Internet Engineering Task Force (IETF) Q. Wu
Request for Comments: 8309 W. Liu
Category: Informational Huawei Technologies
ISSN: 2070-1721 A. Farrel
Juniper Networks
January 2018
Service Models Explained
Abstract
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 3 Virtual Private Network Service Model
(L3SM) 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 interface between the customer and the
provider.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8309.
Wu, et al. Informational [Page 1]RFC 8309 Service Models Explained January 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms and Concepts . . . . . . . . . . . . . . . . . . . . . 4
3. Using Service Models . . . . . . . . . . . . . . . . . . . . 7
3.1. Practical Considerations . . . . . . . . . . . . . . . . 8
4. Service Models in an SDN Context . . . . . . . . . . . . . . 8
5. Possible Causes of Confusion . . . . . . . . . . . . . . . . 11
6. Comparison with Other Work . . . . . . . . . . . . . . . . . 13
6.1. Comparison with Network Service Models . . . . . . . . . 13
6.2. Service Delivery and Network Element Model Work . . . . . 15
6.3. Customer Service Model Work . . . . . . . . . . . . . . . 16
6.4. The MEF Architecture . . . . . . . . . . . . . . . . . . 17
7. Further Concepts . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Technology Agnostic . . . . . . . . . . . . . . . . . . . 18
7.2. Relationship to Policy . . . . . . . . . . . . . . . . . 18
7.3. Operator-Specific Features . . . . . . . . . . . . . . . 19
7.4. Supporting Multiple Services . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. Manageability Considerations . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Wu, et al. Informational [Page 2]RFC 8309 Service Models Explained January 2018
1. Introduction
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", which 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, as well as 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 3
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 is 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 between the customer and
the provider.
Wu, et al. Informational [Page 3]
RFC 8309 Service Models Explained January 2018
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". In this document, Section 6.1
provides a comparison between these two uses of the same terminology.
2. Terms and Concepts
Readers should familiarize themselves with the description and
classification of YANG modules provided in [RFC8199].
The following terms are used in this document:
Network Operator: This term is used to refer to the company that
owns and operates one or more networks that provide Internet
connectivity services and/or other services.
Customer: This term refers to someone who purchases a service
(including connectivity) from a network operator. In the context
of this document, a customer is usually a company that runs their
own network or computing platforms and wishes to connect to the
Internet or between sites. Such a customer may operate an
enterprise network or a data center. Sometimes this term may also
be used to refer to the individual in such a company who contracts
to buy services from a network operator. A customer as described
here is a separate commercial operation from the network operator,
but some companies may operate with internal customers so that,
for example, an IP/MPLS packet network may be the customer of an
optical transport network.
Wu, et al. Informational [Page 4]RFC 8309 Service Models Explained January 2018
Service: A network operator delivers one or more services to a
customer. A service in the context of this document (sometimes
called a Network Service) is some form of connectivity between
customer sites and the Internet or between customer sites across
the network operator's network and across the Internet. However,
a distinction should be drawn between the parameters that describe
a service as included in a customer service model (see the
definition of this term, below) and a Service Level Agreement
(SLA), as discussed in Sections 5 and 7.2.
A service may be limited to simple connectivity (such as IP-based
Internet access), may be a tunnel (such as a virtual circuit), or
may involve more complex connectivity (such as in a multisite
virtual private network). Services may be further enhanced by
additional functions providing security, load balancing,
accounting, and so forth. Additionally, services usually include
guarantees of quality, throughput, and fault reporting.
This document makes a distinction between a service as delivered
to a customer (that is, the service as discussed on the interface
between a customer and the network operator) and the service as
realized within the network (as described in [RFC8199]). This
distinction is discussed further in Section 6.
Readers may also refer to [RFC7297] for an example of how an IP
connectivity service may be characterized.
Data Model: The information model (IM) and data model (DM) concepts
are described in [RFC3444]. That document defines a data model by
contrasting it with the definition of an IM as follows:
The main purpose of an IM is to model managed objects at a
conceptual level, independent of any specific implementations
or protocols used to transport the data. The degree of
specificity (or detail) of the abstractions defined in the IM
depends on the modeling needs of its designers. In order to
make the overall design as clear as possible, an IM should hide
all protocol and implementation details. Another important
characteristic of an IM is that it defines relationships
between managed objects.
DMs, conversely, are defined at a lower level of abstraction
and include many details. They are intended for implementors
and include protocol-specific constructs.
As mentioned in Section 1, this document also uses the terms "data
model" and "YANG module" as defined in [RFC7950].
