  Software-Defined Networking (SDN) is an evolutionary networking paradigm
which has been adopted by large network and cloud providers, among which are
Tech Giants. However, embracing a new and futuristic paradigm as an alternative
to well-established and mature legacy networking paradigm requires a lot of
time along with considerable financial resources and technical expertise.
Consequently, many enterprises can not afford it. A compromise solution then is
a hybrid networking environment (a.k.a. Hybrid SDN (hSDN)) in which SDN
functionalities are leveraged while existing traditional network
infrastructures are acknowledged. Recently, hSDN has been seen as a viable
networking solution for a diverse range of businesses and organizations.
Accordingly, the body of literature on hSDN research has improved remarkably.
On this account, we present this paper as a comprehensive state-of-the-art
survey which expands upon hSDN from many different perspectives.
