiles the configuration into a lower-level form and passes it to all
of the chassis via the southbound database.

Status information in OVN flows from south to north. OVN currently provides only a few
forms of status information. First, ovn-northd populates the up column in the northbound
Logical_Switch_Port table: if a logical port’s chassis column in the southbound
Port_Binding table is nonempty, it sets up to true, otherwise to false. This allows the
CMS to detect when a VM’s networking has come up.

Second, OVN provides feedback to the CMS on the realization of its configuration, that is,
whether the configuration provided by the CMS has taken effect. This feature requires the
CMS to participate in a sequence number protocol, which works the following way:

1. When the CMS updates the configuration in the northbound database, as part of
the same transaction, it increments the value of the nb_cfg column in the
NB_Global table. (This is only necessary if the CMS wants to know when the
configuration has been realized.)

2. When ovn-northd updates the southbound database based on a given snapshot of
the northbound database, it copies nb_cfg from northbound NB_Global into the
southbound database SB_Global table, as part of the same transaction. (Thus, an
observer monitoring both databases can determine when the southbound database
is caught up with the northbound.)

3. After ovn-northd receives confirmation from the southbound database server that
its changes have committed, it updates sb_cfg in the northbound NB_Global table
to the nb_cfg version that was pushed down. (Thus, the CMS or another observer
can determine when the southbound database is caught up without a connection to
the southbound database.)

4. The ovn-controller process on each chassis receives the updated southbound
database, with the updated nb_cfg. This process in turn updates the physical
flows installed in the chassis’s Open vSwitch instances. When it receives
confirmation from Open vSwitch that the physical flows have been updated, it
updates nb_cfg in its own Chassis record in the southbound database.

5. ovn-northd monitors the nb_cfg column in all of the Chassis records in the
southbound database. It keeps track of the minimum value among all the records
and copies it into the hv_cfg column in the northbound NB_Global table. (Thus,
the CMS or another observer can determine when all of the hypervisors have
caught up to the northbound configuration.)

Chassis Setup
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:

• On any chassis, tunnel ports that OVN uses to maintain logical network
connectivity. ovn-controller adds, updates, and removes these tunnel ports.

• On a hypervisor, any VIFs that are to be attached to logical networks. For
instances connected through software emulated ports such as TUN/TAP or VETH
pairs, the hypervisor itself will normally create ports and plug them into
the integration bridge. For instances connected through representor ports,
typically used with hardware offload, the ovn-controller may on CMS
direction consult a VIF plug provider for representor port lookup and plug
them into the integration bridge (please refer to
Documentation/topics/vif-plug-providers/vif-plug-providers.rst
for more information). In both cases the conventions described in
Documentation/topics/integration.rst in the Open vSwitch source tree is
followed to ensure mapping between OVN logical port and VIF. (This is pre-
existing integration work that has already been done on hypervisors that
support OVS.)

• On a gateway, the physical port used for logical network connectivity.
System startup scripts add this port to the bridge prior to starting
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
Avoids switching packets between isolated logical networks before
ovn-controller starts up. See Controller Failure Settings in ovs-vsctl(8)
for more information.

other-config:disable-in-band=true
Suppresses in-band control flows for the integration bridge. It would be
unusual for such flows to show up anyway, because OVN uses a local
controller (over a Unix domain socket) instead of a remote controller. It’s
possible, however, for some other bridge in the same system to have an in-
band remote controller, and in that case this suppresses the flows that in-
band control would ordinarily set up. Refer to the documentation for more
information.

The customary name for the integration bridge is br-int, but another name may be used.

Logical Networks
Logical network concepts in OVN include logical switches and logical routers, the logical
version of Ethernet switches and IP routers, respectively. Like their physical cousins,
logical switches and routers can be connected into sophisticated topologies. Logical
switches and routers are ordinarily purely logical entities, that is, they are not
associated or bound to any physical location, and they are implemented in a distributed
manner at each hypervisor that participates in OVN.

Logical switch ports (LSPs) are points of connectivity into and out of logical switches.
There are many kinds of logical switch ports. The most ordinary kind represent VIFs, that
is, attachment points for VMs or containers. A VIF logical port is associated with the
physical location of its VM, which might change as the VM migrates. (A VIF logical port
can be associated with a VM that is powered down or suspended. Such a logical port has no
location and no connectivity.)

Logical router ports (LRPs) are points of connectivity into and out of logical routers. A
LRP connects a logical router either to a logical switch or to another logical router.
Logical routers only connect to VMs, containers, and other network nodes indirectly,
through logical switches.

Logical switches and logical routers have distinct kinds of logical ports, so properly
speaking one should usually talk about logical switch ports or logical router ports.
However, an unqualified ``logical port’’ usually refers to a logical switch port.

When a VM sends a packet to a VIF logical switch port, the Open vSwitch flow tables
simulate the packet’s journey through that logical switch and any other logical routers
and logical switches that it might encounter. This happens without transmitting the packet
across any physical medium: the flow tables implement all of the switching and routing
decisions and behavior. If the flow tables ultimately decide to output the packet at a
logical port attached to another hypervisor (or another kind of transport node), then that
is the time at which the packet is encapsulated for physical network transmission and
sent.

Logical Switch Port Types

OVN supports a number of kinds of logical switch ports. VIF ports that connect to VMs or
containers, described above, are the most ordinary kind of LSP. In the OVN northbound
database, VIF ports have an empty string for their type. This section describes some of
the additional port types.

A router logical switch port connects a logical switch to a logical router, designating a
particular LRP as its peer.

A localnet logical switch port bridges a logical switch to a physical VLAN. A logical
switch may have one or more localnet ports. Such a logical switch is used in two
scenarios:

• With one or more router logical switch ports, to attach L3 gateway routers
and distributed gateways to a physical network.

• With one or more VIF logical switch ports, to attach VMs or containers
directly to a physical network. In this case, the logical switch is not
really logical, since it is bridged to the physical network rather than
insulated from it, and therefore cannot have independent but overlapping IP
address namespaces, etc. A deployment might nevertheless choose such a
configuration to take advantage of the OVN control plane and features such
as port security and ACLs.

When a logical switch contains multiple localnet ports, the following is assumed.

• Each chassis has a bridge mapping for one of the localnet physical networks
only.

• To facilitate interconnectivity between VIF ports of the switch that are
located on different chassis with different physical network connectivity,
the fabric implements L3 routing between these adjacent physical network
segments.

Note: nothing said above implies that a chassis cannot be plugged to multiple physical
networks as long as they belong to different switches.

A localport logical switch port is a special kind of VIF logical switch port. These ports
are present in every chassis, not bound to any particular one. Traffic to such a port will
never be forwarded through a tunnel, and traffic from such a port is expected to be
destined only to the same chassis, typically in response to a request it received.
OpenStack Neutron uses a localport port to serve metadata to VMs. A metadata proxy process
is attached to this port on every host and all VMs within the same network will reach it
at the same IP/MAC address without any traffic being sent over a tunnel. For further
details, see the OpenStack documentation for networking-ovn.

LSP types vtep and l2gateway are used for gateways. See Gateways, below, for more
information.

Implementation Details

These concepts are details of how OVN is implemented internally. They might still be of
interest to users and administrators.

Logical datapaths are an implementation detail of logical networks in the OVN southbound
database. ovn-northd translates each logical switch or router in the northbound database
into a logical datapath in the southbound database Datapath_Binding table.