Wu, et al. Informational [Page 5]RFC 8309 Service Models Explained January 2018
Service Model: A service model is a specific type of data model. It
describes a service and the parameters of the service in a
portable way that can be used uniformly and independent of the
equipment and operating environment. The service model may be
divided into the two following categories:
Customer Service Model: A customer service model is used to
describe a service as offered or delivered to a customer by a
network operator as shown in Figure 1. It can be used by a
human (via a user interface such as a GUI, web form, or
Command-Line Interface (CLI)) or by software to configure or
request a service and may equally be consumed by a human (such
as via an order fulfillment system) or by a software component.
Such models are sometimes referred to simply as "service
models" [RFC8049]. A customer service model is expressed in a
YANG module as a core set of parameters that are common across
network operators: additional features that are specific to the
offerings of individual network operators would be defined in
extensions or augmentations of the module. Except where
specific technology details are directly pertinent to the
customer (such as encapsulations or mechanisms applied on
access links), customer service models are technology agnostic
so that the customer does not have influence over or knowledge
of how the network operator engineers the service.
An example of where such details are relevant to the customer
is when they describe the behavior or interactions on the
interface between the equipment at the customer site (often
referred to as the Customer Edge or CE equipment) and the
equipment at the network operator's site (usually referred to
as the Provider Edge or PE equipment).
Service Delivery Model: A service delivery model is used by a
network operator to define and manage how a service is
engineered in the network. It can be used by a human operator
(such as via a management station) or by a software tool to
instruct network components. The YANG modules that encode such
models are sometimes referred to as "network service YANG
modules" [RFC8199] and are consumed by "external systems" such
as an Operations Support System (OSS). A service delivery
module is expressed as a core set of parameters that are common
across a network type and technology: additional features that
are specific to the configuration of individual vendor
equipment or proprietary protocols would be defined in
extensions or augmentations of the module. Service delivery
modules include technology-specific modules.
Wu, et al. Informational [Page 6]RFC 8309 Service Models Explained January 2018
The distinction between a customer service model and a service
delivery model needs to be clarified. The modules that encode a
customer service model are not used to directly configure network
devices, protocols, or functions: it is not something that is sent
to network devices (i.e., routers or switches) for processing.
Equally, the modules that encode a customer service model do not
describe how a network operator realizes and delivers the service
described by the module. This distinction is discussed further in
later sections.
3. Using Service Models
As already indicated, customer service models are used on the
interface between customers and network operators. This is shown 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 the customer and network operator is specific
to deployment and implementation. 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 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 when
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.
Wu, et al. Informational [Page 7]RFC 8309 Service Models Explained January 2018
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.
3.1. Practical Considerations
The practicality of customer service models has been repeatedly
debated. It has been suggested that network operators have radically
different business models and widely diverse commercial offerings,
which makes a common customer service model impractical. However,
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 reuse in multiple models.
4. Service Models in an SDN Context
In an 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 specific to deployment.
Wu, et al. Informational [Page 8]
RFC 8309 Service Models Explained January 2018
------------------
| |
| 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 unaware of the technology that the network
operator has available to deliver the service, 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.
Wu, et al. Informational [Page 9]RFC 8309 Service Models Explained January 2018
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 configuration models are used by
a controller to set parameters on an individual network element. The
network configuration models are used by a network orchestrator to
instruct controllers (which may each be responsible for multiple
network elements) how to configure parts of a network.
Wu, et al. Informational [Page 10]RFC 8309 Service Models Explained January 2018
The following examples illustrate the split between control
components that expose a "service interface" that is present in many
figures that show extended SDN architectures:
o Figure 1 of [RFC7426] shows a separation of the "Application
Plane", the "Network Services Abstraction Layer (NSAL)", and the
"Control Plane". It marks the "Service Interface" as situated
between the NSAL and the control plane.
o Figure 1 of [RFC7491] shows an interface between an "Application
Service Coordinator" and an "Application-Based Network Operations
Controller".
o Figure 1 of [RFC8199] shows an interface from an OSS or a Business
Support System (BSS) that is expressed in "Network Service YANG
Modules".
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 the customer and network operator.
Further discussion of this point is provided in Section 5.
5. Possible Causes of Confusion
In discussing service models, there are several possible causes of
confusion:
o The services we are discussing are connectivity services provided
by network operators to customers; the services are achieved by
manipulating the network resources of the operator's network.
This is a completely different thing to "Foo as a Service" (for
example, Storage as a Service (SaaS)) where a service provider
offers reachability to a value-added service that is provided at
some location in the network using other resources (compute,
storage, ...) that are not part of the network itself. The
confusion arises not only because of the use of the word "service"
in both cases, but also because network operators may offer both
types of service to their customers.
