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-.. This work is licensed under a Creative Commons Attribution 4.0 International License.
-.. http://creativecommons.org/licenses/by/4.0
-.. (c) Bin Hu
-
-Hub and Spoke Case
-------------------
-
-Description
-~~~~~~~~~~~
-
-In a traditional Hub-and-spoke topology there are two types of network entities:
-a central hub and multiple spokes. The corresponding VRFs of the hub and the
-spokes are configured to import and export routes such that all traffic is
-directed through the hub. As a result, spokes cannot communicate with each other
-directly, but only indirectly via the central hub. Hence, the hub typically
-hosts central network functions such firewalls.
-
-Furthermore, there is no layer 2 connectivity between the VNFs.
-
-In addition, in this use case, the deployed network infrastructure comprises
-equipment from two different vendors, Vendor A and Vendor B. There are 2 hosts
-(compute nodes). SDN Controller A and vForwarder A are provided by Vendor A, and
-run on host A. SDN Controller B and vForwarder B are provided by Vendor B, and run
-on host B.
-
-There is 1 tenant. Tenant 1 creates L3VPN Blue with 2 subnets: 10.1.1.0/24 and 10.3.7.0/24.
-
-The network topology is shown in :numref:`l3vpn-hub-spoke-figure`:
-
-.. figure:: images/l3vpn-hub-spoke.png
-   :name:  l3vpn-hub-spoke-figure
-   :width: 100%
-
-In L3VPN Blue, vFW(H) is acting the role of ``hub`` (a virtual firewall).
-The other 3 VNF VMs are ``spoke``. vFW(H) and VNF1(S) are spawned on host A,
-and VNF2(S) and VNF3(S) are spawned on host B. vFW(H) (10.1.1.5) and VNF2(S)
-(10.1.1.6) are attached to subnet 10.1.1.0/24. VNF1(S) (10.3.7.9) and VNF3(S)
-(10.3.7.10) are attached to subnet 10.3.7.0/24.
-
-
-Derived Requirements
-~~~~~~~~~~~~~~~~~~~~~
-
-Northbound API / Workflow
-+++++++++++++++++++++++++
-
-.. Georg: this needs to be made more readable / explanatory
-
-Exemplary vFW(H) Hub VRF is as follows:
-
-* RD1 10.1.1.5  IP_vForwarder1 Label1
-* RD1 0/0 IP_vForwarder1 Label1
-* Label 1 Local IF (10.1.1.5)
-* RD3 10.3.7.9  IP_vForwarder1 Label2
-* RD2 10.1.1.6  IP_vForwarder2 Label3
-* RD4 10.3.7.10 IP_vForwarder2 Label3
-
-Exemplary VNF1(S) Spoke VRF is as follows:
-
-* RD1 0/0 IP_vForwarder1 Label1
-* RD3 10.3.7.9  IP_vForwarder1 Label2
-
-Exemplary workflow is described as follows:
-
-1. Create Network
-
-2. Create VRF Policy Resource
-
-  2.1. Hub and Spoke
-
-3. Create Subnet
-
-4. Create Port
-
-  4.1. Subnet
-
-  4.2. VRF Policy Resource, [H | S]
-
-
-
-Current implementation
-++++++++++++++++++++++
-
-Different APIs have been developed to support creating a L3 network topology and
-directing network traffic through specific network elements in specific order,
-for example, [BGPVPN]_ and [NETWORKING-SFC]_. We analyzed those APIs regarding
-the Hub-and-Spoke use case.
-
-
-BGPVPN
-''''''
-
-Support for creating and managing L3VPNs is in general available in OpenStack
-Neutron by means of the BGPVPN API [BGPVPN]_.  The [BGPVPN]_ API currently
-supports the concepts of network- and router-associations. An association maps
-Neutron network objects (networks and routers) to a VRF with the following
-semantics:
-
-* A *network association* interconnects all subnets and ports of a Neutron
-  network by binding them to a given VRF
-* a *router association* interconnects all networks, and hence indirectly all
-  ports, connected to a Neutron router by binding them to a given VRF
-
-It is important to notice that these associations apply to entire Neutron
-networks including all ports connected to a network. This is due to the fact
-that in the Neutron, ports can only exist within a network but not individually.