For the most part, ovn-northd also translates each logical switch port in the OVN
northbound database into a record in the southbound database Port_Binding table. The
latter table corresponds roughly to the northbound Logical_Switch_Port table. It has
multiple types of logical port bindings, of which many types correspond directly to
northbound LSP types. LSP types handled this way include VIF (empty string), localnet,
localport, vtep, and l2gateway.

The Port_Binding table has some types of port binding that do not correspond directly to
logical switch port types. The common is patch port bindings, known as logical patch
ports. These port bindings always occur in pairs, and a packet that enters on either side
comes out on the other. ovn-northd connects logical switches and logical routers together
using logical patch ports.

Port bindings with types vtep, l2gateway, l3gateway, and chassisredirect are used for
gateways. These are explained in Gateways, below.

Gateways
Gateways provide limited connectivity between logical networks and physical ones. They can
also provide connectivity between different OVN deployments. This section will focus on
the former, and the latter will be described in details in section OVN Deployments
Interconnection.

OVN support multiple kinds of gateways.

VTEP Gateways

A ``VTEP gateway’’ connects an OVN logical network to a physical (or virtual) switch that
implements the OVSDB VTEP schema that accompanies Open vSwitch. (The ``VTEP gateway’’ term
is a misnomer, since a VTEP is just a VXLAN Tunnel Endpoint, but it is a well established
name.) See Life Cycle of a VTEP gateway, below, for more information.

The main intended use case for VTEP gateways is to attach physical servers to an OVN
logical network using a physical top-of-rack switch that supports the OVSDB VTEP schema.

L2 Gateways

A L2 gateway simply attaches a designated physical L2 segment available on some chassis to
a logical network. The physical network effectively becomes part of the logical network.

To set up a L2 gateway, the CMS adds an l2gateway LSP to an appropriate logical switch,
setting LSP options to name the chassis on which it should be bound. ovn-northd copies
this configuration into a southbound Port_Binding record. On the designated chassis,
ovn-controller forwards packets appropriately to and from the physical segment.

L2 gateway ports have features in common with localnet ports. However, with a localnet
port, the physical network becomes the transport between hypervisors. With an L2 gateway,
packets are still transported between hypervisors over tunnels and the l2gateway port is
only used for the packets that are on the physical network. The application for L2
gateways is similar to that for VTEP gateways, e.g. to add non-virtualized machines to a
logical network, but L2 gateways do not require special support from top-of-rack hardware
switches.

L3 Gateway Routers

As described above under Logical Networks, ordinary OVN logical routers are distributed:
they are not implemented in a single place but rather in every hypervisor chassis. This is
a problem for stateful services such as SNAT and DNAT, which need to be implemented in a
centralized manner.

To allow for this kind of functionality, OVN supports L3 gateway routers, which are OVN
logical routers that are implemented in a designated chassis. Gateway routers are
typically used between distributed logical routers and physical networks. The distributed
logical router and the logical switches behind it, to which VMs and containers attach,
effectively reside on each hypervisor. The distributed router and the gateway router are
connected by another logical switch, sometimes referred to as a ``join’’ logical switch.
(OVN logical routers may be connected to one another directly, without an intervening
switch, but the OVN implementation only supports gateway logical routers that are
connected to logical switches. Using a join logical switch also reduces the number of IP
addresses needed on the distributed router.) On the other side, the gateway router
connects to another logical switch that has a localnet port connecting to the physical
network.

The following diagram shows a typical situation. One or more logical switches LS1, ...,
LSn connect to distributed logical router LR1, which in turn connects through LSjoin to
gateway logical router GLR, which also connects to logical switch LSlocal, which includes
a localnet port to attach to the physical network.

LSlocal
|
GLR
|
LSjoin
|
LR1
|
+----+----+
| | |
LS1 ... LSn

To configure an L3 gateway router, the CMS sets options:chassis in the router’s northbound
Logical_Router to the chassis’s name. In response, ovn-northd uses a special l3gateway
port binding (instead of a patch binding) in the southbound database to connect the
logical router to its neighbors. In turn, ovn-controller tunnels packets to this port
binding to the designated L3 gateway chassis, instead of processing them locally.

DNAT and SNAT rules may be associated with a gateway router, which provides a central
location that can handle one-to-many SNAT (aka IP masquerading). Distributed gateway
ports, described below, also support NAT.

Distributed Gateway Ports

A distributed gateway port is a logical router port that is specially configured to
designate one distinguished chassis, called the gateway chassis, for centralized
processing. A distributed gateway port should connect to a logical switch that has an LSP
that connects externally, that is, either a localnet LSP or a connection to another OVN
deployment (see OVN Deployments Interconnection). Packets that traverse the distributed
gateway port are processed without involving the gateway chassis when they can be, but
when needed they do take an extra hop through it.

The following diagram illustrates the use of a distributed gateway port. A number of
logical switches LS1, ..., LSn connect to distributed logical router LR1, which in turn
connects through the distributed gateway port to logical switch LSlocal that includes a
localnet port to attach to the physical network.

LSlocal
|
LR1
|
+----+----+
| | |
LS1 ... LSn

ovn-northd creates two southbound Port_Binding records to represent a distributed gateway
port, instead of the usual one. One of these is a patch port binding named for the LRP,
which is used for as much traffic as it can. The other one is a port binding with type
chassisredirect, named cr-port. The chassisredirect port binding has one specialized job:
when a packet is output to it, the flow table causes it to be tunneled to the gateway
chassis, at which point it is automatically output to the patch port binding. Thus, the
flow table can output to this port binding in cases where a particular task has to happen
on the gateway chassis. The chassisredirect port binding is not otherwise used (for
example, it never receives packets).

The CMS may configure distributed gateway ports three different ways. See Distributed
Gateway Ports in the documentation for Logical_Router_Port in ovn-nb(5) for details.

Distributed gateway ports support high availability. When more than one chassis is
specified, OVN only uses one at a time as the gateway chassis. OVN uses BFD to monitor
gateway connectivity, preferring the highest-priority gateway that is online.

A logical router can have multiple distributed gateway ports, each connecting different
external networks. Load balancing is not yet supported for logical routers with more than
one distributed gateway port configured.

Physical VLAN MTU Issues

Consider the preceding diagram again:

LSlocal
|
LR1
|
+----+----+
| | |
LS1 ... LSn

Suppose that each logical switch LS1, ..., LSn is bridged to a physical VLAN-tagged
network attached to a localnet port on LSlocal, over a distributed gateway port on LR1. If
a packet originating on LSi is destined to the external network, OVN sends it to the
gateway chassis over a tunnel. There, the packet traverses LR1’s logical router pipeline,
possibly undergoes NAT, and eventually ends up at LSlocal’s localnet port. If all of the
physical links in the network have the same MTU, then the packet’s transit across a tunnel
causes an MTU problem: tunnel overhead prevents a packet that uses the full physical MTU
from crossing the tunnel to the gateway chassis (without fragmentation).

OVN offers two solutions to this problem, the reside-on-redirect-chassis and redirect-type
options. Both solutions require each logical switch LS1, ..., LSn to include a localnet
logical switch port LN1, ..., LNn respectively, that is present on each chassis. Both
cause packets to be sent over the localnet ports instead of tunnels. They differ in which
packets-some or all-are sent this way. The most prominent tradeoff between these options
is that reside-on-redirect-chassis is easier to configure and that redirect-type performs
better for east-west traffic.

