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SDN Control Plane Scalability Research Proposal
Yatindra Shashi
EIT-ITA, UNITN- 2015
I. INTRODUCTION
In the exponential growth rate of internet users and
devices, traditional network has reached to its bottleneck
causing the requirement of new architecture through
which we can control the network centrally. To tackle
this challenge, experts and companies develop Software
Defined Network (SDN). It not only gives central
authority access to control and manage future large size
network in order to have efficient and flexible packet
flow but also saves lots of money and time that
companies used to invest on operations and maintenance
in network. It works on the principle of decoupling the
control plane with the data plane to reduce the
management complexity. Though the SDN is
programmable and flexible, it faces different challenges
and among it I’m talking in this paper about the
scalability issue of control plane and suggesting a mixed
model for a large network.
II. STATE OF ART:ANALYSING DIFFERENT CONTROL
PLANE ARCHITECTURE
Scaling the centralized control plane is essential to avoid
performance bottleneck and also to avoid single point of
failure. But as the network is very big in case of data
centers & WAN we need to make optimum scalable
model so that higher number of packets are responded in
very less time. To solve this scalability problem,
researcher’s has proposed different architectural modal
among which popular ones based on the approaches are
described in short below:
A. Distributed Multiple Controllers
i) Kandoo:
Kandoo runs on a principal to limit the overhead of
events between the data and control planes, by applying 2
tier controller in a network topology. Local controllers
are located close to the switches for local event
processing and root controllers are located centrally for
network-wide routing solutions. In practice, local
controllers will process the micro flows and trigger the
root controller when an elephant flow arrives [4]. This
flow reduces the packets towards the central controller
and minimizes the traffic overload towards central
controller.
ii)ONIX:
ONIX is a distributed system running a one or more
physical servers each of which may run multiple Onix
instances. In this developers have separated management
functions used for link status and topology monitoring
from the network control to reduce workload for general
control like flow table updating [4]. It introduces the
concept of Network Information Base (NIB), the data
structure that gathers the network state information and
distribute among all ONIX nodes of cluster to ensure
scalability, resiliency and consistency. In order to
calculate global flows, ONIX aggregate information from
multiple area platforms to a global distributed platform
and implement it. The ONIX connects with the switch
through Import/Export module to gather the information
and to manage forwarding rules, flow tables etc. In ONIX
a local area flows are decided by local network controller
whereas global flows are made based on the aggregated
information from multiple area platforms. As the ONIX
controller has divided control plane for separate function
of actions and NIB helps for the global flows, it can
process high number of request with minimum delay.
ii) HyperFlow:
HyperFlow improves the scalability and reliability of
software defined networks by partitioning the network to
several physically distributed controllers. A relatively
large scale network is basically a collection of
interconnected sub-networks. Each sub-network is then
able to be controlled by a controller, and we assume that
the connections among sub-networks have bandwidth
reserved for control messages. Each controller will be
able to make decisions without reactively consulting
others to retrieve relevant information.
B. More Responsibility to Data Plane
i) DIFANE (Distributed Flow Architecture for Networked
Enterprises):
In this model network is partitioned into several
pieces depending on the scale and each piece is managed

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by one authority switch. The controller will proactively
interact with the authority switches, hoping that the rules
can be installed before they become active and imply to
keep all the traffic in the data plane [3]. Switches handle
all the packets in the data plane and access the authority
switches in case of need of forwarding rules. Hence
controlling the number of message towards the central
controller and increasing scalability.
ii)DevoFlow:
In DevoFlow with support from an ASIC, short-lived
flows (flows which have less impact when the control
plane makes decision) are handled in the data path, and
only larger flows are forwarded to the controller to
reduce control plane communication overhead [3].
III. MY PROPOSED SOLUTION: HYBRID MODEL
In my opinion to limit the load on the controller,
frequent event should be managed on the local vicinity
without modifying the switch. Adding new functionality
as in DevoFlow or DIFIANE to the switch breaks the
general principle of the SDN i.e. complete decoupling of
the data plane with the control plane and also increases
the costly process of modification. So I need to search for
another solution which follow SDN principle as well
scale the network.
So, I want to propose the solution for scalability issue
of large networks like WAN or big data center as:
 To limit the load on the central controller we
will introduce local controller for specific
number of switch as in Kandoo architecture.
 And on top of it, logical central controller
which has global view of the network will be
implemented. To manage the remaining work
load towards the central logical controller and
also for failover reliability we will use
distributed multiple physical controllers as in
the ONIX model.
 Central controller will be having cluster of
ONIX instances with the shared NIB and it
also maintains a consistent network-wide
view in the entire controller.
In this architecture we will have hierarchical as well
as distributed controller scaling approach which will
enhance the capability of controller to handle millions of
flow per second with less latency. For the global routing
Onix logical central controllers will make the rules and
forward it to the local controller. In this design, the
computation and storage capacity of a single logical
controller is multiplied as the number of nodes in the
cluster increases, which is ideal for a component
controller for a large scale software-defined network [3].