-Furthermore, Neutron networks were originally designed to represent layer 2
-domains. As a result, ports within the same Neutron network typically have layer
-connectivity among each other. There are efforts to relax this original design
-assumption, e.g. routed networks, which however do not solve the problem at hand
-here (see the gap analysis further down below).
-
-In order to realize the hub-and-spoke topology outlined above, VRFs need to be
-created on a per port basis. Specifically, ports belonging to the same network
-should not be interconnected except through a corresponding configuration of a
-per-port-VRF.  This configuration includes setting up next-hop routing table,
-labels, I-RT and E-RT etc. in order to enable traffic direction from hub to
-spokes.
-
-It may be argued that given the current network- and router-association mechanisms,
-the following workflow establishes a network topology which aims to achieve the desired
-traffic flow from Hub to Spokes. The basic idea is to model separate VRFs per VM
-by creating a dedicated Neutron network with two subnets for each VRF in the
-Hub-and-Spoke topology.
-
-1. Create Neutron network "hub"
-  ``neutron net-create --tenant-id Blue hub``
-
-
-2. Create a separate Neutron network for every "spoke"
-  ``neutron net-create --tenant-id Blue spoke-i``
-
-
-3. For every network (hub and spokes), create two subnets
-  ``neutron subnet-create <hub/spoke-i UUID> --tenant-id Blue 10.1.1.0/24``
-
-  ``neutron subnet-create <hub/spoke-i UUID> --tenant-id Blue 10.3.7.0/24``
-
-
-4. Create the Neutron ports in the corresponding networks
-  ``neutron port-create --tenant-id Blue --name vFW(H) --fixed-ip subnet_id=<hub UUID>,ip_address=10.1.1.5``
-
-  ``neutron port-create --tenant-id Blue --name VNF1(S) --fixed-ip subnet_id=<spoke-i UUID>,ip_address=10.3.7.9``
-
-  ``neutron port-create --tenant-id Blue --name VNF2(S) --fixed-ip subnet_id=<spoke-i UUID>,ip_address=10.1.1.6``
-
-  ``neutron port-create --tenant-id Blue --name VNF3(S) --fixed-ip subnet_id=<spoke-i UUID>,ip_address=10.3.7.10``
-
-
-5. Create a BGPVPN object (VRF) for the hub network with the corresponding import
-   and export targets
-  ``neutron bgpvpn-create --name hub-vrf --import-targets <RT-hub RT-spoke> --export-targets <RT-hub>``
-
-
-6. Create a BGPVPN object (VRF) for every spoke network with the corresponding import
-   and export targets
-  ``neutron bgpvpn-create --name spoke-i-vrf --import-targets <RT-hub> --export-targets <RT-spoke>``
-
-
-7. Associate the hub network with the hub VRF
-  ``bgpvpn-net-assoc-create hub --network <hub network-UUID>``
-
-
-8. Associate each spoke network with the corresponding spoke VRF
-  ``bgpvpn-net-assoc-create spoke-i --network <spoke-i network-UUID>``
-
-
-9. Add static route to direct all traffic to vFW VNF running at the hub.
-
-   **Note:** Support for static routes not yet available.
-
-  ``neutron bgpvpn-static-route-add --tenant-id Blue --cidr 0/0 --nexthop-ip 10.1.1.5 hub``
-
-After step 9, VMs can be booted with the corresponding ports.
-
-The resulting network topology intents to resemble the target topology as shown in
-:numref:`l3vpn-hub-spoke-figure`, and achieve the desired traffic direction from Hub to Spoke.
-However, it deviates significantly from the essence of the Hub-and-Spoke use case as
-described above in terms of desired network topology, i.e. one L3VPN with multiple
-VRFs associated with vFW(H) and other VNFs(S) separately. And this method of using
-the current network- and router-association mechanism is not scalable when there are large
-number of Spokes, and in case of scale-in and scale-out of Hub and Spokes.