Wu, et al. Informational [Page 11]RFC 8309 Service Models Explained January 2018
o Network operation is normally out of scope in the discussion of
services between a network operator and a customer. That means
that the customer service model does not reveal to the customer
anything about how the network operator delivers the service, nor
does the model expose details of technology or network resources
used to provide the service (all of these details could, in any
case, be considered security vulnerabilities). For example, in
the simple case of point-to-point virtual link connectivity
provided by a network tunnel (such as an MPLS pseudowire), the
network operator does not expose the path through the network that
the tunnel follows. Of course, this does not preclude the network
operator from taking guidance from the customer (such as to avoid
routing traffic through a particular country) or from disclosing
specific details (such as might be revealed by a route trace), but
these are not standard features of the service as described in the
customer service model.
o The network operator may use further data models (service delivery
models) that help to describe how the service is realized in the
network. These models might be used on the interface between the
service orchestrator and the network orchestrator, as shown in
Figure 3, and might include many of the pieces of information from
the customer service model alongside protocol parameters and
device configuration information. [RFC8199] also terms these data
models as "service models" and encodes them as "Network Service
YANG Modules"; a comparison is provided in Section 6.1. It is
important that the service orchestrator be able to map from a
customer service model to these service delivery models, but they
are not the same thing.
o Commercial terms (such as "cost per byte", "cost per minute", and
"scoped by quality and type of service", as well as terms related
to payment) are generally not a good subject for standardization.
It is possible that some network operators will enhance standard
customer service models to include commercial information, but the
way this is done is likely to vary widely between network
operators. Thus, this feature is out of scope for standardized
customer service models.
o SLAs have a high degree of overlap with the definition of services
present in customer service models. Requests for specific
bandwidth, for example, might be present in a customer service
model, and agreement to deliver a service is a commitment to the
description of the service in the customer service model.
However, SLAs typically include a number of fine-grained details
about how services are allowed to vary, by how much, and how
often. SLAs are also linked to commercial terms with penalties
and so forth and thus are also not good topics for
Wu, et al. Informational [Page 12]RFC 8309 Service Models Explained January 2018
standardization. As with commercial terms, it is expected that
some network operators will enhance standard customer service
models to include SLA parameters either using their own work or
depending on material from standards bodies that specialize in
this topic, but this feature is out of scope for the IETF's
customer service models.
If a network operator chooses to express an SLA using a data
model, that model might be referenced as an extension or an
augmentation of the customer service model.
6. Comparison with Other Work
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.
6.1. Comparison with Network Service Models
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
include 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 functions such as billing and SLAs.
Wu, et al. Informational [Page 13]RFC 8309 Service Models Explained January 2018
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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 Abstraction Layers Showing
Customer Service Modules
Wu, et al. Informational [Page 14]RFC 8309 Service Models Explained January 2018
6.2. Service Delivery and Network Element Model Work
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
configuration 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:
o [BGP-L3VPN-YANG] defines a YANG module that can be used to
configure and manage BGP L3VPNs.
o [L2VPN-YANG] documents a data model that is expected to be used by
the management tools run by the network operators in order to
manage and monitor the network resources that they use to deliver
L2VPN services.
o [EVPN-YANG] defines YANG modules for delivering an Ethernet VPN
service.
Wu, et al. Informational [Page 15]RFC 8309 Service Models Explained January 2018
6.3. Customer Service Model Work
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 2 VPN Service Model (L2SM) is defined in [L2VPN-SERVICE].
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.
Wu, et al. Informational [Page 16]RFC 8309 Service Models Explained January 2018
----------------------------
| 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
6.4. The MEF Architecture
The MEF Forum (MEF) has developed an architecture for network
management and operation. It is documented as the Lifecycle Service
Orchestration (LSO) Reference Architecture and is illustrated in
Figure 2 of [MEF-55].
The work of the MEF embraces all aspects of Lifecycle Service
Orchestration, including billing, SLAs, order management, and
lifecycle management. The IETF's work on service models is typically
smaller and offers a simple, self-contained service YANG module.
This does not invalidate either approach: it only observes that they
are different approaches.
Wu, et al. Informational [Page 17]RFC 8309 Service Models Explained January 2018
7. Further Concepts
This section introduces a few further, more advanced concepts.
7.1. Technology Agnostic
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 to the customer site and impact the
type of service delivered. Such features do need to be described in
the service model.
Two examples are as follows:
o The data passed between customer equipment and network operator
equipment will be encapsulated in a specific way, and that data-
plane type forms part of the service.
o Protocols that are run between customer equipment and network
operator equipment (for example, Operations, Administration, and
Maintenance (OAM) protocols, protocols for discovery, or protocols
for exchanging routing information) need to be selected and
configured as part of the service description.
