RFC 1228 (rfc1228) - Page 2 of 50
SNMP-DPI: Simple Network Management Protocol Distributed Program Interface
Alternative Format: Original Text Document
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