Overview of the CRNP Protocol

The CRNP defines the Application, Presentation, and Session layers of the standard seven layer Open System Interconnect (OSI) protocol stack. The Transport layer must be TCP and the Network layer must be IP. The CRNP is independent of the Data Link and Physical layers. All Application layer messages that are exchanged in the CRNP are based on XML 1.0.

Semantics of the CRNP Protocol

Clients initiate communication by sending a registration message (SC_CALLBACK_RG) to the server. This registration message specifies the event types for which the clients want to receive notification as well as a port to which the events can be delivered. The source IP of the registration connection and the specified port, taken together, form the callback address.

Whenever an event of interest to a client is generated within the cluster, the server contacts the client on its callback address (IP and port) and delivers the event (SC_EVENT) to the client. The server is highly available, running within the cluster itself. The server stores client registrations in storage that persists even after the cluster is rebooted.

Clients unregister by sending a registration message (SC_CALLBACK_RG, which contains a REMOVE_CLIENT message) to the server. After the client receives an SC_REPLY message from the server, the client closes the connection.

The following diagram shows the flow of communication between a client and a server.

Figure 12–2 Flow of Communication Between a Client and a Server