7.2. Relationship to Policy
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 offerings of
all network operators. Policies that describe which person working
for 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 offerings
of all network operators.
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
Wu, et al. Informational [Page 18]RFC 8309 Service Models Explained January 2018
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.
7.3. Operator-Specific Features
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 upon and
documented in a service model, it is important that that model 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 be varied from one environment to another.
7.4. Supporting Multiple Services
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 that is 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.
Wu, et al. Informational [Page 19]
RFC 8309 Service Models Explained January 2018
8. Security Considerations
The interface between customer and service provider is a commercial
interface, and it 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 function 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 additional protection is provided by the fact that how the
service is delivered is entirely up to the operator.
Equally, all external interfaces, such as any of those between the
functional components in Figure 3, need to be correctly secured.
This document discusses modeling the information, not how it is
exchanged.
9. Manageability Considerations
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.
Wu, et al. Informational [Page 20]
RFC 8309 Service Models Explained January 2018
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.
10. IANA Considerations
This document does not require any IANA actions.
11. References
11.1. Normative References
[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444,
DOI 10.17487/RFC3444, January 2003,
<https://www.rfc-editor.org/info/rfc3444>.
[RFC8199] Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
Classification", RFC 8199, DOI 10.17487/RFC8199, July
2017, <https://www.rfc-editor.org/info/rfc8199>.
11.2. Informative References
[BGP-L3VPN-YANG]
Jain, D., Patel, K., Brissette, P., Li, Z., Zhuang, S.,
Liu, X., Haas, J., Esale, S., and B. Wen, "Yang Data Model
for BGP/MPLS L3 VPNs", Work in Progress, draft-dhjain-
bess-bgp-l3vpn-yang-02, August 2016.
[EVPN-YANG]
Brissette, P., Sajassi, A., Shah, H., Li, Z.,
Tiruveedhula, K., Hussain, I., and J. Rabadan, "Yang Data
Model for EVPN", Work in Progress, draft-ietf-bess-evpn-
yang-03, October 2017.
[L2VPN-SERVICE]
Wen, B., Fioccola, G., Xie, C., and L. Jalil, "A YANG Data
Model for L2VPN Service Delivery", Work in Progress,
draft-ietf-l2sm-l2vpn-service-model-04, October 2017.
[L2VPN-YANG]
Shah, H., Brissette, P., Chen, I., Hussain, I., Wen, B.,
and K. Tiruveedhula, "YANG Data Model for MPLS-based
L2VPN", Work in Progress, draft-ietf-bess-l2vpn-yang-07,
October 2017.
Wu, et al. Informational [Page 21]RFC 8309 Service Models Explained January 2018
[MEF-55] MEF Forum, "Lifecycle Service Orchestration (LSO):
Reference Architecture and Framework", Service Operations
Specification MEF 55, March 2016.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC7149] Boucadair, M. and C. Jacquenet, "Software-Defined
Networking: A Perspective from within a Service Provider
Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
<https://www.rfc-editor.org/info/rfc7149>.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
<https://www.rfc-editor.org/info/rfc7223>.
[RFC7297] Boucadair, M., Jacquenet, C., and N. Wang, "IP
Connectivity Provisioning Profile (CPP)", RFC 7297,
DOI 10.17487/RFC7297, July 2014,
<https://www.rfc-editor.org/info/rfc7297>.
[RFC7407] Bjorklund, M. and J. Schoenwaelder, "A YANG Data Model for
SNMP Configuration", RFC 7407, DOI 10.17487/RFC7407,
December 2014, <https://www.rfc-editor.org/info/rfc7407>.
[RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
Defined Networking (SDN): Layers and Architecture
Terminology", RFC 7426, DOI 10.17487/RFC7426, January
2015, <https://www.rfc-editor.org/info/rfc7426>.
[RFC7491] King, D. and A. Farrel, "A PCE-Based Architecture for
Application-Based Network Operations", RFC 7491,
DOI 10.17487/RFC7491, March 2015,
<https://www.rfc-editor.org/info/rfc7491>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
Wu, et al. Informational [Page 22]RFC 8309 Service Models Explained January 2018
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[RFC8049] Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
Model for L3VPN Service Delivery", RFC 8049,
DOI 10.17487/RFC8049, February 2017,
<https://www.rfc-editor.org/info/rfc8049>.
Acknowledgements
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 their
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 but critical,
one-word typo.
Authors' Addresses
Qin Wu
Huawei Technologies
Email: bill.wu@huawei.com
Will Liu
Huawei Technologies
Email: liushucheng@huawei.com
Adrian Farrel
Juniper Networks
Email: afarrel@juniper.net
Wu, et al. Informational [Page 23]
