SDN Layers and Architecture Terminology
draft-haleplidis-sdnrg-layer-terminology-05
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| Authors | Evangelos Haleplidis , Spyros Denazis , Kostas Pentikousis , Jamal Hadi Salim , David Meyer , Odysseas Koufopavlou | ||
| Last updated | 2014-07-04 | ||
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draft-haleplidis-sdnrg-layer-terminology-05
SDNRG E. Haleplidis
Internet-Draft S. Denazis
Intended status: Informational University of Patras
Expires: January 5, 2015 K. Pentikousis
EICT
J. Hadi Salim
Mojatatu Networks
D. Meyer
Brocade
O. Koufopavlou
University of Patras
July 4, 2014
SDN Layers and Architecture Terminology
draft-haleplidis-sdnrg-layer-terminology-05
Abstract
Software-Defined Networking (SDN) can in general be defined as a new
approach for network programmability. Network programmability refers
to the capacity to initialize, control, change, and manage network
behavior dynamically via open interfaces as opposed to relying on
closed-box solutions and propietary-defined 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 is the layer structure in an SDN architecture
and how do layers interface with each other. This document aims to
answer these questions and provide a concise reference document for
SDNRG, in particular, and the SDN community, in general, based on
relevant peer-reviewed literature and documents in the RFC series.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 5, 2015.
Copyright Notice
Copyright (c) 2014 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
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SDN Layers and Architecture . . . . . . . . . . . . . . . . . 6
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Network Devices . . . . . . . . . . . . . . . . . . . . . 11
3.3. Control Plane . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Management Plane . . . . . . . . . . . . . . . . . . . . 13
3.5. Control vs Management plane . . . . . . . . . . . . . . . 14
3.5.1. Timescale . . . . . . . . . . . . . . . . . . . . . . 14
3.5.2. Ephemeral/Persistent . . . . . . . . . . . . . . . . 14
3.5.3. Locality . . . . . . . . . . . . . . . . . . . . . . 14
3.5.4. CAP Theorem . . . . . . . . . . . . . . . . . . . . . 15
3.6. Network Services Abstraction Layer . . . . . . . . . . . 16
3.7. Application Plane . . . . . . . . . . . . . . . . . . . . 17
4. SDN Model View . . . . . . . . . . . . . . . . . . . . . . . 17
4.1. ForCES . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2. NETCONF . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3. OpenFlow . . . . . . . . . . . . . . . . . . . . . . . . 19
4.4. I2RS . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5. BFD . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.6. SNMP . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Software-Defined Networking (SDN) is a relevant new term for 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]. Another view of what SDN is defined in
[RFC7149] as a set of techniques used to facilitate the design, the
delivery and the operation of network services in a deterministic,
dynamic, and scalable manner.
A key element in SDN is the introduction of an abstraction between
the (traditional) Forwarding and the 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].
Feamster et al. [SDNHistory] review the historical evolution of the
programmable networks research area, starting with earlier efforts
which date back to the 1980s. As the authors document, many of the
ideas, concepts and concerns are applicable to the latest R&D in SDN,
and SDN standardization we may add, 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 data 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 ATM [ITUATM].
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 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 in IETF and the
research community. Of particular interest, for example, is to
determine which layers comprise the SDN architecture and which
interfaces and their corresponding attributes are best suitable to be
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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 document which reflects current approaches
regarding the SDN layers 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 aims to address the potential work item in the SDNRG
charter named "Survey of SDN approaches and Taxonomies", fostering
better understanding of prominent SDN technologies in a technology-
impartial and business-agnostic manner. As such, we do not make any
value statements nor discuss the applicability of any of the
frameworks examined in this draft for any particular purpose.
Instead, we document their characteristics and attributes and
classify them, thus providing a taxonomy. Already there are a number
of survey papers regarding SDN that discuss taxonomies such as
[SLTSDN] and [SDNACS].
This document does not constitute a new IETF standard nor a new
specification, and aims to receive rough consensus within SDNRG to be
published in the IRTF Stream as per [RFC5743].
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
with prominent SDN-enabling technologies
2. Terminology
This document uses the following terms:
Software-Defined Networking (SDN) - A programmable networks
approach that supports the separation of Control and Forwarding
Planes via standardized interfaces.
Resource - A component, physical or virtual, available within a
system. Resources can be very simple or fine-grained, e.g. a
port, a queue or complex, comprised of multiple resources, e.g. a
network device.
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.
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Interface - A point of interaction between two entities. In case
the entities are not in the same physical location, the interface
is usually implemented as a network protocol. In case the
entities are collocated in the same physical location the
interface can be a protocol or an open/proprietary software inter-
process communication Application Programming Interface (API).
