VTN and Service Composition

CORD's support for service composition depends on VTN. The following focuses on the interface VTN exports, and how XOS interacts with VTN interconnect services. For more information about VTN’s internals and its relationship to the CORD fabric, see the Trellis documentation.

VTN-Provided Networks

Each Service is connected to one or two Management Networks and one or more Data Networks. VTN implements both types of networks.

VTN defines two management networks that instances can join:

  • MANAGEMENT_LOCAL: This puts the instance on the network, which is limited to the local compute node. The compute node's root context is always on this local network. Synchronizers currently use this network to SSH into an Instance to configure it. They SSH to the compute node's root context, and then from there into the Instance.

  • MANAGEMENT_HOST: This puts the instance on the network that does span compute nodes and does offer end-to-end connectivity to the head node. This network currently runs over the physical management network.

These two management networks are completely independent. A Slice can choose to participate in either of them, neither of them, or both of them. In the latter case, each instance has two management interfaces, one with a address and one with a address. Instances on different compute nodes can talk to each other on MANAGEMENT_HOST, but they cannot talk to each other on MANAGEMENT_LOCAL.

NOTE: These two management networks are entirely configurable: and are what been set for CORD-in-a-Box but need not necessarily be the same on a physical POD.

The rest of this guide focuses on the Data Network, which connects all the instances in a Service to each other (by default), but optionally can also connect that network to other networks in CORD. VTN is responsible for both implementing the base (default) Data Network and for splicing that network to other networks. These “other” networks are provided by both VTN (i.e., the other network is some other Service’s base Data Network) and by other CORD services (e.g., vOLT, vRouter).

When first created, each Data Network is Private by default, analogous to a private network in OpenStack (i.e., it connects only the Instances in the Service). The network can remain private, or it can be connected onto one or more other networks. These other networks can themselves be the Data Network of some other Service, but it is also possible to connect a Service’s Data Network to the public Internet, making the Instances Internet-routable. This framing of how to connect a Service to the public Internet is unusual (i.e., one connects to a public network by augmenting an existing a Private network), but it is helpful to view the public Internet being just like any other network in CORD: it is provided by some service. In the case of the public Internet, this service is currently provided by vRouter.

Note: While it is natural to expect an Instance to connect directly to a Public network (as opposed to first connecting to a private network and then splicing that network to a public network), that exploits a race condition involving Neutron. Should Neutron have had the opportunity to come up before the public network is connected, a private address rather than a public address would be assigned to each Instance's Port. Instead, our approach is that (a) the private network be established as a first step, and (b) it is possible to assign a second address (in this case public) to each Port. This allows a private network to be made public at any time, and in general, being able to assign multiple addresses to a Port is a requirement.

In addition to adding public connectivity to a private network, it is also possible to create a network that is public by default. This is done by setting the network's template's VTN_KIND to public.

Interconnecting Networks

VTN programs the underlying software switches (e.g., OvS) to forward packets to/from the Ports of the Service’s Instances. Connecting a network onto an existing Data Network means Instances in the two networks can exchange packets according to the parameters of the splicing operation (see below) and it may result in a new address being assigned to each Port.

A Service’s Data Network is interconnected to other networks as a consequence or the corresponding Services being composed in the Service Graph. There are four cases, depending on whether the two services being composed are implemented in the network control plane or the network data plane. We start with the example most people assume (the two services implement VNFs in the data plane), and then show how the same principle generalizes to other possible compositions.

When a dependency is established between a pair of data plane services, denoted A → B, it causes the Data Networks of A and B to be interconnected according to attribute’s assigned to their respective Data Networks. (To simplify the discussion we assume each Service has one Slice and each Slice has one Network. In practice, it is necessary to specify which of the Service’s Networks are being interconnected.) Two attribute vectors are (currently) defined:

  • Direct vs Indirect

  • Unidirectional vs Bidirectional

The first defines whether A uses Direct addressing for the Instances of B (using a unique address for each Instance) or Indirect addressing (using a Service-wide address, where VTN implements load balancing by forwarding the packet to a specific Instance). The second defines whether communication is Unidirectional (A can send packets to B but not vice versa) or Bidirectional (A and B can send packets to each other).

