Internet Research Task Force (IRTF) E. Haleplidis, Ed.
Request for Comments: 7426 University of Patras
Category: Informational K. Pentikousis, Ed.
ISSN: 2070-1721 EICT
University of Patras
J. Hadi Salim
University of Patras
January 2015 Software-Defined Networking (SDN): Layers and Architecture Terminology
Software-Defined Networking (SDN) refers to a new approach for
network programmability, that is, the capacity to initialize,
control, change, and manage network behavior dynamically via open
interfaces. SDN emphasizes the role of software in running networks
through the introduction of an abstraction for the data forwarding
plane and, by doing so, separates it from the control plane. This
separation allows faster innovation cycles at both planes as
experience has already shown. However, there is increasing confusion
as to what exactly SDN is, what the layer structure is in an SDN
architecture, and how layers interface with each other. This
document, a product of the IRTF Software-Defined Networking Research
Group (SDNRG), addresses these questions and provides a concise
reference for the SDN research community based on relevant peer-
reviewed literature, the RFC series, and relevant documents by other
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 Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the Software-
Defined Networking Research Group of the Internet Research Task Force
(IRTF). Documents approved for publication by the IRSG are not a
candidate for any level of Internet Standard; see Section 2 of RFC
Information about the current status of this document, any errata,
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Copyright (c) 2015 IETF Trust and the persons identified as the
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to this document.
"Software-Defined Networking (SDN)" is a term of the programmable
networks paradigm [PNSurvey99] [OF08]. In short, SDN refers to the
ability of software applications to program individual network
devices dynamically and therefore control the behavior of the network
as a whole [NV09]. Boucadair and Jacquenet [RFC7149] point out that
SDN is a set of techniques used to facilitate the design, delivery,
and operation of network services in a deterministic, dynamic, and
A key element in SDN is the introduction of an abstraction between
the (traditional) forwarding and control planes in order to separate
them and provide applications with the means necessary to
programmatically control the network. The goal is to leverage this
separation, and the associated programmability, in order to reduce
complexity and enable faster innovation at both planes [A4D05].
The historical evolution of the research and development area of
programmable networks is reviewed in detail in [SDNHistory]
[SDNSurvey], starting with efforts dating back to the 1980s. As
documented in [SDNHistory], many of the ideas, concepts, and concerns
are applicable to the latest research and development in SDN (and SDN
standardization) and have been under extensive investigation and
discussion in the research community for quite some time. For
example, Rooney, et al. [Tempest] discuss how to allow third-party
access to the network without jeopardizing network integrity or how
to accommodate legacy networking solutions in their (then new)
programmable environment. Further, the concept of separating the
control and forwarding planes, which is prominent in SDN, has been
extensively discussed even prior to 1998 [Tempest] [P1520] in SS7
networks [ITUSS7], Ipsilon Flow Switching [RFC1953] [RFC2297], and
SDN research often focuses on varying aspects of programmability, and
we are frequently confronted with conflicting points of view
regarding what exactly SDN is. For instance, we find that for
various reasons (e.g., work focusing on one domain and therefore not
necessarily applicable as-is to other domains), certain well-accepted
definitions do not correlate well with each other. For example, both
OpenFlow [OpenFlow] and the Network Configuration Protocol (NETCONF)
[RFC6241] have been characterized as SDN interfaces, but they refer
to control and management, respectively.
This motivates us to consolidate the definitions of SDN in the
literature and correlate them with earlier work at the IETF and the
research community. Of particular interest is, for example, to
determine which layers comprise the SDN architecture and which
interfaces and their corresponding attributes are best suited to be
used between them. As such, the aim of this document is not to
standardize any particular layer or interface but rather to provide a
concise reference that reflects current approaches regarding the SDN
layer architecture. We expect that this document would be useful to
upcoming work in SDNRG as well as future discussions within the SDN
community as a whole.
