The IMS: IP Multimedia Concepts And Services, Second Edition

21.2. Message structure

COPS is a binary protocol whose messages consist of the COPS header followed by typed objects. The COPS header (Figure 21.2) consists of the following fields:

• Version – indicates the COPS version number (its default is 1).

• Flags – denotes certain message flags. The only one currently defined is 0x1, which indicates whether the message was solicited by another COPS message.

• OP code – denotes the COPS operation. Table 21.1 lists the defined COPS operation codes.

• Client type – identifies the policy client and determines the interpretation of the accompanying typed objects, with the exception of a Keep-Alive message that always has client type set to zero.

• Message length – denotes the octet length of the message.

Figure 21.2. COPS header.

Table 21.1. COPS operation codes.

Value	Short	Where	Name	Description
1	REQ	PEP → PDP	Request	Requests a decision from a PDP and establishes a client handle that identifies the specific state associated with this PEP
2	DEC	PDP → PEP	Decision	Returns one or more decisions as a response to a request
3	RPT	PEP → PDP	Report state	Reports back to the PDP the PEP's success or failure in carrying out the PDP's decision, or reports a change in state
4	DRQ	PEP → PDP	Delete request state	Indicates to the PDP that the state associated with the client handle is no longer relevant
5	SSQ	PDP → PEP	Synchronize state request	Indicates that the remote PDP wishes the PEP to ...