SDN challenges
Device heterogeneity
SDN Scalability
SDN scalability: distributed controller, distributed apps
NOS: Network abstraction
Programming Abstractions
Configuration abstraction?
Implementation software challenges: Update abstraction
Update abstraction
Composition of modules
Testing and Debugging
SDN security issues

Composition of modules

Many network tasks: routing, access control, traffic monitoring, etc
All require packet handling rules installed in the same flow table
These rules may interact with one another, making it very difficult to decoupling the high level tasks from implementation.
Significant challenges in specifying the tasks and realizing the tasks.