Page 1360 - 5G Basics - Core Network Aspects

3 Signalling aspects

The keywords "can optionally" indicate an optional requirement which is permissible, without implying any
sense of being recommended. This term is not intended to imply that the vendor's implementation must
provide the option, and the feature can be optionally enabled by the network operator/service provider.
Rather, it means the vendor may optionally provide the feature and still claim conformance with this
Supplement.

6 Signalling requirements and scenarios

6.1 SDN-enabled network

In the SDN-enabled network scenario, the centralized SDN controller creates a traffic path from one edge of
the network to the other edge of the network using certain protocols over the southbound interface, such
as OpenFlow [b-ONF], which programs this traffic on each node in the path, including edge, aggregation and
core switches/routers. The first packet of the new traffic is sent to a centralized SDN controller which applies
policy, computes the paths and uses the southbound interface to direct this traffic into each node on the
path.
Considering that this approach brings several problems, the following issues are recommended to be solved:

• It creates an explosion of forwarding states on the physical switches/routers;
• The SDN controller should communicate with each of the physical switches/routers in the path when
a new traffic is needed to be programed;

• This model unavoidably brings extra latency.

6.2 SDN-enabled overlay network
In the SDN-enabled overlay network scenario, the centralized SDN controller uses overlay tunnels to
virtualize the network. These tunnels generally terminate in virtual switches/routers, and can also terminate
in physical switches/routers. This scenario reduces the size of the forwarding states in the physical underlay
nodes and may not touch the physical switches when adding a new tenant or virtual machine (VM). Most
importantly, the SDN controller provides a seamless migration path for introducing SDN into the existing
production networks.
There are multiple data plane protocols which can be used to create overlay tunnels. Taking OpenFlow as an
example, it can just be deployed at the edge of the network and does not touch the aggregation and core
physical switches/routers. In that case, OF-Config [b-ONF] is used to create overlay tunnels and OpenFlow is
used to program traffic into the tunnels.
However, in this scenario, it is very difficult to provide per-tenant or per-VM quality of service (QoS), because
every packet is encapsulated into a tunnel. Support of fine-grained queuing is recommended in order to
isolate tenants and provide per-tenant QoS, respectively.

6.3 SDN controller related requirements and scenarios

6.3.1 Hybrid network
This deployment model allows the co-existence of traditional environments of closed vendors'
router/switches and OpenFlow-enabled devices. This hybrid approach refers to the interconnection of both
the control and data planes of legacy and new network elements, which can be regarded as the smooth
migration for the existing network. Figure 6-1 depicts the hybrid network model. The legacy controller
mentioned in this figure is not limited to the server and can be extended to other device types. The route
reflector for example, which is the most popular way to distribute border gateway patrol (BGP) routes
between routers of the same autonomous system (AS), can be regarded as a legacy controller. It is required
to provide the dedicated gateway-like component between the existing legacy controllers and OpenFlow
controllers in the new control plane.

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