Page 116 - ITU-T Focus Group IMT-2020 Deliverables

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6.3.1.1 Towards a unified model for services

Network softwarization will bring a unifying service modeling whereby SDN services (e.g., controllers), NFV
services (e.g., Virtual Network Functions), and Cloud services are seen as “application” executed on
virtualized resources. In this sense, services are executed in one of more infrastructures “slices”.

A “slice” can be defined as a set of logical resources (e.g., Virtual Machines or Containers) interconnected by
a set of virtual links (e.g., Virtual Networks).

Figure 6.3.1-1 – Example: a service made of a number of service components (S) executed on Virtual
Machines (VM) interconnected via Virtual Network (VN)

According the emerging unifying model, a generic service:

• is made of service components or building block;
• can be composed with other services (e.g., service chain, or more articulated service logics);
• provides a function (both “global” and “local”);
• exports APIs (e.g., REST);
• is available anywhere and anytime (location-time independent);

• is scalable, elastic, and resilient.
TOSCA (Topology and Orchestration Specification for Cloud Applications) [Ref.6.3.1-5] is a standard from
OASIS that targets interoperable deployment and lifecycle management of cloud services. TOSCA uses the
concept of service templates to describe cloud workloads as a topology template. The topology template
describes the structure of a service as a set of node templates and relationship templates modelling the
relations as a directed graph. Node templates and relationship templates (linking different nodes) in fact
specify properties and operations (via interfaces) to manipulate the service components.

In ETSI NFV [Ref.6.3.1-6], a Network Service (NS) is a “composition of Network Functions and it is defined by
its functional and behavioural specification”, being a Network Function (NF) a functional block within a
network infrastructure that has well-defined external interfaces and well-defined functional behaviour.

In IETF, [Ref.6.3.1-7] the term Service Function Chaining (SFC) is used “to describe the definition and
instantiation of an ordered list of instances of such service functions, and the subsequent "steering" of traffic
flows through those service functions”.
Moreover, the YANG declarative data modelling language can be used both to describe deployable instances
of a service (e.g., a VNF) and to configure a network device/element at run time.
Eventually, TOSCA and NETCONF/YANG can be considered as complementary instruments: deployment
templates may trigger the NETCONF/YANG configurations during the instantiation of services, whist in the
Operations OSS can take over configurations at run time [Ref.6.3.1-8].

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