The first solution is the reside-on-redirect-chassis option for logical router ports.
Setting this option on a LRP from (e.g.) LS1 to LR1 disables forwarding from LS1 to LR1
except on the gateway chassis. On chassis other than the gateway chassis, this single
change means that packets that would otherwise have been forwarded to LR1 are instead
forwarded to LN1. The instance of LN1 on the gateway chassis then receives the packet and
forwards it to LR1. The packet traverses the LR1 logical router pipeline, possibly
undergoes NAT, and eventually ends up at LSlocal’s localnet port. The packet never
traverses a tunnel, avoiding the MTU issue.

This option has the further consequence of centralizing ``distributed’’ logical router
LR1, since no packets are forwarded from LS1 to LR1 on any chassis other than the gateway
chassis. Therefore, east-west traffic passes through the gateway chassis, not just north-
south. (The naive ``fix’’ of allowing east-west traffic to flow directly between chassis
over LN1 does not work because routing sets the Ethernet source address to LR1’s source
address. Seeing this single Ethernet source address originate from all of the chassis will
confuse the physical switch.)

Do not set the reside-on-redirect-chassis option on a distributed gateway port. In the
diagram above, it would be set on the LRPs connecting LS1, ..., LSn to LR1.

The second solution is the redirect-type option for distributed gateway ports. Setting
this option to bridged causes packets that are redirected to the gateway chassis to go
over the localnet ports instead of being tunneled. This option does not change how OVN
treats packets not redirected to the gateway chassis.

The redirect-type option requires the administrator or the CMS to configure each
participating chassis with a unique Ethernet address for the logical router by setting
ovn-chassis-mac-mappings in the Open vSwitch database, for use by ovn-controller. This
makes it more difficult to configure than reside-on-redirect-chassis.

Set the redirect-type option on a distributed gateway port.

Using Distributed Gateway Ports For Scalability

Although the primary goal of distributed gateway ports is to provide connectivity to
external networks, there is a special use case for scalability.

In some deployments, such as the ones using ovn-kubernetes, logical switches are bound to
individual chassises, and are connected by a distributed logical router. In such
deployments, the chassis level logical switches are centralized on the chassis instead of
distributed, which means the ovn-controller on each chassis doesn’t need to process flows
and ports of logical switches on other chassises. However, without any specific hint,
ovn-controller would still process all the logical switches as if they are fully
distributed. In this case, distributed gateway port can be very useful. The chassis level
logical switches can be connected to the distributed router using distributed gateway
ports, by setting the gateway chassis (or HA chassis groups with only a single chassis in
it) to the chassis that each logical switch is bound to. ovn-controller would then skip
processing the logical switches on all the other chassises, largely improving the
scalability, especially when there are a big number of chassises.

Life Cycle of a VIF
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.

1. A VIF’s life cycle begins when a CMS administrator creates a new VIF using the
CMS user interface or API and adds it to a switch (one implemented by OVN as a
logical switch). The CMS updates its own configuration. This includes
associating unique, persistent identifier vif-id and Ethernet address mac with
the VIF.

2. The CMS plugin updates the OVN Northbound database to include the new VIF, by
adding a row to the Logical_Switch_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.

3. 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.

4. On every hypervisor, 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.

5. Eventually, a user powers on the VM that owns the VIF. On the hypervisor where
the VM is powered on, the integration between the hypervisor and Open vSwitch
(described in Documentation/topics/integration.rst in the Open vSwitch source
tree) 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.)

6. On the hypervisor where the VM is powered on, ovn-controller notices
external_ids:iface-id in the new Interface. In response, 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. Afterward,
ovn-controller updates the local hypervisor’s OpenFlow tables so that packets
to and from the VIF are properly handled.

7. Some CMS systems, including OpenStack, fully start a VM only when its
networking is ready. To support this, ovn-northd notices the chassis column
updated for the row in Binding table and pushes this upward by updating the up
column in the OVN Northbound database’s Logical_Switch_Port 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.

8. On every hypervisor but the one where the VIF resides, 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.

9. Eventually, a user powers off the VM that owns the VIF. On the hypervisor where
the VM was powered off, the VIF is deleted from the OVN integration bridge.

10. On the hypervisor where the VM was powered off, ovn-controller notices that the
VIF was deleted. In response, it removes the Chassis column content in the
Binding table for the logical port.

11. On every hypervisor, 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.

12. Eventually, when the VIF (or its entire VM) is no longer needed by anyone, an
administrator deletes the VIF using the CMS user interface or API. The CMS
updates its own configuration.

13. The CMS plugin removes the VIF from the OVN Northbound database, by deleting
its row in the Logical_Switch_Port table.

14. 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.

15. On every hypervisor, 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.

Life Cycle of a Container Interface Inside a VM
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.

1. A CIF’s life cycle begins when a container is spawned inside a VM by the either
the same CMS that created the VM or a tenant that owns that VM or even a
container Orchestration System that is different than the CMS that initially
created the VM. Whoever the entity is, it will need to know the vif-id that is
associated with the network interface of the VM through which the container
interface’s network traffic is expected to go through. The entity that creates
the container interface will also need to choose an unused VLAN inside that VM.

2. The container spawning entity (either directly or through the CMS that manages
the underlying infrastructure) updates the OVN Northbound database to include
the new CIF, by adding a row to the Logical_Switch_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.

3. 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.

4. On every hypervisor, 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.

5. One can only start the application inside the container after the underlying
network is ready. To support this, ovn-northd notices the updated chassis
column in Binding table and updates the up column in the OVN Northbound
database’s Logical_Switch_Port table to indicate that the CIF is now up. The
entity responsible to start the container application queries this value and
starts the application.

6. Eventually the entity that created and started the container, stops it. The
entity, through the CMS (or directly) deletes its row in the
Logical_Switch_Port table.

7. 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.

8. On every hypervisor, 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.

Architectural Physical Life Cycle of a Packet
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 key
When OVN encapsulates a packet in Geneve or another tunnel, it attaches
extra data to it to allow the receiving OVN instance to process it
correctly. This takes different forms depending on the particular
encapsulation, but in each case we refer to it here as the ``tunnel key.’’
See Tunnel Encapsulations, below, for details.

logical datapath field
A field that denotes the logical datapath through which a packet is being
processed. OVN uses the field that OpenFlow 1.1+ simply (and confusingly)
calls ``metadata’’ to store the logical datapath. (This field is passed
across tunnels as part of the tunnel key.)

logical input port field
A field that denotes the logical port from which the packet entered the
logical datapath. OVN stores this in Open vSwitch extension register number
14.

Geneve and STT tunnels pass this field as part of the tunnel key. Ramp
switch VXLAN tunnels do not explicitly carry a logical input port, but since
they are used 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. As for
regular VXLAN tunnels, they don’t carry input port field at all. This puts
additional limitations on cluster capabilities that are described in Tunnel
Encapsulations section.

logical output port field
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 Open vSwitch extension register number
15.

Geneve, STT and regular VXLAN tunnels pass this field as part of the tunnel
key. Ramp switch VXLAN tunnels do not transmit the logical output port
field, and since they do not carry a logical output port field in the tunnel
key, when a packet is received from ramp switch VXLAN tunnel by an OVN
hypervisor, the packet is resubmitted to table 8 to determine the output
port(s); when the packet reaches table 39, these packets are resubmitted to
table 40 for local delivery by checking a MLF_RCV_FROM_RAMP flag, which is
set when the packet arrives from a ramp tunnel.

conntrack zone field for logical ports
A field that denotes the connection tracking zone for logical ports. The
value only has local significance and is not meaningful between chassis.
This is initialized to 0 at the beginning of the logical ingress pipeline.
OVN stores this in the lower 16 bits of the Open vSwitch extension register
number 13.

conntrack zone fields for routers
Fields that denote the connection tracking zones for routers. These values
only have local significance and are not meaningful between chassis. OVN
stores the zone information for north to south traffic (for DNATting or ECMP
symmetric replies) in Open vSwitch extension register number 11 and zone
information for south to north traffic (for SNATing) in Open vSwitch
extension register number 12.

