SNMP agents are primarily responsible for responding to SNMP operation requests. An operation request can originate from any entity that supports the management portion of the SNMP protocol. An example of this is z/OS UNIX SNMP command, osnmp, included with this version of TCP/IP. Examples of SNMP operations are GET, GETNEXT, and SET. An operation is performed on an MIB object.

A subagent extends the set of MIB objects provided by the SNMP agent. With the subagent, you define MIB objects useful in your own environment and register them with the SNMP agent.

When the agent receives a request for an MIB object, it passes the request to the subagent. The subagent then returns a response to the agent. The agent creates an SNMP response packet and sends the response to the remote network management station that initiated the request. The existence of the subagent is transparent to the network management station.

For the TCP connections, the agent binds to an arbitrarily chosen TCP port and listens for connection requests. A well-known port is not used. Every invocation of the SNMP agent could potentially use a different TCP port.

For UNIX streams connections, the agent is within the same machine. AF_UNIX connections should be used if possible, because they do not pass into TCP/IP, but flow only within UNIX System Services and hence require fewer system resources.

A DPI SNMP Subagent does not have to directly retrieve a dpiMIB object or objects, but instead uses either DPIconnect_to_agent_TCP() or DPIconnect_to_agent_UNIXstream(). DPIconnect_to_agent_TCP automatically retrieves the object dpiPortForTCP from the dpiMIB through an SNMP agent. DPIconnect_to_agent_TCP then establishes an AF_INET6 or AF_INET TCP socket connection with the SNMP agent.

The query_DPI_port() function issued in Version 1.1 is implicitly run by the DPIconnect_to_agent_TCP() function. The DPI subagent programmer would normally use the DPIconnect_to_agent_TCP() function to connect to the agent, and hence does not need to explicitly retrieve the value of the DPI TCP port.

Conversely, DPIconnect_to_agent_UNIXstream retrieves the value of the object dpiPathNameForUnixStream from the dpiMIB to establish an AF_UNIX connection with the SNMP agent.

After a successful connection to the SNMP agent the subagent registers the MIB trees for the set of variables it supports with the SNMP agent. When all variable classes are registered, the subagent waits for requests from the SNMP agent.

If connections to the SNMP agent are restricted by the security product, then the security product user ID associated with the subagent must be permitted to the agent's security product resource name for the connection to be accepted. See the Simple Network Management Protocol (SNMP) information in z/OS Communications Server: IP Configuration Guide for more information about security product access between subagents and the z/OS® Communications Server SNMP agent.