Page 62 - ITUJournal Future and evolving technologies Volume 2 (2021), Issue 1

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 1

management and con iguration of the telecommunication 3. STATE OF THE ART

infrastructure. Therefore, leveraging SDN to automate

This section presents an analysis of state–of–the–art

management and con iguration tasks is likely to improve
the return on investment (ROI). controller placement solutions.

This work presents a framework which can be used 3.1 Related work

by network operators to optimize their SDN controller

placement during the deployment phase. To date, there has been numerous research studies

directed towards addressing the controller placement

The paper is organized as follows: Section 2 presents problem in SDN. These can be broadly ied into

the problem being addressed by this work, Section two categories: (i) studies that implemented exhaustive

3 describes related work and their drawbacks, and algorithms, as exampled by [8] –[9] and (ii) studies that

highlights our contributions, Section 4 describes implemented heuristic algorithms, as exampled by [16]

the algorithms (based on mathematical modelling) –[30].

used to solve the controller placement problem,

Section 5 provides implementation details of the The controller placement problem was irst introduced

algorithms, Section 6 presents results from our by Heller et al. [8] in 2012. The authors study

mathematical modelling, Section 7 describes the the controller placement problem by investigating the

emulation experiments conducted to verify the outcome impact of uncapacitated controller location on average
and worst–case latency. The algorithm used in this

of the mathematical modelling, Section 9 concludes the

paper. Lastly, Section 10 describes future work. study is k–center. To maintain realism, the authors

tested their algorithm on the Internet2 OS3E topology

2. PROBLEM ST ATEMENT [10]. Their results indicate that increasing the number

of controllers decreases the overall network latency

Although Local Area Networks (LANs) like Data Center with a icant trade‑off between worst–case and

Networks (DCNs) have already ited from SDN, average latency. The authors conclude that deploying

deploying SDN in real Wide Area Networks (WANs) still one controller often ices to meet existing latency

poses several design challenges. As the centralized requirements in campus networks. Expectantly, they also

brain of the network, an SDN controller must be able argue that one controller is not suf icient for large‑scale

to respond to control requests promptly. Moreover, deployments with fault tolerant requirements.

control tasks such as data‑plane monitoring, must

Hu et al. [11] proposes the use of multiple controllers

be performed as iciently as possible to maintain

to ensure reliability in the control‑plane. To optimize

up‑to‑date state information. This requires optimization

on the southbound interface. Due to the signi icant controller placement, the authors carry out a comparative

evaluation of optimization algorithms, namely random

luence of propagation latency (switch‑to‑controller

placement, l–w greedy and brute force. They focus

latency) on WAN performance, controller placement has

their reliability metric on the ”expected percentage of

emerged as a crucial design problem that in luences

valid control paths”, where a control path is de ined

SDN’s southbound performance. Controller placement

as the interface between the switch and controller

ines the location of SDN controllers relative to

(southbound interface) as well as the connection between

the data‑plane elements, that yields better network

perfomance. controllers (east/westbound interface). The algorithms
were evaluated on Internet2 topology as well as various

Another aspect to controller placement has to do with ISP topologies from the Rockefuel database [12]. From

the number of controllers deployed in a given WAN. their simulations, random placement produced the least

Deploying a certain number of controllers has an impact optimal results, while brute force produced optimal

on several objectives such as propagation latency and results after a icantly long runtime. As a result,

reliability. Even though the number of controllers may the authors recommend the l–w greedy as the most

be known in advance, the location of these controllers optimal solution. This work is similar to Gong et al.

usually needs to be optimized to meet user requirements [13] in that they both aim to optimize reliability in
and contraints. the event of node or link failure. However, latency

(both switch–to–controller and inter–controller latency)

Therefore, the overall problem that must be addressed and load balancing are not considered in these research

is: given a real SDN‑enabled WAN, how many SDN works. Moreover, the number of controllers is assumed

controllers are needed and where should they go to to be known in advance.

optimize ined requirements and constraints

while maintaining an acceptable runtime and accuracy. Tanha et al. [9] study the controller placement problem

This is a multi‑objective optimization problem and to optimize network resilience in the event of controller
constitutes competing objectives. It is necessary to

address this problem during the early stages of SDN

planning.
46 © International Telecommunication Union, 2021