Encap ID for logical ports
A field that records an ID that indicates which encapsulation IP should be
used when sending packets to a remote chassis, according to the original
input logical port. This is useful when there are multiple IPs available for
encapsulation. The value only has local significance and is not meaningful
between chassis. This is initialized to 0 at the beginning of the logical
ingress pipeline. OVN stores this in the higher 16 bits of the Open vSwitch
extension register number 13.

logical flow flags
The logical flags are intended to handle keeping context between tables in
order to decide which rules in subsequent tables are matched. These values
only have local significance and are not meaningful between chassis. OVN
stores the logical flags in Open vSwitch extension register number 10.

VLAN ID
The VLAN ID is used as an interface between OVN and containers nested inside
a VM (see 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:

1. 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 8 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 metadata. For tunnel types that support it, they are also annotated
with 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 pieces of information are obtained from the
tunnel encapsulation metadata (see Tunnel Encapsulations for encoding details).
Then the actions resubmit to table 38 to enter the logical egress pipeline.

2. OpenFlow tables 8 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 23 become OpenFlow tables 8 through 31).

Each logical flow maps to one or more OpenFlow flows. An actual packet
ordinarily matches only one of these, although in some cases it can match more
than one of these flows (which is not a problem because all of them have the
same actions). ovn-controller uses the first 32 bits of the logical flow’s UUID
as the cookie for its OpenFlow flow or flows. (This is not necessarily unique,
since the first 32 bits of a logical flow’s UUID is not necessarily unique.)

Some logical flows can map to the Open vSwitch ``conjunctive match’’ extension
(see ovs-fields(7)). Flows with a conjunction action use an OpenFlow cookie of
0, because they can correspond to multiple logical flows. The OpenFlow flow for
a conjunctive match includes a match on conj_id.

Some logical flows may not be represented in the OpenFlow tables on a given
hypervisor, if they could not be used on that hypervisor. For example, if no
VIF in a logical switch resides on a given hypervisor, and the logical switch
is not otherwise reachable on that hypervisor (e.g. over a series of hops
through logical switches and routers starting from a VIF on the hypervisor),
then the logical flow may not be represented there.

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:
Implemented by resubmitting the packet to table 37. If the pipeline
executes more than one output action, then each one is separately
resubmitted to table 37. 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);
get_nd(P, A);
Implemented by storing arguments into OpenFlow fields, then resubmitting
to table 66, which ovn-controller populates with flows generated from the
MAC_Binding table in the OVN Southbound database. If there is a match in
table 66, 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);
put_nd(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.)

R = lookup_arp(P, A, M);
R = lookup_nd(P, A, M);
Implemented by storing arguments into OpenFlow fields, then resubmitting
to table 67, which ovn-controller populates with flows generated from the
MAC_Binding table in the OVN Southbound database. If there is a match in
table 67, then its actions set the logical flow flag MLF_LOOKUP_MAC.

(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.)

3. OpenFlow tables 37 through 41 implement the output action in the logical
ingress pipeline. Specifically, table 37 serves as an entry point to egress
pipeline. Table 37 detects IP packets that are too big for a corresponding
interface. Table 38 produces ICMPv4 Fragmentation Needed (or ICMPv6 Too Big)
errors and deliver them back to the offending port. table 39 handles packets to
remote hypervisors, table 40 handles packets to the local hypervisor, and table
41 checks whether packets whose logical ingress and egress port are the same
should be discarded.

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
40, 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 39.

Each flow in table 39 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 40.) 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 40. Table 39 also includes:

• A higher-priority rule to match packets received from ramp switch
tunnels, based on flag MLF_RCV_FROM_RAMP, and resubmit these packets to
table 40 for local delivery. Packets received from ramp switch tunnels
reach here because of a lack of logical output port field in the tunnel
key and thus these packets needed to be submitted to table 8 to
determine the output port.

• A higher-priority rule to match packets received from ports of type
localport, based on the logical input port, and resubmit these packets
to table 40 for local delivery. Ports of type localport exist on every
hypervisor and by definition their traffic should never go out through a
tunnel.

• A higher-priority rule to match packets that have the MLF_LOCAL_ONLY
logical flow flag set, and whose destination is a multicast address.
This flag indicates that the packet should not be delivered to remote
hypervisors, even if the multicast destination includes ports on remote
hypervisors. This flag is used when ovn-controller is the originator of
the multicast packet. Since each ovn-controller instance is originating
these packets, the packets only need to be delivered to local ports.

• A fallback flow that resubmits to table 40 if there is no other match.

Flows in table 40 resemble those in table 39 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 41. 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 41.

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 39 to reach the remote port, a flow is added in table 40 to
switch the logical outport to the localnet port, and resubmit to table 40 as if
it were unicasted to a logical port on the local hypervisor.

Table 41 matches and drops packets for which the logical input and output ports
are the same and the MLF_ALLOW_LOOPBACK flag is not set. It also drops
MLF_LOCAL_ONLY packets directed to a localnet port. It resubmits other packets
to table 42.

4. OpenFlow tables 42 through 62 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.

5. Table 64 bypasses OpenFlow loopback when MLF_ALLOW_LOOPBACK is set. Logical
loopback was handled in table 41, but OpenFlow by default also prevents
loopback to the OpenFlow ingress port. Thus, when MLF_ALLOW_LOOPBACK is set,
OpenFlow table 64 saves the OpenFlow ingress port, sets it to zero, resubmits
to table 65 for logical-to-physical transformation, and then restores the
OpenFlow ingress port, effectively disabling OpenFlow loopback prevents. When
MLF_ALLOW_LOOPBACK is unset, table 64 flow simply resubmits to table 65.

6. OpenFlow table 65 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.

Logical Routers and Logical Patch Ports
Typically logical routers and logical patch ports do not have a physical location and
effectively reside on every hypervisor. This is the case for logical patch ports between
logical routers and logical switches behind those logical routers, to which VMs (and VIFs)
attach.

Consider a packet sent from one virtual machine or container to another VM or container
that resides on a different subnet. The packet will traverse tables 0 to 65 as described
in the previous section Architectural Physical Life Cycle of a Packet, using the logical
datapath representing the logical switch that the sender is attached to. At table 39, the
packet will use the fallback flow that resubmits locally to table 40 on the same
hypervisor. In this case, all of the processing from table 0 to table 65 occurs on the
hypervisor where the sender resides.

When the packet reaches table 65, the logical egress port is a logical patch port.
ovn-controller implements output to the logical patch is packet by cloning and
resubmitting directly to the first OpenFlow flow table in the ingress pipeline, setting
the logical ingress port to the peer logical patch port, and using the peer logical patch
port’s logical datapath (that represents the logical router).

The packet re-enters the ingress pipeline in order to traverse tables 8 to 65 again, this
time using the logical datapath representing the logical router. The processing continues
as described in the previous section Architectural Physical Life Cycle of a Packet. When
the packet reaches table 65, the logical egress port will once again be a logical patch
port. In the same manner as described above, this logical patch port will cause the packet
to be resubmitted to OpenFlow tables 8 to 65, this time using the logical datapath
representing the logical switch that the destination VM or container is attached to.