As there are some network applications flows which
don’t need to have global view and can be managed in
the local controller, this model will be efficient one for it.
Mathematical modeling of the distributed control
architectures shows that there are lots of improvement in
performance compare to simple OpenFlow structure for
higher number of packets. A single Kandoo controller
can reach a throughput of more than 1Million packet in
per second from 512 switches using a single thread on a
Xeon E7-4807[2]. So I believe that proposed model will
be able to serve more than 1Million packet with less
delay as it’s central controller has shared memory as well
as sync NIB data for fast and efficient global flow.
Proposed model not only increases the number of flow
that it servers but also provide redundant central network
for high reliability.
Fig a. Proposed Architecture sample diagram
There might be a question going on why not mixing
architecture with DevoFlow or DIFIANE as they limit
the load on controller. Reason is that though it limits the
load on the controller, it violates the principle of SDN i.e
complete decoupling of the control plane and data plane.
Lots of modification will be needed to switch which will
increase the cost and on the same time manufacturer may
need to make switch based on the architecture rather than
open standard switch. In my opinion development of
switch should be kept independent of control plane
architecture and requirement. So I don’t support.
IV. BUSINESS AND INNOVATION IMPACT
In the current scenario of communication technology,
there is always growth in demand of high speed data and
fulfilling demand by communication operators to earn
has always increased cost burden to them. Investigating
to find innovative mechanism which will reduce the
capital investment and operating cost has been essential
to have profitable business.

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Industries and researcher’s after long investigation to
find innovative mechanism has found SDN, a new
approach towards networking dedicated to transform the
society. Techno-economic analysis of SDN put forward
following major aspect as supporting fact for SDN as
new technical business innovation in networking:
 It provides platform for more innovation in
the networking domain by allowing freedom
to entrepreneurs to develop network
applications on the top of OS for efficient
and cost effective management of the
network. As it is open standard based,
innovators can develop and use their own
methods to design network and operate it.
 Separating the control and forwarding logic
in SDN has reduced vendor dependency and
increased the competition among them to
produce efficient hardware in low cost.
 It also imposed independent management
control which has opened the concept of
sharing hardware between operators and
running their own applications. As the
sharing hardware will reduce the cost of
investment and operation of infrastructure it
will encourage new innovating entrant to
work on communication domain stimulating
high degree of competition in this area [5].
Now talking about the proposed architecture, there is
more scaling in the software than in hardware which is
comparatively less costly than other architecture as
investment and maintenance cost of software is less than
other. Additionally, in this architecture cost of power and
cooling will be also reduced as used to be for the control
plane in the network switches. It also gives better traffic
steering, hence reducing the number of the switches as
well. Its faster testing time, less devices to update and
robustness gives smooth initial implementation. Thus it
reduces initial cost and time which is precious to any
operator or network service provider.
Now days skilled networking personnel are required
to set up, configure and maintain the network and that
also on different locations which has increased the cost to
the company. As proposed network gives global view of
network that also with the cluster of controller, a skilled
authority can access network centrally and do required
changes with in less time and budget. As proposed
architecture have always updated global traffic routing
data and control over all the switch of cluster it reduce
the current practice of over provisioning done on the
switch of local router.
As we all know, in the networking industry, 80% of
the expanses are in operation and support, by
implementation of the SDN will bring revolution not
only in cost of operation but also in the performance. I
believe that, one day SDN will do market transformation
towards new era of networking with the open source
community. This innovative mechanism will reduce the
expanses of network operator, hence giving high profit
which can be further used for not only its development
but also for the society.
REFERENCES
[1] Bruno Astuto A. Nunes, Marc Mendonca, Xuan-Nam Nguyen,
Katia Obraczka, and Thierry Turletti, “A Survey of Software-
Defined Networking: Past,Present, and Future of Programmable
Networks” IEEE Communication surveys & Tutorials, Vol. 16,
No. 3, third quarter2014
[2] Soheil Hassas Yeganeh, Yashar Ganjali, “Kandoo: A
Framework for Efficient and Scalable Offloading of Control
Applications” HotSDN’12, August 13, 2012, Helsinki, Finland
[3] Shuhao Liu and Baochun Li , “On Scaling Software-Defined
Networking in Wide-Area Networks” Tsinghua Science and
Technology ISSNll1007-0214ll01/10llpp221-232 Volume 20,
Number 3, June 2015
[4] Benjamin J. van Asten, Niels L. M. van Adrichem and
Fernando A. Kuipers, “Scalability and Resilience of Software-
Defined Networking: An Overview”, arXiv:1408.6760v1 [cs.NI]
28 Aug 2014
[5] Bram Naudts_, Mario Kindy, Fritz-Joachim Westphaly, Sofie
Verbrugge_, Didier Colle_ Mario Pickavet. “Techno-economic
analysis of software defined networking as architecture for the
virtualization of a mobile network” IEEE SDN European
Workshop,ISBN 978-1-4673-4554-5, 25-26 Oct. 2012
.