Software-defined networking (SDN) is a well-known example of a research idea that has been reduced to practice in numerous settings. Network virtualization has been successfully developed commercially using SDN techniques. This paper describes our experience in developing production-ready, multi-vendor implementations of a complex network virtualization system. Having struggled with a traditional network protocol approach (based on OpenFlow) to achieving interoperability among vendors, we adopted a new approach. We focused first on defining the control information content and then used a generic database protocol to synchronize state between the elements. Within less than nine months of starting the design, we had achieved basic interoperability between our network virtualization controller and the hardware switches of six vendors. This was a qualitative improvement on our decidedly mixed experience using OpenFlow. We found a number of benefits to the database approach, such as speed of implementation, greater hardware diversity, the ability to abstract away implementation details of the hardware, clarified state consistency model, and extensibility of the overall system.
In this paper we consider the formally symmetric differential expression M[·] of any order (odd or even) ≥ 2. We characterise the dimension of the quotient spaceis the sesquilinear form in f and g associated with M. These results generalise the wellknown theorem that M is in the limit-point case at ∞ if and only if [ f g](∞) = 0 for every f , g ∈ the maximal domain ∆ associated with M.Keywords. Limit classification, minimal and maximal closed operators; symmetric operators, self-adjoint operators; quotient space D(T max )/D(T min ).
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