This document addresses the work item in the SDNRG charter titled
"Survey of SDN approaches and Taxonomies", fostering better
understanding of prominent SDN technologies in a technology-impartial
and business-agnostic manner but does not constitute a new IETF
standard. It is meant as a common base for further discussion. As
such, we do not make any value statements nor discuss the
applicability of any of the frameworks examined in this document for
any particular purpose. Instead, we document their characteristics
and attributes and classify them, thus providing a taxonomy. This
document does not intend to provide an exhaustive list of SDN
research issues; interested readers should consider reviewing
[SLTSDN] and [SDNACS]. In particular, Jarraya, et al. [SLTSDN]
provide an overview of SDN-related research topics, e.g., control
partitioning, which is related to the Consistency, Availability and
Partitioning (CAP) theorem discussed in Section 3.5.4.
This document has been extensively reviewed, discussed, and commented
by the vast majority of SDNRG members, a community that certainly
exceeds 100 individuals. It is the consensus of SDNRG that this
document should be published in the IRTF stream of the RFC series
The remainder of this document is organized as follows. Section 2
explains the terminology used in this document. Section 3 introduces
a high-level overview of current SDN architecture abstractions.
Finally, Section 4 discusses how the SDN layer architecture relates
to prominent SDN-enabling technologies.
This document uses the following terms:
o Software-Defined Networking (SDN) - A programmable networks
approach that supports the separation of control and forwarding
planes via standardized interfaces.
o Resource - A physical or virtual component available within a
system. Resources can be very simple or fine-grained (e.g., a
port or a queue) or complex, comprised of multiple resources
(e.g., a network device).
o Network Device - A device that performs one or more network
operations related to packet manipulation and forwarding. This
reference model makes no distinction whether a network device is
physical or virtual. A device can also be considered as a
container for resources and can be a resource in itself.
o Interface - A point of interaction between two entities. When the
entities are placed at different locations, the interface is
usually implemented through a network protocol. If the entities
are collocated in the same physical location, the interface can be
implemented using a software application programming interface
(API), inter-process communication (IPC), or a network protocol.
o Application (App) - An application in the context of SDN is a
piece of software that utilizes underlying services to perform a
function. Application operation can be parameterized, for
example, by passing certain arguments at call time, but it is
meant to be a standalone piece of software; an App does not offer
any interfaces to other applications or services.
o Service - A piece of software that performs one or more functions
and provides one or more APIs to applications or other services of
the same or different layers to make use of said functions and
returns one or more results. Services can be combined with other
services, or called in a certain serialized manner, to create a
o Forwarding Plane (FP) - The collection of resources across all
network devices responsible for forwarding traffic.
o Operational Plane (OP) - The collection of resources responsible
for managing the overall operation of individual network devices.
o Control Plane (CP) - The collection of functions responsible for
controlling one or more network devices. CP instructs network
devices with respect to how to process and forward packets. The
control plane interacts primarily with the forwarding plane and,
to a lesser extent, with the operational plane.
o Management Plane (MP) - The collection of functions responsible
for monitoring, configuring, and maintaining one or more network
devices or parts of network devices. The management plane is
mostly related to the operational plane (it is related less to the
o Application Plane - The collection of applications and services
that program network behavior.
o Device and resource Abstraction Layer (DAL) - The device's
resource abstraction layer based on one or more models. If it is
a physical device, it may be referred to as the Hardware
Abstraction Layer (HAL). DAL provides a uniform point of
reference for the device's forwarding- and operational-plane
o Control Abstraction Layer (CAL) - The control plane's abstraction
layer. CAL provides access to the Control-Plane Southbound
o Management Abstraction Layer (MAL) - The management plane's
abstraction layer. MAL provides access to the Management-Plane
o Network Services Abstraction Layer (NSAL) - Provides service
abstractions that can be used by applications and services.
