White Paper - Remote Management SNMP SubAgent

1. Basics of SNMP
The SNMP defines how the communication between two network nodes happens, message type and
format, and message interpretation in accordance with its standards. The SNMP management
application communicates to the SNMP agent to get management information from the managed
device. The SNMP message is wrapped in the UDP packet and that inturn is wrapped in the IP packet.
The SNMP resides in the application layer. The layered communication model for SNMP message is
shown in Exhibit 1. The SNMP manager issues GET, GETNEXT and SET messages to the agent. The
GET and GET-NEXT messages are used by management application as requests to the agent. The
agent issues GET-RESPONSE message to the manager with either the information requested or an
error indication as to why the request could not be processed. A SET message allows management
application to modify the value of a specific variable. Again agent would respond with the GET-
RESPONSE message with an indication whether the variable is modified or not. The general format
for all types of SNMP message is shown in Exhibit 2.
Exhibit 1: Layered Communication of SNMP Messages
SNMP manager and agent use management information base (MIB) along with a small set of
commands to communicate with each other. On the managed device, there are few variables in which
management application is interested. These variables are represented in the tree structure in the
MIB. Each variable in the MIB is termed as OID (Object Identifier). The OID for every variable is unique
within the MIB. The MIB is also a guide the capabilities of managed device or node.
2. SNMP Notification Messages
The SNMP management application may be responsible for managing a large number of network
devices. The management application is always interested in the change of state of managed device.
This is certainly not practical for management application to poll all the managed devices for a specific
event. Therefore, the SNMP agent on each managed device notifies the management application
without solicitation. For SNMP v1 and v2c, these event messages are SNMP trap of the event. For
SNMP v3, trap and inform are two types of notification message for such event. The SNMP inform
message is reliable. The receiver management application sends the acknowledgement. The default
values for retries and timeout is 3 and 30 seconds, respectively. This is the only message that is
generated by the agent to inform or alert the management application if a specific variable is
behaving unexpectedly. After receiving the SNMP trap, the management application can choose to
take an action. The SNMP traps and details of its format are defined in RFC 1157 [2].
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