  As networks expand in size and complexity, they pose greater administrative
and management challenges. Software Defined Networks (SDN) offer a promising
approach to meeting some of these challenges. In this paper, we propose a
policy driven security architecture for securing end to end services across
multiple SDN domains. We develop a language based approach to design security
policies that are relevant for securing SDN services and communications. We
describe the policy language and its use in specifying security policies to
control the flow of information in a multi-domain SDN. We demonstrate the
specification of fine grained security policies based on a variety of
attributes such as parameters associated with users and devices/switches,
context information such as location and routing information, and services
accessed in SDN as well as security attributes associated with the switches and
Controllers in different domains. An important feature of our architecture is
its ability to specify path and flow based security policies, which are
significant for securing end to end services in SDNs. We describe the design
and the implementation of our proposed policy based security architecture and
demonstrate its use in scenarios involving both intra and inter-domain
communications with multiple SDN Controllers. We analyse the performance
characteristics of our architecture as well as discuss how our architecture is
able to counteract various security attacks. The dynamic security policy based
approach and the distribution of corresponding security capabilities
intelligently as a service layer that enable flow based security enforcement
and protection of multitude of network devices against attacks are important
contributions of this paper.
