| OPS Area Working Group | Q. Wu |
| Internet-Draft | W. Liu |
| Intended status: Informational | Huawei Technologies |
| Expires: April 15, 2018 | A. Farrel |
| Juniper Networks | |
| October 12, 2017 |
Service Models Explained
draft-ietf-opsawg-service-model-explained-05
The IETF has produced many modules in the YANG modeling language. The majority of these modules are used to construct data models to model devices or monolithic functions.
A small number of YANG modules have been defined to model services (for example, the Layer Three Virtual Private Network Service Model produced by the L3SM working group and documented in RFC 8049).
This document describes service models as used within the IETF, and also shows where a service model might fit into a Software Defined Networking architecture. Note that service models do not make any assumption of how a service is actually engineered and delivered for a customer; details of how network protocols and devices are engineered to deliver a service are captured in other modules that are not exposed through the Customer-Provider Interface.
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In recent years the number of modules written in the YANG modeling language [RFC6020] for configuration and monitoring has blossomed. Many of these are used for device-level configuration (for example, [RFC7223]) or for control of monolithic functions or protocol instances (for example, [RFC7407]).
[RFC7950] makes a distinction between a "data model" which it defines as describing how data is represented and accessed, and a "YANG module" that defines hierarchies of schema nodes to make a self-contained and compilable block of definitions and inclusions. YANG structures data models into modules for ease of use.
Within the context of Software Defined Networking (SDN) [RFC7149] [RFC7426], YANG data modules may be used on the interface between a controller and network devices, and between network orchestrators and controllers. There may also be a hierarchy of such components with super controllers, domain controllers, and device controllers all exchanging information and instructions using YANG modules.
There has been interest in using YANG to define and document data models that describe services in a portable way that is independent of which network operator uses the model. For example, the Layer Three Virtual Private Network Service Model (L3SM) [RFC8049]. Such models may be used in manual and even paper-driven service request processes with a gradual transition to IT-based mechanisms. Ultimately they could be used in online, software-driven dynamic systems, and may be used as part of an SDN system.
This document explains the scope and purpose of service models within the IETF (and limited to within the scope of the IETF) and describes how a service model can be used by a network operator. Equally, this document clarifies what a service model is not, and dispels some common misconceptions.
The document also shows where a service model might fit into an SDN architecture, but it is important to note that a service model does not require or preclude the use of SDN. Note that service models do not make any assumption of how a service is actually engineered and delivered to a customer; details of how network protocols and devices are engineered to deliver a service are captured in other modules that are not exposed through the interface betweeen the customer and the provider.
In summary, a service model is a formal representation of the data elements that describe a network service as that service is described to or requested by a customer of a network operator. Details included in the service model include a description of the service as experienced by the customer, but not features of how that service is delivered or realized by the service provider.
Other work on classifying YANG modules has been done in [RFC8199]. That document provides an important reference for this document, and also uses the term "service module". Section 6.1 in this document provides a comparison between these two uses of the same terminology.
Readers should familiarize themselves with the description and classification of YANG modules provided in [RFC8199].
The following terms are used in this document:
As already indicated, customer service models are used on the interface between customers and network operators. This is shown simply in Figure 1.
The language in which a customer service model is described is a choice for whoever specifies the model. The IETF uses the YANG data modeling language defined in [RFC6020].
The encoding and communication protocol used to exchange a customer service model between customer and network operator are deployment- and implementation-specific. The IETF has standardized the NETCONF protocol [RFC6241] and the RESTCONF protocol [RFC8040] for interactions "on the wire" between software components with data encoded in XML or JSON. However, co-located software components might use an internal API, while systems with more direct human interactions might use web pages or even paper forms. Where direct human interaction comes into play, interface interactions may be realized via business practices and that may introduce some margin of error, thus raising the priority for automated, deterministic interfaces.
-------------- Customer ----------------------
| | Service Model | |
| Customer | <-----------------> | Network Operator |
| | | |
-------------- ----------------------
Figure 1: The Customer Service Models used on the Interface between Customers and Network Operators
How a network operator processes a customer's service request described with a customer service model depends on the commercial and operational tools, processes, and policies used by the network operator. These may vary considerably from one network operator to another.
However, the intent is that the network operator maps the service request into configuration and operational parameters that control one or more networks to deliver the requested services. That means that the network operator (or software run by the network operator) takes the information in the customer service model and determines how to deliver the service by enabling and configuring network protocols and devices. They may achieve this by constructing service delivery models and passing them to network orchestrators or controllers. The use of standard customer service models eases service delivery by means of automation.
