What is Softwae Defined Network

(This is mainly Scott Shenkar’s Definition, but I wholeheartedly agree)

SDN is three abstractions aiming at abstract out simplicity on the network control plane (which is currently ad-hot ACL, middleboxes, DPI, and other functionalities).They are distributed state abstraction, specification abstraction, and configuration abstraction.

1. Distributed State abstraction -- centralized state
network states are physically distributed over many many switches. But that doesn’t mean we have to always deal with this. This distributed states should be abstracted out into a logically centralized task, where you are dealing with a global network view, i.e., some data structure, not some distributed states. Then this logically centralized task could be dealt with in whatever way you like, you could even distributedly do it for scalability when approiprate. But that is a distributed system problem, not a networking problem with inherently distributed states. And you are not forced to deal with network scale complexity.


2. Specification abstraction (or network virtualization) -- simple network view
Control program should describe functionality, not how to realize it in the particular physical network. So what the control program see should be virtual network view which is only complex enough to express its desire, not as complex as the actual underlying physical network.
e.g., for ACL problem, program should only see endpoint-to-endpoint network.


3. Configuration abstraction (or forwarding abstraction) -- hardware oblivious forwarding specification.
Configuration abstraction should expose enough to enable flexible forwarding decisions, but it should NOT expose the hardware details. (OpenFlow comes in, but only partially solve the problem here. It assumes switches are the unit of forwaring abstraction, instead of , say, a fabric).

All in all, SDN is NOT OpenFlow. SDN doesn’t have to happen in a datacenter neither. SDN is just reexamining how we manage the control plane of our network.


How to realize SDN (not that important, and you probably have seen this dozen of times...):

            control programs
     -----------------------------------------  (control program’s network view, or virtualized network)      
          virtualization layer
    ---------------------------------------------  (centralized network view, i.e, one data structure)      
common distribution layer (network OS)
     -------------------------------------------------  (physical, distributed network states)
physical network + switches