3. SDN Layers and Architecture
Figure 1 summarizes the SDN architecture abstractions in the form of
a detailed, high-level schematic. Note that in a particular
implementation, planes can be collocated with other planes or can be
physically separated, as we discuss below.
SDN is based on the concept of separation between a controlled entity
and a controller entity. The controller manipulates the controlled
entity via an interface. Interfaces, when local, are mostly API
invocations through some library or system call. However, such
interfaces may be extended via some protocol definition, which may
use local inter-process communication (IPC) or a protocol that could
also act remotely; the protocol may be defined as an open standard or
in a proprietary manner.
Day [PiNA] explores the use of IPC as the mainstay for the definition
of recursive network architectures with varying degrees of scope and
range of operation. The Recursive InterNetwork Architecture [RINA]
outlines a recursive network architecture based on IPC that
capitalizes on repeating patterns and structures. This document does
not propose a new architecture -- we simply document previous work
through a taxonomy. Although recursion is out of the scope of this
work, Figure 1 illustrates a hierarchical model in which layers can
be stacked on top of each other and employed recursively as needed.
This document follows a network-device-centric approach: control
mostly refers to the device packet-handling capability, while
management typically refers to aspects of the overall device
operation. We view a network device as a complex resource that
contains and is part of multiple resources similar to [DIOPR].
Resources can be simple, single components of a network device, for
example, a port or a queue of the device, and can also be aggregated
into complex resources, for example, a network card or a complete
The reader should keep in mind that we make no distinction between
"physical" and "virtual" resources or "hardware" and "software"
realizations in this document, as we do not delve into implementation
or performance aspects. In other words, a resource can be
implemented fully in hardware, fully in software, or any hybrid
combination in between. Further, we do not distinguish whether a
resource is implemented as an overlay or as a part/component of some
other device. In general, network device software can run on so-
called "bare metal" or on a virtualized substrate. Finally, this
document does not discuss how resources are allocated, orchestrated,
and released. Indeed, orchestration is out of the scope of this
SDN spans multiple planes as illustrated in Figure 1. Starting from
the bottom part of the figure and moving towards the upper part, we
identify the following planes:
o Forwarding Plane - Responsible for handling packets in the data
path based on the instructions received from the control plane.
Actions of the forwarding plane include, but are not limited to,
forwarding, dropping, and changing packets. The forwarding plane
is usually the termination point for control-plane services and
applications. The forwarding plane can contain forwarding
resources such as classifiers. The forwarding plane is also
widely referred to as the "data plane" or the "data path".
o Operational Plane - Responsible for managing the operational state
of the network device, e.g., whether the device is active or
inactive, the number of ports available, the status of each port,
and so on. The operational plane is usually the termination point
for management-plane services and applications. The operational
plane relates to network device resources such as ports, memory,
and so on. We note that some participants of the IRTF SDNRG have
a different opinion in regards to the definition of the
operational plane. That is, one can argue that the operational
plane does not constitute a "plane" per se, but it is, in
practice, an amalgamation of functions on the forwarding plane.
For others, however, a "plane" allows one to distinguish between
different areas of operations; therefore, the operational plane is
included as a "plane" in Figure 1. We have adopted this latter
view in this document.
o Control Plane - Responsible for making decisions on how packets
should be forwarded by one or more network devices and pushing
such decisions down to the network devices for execution. The
control plane usually focuses mostly on the forwarding plane and
less on the operational plane of the device. The control plane
may be interested in operational-plane information, which could
include, for instance, the current state of a particular port or
its capabilities. The control plane's main job is to fine-tune
the forwarding tables that reside in the forwarding plane, based
on the network topology or external service requests.
o Management Plane - Responsible for monitoring, configuring, and
maintaining network devices, e.g., making decisions regarding the
state of a network device. The management plane usually focuses
mostly on the operational plane of the device and less on the
forwarding plane. The management plane may be used to configure
the forwarding plane, but it does so infrequently and through a
more wholesale approach than the control plane. For instance, the
management plane may set up all or part of the forwarding rules at
once, although such action would be expected to be taken
o Application Plane - The plane where applications and services that
define network behavior reside. Applications that directly (or
primarily) support the operation of the forwarding plane (such as
routing processes within the control plane) are not considered
part of the application plane. Note that applications may be
implemented in a modular and distributed fashion and, therefore,
can often span multiple planes in Figure 1.