The packet traverses tables 8 to 65 a third and final time. If the destination VM or
container resides on a remote hypervisor, then table 39 will send the packet on a tunnel
port from the sender’s hypervisor to the remote hypervisor. Finally table 65 will output
the packet directly to the destination VM or container.

The following sections describe two exceptions, where logical routers and/or logical patch
ports are associated with a physical location.

Gateway Routers

A gateway router is a logical router that is bound to a physical location. This includes
all of the logical patch ports of the logical router, as well as all of the peer logical
patch ports on logical switches. In the OVN Southbound database, the Port_Binding entries
for these logical patch ports use the type l3gateway rather than patch, in order to
distinguish that these logical patch ports are bound to a chassis.

When a hypervisor processes a packet on a logical datapath representing a logical switch,
and the logical egress port is a l3gateway port representing connectivity to a gateway
router, the packet will match a flow in table 39 that sends the packet on a tunnel port to
the chassis where the gateway router resides. This processing in table 39 is done in the
same manner as for VIFs.

Distributed Gateway Ports

This section provides additional details on distributed gateway ports, outlined earlier.

The primary design goal of distributed gateway ports is to allow as much traffic as
possible to be handled locally on the hypervisor where a VM or container resides. Whenever
possible, packets from the VM or container to the outside world should be processed
completely on that VM’s or container’s hypervisor, eventually traversing a localnet port
instance or a tunnel to the physical network or a different OVN deployment. Whenever
possible, packets from the outside world to a VM or container should be directed through
the physical network directly to the VM’s or container’s hypervisor.

In order to allow for the distributed processing of packets described in the paragraph
above, distributed gateway ports need to be logical patch ports that effectively reside on
every hypervisor, rather than l3gateway ports that are bound to a particular chassis.
However, the flows associated with distributed gateway ports often need to be associated
with physical locations, for the following reasons:

• The physical network that the localnet port is attached to typically uses L2
learning. Any Ethernet address used over the distributed gateway port must
be restricted to a single physical location so that upstream L2 learning is
not confused. Traffic sent out the distributed gateway port towards the
localnet port with a specific Ethernet address must be sent out one specific
instance of the distributed gateway port on one specific chassis. Traffic
received from the localnet port (or from a VIF on the same logical switch as
the localnet port) with a specific Ethernet address must be directed to the
logical switch’s patch port instance on that specific chassis.

Due to the implications of L2 learning, the Ethernet address and IP address
of the distributed gateway port need to be restricted to a single physical
location. For this reason, the user must specify one chassis associated with
the distributed gateway port. Note that traffic traversing the distributed
gateway port using other Ethernet addresses and IP addresses (e.g. one-to-
one NAT) is not restricted to this chassis.

Replies to ARP and ND requests must be restricted to a single physical
location, where the Ethernet address in the reply resides. This includes ARP
and ND replies for the IP address of the distributed gateway port, which are
restricted to the chassis that the user associated with the distributed
gateway port.

• In order to support one-to-many SNAT (aka IP masquerading), where multiple
logical IP addresses spread across multiple chassis are mapped to a single
external IP address, it will be necessary to handle some of the logical
router processing on a specific chassis in a centralized manner. Since the
SNAT external IP address is typically the distributed gateway port IP
address, and for simplicity, the same chassis associated with the
distributed gateway port is used.

The details of flow restrictions to specific chassis are described in the ovn-northd
documentation.

While most of the physical location dependent aspects of distributed gateway ports can be
handled by restricting some flows to specific chassis, one additional mechanism is
required. When a packet leaves the ingress pipeline and the logical egress port is the
distributed gateway port, one of two different sets of actions is required at table 39:

• If the packet can be handled locally on the sender’s hypervisor (e.g. one-
to-one NAT traffic), then the packet should just be resubmitted locally to
table 40, in the normal manner for distributed logical patch ports.

• However, if the packet needs to be handled on the chassis associated with
the distributed gateway port (e.g. one-to-many SNAT traffic or non-NAT
traffic), then table 39 must send the packet on a tunnel port to that
chassis.

In order to trigger the second set of actions, the chassisredirect type of southbound
Port_Binding has been added. Setting the logical egress port to the type chassisredirect
logical port is simply a way to indicate that although the packet is destined for the
distributed gateway port, it needs to be redirected to a different chassis. At table 39,
packets with this logical egress port are sent to a specific chassis, in the same way that
table 39 directs packets whose logical egress port is a VIF or a type l3gateway port to
different chassis. Once the packet arrives at that chassis, table 40 resets the logical
egress port to the value representing the distributed gateway port. For each distributed
gateway port, there is one type chassisredirect port, in addition to the distributed
logical patch port representing the distributed gateway port.

High Availability for Distributed Gateway Ports

OVN allows you to specify a prioritized list of chassis for a distributed gateway port.
This is done by associating multiple Gateway_Chassis rows with a Logical_Router_Port in
the OVN_Northbound database.

When multiple chassis have been specified for a gateway, all chassis that may send packets
to that gateway will enable BFD on tunnels to all configured gateway chassis. The current
active chassis for the gateway is the highest priority gateway chassis that is currently
viewed as active based on BFD status.

For more information on L3 gateway high availability, please refer to
http://docs.ovn.org/en/latest/topics/high-availability.html.

Restrictions of Distributed Gateway Ports

Distributed gateway ports are used to connect to an external network, which can be a
physical network modeled by a logical switch with a localnet port, and can also be a
logical switch that interconnects different OVN deployments (see OVN Deployments
Interconnection). Usually there can be many logical routers connected to the same external
logical switch, as shown in below diagram.

In this diagram, there are n logical routers connected to a logical switch LS-EXT, each
with a distributed gateway port, so that traffic sent to external world is redirected to
the gateway chassis that is assigned to the distributed gateway port of respective logical
router.

In the logical topology, nothing can prevent an user to add a route between the logical
routers via the connected distributed gateway ports on LS-EXT. However, the route works
only if the LS-EXT is a physical network (modeled by a logical switch with a localnet
port). In that case the packet will be delivered between the gateway chassises through the
localnet port via physical network. If the LS-EXT is a regular logical switch (backed by
tunneling only, as in the use case of OVN interconnection), then the packet will be
dropped on the source gateway chassis. The limitation is due the fact that distributed
gateway ports are tied to physical location, and without physical network connection, we
will end up with either dropping the packet or transferring it over the tunnels which
could cause bigger problems such as broadcast packets being redirect repeatedly by
different gateway chassises.

With the limitation in mind, if a user do want the direct connectivity between the logical
routers, it is better to create an internal logical switch connected to the logical
routers with regular logical router ports, which are completely distributed and the
packets don’t have to leave a chassis unless necessary, which is more optimal than routing
via the distributed gateway ports.

ARP request and ND NS packet processing

Due to the fact that ARP requests and ND NA packets are usually broadcast packets, for
performance reasons, OVN deals with requests that target OVN owned IP addresses (i.e., IP
addresses configured on the router ports, VIPs, NAT IPs) in a specific way and only
forwards them to the logical router that owns the target IP address. This behavior is
different than that of traditional switches and implies that other routers/hosts connected
to the logical switch will not learn the MAC/IP binding from the request packet.

All other ARP and ND packets are flooded in the L2 broadcast domain and to all attached
logical patch ports.

VIFs on the logical switch connected by a distributed gateway port

Typically the logical switch connected by a distributed gateway port is for external
connectivity, usually to a physical network through a localnet port on the logical switch,
or to a remote OVN deployment through OVN Interconnection. In these cases there is no VIF
ports required on the logical switch.

