RFC 1228 (rfc1228) - Page 2 of 50

SNMP-DPI: Simple Network Management Protocol Distributed Program Interface

RFC 1228                        SNMP-DPI                        May 1991


   potentially interesting variable which is not in the core MIB would
   be CPU utilization (percent busy).  Unfortunately, conventional SNMP
   agent implementations provide no means for an end-user to make
   available new variables.

   The SNMP DPI addresses this issue by providing a light-weight
   mechanism by which a process can register the existence of a MIB
   variable with the SNMP agent.  When requests for the variable are
   received by the SNMP agent, it will pass the query on to the process
   acting as a sub-agent.  This sub-agent then returns an appropriate
   answer to the SNMP agent.  The SNMP agent eventually packages an SNMP
   response packet and sends the answer back to the remote network
   management station that initiated the request.

   None of the remote network management stations have any knowledge
   that the SNMP agent calls on other processes to obtain an answer.  As
   far as they can tell, there is only one network management
   application running on the host.

THEORY OF OPERATION

CONNECTION ESTABLISHMENT

   Communication between the SNMP Agent and its clients (sub-agents)
   takes place over a stream connection.  This is typically a TCP
   connection, but other stream-oriented transport mechanisms can be
   used.  As an example, the VM SNMP agent allows DPI connections over
   IUCV (Inter-User Communications Vehicle) [6, 7].  Other than the
   connection establishment procedure, the protocol used is identical in
   these environments.

REGISTRATION

   Regardless of the connection-oriented transport mechanism used, after
   establishing a connection to the SNMP agent, the sub-agent registers
   the set of variables it supports.  Finally, when all the variable
   classes have been registered, the sub-agent then waits for requests
   from the SNMP agent or generates traps as required.

DPI ARCHITECTURE

   There are three requests that can be initiated by the SNMP agent:
   GET, GET-NEXT and SET.  These correspond directly to the three SNMP
   requests that a network management station can make.  The sub-agent
   responds to a request with a RESPONSE packet.

   There are currently two requests that can be initiated by a sub-
   agent: REGISTER and TRAP.



Carpenter & Wijnen