[RFC7276] has defined the data, control, and management planes in
terms of Operations, Administration, and Maintenance (OAM). This
document attempts to broaden the terms defined in [RFC7276] in order
to reflect all aspects of an SDN architecture.
All planes mentioned above are connected via interfaces (indicated
with "Y" in Figure 1. An interface may take multiple roles depending
on whether the connected planes reside on the same (physical or
virtual) device. If the respective planes are designed so that they
do not have to reside in the same device, then the interface can only
take the form of a protocol. If the planes are collocated on the
same device, then the interface could be implemented via an open/
proprietary protocol, an open/proprietary software inter-process
communication API, or operating system kernel system calls.
Applications, i.e., software programs that perform specific
computations that consume services without providing access to other
applications, can be implemented natively inside a plane or can span
multiple planes. For instance, applications or services can span
both the control and management planes and thus be able to use both
the Control-Plane Southbound Interface (CPSI) and Management-Plane
Southbound Interface (MPSI), although this is only implicitly
illustrated in Figure 1. An example of such a case would be an
application that uses both [OpenFlow] and [OF-CONFIG].
Services, i.e., software programs that provide APIs to other
applications or services, can also be natively implemented in
specific planes. Services that span multiple planes belong to the
application plane as well.
While not shown explicitly in Figure 1, services, applications, and
entire planes can be placed in a recursive manner, thus providing
overlay semantics to the model. For example, application-plane
services can be provided to other applications or services through
NSAL. Additional examples include virtual resources that are
realized on top of a physical resources and hierarchical control-
plane controllers [KANDOO].
Note that the focus in this document is, of course, on the north/
south communication between entities in different planes. But this,
clearly, does not exclude entity communication within any one plane.
It must be noted, however, that in Figure 1, we present an abstract
view of the various planes, which is devoid of implementation
details. Many implementations in the past have opted for placing the
management plane on top of the control plane. This can be
interpreted as having the control plane acting as a service to the
management plane. Further, in many networks, especially in Internet
routers and Ethernet switches, the control plane has been usually
implemented as tightly coupled with the network device. When taken
as a whole, the control plane has been distributed network-wide. On
the other hand, the management plane has been traditionally
centralized and has been responsible for managing the control plane
and the devices. However, with the adoption of SDN principles, this
distinction is no longer so clear-cut.
Additionally, this document considers four abstraction layers:
o The Device and resource Abstraction Layer (DAL) abstracts the
resources of the device's forwarding and operational planes to the
control and management planes. Variations of DAL may abstract
both planes or either of the two and may abstract any plane of the
device to either the control or management plane.
o The Control Abstraction Layer (CAL) abstracts the Control-Plane
Southbound Interface and the DAL from the applications and
services of the control plane.
o The Management Abstraction Layer (MAL) abstracts the Management-
Plane Southbound Interface and the DAL from the applications and
services of the management plane.
o The Network Services Abstraction Layer (NSAL) provides service
abstractions for use by applications and other services.
At the time of this writing, SDN-related activities have begun in
other SDOs. For example, at the ITU, work on architectural [ITUSG13]
and signaling requirements and protocols [ITUSG11] has commenced, but
the respective study groups have yet to publish their documents, with
the exception of [ITUY3300]. The views presented in [ITUY3300] as
well as in [ONFArch] are well aligned with this document.