Application (App) - A piece of software that utilizes underlying
services to perform a function. Application operation can be
parametrized, 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.
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
new service.
Forwarding Plane (FP) - The network device part responsible for
forwarding traffic.
Operational Plane (OP) - The network device part responsible for
managing the overall device operation.
Control Plane (CP) - Part of the network functionality that is
assigned to control one or more network devices. CP instructs
network devices with respect to how to treat and forward packets.
The control plane interacts primarily with the forwarding plane
and less with the operational plane.
Management Plane (MP) - Part of the network functionality
responsible for monitoring, configuring and maintaining one or
more network devices. The management plane is mostly related with
the operational plane and less with the forwarding plane.
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
resources.
Control Abstraction Layer (CAL) - The control plane's abstraction
layer. CAL provides access to the control plane southbound
interface.
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Management Abstraction Layer (MAL) - The management plane's
abstraction layer. MAL provides access to the management plane
southbound interface.
3. SDN Layers and Architecture
Figure 1 provides a detailed high-level overview of the current SDN
architecture abstractions. 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
calls 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.
The concept of separation via IPCs is explored in RINA [RINA] where
the premise is that all network communications is considered an IPC
and that allows a recursive approach on creating hierarchical network
connections. RINA's [RINA] approach has a lot of commonalities with
the described SDN layer abstractions as we can also view these layers
as being hierarchical stacked on top of each other as needed.
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o--------------------------------o
| |
| +-------------+ +----------+ |
| | Application | | Service | |
| +-------------+ +----------+ |
| Application Plane |
o---------------Y----------------o
|
*-----------------------------Y---------------------------------*
| Network Services Abstraction Layer (NSAL) |
*------Y------------------------------------------------Y-------*
| |
| Service Interface |
| |
o------Y------------------o o---------------------Y------o
| | Control Plane | | Management Plane | |
| +----Y----+ +-----+ | | +-----+ +----Y----+ |
| | Service | | App | | | | App | | Service | |
| +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ |
| | | | | | | |
| *----Y-----------Y----* | | *---Y---------------Y----* |
| | Control Abstraction | | | | Management Abstraction | |
| | Layer (CAL) | | | | Layer (MAL) | |
| *----------Y----------* | | *----------Y-------------* |
| | | | | |
o------------|------------o o------------|---------------o
| |
| CP | MP
| Southbound | Southbound
| Interface | Interface
| |
*------------Y---------------------------------Y----------------*
| Device and resource Abstraction Layer (DAL) |
*------------Y---------------------------------Y----------------*
| | | |
| o-------Y----------o +-----+ o--------Y----------o |
| | Forwarding Plane | | App | | Operational Plane | |
| o------------------o +-----+ o-------------------o |
| Network Device |
+---------------------------------------------------------------+
Figure 1: SDN Layer Architecture
3.1. Overview
This document follows a network device centric approach: Control
refers to the device packet handling capability, while Management
refers to the overall device operation aspects. We view a network
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device as a complex resource which 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 device.
The reader should keep in mind throughout this document that we make
no distinction between "physical" and "virtual" resources, 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 on whether a resource is implemented as an overlay or as
a part/component of some other device. Finally, network device
software can run on so-called "bare metal" or on a virtualized
substrate.
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
datapath. 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.
o Operational Plane - Responsible for managing the operational state
of the Network Device, e.g. active/inactive, number of ports, port
status, etc. The Operational Plane is usually the termination
point for management plane services and applications. The
operational plane relates to (operational aspects of) Network
Device resources such as ports, memory, and so on. Members of the
SDNRG have different opinions in regards to the operational plane,
with the argument being that it does not constitute a plane by
itself but an amalgamation of functions in the data plane. For
others it is considered a plane similar to the rest of the planes
as a plane may be considered as a distinction between different
areas of operations.
o Control Plane - Responsible for taking decisions on how packets
should be forwarded by one or more Network Devices and pushing
such decisions down to the Network Devices to be executed. 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 example, the current state of a particular port or
its capabilities. The control plane's main job is to fine-tune
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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. taking 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
sparingly.
o Application Plane - The plane where applications that rely on the
network to provide services for end users and processes 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.
All planes mentioned above are connected via Interfaces (as 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 co-located 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 plane and, thus, be able to use both
the CPSI and MPSI. 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 in Figure 1, services, applications and entire
planes, can be placed in a recursive manner thus providing overlay
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semantics to the model. For example, application plane services can
provide through NSAL services to other applications or services.