The practicality of customer service models has been repeatedly debated. It has been suggested that network operators have radically different business modes and widely diverse commercial offerings making a common customer service model is impractical. However, the L3SM [RFC8049] results from the consensus of multiple individuals working at network operators and offers a common core of service options that can be augmented according to the needs of individual network operators.
It has also been suggested that there should be a single, base customer service module, and that details of individual services should be offered as extensions or augmentations of this. It is quite possible that a number of service parameters (such as the identity and postal address of a customer) will be common and it would be a mistake to define them multiple times, once in each customer service model. However, the distinction between a 'module' and a 'model' should be considered at this point: modules are how the data for models is logically broken out and documented especially for re-use in multiple models.
In a Software Defined Networking (SDN) system, the management of network resources and protocols is performed by software systems that determine how best to utilize the network. Figure 2 shows a sample architectural view of an SDN system where network elements are programmed by a component called an "SDN controller" (or "controller" for short), and where controllers are instructed by an orchestrator that has a wider view of the whole of, or part of, a network. The internal organization of an SDN control plane is deployment-specific.
------------------
| |
| Orchestrator |
| |
.------------------.
. : .
. : .
------------ ------------ ------------
| | | | | |
| Controller | | Controller | | Controller |
| | | | | |
------------ ------------ ------------
: . . :
: . . :
: . . :
--------- --------- --------- ---------
| Network | | Network | | Network | | Network |
| Element | | Element | | Element | | Element |
--------- --------- --------- ---------
Figure 2: A Sample SDN Architecture
A customer's service request is (or should be) technology-agnostic. That is, a customer is unware of the technology that the network operator has available to deliver the service and so the customer does not make requests specific to the underlying technology, but is limited to making requests specific to the service that is to be delivered. The orchestrator must map the service request to its view, and this mapping may include a choice of which networks and technologies to use depending on which service features have been requested.
One implementation option to achieve this mapping is to split the orchestration function between a "Service Orchestrator" and a "Network Orchestrator" as shown in Figure 3.
Customer
------------------ Service ----------
| | Model | |
| Service |<-------->| Customer |
| Orchestrator | (a) | |
| | ----------
------------------
. .
. . (b) -----------
. (b) . ......|Application|
. . : | BSS/OSS |
. . : -----------
. Service Delivery . :
. Model . :
------------------ ------------------
| | | |
| Network | | Network |
| Orchestrator | | Orchestrator |
| | | |
.------------------ ------------------.
. : : .
. : Network Configuration : .
. : Model : .
------------ ------------ ------------ ------------
| | | | | | | |
| Controller | | Controller | | Controller | | Controller |
| | | | | | | |
------------ ------------ ------------ ------------
: . . : :
: . . Device : :
: . . Configuration : :
: . . Model : :
--------- --------- --------- --------- ---------
| Network | | Network | | Network | | Network | | Network |
| Element | | Element | | Element | | Element | | Element |
--------- --------- --------- --------- ---------
Figure 3: An Example SDN Architecture with a Service Orchestrator
Figure 3 also shows where different data models might be applied within the architecture. The Device Confguration Models are used by a Controller to set parameters on an individual network element. The Network Configuration Models are use by a Network Orchestrator to instruct Controllers (that may each be responsible for multiple network elements) how to configure parts of a network.
The split between control components that exposes a "service interface" that is present in many figures showing extended SDN architectures:
This can all lead to some confusion around the definition of a "service interface" and a "service model". Some previous literature considers the interface northbound of the Network Orchestrator (labeled "(b)" in Figure 3) to be a "service interface" used by an application, but the service described at this interface is network-centric and is aware of many features such as topology, technology, and operator policy. Thus, we make a distinction between this type of service interface and the more abstract service interface (labeled "(a)" in Figure 3) where the service is described by a service model and the interaction is between customer and network operator. Further discussion of this point is provided in Section 5.
In discussing service models, there are several possible causes of confusion:
Other work has classified YANG modules, produced parallel architectures, and developed a range of YANG modules. This section briefly examines that other work and shows how it fits with the description of service models introduced in this document.
As previously noted, [RFC8199] provides a classification of YANG modules. It introduces the term "Network Service YANG Module" to identify the type of module used to "describe the configuration, state data, operations and notifications of abstract representations of services implemented on one or multiple network elements." These modules are used to construct the service delivery models as described in this document; that is, they are the modules used on the interface between the Service Orchestrator or OSS/BSS and the Network Orchestrator as shown in Figure 3.