3.2. Network Devices
A network device is an entity that receives packets on its ports and
performs one or more network functions on them. For example, the
network device could forward a received packet, drop it, alter the
packet header (or payload), forward the packet, and so on. A network
device is an aggregation of multiple resources such as ports, CPU,
memory, and queues. Resources are either simple or can be aggregated
to form complex resources that can be viewed as one resource. The
network device is in itself a complex resource. Examples of network
devices include switches and routers. Additional examples include
network elements that may operate at a layer above IP (such as
firewalls, load balancers, and video transcoders) or below IP (such
as Layer 2 switches and optical or microwave network elements).
Network devices can be implemented in hardware or software and can be
either physical or virtual. As has already been mentioned before,
this document makes no such distinction. Each network device has a
presence in a forwarding plane and an operational plane.
The forwarding plane, commonly referred to as the "data path", is
responsible for handling and forwarding packets. The forwarding
plane provides switching, routing, packet transformation, and
filtering functions. Resources of the forwarding plane include but
are not limited to filters, meters, markers, and classifiers.
The operational plane is responsible for the operational state of the
network device, for instance, with respect to status of network ports
and interfaces. Operational-plane resources include, but are not
limited to, memory, CPU, ports, interfaces, and queues.
The forwarding and the operational planes are exposed via the Device
and resource Abstraction Layer (DAL), which may be expressed by one
or more abstraction models. Examples of forwarding-plane abstraction
models are Forwarding and Control Element Separation (ForCES)
[RFC5812], OpenFlow [OpenFlow], YANG model [RFC6020], and SNMP MIBs
[RFC3418]. Examples of the operational-plane abstraction model
include the ForCES model [RFC5812], the YANG model [RFC6020], and
SNMP MIBs [RFC3418].
Note that applications can also reside in a network device. Examples
of such applications include event monitoring and handling
(offloading) topology discovery or ARP [RFC0826] in the device itself
instead of forwarding such traffic to the control plane.
3.3. Control Plane
The control plane is usually distributed and is responsible mainly
for the configuration of the forwarding plane using a Control-Plane
Southbound Interface (CPSI) with DAL as a point of reference. CP is
responsible for instructing FP about how to handle network packets.
Communication between control-plane entities, colloquially referred
to as the "east-west" interface, is usually implemented through
gateway protocols such as BGP [RFC4271] or other protocols such as
the Path Computation Element (PCE) Communication Protocol (PCEP)
[RFC5440]. These corresponding protocol messages are usually
exchanged in-band and subsequently redirected by the forwarding plane
to the control plane for further processing. Examples in this
category include [RCP], [SoftRouter], and [RouteFlow].
Control-plane functionalities usually include:
o Topology discovery and maintenance
o Packet route selection and instantiation
o Path failover mechanisms
The CPSI is usually defined with the following characteristics:
o time-critical interface that requires low latency and sometimes
high bandwidth in order to perform many operations in short order
o oriented towards wire efficiency and device representation instead
of human readability
Examples include fast- and high-frequency of flow or table updates,
high throughput, and robustness for packet handling and events.
CPSI can be implemented using a protocol, an API, or even inter-
process communication. If the control plane and the network device
are not collocated, then this interface is certainly a protocol.
Examples of CPSIs are ForCES [RFC5810] and the OpenFlow protocol
The Control Abstraction Layer (CAL) provides access to control
applications and services to various CPSIs. The control plane may
support more than one CPSI.
Control applications can use CAL to control a network device without
providing any service to upper layers. Examples include applications
that perform control functions, such as OSPF, IS-IS, and BGP.
Control-plane service examples include a virtual private LAN service,
service tunnels, topology services, etc.
3.4. Management Plane
The management plane is usually centralized and aims to ensure that
the network as a whole is running optimally by communicating with the
network devices' operational plane using a Management-Plane
Southbound Interface (MPSI) with DAL as a point of reference.
Management-plane functionalities are typically initiated, based on an
overall network view, and traditionally have been human-centric.
However, lately, algorithms are replacing most human intervention.