While not very common, it is still possible to create VIF ports on the logical switch
connected by a distributed gateway port, but there is a limitation that the logical ports
need to reside on the gateway chassis where the distributed gateway port resides to get
connectivity to other logical switches through the distributed gateway port. There is no
limitation for the VIFs to connect within the logical switch, or beyond the logical switch
through other regular distributed logical router ports.

A special case is when using distributed gateway ports for scalability purpose, as
mentioned earlier in this document. The logical switches connected by distributed gateway
ports are not for connectivity but just for regular VIFs. However, the above limitation
usually does not matter because in this use case all the VIFs on the logical switch are
located on the same chassis with the distributed gateway port that connects the logical
switch.

Multiple localnet logical switches connected to a Logical Router
It is possible to have multiple logical switches each with a localnet port (representing
physical networks) connected to a logical router, in which one localnet logical switch may
provide the external connectivity via a distributed gateway port and rest of the localnet
logical switches use VLAN tagging in the physical network. It is expected that
ovn-bridge-mappings is configured appropriately on the chassis for all these localnet
networks.

East West routing

East-West routing between these localnet VLAN tagged logical switches work almost the same
way as normal logical switches. When the VM sends such a packet, then:

1. It first enters the ingress pipeline, and then egress pipeline of the source
localnet logical switch datapath. It then enters the ingress pipeline of the
logical router datapath via the logical router port in the source chassis.

2. Routing decision is taken.

3. From the router datapath, packet enters the ingress pipeline and then egress
pipeline of the destination localnet logical switch datapath and goes out of
the integration bridge to the provider bridge ( belonging to the destination
logical switch) via the localnet port. While sending the packet to provider
bridge, we also replace router port MAC as source MAC with a chassis unique
MAC.

This chassis unique MAC is configured as global ovs config on each chassis (eg.
via "ovs-vsctl set open . external-ids:
ovn-chassis-mac-mappings="phys:aa:bb:cc:dd:ee:$i$i""). For more details, see
ovn-controller(8).

If the above is not configured, then source MAC would be the router port MAC.
This could create problem if we have more than one chassis. This is because,
since the router port is distributed, the same (MAC,VLAN) tuple will seen by
physical network from other chassis as well, which could cause these issues:

• Continuous MAC moves in top-of-rack switch (ToR).

• ToR dropping the traffic, which is causing continuous MAC moves.

• ToR blocking the ports from which MAC moves are happening.

4. The destination chassis receives the packet via the localnet port and sends it
to the integration bridge. Before entering the integration bridge the source
mac of the packet will be replaced with router port mac again. The packet
enters the ingress pipeline and then egress pipeline of the destination
localnet logical switch and finally gets delivered to the destination VM port.

External traffic

The following happens when a VM sends an external traffic (which requires NATting) and the
chassis hosting the VM doesn’t have a distributed gateway port.

1. The packet first enters the ingress pipeline, and then egress pipeline of the
source localnet logical switch datapath. It then enters the ingress pipeline of
the logical router datapath via the logical router port in the source chassis.

2. Routing decision is taken. Since the gateway router or the distributed gateway
port doesn’t reside in the source chassis, the traffic is redirected to the
gateway chassis via the tunnel port.

3. The gateway chassis receives the packet via the tunnel port and the packet
enters the egress pipeline of the logical router datapath. NAT rules are
applied here. The packet then enters the ingress pipeline and then egress
pipeline of the localnet logical switch datapath which provides external
connectivity and finally goes out via the localnet port of the logical switch
which provides external connectivity.

Although this works, the VM traffic is tunnelled when sent from the compute chassis to the
gateway chassis. In order for it to work properly, the MTU of the localnet logical
switches must be lowered to account for the tunnel encapsulation.

Centralized routing for localnet VLAN tagged logical switches connected to a Logical Router
To overcome the tunnel encapsulation problem described in the previous section, OVN
supports the option of enabling centralized routing for localnet VLAN tagged logical
switches. CMS can configure the option options:reside-on-redirect-chassis to true for each
Logical_Router_Port which connects to the localnet VLAN tagged logical switches. This
causes the gateway chassis (hosting the distributed gateway port) to handle all the
routing for these networks, making it centralized. It will reply to the ARP requests for
the logical router port IPs.

If the logical router doesn’t have a distributed gateway port connecting to the localnet
logical switch which provides external connectivity, or if it has more than one
distributed gateway ports, then this option is ignored by OVN.

The following happens when a VM sends an east-west traffic which needs to be routed:

1. The packet first enters the ingress pipeline, and then egress pipeline of the
source localnet logical switch datapath and is sent out via a localnet port of
the source localnet logical switch (instead of sending it to router pipeline).

2. The gateway chassis receives the packet via a localnet port of the source
localnet logical switch and sends it to the integration bridge. The packet then
enters the ingress pipeline, and then egress pipeline of the source localnet
logical switch datapath and enters the ingress pipeline of the logical router
datapath.

3. Routing decision is taken.

4. From the router datapath, packet enters the ingress pipeline and then egress
pipeline of the destination localnet logical switch datapath. It then goes out
of the integration bridge to the provider bridge ( belonging to the destination
logical switch) via a localnet port.

5. The destination chassis receives the packet via a localnet port and sends it to
the integration bridge. The packet enters the ingress pipeline and then egress
pipeline of the destination localnet logical switch and finally delivered to
the destination VM port.

The following happens when a VM sends an external traffic which requires NATting:

3. Routing decision is taken and NAT rules are applied.

4. From the router datapath, packet enters the ingress pipeline and then egress
pipeline of the localnet logical switch datapath which provides external
connectivity. It then goes out of the integration bridge to the provider bridge
(belonging to the logical switch which provides external connectivity) via a
localnet port.

The following happens for the reverse external traffic.

1. The gateway chassis receives the packet from a localnet port of the logical
switch which provides external connectivity. The packet then enters the ingress
pipeline and then egress pipeline of the localnet logical switch (which
provides external connectivity). The packet then enters the ingress pipeline of
the logical router datapath.

2. The ingress pipeline of the logical router datapath applies the unNATting
rules. The packet then enters the ingress pipeline and then egress pipeline of
the source localnet logical switch. Since the source VM doesn’t reside in the
gateway chassis, the packet is sent out via a localnet port of the source
logical switch.

3. The source chassis receives the packet via a localnet port and sends it to the
integration bridge. The packet enters the ingress pipeline and then egress
pipeline of the source localnet logical switch and finally gets delivered to
the source VM port.

As an alternative to reside-on-redirect-chassis, OVN supports VLAN-based redirection.
Whereas reside-on-redirect-chassis centralizes all router functionality, VLAN-based
redirection only changes how OVN redirects packets to the gateway chassis. By setting
options:redirect-type to bridged on a distributed gateway port, OVN redirects packets to
the gateway chassis using the localnet port of the router’s peer logical switch, instead
of a tunnel.

Following happens for bridged redirection:

1. On compute chassis, packet passes though logical router’s ingress pipeline.

2. If logical outport is gateway chassis attached router port then packet is
"redirected" to gateway chassis using peer logical switch’s localnet port.

3. This redirected packet has destination mac as router port mac (the one to which
gateway chassis is attached). Its VLAN id is that of localnet port (peer
logical switch of the logical router port).

4. On the gateway chassis packet will enter the logical router pipeline again and
this time it will passthrough egress pipeline as well.

5. Reverse traffic packet flows stays the same.

Some guidelines and expections with bridged redirection:

1. Since router port mac is destination mac, hence it has to be ensured that
physical network learns it on ONLY from the gateway chassis. Which means that
ovn-chassis-mac-mappings should be configure on all the compute nodes, so that
physical network never learn router port mac from compute nodes.

