Northbound, Southbound, and East/Westbound. What do they mean?

We hear a lot of directions when we are talking about Data Center technologies: Northbound, Southbound, and even Eastbound/Westbound. What do they mean? Why are they called this way?

Software Defined Networking (SDN)

Source: Wikipedia

For minimum discussion of SDN, three layers are defined inside an SDN. They are SDN Service Layer, Controller, and Physical Network.

Controller is like a Quarterback in Football

In Football, a Quarterback leads the attacks. All players of the same team should listen to and act as a Quarterback signals them to do.

In SDN, all operations of Physical Network objects should listen to and act as Controller signals them to do. So, Controller is so similar to a Quarterback. Controller is always the focus of the game.

Upper side of the map is the north. Lower side is the south.

From the point of Controller's view, anything above (e.g. Service Layer) is at the north of Controller. Anything below (e.g. Physical Network) is at the south of Controller.

Therefore, for Application Programming Interface (API) communicating between the Controller and Physical Network is called Southbound API. On the other hand, API communicating the Service Layer and Controller is call the Northbound API.

The most discussed and implemented Southbound API is OpenFlow, which is defined by Open Network Foundation (ONF).

The most discussed and implemented Northbound API is RESTful Open Daylight project.

OK, then the rest of this post is NOT related to SDN at all!


For "traffic going Northbound/Southbound", it refers to the traffic among clients and servers.

When we draw a diagram, most of the time we put the clients at the north side, and servers at the south side. Therefore, "traffic going Northbound/Southbound" is indeed the traffic among clients and servers.

Northbound/Southbound Traffic = Client/Server Traffic

"Traffic going Eastbound/Westbound" refers to the traffic among the servers.

Just like a map, most of the time the left hand side is the west, and right hand side is the east. So the traffic going Eastbound/Westbound is the traffic going horizontally inside a diagram.

"Traffic going Eastbound/Westbound is indeed the traffic among the servers.

Eastbound/Westbound Traffic = Server to Server Traffic

With new computing models such as Big Data, traffic among servers is increasing much more rapidly than before. Traditional network designs are serving well only for huge client/server traffic but not for huge server/server traffic. Many solution providers such as Cisco Systems are developing new solutions to this problem.