Additional examples include virtual resources that are realized on
top of a physical resources and hierarchical control plane
controllers [KANDOO].
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 tend to place the management plane on
top of the control plane, which may be interpreted as having the
control plane acting as a service to the management plane.
Traditionally, the control plane was tightly coupled with the device.
When taken as whole, the control plane was distributed network-wide.
On the other hand, the management plane has been traditionally
centralized and was 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:
The Device and resource Abstraction Layer (DAL) abstracts the
device's forwarding and operational plane resources to the control
and management plane. 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.
The Control Abstraction Layer (CAL) abstracts the CP southbound
interface and the DAL from the applications and services of the
Control Plane.
The Management Abstraction Layer (MAL) abstracts the MP southbound
interface and the DAL from the applications and services of the
Management Plane.
The Network Services Abstraction Layer (NSAL) provides service
abstractions for use by applications and other services.
We observe that the view presented in this document is quite well-
aligned with recently published work by the ONF; see [ONFArch]. A
key difference, however, is that the ONF architecture does not
include the management plane in its scope.
SDN related activities have begun in many other SDO's, such as:
ITU: Architectural work [ITUSG13] but have not been published at
the time this document was written.
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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) and 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.
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
both 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 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 ForCES [RFC5812] and OpenFlow [OpenFlow]. Examples of the
Operational Plane abstraction model include the ForCES model
[RFC5812], the YANG model [RFC6020] and SNMP MIBs [RFC3418].
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.
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
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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 planes, colloquially referred to as the
"east-west" interface, is usually implemented through gateway
protocols like BGP [RFC4271]. However, the corresponding protocol
messages are in fact 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 which 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 interprocess
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 [OpenFlow].
The Control Abstraction Layer (CAL) provides access to control
applications and services to various CPSIs. The Control Plane may
support more than one CPSIs.
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, BGP, etc.
Control Plane service examples include a virtual private LAN service,
service tunnels, topology services, etc.
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3.4. Management Plane
The Management Plane is usually centralized and aims to ensure that
the network, which consists of network devices, is running optimally
by communicating with the network devices's 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] [RFC3535] usually include:
o Fault and Monitoring management
o Configuration management
Normally MSPI, in contrast to the CPSI, is not a time-critical
interface and does not share the CPSI requirements.
MSPI is [RFC3535] typically closer to human interaction than the
control plane and therefore the MSPI usually has the following
characteristics:
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], as mentioned in
[RFC6632], where 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 require more processing capabilities to parse.
The MPSI can range from a protocol, to an API or even interprocess
communication. If the Management Plane is not embedded in the
network device, the MSPI is certainly a protocol. Examples of MPSIs
are ForCES [RFC5810], NETCONF [RFC6241], 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
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management applications include network monitoring and fault
detection and recovery applications.
Management Plane Services provide access to other services or
applications above the Management Plane.
3.5. Control vs Management plane
During the SDNRG meetings as well via the list, one of the most
commonly discussed topic, in regards to this document, was the clear
distinction between control and management. We have identified the
following characteristics that together or each one may provide the
necessary differentiator between the planes.
3.5.1. Timescale
Timescale refers to how fast an application in the respective plane
react or need to manipulate the forwarding or operational plane of
the device. In general, the control plane needs to send updates very
often within the range of milliseconds and that requires a high
bandwidth and low latency links. In contrast the management plane
reacts generally at very slow timeframes, minutes, hours or even
days, e.g. in the case of changing the configuration state of the
device, or and thus do not need to be very efficient on the wire.
3.5.2. Ephemeral/Persistent
Another distinction that discussed was the distinction between
ephemeral versus persistent state. Ephemeral state is state that may
have a very limited lifespan, such as routing decisions, and thus is
usually associated with the control plane. On the other hand,
persistent state is state that may have a larger and extended
lifespan which may range from hours to days and months and is usually
associated with the management plane. Persistent state is also
usually associated with data store of the state.
3.5.3. Locality
Before the concept of centralizing the controller, usually the
control plane is local to device and distributed whilst the
management plane is usually centralized and remote from the device.
However, as has been noted before, centralizing, or "locally
centralizing" the controller tends to muddle the distinction of the
control and management plane on locality.
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3.5.4. CAP Theorem
An additional distinction was introduced in the 89th IETF and refers
to the CAP theorem.