2. Since packet enters logical router ingress pipeline twice (once on compute
chassis and again on gateway chassis), hence ttl will be decremented twice.

3. Default redirection type continues to be overlay. User can switch the redirect-
type between bridged and overlay by changing the value of options:redirect-type

Life Cycle of a VTEP gateway
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.

1. A VTEP gateway’s life cycle begins with the administrator registering the VTEP
gateway as a 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.

2. The administrator can then create a new 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 corresponding vtep logical switch is bound to an OVN logical
network.

3. Now, the administrator can use the CMS to add a VTEP logical switch to the OVN
logical network. To do that, the CMS must first create a new
Logical_Switch_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. Next, the
addresses column of this logical port must be set to "unknown", it will add a
priority 0 entry in "ls_in_l2_lkup" stage of logical switch ingress pipeline.
So, traffic with unrecorded mac by OVN would go through the Logical_Switch_Port
to physical network.

4. The newly created logical port in the 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.

5. Beside binding to the VTEP gateway chassis, the 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.

6. Next, the 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.

7. Eventually, the VTEP gateway’s life cycle ends when the administrator
unregisters the VTEP gateway from the 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.

8. When the 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 Deployments Interconnection
It is not uncommon for an operator to deploy multiple OVN clusters, for two main reasons.
Firstly, an operator may prefer to deploy one OVN cluster for each availability zone, e.g.
in different physical regions, to avoid single point of failure. Secondly, there is always
an upper limit for a single OVN control plane to scale.

Although the control planes of the different availability zone (AZ)s are independent from
each other, the workloads from different AZs may need to communicate across the zones. The
OVN interconnection feature provides a native way to interconnect different AZs by L3
routing through transit overlay networks between logical routers of different AZs.

A global OVN Interconnection Northbound database is introduced for the operator (probably
through CMS systems) to configure transit logical switches that connect logical routers
from different AZs. A transit switch is similar to a regular logical switch, but it is
used for interconnection purpose only. Typically, each transit switch can be used to
connect all logical routers that belong to same tenant across all AZs.

A dedicated daemon process ovn-ic, OVN interconnection controller, in each AZ will consume
this data and populate corresponding logical switches to their own northbound databases
for each AZ, so that logical routers can be connected to the transit switch by creating
patch port pairs in their northbound databases. Any router ports connected to the transit
switches are considered interconnection ports, which will be exchanged between AZs.

Physically, when workloads from different AZs communicate, packets need to go through
multiple hops: source chassis, source gateway, destination gateway and destination
chassis. All these hops are connected through tunnels so that the packets never leave
overlay networks. A distributed gateway port is required to connect the logical router to
a transit switch, with a gateway chassis specified, so that the traffic can be forwarded
through the gateway chassis.

A global OVN Interconnection Southbound database is introduced for exchanging control
plane information between the AZs. The data in this database is populated and consumed by
the ovn-ic, of each AZ. The main information in this database includes:

• Datapath bindings for transit switches, which mainly contains the tunnel
keys generated for each transit switch. Separate key ranges are reserved for
transit switches so that they will never conflict with any tunnel keys
locally assigned for datapaths within each AZ.

• Availability zones, which are registered by ovn-ic from each AZ.

• Gateways. Each AZ specifies chassises that are supposed to work as
interconnection gateways, and the ovn-ic will populate this information to
the interconnection southbound DB. The ovn-ic from all the other AZs will
learn the gateways and populate to their own southbound DB as a chassis.

• Port bindings for logical switch ports created on the transit switch. Each
AZ maintains their logical router to transit switch connections
independently, but ovn-ic automatically populates local port bindings on
transit switches to the global interconnection southbound DB, and learns
remote port bindings from other AZs back to its own northbound and
southbound DBs, so that logical flows can be produced and then translated to
OVS flows locally, which finally enables data plane communication.

• Routes that are advertised between different AZs. If enabled, routes are
automatically exchanged by ovn-ic. Both static routes and directly connected
subnets are advertised. Options in options column of the NB_Global table of
OVN_NB database control the behavior of route advertisement, such as
enable/disable the advertising/learning routes, whether default routes are
advertised/learned, and blacklisted CIDRs. See ovn-nb(5) for more details.

The tunnel keys for transit switch datapaths and related port bindings must be agreed
across all AZs. This is ensured by generating and storing the keys in the global
interconnection southbound database. Any ovn-ic from any AZ can allocate the key, but race
conditions are solved by enforcing unique index for the column in the database.

Once each AZ’s NB and SB databases are populated with interconnection switches and ports,
and agreed upon the tunnel keys, data plane communication between the AZs are established.

When VXLAN tunneling is enabled in an OVN cluster, due to the limited range available for
VNIs, Interconnection feature is not supported.

A day in the life of a packet crossing AZs

1. An IP packet is sent out from a VIF on a hypervisor (HV1) of AZ1, with
destination IP belonging to a VIF in AZ2.

2. In HV1’s OVS flow tables, the packet goes through logical switch and logical
router pipelines, and in a logical router pipeline, the routing stage finds out
the next hop for the destination IP, which belongs to a remote logical router
port in AZ2, and the output port, which is a chassis-redirect port located on
an interconnection gateway (GW1 in AZ1), so HV1 sends the packet to GW1 through
tunnel.

3. On GW1, it continues with the logical router pipe line and switches to the
transit switch’s pipeline through the peer port of the chassis redirect port.
In the transit switch’s pipeline it outputs to the remote logical port which is
located on a gateway (GW2) in AZ2, so the GW1 sends the packet to GW2 in
tunnel.

4. On GW2, it continues with the transit switch pipeline and switches to the
logical router pipeline through the peer port, which is a chassis redirect port
that is located on GW2. The logical router pipeline then forwards the packet to
relevant logical pipelines according to the destination IP address, and figures
out the MAC and location of the destination VIF port - a hypervisor (HV2). The
GW2 then sends the packet to HV2 in tunnel.

5. On HV2, the packet is delivered to the final destination VIF port by the
logical switch egress pipeline, just the same way as for intra-AZ
communications.

Native OVN services for external logical ports
To support OVN native services (like DHCP/IPv6 RA/DNS lookup) to the cloud resources which
are external, OVN supports external logical ports.

Below are some of the use cases where external ports can be used.

• VMs connected to SR-IOV nics - Traffic from these VMs by passes the kernel
stack and local ovn-controller do not bind these ports and cannot serve the
native services.

• When CMS supports provisioning baremetal servers.

OVN will provide the native services if CMS has done the below configuration in the OVN
Northbound Database.

• A row is created in Logical_Switch_Port, configuring the addresses column
and setting the type to external.

• ha_chassis_group column is configured.

• The HA chassis which belongs to the HA chassis group has the
ovn-bridge-mappings configured and has proper L2 connectivity so that it can
receive the DHCP and other related request packets from these external
resources.

• The Logical_Switch of this port has a localnet port.

• Native OVN services are enabled by configuring the DHCP and other options
like the way it is done for the normal logical ports.

It is recommended to use the same HA chassis group for all the external ports of a logical
switch. Otherwise, the physical switch might see MAC flap issue when different chassis
provide the native services. For example when supporting native DHCPv4 service, DHCPv4
server mac (configured in options:server_mac column in table DHCP_Options) originating
from different ports can cause MAC flap issue. The MAC of the logical router IP(s) can
also flap if the same HA chassis group is not set for all the external ports of a logical
switch.