Technically, these attributes are associated with Service B’s Network rather than with the interconnection of A to B. When A interconnects with B, the properties of B’s Data Network dictate the terms of the interconnection. But as we expand the capability, it may be possible that attributes of the interconnection will be specified with the interconnection model rather than one or other of the networks being interconnected.

The above corresponds to a service implemented by one set of Instances being composed with a service implemented by another set of Instances. This results in two VTN-implemented Data Networks being connected together. Because VTN allocates a disjoint block of addresses drawn from a common private address space to each Data Network, this means a new block of addresses becomes visible to (routable from) each Service; each Instance continues to have the same private IP address assigned to its Ports.

In general, however, one or both of A and B might be “control plane” Services, in which case it implements a network rather than uses a network. Two examples are vOLT and vRouter, meaning we have two “mixed” service interconnections:

  • vOLT → vSG (a control plane service connects to a data plane service)

  • vSG → vRouter (a data plane service connects to a control plane service)

What this means is that rather than VTN splicing together two VTN-based Data Networks, we are asking VTN to connect some “other” network to the VTN-defined Data Network.

Similarly, this generalizes to account for other Networks-as-a-Service; e.g.,

  • vSG → vNaaS (a container connects to a wide-area virtual Network-as-a-Service)

Finally, although VTN does not yet support this case, one can imagine a situation where one control plane service connects to another control plane service, in which case VTN will need to interconnect two networks (neither of which VTN implements itself). For example:

  • vOLT → vRouter (from R-CORD)

  • vEE → vNaaS (from E-CORD)

Looking across this set of examples, there are two subcases. In the first, when interconnecting two VTN-based networks, the result is basically the union of the two original networks (with restrictions). In the second, when interconnecting a Service to some ONOS-provided network, the result is to dynamically add the ServiceInstances to that new network, with the side-effect of the instances being assigned a new address on that network. These two subcases can be traced back to the two roles VTN plays: (1) it connects instances to networks, and (2) it provides a private network for a set of instances.

Components and Interfaces

Two interfaces (one provided by VTN and the other provided by XOS) are necessary to support service composition. XOS invokes the VTN-provided API to interconnect networks belonging to two composed services. VTN invokes the XOS-provided API to restore interconnection state (e.g., if VTN restarts), but this interface is not involved in typical XOS/VTN interaction. The following diagram shows the relationship between XOS, OpenStack Neutron, and the various sub-systems of VTN.

VTN relationships

In a configuration that includes OpenStack, XOS indirectly calls VTN via Neutron. In this case, Neutron's ML2 plugin informs OpenStack Nova about the virtual network connecting a set of instances. XOS can also create a virtual network by directly calling VTN without Neutron's involvement (e.g,. to interconnect Docker containers).

Because VTN provides a CLI to purge its internal state, it uses the XOS-provided API to resync with XOS. This VTN-to-XOS interface is not shown in the figure.

XOS Provided API

  • GET xosapi/v1/vtn/vtnservices Get a list of VTN services
  • PUT xosapi/v1/vtn/vtnservices/{service_id} Update a VTN service

To cause VTN to be resynchronized from XOS to the VTN app, the following steps are performed:

  1. GET xosapi/v1/vtn/vtnservices/ This will provide a list of registered VTN services. There's usually only one, and it's id is typically set to 1, but we recommend always getting the list of services rather than assuming an the id.

  2. PUT xosapi/v1/vtn/vtnservices/{service_id} with data {"resync": true}. {service_id} is the identifier you retrieved in step (1).