Management-plane functionalities [FCAPS] typically include:
o Fault and monitoring management
o Configuration management
In addition, management-plane functionalities may also include
entities such as orchestrators, Virtual Network Function Managers
(VNF Managers) and Virtualised Infrastructure Managers, as described
in [NFVArch]. Such entities can use management interfaces to
operational-plane resources to request and provision resources for
virtual functions as well as instruct the instantiation of virtual
forwarding functions on top of physical forwarding functions. The
possibility of a common abstraction model for both SDN and Network
Function Virtualization (NFV) is explored in [SDNNFV]. Note,
however, that these are only examples of applications and services in
the management plane and not formal definitions of entities in this
document. As has been noted above, orchestration and therefore the
definition of any associated entities is out of the scope of this
The MPSI, in contrast to the CPSI, is usually not a time-critical
interface and does not share the CPSI requirements.
MPSI is typically closer to human interaction than CPSI (cf.
[RFC3535]); therefore, MPSI usually has the following
o It is oriented more towards usability, with optimal wire
performance being a secondary concern.
o Messages tend to be less frequent than in the CPSI.
As an example of usability versus performance, we refer to the
consensus of the 2002 IAB Workshop [RFC3535]: the key requirement for
a network management technology is ease of use, not performance. As
per [RFC6632], textual configuration files should be able to contain
international characters. Human-readable strings should utilize
UTF-8, and protocol elements should be in case-insensitive ASCII,
which requires more processing capabilities to parse.
MPSI can range from a protocol, to an API or even inter-process
communication. If the management plane is not embedded in the
network device, the MPSI is certainly a protocol. Examples of MPSIs
are ForCES [RFC5810], NETCONF [RFC6241], IP Flow Information Export
(IPFIX) [RFC7011], Syslog [RFC5424], Open vSwitch Database (OVSDB)
[RFC7047], and SNMP [RFC3411].
The Management Abstraction Layer (MAL) provides access to management
applications and services to various MPSIs. The management plane may
support more than one MPSI.
Management applications can use MAL to manage the network device
without providing any service to upper layers. Examples of
management applications include network monitoring, fault detection,
and recovery applications.
Management-plane services provide access to other services or
applications above the management plane.
3.5. Discussion of Control and Management Planes
The definition of a clear distinction between "control" and
"management" in the context of SDN received significant community
attention during the preparation of this document. We observed that
the role of the management plane has been earlier largely ignored or
specified as out-of-scope for the SDN ecosystem. In the remainder of
this subsection, we summarize the characteristics that differentiate
the two planes in order to have a clear understanding of the
mechanics, capabilities, and needs of each respective interface.
A point has been raised regarding the reference timescales for the
control and management planes regarding how fast the respective plane
is required to react to, or how fast it needs to manipulate, the
forwarding or operational plane of the device. In general, the
control plane needs to send updates "often", which translates roughly
to a range of milliseconds; that requires high-bandwidth and low-
latency links. In contrast, the management plane reacts generally at
longer time frames, i.e., minutes, hours, or even days; thus, wire
efficiency is not always a critical concern. A good example of this
is the case of changing the configuration state of the device.
Another distinction between the control and management planes relates
to state persistence. A state is considered ephemeral if it has a
very limited lifespan and is not deemed necessary to be stored on
non-volatile memory. A good example is determining routing, which is
usually associated with the control plane. On the other hand, a
persistent state has an extended lifespan that may range from hours
to days and months, is meant to be used beyond the lifetime of the
process that created it, and is thus used across device reboots.
Persistent state is usually associated with the management plane.
