The IMS: IP Multimedia Concepts And Services, Second Edition

21.1. Introduction

The COPS protocol is an Internet Engineering Task Force (IETF) protocol used for the general administration, configuration and enforcement of policies. It defines a simple query and response protocol for exchanging policy information between a policy server and its clients. The clients are denoted as Policy Enforcement Points (PEPs) and the server as a Policy Decision Point (PDP),^[] respectively. The protocol employs a client/server model in which a PEP sends requests, updates and deletes to the PDP, which in turn returns policy decisions back to the PEP. A special type of PDP is the Local Policy Decision Point (LPDP), which is used by PEPs to request local policy decisions when there is no available PDP with which to communicate. Figure 21.1 illustrates the model.

^[] In this chapter PDP stands for Policy Decision Point and should not be confused with Packet Data Protocol.

There are two main models for COPS policy control:

• Outsourcing – the PEP assigns (outsources) responsibility for authorizing certain events at the PEP to an external entity (PDP). This model assumes a one-to-one correlation between events at a PEP and decisions from a PDP.

• Configuration – unlike the previous model, there exists no direct mapping between events at the PEP and decisions from the PDP. The PDP may proactively configure the PEP based on any external events as well as events originating at the PEP. This may be performed by the PDP in bulk or in portions, but the overall timing is ...