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 and identifies three characteristics of distributed systems that
are universally desirable:
Consistency, meaning that the system responds identically to a
query no matter which node receives the request (or does not
respond at all)
Availability, i.e., that the system always responds to a request
(although the response may not be consistent or correct)
Partition tolerance, namely that the system continues to function
even when 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 called the CAP theorem
Correctly forwarding a packet through a network, is a computational
problem. One of the major abstractions that SDN posits - all network
elements are computational resources that perform the single
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, control plane operations are
usually local and fast (available), while management plane operations
are usually centralized (consistent) and slow.
The CAP theorem provides insights on SDN performance. For example,
in regards to locality, although not explicitly stated, modern SDN
philosophy, centralizing the controller, stresses consistency. The
controller acts as a consistent global database, and specific
mechanisms ensure that a packet entering the network is handled
consistently by all switches and acts like a management entity. The
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issue of tolerance to loss of connectivity to the controller is not
addressed by the basic SDN model. When an SDN switch can't 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 improves partition tolerance, but at the cost of
loss of absolute consistency
3.6. Network Services Abstraction Layer
The Network Services Abstraction Layer (NSAL) provides access from
services of the control, management and application planes to
services and applications of the application plane. 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 prefixed it with "Network" 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 or proprietary protocols such as
NETCONF, inter-process communications, CORBA interfaces, etc.
Two leading standards of service interface are RESTful interfaces and
RPC interfaces. Both follow a client-server architecture and use XML
or JSON to pass messages but each have some slightly different
characteristics.
RESTful interfaces, designed with the Representational state transfer
design paradigm [REST], have the following characteristics:
Resource identification - individual resources are identified
using a resource identifier, for example a URI.
Manipulation of resources through representations - Resources are
represented in a format like JSON, XML or HTML.
Self-descriptive messages - Each message has enough information to
describe how the message is to be processed.
Hypermedia as the engine of application state - a client needs no
prior knowledge of how to interact with a server, not through a
fixed interface.
Remote procedure calls (RPC), e.g. [RFC5531], XML-RPC etc., have the
following characteristics:
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Individual procedures are identified using an identifier
A client needs to know the procedure name and the parameters
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.
4. SDN Model View
We advocate that the SDN southbound interface should encompass both
CSPI and MSPI.
The SDN northbound interface is implemented in the Network Services
Abstraction Layer of Figure 1.
The above model can be used to describe in a concise manner all
prominent SDN-enabling technologies, as we explain in the following
subsections.
4.1. ForCES
The IETF-standardized Forwarding and Control Element Separation
(ForCES [RFC5810]) framework consists of one model and two protocols.
ForCES separates the Forwarding from the Control Plane via an open
interface, namely the ForCES protocol which operates on entities of
the forwarding plane that have been modeled using the ForCES model.
The ForCES model is based on the fact that a network element is
composed of numerous logically separate entities that cooperate to
provide a given functionality -such as routing or IP switching- and
yet appear as a normal integrated network element to external
entities and secondly with a protocol to transport information.
ForCES models the Forwarding Plane using Logical Functional Blocks
(LFBs) which are connected in a graph, composing the Forwarding
Element (FE). LFBs are described in an XML language, based on an XML
schema.
LFB definitions include:
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o Base and custom-defined datatypes
o Metadata definitions
o Input and Output ports
o Operational parameters, or components
o Capabilities
o Event definitions
The ForCES model can be used to define LFBs from fine- to coarse-
grained as needed irrelevant of whether they are physical or virtual.
The ForCES protocol is agnostic to the model and can be used to
monitor, configure and control any ForCES-modeled element. The
protocol has very simple commands: Set, Get and Del(ete). ForCES is
a protocol designed for high throughput and fast updates.
ForCES [RFC5810] can be mapped to the framework illustrated in
Figure 1 as follows:
o The ForCES model can be used to describe DAL, both for the
Operational and the Forwarding Plane, using LFBs.
o The ForCES protocol can then be both the CPSI and the MPSI.
ForCES is inherently specified for the CPSI and satisfies its
requirements, however it can also be utilized for the MPSI.
o CAL and MAL must be able to utilize the ForCES protocol.
4.2. NETCONF
The Network Configuration Protocol (NETCONF [RFC6241]), is an IETF-
standardized network management protocol [RFC6632]. NETCONF provides
mechanisms to install, manipulate, and delete the configuration of
network devices.
NETCONF protocol operations are realized as remote procedure calls
(RPCs). The NETCONF protocol uses an Extensible Markup Language
(XML) based data encoding for the configuration data as well as the
protocol messages. Recent studies, such as [ESNet] and [PENet], have
shown that NETCONF performs better than SNMP [RFC3411].
Additionally, the YANG data modeling language [RFC6020] has been
developed for specifying NETCONF data models and protocol operations.
