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Control and Management Planes – Part 2
As mentioned last time, SDN theorists declare that SDN is the networking technology that separates the forwarding plane from the control plane. Of course the logical separation of networking into planes has been conventional for three decades, and time-honored tradition distinguishes three planes – those of forwarding, control, and management.
SDN’s innovation is that its separation into planes is physical, not merely logical. Conventional network elements comprise both forwarding hardware (for real-time handling of user packets) and control protocol software (for interaction with other network elements), which although logically distinct, are collocated in the same box. SDN’s white box switches retain only unadorned forwarding functionality, while the control functionality is physically, indeed geographically, removed from the network location. In particular SDN white box switches do not participate in any distributed protocols, the kind used by routers to exchange information with each other. In an SDN network the control functionality is relocated to a centralized omniscient God Box (called the SDN controller) and local forwarding behavior is dynamically configured from there. SDN reinstates the PSTN’s architecture of centralized intelligence, which had been previously abandoned by IP networks for one of completely distributed intelligence.
Incidentally, other networking technologies don’t align as well with this dichotomy. Basic Ethernet switches are plug-and-play, and ergo no centralized entity is needed. So basic Ethernet can be considered to have distributed intelligence. However, the forwarding behavior of Carrier Ethernet networks is dictated by a centralized Network Management System, and so they are definitely in the same camp as SDN networks. In similar fashion, LSPs of LDP-based MPLS follow the paths found by distributed routing protocols, but PCE-based traffic engineered MPLS presaged SDN well before the term SDN was invented.
Now that we understand the novelty of SDN’s separation of planes, we need to inquire into the relationship between these planes and the conventional ones. Is SDN’s control plane to be equated with networking theory’s control plane, or its management plane, or some combination of both?
On the one hand, SDN enthusiasts call the connection between the controller and the SDN switches the control plane, a term reserved in networking theory for dialogs between intelligent entities. On the other hand, in order to emphasize the lack of distributed routing protocols, SDN designates as the southbound API. The software term Application Programming Interface is conjectured to be in sharp contrast with the networking term protocol. An interface is merely a conduit for commands and responses, not a dialog between equally intelligent peers. Such an interface is what networking theory would consider a management entity. Although the original raison d’etre for this interface may have been to allow experimentation into new networking concepts, the business driver for such an interface is its facilitation of automation.
Taking this into account, we can now reword the definition of SDN. SDN is a networking technology that entirely eliminates the distributed control plane, and moves its functionality into a centralized management plane (called the control plane in SDN terminology).
If two planes suffice for SDN, why are three planes still deemed necessary by networking theorists? Distributed control protocols are robust and continue to function in the presence of network failures; however, they are limited in capability, especially in the optimizations they can achieve. Centralizing all the network information leads to the ability to perform arbitrary optimizations, but at the expense of a single point of failure.
We will delve more deeply into these issues in the next and final blog post.
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