Figure 1 of [RFC8199] can be modified to make this more clear and to add an additional example of a Network Service YANG module as shown in Figure 4. As can be seen, the highest classification of modules collects those that are used to deliver operations support and business support. These might be consumed by an Operations Support System (OSS) or a Business Support System (BSS), and a Service Orchestrator may form part of an OSS/BSS or may be a separate component. This highest layer in the figure is divided into the Customer Service Modules that are used to describe services to a customer as discussed in this document, and other modules that describe further OSS/BSS function such as billing and SLAs.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Operations Support and Business Support YANG Modules
+-----------------------+ +-----------------------+
| | | |
| Customer Service | | Other |
| YANG Modules | | Operations Support |
| | | and |
| +------+ +------+ | | Business Support |
| | | | | | | YANG Modules |
| | L2SM | | L3SM | | | |
| | | | | | | |
| +------+ +------+ | | |
| | | |
+-----------------------+ +-----------------------+
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Network Service YANG Modules
+------------+ +-------------+ +-------------+ +-------------+
| | | | | | | |
| - L2VPN | | - L2VPN | | EVPN | | L3VPN |
| - VPWS | | - VPLS | | | | |
| | | | | | | |
+------------+ +-------------+ +-------------+ +-------------+
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Network Element YANG Modules
+------------+ +------------+ +-------------+ +------------+
| | | | | | | |
| MPLS | | BGP | | IPv4 / IPv6 | | Ethernet |
| | | | | | | |
+------------+ +------------+ +-------------+ +------------+
Key:
EVPN: Ethernet Virtual Private Network
L2SM: Layer 2 Virtual Private Network Service Model
L2VPN: Layer 2 Virtual Private Network
L3SM: Layer 3 Virtual Private Network Service Model
L3VPN: Layer 3 Virtual Private Network
VPLS: Virtual Private LAN Service
VPWS: Virtual Private Wire Service
Figure 4: YANG Module Abstaction Layers Showing Customer Service Modules
A number of IETF working groups are developing YANG modules related to services. These models focus on how the network operator configures the network through protocols and devices to deliver a service. Some of these models are classified as network service delivery models (also called service delivery models or network configuation models depending on the level at which they are pitched), while others have details that are related to specific element configuration and so are classed as network element models (also called device models).
A sample set of these models is listed here:
Several initiatives within the IETF are developing customer service models. The L3SM presents the L3VPN service as described by a network operator to a customer. Figure 5, which is reproduced from [RFC8049], shows that the L3SM is a customer service model as described in this document.
L3VPN-SVC |
MODEL |
|
+------------------+ +-----+
| Orchestration | < --- > | OSS |
+------------------+ +-----+
| |
+----------------+ |
| Config manager | |
+----------------+ |
| |
| Netconf/CLI ...
| |
+------------------------------------------------+
Network
Figure 5: The L3SM Service Architecture
A Layer Two VPN service model (L2SM) is defined in [I-D.ietf-l2sm-l2vpn-service-model]. That model's usage is described as in Figure 6 which is a reproduction of Figure 5 from that document. As can be seen, the L2SM is a customer service model as described in this document.
----------------------------
| Customer Service Requester |
----------------------------
|
L2VPN |
Service |
Model |
|
-----------------------
| Service Orchestration |
-----------------------
|
| Service +-------------+
| Delivery +------>| Application |
| Model | | BSS/OSS |
| V +-------------+
-----------------------
| Network Orchestration |
-----------------------
| |
+----------------+ |
| Config manager | |
+----------------+ | Device
| | Models
| |
--------------------------------------------
Network
Figure 6: The L2SM Service Architecture
The MEF Forum has developed an architecture for network management and operation. It is documented as the Lifecycle Service Orchestration (LSO) Reference Architecture and illustrated in Figure 2 of [MEF-55].
The work of the MEF Forum embraces all aspects of Lifecycle Service Orchestration including billing, SLAs, order management, and life-cycle management. The IETF's work on service models is typically smaller offering a simple, self-contained service YANG module. This does not invalidate either approach: it only observes that they are different approaches.
This section introduces a few further, more advanced concepts
Service models should generally be technology agnostic. That is to say, the customer should not care how the service is provided so long as the service is delivered.
However, some technologies reach the customer site and make a difference to the type of service delivered. Such features do need to be described in the service model.