YANG is a data modeling language used to model configuration and
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state data manipulated by NETCONF, NETCONF remote procedure calls,
and NETCONF notifications.
YANG models the hierarchical organization of data as a tree, in which
each node has either a value or a set of child nodes. Additionally,
YANG structures data models into modules and submodules allowing
reusability and augmentation. YANG models can describe constraints
to be enforced on the data. Additionally YANG has a set of base
datatype and allows custom defined datatypes as well.
YANG allows the definition of NETCONF RPCs allowing the protocol to
have an extensible number of commands. For RPC definition, the
operations names, input parameters, and output parameters are defined
using YANG data definition statements.
NETCONF can be mapped to the framework illustrated in Figure 1 as
follows:
o The YANG model [RFC6020] is suitable for specifying DAL for the
operational plane and NETCONF [RFC6241] for the MPSI.
o Technically, the YANG model [RFC6020] can be used to specify DAL
for the Forwarding plane as well. That said, in principle NETCONF
[RFC6241] is a management protocol which was not (originally)
designed for fast CP updates, and it might not be suitable for
addressing the requirements of CPSI.
4.3. OpenFlow
[OpenFlow] is a framework originally developed by Standford, and
currently under active standards development through the Open
Networking Foundation. Initially, the goal was to provide a way for
researchers to run experimental protocols in a production network
[OFSIGC]. OpenFlow provides a protocol with which a controller may
manage a static model of an OpenFlow switch.
An OpenFlow switch consists of one or more flow tables which perform
packet lookups, actions on a success packet lookup and forwarding, a
group table and an OpenFlow channel to an external controller. The
switch communicates with the controller which manages the switch via
the OpenFlow protocol.
OpenFlow has undergone many revisions. The current version is 1.4
[OpenFlow] and supports amongst others, multiple controllers for high
availability and extensible flow match field protocol messages to
support arbitraty match fields. Efforts to define OpenFlow 2.0
[PPIPP] are already underway aiming to provide an abstract forwarding
model to provide protocol independence and device programmability.
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OpenFlow can be mapped to the framework illustrated in Figure 1 as
follows:
o The Openflow switch specifications [OpenFlow] covers DAL for the
Forwarding Plane and provides the specification for CPSI.
o The OF-CONFIG protocol [OF-CONFIG] based on the YANG model
[RFC6020], provides DAL for the Operational Plane and specifies
NETCONF [RFC6241] as the MPSI. OF-CONFIG overlaps with the
OpenFlow DAL, but with NETCONF [RFC6241] as the transport protocol
it shares the limitations described in the previous section.
o CAL must be able to utilize the OpenFlow protocol.
o MAL must be able to utilize the NETCONF protocol.
4.4. I2RS
I2RS is currently developed by a recently-established IETF working
group. The intention is to provide a standard interface to the
routing system for real-time or event-driven interaction through a
collection of protocol-based control or management interfaces.
Essentially, I2RS aims to make the routing information base (RIB)
programmable thus enabling new kinds of network provisioning and
operation.
I2RS does not initially intend to create new interfaces, but rather
leverage or extend existing ones and define informational models for
the routing system. For example, the latest I2RS problem statement
[I-D.ietf-i2rs-problem-statement] discusses previously-defined IETF
protocols and data models such as ForCES, YANG, NETCONF, and SNMP.
Currently the I2RS working group is developing an Information Model
[I-D.ietf-i2rs-rib-info-model] in regards to the Network Services
Abstraction Layer for the I2RS agent.
I2RS can be mapped to the framework illustrated in Figure 1 as
follows:
o The I2RS architecture [I-D.ietf-i2rs-architecture] encompasses the
Control and Application Planes and uses any CPSI and DAL that is
available, whether that may be ForCES, OpenFlow or another
Interface.
o The I2RS agent is a Control Plane Service. All services or
applications on top of that belong to either the Control,
Management or the Application plane. In the I2RS documents,
management access to the agent may be provided by management
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protocols like SNMP and NETCONF. The I2RS protocol may also be
mapped to the Service Interface as it will provide access even to
other than control applications.
4.5. BFD
Bidirectional Forwarding Detection (BFD) [RFC5880], is an IETF
network protocol designed for detecting communication failures
between two forwarding elements which are directly connected. It is
intended to be implemented in some component of the forwarding engine
of a system, in cases where the forwarding and control engines are
separated.
BFD provides low-overhead detection of faults even on physical media
that do not support failure detection of any kind, such as Ethernet,
virtual circuits, tunnels and MPLS Label Switched Paths.