-
-The gap analysis in the next section describes the technical reasons for this.
-
-
-Network SFC
-'''''''''''
-
-Support of Service Function Chaining is in general available in OpenStack Neutron through
-the Neutron API for Service Insertion and Chaining project [NETWORKING-SFC]_.
-However, the [NETWORKING-SFC]_ API is focused on creating service chaining through
-NSH at L2, although it intends to be agnostic of backend implementation. It is unclear whether
-or not the service chain from vFW(H) to VNFs(S) can be created in the way of L3VPN-based
-VRF policy approach using [NETWORKING-SFC]_ API.
-
-Hence, it is currently not possible to configure the networking use case as described above.
-
-.. **Georg: we need to look deeper into SFC to substantiate our claim here.**
-
-
-Gaps in the Current Solution
-++++++++++++++++++++++++++++
-
-Given the use case description and the currently available implementation in
-OpenStack provided by [BGPVPN]_ project and [NETWORKING-SFC]_ project,
-we identify the following gaps:
-
-
-[L3VPN-HS-GAP1] No means to disable layer 2 semantic of Neutron networks
-''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
-
-Neutron networks were originally designed to represent layer 2 broadcast
-domains. As such, all ports connected to a network are in principle
-inter-connected on layer 2 (not considering security rules here). In contrast,
-in order to realize L3VPN use cases such as the hub-and-spoke topology,
-connectivity among ports must be controllable on a per port basis on layer 3.
-
-There are ongoing efforts to relax this design assumption, for instance by means
-of routed networks ([NEUTRON-ROUTED-NETWORKS]_). In a routed network, a Neutron network
-is a layer 3 domain which is composed of multiple layer 2 segments. A routed
-network only provides layer 3 connectivity across segments, but layer 2
-connectivity across segments is **optional**. This means, depending on the
-particular networking backend and segmentation technique used, there might be
-layer 2 connectivity across segments or not. A new flag ``l2_adjacency``
-indicates whether or not a user can expect layer 2 connectivity or not across
-segments.
-
-This flag, however, is ready-only and cannot be used to overwrite or disable the
-layer 2 semantics of a Neutron network.
-
-
-[L3VPN-HS-GAP2] No port-association available in the BGPVPN project yet
-'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
-
-Due to gap [L3VPN-HS-GAP1], the [BGPVPN]_ project was not yet able to implement
-the concept of a port association. A port association would allow to associate
-individual ports with VRFs and thereby control layer 3 connectivity on a per
-port basis.
-
-The workflow described above intents to mimic port associations by means of
-separate Neutron networks. Hence, the resulting workflow is overly complicated
-and not intuitive by requiring to create additional Neutron entities (networks)
-which are not present in the target topology. Moreover, creating large numbers
-of Neutron networks limits scalability.
-
-Port associations are on the road map of the [BGPVPN]_ project, however, no
-design that overcomes the problems outlined above has been specified yet.
-Consequently, the time-line for this feature is unknown.
-
-As a result, creating a clean Hub-and-Spoke topology is current not yet
-supported by the [BGPVPN]_ API.
-
-
-[L3VPN-HS-GAP3] No support for static routes in the BGPVPN project yet
-''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
-
-In order to realize the hub-and-spoke use case, a static route is needed to
-attract the traffic at the hub to the corresponding VNF (direct traffic to the
-firewall). Support for static routes in the BGPVPN project is available for the
-router association by means of the Neutron router extra routes feature. However,
-there is no support for static routes for network and port associations yet.
-
-Design work for supporting static routes for network associations has started,
-but no final design has been proposed yet.
-
-..
-.. [L3VPN-HS-GAP4] Creating a clean hub-and-spoke topology is current not yet supported by the NETWORKING-SFC API.
-.. [Georg: We need to look deeper into SFC before we can substantiate our claim]
-