VTN Provided API


  • POST onos/cordvtn/servicePorts Create a service port

  • GET onos/cordvtn/servicePorts List service ports including service port details

  • GET onos/cordvtn/servicePorts/{port_id} Show service port details

  • DELETE onos/cordvtn/servicePorts/{port_id} Delete a service port

Service Port Details

Parameters Type Description
id * UUID The UUID of the service port.
network_id * UUID The UUID of the attached service network.
name * string The name of the port on the switch.
vlan_id number VLAN ID of the port interface.
mac_address string The MAC address of the port interface.
ip_address string The IP address of the port interface.
floating_address_pairs list Additional public addresses allowed to the port interface.
ip_address string Additional public IP address.
mac_address string Additional MAC address mapped to the public IP address.

NOTE: * fields are mandatory for creating a new service port.

Example json request:



  • POST onos/cordvtn/serviceNetworks Create a service network

  • GET onos/cordvtn/serviceNetworks List service networks including the details

  • GET onos/cordvtn/serviceNetworks/{network_id} Show service network details

  • PUT onos/cordvtn/serviceNetworks/{network_id} Update a service network dependencies

  • DELETE onos/cordvtn/serviceNetworks/{network_id} Delete a service network

Service Network Details

Parameters Type Description
id * UUID The UUID of the service network.
name string The name of the service network.
type * string The type of the service network
segment_id integer The ID of the isolated segment on the physical network. Currently, only VXLAN based isolation is supported and this ID is a VNI.
subnet string The associated subnet.
providers list The list of the provider service networks.
id string The UUID of the provider service network.
bidirectional boolean The dependency, which is bidirectional (true) or unidirectional (false).

NOTE: * fields are mandatory for creating a new service port.

Service Network Types

  • PRIVATE: virtual network for the instances in the same service
  • PUBLIC: externally accessible network
  • MANAGEMENT_LOCAL: instance management network which does not span compute nodes, only accessible from the host machine
  • MANAGEMENT_HOST: real management network which spans compute and head nodes
  • ACCESS_AGENT: network for access agent infrastructure service

Example json request:

             "id": "71cc8c93-f809-42ff-b1d6-0c8d92c6cd2b",
             "bidirectional": true

Relationship to Core Models

Two of CORD's core models play a role in service composition, and hence, in the XOS/VTN interaction.

The first is the ServiceDependency model, which defines an edge in the Service Graph, connecting a consumer service to a provider service. XOS uses this model to instruct VTN in how to interconnect the two services in the underlying data plane. This interconnection is specified by a connect_method field, with the following values currently supported:

  • None No network connectivity is provided (services not connected in data plane)

  • Public Connected via a public network (currently implemented by vRouter)

  • Private-unidirectional Connected via a private network with unidirectional connectivity (currently implemented by VTN)

  • Private-bidirectional Connected via a private network with bidirectional connectivity (currently implemented by VTN)

  • Other Connected via some other network (how specified is TBD)

NOTE: The Other choice does not currently exist. We expect to add it in the near future as we reconcile how networks are parameterized (see below).

The second is the NetworkTemplate model, which defines the parameters by which all networks are set up, including any VTN-provided networks (which corresponds to the situation where connect_method = Private). This model includes a vtn_kind field, with the following values currently supported:

  • PRIVATE Provides a private network for the instances in the same service

  • PUBLIC Provides an externally accessible network public network

  • MANAGEMENT_LOCAL Provides a node-local (virtual) management network

  • MANAGEMENT_HOST Provides a CORD-wide (physical) management network

  • VSG Provides an access-side network

  • ACCESS_AGENT Provides a network for access agent infrastructure service

NOTE: The NetworkTemplate model needs to be cleaned up and reconciled with the ServiceDependency model. For example, there are currently three different places one can specify some version of public versus private, and the choices imposed on various fields are not currently enforced. The logic that controls how XOS invokes VTN can be found in the VTN synchronizer, and can be summarized as follows: If a ServiceDependency exists between Services A and B, then VTN will connect every eligible Network in A to every eligible network in B, where a network is eligible if its NetworkTemplate's vtn_kind field is set of VSG or Private.

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