BFD could be mapped to the framework illustrated in Figure 1 either
as:
1. A control plane service or application that would use the CPSI
towards the forwarding plane to send/receive BFD packets.
2. Or, better, as it was intended for, i.e. as an application that
runs on the device itself and uses the forwarding plane to send/
receive BFD packets and update the operational plane resources
accordingly.
4.6. SNMP
The Simple Network Management Protocol (SNMP) is an IETF management
protocol and is currently at the third version called SNMPv3 and
described in STD 62, RFC 3417 [RFC3417], RFC 3412 [RFC3412] and RFC
3414 [RFC3414]. It consists of a set of standards for network
management, including an application layer protocol, a database
schema, and a set of data objects. SNMP exposes management data
(managed objects) in the form of variables on the managed systems,
which describe the system configuration. These variables can then be
queried and set by managing applications.
SNMP uses an extensible design for describing data, defined by
management information bases (MIBs). MIBs describe the structure of
the management data of a device subsystem. MIBs use a hierarchical
namespace containing object identifiers (OID). Each OID identifies a
variable that can be read or set via SNMP. MIBs use the notation
defined by Structure of Management Information Version 2 SMIv2
[RFC2578]
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SNMP could be mapped to the framework illustrated in Figure 1 as:
1. SNMP MIBs are usually used to describe DAL for the Operational
Plane.
2. SNMP is usually used for the MPSI.
5. Acknowledgements
The authors would like to acknowledge Salvatore Loreto and Sudhir
Modali for the initial discussion on the SDNRG mailing list as well
as their draft-specific comments that helped put this document in a
better shape.
Additionally the authors would like to acknowledge Russ White, Linda
Dunbar, Robert Raszuk, Pedro Martinez-Julia, Lee Young, Yaakov Stein,
Shivleela Arlimatti, Gurkan Deniz, Scott Brim, Carlos Pignataro,
Ramki Krishnan, Bless Roland, Tim Copley, Francisco Javier Ros Munoz,
Sriganesh Kini, Alan Clark, Erik Nordmark, Scott Mansfield, Dirk
Kutscher, Roland Bless, David E Mcdysan, Bhumip Khasnabish and
Georgios Karagiannis for their critical comments and discussions at
the IETF 88 and 89 meetings (and the SDNRG mailing list), which we
took into consideration while revising this document.
Special thanks to Yaakov Stein for providing text related to the CAP
theorem and Scott Mansfield for information regarding ITU status on
SDN
6. IANA Considerations
This memo makes no requests to IANA.
7. Security Considerations
TBD
8. Informative References
[A4D05] Greenberg, Albert, et al., "A clean slate 4D approach to
network control and management", ACM SIGCOMM Computer
Communication Review 35.5 (2005): 41-54 , 2005.
[CAPBR] Eric A. Brewer, "Towards robust distributed systems.",
Symposium on Principles of Distributed Computing (PODC).
2000 , 2000.
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[CAPGL] Seth Gilbert, and Nancy Ann Lynch., "Brewer's conjecture
and the feasibility of consistent, available, partition-
tolerant web services", ACM SIGACT News 33.2 (2002):
51-59. , 2002.
[DIOPR] Denazis, Spyros, Kazuho Miki, John Vicente, and Andrew
Campbell., "Designing interfaces for open programmable
routers.", In Active Networks, pp. 13-24. Springer Berlin
Heidelberg, 1999 , 1999.
[ESNet] Yu, James, and Imad Al Ajarmeh., "An empirical study of
the NETCONF protocol.", In Networking and Services (ICNS),
2010 Sixth International Conference on, pp. 253-258. IEEE,
2010. , 2010.
[FCAPS] International Telecommunication Union, "X.700: Management
Framework For Open Systems Interconnection (OSI) For CCITT
Applications", September 1992,
<http://www.itu.int/rec/T-REC-X.700-199209-I/en>.
[I-D.ietf-i2rs-architecture]
Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
Nadeau, "An Architecture for the Interface to the Routing
System", draft-ietf-i2rs-architecture-04 (work in
progress), June 2014.
[I-D.ietf-i2rs-problem-statement]
Atlas, A., Nadeau, T., and D. Ward, "Interface to the
Routing System Problem Statement", draft-ietf-i2rs-
problem-statement-04 (work in progress), June 2014.
[I-D.ietf-i2rs-rib-info-model]
Bahadur, N., Folkes, R., Kini, S., and J. Medved, "Routing
Information Base Info Model", draft-ietf-i2rs-rib-info-
model-03 (work in progress), May 2014.
[ITUATM] CCITT, Geneva, Switzerland, "CCITT Recommendation 1.361,
B-ISDN ATM Layer Specification", 1990.
