Hence the requirement for a different approach to routing, one that allows the network to transport data based not only on destination, hopcount and bandwidth but using rules defined externally in accordance with service requirements. The methodology of network policy is complex. Terms like policy routing, DEN, CIM, LDAP, RSVP, Cops, MPLS and class of service are all used to describe offerings and future developments. With such a plethora of technologies it is difficult to understand the role each one plays in the context of reliable service differentiation and provision.
Policies are the rules of preference that are assigned either to applications or users on a network.
Alternatively, certain users in an organisation, such as share dealers or executives, may perform business-critical functions so a policy might be defined to give their data priority. QoS is the measurable benefit delivered by policy networking in such cases, in terms of throughput and latency for each service. It can be enforced in different ways, including source node, application port, predefined class or type, labelling, bandwidth requirement or maximum allowed delay. In practice, provision for transport-sensitive applications requires both the application of a policy and the QoS mechanism to enforce it. The ACL rule can then be applied to give traffic on that port a faster interface, or Cisco's Weighted Fair Queuing can be used to give it preference.