ONC+ Developer's Guide

Message Classes

The concept of message class has been included to accommodate possible SAF extensions. The preceding messages are all class 1 messages. None of these messages contains a variable data portion. All pertinent information is contained in the message header.

If new messages are added to the protocol, they are defined as new message classes, for example, class 2. The first message that the SAC sends to a port monitor is always a class 1 message. Because all port monitors by definition understand class 1 messages, the first message that the SAC sends is guaranteed to be understood. In its response to the SAC, the port monitor sets the pm_maxclass field to the maximum message class number for that port monitor. The SAC does not send messages to a port monitor from a class with a larger number than the value of pm_maxclass. Requests fail if they require messages of a higher class than the port monitor can understand. For the Solaris environment, always set pm_maxclass to 1.

For any given port monitor, messages of class pm_maxclass and messages of all classes with values lower than pm_maxclass are valid. Thus, if the pm_maxclass field is set to 3, the port monitor understands messages of classes 1, 2, and 3. Port monitors cannot generate messages; they can only respond to messages. A port monitor's response must be of the same class as the originating message.

Because only the SAC can generate messages, this protocol function— even if the port monitor is capable of dealing with messages of a higher class than the SAC can generate.

pm_size is an element of the pmmsg structure. sc_size is an element of the sacmsg structure. These elements indicate the size of the optional-data part of the message. The format of this part of the message is undefined. Its definition is inherent in the type of message. For the Solaris environment, always set both sc_size and pm_size to 0.