[ITUSG13] Telecommunication Standardization sector of ITU, "ITU,
Study group 13", 2013, <http://www.itu.int/en/ITU-T/
studygroups/2013-2016/13/Pages/default.aspx>.
[ITUSS7] Telecommunication Standardization sector of ITU, "ITU,
Q.700 : Introduction to CCITT Signalling System No. 7",
1993.
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Internet-Draft SDN Layers and Architecture Terminology July 2014
[KANDOO] Hassas Yeganeh, Soheil, and Yashar Ganjali., "Kandoo: a
framework for efficient and scalable offloading of control
applications.", In Proceedings of the first workshop on
Hot topics in software defined networks, pp. 19-24. ACM
SIGCOMM, 2012. , 2012.
[NV09] Chowdhury, NM Mosharaf Kabir, and Raouf Boutaba, "Network
virtualization: state of the art and research challenges",
Communications Magazine, IEEE 47.7 (2009): 20-26 , 2009.
[OF-CONFIG]
Open Networking Foundation, "OpenFlow Management and
Configuration Protocol 1.1.1", March 2013,
<https://www.opennetworking.org/images/stories/downloads/
sdn-resources/onf-specifications/openflow-config/of-
config-1-1-1.pdf>.
[OF08] McKeown, Nick, et al., "OpenFlow: enabling innovation in
campus networks", ACM SIGCOMM Computer Communication
Review 38.2 (2008): 69-74 , 2008.
[OFSIGC] McKeown, Nick, Tom Anderson, Hari Balakrishnan, Guru
Parulkar, Larry Peterson, Jennifer Rexford, Scott Shenker,
and Jonathan Turner., "OpenFlow: enabling innovation in
campus networks.", ACM SIGCOMM Computer Communication
Review 38, no. 2 (2008): 69-74. , 1998.
[ONFArch] Open Networking Foundation, "SDN Architecture Overview",
December 2013,
<https://www.opennetworking.org/images/stories/downloads/
sdn-resources/technical-reports/SDN-architecture-overview-
1.0.pdf>.
[OpenFlow]
Open Networking Foundation, "The OpenFlow 1.4
Specification.", October 2013,
<https://www.opennetworking.org/images/stories/downloads/
sdn-resources/onf-specifications/openflow/openflow-spec-
v1.4.0.pdf>.
[P1520] Biswas, Jit, Aurel A. Lazar, J-F. Huard, Koonseng Lim,
Semir Mahjoub, L-F. Pau, Masaaki Suzuki, Soren
Torstensson, Weiguo Wang, and Stephen Weinstein., "The
IEEE P1520 standards initiative for programmable network
interfaces.", Communications Magazine, IEEE 36, no. 10
(1998): 64-70. , 1998.
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Internet-Draft SDN Layers and Architecture Terminology July 2014
[PENet] Hedstrom, Brian, Akshay Watwe, and Siddharth Sakthidharan,
"Protocol Efficiencies of NETCONF versus SNMP for
Configuration Management Functions", PhD dissertation,
Master's thesis, University of Colorado, 2011 , 2011.
[PNSurvey99]
Campbell, Andrew T., et al, "A survey of programmable
networks", ACM SIGCOMM Computer Communication Review 29.2
(1999): 7-23 , September 1992.
[PPIPP] Bosshart, Pat, Dan Daly, Martin Izzard, Nick McKeown,
Jennifer Rexford, Dan Talayco, Amin Vahdat, George
Varghese, and David Walker., "Programming Protocol-
Independent Packet Processors.", arXiv preprint
arXiv:1312.1719 (2013). , 2013.
[RCP] Caesar, Matthew, Donald Caldwell, Nick Feamster, Jennifer
Rexford, Aman Shaikh, and Jacobus van der Merwe., "Design
and implementation of a routing control platform.", In
Proceedings of the 2nd conference on Symposium on
Networked Systems Design & Implementation-Volume 2, pp.
15-28. USENIX Association, 2005. , 2005.
[REST] Fielding, Roy, "Fielding Dissertation: Chapter 5:
Representational State Transfer (REST).", 2000.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC1953] Newman, P., Edwards, W., Hinden, R., Hoffman, E., Ching
Liaw, F., Lyon, T., and G. Minshall, "Ipsilon Flow
Management Protocol Specification for IPv4 Version 1.0",
RFC 1953, May 1996.