SECURITY

Role-Based Access Controls for the Southbound DB
In order to provide additional security against the possibility of an OVN chassis becoming
compromised in such a way as to allow rogue software to make arbitrary modifications to
the southbound database state and thus disrupt the OVN network, role-based access controls
(see ovsdb-server(1) for additional details) are provided for the southbound database.

The implementation of role-based access controls (RBAC) requires the addition of two
tables to an OVSDB schema: the RBAC_Role table, which is indexed by role name and maps the
the names of the various tables that may be modifiable for a given role to individual rows
in a permissions table containing detailed permission information for that role, and the
permission table itself which consists of rows containing the following information:

Table Name
The name of the associated table. This column exists primarily as an aid for
humans reading the contents of this table.

Auth Criteria
A set of strings containing the names of columns (or column:key pairs for
columns containing string:string maps). The contents of at least one of the
columns or column:key values in a row to be modified, inserted, or deleted
must be equal to the ID of the client attempting to act on the row in order
for the authorization check to pass. If the authorization criteria is empty,
authorization checking is disabled and all clients for the role will be
treated as authorized.

Insert/Delete
Row insertion/deletion permission; boolean value indicating whether
insertion and deletion of rows is allowed for the associated table. If true,
insertion and deletion of rows is allowed for authorized clients.

Updatable Columns
A set of strings containing the names of columns or column:key pairs that
may be updated or mutated by authorized clients. Modifications to columns
within a row are only permitted when the authorization check for the client
passes and all columns to be modified are included in this set of modifiable
columns.

RBAC configuration for the OVN southbound database is maintained by ovn-northd. With RBAC
enabled, modifications are only permitted for the Chassis, Encap, Port_Binding, and
MAC_Binding tables, and are restricted as follows:

Chassis
Authorization: client ID must match the chassis name.

Insert/Delete: authorized row insertion and deletion are permitted.

Update: The columns nb_cfg, external_ids, encaps, and vtep_logical_switches
may be modified when authorized.

Encap Authorization: client ID must match the chassis name.

Insert/Delete: row insertion and row deletion are permitted.

Update: The columns type, options, and ip can be modified.

Port_Binding
Authorization: disabled (all clients are considered authorized. A future
enhancement may add columns (or keys to external_ids) in order to control
which chassis are allowed to bind each port.

Insert/Delete: row insertion/deletion are not permitted (ovn-northd
maintains rows in this table.

Update: Only modifications to the chassis column are permitted.

MAC_Binding
Authorization: disabled (all clients are considered to be authorized).

Insert/Delete: row insertion/deletion are permitted.

Update: The columns logical_port, ip, mac, and datapath may be modified by
ovn-controller.

IGMP_Group
Authorization: disabled (all clients are considered to be authorized).

Insert/Delete: row insertion/deletion are permitted.

Update: The columns address, chassis, datapath, and ports may be modified by
ovn-controller.

Enabling RBAC for ovn-controller connections to the southbound database requires the
following steps:

1. Creating SSL certificates for each chassis with the certificate CN field set to
the chassis name (e.g. for a chassis with external-ids:system-id=chassis-1, via
the command "ovs-pki -u req+sign chassis-1 switch").

2. Configuring each ovn-controller to use SSL when connecting to the southbound
database (e.g. via "ovs-vsctl set open .
external-ids:ovn-remote=ssl:x.x.x.x:6642").

3. Configuring a southbound database SSL remote with "ovn-controller" role (e.g.
via "ovn-sbctl set-connection role=ovn-controller pssl:6642").

Encrypt Tunnel Traffic with IPsec
OVN tunnel traffic goes through physical routers and switches. These physical devices
could be untrusted (devices in public network) or might be compromised. Enabling
encryption to the tunnel traffic can prevent the traffic data from being monitored and
manipulated.

The tunnel traffic is encrypted with IPsec. The CMS sets the ipsec column in the
northbound NB_Global table to enable or disable IPsec encrytion. If ipsec is true, all OVN
tunnels will be encrypted. If ipsec is false, no OVN tunnels will be encrypted.

When CMS updates the ipsec column in the northbound NB_Global table, ovn-northd copies the
value to the ipsec column in the southbound SB_Global table. ovn-controller in each
chassis monitors the southbound database and sets the options of the OVS tunnel interface
accordingly. OVS tunnel interface options are monitored by the ovs-monitor-ipsec daemon
which configures IKE daemon to set up IPsec connections.

Chassis authenticates each other by using certificate. The authentication succeeds if the
other end in tunnel presents a certificate signed by a trusted CA and the common name (CN)
matches the expected chassis name. The SSL certificates used in role-based access controls
(RBAC) can be used in IPsec. Or use ovs-pki to create different certificates. The
certificate is required to be x.509 version 3, and with CN field and subjectAltName field
being set to the chassis name.

The CA certificate, chassis certificate and private key are required to be installed in
each chassis before enabling IPsec. Please see ovs-vswitchd.conf.db(5) for setting up CA
based IPsec authentication.

DESIGN DECISIONS

Tunnel Encapsulations
In general, 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:

• 24-bit logical datapath identifier, from the tunnel_key column in the OVN
Southbound Datapath_Binding table.

• 15-bit logical ingress port identifier. ID 0 is reserved for internal use
within OVN. IDs 1 through 32767, inclusive, may be assigned to logical ports
(see the tunnel_key column in the OVN Southbound Port_Binding table).

• 16-bit logical egress port identifier. IDs 0 through 32767 have the same
meaning as for logical ingress ports. IDs 32768 through 65535, inclusive,
may be assigned to logical multicast groups (see the tunnel_key column in
the OVN Southbound Multicast_Group table).

When VXLAN is enabled on any hypervisor in a cluster, datapath and egress port identifier
ranges are reduced to 12-bits. This is done because only STT and Geneve provide the large
space for metadata (over 32 bits per packet). The mode with reduced ranges is called VXLAN
mode. To accommodate for VXLAN, 24 bits available are split as follows:

• 12-bit logical datapath identifier, derived from the tunnel_key column in
the OVN Southbound Datapath_Binding table.

• 12-bit logical egress port identifier. IDs 0 through 2047 are used for
unicast output ports. IDs 2048 through 4095, inclusive, may be assigned to
logical multicast groups (see the tunnel_key column in the OVN Southbound
Multicast_Group table). For multicast group tunnel keys, a special mapping
scheme is used to internally transform from internal OVN 16-bit keys to
12-bit values before sending packets through a VXLAN tunnel, and back from
12-bit tunnel keys to 16-bit values when receiving packets from a VXLAN
tunnel.

• No logical ingress port identifier.

The limited space available for metadata when VXLAN tunnels are enabled in a cluster put
the following functional limitations onto features available to users:

• The maximum number of networks is reduced to 4096.

• The maximum number of ports per network is reduced to 2048.

• ACLs matching against logical ingress port identifiers are not supported.

• OVN interconnection feature is not supported.

In addition to functional limitations described above, the following should be considered
before enabling it in your cluster:

• STT and Geneve use randomized UDP or TCP source ports that allows efficient
distribution among multiple paths in environments that use ECMP in their
underlay.

• NICs are available to offload STT and Geneve encapsulation and
decapsulation.

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
0x80, and a 32-bit value encoded as follows, from MSB to LSB:

1 15 16
+---+------------+-----------+
|rsv|ingress port|egress port|
+---+------------+-----------+
0

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.

VXLAN mode is recommended to be disabled if VXLAN encap at hypervisors is needed only to
support HW VTEP L2 Gateway functionality. See man ovn-nb(5) for table NB_Global column
options key vxlan_mode for more details.