As mentioned earlier, traditionally, the control plane has been
executed locally on the network device and is distributed in nature
whilst the management plane is usually executed in a centralized
manner, remotely from the device. However, with the advent of SDN
centralizing, or "logically centralizing", the controller tends to
muddle the distinction of the control and management plane based on
3.5.4. CAP Theorem Insights
The CAP theorem views a distributed computing system as composed of
multiple computational resources (i.e., CPU, memory, storage) that
are connected via a communications network and together perform a
task. The theorem, or conjecture by some, identifies three
characteristics of distributed systems that are universally
o Consistency, meaning that the system responds identically to a
query no matter which node receives the request (or does not
respond at all).
o Availability, i.e., that the system always responds to a request
(although the response may not be consistent or correct).
o Partition tolerance, namely that the system continues to function
even when specific nodes or the communications network fail.
In 2000, Eric Brewer [CAPBR] conjectured that a distributed system
can satisfy any two of these guarantees at the same time but not all
three. This conjecture was later proven by Gilbert and Lynch [CAPGL]
and is now usually referred to as the CAP theorem [CAPFN].
Forwarding a packet through a network correctly is a computational
problem. One of the major abstractions that SDN posits is that all
network elements are computational resources that perform the simple
computational task of inspecting fields in an incoming packet and
deciding how to forward it. Since the task of forwarding a packet
from network ingress to network egress is obviously carried out by a
large number of forwarding elements, the network of forwarding
devices is a distributed computational system. Hence, the CAP
theorem applies to forwarding of packets.
In the context of the CAP theorem, if one considers partition
tolerance of paramount importance, traditional control-plane
operations are usually local and fast (available), while management-
plane operations are usually centralized (consistent) and may be
The CAP theorem also provides insights into SDN architectures. For
example, a centralized SDN controller acts as a consistent global
database and specific SDN mechanisms ensure that a packet entering
the network is handled consistently by all SDN switches. The issue
of tolerance to loss of connectivity to the controller is not
addressed by the basic SDN model. When an SDN switch cannot reach
its controller, the flow will be unavailable until the connection is
restored. The use of multiple non-collocated SDN controllers has
been proposed (e.g., by configuring the SDN switch with a list of
controllers); this may improve partition tolerance but at the cost of
loss of absolute consistency. Panda, et al. [CAPFN] provide a first
exploration of how the CAP theorem applies to SDN.
3.6. Network Services Abstraction Layer
The Network Services Abstraction Layer (NSAL) provides access from
services of the control, management, and application planes to other
services and applications. We note that the term "SAL" is
overloaded, as it is often used in several contexts ranging from
system design to service-oriented architectures; therefore, we
explicitly add "Network" to the title of this layer to emphasize that
this term relates to Figure 1, and we map it accordingly in Section 4
to prominent SDN approaches.
Service interfaces can take many forms pertaining to their specific
requirements. Examples of service interfaces include, but are not
limited to, RESTful APIs, open protocols such as NETCONF, inter-
process communication, CORBA [CORBA] interfaces, and so on. The two
leading approaches for service interfaces are RESTful interfaces and
Remote Procedure Call (RPC) interfaces. Both follow a client-server
architecture and use XML or JSON to pass messages, but each has some
slightly different characteristics.
RESTful interfaces, designed according to the representational state
transfer design paradigm [REST], have the following characteristics:
o Resource identification - Individual resources are identified
using a resource identifier, for example, a URI.
o Manipulation of resources through representations - Resources are
represented in a format like JSON, XML, or HTML.
o Self-descriptive messages - Each message has enough information to
describe how the message is to be processed.
o Hypermedia as the engine of application state - A client needs no
prior knowledge of how to interact with a server, as the API is
not fixed but dynamically provided by the server.
Remote procedure calls (RPCs) [RFC5531], e.g., XML-RPC and the like,
have the following characteristics:
o Individual procedures are identified using an identifier.
o A client needs to know the procedure name and the associated
3.7. Application Plane
Applications and services that use services from the control and/or
management plane form the application plane.
Additionally, services residing in the application plane may provide
services to other services and applications that reside in the
application plane via the service interface.
Examples of applications include network topology discovery, network
provisioning, path reservation, etc.