[RFC2297] Newman, P., Edwards, W., Hinden, R., Hoffman, E., Liaw,
F., Lyon, T., and G. Minshall, "Ipsilon's General Switch
Management Protocol Specification Version 2.0", RFC 2297,
March 1998.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
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Internet-Draft SDN Layers and Architecture Terminology July 2014
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412, December
2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3417] Presuhn, R., "Transport Mappings for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3417, December
2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62, RFC
3418, December 2002.
[RFC3535] Schoenwaelder, J., "Overview of the 2002 IAB Network
Management Workshop", RFC 3535, May 2003.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, May 2009.
[RFC5743] Falk, A., "Definition of an Internet Research Task Force
(IRTF) Document Stream", RFC 5743, December 2009.
[RFC5810] Doria, A., Hadi Salim, J., Haas, R., Khosravi, H., Wang,
W., Dong, L., Gopal, R., and J. Halpern, "Forwarding and
Control Element Separation (ForCES) Protocol
Specification", RFC 5810, March 2010.
[RFC5812] Halpern, J. and J. Hadi Salim, "Forwarding and Control
Element Separation (ForCES) Forwarding Element Model", RFC
5812, March 2010.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
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Internet-Draft SDN Layers and Architecture Terminology July 2014
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6632] Ersue, M. and B. Claise, "An Overview of the IETF Network
Management Standards", RFC 6632, June 2012.
[RFC7047] Pfaff, B. and B. Davie, "The Open vSwitch Database
Management Protocol", RFC 7047, December 2013.
[RFC7149] Boucadair, M. and C. Jacquenet, "Software-Defined
Networking: A Perspective from within a Service Provider
Environment", RFC 7149, March 2014.
[RINA] John Day, Ibrahim Matta, and Karim Mattar., "Networking is
IPC: a guiding principle to a better internet.", In
Proceedings of the 2008 ACM CoNEXT Conference, p. 67. ACM,
2008. , 2008.
[RouteFlow]
Nascimento, Marcelo R., Christian E. Rothenberg, Marcos R.
Salvador, Carlos NA Correa, Sidney C. de Lucena, and
Mauricio F. Magalhaes., "Virtual routers as a service: the
routeflow approach leveraging software-defined networks.",
In Proceedings of the 6th International Conference on
Future Internet Technologies, pp. 34-37. ACM, 2011. ,
2011.
[SDNACS] Diego Kreutz, Fernando M. V. Ramos, Paulo Verissimo,
Christian Esteve Rothenberg, Siamak Azodolmolky, Steve
Uhlig, "Software-Defined Networking: A Comprehensive
Survey.", arXiv preprint arXiv:1406.0440 , 2014.
[SDNHistory]
Feamster, Nick, Jennifer Rexford, and Ellen Zegura., "The
Road to SDN", ACM Queue11, no. 12 (2013): 20. , 2013.
[SLTSDN] Yosr Jarraya, Taous Madi, and Mourad Debbabi, "A Survey
and a Layered Taxonomy of Software-Defined Networking", To
be published in Communications Surveys and Tutorials, IEEE
Issue: 99 , 2014.
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[SoftRouter]
Lakshman, T. V., T. Nandagopal, R. Ramjee, K. Sabnani, and
T. Woo., "The softrouter architecture.", In Proc. ACM
SIGCOMM Workshop on Hot Topics in Networking. 2004. ,
2004.
[Tempest] Rooney, Sean, Jacobus E. van der Merwe, Simon A. Crosby,
and Ian M. Leslie., "The Tempest: a framework for safe,
resource assured, programmable networks.", Communications
Magazine, IEEE 36, no. 10 (1998): 42-53 , 1998.
Authors' Addresses
Evangelos Haleplidis
University of Patras
Department of Electrical and Computer Engineering
Patras 26500
Greece
Email: ehalep@ece.upatras.gr
Spyros Denazis
University of Patras
Department of Electrical and Computer Engineering
Patras 26500
Greece
Email: sdena@upatras.gr
Kostas Pentikousis
EICT GmbH
Torgauer Strasse 12-15
10829 Berlin
Germany
Email: k.pentikousis@eict.de
Jamal Hadi Salim
Mojatatu Networks
Suite 400, 303 Moodie Dr.
Ottawa, Ontario K2H 9R4
Canada
Email: hadi@mojatatu.com
Haleplidis, et al. Expires January 5, 2015 [Page 28]
Internet-Draft SDN Layers and Architecture Terminology July 2014
David Meyer
Brocade
Email: dmm@1-4-5.net
Odysseas Koufopavlou
University of Patras
Department of Electrical and Computer Engineering
Patras 26500
Greece
Email: odysseas@ece.upatras.gr
Haleplidis, et al. Expires January 5, 2015 [Page 29]