Two examples are:
Policy appears as a crucial function in many places during network orchestration. A Service Orchestrator will, for example, apply the network operator's policies to determine how to provide a service for a particular customer (possibly considering commercial terms). However, the policies within a service model are limited to those over which a customer has direct influence and that are acted on by the network operator.
The policies that express desired behavior of services on occurrence of specific events are close to SLA definitions: they should only be included in the base service model where they are common to all network operators' offerings. Policies that describe who at a customer may request or modify services (that is, authorization) are close to commercial terms: they, too, should only be included in the base service model where they are common to all network operators' offerings.
As with commercial terms and SLAs discussed in Section 5, it is expected that some network operators will enhance standard customer service models to include policy parameters either using their own work or depending on specific policy models built in the IETF or other standards bodies.
Nevertheless, policy is so important that all service models should be designed to be easily extensible to allow policy components to be added and associated with services as needed.
When work on the L3SM was started, there was some doubt as to whether network operators would be able to agree on a common description of the services that they offer to their customers because, in a competitive environment, each markets the services in a different way with different additional features. However, the working group was able to agree on a core set of features that multiple network operators were willing to consider as "common". They also understood that, should an individual network operator want to describe additional features (operator-specific features), they could do so by extending or augmenting the L3SM model.
Thus, when a basic description of a core service is agreed and documented in a service model, it is important that that model should be easily extended or augmented by each network operator so that the standardized model can be used in a common way and only the operator-specific features varied from one environment to another.
Network operators will, in general, offer many different services to their customers. Each would normally be the subject of a separate service model.
Whether each service model is handled by a specialized Service Orchestrator able to provide tuned behavior for a specific service or whether all service models are handled by a single Service Orchestrator is an implementation and deployment choice.
It is expected that, over time, certain elements of the service models will be seen to repeat in each model. An example of such an element is the postal address of the customer.
It is anticipated that, while access to such information from each service model is important, the data will be described in its own module and may form part of the service model either by inclusion or by index.
The interface between customer and service provider is a commercial interface and needs to be subject to appropriate confidentiality. Additionally, knowledge of what services are provided to a customer or delivered by a network operator may supply information that can be used in a variety of security attacks. The service model itself will expose security-related parameters for the specific service where the related functon is available to the customer.
Clearly, the ability to modify information exchanges between customer and network operator may result in bogus requests, unwarranted billing, and false expectations. Furthermore, in an automated system, modifications to service requests or the injection of bogus requests may lead to attacks on the network and delivery of customer traffic to the wrong place.
Therefore it is important that the protocol interface used to exchange service request information between customer and network operator is subject to authorization, authentication, and encryption. Clearly, the level of abstraction provided by a service model protects the operator from unwarranted visibility into their network, and the fact that it is entirely up to the operator how they deliver the service provides additional protection.
Equally, all external interfaces, such as any of those between the functional components in Figure 3 needs to be correctly secured. This document discusses modeling the information, not how it is exchanged.
This whole document discusses issues related to network management and control.
It is important to observe that automated service provisioning resulting from use of a customer service model may result in rapid and significant changes in traffic load within a network and that that might have an effect on other services carried in a network.
It is expected, therefore, that a Service Orchestration component has awareness of other service commitments, that the Network Orchestration component will not commit network resources to fulfill a service unless doing so is appropriate, and that a feedback loop will be provided to report on degradation of the network that will impact the service.
The operational state of a service does not form part of a customer service model. However, it is likely that a network operator may want to report some state information about various components of the service, and that could be achieved through extensions to the core service model just as SLA extensions could be made as described in Section 5.
This document makes no requests for IANA action.
Thanks to Daniel King, Xian Zhang, Michael Scharf, Med Boucadair, Luis Miguel Contreras Murillo, Joe Salowey, Benoit Claise, Robert Sparks, Tom Petch, David Sinicrope, and Deborah Brungard for useful review and comments.
Thanks to Dean Bogdanovic, Tianran Zhou, and Carl Moberg for their help coordinating with [RFC8199].
Many thanks to Jerry Bonner for spotting a tiny, one-word, but critical typo.
| [RFC3444] | Pras, A. and J. Schoenwaelder, "On the Difference between Information Models and Data Models", RFC 3444, DOI 10.17487/RFC3444, January 2003. |
| [RFC8199] | Bogdanovic, D., Claise, B. and C. Moberg, "YANG Module Classification", RFC 8199, DOI 10.17487/RFC8199, July 2017. |